February 4, 2006 12:02am by Steve | 
I’ve come across a thought experiment that SHARPLY divides people. The funny thing is that there is only one right answer. Physics dictates this to be so. Here it is:
A plane is standing on a runway that can move (a conveyor belt). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane’s speed and tunes the speed of the conveyor to be exactly the same, but in the opposite direction. The pilot begins to add thrust to the engines…
The question is:
Will the plane take off or not?
Please answer via comments.
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The plane isn’t moving, and therefore there is no wind under the wings to raise it into the air.
This is the intuitive answer, but it is incorrect. The plane does, in fact, move down the conveyor just as any normal plane would travel down a runway. I’ll leave it up to you to understand exactly why.
I’ll post an explanation in a few days.
Yep it will take off. Not sure I should ruin it with an explanation.
Interestingly, it’s landing (well, stopping) that it would have a problem with!
Ehh, I take that landing comment back. I misread the original post.
I’m stuck. A force moving west that is equal to a force moving east should cancel each other out, thus no friction and the plane doesn’t move.
I did a web search and there are a lot of arguments either way, but I couldn’t find an actual explanation for the answer.
BTW, I got a video camera …. do you still fly model airplanes?
All we need is a treadmill….
How is this different from a car running on a dyno? The wheels of the airplane, like the car, move in the opposite direction of the conveyor. How is lift generated when in essence you have a set of wheels moving while the plane stands still?
Take out the engines from the equation. What if it’s a model airplane that you’re holding on a treadmill. If you hold the airplane in place(like the engines are doing) would the plane suddenly take off? No, it would stay in your hands because just the wheels are moving.
Now if you generate more thrust than is neccessary to keep the plane still, I would assume that the plane takes off.
Soooo cool. I LOVE this thought experiment. It is amazingly counter-intuitive. Actually it’s not, but you have to clearly think about what is going on when an aircraft applies thrust to start moving down a runway. Think about the different forces and isolate them. It will become clear.
I’ve been doing a lot of thinking about this and I’ve actually even developed proofs that essentially “force” the airplane to take off. It can be no other way.
Funny how the answer was very apparent to me as soon as I read it. I don’t see how people could think the plane would NOT take off.
Not changing the plane to a car (which is much more understood by people) , is key since a plane is NOT a car, especially in the area of propulsion. (hint hint)
Keep the brain teasers coming.
Imagine a tiny jet instead. Hold it in your hand while it fires up its thrusters exactly as it would if it were taking off down a runway normally. Let it go when it is exerting the same thrust it would when it takes off on a runway.
What happens?
This should be the same answer to the riddle.
Hint: I would not want to be inside that plane.
Regarding the “jet in the hand” idea…
Achieving “take-off thrust” (power needed to achieve take-off) and “take-off velocity” (enough air flowing over the wings to generate lift) are two different things. I believe take-off velocity in pilot terms is known as V1 (begin rotation) and V2 (committed to lift off the ground).
Regardless, by the description of your experiment I don’t think you understand that nothing out of the ordinary is happening in our Conveyor Belt Aiplane take-off. The plane doesn’t just rise off the ground like an elevator. It travels down the runway at normal speed and takes off like any other plane should.
I guess I don’t then. I was under the impression that the belt was able to keep the jet stationary relative to the ground.
For the “jet in hand”, if the jet engines were able to get the air moving past it at the right speed why wouldn’t it be able to keep itself in the air after letting it go?
If a jet on the ground suddenly experienced increased wind speed at the right heading wouldn’t it essentially be “flying” even if it was stationary relative to the ground?
Bill
Yep, that could happen. This is exactly what happens in a wind tunnel… with a nice controlled environment.
You could put a jetliner in a giant wind tunnel and bring the air velocity through the tunnel to 300mph and the aircraft will certainly lift into the air while remaining over the same spot on the ground.
However, this is not what is happening in our thought experiment.
OK, I guess I am comparing two (or more) completely different scenarios.
Anyway, with the original runway scenario, yes, it’ll take off.
So as thrust is added, does the plane physically move forward or is it stationary? In reading the question I understood that the control system on the conveyor kept the plane stationary because it was matching the speed of the plane. Thus the speed “forward” of the plane and the speed “backwards” of the conveyor belt leave the plane stationary. Is this correct or not?
If this is correct, I’m not seeing how any air is passing over the plane. Unless the pilot adds thrust which is not matched by the conveyor. How can a stationary plane gain lift if there is no movement of air around the plane. And if it does gain lift – where is the moving air coming from?
Yes, it still has velocity relative to a stationary observer- it moves forward. That’s the trick I missed. I see your post about the dyno- that’s what I also assumed and what had me going to the “jet in hand” scenario.
You could rewrite the riddle as follows and get the jist of the puzzle with the same reasoning:
“A conventional jet takes off from a runway, it’s wheels are not moving or damaged by the time it takes off. How is this possible?” Answer would be the conveyer belt (but it would be matching the thrust speed in a foward motion instead of going backwards) .
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