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

[The_Russian]

XPOO,

Is that a pic of Brittney Cabbin' me?

 

[FlyinScotsman]

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.

I was once one of you naysayers, now I am a believer, It will fly!!

This dude explains it very well......

Read and Repeat!!!

 

[aussieflyboy]

Someone just rig up a big treadmill and use a RC airplane and see if it works or not. You don't have to see it take off, just if it moves forward at all.


...and this thread will never end.

Nobody can ever recreate this scenario, megreene... that's the whole point.

The way the question was worded says that the plane holds its position in space, but physics has the plane accelerating down the treadway and taking off.

You cannot recreate this.

 

[aussieflyboy]

flying scotsman,

we all agree that it would fly... however that's not what the scenario specified. This is a hypothetical question, and it clearly stated that the airplane's position in space was fixed. How do you get around that?

 

[Flyer1015]

The question was written in a flawed manner.

 

[temcgrew]

How about a new twist? What if the aircraft were landing on a big treadmill?
Would the landing distance be shorter?

As for the original question, I would like to see the treadmill seize up.

 

[YourPilotFriend]

As far as I can tell, THIS was the only guy dopey enough to suggest that the plane would stay in the same position AND take off!

Congratulations mate!!!

By the way, it's clear that we were played for a fool.

The correct answer is that there's no answer, the question's f**ked.

Time for bed

Ahh no, the last sentence states that if the wheels didn't have any friction the airplane would accelerate to takeoff speed rather than apply a force opposite to the direction of the thrust vector.

In all the posts that suggest it will fly, they apply theoretical models that are simplified to exclude friction. So the answer to this question is a theoretical plane can takeoff, but a real one would not.

If the treadmill were calibrated to provide enough velocity to overcome the forward motion of the aircraft through friction. A real world airplane would be destroyed before it even reached a 1/2 knot of forward airspeed.

 

[Edge540]

_____________:smash:
:uzi:***this thread***:bomb::bomb::bomb:
_________uke:

 

[aussieflyboy]

None of us who say it won't fly disagree with the principle of it flying... I know it would accelerate anyway with the wheels spinning at hypersonic speed. However that doesn't satisfy the question which states that it remains in the same place.

Who gives a f**k anyway. These are precious hours I'll never get back!

 

[ImbracableCrunk]

Garbage in, garbage out.

Motion to close.