ReallyReallyGoodMeat

Not strictly true, as friction will be at play.
If you put an aircraft on a conveyer belt and then turn it on, the plane will move with it, the wheels wouldn't just spin round.
But I accept the overall argument.

JackClark

There's another video that shows friction having an effect up to a certain low speed then making no difference at all.

TelBoy

Yes, absolutely i am. If your grip on the rope cannot change, and the wheels under your feet are being exactly counteracted by the treadmill, then you go precisely nowhere, no matter how fast everything is spinning. Jack's video clip demonstrates it perfectly.

As i've mentioned earlier, planes need groundspeed and thrust to take off. If there's no groundspeed, there's no take off. The only way that VTOL planes overcome this is by eliminating the need for groundspeed, then the wheels/conveyor combination could be spinning at a million miles an hour, it wouldn't matter. But for a conventional plane, it does.

JackClark

Planes don't need groundspeed to take off, stick a 747 in a 400mph wind - airspeed - and it'll get off the ground.

TelBoy

Yep ok, wind over the wings if we're being pedantic. So unless we're introducing a windy day into the initial parameters, the plane won't take off.

Devildog

TelBoy

Yes the irony hasn't been lost on me

JackClark

The plane will take off, there's nothing stopping it gaining airspeed. The wheels spin faster to make up for the moving runway, that's all. Mythbusters nailed it.

ReallyReallyGoodMeat

The acceleration will come from the engine pushing against the air behind it, not the wheels. The wheels will just stop it from dragging along the ground. There is negligible friction at the ground, so unless the conveyer belt is made of maple syrup then the aircraft will definitely move relative to the ground.

And I'm out.

TelBoy

Then the assumtion, Jack, is that every plane's engines are powerful enough to negate the need for a runway, and that a runway is just a helping hand. If that's a technical fact then ok, fair enough. In this scenario it would mean the plane would begin to hover, and then move forward as more thrust is applied. Is that what Mythbusters demonstrated?

J4CKO

iTrader: (1)

The plane will still take off, the engines pull it throught the air until it generates enough lift, i.e. the lift is greater than the planes Mass and it takes off, the treadmill would have a minimal effect in terms of additional friction but not enough to make any difference to the final result.

JackClark

What he said.

markjmd

iTrader: (11)

While the Mythbusters video does on the face of it appear to provide very compelling evidence, I'd still be very interested to see an explanation in your own words for the following:
- flight is normally achieved by thrust from the jet or propeller driving the aircraft forwards against the ground and through the surrounding air, which causes air to flow over its wings, providing lift.
- in the case-scenario where the wheels are sitting on ground which is slipping backwards, the aircraft not only fails to move forwards relative to the solid earth beneath the slipping ground, but also relative to the surrounding air above it. Under these conditions, where is the lift derived from?

JackClark

You're assuming the wheels are driven. They're not. The propeller doesn't care what speed the wheels are turning, it'll pulls the plane forward using the air not the runway.

markjmd

iTrader: (11)

If the plane is moving forwards through the air at all, doesn't that simply mean the conveyor isn't in reality matching the acceleration of the plane's engines?

JackClark

http://www.airplaneonatreadmill.com

markjmd

iTrader: (11)

To sum up then:
airplane fuselage speed=150mph
conveyor speed=-150mph
airplane wheel speed=300mph

It'll take off, but the landings might eventually be a bit iffy on that undercarriage, if the experiment was repeated often enough.

ReallyReallyGoodMeat

Indeed, because it cannot. All it can do is spin the wheels.

And now I'm really out

TelBoy

So to summarise

All planes have engines powerful enough to generate enough lift to get them off the ground and stay in the air without the need for any actual movement relative to the ground.



I'm surprised by that fact i have to say.

JackClark

Did you think they rolled them downhill to get them going?

TelBoy

ReallyReallyGoodMeat

It will move relative to the ground though!

TelBoy

Yes i get the point it's making now. We could go into the semantics of it, in terms of whether the conveyor belt prevents the plane moving, which it could. The original question implies that this is what happens, in my mind at least. What Jack's article adds is that the premise is that the conveyor merely attempts to stop the plane moving. I know it seems like a minor point but it changes everything. So yes if the plane is allowed to physically move it WILL take off. Trust me i know what i mean even if nobody else does

Quite a few "won't fly" posts have been deleted from this thread now. I'm happy to keep all my thoughts on show, for better or worse

Jay m A

iTrader: (2)

This is it though. The statement in the OP suggests that the scenario will make the plane stand still relative to the observer on the ground, but that cannot happen since the planes engines do not drive the wheels, so all the conveyor does is spin the wheels twice as fast as the planes groundspeed along the conveyor.

jonc

The reason why you would not be moving is not because the treadmill is counter acting your movement, it's because you've stopped pulling on the rope and if you've stopped pulling on the rope, neither the treadmill nor the wheels would be spinning.

You don't need ground speed to take off, you need air speed acting on the wing to generate lift. A plane will take off in a wind tunnel and still remain stationary in relation to the ground.

TelBoy

Exactly as i said on my post after that

I think there's enough thinking out loud on this thread now for everyone to get the jist of the question and the outcome

ReallyReallyGoodMeat

This thread should be locked away and never visited again...

jonc

But we only got to three pages! Fug knows how it managed to get over 80 pages on another forum!!!

TelBoy

Oh i can. If the question said *can* a conveyor always stop a plane taking off then i think we'd have got there a lot sooner, but if people are answering two different questions with two different answers then i reckon they did well not to get to page 100