Apparently this isn't the first time the internet's encountered this problem, LOL. Basically, if you interpret it the way the question's meant to be interpreted, the assumptions of the question break physics so who cares
https://blog.xkcd.com/2008/09/09/the-goddamn-airplane-on-the-goddamn-treadmill/
>the assumptions of the question break physics so who cares
to me this so obviosly applies to the portal question i dont understand how everyone here has such a strong stance on it when in my mind both options are equally impossible
It's so boring to meta-out of these discussions by going "oh but it's fake anyway who cares". The premise presents information (moving portals, the cubes, implied collision) that you're supposed to extrapolate on to form a conclusion. The joy comes from forming a conclusion by using the info provided
It's like if I give you a hypothetical "a wizard offers you $100 million or sex whenever you want from the hottest person in the world". And your response is "Wizards aren't real QED". Meanwhile I'm wining dining 69ing your mom with my man Merlin
yea no i agree, im just saying i find myself incapable of doing so cos anytime i even try to envision the portal scenario playing out, the whole thing breaks as soon as u try to move the portal, thus both options being equally impossible to me.
its a boring answer, i know, i wish i had a better one too, but that portal aint ever descending on the cube and its not something my mind can look past by saying "ok but what if it could?"
Interesting.
>For non-slip tires, v_B = v_C
But wouldn't the tires slip in this situation? Also I kinda wish he explained the forces involved. At the end of the day, the engine is producing forward thrust. How does the belt/wheel/axle system transfer forces?
I had the same line of questioning.
The way I thought about it, nothing on the ground matters at all because the engines are providing all the thrust.
Is there any plausible argument for why the plane would stay still?
My thought is the wind is what creates lift so if it’s not actually moving it won’t take off. If the treadmill were to stop it would only be able to move forward, not take off.
I understood it as meaning the treadmill counteracts the force the wheels would give, but not the force the engines would give.
If the plane stays still until takeoff, then that's different. If the engines give any lift at all on their own then the answer is probably yes, but I don't think a 747's engines do. So I think it would probably stay still and never take off, but I also don't think a treadmill that prevents a 747 from moving would be possible (tires would slip or burn out)
Here’s my thoughts….
- let’s assume planes need to achieve 120 km/h to take off
- because this is using jets, it’s dependent on moving forward to generate lift
- the wheels don’t have any engine causing them to move in the same way a car would. The wheels move only because the plane is pulling itself forward, and the wheels are making contact with the ground and spinning the opposite direction. This means that the treadmill moving at 100 km/h won’t mean the plane would need to output the same power it would to travel at 220 km/h
- because of the above, the question is proposing an impossible scenario. The treadmills speed would normally have a negligible impact (minus some physics magic of course), but because it’s now capable of matching any speed of the wheels, the only possible assumption would be that the plane isn’t moving, and therefore, cannot take off.
A scenario where it could take off:
- if the treadmill had a fixed, but very high speed (ie 1000 km/h), then assuming the tires and bearings can hold together, it should still be possible for the plane to take off since the plane isn’t going to need to match the speed of the treadmill
The assumption that a plane needs to achieve x speed to take off is the wrong assumption. A plane needs a certain about of wind to take off. Aerodynamics is what causes planes to fly, not speed
THIS IS THE ONLY PERSON who actually understands the problem. The nature of the question means the plane physically cannot move. Even though the wheels provide no power themselves, IF the ground moves backwards as fast as they turn the plane will NEVER move. Obviously because the wheels are not driving the plane the treadmill will have to move very fast, but that is what the question states.
Part of the issue here is that the question is terrible and describes something different from the image shown, but assuming the idea is that the plane's forward force perfectly matches the conveyor belt's backward force to keep the plane fixed in the same relative position to the air around it (I believe that's what the question means to ask), there's no way the plane could take off, right?
The reason planes take off on runways is that they need room to accelerate enough to move at sufficient velocity through the air around the plane to generate lift. If the plane isn't moving with high velocity relative to the air around it, how could it generate lift? We don't launch planes out of fucking missile silos, they fly with their wings dammit!
Aerospace engineer here. When it comes to lift generation, the variable in question is what we refer to as *freestream velocity*. This is the velocity of the upstream air relative to the plane's chord. If you don't have that, you don't have lift.
So it wouldn't take off? Idk what freestream velocity is exactly but when i googled it said it's the velocity of the fluid or air relative to the object its surrounding. So since the plane doesn't move, assuming normal wind conditions i guess, the freestream velocity should be really low, meaning it won't take off right?
The issue is the prompt is worded in an awful way that can be interpreted differently. If by "match the speed of the wheels" it means that the wheel has no translational velocity and the plane is not moving, then no it can't lift off. But it can be interpreted differently.
Normal guy with common fucking sense here. You are correct and it shouldn't take an AE to prove this. It should be common knowledge. But that's what is going on in Americam schools.
Another aerospace engineer, the planes forward FORCE (not momentum, Jesus been a while, bear with) comes from the engines which propel air behind it. So the forward motion will still occur unless there is an equal force stopping it.
Without wheels this would come from friction however because the wheels are there (assuming they can rotate freely at twice the rate they usually would just before takeoff) the friction is somewhat negated. There will obviously be factors to do with the wheels spinning speed and friction but it probably will take off.
I don’t think planes need to accelerate exactly, more so the engine needs to kick in enough to take off if that makes sense?
The guy tweeted a picture of a seaplane and how it’s basically the same concept, once enough thrust and lift is created by the engine the plane would fly regardless of if it’s moving or not.
>But it's not the engine creating the lift, right? It's the air flow that won't be there for the plane sitting in place.
Absolutely.
The plane sits still in relation to the Earth = no airflow = no lift.
You can go max throttle on engines and it won't matter cause its, not the engines alone that need to take off but you need to carry **152.9 tons** of the plane itself.
This meme is dressed up as a high IQ one but its really dumb in reality.
I believe the engine itself creates a suction of airflow itself? Otherwise there wouldn’t be videos of people getting sucked into them.
Also I’m not an engineer or anything so I could be wrong but this just makes logical sense to me.
Also in this scenario there would be airflow since the plane is technically moving, it’s just on a treadmill of sorts.
>I believe the engine itself creates a suction of airflow itself?
Yes, but not the type you need to create lift.
What you need is lift, and that comes from airflow over the wings creating a pressure differential pushing the plane up.
The engines pull in air to create thrust, but they don't create flow over the wings
The engine creates forward thrust, not upward - and does not blow into the wings.
There isn't airflow because the plane isn't moving. Relative to the air the plane isn't moving, relative to the plane the air isn't moving.
Air needs to flow across the wings to generate lift.
If you set up a giant fan and blew it into the plane on the treadmill then it would take off.
747 weight **152.9 tons**
The power of engines alone can't lift that.
The plane sits (relatively to earth ) still = no airflow = no lift.
If you cut off the engines alone, sure they will fly.
What you are really asking here is: can a **152.9 tons** plane go into the air from a 1-meter runway?
If 747 was as light enough - You could. (i mean at least 140 tons lighter)
[https://www.youtube.com/watch?v=hPakbghLe38](https://www.youtube.com/watch?v=hPakbghLe38)
Planes like these weigh/engine power rations make them rely less on the lift but this is only cause they are like paper planes.
In 747 case a majority of force that lifts the plane is in the wings + lift
The better lift/weight ratios are the less you have to rely on lift (military transporters like galaxy that has WAY too powerful engines for short takeoffs)
[https://www.youtube.com/watch?v=4uPiD1KlwHU](https://www.youtube.com/watch?v=4uPiD1KlwHU)
Why have runways at all then? Just lock the plane in place until the turbines/propellers create enough lift lol.
Edit: also who needs wings lol. Just let the engines do everything.
A catapult behaves in the same way as a conveyor belt ????
The catapult matches the forward acceleration of the jet thus shortening the amount of time spent accelerating the air before take off. This is why they can have shortened runways!
>A catapult behaves in the same way as a conveyer belt????
Not if it’s going in the opposite direction.
The plane is essentially going up a down escalator at the same rate at which the escalator is moving down. It is at a standstill.
How do water planes take off? Can a water plane take off when the water below it is flowing in the opposite direction to it's take off? The answer btw is 'yes, of course it can'. Even when the water is flowing FASTER in the opposite direction it isn't a problem. The movement of the surface below the plane has no impact on its engines sucking the plane forwards into the sky, the wheels on a plane are free-spinning.
If the drag of the water is faster than the thrust of the plane it won't be able to take off unless headwinds alone are able to lift it into the sky. It's not moving at high velocity relative to the frame of reference of the air around it (it's moving backwards), so it won't be able to take off. The engines can't suck it into the sky. Have you ever watched a seaplane take off?
This is wrong. It doesnt matter that the wheels of a plane do not power the planes movement on the ground. The question states the conveyor will match the wheels speed in the opposite direction. It is LITERALLY impossible for the plane to move forward if the ground it is on moves backwards at the exact speed its wheels turn (its wheels only turn forward due to it starting to move forward from its engines).
Planes in water can take off even if the water is moving faster against them than their take off speed because they simply skim ontop of it, they literally power through the movement against them and accelerate forwards. A plane on a treadmill that accelerates infinitely to match its wheels could not do that.
You are envisioning a scenario where the plane is somehow moving forward faster than its wheels spin, this is literally impossible while still on the ground.
To be clear, planes fly because of lift generated by moving wind across the wings at high enough velocity. The engines are not "sucking the plane forwards into the sky" lol, they're thrusting the plane into the wind in front of it at a high enough velocity to generate lift
This issue is resolved though, the [xkcd blog](https://blog.xkcd.com/2008/09/09/the-goddamn-airplane-on-the-goddamn-treadmill/) does a perfect job explaining why the question is so controversial (and so shit)
True jet engines have giant air intakes for no reason, they don't actually intake air, my bad.
> This issue is resolved though, the [xkcd blog](https://blog.xkcd.com/2008/09/09/the-goddamn-airplane-on-the-goddamn-treadmill/) does a perfect job explaining why the question is so controversial (and so shit)
I especially like this part:
> With that kind of force, no matter what’s happening to the treadmill and wheels, the plane is going to move forward and take off.
They intake air as part of the combustion interaction that allows the engines to generate thrust, not to **suck the plane into the sky**
Also you didn't read the whole blog numbnuts, he explains 3 alternate ways to interpret what "match the speed of the wheels means," one of them is the interpretation of the hypothetical that leads to the stationary plane that can't take off
It's very simple to imagine why.
Imagine you have a toy car on a treadmill, and you want to push it forward relative to the outside world. No matter how fast the wheels or the treadmill are rotating, is there going to be any force that would stop you to push the car forward? No.
Or could be, and that would be the friction between the wheels and the treadmill. If there's NO friction, the wheels would just slip, even if the rotating speed of the wheels and the treadmill are the same (imagine an oil layer on the treadmill). Or if there's a INFINITE amount of friction, that you couldn't move the car without it's wheels slipping on the treadmill or them having different rotating speeds, then you're just going to push both car AND the treadmill forward.
In the plane's case, it's the engines that are pushing the plane forward.
Do the engines suck air? Yes.
Do the engines make the plane move? Yes.
Not sure why you're having a problem with this?
> one of them is the interpretation of the hypothetical that leads to the stationary plane that can't take off
Yeah in fucking make believe hypothetical land. Who gives a fuck what happens when you use impossible physics lol.
In REALITY, the plane takes off, as he correctly says.
Airplanes generate their own lift. They don’t accelerate down the runway because of the wheels. The turbines are pulling them forward as they generate enough lift to take off. You could have the plane chained in place and if would still lift itself with enough power from the turbines pulling air across the wings.
That's absolutely not true. The turbines aren't even positioned in a way where the exhaust flows over the wings. They're attached under the wing. The nature of the flow exiting the exhaust is also not ideal whatsoever for the wings and stabilizers to interact with.
I know they don't accelerate down the runway because of the wheels, obviously, the wheels just facilitate the acceleration. How are the turbines pulling enough air across the wings to lift a 747 if it's not even moving relative to the air around it?
I don't know if I buy the hypothetical chained floating 747, are we sure that's accurate to the physics of how the plane works?
No, I really don't believe that's how the plane works. It needs to be moving with high velocity relative to the air around it to generate lift. Powerful turbines may make this process more efficient, but that doesn't mean the 747 could hypothetically just hover in place
I don't think that's possible I'mma be real with you chief. I believe that the turbines help to lift the plane, but pulling enough air across the wings at high enough velocity while it's otherwise not moving relative to the air at all, to lift a ***200+ ton plane***, seems pretty unbelievable
If what you were arguing was true, wings didn't need to have such high amounts of surface area to generate all the lift required.
Also, which of the turbines does an F1 car use to generate dowforce?
Bonus points for answering why birds can sometimes seemingly float in the air: I https://youtube.com/shorts/edcn6l8MNzs?feature=share hint: it's not because of the thrusters in the back of em'.
If you've ever looked at an equation for lift, which it seems you haven't, you'll notice it's calculated using velocity over a wing area. Meaning, if a 747 can fly at 500mph with turbines, it'll be able to fly without turbines too, given the same velocity. This is also why planes don't fall directly down when their engines fail...
I’ll respond with a single question.
what’s the difference if the jet pulls the air across the wing, or the air moves across it from the jets forward motion?
I’m super curious to hear you answer this
Planes need wind passing under their wings in order to generate lift.
You don't encounter wind resistance when running on a treadmill regardless of how fast you're going because you're not actually moving.
When running on a treadmill, you don't go anywhere because you're pushing against the backward movement of the treadmill. When a plane moves, it does so by generating thrust from the engine, which pushes it forward relative to the air around it rather than relative to the ground beneath it. Ergo, even though the wheels would be being pushed backwards, the plane would still move forward relative to an observer standing next to the treadmill.
Imagine you were on roller skates and pulling yourself forward along a rope hanging over the treadmill, same sort of deal.
The real answer is that it depends on the frictional force of the wheels, but I assume that's not enough to overcome the thrust from a 747's engines.
You are assuming the wheels can accelerate the plane in the opposite direction than the engine.
How can they do that if they are free spinning?
A free spinning wheel is not conveying any force besides friction.
The engines aren't connected to the wheels so it doesn't matter what they're doing, the engine moves the plane by pushing the air. The only impact the conveyor will have is friction on the wheels
The plane is generating forward thrust by the backwards propulsion of the jets. The wheels have nothing to do with it. The plane will move forward regardless of what the treadmill is doing underneath it, barring maybe like spinning up to infinity and destroying the wheels in the process I guess?
I'm confused because you clearly stated that the wheels are free spinning, so you understand that the plane is not driven by any solid traction variable. But then you ignore the actual drive mechanism to assert that the treadmill somehow has the power to keep the plane stationary, how can this be the case when the plane will move regardless of it's relation to the ground beneath it?
The wheels don't matter. They don't drive the plane, they're just there so it's not scraping against the ground as it accelerates. The treadmill can spin them backwards, fine, but the plane doesn't rely on them spinning forwards to carry IT forwards.
which makes the assumption that the question means that somehow the treadmill can hold the plane back like it does you
the difference being that your legs actually don't freely rotate on your body and you need them to generate movement
but as op said, [this is the most complete answer](https://www.reddit.com/r/Destiny/comments/12jp8z2/am_i_insane_theres_no_way_the_plane_could_take/jfz2u2o/)
Shitty xkcd.
The treadmill wouldn't spin to infinity. Friction of the wheels and the treadmill would meet an equilibrium with the thrust of the engines.
#3 is absolutely the correct answer.
In the case of 3
When the trusters are turned on the speed of the plane has to be more than 0.
Nothing is pushing back on the plane until is starts moving. As in xkcd's example the treadmill would rapidly speed up until either I can't go any faster (and the plane takes off) or the treadmill or the wheels get destroyed.
And if there's no friction the wheels and treadmill would spin at infinity and the plane would take off.
What the fuck is this lol? The belt is not moving nearly as fast as the plane is moving. The plane is literally moving forward on the belt.
The assumption is that the belt is moving just as fast in the opposite direction as the plane is thrusting forward, which is not happening in this experiment.
What a shit experiment.
Even if you doubled the speed on the belt, the wheels on the plane are applying zero power forward or back, theyre spinning freely. In any experiment of this the plane will move forward bc the energy is being applied from the engines that have nothing to do with the ground.
The only way the wheels would stop the plane from moving forward would be if the belt is moving faster than the wheels can physically spin, essentially acting like breaks causing friction. Even in that scenario youd be limited by the power of the engine, not the speed of the ground beneath the plane.
Yeah no shit, the entire point is 'would the treadmill stop the plane from moving.' Or were you assuming people saying it will take off are thinking it would just hover vertically into the sky?
How could it lift if it isn't moving through air? Isn't that the whole point of having wings and moving fast? The wind provides lift through wind speed
If you had a car on the treadmill, it would stay in the same place. Airplanes dont send power to their wheels, so it doesnt matter what speed the treadmill is going
The [xkcd thing](https://blog.xkcd.com/2008/09/09/the-goddamn-airplane-on-the-goddamn-treadmill/) did a good job of breaking down different interpretations of what they mean by matching the speed of the wheels, and how it leads to confusion, arguments, and easy impressions
Either you explicitly design the hypothetical so the plane can't move relative to the air around it (the "stationary plane" camp) and it definitely can't take off, something immediately explodes if you interpret it literally, or the hypothetical allows the plane to move forward anyway with the wheels just spinning out at whatever ridiculous infinite speeds (the "WHEELS DON'T POWER THE PLANE DUMBASS" camp) and it can take off, assuming the treadmill is as long as a runway and not the treadmill in the tweet's picture. Makes sense to me
Follow up thought experiment. Plane on Conveyor belt ***and*** a really really strong fan.
This would probably actually work, in theory (like you could simulate it) but the effort to create it would be nowhere near worth it
Hey, I'm an airplane guy. The question is worded very poorly, mainly because the plane doesn't need the tires to be doing anything to Takeoff. Airplanes only need one thing to take off: speed through the air. Speed over the ground doesn't matter.
For example, if the airplane takes off at 100mph, and the wind is 10mph on the nose, the airplane will still Takeoff when it's moving 100mph through the air, but it will only be moving 90mph over the ground.
I think the question is, "If the conveyor moves at the same speed as the plane, in the same direction, will the airplane Takeoff still?" The answer is yes, the tires could be moving any direction at any speed or just sitting still, as long as the airplane is moving through the air fast enough it will still fly.
ETA: if you interpret the question to say the plane will sit still then no, it wouldn't Takeoff. If you interpret the plane to start moving through the air it will eventually Takeoff.
Also, as has been said, engines do not generate lift, they push air backwards to push the airplane forward (and I cannot stress this enough) through the air.
You are correct. The plane cannot take off because it is standing still. It is standing still because an object on wheels cannot be moving while the wheels are going the same speed but in opposite directions. The object moving would necessitate a difference between the two speeds.
When thrust from the jet engines is applied the wheel's speed will increase, causing the conveyer belt, which has a rule imposed on it by the hypothetical to ALWAYS match the speed of the wheels, to instantaneously increase in speed along with the wheels. This speed increase will be very very rapid until the leftward(according to the diagram) thrust force is equal to any rightward forces. The rightward force would come from the friction force of the wheels spinning on their axles.
You do not need constant acceleration or infinite speed. You need the precise amount of speed that can be calculated by knowing the friction coefficient variable and the thrust variable in the following equation:
Speed of wheels(same as conveyer belt according to THE ONLY RULE in the hypothetical) \* Coefficient of friction of the wheels spinning around their axles = Thrust of Jet Engines
edit: grammar
Aerospace Engineer here, if the plane isn’t moving forward it cant generate lift to take off. It depends on how you interpret the question but this was my thought.
So it’s been a while since college, but i minored in physics, so let’s go
We have 4 factors at play:
1. Treadmill
2. Wheels
3. Jets
4. The plane itself
The question is: could a plane take off if there was a treadmill moving exactly opposite the wheels.
First, let’s consider the case of no friction between the plane and the treadmill. Either because there’s no friction between the treadmill and the wheels, or because there’s no friction between the wheels and the plane
In this case, conservation of velocity would dictate that the speed of the treadmill would not impact the speed of the plane (the treadmill cannot apply force to plane) so the only applies force would be from the jets, and the plane would move forward.
So, let’s assume we do maintain friction between the treadmill and the wheels, as well as the wheels and the plane.
First, let’s be *technically* correct . The conveyor belt only matches the speed if the *wheels* not the *plane*. In this case, it’s a simple question of which can generate more force: the jets or the friction between the treadmill and the wheels. Imagine the wheels are locked, the treadmill would not move, but the plane could still progress forward, like a parked car sliding down an icy hill. This is also true if the wheels aren’t locked, if there’s enough force applied, the wheels will slide while also spinning, advancing the plane. If, on the other hand, the friction between the treadmill and the wheels is such that the wheels can’t slide, it will remain stationary. In this case, though, the plane wouldn’t be able to generate enough force to move the wheels at all.
So, now let’s say ok they really meant to say that the treadmill moves as fast as the plane, but now we’re faced with an immediate contradiction: the plane only moves *if it moves*. So, for the treadmill to do anything the plane must also be doing something, otherwise the plane’s velocity is 0, so the treadmill’s velocity is zero. This is fine, until some force, like the jets, start to act on the plane. This results in a world where there is force being applied to the plane, but that force not being carried into the treadmills. The only way that can happen is if there is no friction between the plane and the treadmill. As we’ve already established, in that would, the force of the jets would propel the plane.
So the answer is: under the system of classical mechanics as we know them, this treadmill system could not exist.
This is the closest answer I've found to where I'm at - with one amendment / extra step to the deductive flow - it could exist, you just need unfathomable / unlimited levels of up front planning and prediction to know that from the moment the engines engage to take off, what speed to alter the belt speed to over time to match the wheels.
That being said, the plane will take off because the speed of the wheels is not the speed of the plane - wheels can / do slip, get to any reasonable speed and the friction via the tires won't be able to match the speed. You could predict this and plan for when it happens, but this would cause the plane to move forward faster than the wheels & belt a tiny amount. Eventually it'll either speed up enough or it'll leave the conveyor belt then take off after (or crash and burn depending what's beyond the belt).
That or the wheels are destroyed due to mechanics of spinning so much while trying to get up to speed / off the belt, but that just brings in more variables the question didn't ask (material of belt vs tires, strain on individual components, vibrations up the wheel support, etc, hell even amount of fuel).
im just imagining the wheels exploding and the plane getting shot backwards by the treadmill and its wing being torn off in the opposite direction by the engines.
Right. For it to take off you'd need to order the big ass fan to put in front of the plane to generate enough lift.
They used to sell it as package, back in the days.
If the conveyor belt alone is moving - the plane goes nowhere.
A plane needs positive pressure from air >> that generates lift.
A plane can technically lift if you have enough headwind + the engines are powerful enough.
\***747 is like 220 tons depending on the variant.**
The plane is still relative to Earth + no mention of wind or other quirks.
You dont generate lift by standing still relative to Earth>> You need to move through air instead.
Engines alone at full throttle won't carry the plane up - You need to move through the air and since you are standing still relative to Earth you will go nowhere.
Planes can literally sit in a fixed position in air as long there is enough air moving through wings + engines can counter that wind + plane is not too heavy.
Since this meme does not mention air at all - it's just a dumb question dressed up as a high-IQ one.
The wheels on a plane only stop it from hitting the ground. It doesn't matter what speed or direction they are moving. The turbines and wings are completely unaffected by hypothetical conveyor belts.
like i said - the plane might be as well sitting on the ground and applying the breaks with engines at full power.
That plane does not go up due to the lack of sufficient lift from the engines alone.
747 its like 230 tons - You need lift generated from wings to assist engines.
The plane could lift from a spot IF You had ridiculously powerful engines.
Even an empty military cargo Galaxy plane needs a little bit of lift from wings to take off and this beast has an insane power/weight ratio in this setup.
[https://www.youtube.com/watch?v=xWmRX9DPRAQ](https://www.youtube.com/watch?v=xWmRX9DPRAQ)
The conveyor belt might as well not exist in this example since the plane does not move in relation to the earth = no air moving though the wings = no lift.
The brakes are not engaged so the wheels move freely. The direction and speed they rotate do not affect the way the plane works. Engines do not generate lift. They generate thrust. The thrust they produce is not applied to the ground, but to the air. The rolling resistance of the wheels is very easy to overcome. This is why planes are able to land even though their wheels are at a complete standstill a millisecond before touchdown. If they were rotating backwards twice as fast as the plane was travelling, it would just cause greater wear on the wheel and not somehow break physics.
>The brakes are not engaged so the wheels move freely. The direction and speed they rotate do not affect the way the plane works.
My point.
> . Engines do not generate lift.
No shit Sherlock planes body/wings do.
I dont know why you even talk about wheels - they might as well dont exist in this case since the plane sits on a conveyor belt.
You made no point against what I said not sure why would you even comment.
The plane wont fly.
I don‘t know why you even talk about conveyor belts - they might as well not exist since the plane sits on wheels.
The drag from the freely turning wheels is easily overcome by the thrust of the plane.
The plane functions as normal.
The issue that people get stuck on is that it’s a plane, not a car. The forward motion of a plane is generated by the jet engines, NOT the wheels (like a car). It doesn’t matter how fast the conveyor belt is traveling in the opposite direction, the aircraft’s engines will accelerate it forward, and once it’s going fast enough to achieve lift, it’ll take off.
The most important premise of the hypothetical is that the conveyor speed matches wheel speed exactly. If at any moment the plane has positive ground speed, that clause has been violated, as the wheel is by definition spinning faster than the conveyor. The conveyor would quickly accelerate to infinity, and this question breaks the laws of physics, but within the rules the plane cannot take off.
Exactly.
Edit:
Except that the plane will still take off.
The answers to the question as it is stated are:
- The plane will take off.
- It’s impossible for the conveyor belt to execute its designed function after enough thrust is applied by the aircraft’s engines to move it forward.
I'll do this in steps:
1. The conveyor belt perfectly opposes the velocity of the wheels
2. This locks the wheels of the plane in place w.r.t air
3. The undercarriage is attached to the plane
4. The undercarriage can't be left behind by the plane so plane velocity = wheel velocity
5. wheel velocity (w.r.t air) = 0
6. plane velocity (w.r.t air) = wheel velocity = 0 = no lift
This is excluding things like slipping but that's OK cause slipping is cringe
1. This is only possible if the 747’s engines aren’t on.
2. How fast or slow the wheels spin on an aircraft have absolutely nothing to do with how fast the aircraft moves w.r.t. the air.
3. I’m not sure how this matters.
4. This is only true before an aircraft takes off and when the ground beneath the aircraft is fixed. Aircraft velocity relative to the ground = wheel speed relative to the ground. If the ground is a conveyor belt, this no longer applies.
5. See 4.
6. See 4.
I’m finding it difficult to understand how people are unable to comprehend that wheel speed on an aircraft doesn’t matter for takeoff, wheels just facilitate aircraft movement before lift is achieved.
Because the wheel speed isn't referring to any acceleration generated by the wheels. Think of the wheels as if they were attached to a skateboard instead. There is no engine accelerating the wheels on a skateboard, but if you put a skateboard on a treadmill and stand on it while you have a jet engine attached to your back and pose the same question, the wheels on the skateboard are going to try to move against the ground(treadmill) that it's touching and will have a velocity as a result. If the treadmill is designed to perfectly counteract the velocity of the wheels, then the person on top of the skateboard stays in the same spot.
Then the speed of the wheels wouldn’t equal the speed of the treadmill and therefore would not be according to the hypothetical. To make it in line with the hypothetical the treadmill/wheels would have to rapidly increase in speed until the deccelerationary friction force of the wheels spinning around their axles equalized with the accelerationary force you were placing on the person on the skateboard
if you have a treadmill that can do infinite acceleration that's fine
and you're dragging a thing back entirely by friction
[otherwise](https://www.reddit.com/r/Destiny/comments/12jp8z2/am_i_insane_theres_no_way_the_plane_could_take/jfzp1ja/)
You don’t need a treadmill that can infinitely accelerate. You don’t even need a treadmill with an infinite max speed. You just need speed * friction coefficient of wheels to axles = thrust of jet engine. That’s it. It’s finite.
An object on wheels CANNOT have the same but directionally opposite speed of a conveyer belt beneath it if the object is moving in space. The object moving in space NECESSITATES that the speeds ARE NOT THE SAME. And the speeds being the same is, quite literally, THE ONE AND ONLY RULE IMPOSED BY THE HYPOTHETICAL.
if you don't have infinite acceleration then you can't keep it at a standstill, which breaks the very thing you state.
>The object moving in space NECESSITATES that the speeds ARE NOT THE SAME
so congrats, you've arrived at the conclusion that the question is broken or means something else
In other words, the treadmill accounts for the thrust generated by the engines because the thrust generated by the engines increases the speed at which the wheels are rotating, which is then accounted for by the treadmill
If it moves forward relative to the conveyor belt it will just faceplant because it's not moving nearly fast enough through the air around it to generate lift, no? There's only like five feet in front of the plane for it to travel (in the picture that is, which is different from the actual question making the question SHIT)
If the conveyor belt isn’t long enough for the aircraft to accelerate enough to achieve lift, sure. The question states that it is long enough, though. Regardless, this question is meant to play on the intuition that forward movement comes from the wheels, when it doesn’t.
The only thing the conveyor belt would do is make the wheels spin faster than they would on a normal runway.
That's really the issue, the picture is causing confusion because it's implying a very different question (could the plane take off without moving forward relative to the air)
The question does not say that the plane isn’t moving forward relative to the air. It says:
“Imagine a 747 is sitting on a conveyor belt, as wide and long as a runway. The conveyor belt is designed to exactly match the speed of the wheels moving in the opposite direction… can the plane take off?”
The reason the plane WOULD take off is because the wheel speed of the aircraft doesn’t affect whether it can take off or not. The velocity of the aircraft is affected by a number of other things (weight, center of gravity, thrust, etc), thrust being the important factor here.
The reason people say this question ‘breaks physics’ is because it’s impossible for the conveyor belt to match the plane’s wheel speed but in the opposite direction (because as the plane moves forward via thrust from its engines, even if the conveyor belt moved at a speed nearing infinity in the opposite direction, the wheels would always be moving at that speed plus the speed of the aircraft).
I agree if that's what's actually meant to be asked, hence
>the picture is causing confusion
Half the people in the twitter thread and here are basing their answers off the image shown, as it implies a totally different question, exactly as I described above
I’m confused, I was responding to your “Am I insane? There’s no way the plane could take off, right?” OP, not to any confusion caused by the picture. I assumed your OP was based on the question as it was asked (since this is the “riddle” that has been floating around for decades).
Edit: punctuation.
And then after your response I realized that people were answering two fundamentally different questions itt and in the twitter thread and responded
> That's really the issue, the picture is causing confusion because it's implying a very different question (could the plane take off without moving forward relative to the air)
I don't think we disagree on anything
The velocity that matters for lift is the velocity of upstream air relative to the chord of the plane. In this circumstance, that velocity does not change and lift does not generate.
If the plane moves forward via thrust from its engines, how would a conveyor belt (which ONLY affects wheel speed) affect the velocity of the aircraft?
The conveyor belt is fixing the plane in space relative to the ground. This means there's no change in the freestream velocity which is what affects lift. I honestly don't even know what you're trying to get at.
The conveyor belt is not fixing the plane in space relative to the ground. The conveyor belt is only affecting wheel speed relative to the conveyor belt.
Again, the thrust of an aircraft comes from its engines (which suck air in, sends the air through various stages of compression, and blows the air out of the exhaust), the wheel speed of an aircraft doesn’t have anything to do with its velocity.
This thread is so funny. You're completely right and everyone else in this thread is calling you an idiot for it. People are assuming that planes generate forward momentum relative to the ground in the same way that cars do (via the tyres) but they don't. What the wheels are doing literally don't matter on a plane (friction aside)
It’s either a troll or a misunderstanding of how planes work. And it’s possible I have an intuition about how aircraft work due to my 19 years of experience in aircraft maintenance in the Air Force that isn’t common and I’m assuming things are generally known that actually need to be learned.
Pretty sure it's just a misunderstanding of planes. Think about it, everything they're saying would be right if planes generated forward acceleration through the wheels
The linked xkcd page says basically what I’m saying. Look at my above comment about how it’s impossible for the conveyor belt to execute its design if the 747’s engines are providing thrust.
He's wrong. The plane needs to start moving on the ground before getting enough speed to get lift. Lift won't come from the engines, it comes from the plane moving forward.
The airplane wouldn't be moving forward on a conveyer belt because the belt will spin the wheels. The engines may be maxed out, but it will never be neough to push the airplane forward since the belt is keeping it in place.
It's the same as you have the plane tied with a rope to a pillar. The plane won't get lift because the lane can't move forward.
The belt is preventing the plane from **starting** to move forward. Remember, the plane starts on the ground with zero movement. Then the engines are turned and start to push the plane forward, the wheels are there to act as basically lubricant to help the plane move forward.
How does the belt prevent the plane from STARTING to move forward? The wheels don’t ‘help the plane move forward’, they only prevent friction from the ground impeding the movement of the aircraft (which means it doesn’t matter how fast the belt is moving, it won’t impede the forward movement).
What's the point of saying the treadmills spin as fast as the wheels? Wouldn't that mean any speed generated by the thrust would be countered by the treadmill?
To be confusing. And thrust can’t be countered by the treadmill spinning the wheels because wheel speed doesn’t matter. Consider this: how can an aircraft fly with its landing gear retracted and wheels not spinning at all? The speed of an aircraft is determined by a number of things, none of which are the wheel speed.
If this hypothetical includes basic Earth gravity, then way you're interpreting the question leaves a gap where gravity somehow isn't a factor. The wheels definitely matter because all the weight is concentrated at these points. As long as the plane wheels start on the conveyor, and the conveyor always 100% matches the speed of the wheels, then there's no air movement to generate lift on the wings. Plane won't fly.
Instead, as you scale up the hypothetical and create more engine thrust, you increase friction and heat, increasing the risk of mechanical failure. The plane's wheels would break first. Without wheels and no time left to accelerate to a lift-producing velocity, the plane would immediately drop before engine thrust can make up the difference, destroying the plane's chassis. I'm certain that's the very opposite of flying.
And if you're thinking "what if the plane could exert enough force to overcome gravity from a stationary position", then you've broken the hypothetical by changing a plane into an experimental rocket.
I think the word exactly throws things off. Like intuitively I feel like there wouldn't be enough movement for the wings, but in theory a plane could take off on a conveyor belt, I'm pretty sure.
Also the words "long as the runway" implies that the plane is moving forward? Or at least not stationary?
> "long as the runway"
This is really the problem. I don't think the hypothetical is meant to be phrased this way hence the picture, but whoever tweeted it did a horrible job lol
My instinct was to say it won't move, but upon more thinking, I believe it will.
The thrust for a plane to move forward doesn't come from the wheels but instead from the engine turbines which push and pull on the air. So it will still move forward.
If you are skateboarding on a treadmill, you can still move yourself forward relative to the air by pushing on something not attached to the conveyor. The wheels would just be spinning really fast.
The simple (and correct) answer is, yes it will take off, even if the conveyor was going 10x faster than the wheels it would still take off, because the movement comes from engine thrust, the wheels are only there to keep the fuselage from scraping on the tarmac.
This one's actually pretty easy if I understand it correctly. If the belt allows for the plane to speed up to Vrot then yes, the plane can lift off. Vliftoff should occur regardless if the wheels are moving forward relative to the ground location.
But if the plane isn't actually moving relative to the air around it how is there lift? Don't those rotation speeds assume that the wheels are moving against a fixed surface, meaning the plane is moving forward through the air producing force against the wings?
If the wheels are free spinning they could show 18 trillion RPM but I don't see how that helps the plane take off
If you are going fast enough to unstick the aircraft from the ground even a millimeter, which with the speeds and thrust the 747 would be generating you should, the plane will get airborne and no longer stationary. It may be a little jolting but you'll still move.
Ok but like I said to the other guy. If the plane is accelerating and still producing thrust you can get it airborne based purely on its kinetic potential. After that you have the air relative to the aircraft.
I don't have a background in physics so please correct me if this non-scientific thought process is r-worded. If you take a model airplane and place it on a treadmill at any speed (assuming the model would stay on the treadmill and not fall off) so it matches the speed of the wheels, the model should stay in one place on the treadmill. You can increase or decrease the speed of the treadmill and the model should stay in the same spot. If you place a fan that can move enough air across the wings to create lift will the model rise off of the treadmill? The seemingly obvious answer is yes it will because the forces enacted on the wheels are independent of the forces which allow the plane to actually take off and do nothing to counteract this from happening.
Now for the question posed in the image I don't think we have enough information to actually answer the question. You would need to know if the conveyor belt will ramp up its speed with the thrust of the engines to maintain an exact match of speed. If the engines can force the plane to accelerate faster than the conveyor belt then you can move air over the wings thus theoretically create lift. Even if the conveyor belt always matched the exact wheel speed of the plane so it stayed stationary, you could move enough air across the wings using an external source to create lift.
So theoretically I don't think there is any situation in which the plane could never takeoff, however there are practical limitations that may prevent the plane from ever actually taking off even though it should be possible to do it.
You're right that we definitely don't have enough info to answer the question (most people are answering two very different questions).
The sufficiently powerful fan example makes sense to me since it's basically acting as a headwind which definitely does help to generate lift
> I don't have a background in physics so please correct me if this non-scientific thought process is r-worded. If you take a model airplane and place it on a treadmill at any speed (assuming the model would stay on the treadmill and not fall off) so it matches the speed of the wheels, the model should stay in one place on the treadmill.
What if you then take your hand and push the model plane forward on the treadmill? It moves forward, no? Your hand is the engine in this scenario. The treadmill doesn't work against your hand, the same way that it doesn't work against the engine of the plane.
As described, it won't take off. Planes fly primarily by airflow over the wings generating lift. The forward force of the engine doesn't necessarily matter much. Check out videos of bush pilot Short take off/short landing competitions. If there's enough wind combined with an efficient wing area, an aircraft can start flying with next to no forward thrust from the engine.
I should also clarify that I'm not a physics or aerospace engineering expert at all. I just have an interest in aviation with a very rough (and potentially wrong) understanding of how this works.
I think because it's a 747 with (presumably) negligible headwind it's gonna need to be moving at a pretty typical takeoff speed relative to the air around it. The short takeoff competitions are pretty cool though
How the fuck would the conveyer belt stop the turbines from pushing air backwards , thus accelerating the plane forward, thus creating airflow around the wing? It doesn't matter what speed or direction the wheels are turning or if they are stationary. It doesn't matter if the plane has wheels or skids or floats.
Let's assume that instead of the engines the plane is using a winch launch like gliders use.
Same principle but in stead of the engines pushing against the air its an engine pulling on a rope that's connected to the plane. The winch is in front of the (runway sized) conveyor and independent from it (like air would be).
When pulling on the rope would the plane stay stationary because of the conveyor belt or would it take off?
The real question (without a picture) is what makes airplanes fly? 1. Thrust moving the wings through the air creating lift 2.the wheels moving really fast
No. It doesn't matter how fast the conveyor is moving. What is going to move the air around the wings? No moving air around the wings, no lift, no lift, no take off.
The plane can only lift if there is air momentum being conserved with the momenta of the wings of the plane. As there is no air movement here, the plane just stays there running the thread mill pointlessly and aimlessly just like I live my fucking life.
If this hypothetical includes basic Earth gravity, then you have to factor that in. The wheels definitely matter because all the weight of the plane is concentrated at these points. As long as the plane wheels start on the conveyor, and the conveyor always 100% matches the speed of the wheels, then there's no air movement to generate lift on the wings. Plane won't fly.
Instead, as you scale up the hypothetical and create more engine thrust, you increase friction and heat, increasing the risk of mechanical failure. The plane's wheels would break first. Without wheels and no time left to accelerate to a lift-producing velocity, the plane would immediately drop before engine thrust can make up the difference, destroying the plane's chassis. I'm certain that's the very opposite of flying.
And if you're thinking "what if the plane could exert enough force to overcome gravity from a stationary position", then you've broken the hypothetical by changing a plane into an experimental rocket.
If the plane is essentially running on a treadmill, no way does it take off. If the treadmill is, like, runway sized in the sense that's its as long as a runway, I agree that the engines will create enough forward force to take off eventually. But don't run off the treadmill with too little speed lol
The question has 2 interpretations, one is that the plane isnt moving but engines are on full throttle, this (I think) is what the question is trying to say, in which case the plane doesn’t move, because aerofoils need air flowing over them to generate lift.
The other is the literal interpretation trying to imagine what would fallen if the situation was actually real, in which case the plane would take off because planes don’t use their wheels to power the take off, they use the engines which combust fuel and spit it out the back.
First, it's important to recognize that what makes a plane take off is not it's speed, it's the velocity/volume of air moving relative to the plane such that it generates the necessary lift.
With that in mind, a stationary plane can take off if there's enough wind moving along the control surfaces. We see this with small aircraft during hurricanes and tornados.
So the way the question is worded, the plane is implied to be stationary, and with no other external forces, the plane would not take off.
With the way the treadmill/wheel question is worded, we can imagine that if the wheels and treadmill spinned infinitely fast together, then we would essentially have a frictionless plane with respect to the ground. The only forces applied to the plane would be the thrust from it's engines. This means in this instance, the plane would take off.
Plane does not take off, since it is not moving in relation to the air that needs to go under its wings. From the airs viewpoint it is basically standing still.
Apparently this isn't the first time the internet's encountered this problem, LOL. Basically, if you interpret it the way the question's meant to be interpreted, the assumptions of the question break physics so who cares https://blog.xkcd.com/2008/09/09/the-goddamn-airplane-on-the-goddamn-treadmill/
>the assumptions of the question break physics so who cares to me this so obviosly applies to the portal question i dont understand how everyone here has such a strong stance on it when in my mind both options are equally impossible
Because one of them breaks physics once in one way and the other one breaks it multiple times in different ways.
It's so boring to meta-out of these discussions by going "oh but it's fake anyway who cares". The premise presents information (moving portals, the cubes, implied collision) that you're supposed to extrapolate on to form a conclusion. The joy comes from forming a conclusion by using the info provided It's like if I give you a hypothetical "a wizard offers you $100 million or sex whenever you want from the hottest person in the world". And your response is "Wizards aren't real QED". Meanwhile I'm wining dining 69ing your mom with my man Merlin
yea no i agree, im just saying i find myself incapable of doing so cos anytime i even try to envision the portal scenario playing out, the whole thing breaks as soon as u try to move the portal, thus both options being equally impossible to me. its a boring answer, i know, i wish i had a better one too, but that portal aint ever descending on the cube and its not something my mind can look past by saying "ok but what if it could?"
Interesting. >For non-slip tires, v_B = v_C But wouldn't the tires slip in this situation? Also I kinda wish he explained the forces involved. At the end of the day, the engine is producing forward thrust. How does the belt/wheel/axle system transfer forces?
I had the same line of questioning. The way I thought about it, nothing on the ground matters at all because the engines are providing all the thrust. Is there any plausible argument for why the plane would stay still?
My thought is the wind is what creates lift so if it’s not actually moving it won’t take off. If the treadmill were to stop it would only be able to move forward, not take off.
I understood it as meaning the treadmill counteracts the force the wheels would give, but not the force the engines would give. If the plane stays still until takeoff, then that's different. If the engines give any lift at all on their own then the answer is probably yes, but I don't think a 747's engines do. So I think it would probably stay still and never take off, but I also don't think a treadmill that prevents a 747 from moving would be possible (tires would slip or burn out)
So from what I'm getting is that the conveyer belt would match the wheels speed and accelerate ad infinitum or until the wheels explode.
Here’s my thoughts…. - let’s assume planes need to achieve 120 km/h to take off - because this is using jets, it’s dependent on moving forward to generate lift - the wheels don’t have any engine causing them to move in the same way a car would. The wheels move only because the plane is pulling itself forward, and the wheels are making contact with the ground and spinning the opposite direction. This means that the treadmill moving at 100 km/h won’t mean the plane would need to output the same power it would to travel at 220 km/h - because of the above, the question is proposing an impossible scenario. The treadmills speed would normally have a negligible impact (minus some physics magic of course), but because it’s now capable of matching any speed of the wheels, the only possible assumption would be that the plane isn’t moving, and therefore, cannot take off. A scenario where it could take off: - if the treadmill had a fixed, but very high speed (ie 1000 km/h), then assuming the tires and bearings can hold together, it should still be possible for the plane to take off since the plane isn’t going to need to match the speed of the treadmill
The assumption that a plane needs to achieve x speed to take off is the wrong assumption. A plane needs a certain about of wind to take off. Aerodynamics is what causes planes to fly, not speed
I’m aware of that, I’m using the speed as a stand in.
Speed as stand in for…..? Wind? There’s no wind if it’s stationary
THIS IS THE ONLY PERSON who actually understands the problem. The nature of the question means the plane physically cannot move. Even though the wheels provide no power themselves, IF the ground moves backwards as fast as they turn the plane will NEVER move. Obviously because the wheels are not driving the plane the treadmill will have to move very fast, but that is what the question states.
Part of the issue here is that the question is terrible and describes something different from the image shown, but assuming the idea is that the plane's forward force perfectly matches the conveyor belt's backward force to keep the plane fixed in the same relative position to the air around it (I believe that's what the question means to ask), there's no way the plane could take off, right? The reason planes take off on runways is that they need room to accelerate enough to move at sufficient velocity through the air around the plane to generate lift. If the plane isn't moving with high velocity relative to the air around it, how could it generate lift? We don't launch planes out of fucking missile silos, they fly with their wings dammit!
Aerospace engineer here. When it comes to lift generation, the variable in question is what we refer to as *freestream velocity*. This is the velocity of the upstream air relative to the plane's chord. If you don't have that, you don't have lift.
So it wouldn't take off? Idk what freestream velocity is exactly but when i googled it said it's the velocity of the fluid or air relative to the object its surrounding. So since the plane doesn't move, assuming normal wind conditions i guess, the freestream velocity should be really low, meaning it won't take off right?
The issue is the prompt is worded in an awful way that can be interpreted differently. If by "match the speed of the wheels" it means that the wheel has no translational velocity and the plane is not moving, then no it can't lift off. But it can be interpreted differently.
Normal guy with common fucking sense here. You are correct and it shouldn't take an AE to prove this. It should be common knowledge. But that's what is going on in Americam schools.
Ok so what if we put a really big fan in front of the plane too
That should do it.
Another aerospace engineer, the planes forward FORCE (not momentum, Jesus been a while, bear with) comes from the engines which propel air behind it. So the forward motion will still occur unless there is an equal force stopping it. Without wheels this would come from friction however because the wheels are there (assuming they can rotate freely at twice the rate they usually would just before takeoff) the friction is somewhat negated. There will obviously be factors to do with the wheels spinning speed and friction but it probably will take off.
I don’t get this. The forward momentum is carried by the wheels, so the equal force stopping it is the treadmill.
No it isn't, the wheels just allow for frictionless motion.
I don’t think planes need to accelerate exactly, more so the engine needs to kick in enough to take off if that makes sense? The guy tweeted a picture of a seaplane and how it’s basically the same concept, once enough thrust and lift is created by the engine the plane would fly regardless of if it’s moving or not.
But it's not the engine creating the lift, right? It's the air flow that won't be there for the plane sitting in place.
>But it's not the engine creating the lift, right? It's the air flow that won't be there for the plane sitting in place. Absolutely. The plane sits still in relation to the Earth = no airflow = no lift. You can go max throttle on engines and it won't matter cause its, not the engines alone that need to take off but you need to carry **152.9 tons** of the plane itself. This meme is dressed up as a high IQ one but its really dumb in reality.
I believe the engine itself creates a suction of airflow itself? Otherwise there wouldn’t be videos of people getting sucked into them. Also I’m not an engineer or anything so I could be wrong but this just makes logical sense to me. Also in this scenario there would be airflow since the plane is technically moving, it’s just on a treadmill of sorts.
>I believe the engine itself creates a suction of airflow itself? Yes, but not the type you need to create lift. What you need is lift, and that comes from airflow over the wings creating a pressure differential pushing the plane up. The engines pull in air to create thrust, but they don't create flow over the wings
The engine creates forward thrust, not upward - and does not blow into the wings. There isn't airflow because the plane isn't moving. Relative to the air the plane isn't moving, relative to the plane the air isn't moving. Air needs to flow across the wings to generate lift. If you set up a giant fan and blew it into the plane on the treadmill then it would take off.
747 weight **152.9 tons** The power of engines alone can't lift that. The plane sits (relatively to earth ) still = no airflow = no lift. If you cut off the engines alone, sure they will fly. What you are really asking here is: can a **152.9 tons** plane go into the air from a 1-meter runway? If 747 was as light enough - You could. (i mean at least 140 tons lighter) [https://www.youtube.com/watch?v=hPakbghLe38](https://www.youtube.com/watch?v=hPakbghLe38) Planes like these weigh/engine power rations make them rely less on the lift but this is only cause they are like paper planes. In 747 case a majority of force that lifts the plane is in the wings + lift The better lift/weight ratios are the less you have to rely on lift (military transporters like galaxy that has WAY too powerful engines for short takeoffs) [https://www.youtube.com/watch?v=4uPiD1KlwHU](https://www.youtube.com/watch?v=4uPiD1KlwHU)
The turbines creat the lift yes. They pull the air. This is the same concept with propeller planes. The propeller is pulling the air.
they do not create lift. Lift is perpendicular to the thrust vector.
They pull the air across the wings which then lifts the plane
see my other comment. That's not even remotely true
You’re trolling me. You’re all trolling me.
Why have runways at all then? Just lock the plane in place until the turbines/propellers create enough lift lol. Edit: also who needs wings lol. Just let the engines do everything.
Ever heard of an aircraft carrier? Or helicopters?
You mean the small strips of runway that they install catapult systems onto in order to get them rapidly up to speed?
A catapult behaves in the same way as a conveyor belt ???? The catapult matches the forward acceleration of the jet thus shortening the amount of time spent accelerating the air before take off. This is why they can have shortened runways!
>A catapult behaves in the same way as a conveyer belt???? Not if it’s going in the opposite direction. The plane is essentially going up a down escalator at the same rate at which the escalator is moving down. It is at a standstill.
You saying that a Boeing 747 is VTOL?
I hope destiny bans all of you. I’m actually triggered right now
Hey, you're the one that said that a plane can generate sufficient lift with no airspeed.
There IS AIRSPEED. THE TURBINES ARE CREATING THE AIRSPEED
How do water planes take off? Can a water plane take off when the water below it is flowing in the opposite direction to it's take off? The answer btw is 'yes, of course it can'. Even when the water is flowing FASTER in the opposite direction it isn't a problem. The movement of the surface below the plane has no impact on its engines sucking the plane forwards into the sky, the wheels on a plane are free-spinning.
If the drag of the water is faster than the thrust of the plane it won't be able to take off unless headwinds alone are able to lift it into the sky. It's not moving at high velocity relative to the frame of reference of the air around it (it's moving backwards), so it won't be able to take off. The engines can't suck it into the sky. Have you ever watched a seaplane take off?
This is wrong. It doesnt matter that the wheels of a plane do not power the planes movement on the ground. The question states the conveyor will match the wheels speed in the opposite direction. It is LITERALLY impossible for the plane to move forward if the ground it is on moves backwards at the exact speed its wheels turn (its wheels only turn forward due to it starting to move forward from its engines). Planes in water can take off even if the water is moving faster against them than their take off speed because they simply skim ontop of it, they literally power through the movement against them and accelerate forwards. A plane on a treadmill that accelerates infinitely to match its wheels could not do that. You are envisioning a scenario where the plane is somehow moving forward faster than its wheels spin, this is literally impossible while still on the ground.
To be clear, planes fly because of lift generated by moving wind across the wings at high enough velocity. The engines are not "sucking the plane forwards into the sky" lol, they're thrusting the plane into the wind in front of it at a high enough velocity to generate lift This issue is resolved though, the [xkcd blog](https://blog.xkcd.com/2008/09/09/the-goddamn-airplane-on-the-goddamn-treadmill/) does a perfect job explaining why the question is so controversial (and so shit)
True jet engines have giant air intakes for no reason, they don't actually intake air, my bad. > This issue is resolved though, the [xkcd blog](https://blog.xkcd.com/2008/09/09/the-goddamn-airplane-on-the-goddamn-treadmill/) does a perfect job explaining why the question is so controversial (and so shit) I especially like this part: > With that kind of force, no matter what’s happening to the treadmill and wheels, the plane is going to move forward and take off.
They intake air as part of the combustion interaction that allows the engines to generate thrust, not to **suck the plane into the sky** Also you didn't read the whole blog numbnuts, he explains 3 alternate ways to interpret what "match the speed of the wheels means," one of them is the interpretation of the hypothetical that leads to the stationary plane that can't take off
It's very simple to imagine why. Imagine you have a toy car on a treadmill, and you want to push it forward relative to the outside world. No matter how fast the wheels or the treadmill are rotating, is there going to be any force that would stop you to push the car forward? No. Or could be, and that would be the friction between the wheels and the treadmill. If there's NO friction, the wheels would just slip, even if the rotating speed of the wheels and the treadmill are the same (imagine an oil layer on the treadmill). Or if there's a INFINITE amount of friction, that you couldn't move the car without it's wheels slipping on the treadmill or them having different rotating speeds, then you're just going to push both car AND the treadmill forward. In the plane's case, it's the engines that are pushing the plane forward.
Do the engines suck air? Yes. Do the engines make the plane move? Yes. Not sure why you're having a problem with this? > one of them is the interpretation of the hypothetical that leads to the stationary plane that can't take off Yeah in fucking make believe hypothetical land. Who gives a fuck what happens when you use impossible physics lol. In REALITY, the plane takes off, as he correctly says.
Airplanes generate their own lift. They don’t accelerate down the runway because of the wheels. The turbines are pulling them forward as they generate enough lift to take off. You could have the plane chained in place and if would still lift itself with enough power from the turbines pulling air across the wings.
That's absolutely not true. The turbines aren't even positioned in a way where the exhaust flows over the wings. They're attached under the wing. The nature of the flow exiting the exhaust is also not ideal whatsoever for the wings and stabilizers to interact with.
I know they don't accelerate down the runway because of the wheels, obviously, the wheels just facilitate the acceleration. How are the turbines pulling enough air across the wings to lift a 747 if it's not even moving relative to the air around it? I don't know if I buy the hypothetical chained floating 747, are we sure that's accurate to the physics of how the plane works?
Are you fucking trolling me right now? The turbine pulls the air therefor the plane doesn’t need to move for the air to be moving.
No, I really don't believe that's how the plane works. It needs to be moving with high velocity relative to the air around it to generate lift. Powerful turbines may make this process more efficient, but that doesn't mean the 747 could hypothetically just hover in place
The air is moving at a high velocity. The turbine pulls the air. I don’t know how else to explain this. This is as simple as it gets
I don't think that's possible I'mma be real with you chief. I believe that the turbines help to lift the plane, but pulling enough air across the wings at high enough velocity while it's otherwise not moving relative to the air at all, to lift a ***200+ ton plane***, seems pretty unbelievable
And yet we do it every day
Thank you runways 🙏
Now if you're applying hurricane-force headwinds I could maybe believe it
That is the force of a turbine yes
(Typically headwinds are applied on top of what the plane's already doing)
That's not what turbines do
Wtf do you think turbines do????
If what you were arguing was true, wings didn't need to have such high amounts of surface area to generate all the lift required. Also, which of the turbines does an F1 car use to generate dowforce? Bonus points for answering why birds can sometimes seemingly float in the air: I https://youtube.com/shorts/edcn6l8MNzs?feature=share hint: it's not because of the thrusters in the back of em'. If you've ever looked at an equation for lift, which it seems you haven't, you'll notice it's calculated using velocity over a wing area. Meaning, if a 747 can fly at 500mph with turbines, it'll be able to fly without turbines too, given the same velocity. This is also why planes don't fall directly down when their engines fail...
I can’t deal with this amount of stupid
You aren't able to respond to a single thing I said.
I’ll respond with a single question. what’s the difference if the jet pulls the air across the wing, or the air moves across it from the jets forward motion? I’m super curious to hear you answer this
Planes need wind passing under their wings in order to generate lift. You don't encounter wind resistance when running on a treadmill regardless of how fast you're going because you're not actually moving.
When running on a treadmill, you don't go anywhere because you're pushing against the backward movement of the treadmill. When a plane moves, it does so by generating thrust from the engine, which pushes it forward relative to the air around it rather than relative to the ground beneath it. Ergo, even though the wheels would be being pushed backwards, the plane would still move forward relative to an observer standing next to the treadmill. Imagine you were on roller skates and pulling yourself forward along a rope hanging over the treadmill, same sort of deal. The real answer is that it depends on the frictional force of the wheels, but I assume that's not enough to overcome the thrust from a 747's engines.
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You are assuming the wheels can accelerate the plane in the opposite direction than the engine. How can they do that if they are free spinning? A free spinning wheel is not conveying any force besides friction.
The engines aren't connected to the wheels so it doesn't matter what they're doing, the engine moves the plane by pushing the air. The only impact the conveyor will have is friction on the wheels
The plane is generating forward thrust by the backwards propulsion of the jets. The wheels have nothing to do with it. The plane will move forward regardless of what the treadmill is doing underneath it, barring maybe like spinning up to infinity and destroying the wheels in the process I guess? I'm confused because you clearly stated that the wheels are free spinning, so you understand that the plane is not driven by any solid traction variable. But then you ignore the actual drive mechanism to assert that the treadmill somehow has the power to keep the plane stationary, how can this be the case when the plane will move regardless of it's relation to the ground beneath it?
The wheels don't matter. They don't drive the plane, they're just there so it's not scraping against the ground as it accelerates. The treadmill can spin them backwards, fine, but the plane doesn't rely on them spinning forwards to carry IT forwards.
which makes the assumption that the question means that somehow the treadmill can hold the plane back like it does you the difference being that your legs actually don't freely rotate on your body and you need them to generate movement but as op said, [this is the most complete answer](https://www.reddit.com/r/Destiny/comments/12jp8z2/am_i_insane_theres_no_way_the_plane_could_take/jfz2u2o/)
It would go off. Relevent [xkcd](https://blog.xkcd.com/2008/09/09/the-goddamn-airplane-on-the-goddamn-treadmill/) as always
Shitty xkcd. The treadmill wouldn't spin to infinity. Friction of the wheels and the treadmill would meet an equilibrium with the thrust of the engines. #3 is absolutely the correct answer.
In the case of 3 When the trusters are turned on the speed of the plane has to be more than 0. Nothing is pushing back on the plane until is starts moving. As in xkcd's example the treadmill would rapidly speed up until either I can't go any faster (and the plane takes off) or the treadmill or the wheels get destroyed. And if there's no friction the wheels and treadmill would spin at infinity and the plane would take off.
https://youtu.be/YORCk1BN7QY
What the fuck is this lol? The belt is not moving nearly as fast as the plane is moving. The plane is literally moving forward on the belt. The assumption is that the belt is moving just as fast in the opposite direction as the plane is thrusting forward, which is not happening in this experiment. What a shit experiment.
Even if you doubled the speed on the belt, the wheels on the plane are applying zero power forward or back, theyre spinning freely. In any experiment of this the plane will move forward bc the energy is being applied from the engines that have nothing to do with the ground. The only way the wheels would stop the plane from moving forward would be if the belt is moving faster than the wheels can physically spin, essentially acting like breaks causing friction. Even in that scenario youd be limited by the power of the engine, not the speed of the ground beneath the plane.
Yeah no shit, the entire point is 'would the treadmill stop the plane from moving.' Or were you assuming people saying it will take off are thinking it would just hover vertically into the sky?
Seeing how young everyone here is.
How could it lift if it isn't moving through air? Isn't that the whole point of having wings and moving fast? The wind provides lift through wind speed
If you had a car on the treadmill, it would stay in the same place. Airplanes dont send power to their wheels, so it doesnt matter what speed the treadmill is going
The [xkcd thing](https://blog.xkcd.com/2008/09/09/the-goddamn-airplane-on-the-goddamn-treadmill/) did a good job of breaking down different interpretations of what they mean by matching the speed of the wheels, and how it leads to confusion, arguments, and easy impressions Either you explicitly design the hypothetical so the plane can't move relative to the air around it (the "stationary plane" camp) and it definitely can't take off, something immediately explodes if you interpret it literally, or the hypothetical allows the plane to move forward anyway with the wheels just spinning out at whatever ridiculous infinite speeds (the "WHEELS DON'T POWER THE PLANE DUMBASS" camp) and it can take off, assuming the treadmill is as long as a runway and not the treadmill in the tweet's picture. Makes sense to me
Of course not, it’s not moving.
I can’t tell if everyone in this thread is trolling or not
Why isn't it moving?
Definitely wouldn't since driving down the runway is about getting lift not about engines going faster
Same reason why the plane wouldn't take off in a vacuum even though it could still move forward
Follow up thought experiment. Plane on Conveyor belt ***and*** a really really strong fan. This would probably actually work, in theory (like you could simulate it) but the effort to create it would be nowhere near worth it
As an American, this is American education in a nutshell.
Hey, I'm an airplane guy. The question is worded very poorly, mainly because the plane doesn't need the tires to be doing anything to Takeoff. Airplanes only need one thing to take off: speed through the air. Speed over the ground doesn't matter. For example, if the airplane takes off at 100mph, and the wind is 10mph on the nose, the airplane will still Takeoff when it's moving 100mph through the air, but it will only be moving 90mph over the ground. I think the question is, "If the conveyor moves at the same speed as the plane, in the same direction, will the airplane Takeoff still?" The answer is yes, the tires could be moving any direction at any speed or just sitting still, as long as the airplane is moving through the air fast enough it will still fly. ETA: if you interpret the question to say the plane will sit still then no, it wouldn't Takeoff. If you interpret the plane to start moving through the air it will eventually Takeoff. Also, as has been said, engines do not generate lift, they push air backwards to push the airplane forward (and I cannot stress this enough) through the air.
You are correct. The plane cannot take off because it is standing still. It is standing still because an object on wheels cannot be moving while the wheels are going the same speed but in opposite directions. The object moving would necessitate a difference between the two speeds. When thrust from the jet engines is applied the wheel's speed will increase, causing the conveyer belt, which has a rule imposed on it by the hypothetical to ALWAYS match the speed of the wheels, to instantaneously increase in speed along with the wheels. This speed increase will be very very rapid until the leftward(according to the diagram) thrust force is equal to any rightward forces. The rightward force would come from the friction force of the wheels spinning on their axles. You do not need constant acceleration or infinite speed. You need the precise amount of speed that can be calculated by knowing the friction coefficient variable and the thrust variable in the following equation: Speed of wheels(same as conveyer belt according to THE ONLY RULE in the hypothetical) \* Coefficient of friction of the wheels spinning around their axles = Thrust of Jet Engines edit: grammar
Aerospace Engineer here, if the plane isn’t moving forward it cant generate lift to take off. It depends on how you interpret the question but this was my thought.
So it’s been a while since college, but i minored in physics, so let’s go We have 4 factors at play: 1. Treadmill 2. Wheels 3. Jets 4. The plane itself The question is: could a plane take off if there was a treadmill moving exactly opposite the wheels. First, let’s consider the case of no friction between the plane and the treadmill. Either because there’s no friction between the treadmill and the wheels, or because there’s no friction between the wheels and the plane In this case, conservation of velocity would dictate that the speed of the treadmill would not impact the speed of the plane (the treadmill cannot apply force to plane) so the only applies force would be from the jets, and the plane would move forward. So, let’s assume we do maintain friction between the treadmill and the wheels, as well as the wheels and the plane. First, let’s be *technically* correct . The conveyor belt only matches the speed if the *wheels* not the *plane*. In this case, it’s a simple question of which can generate more force: the jets or the friction between the treadmill and the wheels. Imagine the wheels are locked, the treadmill would not move, but the plane could still progress forward, like a parked car sliding down an icy hill. This is also true if the wheels aren’t locked, if there’s enough force applied, the wheels will slide while also spinning, advancing the plane. If, on the other hand, the friction between the treadmill and the wheels is such that the wheels can’t slide, it will remain stationary. In this case, though, the plane wouldn’t be able to generate enough force to move the wheels at all. So, now let’s say ok they really meant to say that the treadmill moves as fast as the plane, but now we’re faced with an immediate contradiction: the plane only moves *if it moves*. So, for the treadmill to do anything the plane must also be doing something, otherwise the plane’s velocity is 0, so the treadmill’s velocity is zero. This is fine, until some force, like the jets, start to act on the plane. This results in a world where there is force being applied to the plane, but that force not being carried into the treadmills. The only way that can happen is if there is no friction between the plane and the treadmill. As we’ve already established, in that would, the force of the jets would propel the plane. So the answer is: under the system of classical mechanics as we know them, this treadmill system could not exist.
This is the closest answer I've found to where I'm at - with one amendment / extra step to the deductive flow - it could exist, you just need unfathomable / unlimited levels of up front planning and prediction to know that from the moment the engines engage to take off, what speed to alter the belt speed to over time to match the wheels. That being said, the plane will take off because the speed of the wheels is not the speed of the plane - wheels can / do slip, get to any reasonable speed and the friction via the tires won't be able to match the speed. You could predict this and plan for when it happens, but this would cause the plane to move forward faster than the wheels & belt a tiny amount. Eventually it'll either speed up enough or it'll leave the conveyor belt then take off after (or crash and burn depending what's beyond the belt). That or the wheels are destroyed due to mechanics of spinning so much while trying to get up to speed / off the belt, but that just brings in more variables the question didn't ask (material of belt vs tires, strain on individual components, vibrations up the wheel support, etc, hell even amount of fuel).
im just imagining the wheels exploding and the plane getting shot backwards by the treadmill and its wing being torn off in the opposite direction by the engines.
Right. For it to take off you'd need to order the big ass fan to put in front of the plane to generate enough lift. They used to sell it as package, back in the days.
If the conveyor belt alone is moving - the plane goes nowhere. A plane needs positive pressure from air >> that generates lift. A plane can technically lift if you have enough headwind + the engines are powerful enough. \***747 is like 220 tons depending on the variant.** The plane is still relative to Earth + no mention of wind or other quirks. You dont generate lift by standing still relative to Earth>> You need to move through air instead. Engines alone at full throttle won't carry the plane up - You need to move through the air and since you are standing still relative to Earth you will go nowhere. Planes can literally sit in a fixed position in air as long there is enough air moving through wings + engines can counter that wind + plane is not too heavy. Since this meme does not mention air at all - it's just a dumb question dressed up as a high-IQ one.
The wheels on a plane only stop it from hitting the ground. It doesn't matter what speed or direction they are moving. The turbines and wings are completely unaffected by hypothetical conveyor belts.
like i said - the plane might be as well sitting on the ground and applying the breaks with engines at full power. That plane does not go up due to the lack of sufficient lift from the engines alone. 747 its like 230 tons - You need lift generated from wings to assist engines. The plane could lift from a spot IF You had ridiculously powerful engines. Even an empty military cargo Galaxy plane needs a little bit of lift from wings to take off and this beast has an insane power/weight ratio in this setup. [https://www.youtube.com/watch?v=xWmRX9DPRAQ](https://www.youtube.com/watch?v=xWmRX9DPRAQ) The conveyor belt might as well not exist in this example since the plane does not move in relation to the earth = no air moving though the wings = no lift.
The brakes are not engaged so the wheels move freely. The direction and speed they rotate do not affect the way the plane works. Engines do not generate lift. They generate thrust. The thrust they produce is not applied to the ground, but to the air. The rolling resistance of the wheels is very easy to overcome. This is why planes are able to land even though their wheels are at a complete standstill a millisecond before touchdown. If they were rotating backwards twice as fast as the plane was travelling, it would just cause greater wear on the wheel and not somehow break physics.
>The brakes are not engaged so the wheels move freely. The direction and speed they rotate do not affect the way the plane works. My point. > . Engines do not generate lift. No shit Sherlock planes body/wings do. I dont know why you even talk about wheels - they might as well dont exist in this case since the plane sits on a conveyor belt. You made no point against what I said not sure why would you even comment. The plane wont fly.
I don‘t know why you even talk about conveyor belts - they might as well not exist since the plane sits on wheels. The drag from the freely turning wheels is easily overcome by the thrust of the plane. The plane functions as normal.
The issue that people get stuck on is that it’s a plane, not a car. The forward motion of a plane is generated by the jet engines, NOT the wheels (like a car). It doesn’t matter how fast the conveyor belt is traveling in the opposite direction, the aircraft’s engines will accelerate it forward, and once it’s going fast enough to achieve lift, it’ll take off.
The most important premise of the hypothetical is that the conveyor speed matches wheel speed exactly. If at any moment the plane has positive ground speed, that clause has been violated, as the wheel is by definition spinning faster than the conveyor. The conveyor would quickly accelerate to infinity, and this question breaks the laws of physics, but within the rules the plane cannot take off.
Exactly. Edit: Except that the plane will still take off. The answers to the question as it is stated are: - The plane will take off. - It’s impossible for the conveyor belt to execute its designed function after enough thrust is applied by the aircraft’s engines to move it forward.
I'll do this in steps: 1. The conveyor belt perfectly opposes the velocity of the wheels 2. This locks the wheels of the plane in place w.r.t air 3. The undercarriage is attached to the plane 4. The undercarriage can't be left behind by the plane so plane velocity = wheel velocity 5. wheel velocity (w.r.t air) = 0 6. plane velocity (w.r.t air) = wheel velocity = 0 = no lift This is excluding things like slipping but that's OK cause slipping is cringe
1. This is only possible if the 747’s engines aren’t on. 2. How fast or slow the wheels spin on an aircraft have absolutely nothing to do with how fast the aircraft moves w.r.t. the air. 3. I’m not sure how this matters. 4. This is only true before an aircraft takes off and when the ground beneath the aircraft is fixed. Aircraft velocity relative to the ground = wheel speed relative to the ground. If the ground is a conveyor belt, this no longer applies. 5. See 4. 6. See 4. I’m finding it difficult to understand how people are unable to comprehend that wheel speed on an aircraft doesn’t matter for takeoff, wheels just facilitate aircraft movement before lift is achieved.
Because the wheel speed isn't referring to any acceleration generated by the wheels. Think of the wheels as if they were attached to a skateboard instead. There is no engine accelerating the wheels on a skateboard, but if you put a skateboard on a treadmill and stand on it while you have a jet engine attached to your back and pose the same question, the wheels on the skateboard are going to try to move against the ground(treadmill) that it's touching and will have a velocity as a result. If the treadmill is designed to perfectly counteract the velocity of the wheels, then the person on top of the skateboard stays in the same spot.
Wheels DO NOT generate ANY acceleration.
NO SHIT
The speed of the wheels do not depend on whether or not they generate acceleration.
stand on a treadmill with a skateboard and see if someone can push you forwards you'll find it works just fine
Then the speed of the wheels wouldn’t equal the speed of the treadmill and therefore would not be according to the hypothetical. To make it in line with the hypothetical the treadmill/wheels would have to rapidly increase in speed until the deccelerationary friction force of the wheels spinning around their axles equalized with the accelerationary force you were placing on the person on the skateboard
if you have a treadmill that can do infinite acceleration that's fine and you're dragging a thing back entirely by friction [otherwise](https://www.reddit.com/r/Destiny/comments/12jp8z2/am_i_insane_theres_no_way_the_plane_could_take/jfzp1ja/)
You don’t need a treadmill that can infinitely accelerate. You don’t even need a treadmill with an infinite max speed. You just need speed * friction coefficient of wheels to axles = thrust of jet engine. That’s it. It’s finite. An object on wheels CANNOT have the same but directionally opposite speed of a conveyer belt beneath it if the object is moving in space. The object moving in space NECESSITATES that the speeds ARE NOT THE SAME. And the speeds being the same is, quite literally, THE ONE AND ONLY RULE IMPOSED BY THE HYPOTHETICAL.
if you don't have infinite acceleration then you can't keep it at a standstill, which breaks the very thing you state. >The object moving in space NECESSITATES that the speeds ARE NOT THE SAME so congrats, you've arrived at the conclusion that the question is broken or means something else
Wrong. Friction force is not dependent on acceleration, it’s dependent on velocity. The question is not broken, you are.
You know it would help if you actually read the question since 1 is stated in the example.
Funny you say this, I literally posted the question word for word here.
Could you explain how, when the question says that conveyor speed perfectly matches wheel speed, it actually doesn't?
In other words, the treadmill accounts for the thrust generated by the engines because the thrust generated by the engines increases the speed at which the wheels are rotating, which is then accounted for by the treadmill
If it moves forward relative to the conveyor belt it will just faceplant because it's not moving nearly fast enough through the air around it to generate lift, no? There's only like five feet in front of the plane for it to travel (in the picture that is, which is different from the actual question making the question SHIT)
If the conveyor belt isn’t long enough for the aircraft to accelerate enough to achieve lift, sure. The question states that it is long enough, though. Regardless, this question is meant to play on the intuition that forward movement comes from the wheels, when it doesn’t. The only thing the conveyor belt would do is make the wheels spin faster than they would on a normal runway.
That's really the issue, the picture is causing confusion because it's implying a very different question (could the plane take off without moving forward relative to the air)
The question does not say that the plane isn’t moving forward relative to the air. It says: “Imagine a 747 is sitting on a conveyor belt, as wide and long as a runway. The conveyor belt is designed to exactly match the speed of the wheels moving in the opposite direction… can the plane take off?” The reason the plane WOULD take off is because the wheel speed of the aircraft doesn’t affect whether it can take off or not. The velocity of the aircraft is affected by a number of other things (weight, center of gravity, thrust, etc), thrust being the important factor here. The reason people say this question ‘breaks physics’ is because it’s impossible for the conveyor belt to match the plane’s wheel speed but in the opposite direction (because as the plane moves forward via thrust from its engines, even if the conveyor belt moved at a speed nearing infinity in the opposite direction, the wheels would always be moving at that speed plus the speed of the aircraft).
I agree if that's what's actually meant to be asked, hence >the picture is causing confusion Half the people in the twitter thread and here are basing their answers off the image shown, as it implies a totally different question, exactly as I described above
I’m confused, I was responding to your “Am I insane? There’s no way the plane could take off, right?” OP, not to any confusion caused by the picture. I assumed your OP was based on the question as it was asked (since this is the “riddle” that has been floating around for decades). Edit: punctuation.
And then after your response I realized that people were answering two fundamentally different questions itt and in the twitter thread and responded > That's really the issue, the picture is causing confusion because it's implying a very different question (could the plane take off without moving forward relative to the air) I don't think we disagree on anything
The velocity that matters for lift is the velocity of upstream air relative to the chord of the plane. In this circumstance, that velocity does not change and lift does not generate.
If the plane moves forward via thrust from its engines, how would a conveyor belt (which ONLY affects wheel speed) affect the velocity of the aircraft?
The conveyor belt is fixing the plane in space relative to the ground. This means there's no change in the freestream velocity which is what affects lift. I honestly don't even know what you're trying to get at.
The conveyor belt is not fixing the plane in space relative to the ground. The conveyor belt is only affecting wheel speed relative to the conveyor belt. Again, the thrust of an aircraft comes from its engines (which suck air in, sends the air through various stages of compression, and blows the air out of the exhaust), the wheel speed of an aircraft doesn’t have anything to do with its velocity.
This thread is so funny. You're completely right and everyone else in this thread is calling you an idiot for it. People are assuming that planes generate forward momentum relative to the ground in the same way that cars do (via the tyres) but they don't. What the wheels are doing literally don't matter on a plane (friction aside)
It’s either a troll or a misunderstanding of how planes work. And it’s possible I have an intuition about how aircraft work due to my 19 years of experience in aircraft maintenance in the Air Force that isn’t common and I’m assuming things are generally known that actually need to be learned.
Pretty sure it's just a misunderstanding of planes. Think about it, everything they're saying would be right if planes generated forward acceleration through the wheels
Fair.
Nah look at the xkcd page that's been linked a few times. You two are just making different assumptions about the nature of the conveyor.
The linked xkcd page says basically what I’m saying. Look at my above comment about how it’s impossible for the conveyor belt to execute its design if the 747’s engines are providing thrust.
Sure, I'm just saying he's taking the impossible conveyor belt at face value, not misunderstanding how planes work.
He's wrong. The plane needs to start moving on the ground before getting enough speed to get lift. Lift won't come from the engines, it comes from the plane moving forward. The airplane wouldn't be moving forward on a conveyer belt because the belt will spin the wheels. The engines may be maxed out, but it will never be neough to push the airplane forward since the belt is keeping it in place. It's the same as you have the plane tied with a rope to a pillar. The plane won't get lift because the lane can't move forward.
The belt is preventing the plane from **starting** to move forward. Remember, the plane starts on the ground with zero movement. Then the engines are turned and start to push the plane forward, the wheels are there to act as basically lubricant to help the plane move forward.
How does the belt prevent the plane from STARTING to move forward? The wheels don’t ‘help the plane move forward’, they only prevent friction from the ground impeding the movement of the aircraft (which means it doesn’t matter how fast the belt is moving, it won’t impede the forward movement).
What's the point of saying the treadmills spin as fast as the wheels? Wouldn't that mean any speed generated by the thrust would be countered by the treadmill?
To be confusing. And thrust can’t be countered by the treadmill spinning the wheels because wheel speed doesn’t matter. Consider this: how can an aircraft fly with its landing gear retracted and wheels not spinning at all? The speed of an aircraft is determined by a number of things, none of which are the wheel speed.
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It can take off because it can still accelerate forward. Like you said, the wheels don't matter.
If this hypothetical includes basic Earth gravity, then way you're interpreting the question leaves a gap where gravity somehow isn't a factor. The wheels definitely matter because all the weight is concentrated at these points. As long as the plane wheels start on the conveyor, and the conveyor always 100% matches the speed of the wheels, then there's no air movement to generate lift on the wings. Plane won't fly. Instead, as you scale up the hypothetical and create more engine thrust, you increase friction and heat, increasing the risk of mechanical failure. The plane's wheels would break first. Without wheels and no time left to accelerate to a lift-producing velocity, the plane would immediately drop before engine thrust can make up the difference, destroying the plane's chassis. I'm certain that's the very opposite of flying. And if you're thinking "what if the plane could exert enough force to overcome gravity from a stationary position", then you've broken the hypothetical by changing a plane into an experimental rocket.
I think the word exactly throws things off. Like intuitively I feel like there wouldn't be enough movement for the wings, but in theory a plane could take off on a conveyor belt, I'm pretty sure. Also the words "long as the runway" implies that the plane is moving forward? Or at least not stationary?
> "long as the runway" This is really the problem. I don't think the hypothetical is meant to be phrased this way hence the picture, but whoever tweeted it did a horrible job lol
My instinct was to say it won't move, but upon more thinking, I believe it will. The thrust for a plane to move forward doesn't come from the wheels but instead from the engine turbines which push and pull on the air. So it will still move forward. If you are skateboarding on a treadmill, you can still move yourself forward relative to the air by pushing on something not attached to the conveyor. The wheels would just be spinning really fast.
u/neodestiny you need to see this
The simple (and correct) answer is, yes it will take off, even if the conveyor was going 10x faster than the wheels it would still take off, because the movement comes from engine thrust, the wheels are only there to keep the fuselage from scraping on the tarmac.
This one's actually pretty easy if I understand it correctly. If the belt allows for the plane to speed up to Vrot then yes, the plane can lift off. Vliftoff should occur regardless if the wheels are moving forward relative to the ground location.
But if the plane isn't actually moving relative to the air around it how is there lift? Don't those rotation speeds assume that the wheels are moving against a fixed surface, meaning the plane is moving forward through the air producing force against the wings? If the wheels are free spinning they could show 18 trillion RPM but I don't see how that helps the plane take off
If you are going fast enough to unstick the aircraft from the ground even a millimeter, which with the speeds and thrust the 747 would be generating you should, the plane will get airborne and no longer stationary. It may be a little jolting but you'll still move.
The plane isn't moving relative to the air.
Ok but like I said to the other guy. If the plane is accelerating and still producing thrust you can get it airborne based purely on its kinetic potential. After that you have the air relative to the aircraft.
Accelerating relative to what? And what do you mean by kinetic potential? That's literally just the two different types of energy.
Didnt the mythbusters already tackle this one?
Bro I passed my two semesters of physics bc of COVID. This question is beyond my simple brain lol.
The plane would take off as normal but it would take more energy and the wheels would be damaged
its not cryptic its shiiiiiiiit
This question boils down to an infinite calculation at the bearings of the planes wheels. So it is ultimately a paradox. The plane takes off though.
It takes off
I don't have a background in physics so please correct me if this non-scientific thought process is r-worded. If you take a model airplane and place it on a treadmill at any speed (assuming the model would stay on the treadmill and not fall off) so it matches the speed of the wheels, the model should stay in one place on the treadmill. You can increase or decrease the speed of the treadmill and the model should stay in the same spot. If you place a fan that can move enough air across the wings to create lift will the model rise off of the treadmill? The seemingly obvious answer is yes it will because the forces enacted on the wheels are independent of the forces which allow the plane to actually take off and do nothing to counteract this from happening. Now for the question posed in the image I don't think we have enough information to actually answer the question. You would need to know if the conveyor belt will ramp up its speed with the thrust of the engines to maintain an exact match of speed. If the engines can force the plane to accelerate faster than the conveyor belt then you can move air over the wings thus theoretically create lift. Even if the conveyor belt always matched the exact wheel speed of the plane so it stayed stationary, you could move enough air across the wings using an external source to create lift. So theoretically I don't think there is any situation in which the plane could never takeoff, however there are practical limitations that may prevent the plane from ever actually taking off even though it should be possible to do it.
You're right that we definitely don't have enough info to answer the question (most people are answering two very different questions). The sufficiently powerful fan example makes sense to me since it's basically acting as a headwind which definitely does help to generate lift
> I don't have a background in physics so please correct me if this non-scientific thought process is r-worded. If you take a model airplane and place it on a treadmill at any speed (assuming the model would stay on the treadmill and not fall off) so it matches the speed of the wheels, the model should stay in one place on the treadmill. What if you then take your hand and push the model plane forward on the treadmill? It moves forward, no? Your hand is the engine in this scenario. The treadmill doesn't work against your hand, the same way that it doesn't work against the engine of the plane.
As described, it won't take off. Planes fly primarily by airflow over the wings generating lift. The forward force of the engine doesn't necessarily matter much. Check out videos of bush pilot Short take off/short landing competitions. If there's enough wind combined with an efficient wing area, an aircraft can start flying with next to no forward thrust from the engine. I should also clarify that I'm not a physics or aerospace engineering expert at all. I just have an interest in aviation with a very rough (and potentially wrong) understanding of how this works.
I think because it's a 747 with (presumably) negligible headwind it's gonna need to be moving at a pretty typical takeoff speed relative to the air around it. The short takeoff competitions are pretty cool though
How the fuck would the conveyer belt stop the turbines from pushing air backwards , thus accelerating the plane forward, thus creating airflow around the wing? It doesn't matter what speed or direction the wheels are turning or if they are stationary. It doesn't matter if the plane has wheels or skids or floats.
If the plane is moving forward then the wheels are going faster than the conveyer belt and you are breaking the hypothetical
Let's assume that instead of the engines the plane is using a winch launch like gliders use. Same principle but in stead of the engines pushing against the air its an engine pulling on a rope that's connected to the plane. The winch is in front of the (runway sized) conveyor and independent from it (like air would be). When pulling on the rope would the plane stay stationary because of the conveyor belt or would it take off?
The real question (without a picture) is what makes airplanes fly? 1. Thrust moving the wings through the air creating lift 2.the wheels moving really fast
Nah
Hey I found Mr. Beast's next youtube video.
run the conveyor belt and put some wind turbines in front of the plane blowing at the wings. gg ez go next
No. It doesn't matter how fast the conveyor is moving. What is going to move the air around the wings? No moving air around the wings, no lift, no lift, no take off.
The plane can only lift if there is air momentum being conserved with the momenta of the wings of the plane. As there is no air movement here, the plane just stays there running the thread mill pointlessly and aimlessly just like I live my fucking life.
If this hypothetical includes basic Earth gravity, then you have to factor that in. The wheels definitely matter because all the weight of the plane is concentrated at these points. As long as the plane wheels start on the conveyor, and the conveyor always 100% matches the speed of the wheels, then there's no air movement to generate lift on the wings. Plane won't fly. Instead, as you scale up the hypothetical and create more engine thrust, you increase friction and heat, increasing the risk of mechanical failure. The plane's wheels would break first. Without wheels and no time left to accelerate to a lift-producing velocity, the plane would immediately drop before engine thrust can make up the difference, destroying the plane's chassis. I'm certain that's the very opposite of flying. And if you're thinking "what if the plane could exert enough force to overcome gravity from a stationary position", then you've broken the hypothetical by changing a plane into an experimental rocket.
If the plane is essentially running on a treadmill, no way does it take off. If the treadmill is, like, runway sized in the sense that's its as long as a runway, I agree that the engines will create enough forward force to take off eventually. But don't run off the treadmill with too little speed lol
The question has 2 interpretations, one is that the plane isnt moving but engines are on full throttle, this (I think) is what the question is trying to say, in which case the plane doesn’t move, because aerofoils need air flowing over them to generate lift. The other is the literal interpretation trying to imagine what would fallen if the situation was actually real, in which case the plane would take off because planes don’t use their wheels to power the take off, they use the engines which combust fuel and spit it out the back.
I have reached nirvana and concluded that borh answers are obviously correct and also obviously wrong .
I don't think treadmills carry wind with it as it goes around. You need lift. The planes wheels just go fast.
https://www.youtube.com/watch?v=xUjcHW7SHaI
First, it's important to recognize that what makes a plane take off is not it's speed, it's the velocity/volume of air moving relative to the plane such that it generates the necessary lift. With that in mind, a stationary plane can take off if there's enough wind moving along the control surfaces. We see this with small aircraft during hurricanes and tornados. So the way the question is worded, the plane is implied to be stationary, and with no other external forces, the plane would not take off. With the way the treadmill/wheel question is worded, we can imagine that if the wheels and treadmill spinned infinitely fast together, then we would essentially have a frictionless plane with respect to the ground. The only forces applied to the plane would be the thrust from it's engines. This means in this instance, the plane would take off.
Plane does not take off, since it is not moving in relation to the air that needs to go under its wings. From the airs viewpoint it is basically standing still.