Thursday, 18 June 2015

Can a 747 take off from a conveyor belt runway?

This question is currently going round on Facebook; one of my friends posted it, I answered it, and a few hours later entered a lengthy and circular debate in the comments section.  Here, with more space than a Facebook comments section, I'd like to pose the question, answer it and then address some of the misconceptions.

Here's the question:  Imagine a 747 is sitting on a conveyor belt, as wide and long as a runway.  The conveyor belt is design to exactly match the speed of the wheels, moving in the opposite drection.  Can the plane take off?
Shared originally by Aviwxchasers.Com a news and media site from the US.

My friend shared the post, and answered, "No - there's no way to generate lift."

My original answer:  

"I think you're assuming that the 747 will be moving because its wheels are being driven. The forward thrust of the aircraft comes from its engines, not from making its wheels turn (like a car). So the engines push the aircraft forward and the wheels just slide and skid along the conveyor belt , while the engines push the aircraft to take-off speed. The wheels don't have to turn to allow the aircraft to move."

However, this wasn't deemed sufficient by some other commentators, who (to summarise) posted the following questions or objections.

1. The conveyer would counteract the forward movement produced by the engines' thrust.  The conveyor matches the speed of the wheels in the opposite direction, so there can be no forward motion.

2. Lift is created by air flow over the wings. It doesn't matter how much thrust you have , if doesn't generate airflow over the wing, it won't fly.

3. The method of propulsion should be irrelevant. If the speed of the conveyor matches the speed of the wheels in the opposite direction, there can be no forward motion.

4. Take the vehicle out of the equation, it's all about the wheels and the conveyor. The faster the wheels turn, the faster the conveyor goes. Stick anything you want on top of the wheels, the principles are the same. Newton's third law
5. Indeed it's not about the wheels it's about forward movement (thrust from the engine) needed to take off but that forward movement is being counteracted by the conveyer    

Here, I propose to look at these arguments individually and collectively, and explain why the points which are raised aren't sufficient to stop the aircraft from moving and taking off.

1.  The conveyor would counteract the forward movement produced by the engines' thrust.  The conveyoy matches the speed of the wheels in the opposite direction, so there can be no forward motion.

1A.  The conveyor will only stop the wheels' rotation from generating forward movement.  However, it is not the wheels which are being driven - this is not a car or a truck.  The forward movement of the plane is not a result of the wheels successfully pushing against the conveyor belt; the forward movement of the plane comes from the push of the engines.  The result is that the plane will move forwards (the engine pushes against the air flowing through it, the air pushes back - Newton's Third Law) and the wheels will spin and skid down the track.

It's not "The plane moves forwards because the wheels go round", it's, "The wheels go round because the plane goes forwards [over a surface which has sufficient friction] ."


2. Lift is created by air flow over the wings. It doesn't matter how much thrust you have , if doesn't generate airflow over the wing, it won't fly.

True.  But there is thrust, and it is generating airflow over the wing.  The conveyor belt does not have the ability to resist the movement of the plane, only to counteract the turning of the wheels.  

The plane can move forwards -even along the ground - without its wheels turning.  The engine isn't driving the wheels.

3.  The method of propulsion should be irrelevant. If the speed of the conveyor matches the speed of the wheels in the opposite direction, there can be no forward motion.
and 
4. Take the vehicle out of the equation, it's all about the wheels and the conveyor. The faster the wheels turn, the faster the conveyor goes. Stick anything you want on top of the wheels, the principles are the same. Newton's third law


3A and 4A.  Comment 3 was a response to my question, "What happens if we replace the 747 with a space rocket, aligned horizontally on the conveyor belt runway, on wheels?"  and the comment suggests a misunderstanding about what's causing the forward motion of the plane.  The thrust of a space rocket is completely independent of the any wheels (they normally fly fine without them) and the wheels will just get dragged along the conveyor belt, without turning. 

Yes, the faster the wheels turn, the faster the conveyor belt goes.  But the wheels don't have to turn for the plane to move (as I said in response to 2).  The conveyor belt does not have some property which prevents something from skidding along it, just from preventing any forward motion due to wheels turning on it. 


You can only take the vehicle out of the equation when you realise that the vehicle isn't a car, with the limitations that a car has.

Newton's third law applies to rotating wheels and conveyor belts.  It also applies to the aircraft engines and the air flowing through them, or to space rockets and the fuel burning inside them.  I wonder if the Starship Enterprise (on wheels) would have this problem?


5.  It's all about forward movement (thrust from the engine) needed to take off but that forward movement is being counteracted by the conveyer  

The forward movement is not counteracted by the conveyor.  The conveyor just stops rotating wheels from generating any forward movement.  But if the wheels aren't rotating (because they're not being driven, such as in a car) then there's no movement to resist.

In conclusion, I'd like to offer this video from "Mythbusters" which shows a light aircraft attempting to take off against a conveyor belt (which in this case is being pulled by a pickup truck to match the plane's speed).  To quote one of the engineers, "People just can't wrap their heads around the fact that the plane's engine drives the propellor, not the wheels."




23 comments:

  1. I love this as nothing could prob solve it its a yes/no FLAWED QESTION.
    I always find no counter force if the wheel spins to say the 5000lb thrust the planes give out as push nothing can touch the plane its self.
    my reaction was no due to what it is but as it was a plane i changed it to a yes ?
    just because it was a plane and thinking of flight and free wheeling.
    So trying to prove the no answer wrong as who would think you can do the things we have today like me now typing on a screen.
    so with that in mind the quest was set ! lol
    the problem that always done my head in was...
    if i had 2 cars roof to roof wheels up wheels down both things can basically move, but can i move the car if i push on the wheel of the top car, no i can not as wheel counters my force no matter what i do with the wheel i just would'nt move the cars. now if put a rockit on it can i stop it by those wheels that i am spinning the wheel on ground are to simulate something to move freely like the top of the plane it can move.
    the bit i can not find is the power to stop the actual mass of the jet, wheel can not push my simple car :-(
    what is the counter force if a wheel can just spin and counter any force so i can not push the car.
    is it just force of the belt but that can do what it like can only spin as i can not put force on the car same speed aswheels or faster or slower my hand is the belt as i see it.
    it just the plane upside down version ish.
    im trying to find the force to push the actual mass backwards if i touch the actual car i can push it backwards easy as i have force but no counter force.
    i so understand it can not take off but its that which bugs me.
    if a plane sat at full thrust and let its breaks off would the plane move ?
    i can only say the plane tells the wheel to move first is it exact or is ther a very slight delay if there is will the treadmill only ever be behind as that reacts to the wheel then causing anohther delay, as the inistial plane thrust happens first evrything reacts after is it instant if you drop the break im asking is there delay from that before wheels move would the force over come the drag and friction from instant full power of 5000lb thust ?
    if for any reason the plane can move faster at the start the belt and wheels would have to catch up to which can only do that if the plane can not keep the advantage it had ie no more thrust.
    i am looking at all the angles from it all and this is just another i havnt read about as everyone talk the same things.
    its always tho what is the 5000lb thrust going against that bloody plane if i can not push a simple car anywhere without 5000lb of thrust see.
    if i find that im happy the natural answers of no are right, but way is see it is if you dont challenge it you will never no as we think its impossible to happen just like 100 years ago and me typing on here if i said that.
    lo i love it but people dont talk together to try and solve it rather it there way or nothing.

    great minds challenge weak mind just go with it as it what they no but what if you find a small thing and makes it possible.
    it says can i take of from a belt if it goes same speed as the wheels yes i take tyres of so no grip aplly brakes and slide power over grip it just like a plane can without wheels.
    wheel can not move so neither can the road but it can take of on the belt :-) i think so will be yes that way but everything in the qeustion has been answerd its still the same plane just no tyres.

    lol
    wish people would talk to solve things not just go with having to be right all the time im not saying im right at all i just say there a few issues find if any varible of boubt.

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    1. That was really hard to read, without any punctuation.

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  2. im not sure the video of mythbusters was this challenge rather a diffrent one.
    but the most intresting is did that plane take of in less space than a normal take of ?
    if it was a shorter distance then you have a new force on that plane that puts it in the air faster than a normal take off.
    now is that the counter act on the belt moving as the air is a drag on the belt.
    so say the plane can not actually move and spin wheels at same speed as the the belt they have to go at the speed of the thrust if the belt has to do that how much air can it drag back towards the wings in speed of air moving.
    now if you take it that the planes wheels to move forward would go twice as fast to move along the belt then the belty has to do that too, but in doing that are you double the drag of the air see and if that air can move now what does it need from the belt to lift the plane relative to the air being dragged ??

    another angle if that mythbusters plane took off shorter on the belt proving the extra force is helping it as it took off shorter see.

    sorry for typing aswell before you all start at me for that.

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    1. I think the myth buster video was flawed, because the truck generates ground speed and the aircraft indicated airspeed which is higher, once the propeller generated enough relative airflow over the wing and overcame the drag and produced excess power then the aircraft moved forward and became airborne.

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    2. The propeller does not make the airplane take please! The propeller "pulls" the plane forward untill the wings get enough air under them to take off. The air the propellers push back doesn't pass over enough of the wing to generate lift.

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  4. This comment has been removed by a blog administrator.

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  5. Am I missing something? The Myth Buster's video seems irrelevant to me. With a conveyor belt there is no forward motion, therefore no air flow over/under wings, therefore it wouldn't work. The video shows them pulling the plane on the tarp, creating ground speed, airflow, and lift. Two completely different sets of circumstances.

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  6. Seriously? This is a purely hypothetical question. If you're going to be technical or realistic, according to an Aerospace engineer, the plane would never take off because its tires would blow out, not being made to withstand that kind of force.
    But the question really is, Can a plane take off if it is going as fast as it can, yet not moving relative to the ground? And the answer is no.
    And the Mythbusters thing is completely different, because the plane does move relative to the ground.

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    1. No that's not the question, and if you are an Aerospace Engineer then you would know that this involves action and reaction, whereas irrelevant of the conveyor belt the thrust generated by the engines creates an opposite force pushing the plane forward moving the plane relative to the ground!

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    2. The point of this is if the belt moves as fast backwards as the planes max V then of course it won't move relative to the ground thus ZERO airspeed ZERO lift.

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  7. Since the wheels and conveyor cancel eaxh other out, it seems we can treat the jet as a hovercraft. Can engines push the craft forward on a frictionless surface. And yes...thrust can certainly do that! Eventually the plane takes off.

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  8. Since the wheels and conveyor cancel eaxh other out, it seems we can treat the jet as a hovercraft. Can engines push the craft forward on a frictionless surface. And yes...thrust can certainly do that! Eventually the plane takes off.

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    1. The current design of the plane makes the possibility of belt / bernouli effect impossible, and the question is if the wheels accelerating exponentially faster would keep the plane from taking off.

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  9. We're faced with analyzing data as it passes by on a high-speed conveyor belt.

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    1. The simple logic is that The plane would have the same relative takeoff as normal. Your data observed onboard and from the ground would be indistinguishable. Assuming that The wheels are not on fire or disintegrating. Possible variations that are not in the question are irrelevant.

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  10. Actually, they just need to turn the plane around, the plane would take off as usual. Save a lot of wear and tear on the wheel!

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    1. Yes turn the plane around and it would take off without the wheel even rolling provided the belt moves as fast as take off speed :-)

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  11. If the conveyor belt could instantly match the wheel speed the airplane could not takeoff. Think about it. If an airplane needs 130 KTS of airspeed to take off then that means that the aircraft needs to be moving through the air at 130 KTS (not counting for wind). So let's say that A is the wheel speed, B is the belt speed, and C is the air speed. It is IMPOSSIBLE for this equation to work. The original problem is this. A = -1(B). So is the wheels are spinning at 100 KTS then B is -100 KTS 100kts = -1(-100). However in order for the plane to move forward through the air fast enough to takeoff the aircraft wheels will need to be going 130 KTS FASTER than the belt. So to take off its A + B = C. Or say the belt is doing 200 mph then A + (-200) = 130. A would have to equal 330. Which does not work with A = -1(B). Long story short, if there is no relative motion there can be no relative wind to produce lift.

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  12. Those who answer that the plane would still take off regardless of the conveyor, are ignoring the specified facts of the matching speed of the wheels and the conveyor. If the plane has any ground speed, then its wheels are travelling at a different speed to the conveyor.

    The specification that the plane wheels and the conveyor always travel at matching speeds, in opposite directions, tells us that by some means, the conveyor is designed to prevent the plane from gaining ground speed.

    It’s logical to say that this is impossible, and I agree that this is a satisfactory solution. But, is it a complete solution?
    A little lateral thinking provides two possible means that the described situation can be achieved:

    The first (and most practical) is that the conveyor is situated in a wind tunnel. The plane would then be able to gain air speed within the wind tunnel, whilst maintaining zero ground speed.
    There is, in fact, a wind tunnel large enough to accommodate a large plane: the NASA wind tunnel at Moffett Field. Of course, in this situation, the conveyor belt is redundant, but this set-up satisfies nearly all the conditions of the question. However, the wind tunnel would not be able to accommodate an entire runway, so this solution doesn't really work

    The second solution is far more dangerous and impractical. The aircraft could be shackled to the static part of the conveyor by some means, meaning that if the engines are fired up, it will remain stationary relative to the ground.

    Of course, this would mean that putting the engines into full throttle would inevitably cause a catastrophic failure of the plane, and tear it apart.
    A third, but ludicrous, solution, would be to have the entire conveyor able to somehow travel along the ground, and match the ground speed of the plane. However, since the conveyor is supposed to be the size of a runway, this solution is absurd.

    So my answer is "no, the plane cannot take off". :)

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  15. TOS 130Kts. Thrust provides adequate to attain TOS, any "forward motion" up to and including 130Kts is matched EXACTLY in the opposite direction 130Kts "subtract" 130Kts = 0 kts, ground and airspeed, therefore no airflowto provide "lift" over the wing area.

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