Is it just me or are people fucking thick?

[Jedburgh]

That bit was rhetorical.

Really? Was it?

You askin'?

[fred_zen]

"if you took the wheels and legs off a plane so it just sat on the ground. would it achieve lift?"

Yes, if the engine was powerful enough and it survived the damage.

how about if the plane has no wheels or legs and has a bloody great rope tied to the tail to restrict forwards motion so the only airflow is provided by the engine?

We should also cover it in tar and feathers, just to make sure.

and as I'm trying to provide a real world example we'll say the plane is a cessna.

A no wheeld, no legged, bound-by-the-tail cessna. will it fly^H^H^H^H^H^^H^H? edit: achive lift.

Hell no, of course not, The engines don't blow air around the wings, they just drag the plane (including wings) through the air, creating lift.

[teamonkey]

so here we have an idea of a better question.

"if you took the wheels and legs off a plane so it just sat on the ground. would it achieve lift?"

Yes, if it was facing into the wind and the wind was strong enough. Stall speed for a Cessna is what, 90Mph? So you'd need a >90Mph wind to gain lift. In order to hover the thrust of the engine would have to perfectly counteract the force of friction caused by the passing wind.

So the answer is to forget about the magical treadmill and think about the magical desk fan.

[zip accident]

phattslagg wrote:

would the energy from the plane's engines be completely spent in that particular exchange,

Depends how fast you spin the wheels in the other direction. There is probably some allowance in the design for running at speeds above takeoff revolutions, but twice that would probably be pushing it, and three times would very probably break them. If you didn't break the wheels (which is pretty much impossible), there would be a speed at which they could be run backwards where the friction from that would stop the plane, but it would probably have to be a supersonic speed backward.

[fred_zen]

What about an aircraft on a magical conveyor belt, with a cat strapped to a piece of buttered toast sitting on top?

Will it fly, hover, turn inside out or end the universe?

I'd say it would end the universe.

Not sure what would happen if it was margarine instead, though.

Would still end the universe of the person trying, assuming they don't believe in an afterlife, and you and I mean the same cuddly-yet-fierce ball of fluff and claws by "cat".

[phattslagg]

phattslagg wrote:

would the energy from the plane's engines be completely spent in that particular exchange,

Depends how fast you spin the wheels in the other direction. There is probably some allowance in the design for running at speeds above takeoff revolutions, but twice that would probably be pushing it, and three times would very probably break them. If you didn't break the wheels (which is pretty much impossible), there would be a speed at which they could be run backwards where the friction from that would stop the plane, but it would probably have to be a supersonic speed backward.

If that is the case, then we have answered the question, since 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.

[zip accident]

phattslagg wrote:

I think it's fair to assume

In this case, I don't agree.

[Mark]

Assuming the plane's wheels can spin at unlimited speed, doesn't the treadmill become a frictionless surface essentially? In which case the engines will push the plane forward in the same way they do when it's airborne, so it'll take off. Possibly. Confused now.

This has set a little bit of my head going off though, I saw or heard a nice bit of comedy that compounded all the three-doorways and a goat, walking on the south pole, doctor operation on her son-type brainteasers into a monologue. But I can't remember where/when I heard it. I *think* it was on the Mitchell and Webb show, or Peep Show, but now I just don't know.

This, perhaps?
http://www.michaelkelly.fsnet.co.uk/lateral.htm

[Mir Nyet]

Reading that, it seems to me that they are speaking about a rolling road which would negagate the thrust from the planes engines (i.e. if the planes engines were switched off, the plane would move backwards if the rolling road was active)... Regardless of the thrust from the engines, if the plane has no forward motion, then there is zero air being accelerated over the planes wings, therefore no pressure drop over the wings, and no lift...

[tufty]

and as I'm trying to provide a real world example we'll say the plane is a cessna.

A no wheeld, no legged, bound-by-the-tail cessna. will it fly^H^H^H^H^H^^H^H? edit: achive lift.

Hell no, of course not, The engines don't blow air around the wings, they just drag the plane (including wings) through the air, creating lift.

No, they don't blow air over the wings. But, by entrainment, the propeller would ensure that air was flowing over the wings. So, if one could run the engine fast enough and generate enough flow, yes, the engine could cause the aircraft to take off.

Also, consider the case where this held-by-the-tail cessna happens to be parked facing a wind greater than its stall speed. Again, assuming that the engine could be used to "pull" then aircraft into that wind and hold it in place, yes, it would "fly".

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

[phattslagg]

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.

[kalmar]

But it's a 'magical' treadmill, so I think we can assume it will perfectly match the plane's wheel rotation at every stage of the process?

It's even more nonsensical when you add the word "magical"
The wheels still aren't anything to do with propelling the plane.

No, but they are to do with allowing the plane to be propelled. If the plane just had metal spikes which scraped across the tarmac, then takeoff would be a little more..interesting, wouldn't you say?

True, but it doesn't alter my point.
Also, ski-planes and water planes manage without wheels. Some gliders take off on a skid on grass. Wheels obviously have the lowest friction on tarmac though.

Well, yes. But the whole point of this question is:

If one could completely negate the [rotation>forward movement] of the planes wheels using an admittedly non-existent technology, would the energy from the plane's engines be completely spent in that particular exchange, or would they still somehow create lift?

What, non-existant technology like a couple of house bricks? If you stop the plane, it won't take off, all else being equal. A treadmill wouldn't stop a plane from moving, so that's why it's a daft question.

Like everyone keeps saying, the wheels are not driven, they have no part in propelling the plane, so all this talk of "negating their energy" is starting to get silly.

[Mir Nyet]

and as I'm trying to provide a real world example we'll say the plane is a cessna.

A no wheeld, no legged, bound-by-the-tail cessna. will it fly^H^H^H^H^H^^H^H? edit: achive lift.

Hell no, of course not, The engines don't blow air around the wings, they just drag the plane (including wings) through the air, creating lift.

No, they don't blow air over the wings. But, by entrainment, the propeller would ensure that air was flowing over the wings. So, if one could run the engine fast enough and generate enough flow, yes, the engine could cause the aircraft to take off.

Also, consider the case where this held-by-the-tail cessna happens to be parked facing a wind greater than its stall speed. Again, assuming that the engine could be used to "pull" then aircraft into that wind and hold it in place, yes, it would "fly".

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

But unfortunately, airspeed and ground speed when the plane is actually on the ground are rather tied together Also, the backwash from a planes props is too dirty to generate lift as the wing is designed to maximize airflow flowing from a certain angle, in a certain pattern, etc etc etc - quite similar as to why modern F1 cars cannot closely follow one another through a corner at speed.

[phattslagg]

If one could completely negate the [rotation>forward movement] of the planes wheels using an admittedly non-existent technology, would the energy from the plane's engines be completely spent in that particular exchange, or would they still somehow create lift?

What, non-existant technology like a couple of house bricks? If you stop the plane, it won't take off, all else being equal. A treadmill wouldn't stop a plane from moving, so that's why it's a daft question.

Like everyone keeps saying, the wheels are not driven, they have no part in propelling the plane, so all this talk of "negating their energy" is starting to get silly.

Please re-read what I wrote. I didn't say anything of the sort.

[Mir Nyet]

If one could completely negate the [rotation>forward movement] of the planes wheels using an admittedly non-existent technology, would the energy from the plane's engines be completely spent in that particular exchange, or would they still somehow create lift?

What, non-existant technology like a couple of house bricks? If you stop the plane, it won't take off, all else being equal. A treadmill wouldn't stop a plane from moving, so that's why it's a daft question.

Like everyone keeps saying, the wheels are not driven, they have no part in propelling the plane, so all this talk of "negating their energy" is starting to get silly.

Please re-read what I wrote. I didn't say anything of the sort.

A very (very) large rolling road could do this, if the speed of the road was linked to some sort of measurement of the planes position (lasers?) then any forward motion of the plane could be negated by an increase in the speed of the rolling road...