This XKCD.com comic’s “alt” text struck a chord with me as a pilot (or pilot in training to be more exact). Here is the comic:
If you head on over to XKCD.com and hover your cursor over the image, the text will read:
We actually divorced once over the airplane/treadmill argument. (Preemptive response to the inevitable threads arguing about it: you’re all wrong on the internet.)
I had head of the plane on a treadmill “brain teaser” (if it can be called that) before but it didn’t occur to me that it could be such a big deal (I yet again underestimated the internet). I Googled “airplane on a conveyor belt” and came up with quite a few results, including a Mythbusters video.
Unfortunately, as with many such things, the original wording of the brain teaser is lost. The scenario I will attempt to shed some light on in this post is the following:
Take an airplane (be it a 777, A380, Cherokee, Cessna, etc.) and place it on a large enough conveyor belt. As power is increased to the plane’s engine(s), start the conveyor belt and have it move in the exact opposite direction the plane’s nose is pointing, accelerating at EXACTLY the same rate the plane would be accelerating at if it was on regular ground.
The questions is “Does the airplane take off?”
Many people on the internet (and even some I’ve spoken to) say “yes.” My answer (and that of a few pilots with whom I’ve consulted): “NO, NO, NO, a thousand times NO!”
Why do airplanes fly?
There are four forces acting on an aircraft in flight: thrust, drag, lift and weight:
An airplane’s wing produces lift as air goes by it. As air is accelerated over the wing, a low pressure area develops on the top side and on the bottom, a high pressure area. This strength of the relative airflow and its angle to the wing dictate how much lift a wing will produce.
An airplane flies because its engines push its wings forward, forcing air back, across the wings and creating lift. Lift is produced by the wing being thrust foward.
How does an engine affect an airplane?
The engine, be it jet or propeller, provides a force to move the airplane forward. Just like the string tied to a kite, the engine pulls (or rather pushes) the airframe forward. This causes the air to flow backwards over the wing and create lift as explained above. In terms of the four forces, thrust, drag, weight and lift, the engines produce the thrust.
To push the aircraft forward, the vector of thrust must overcome that of drag (aka a type of friction).
What happens when an airplane takes off?
Whether on water, snow, ice or a regular runway, when an airplane takes off, the pilot applies power to the engine. This generates thrust and accelerates the plane forward through the air mass around it. As the plane moves forward, the airflow over the wing creates lift as explained above. The airflow is created because the air mass through which the airplane moves is so large that it is relatively unmoving with respect to the plane (unless you take off into the wind, in which case it is moving backwards).
So, what happens on a conveyor belt?
As mentioned above:
As power is increased to the plane’s engine(s), start the conveyor belt and have it move in the exact opposite direction the plane’s nose is pointing, accelerating at EXACTLY the same rate the plane would be accelerating at if it was on regular ground.
The engines, push the airplane forward which, on regular ground would cause it to roll forward on its wheels. The wheels would roll because of the friction between them and the surface.
Since the conveyor belt moves backwards, the friction between it and wheels causes them to roll just as they would on the takeoff roll. The interaction between the wheels and the conveyor belt provides enough force to keep the plane in place. Since the airplane is not moving forward, there is no lift created by the wing and therefore the plane does not take off.
Think of it this way: You have a kite and you’re running on a treadmill. The kite will never leave the ground.