Posted on by Jeff Schaefer

The Airplane Conveyor Problem

This problem has been around for a long time!

Imagine a 747 is sitting on a conveyor belt, as wide and long as a runway. The conveyor belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Can the plane take off?

The correct answer is YES!

(If you answered no then the last section of this post is for you)

How does it take off?

The pilot of the 747 will set takeoff thrust and the huge jet engines will start pushing air out the back. The equal and opposite reaction of the air will push the engines forward. Since the engines are mounted to the plane it will also push the whole plane forward. The plane will begin to move forward.

Once it reaches 5 knots the conveyer will be moving backward at 5 knots, so the wheels will roll as if they are moving at 10 knots.

Likewise, when it reaches 50 knots, the wheels will be rolling at 100.

The drag from the belt would be negligible compared to the aerodynamic drag. This is because wheels are designed to roll. If you don’t hit the brakes they will keep on rolling.

The pilots might not even be aware that this conveyer belt thing is happening at all because airplanes do not have wheel speed sensors. This is because the wheel speed doesn’t matter at all.

At takeoff speed (assume 150 knots), the wheels will feel 300 knots because the belt speed matches the wheel speed. Hopefully, the wheels don’t explode or burst into flames at this speed.

The plane will be moving at 150 knots indicated airspeed and the pilot can rotate and climb as normal.

See the mythbusters do it here:

Important Concepts

  • Airplane wheels are not powered
  • Airplane engines move air
  • Airplane wheels are really just there to keep the plane from touching the ground (and for braking)
  • The problem does not say anything about the engines being unusable

Why do people say no?

Many people answer this question with no. I believe that they are correct!

That’s right, those that say no are also often correct! This is because I believe they are interpreting the question differently and answering this question:

Imagine a 747 is sitting on a conveyor belt, as wide and long as a runway. The conveyor belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Is the action of the conveyer belt sufficient for the plane to take off without the engines?

They are correct in saying that, NO, the aircraft could not take off because it is not moving forward through the air!