Let's say you had a giant treadmill with an airplane on it

[Edge540]

As you add power and begin to move for the takoff roll, the treadmill speeds up and keeps the plane in the same relative position. As the plane goes faster, the treadmill speeds up, always maintaining the aircraft's relative position.

No movement.

 

[ImbracableCrunk]

I thought I would dust off this old gem to kill the PIC thread.

Let's say you had a giant treadmill with an airplane on it.

As you add power and begin to move for the takoff roll, the treadmill speeds up and keeps the plane in the same relative position. As the plane goes faster, the treadmill speeds up, always maintaining the aircraft's relative position.

Would you ever be able to take off?

GO!!

It seems to me that there are three camps:

1. The This is a joke camp.

2. The It'll eventually accelerate because I know more about physics than you camp.

3. The This question clearly states the plane remains in the same spot (ignoring wheels and friction and z-ratings) camp.

Why does this question make me think of dihydrogen monoxide?

 

[PCL_128]

Yes..One:

If the airplane is not moving forward, where is the air coming from to provide lift? No relative wind = no lift

The airplane does move forward. The treadmill can't counteract the thrust provided by the prop/jet. Simple.

 

[ImbracableCrunk]

The airplane does move forward.

Hmmm. The question clearly states that the airplane stays in the same relative position. How do you respond to that given?

 

[PCL_128]

Hmmm. The question clearly states that the airplane stays in the same relative position. How do you respond to that given?

The question is written incorrectly. Quite simply, it's impossible for the treadmill to hold the airplane in the same relative position. The airplane will move forward despite the opposite direction of the treadmill.

 

[ImbracableCrunk]

Ahhhhhhhh. The fourth camp:

The I know what question was really asking camp, even though it clearly states something else camp.

 

[belchfire]

Ahhhhhhhh. The fourth camp:

The I know what question was really asking camp, even though it clearly states something else camp.

Na, now there's a fifth camp...the camp that says the question was posed by someone that was educated beyond their native intellectual capacity...that knew little about the limitations of treadmills and even less about aircraft!

What, the Cylons are building giant fracking treadmills now instead of more Cylons???

The chemtrails are falling the chemtrails are falling!

 

[Edge540]

Ahhhhhhhh. The fourth camp:

The I know what question was really asking camp, even though it clearly states something else camp.

:beer:

 

[runwayjockey]

You guys have way too much time on your hands!

 

[runwayjockey]

JHC, I cannot believe this thread came back...which once again proves that it doesn't take an engineering degree to become a pilot, but that likewise it doesn't take full knowledge of why an airplane flies for somebody to take off and land without disclosing that fact ...SCARY!!!

Listen up. The only relevance a moving surface would have on the aircraft's ability to positively affect its relative motion is if the aircraft's thrust was enacted through the point of contact in question, the treadmill in this case. If it wasn't clear in PPL ground school, let me refresh the class: that cessna is NOT a car....repeat with me, IT'S NOT A CAR.

The thrust created by the engine, whether the propeller driven or jet persuasion alike, is translated through a different medium, not the ground, but the fluid we call air. Therefore, the only force the aircraft has to overcome on initial takeoff is the static friction force to make the tires begin to roll. As you can see the fact that the airplane is sitting on a moving or stationary platform is irrelevant to it. Taking into consideration the assumption that tire limit rotational speed is neglected, it doesn't matter how fast that treadmill is moving, all the airplanes sees is air beginning to rush through its wings once you push up that/those throttle(s).

To further illustrate the point of why force medium matters, clamp a rope to the front end of the aircraft to a post at the end of the treadmill, a stationary fix. Now, crank that treadmill into uber-ridiculous speed...what happens to the aircraft's relative motion with respect to the air? Nothing, the airplane sits there fat dumb and happy, 0 KIAS, with the tires spinning at the tire equivalent rpm for uber-ridiculous speed. Now what about the rope? hmm...yep, once again nothing. Furthermore, the tension force will STILL be nothing more than the friction force required to make the tires begin to roll from rest.

Now, push up the throttle, what happens? Holy crap a miracle! The rope slacks and the mighty cessna moves forward with respect to the air and the tires still on fire at uber-ridiculous speed-rpm+(rpm equivalent to X KIAS). But wait. Let's put skids on that son'b%tch, assume the treadmill is now an icy treadmill, and what happens? No way! It still moves relative to the air REGARDLESS of that moving treadmill at uber-ridiculous speed.

In a real world scenario, what would happen is that the takeoff roll would be increased by the initial head-start you gave the treadmill as it began moving the aircraft backwards with respect to the air as you sat there wondering to yourself if this experiment was going to work or not. A takeoff with an initial tailwind if you want to look at it in more familiar fashion. After brake release, throttles up, the wheels start rolling and you're moving forward w.r.t. the air son.

IF the aircraft had to overcome the speed of the treadmill by means of enacting its thrust by rotating the wheels, then the treadmill would be relevant. Look at it this way. Your bass boat is going up the river and the stream is wayyy too fast for that mercury, your buddies at both banks see you sitting there idling with respect to them, at max power, and throw you a rope, then pull you in. Are they able to pull you in in spite of the wicked fast stream? Sure. And that's a worse case scenario than the airplane on a treadmill, as the water will create higher parasite drag the faster the stream, which means billy bob needs to pull a little harder, whereas the aircraft tires will keep the friction force constant no matter the speed of the treadmill. So, if you can at least buy the "buddies pull you with positive relative motion in spite of the river speed" scenario then convince yourself that strapping an IO-360, with included propeller, to your head while you duct tape yourself to your bass boat is AN EQUIVALENT SYSTEM PROBLEM!!!!

If you cannot convince yourself of the latter, you need to go back to high school physics, for you will never understand why the airplane WILL attain flying airspeed regardless of a moving ground surface. F%ck, I need my g/f...and a drink.

The original question assumes the aircraft does not move, It states "As you add power and begin to move for the takoff roll, the treadmill speeds up and keeps the plane in the same relative position. As the plane goes faster, the treadmill speeds up, always maintaining the aircraft's relative position.

An aircraft with no relative wind will not fly! This question clearly states the aircraft has no relative wind.