The old airplane/treadmill revisited

[sailor55330]

Ok, SO we have all heard about the plane on the conveyor belt taking off even though it is stationary. Mythbusters even tried to do it on a full scale experiment. The overwelming conclusion was that the plane would take off because the airflow over the wings provided the lift. I think I have a new wrinkle that has been bothering me.

The planes that are always tested seem to be prop planes that have the propellers at the front of the plane, which would force airflow over the wings, providing lift.

My question is would a jet behave the same way? Jet engines are typically positioned under the wings or often times behind the wings, which would not necessarily provide the same airflow over the wings as prop in front, thus possibly not producing the lift needed for flight.

Am I wrong? I'm not sure.

[dbhost]

Write Mythbusters and have them do an episode on that. That would be cool to see...

[ke4fsm]

The engine whether prop or jet do nothing to help with the lift (they are providing the speed of the air over the wing by propelling the plane), It is the wing shape and Bernoulli's principle that get the jet up in the air. Since you have "sailor" in your user name, the main and jib on a sloop (as well as the keel) form "wings" that fly in the air (and water) to allow the sloop to sail to windward with no propulsion other than the wind.

Sam

[docrowan]

Sailor,

You raise a good point. Another way to test this would be to measure the uplift using tension scales on a prop airplane that is shackled to the ground with engine running at full speed.

[crokett]

The plane would take off, even if on a treadmill. The prop/jet engine is pushing against the air and have nothing to do with the treadmill. If the treadmill is moving, the wheels on the plane would just spin faster than they would if the plan were on a regular runway. From the perspective of a stationary observer, the plane would move along the treadmill and then take off when air flowing around the wings got fast enough to provide enough lift.

So yes, if a jet were on a treadmill it would take off.

A car or other wheeled vehicle on a treadmill would remain stationary because the wheels are pushing against the treadmill, which is moving. This is why the movie stunts showing cars driving onto trailers at highway speeds are so silly. If the trailer is moving at 60MPH and the car is at 65, the speed differential is only 5MPH, at least until the drive wheels of the car hit the trailer, then the speed differential is 65MPH and the car crashes into the cab, unless there are rollers on the trailer to let the wheels spin.

[billwmeyer]

I am still confused from the last one!
Bill

[phi1l]

The important think is the AIR SPEED, not how fast teh wheels are going around.

[Mr__Bill]

I am still confused from the last one!
Bill

It's simple, just remember that the Laws of Physics are just suggestions.




Bill

[docrowan]

The plane would take off, even if on a treadmill. The prop/jet engine is pushing against the air and have nothing to do with the treadmill. If the treadmill is moving, the wheels on the plane would just spin faster than they would if the plan were on a regular runway. From the perspective of a stationary observer, the plane would move along the treadmill and then take off when air flowing around the wings got fast enough to provide enough lift.

So yes, if a jet were on a treadmill it would take off.

I FINALLY get what you're saying (after about two years of discussing this scenario.) The point is, putting a plane on a treadmill does no good. It will still move forward at its normal speed, but the wheels will spin twice as fast as if it were not on a treadmill running in reverse at the plane's forward speed.

Man I feel dumb.

[cgallery]

Wait, I'm confused.

The Mythbusters experiment shows the plane was making forward progress. The conveyor belt wasn't moving fast enough in the opposite direction to keep the plane stationary. You can tell because the plane is moving relative to the cones while it is still on the ground. Here, watch the video:

http://www.youtube.com/watch?v=YORCk1BN7QY

Am I misunderstanding the question? What the heck did Mythbusters actually prove here?

Why do you need a conveyor belt at all for this experiment? Why not just hold the brakes on the plane and hit the throttle?

[gsmittle]

This is why the movie stunts showing cars driving onto trailers at highway speeds are so silly. If the trailer is moving at 60MPH and the car is at 65, the speed differential is only 5MPH, at least until the drive wheels of the car hit the trailer, then the speed differential is 65MPH and the car crashes into the cab, unless there are rollers on the trailer to let the wheels spin.

Mythbusters tried this one, too, and discovered that it was relatively easy to drive into the trailer and stop. I'm not sure why, since I too would think the speed differential would add up once the drive wheels hit the ramp. OTOH, most physics makes no sense to my math-deficient brain.

g.

[phi1l]

It's simple, just remember that the Laws of Physics are just suggestions.




Bill

Is that the theory of suggestitivity?

[ke4fsm]

The plane may be accelerating at, say 40 MPH and if the conveyor belt was moving at the same 40 MPH pace. The key to understanding it is because the prop is pushing the air, not the wheels, pulling the plane through the air at its normal takeoff speed the plane will take off normally. The wheels will just turn twice as fast as a usual takeoff. That is why the plane was moving forward. Once the plane reaches its takeoff speed, the plane will rotate as normal an lift off.

Sam

[cgallery]

The plane may be accelerating at, say 40 MPH and if the conveyor belt was moving at the same 40 MPH pace. The key to understanding it is because the prop is pushing the air, not the wheels, pulling the plane through the air at its normal takeoff speed the plane will take off normally. The wheels will just turn twice as fast as a usual takeoff. That is why the plane was moving forward. Once the plane reaches its takeoff speed, the plane will rotate as normal an lift off.

Sam

But I thought the purpose of the experiment was to show that a plane not moving relative to the ground was able to take off.

In the Mythbusters experiment, the plane was definitely moving relative to the ground.

[ke4fsm]

The myth they were busting was that the plane would NOT stand still.

Air speed and ground speed are two different concepts. If the plane was propelled by the wheels, the plane would stand still and would NOT takeoff as the air speed would be zero--not enough air going over the top of the wings to create lift.

However the prop is pulling/pushing (another discussion) relative to the air, not the ground so it will continue to take off normally only the wheels are affected--and turn twice as fast.

Sam