Is it just me or are people fucking thick?

[zip accident]

Mir Nyet wrote:

How can the plane take off if it is effectively standing still?

The example you gave was in a wind tunnel. In that case, the roadspeed is 200, the groundspeed is 0, the airspeed is 200, so the plane will fly, since airspeed is what matters.

[tufty]

Yes, Jedburgh. Some of them are.

[Istar]

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...

Concisely put, and absolutely correct.

Aircraft fly due to a difference in air pressure across the ventral and dorsal wing surfaces as air flows across them. If the aircraft is not physically moving forwards, airflow does not occur (headwinds discounted - we're assuming still air conditions) and the aircraft cannot maintain lift.

A plane takes off because its engines drive it forward. Implicitly, it moves forward, and the wheels turn in order to allow it to continue to move forward. If the surface on which the wheels are placed is moving in the opposite direction at exactly the same speed as the forward motion of the aircraft, then it moves back to its starting position. For all intents and purposes, it has not moved - ergo, there is no flow of air over the wings, and no lift; hence, no take-off and flight. QED.

[kalmar]

Oh for goodness sake.

[Mir Nyet]

Yes, I agree than the tires would be under huge stress, but the plane would be stationary... No air being accelerated over the winds means no lift...

Yes, do try to keep up! But this is a silly and impossible scenario, just like the original question. And while we're here, it's just been pointed out that the draught from the magical rolling road going at nearly infinity miles per hour would allow the plane to lift anyway.

The back of the plane would likely buck around due to the dirty air replusing the tail, but the accelerated air at the forward part of the rolling road would likely cause an area of lowered pressure which would actually suck the planes nose down

[tufty]

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...

Concisely put, and absolutely correct.

Aircraft fly due to a difference in air pressure across the ventral and dorsal wing surfaces as air flows across them. If the aircraft is not physically moving forwards, airflow does not occur (headwinds discounted - we're assuming still air conditions) and the aircraft cannot maintain lift.

A plane takes off because its engines drive it forward. Implicitly, it moves forward, and the wheels turn in order to allow it to continue to move forward. If the surface on which the wheels are placed is moving in the opposite direction at exactly the same speed as the forward motion of the aircraft, then it moves back to its starting position. For all intents and purposes, it has not moved - ergo, there is no flow of air over the wings, and no lift; hence, no take-off and flight. QED.

Nope, if the surface over which it is moving moves back at the exact speed that the plane would otherwise have moved, the *undriven* wheels will merely rotate twice as fast, the plane will continue to move forwards, albeit at a microscopically smallr speed due to extra rolling friction, and take off.

A vehicle moves relative to the medium on which it applies a force. A car applies a force relative to the road, thus if you move the road, you alter the speed of the car. And aircraft apples a force to the air itself, thus in order to slow it you would have to move the air, not the surface on which it is sitting.

For further explanation, I refer you to my previous reply in the thread.

Simon

[fdave]

This appears to be a popular subject for discussion at the moment.

In brief, an aeroplane is put on a magical conveyor belt that automatically matches the plane's speed and keeps it stationary. The question is, can the aeroplane take off?

Yeah seems like people are just overcomplicating things. If the plane is stationary, (and the conveyor belt isn't chucking air over the wings), there's no air pushing the plane upwards. So it wouldn't take off

[kalmar]

So the "magical treadmill" is really just a couple of breezeblocks bolted to the runway. That'd stop the plane, and it wouldn't take off. OK, is everyone happy now?

[Istar]

Nope, if the surface over which it is moving moves back at the exact speed that the plane would otherwise have moved, the *undriven* wheels will merely rotate twice as fast, the plane will continue to move forwards, albeit at a microscopically smallr speed due to extra rolling friction, and take off.

Oddly enough, almost as soon as I posted I finished working out the maths and realised exactly that.

Ergo, I retract my earlier assertion and posit precisely the opposite. The aircraft will drive forward, generate lift, and take flight.

Colour me embarrassed.

[zip accident]

Istar wrote:

If the surface on which the wheels are placed is moving in the opposite direction at exactly the same speed as the forward motion of the aircraft, then it moves back to its starting position.

That is bollocks. For aircraft. Though something very like it would work for cars.

tufty wrote:

For further explanation, I refer you to my previous reply in the thread.

And I refer you to mine, as well.

[Istar]

And I refer you to mine equally.

[tufty]

Imagine a plane is sitting on a massive conveyor belt, as wide and as long as a runway. The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Can the plane take off?

"The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction."

Note, there's nothing about holding the aircraft in the same place. So all the aircraft has to do is get to airspeed, and take off. And the only thing "holding it back" is the rolling resistance of the wheels.

Let's assume that the air is dead calm, and that airspeed = groundspeed. We'll assume airspeeds in terms of % takeoff speed, so 100 = takeoff speed.

At airspeed 0, speed relative to the treadmill is 0. Easy
At airspeed 25, speed relative to treadmill is 50. Good so far. The treadmill can't be offering more rolling resistive force than the engines can overcome, *because in normal operation we see airspeed / groundspeed 50 and we are still accelerating*
At airspeed 50, speed relative to treadmill is 100. Again, the argument above applies, in normal, non-magical treadmill operation, we accelerate past a groundspeed of 100% takeoff speed

So, what happens from 50 to 100? Well, aviation bearings are rated to operate near-frictionless up to speeds well above their nominal operating speed. I can't give exact figures, because it's been a while, but let's just say that I used to work for "Britain's Favourite Airline", in the engineering department at Hatton Cross; most aircraft components are rated at over 20x expected load.

The plane will take off. There might be some edge cases that could be argued if we were to suspend the laws of physics, but the plane *will* take off.

Simon

[zip accident]

Istar wrote:

I refer you to mine

Yeah, I wrote before reading that. I was about to edit that once I did, but a reply here seems to be as good. No offence intended.

[tufty]

Istar wrote:
I refer you to mine

Yeah, I wrote before reading that. I was about to edit that once I did, but a reply here seems to be as good. No offence intended.

None really intended here, either, and the same situation.

Simon

[gilesgoat]

I repeat ..

If I tie your car to a rope and I am pulling it away, even if I put a
conveyour belt under it making the weels spin however you like do you
think the car is staying there over the belt ?

And/or you think an airplane would not fly if the engines would be full
power and the wheels are unable to spin ? It would destroy all the rubber
properly in scraping the runway but it could take off the same.

It's the turbines that materially are "screwing into the air fux" causing
the airplane to move, not the weels, you can't keep "the airplane in place"
as long as there's air around.