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We have an answer from NASA

Well folks, do you remember the whole discussion about whether an airplane sitting on a conveyer belt runway would take off? It just so happens that someone from NASA posted a comment today at BrainFuel.

The comment from a NASA engineer reflects the viewpoint I had about this topic. Here is his answer:

This problem can be understood quite simply by considering the example of a plane with frictionless bearings in the wheels. Examine the case when no thrust is applied and the conveyer moves at any speed. The plane will remain motionless with respect to the ground while the wheels spin at the conveyer speed. Now apply some thrust and the plane will start to move forward relative to the ground because in this case the speed of the conveyer (with frictionless bearings) can have no effect on the plane motion. As you increase thrust the plane will take off just like a normal plane. In fact from an observer on the ground the take-off will look perfectly normal. Now add some friction to the bearings and the only thing you will have to do is add a little more thrust to overcome this friction. Again from an observer on the ground the take-off looks exactly the same with the exception that the wheels seem to be spinning too fast. This is different from a car because the car must have locked (not frictionless) bearings (at least in one axel) in order to move.

The ip address of the poster: 163.205.211.20

By Chris Tingom

Principal of Tornado Design, a Phoenix, AZ based web consultancy

7 replies on “We have an answer from NASA”

What’s really cool is how he’s standing on the earth, but the moon is reflected in his visor.

Just goes to show that all that moon landing stuff really was just a farce perputrated by the government.

Aren’t NASA guys clever! Yet, not being a scientist I wonder if he might be wrong nonetheless? Given 0% friction (resistance/ drag?) he must be right. Given 100% resistance (wheels tied/fixed to the ground/conveyer) the plane would move in direction of conveyer unless sufficient thrust could be generated to tear the plan apart! Since the first is impossible, and the second is not described, yet it is assumed not to apply, the answer must lie in the hinterland. In that case however much energy is converted into forward motion by the thrust of the engines, the friction effect of the wheel bearings must be calculated into the equation by whatever mechanism (computer?) is used to control the rear-ward motion of the conveyer. The consequence is that however much ‘action’ (forward motion) results from the thrust of the engines, if the opposite reaction (if instantaneous or slightly greater to create the effect of instantaneous) is equal then the total forward motion must be nill, as long as the equilibrium is maintained. In this case there would be no airflow over the wings and no lift. The plane would not take-off. This all assumes a jet engine. A prop’ driven plane is a different matter, if the airflow over the wings created by an appropriately positioned prop’ were sufficient to generate lift then the foreward motion, or lack of it, would be immaterial. The plane would take-off.

There is one other scenario which comes to mind: that the plane in question is not an ‘aeroplane’ at all but exactly as stated in the puzzle, a ‘plane’, that is a wood-working tool unusually, but not impossibly placed, on a conveyer type runway. In that case, it could never take-off!

Of course, as a non-scientist I may be completely wrong, but that would be nothing new!
Ed Straszak

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