Is it just me or are people fucking thick?

[kalmar]

But this is all more nonsensical than the original question, which is badly worded but all about much the same thing - the difference between airspeed and ground speed. Ground speed can be positive, negative, or even tracking sideways, it doesn't matter as long as airspeed is enough to keep the plane in the air.

simon

No, this is only true once the plane is airborne. this must be achieved by first running along a suitable surface until enough lift is created for the plane to achieve takeoff.

Tarmac offers considerably more resistance due to friction than water or grass, hence the need for wheels, the rotational properties of which are the basis for this hypothetical situation, since forward momentum via rotation+friction in one direction can, theoretically, be compensated by rotation+friction in the opposite direction.

I see what you're getting at. You'd be opposing several tonnes of propulsive force with rolling resistance and bearing / brake drag. The treadmill would probably have to run at a million miles per hour, and the tyres would explode pretty early on in the scenario.

However, it wouldn't take off.

[tufty]

But this is all more nonsensical than the original question, which is badly worded but all about much the same thing - the difference between airspeed and ground speed. Ground speed can be positive, negative, or even tracking sideways, it doesn't matter as long as airspeed is enough to keep the plane in the air.

simon

No, this is only true once the plane is airborne. this must be achieved by first running along a suitable surface until enough lift is created for the plane to achieve takeoff.

Nope. Not at all. Ground speed is utterly, utterly, irrelevant except that, in the normal course of things, when you are at airspeed x, you are also at ground speed x, more or less. Therefore, to achieve airspeed x, you also need at achieve groundspeed x. Take the hypothetical Cessna, but it could be any fixed-wing aircraft, and blast a wind at it that is in excess of its stall speed, and it *will* lift.

Simon

[zip accident]

phattslagg wrote:

I think it's fair to assume that for this hypothesis the wheels and treadmill are able to spin as fast as necessary without damage.

The reason I don't agree is that as I said above, once the plane is moving, the conveyer is chasing it's own input, which by definition means it can never work to specification, even if it is magical. Further, before it catches up with itself it exceeds the speed of light, and that is a non-trivial barrier, exceeding which, so far as we can currently tell, is a physical impossiblity.

This question was silly to start with, but interesting (my thanks Jedburgh). However, the more you examine it, the sillier it gets.

[Mir Nyet]

http://forum.nifty.com/fmotor/f1/pics/w ... ber_l1.jpg

F1 wind tunnel, with a full sized car on a rolling road. During testing, the get the road up to race speeds - 170-215mph to demostrate how the air reacts going over the tires at speed. This rolling road allows the wheels to spin at race speed, while keeping the car perfectly stationary.

[phattslagg]

Ground speed is utterly, utterly, irrelevant except that to achieve airspeed x, you also need at achieve groundspeed x

So we agree then?

[WiteWulf]

Meh...everyone knows it's physically impossible for bumble bees to fly anyways...

/troll

[phattslagg]

phattslagg wrote:

I think it's fair to assume that for this hypothesis the wheels and treadmill are able to spin as fast as necessary without damage.

The reason I don't agree is that as I said above, once the plane is moving

But isn't that the nub of it? If the treadmill was to match the wheels' rotation right from the start (and therein lies possibly the biggest problem of all) then the plane would never move.

[skweek]

That bit was rhetorical.

Really? Was it?

You askin'?

If you're askin' then I'm dancin'

[kalmar]

phattslagg wrote:

I think it's fair to assume that for this hypothesis the wheels and treadmill are able to spin as fast as necessary without damage.

The reason I don't agree is that as I said above, once the plane is moving

But isn't that the nub of it? If the treadmill was to match the wheels' rotation right from the start (and therein lies possibly the biggest problem of all) then the plane would never move. Nothing like the speed of light would be reached anyway, since the engines would max out much earlier than this.

In this scenario, I'd have to agree the plane would not take off. But it's a bit of a stretch, because you're having to assume your way past so many physical limitations and "magical" explanations to reach the point where the plane is at full thrust and still not taking off, that it all just becomes pointless and you have to say "think of a better question please".

[tufty]

Ground speed is utterly, utterly, irrelevant except that ... to achieve airspeed x, you also need at achieve groundspeed x

So we agree then?

No. Speed on your magical rolling road is *still* completely different to the airspeed.

Even in the best case, where you say "we'll use the magical rolling road to negate the force of the motors", the only forces opposing the engine you would be able to apply would be *marginal* frictional forces from the wheels. Given that, on full throttle, even the feeblest cessna can overcome the frictional resistance of its tyres and just slide the rubber along the tarmac, no, you are not going to be able to do it.

Oh, and I fixed the quoting above, as you made is say something that it didn't say before.

Simon

[zip accident]

phattslagg wrote:

Ground speed is utterly, utterly, irrelevant except that to achieve airspeed x, you also need at achieve groundspeed x

So we agree then?

No, because the conveyer isn't moving at groundspeed, it's going backward.

If the treadmill was to match the wheels' rotation right from the start

No. The treadmill exactly matches the rotation of the wheels when the plane is still, because it is also still. When the plane starts moving, the treadmill has to move to catch up, and then it immediately gets into a race with itself that it must always lose.

[phattslagg]

Ground speed is utterly, utterly, irrelevant except that ... to achieve airspeed x, you also need at achieve groundspeed x

So we agree then?

No. Speed on your magical rolling road is *still* completely different to the airspeed.

Hey - steady on! I never invented the magical rolling road!

Even in the best case, where you say "we'll use the magical rolling road to negate the force of the motors", the only forces opposing the engine you would be able to apply would be *marginal* frictional forces from the wheels. Given that, on full throttle, even the feeblest cessna can overcome the frictional resistance of its tyres and just slide the rubber along the tarmac, no, you are not going to be able to do it.

Not if they use my magical super-frictional rubber they don't

[kalmar]

If the treadmill was to match the wheels' rotation right from the start

No. The treadmill exactly matches the rotation of the wheels when the plane is still, because it is also still. When the plane starts moving, the treadmill has to move to catch up, and then it immediately gets into a race with itself that it must always lose.

Nah, phattslag has a different interpretation: the treadmill works like a treadmill, and can go so fast that the rolling resistance of the tyres equals the output of the engines.
You have to ignore that they would explode practically as soon as you open the throttle, but he's come up with an answer for it.

[phattslagg]

The treadmill exactly matches the rotation of the wheels when the plane is still, because it is also still. When the plane starts moving, the treadmill has to move to catch up, and then it immediately gets into a race with itself that it must always lose.

Even if the treadmill is capable of exceeding the maximum velocity created by the plane's engines?

[WiteWulf]

Even if the treadmill is capable of exceeding the maximum velocity created by the plane's engines?

In which case wouldn't we all be going on our holidays by treadmill rather than jetplane?

/troll