Originally published at: Ask Adam Savage: plane on a conveyor belt controversy | Boing Boing

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And I learned something today! (I knew the plane took off, but this was an extremely lucid explanation of why)

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I’m failing to see how this ever rose to level of “controversy”…I mean the wings are the thing providing lift, not the wheels. How was this such a difficult concept to grasp?

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I think I could explain it in about 60 seconds: Because the propeller is what pulls the plane forward, it doesn’t matter at all what’s happening on the ground once the propeller starts pulling the plane forward.

My problem was, I assumed the forward movement of the plane would be countered by the backwards movement of the conveyor and therefore there’d be no air rushing around the wings to create lift. But once he pointed out that the plane would move forward regardless of what was going on underneath it, I understood.

Another way to imagine this would be, let’s say you put a box on a conveyor belt. You attach a rope to the box and hold the other end, but you yourself aren’t on the conveyor belt. You turn the conveyor belt on and start running in the opposite direction at the exact same speed. (Assume you can do so in a way that won’t pull the box off the belt.)

Does the box sit still because you’re pulling at a constant speed in one direction and the ground is moving the same speed in the opposite direction?

Of course not. It moves forward at the speed you’re running, regardless of the speed of the conveyor belt, because the movement of the box is due to the rope and has nothing to do with the ground.

In this same way, the movement of the plane is due to the propeller moving air and has nothing to do with the ground.

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I think I get what you are saying. The landing gear of the plane will over speed and fail due to exceeding structural limitations and everyone on board dies because they careen into a high speed moving belt.

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Something that never occurred to me in such plain terms- not an exact quote- “The car is moving because of friction with the ground, and the plane is moving because of friction with the air.” That man is a national treasure.

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I was also sucked into the ‘well, maybe…’ until the DUH moment that the wheels aren’t actually propelling shit. But now I kinda want to troll non-believers wherever they are.

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Quite correct (i think) so what i always wanted for experiment #2 is: lose the @#! conveyor belt; can the propeller itself move enough air past the wings to lift a plane? can a stationary plane (somehow fixed in xy but free in z) lift off without moving, just with propeller provided wind? i think the answer must be ‘yes’, … a plane stationary on the ground, (wheel brakes on), with a tornado nearby will (clumsily) rise into the air.

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I’ll watch this later, but wasn’t this pretty conclusively debunked on Mythbusters?

Yep. Whatever, Savage can have all the clicks as far as I’m concerned.

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I suppose if you got the conveyor belt running fast enough, it would create enough mechanical resistnce in the wheels’ bearings to act as a brake.

How about this: if the conveyor belt is moving forward with the plane, and the boundary layer of air moving with the conveyor belt is deep enough, there will be no net flow of air over the wings and the plane won’t be able to take off.

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You might find this interesting.

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what is often not described when this controversy is presented is how long is the conveyor? As long as a runway?

The wheels are not connected to the propeller in any way - they just roll. It does not matter how fast the conveyor is moving in the opposite direction - the wheels will just spin faster than normal. The plane will take off in the same manner as a fixed runway.

The mistake is not in thinking the plane won’t take off - the mistake is thinking the plane will stay in one place on the conveyor. That is the mistake that people make.

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That was the way I understood it. Ah well, we learn.

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Very useful for those runways that are very, very short and extremely wide (often hundreds of yards with a dotted line denoting the halfway point)

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At that point it would be simpler for the propeller to directly lift the plane with its thrust, no? so the question is simply whether the engine’s thrust exceeds the weight of the plane.

It can, and they do. Small planes frequently have to be tied down when parked outside because large enough gusts of wind can lift them off the ground. My father was an aircraft mechanic who would make note of particularly windy days when you could watch all the tie-down straps pay for themselves hundreds of times over.

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So with that evidence alone; it makes the whole conveyor-belt experiment gratuitous (sez i).

((and/or) don’t they teach about “air foils” in schools no mo’?)

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There are conventional planes that use the airflow generated by propellers (called prop wash) to create additional lift, but I don’t believe they are able to lift the plane solely with the prop wash, without any other forward movement through the air.

As far as thrust exceeding the weight of the plane, there are “jump jets” like the Harrier that can take off in that way.

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The simplest answer to this “controversy” is that planes take off on skis and pontoons all the time. The plane is moving through the atmosphere, what is temporarily supporting its weight until lift takes over, doesn’t matter.

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Agreed. Perhaps controversy is a strong word here.