plane on a conveyor belt?

Question

If a plane is traveling at takeoff speed on a conveyor belt, and the belt is matching that speed in the opposite direction, can the plane take off?

Discuss

Answer 1

The plane will not stay on the conveyor belt. The power of the plane comes from the jet engines or the propellors, not the wheels.

The conveyor belt, no matter how fast it is going, will have no effect on the motion of the plane. The plane will move forward because it is pushing air, not because the wheels are making it move.

Therefore: yes, the plane can and will take off.

Answer 2

Hello Brian -

Brian G is correct. Let's start from square one. We park the aircraft on the belt and turn the belt on. A large aircraft weighs tons. As the belt moves backward, it carries the aircraft with it. We now start the engines. The aircraft begins to move forward relative to the belt, which means that it is really moving backward more slowly relative to observers on the ground. As "takeoff speed" is approached, the weight of the aircraft is still on the belt, the wheels are spinning and the tires are under load, but the aircraft is only moving relative to the belt, not to the surrounding area and not to the air. Therefore, the engines are pushing like crazy just to stand still. The wings will not have any effect until they move relative to the air.

It is a common misconception that the engines push against the surrounding air. They do not. They provide a momentum that forces the mass of the aircraft in the opposite direction. It's sort of like standing on a wheeled cart and throwing a bowling ball off the back. When you release the bowling ball, the cart will move forward. But it's not pushing against the air. Think of the engines as pushing thousands of bowling balls out the back at high speed.

The conveyor belt is pulling the big mass of the aircraft aft at - say, 130 knots. In order to get the aircraft mass back to standing still relative to observers on the ground, a lot of momentum must be created in the form of the engine thrust. This still only gets the aircraft to stand still, and no air is flowing over the wings to create lift. The engine thrust is equal and opposite to the motion of the belt and the friction on the wheels, but no flight will occur. Interesting question.

Added:

Kirchwey has a point. If we start by getting the aircraft up to takeoff speed first, then there will be minimal weight on the wheels and when you turn the belt on, the wheels will just spin faster. If you throttle up the engines, you can still take off, but it will always take more energy than if the belt wasn't there.

If you accelerate the engines and the belt at the same time, and you push the throttles to where you would normally get airborne, you would still not be moving relative to the surrounding air and you would not fly.

Added 2: It all depends on what you mean by takeoff speed.

Answer 3

This is a badly phrased question. We don't know if the plane's "takeoff speed" is relative to the belt or absolute. And anyway, takeoff speed is an airspeed.
Paraphrasing what previous answers have correctly said, if it's absolute speed (and there's no wind) then the belt's speed is irrelevant.
However it is wrong to bring in the argument about wheels vs. jets and propellors. The question simply says it is traveling at takeoff speed; how it got to that speed doesn't matter. But what the question means by the plane traveling at that speed does matter, and isn't at all clear.

Answer 4

No. This is a question of relativity. If we have a conveyor belt traveling at, say, 150mph and we have the plane matching the speed in the opposite direction, the net sum is zero. (conveyor speed = 150mph, plane speed = -150mph.) Zero is the answer both mathematically and visually/intuitively as the plane is not moving relative to the region around it, only to the belt. Since an airplane's wings develop lift by their moving through the air, a positive displacement, no lift is created by the zero displacement.

Therefore, no lift means no take off.

Answer 5

you assume (and so have others) that the plane propells itself by the wheels in contact with the conveyor belt


planes do NOT move by rotation of the wheels (like a car does)

so the conveyor belt merely adds a little more friction to the wheel bearings

let's say the 'plane is doing 150 mph as the wheels leave the ground

but the wheels were doing 300 mph (if they survived)

Answer 6

It's all about relativity! - and I don't mean Einsteinian.

If you mean the plane is at take-off speed RELATIVE TO THE BELT, then it's actually stationary and won't take off.

But why should that be? The aircraft's engines propel it through the air, regardless of what's under the plane, so it should reach take-off speed RELATIVE TO THE AIR and will take off. The belt can do what it likes - including go backwards at take-off speed - and it will affect only the speed at which the aircraft's wheels rotate (before take-off, of course).

Answer 7

The engines use air to push the plane. They do not move enough air across the wings to create lift. The friction between the air and the belt would move more air, but not enough to do the job.

Answer 8

It WILL take off since its speed with respect to air is at take off speed. Once just off the conveyor, the movement of the conveyor will have no bearing on its movement.