Think sea plane. No wheels at all!
can a plane take off from a conveyor moving at the same rate as the plane
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Nytrunner
Pop lugs, not drugs
The wheels are not driving the plane, they are literally just a bearing between the plane and the ground.
Once rolling friction is broken, the plane's thrust will accelerate it forward regardless of the indescribably stupid reverse velocity of the treadmill.
What WILL happen if the treadmill matches plane speed, is that the wheels will be spinning twice as fast.
Unless the brakes are on, the only connection between the plane and the surface of the treadmill is rolling friction and bearing friction. Those are minor and may be ignored. In other words the speed of the treadmill has nothing whatsoever to do with the airspeed of the plane. The treadmill has almost no effect on the air that travels over the wings. Air traveling over the wings (more accurately the wings traveling through the air) is the only thing that causes the airplane to lift.
As the planes picks up speed to takeoff velocity, don't the wheels turn faster?
I agree that the lift force is caused by air travelling over the air foil (wings). But if the forward motion of the plane is matched by the rearward motion of the conveyor belt, isn't the air speed 0?
Nytrunner
Pop lugs, not drugs
Not at all. Any extra rolling friction from the rearward motion of the conveyor is negligible compared to the forward thrust of the engine
In rocket terms, it'd be like saying "If an infinitely long launch rod/rail were moving downwards at the exact opposite speed of the rocket, would the rocket move at all?" Yes it would, the buttons would melt, but it would indeed move
No - they are not related. The wheels spin freely.
Great analogy. In this case the wheels could spin apart.
OK, let me see if I've got this straight: once the rolling resistance is overcome, the forward acceleration of the plane will happen regardless of what is happening to the ground. Is that right?
Exactly. The plane is accelerated by a force that’s disconnected from the ground.
dr wogz
Fly caster
I think there's a reason why planes take off into the wind..
I'm perplexed why this is even a question worthy of discussion. Once you know that airplane wheels are not used for locomotion, and that forward thrust comes only from the props or jets or whatever, that's the end of it. Where's the mystery here?
It’s just that not everyone understands that.
Nytrunner
Pop lugs, not drugs
You owe me a new plane, and probably a new magic conveyor too....
It travels the same speed and direction as the treadmill surface. If the treadmill is going fast enough it could even lift off, but if the plane isn’t creating its own propulsion it will begin losing airspeed as soon as it leaves the treadmill.
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beings how no mention of the planes engine being used, pretty much what i thought here.
It's easy to make the wrong assumption that the conveyor is moving at the same rate as the wheels always and not just a constant 50 knots.
Boeing C-17 Globemaster is one I know of that can drive on its wheels. It can do a three-point turn on the runway when needed. Impressive. I am sure there are others, but not many. Carrying the extra mass of drive motors is detrimental to fuel economy every time they fly. That is not tolerable on commercial aircraft currently.
Imagine it's a toy plane on a treadmill in a gym. You're sitting on the floor next to it, holding the plane in your hand with the wheels on the treadmill.
Could your hand move the plane forwards if someone kept turning the speed of the treadmill up?
My recollection from flight test videos is that it’s ability to turn on the runway is from thrust reversers, not from anything in the landing gear.
The least fuel use will result from comparing the stream speed and direction to the wind speed and direction and selecting whichever results in an into-the-wind condition while floating along with the stream.
You may be right with that, but I don't remember the jet engines changing pitch when I saw the maneuver. I don't have definitive knowledge.
Whatever gets you into the air quickest. Depends on river and wind conditions on the day. I am guessing they have tables or a calculation for that for each aircraft and load configuration. Maybe they just take off from lakes
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what about when landing?
Tony
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My point was that the plane is now "stuck" to the treadmill and will move with it. I should have added that I meant for the treadmill to be moving backwards relative to the plane.
Let's imagine the plane sitting on a runway on really sturdy skis. I would think most people can see how the plane will move forward if it can generate enough power to overcome the static friction between the skis and the runway. It may make a helluva racket and throw sparks everywhere, but it could takeoff if it had enough power.
So now put the plane on a treadmill that is locked down and won't move. The plane took off from a runway so it should take off from a stationary treadmill. Right? Now move the treadmill backwards at 1 foot per second. It is a little more difficult, but again the plane should still take off if it can generate enough power. At what speed of the treadmill can't the plane take off? That would be when the treadmill is moving backwards at a speed that prevents the plane from generating enough forward thrust to attain lift off.
Now swap out wheels for the skis. What has changed? I have removed a tremendous amount of friction, but nothing else. Think about it.
No, you’re wrong. Once static friction has been overcome the friction actually goes down and sliding faster, although generating more heat, does not create more drag. That’s why spinning your tires faster and faster on ice doesn’t get you unstuck.
Think about that. [emoji16]
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