Physics question.

This is an ongoing debate on another forum , I figured I'd bring it to a more intellectual crowd to see what the result here was.

If you put an airplane on a conveyor system that was able to equal it's speed in the opposite direction as it atttempted to take off , would it be able to take off and why?

EDIT: The conveyor ,for this purpose, will automatically move against any forward motion of the plane , therefore ,the plane will never move forward.

What the conveyor belt is doing is immaterial. What matters is the windspeed over the wings.

The "conveyor system" can move in whatever direction it wants. As long as the plane is free to roll along, and is not controlled by the conveyor, then it builds its own airspeed. This builds windspeed and allows it to take off. Until it loses contact with the conveyor system, it's wheels may be rotating a lot faster than it would without contact with the conveyor system. But, that does not control it's ability to take off.

Now, if the conveyor belt actually controlled the AIRspeed (and thus windspeed) of the plane, then things would be different.

Alex

No, it wouldn't it take off. The reason; insufficient air density flow across the wings.

This comes into play, fatally, in many instances. It's know as density altitude. I experienced this for the first time in a take off from Bend, OR in the middle of the summer. The temperature had risen significantly and although the Cessna, from the feeling as the tarmac passed beneath us, felt like it should be ready to be air born wasn't. When I tried to rotate, nothing happened!!

Fortunately for me and my wife I was able to inch it off before the runway ended.

You only get scared like that once in your life. After that you check before you try to fly!

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Okay--Situation may be different from when the question was first posed.

You are saying that the plane cannot move forward.

We need to establish some points here or we are stuck in a question of relativity.

The plane cannot move forward relative to what?

Let's build a conveyer belt covering the whole east-west runway. Paint two big dots, a green one and a red one on the conveyer belt right in front of the control tower. Place an airplane on the green dot and a car on the red dot.

The control tower--which is not on the runway or conveyer belt-- stays put. We call that point directly in front of the control tower POINT ZERO.

Now the car and the airplane start to run. Simultaneously the airplane's engine and the cars engine start to drive the respective vehicles west at what should be 200 miles per hour (relative to POINT ZERO). The conveyer bels is dong nothing. Both vehicles move forward (west) at 200 miles per hour relative to POINT ZERO. They leave their respective dots (which are parked at ground zero because the conveyer is not running) behind at that rate.

Now, let's run it again with the conveyer belt. We start the car and the airplane off again to the west at 200 miles per hour. But the COnveyer belt starts to run east at precisely the same time at 200 miles per hour (or 200 miles per hour to the east). SO, the car engine and the airplane each speed up to 300 miles per hour worth of output west. But exactly simultaneously, the conveyer belt speeds up also to 300 miles per hour east.

Is this the situation you are describing?

In this situation, The dots on the conveyer belt are next to each other heading 300 miles per hour east in relation to POINT ZERO. The car is going three hundred miles per hour west in relation to its red dot, and zero miles per hour in relation to the control tower at POINT ZERO. The airplane is going three hundred miles per hour west in relation to the automobile and the control tower at point zero, and six hundred miles an hour west in relation to its green dot.

Now, let us posit another example--the one that you were imagining it appears: Let us say the the conveyer belt is now going to head east at exactly whatever rate the plane is travelling west. If the plane speeds up to a million miles an hour west, the conveyer belt with its little dot will be heading a million miles an hour east.

Unless the conveyer belt itself is attached to the airplane, and vice-versa, then it does not matter how fast the conveyer belt is going. Planes are not attached to conveyer belts.

It is as if you are asking the question: "If I had a perpetual motion machine, could it stay in motion forever." Well, by definition, yes. But, of course, there is no such thing as a perpetual motion machine.

(Note, all of this gedanken experiment assumes efficient wheels and engines, and that there can actually be matching accelerations, and so forth.)

Now, let us posit more than just a conveyer belt. Your conveyer "system" includes the air mass around the airplane. Now, you race the airplane west at exactly 600 miles an hour, but (relative to some imagined external reference point) it appears to be staying perfectly still because everything else out there is moving exactly east at exactly the same 600 miles per hour. That everything includes an airmass that must move east at 600 miles per hour, and if the plane is staying fixed in space, with a six hundred miles per hour headwind, it will fly. So, the plane flies.

Fact is, if the air of sufficient density is flowing with sufficient speed over properly designed wings, the airplane will lift. Whether that airflow comes from the plane moving through the air or the air moving past the wing (after for instance, being forced by a fan in a wind tunnel), the wing will lift the plane.


Alex

Hmmm. Seems to me that the engines buy themselves would produce enough air flow to lift the aircraft whether it was moving or not. maybe more possible with a propeller drive aircraft