Prandtl Glauert Singularity

[tigisfat]

Not only that, but without prop thrust the flight (even if it happened) would be mighty short because as soon as the wheels broke contact with the treadmill, the thrust would go immediately to zero and the best outcome would be an instant touch down, or worse, a very hard one!Ken

Not only did you fail to outline how I was wrong, but you're agreeing with him (saying he's right) and then telling him he's wrong. Either the plane in his scenario takes off or it doesn't. Going back to my magical machine, it'd be stupid to assume that you can't hold an aircraft on the ground any faster than liftoff speed, and any excess speed would make the flight longer.

Try this on for size, I already know it fits: you're both wrong.

 

[AckAck]

Yeah, but how about that neat effect when the aircraft exceeds the speed of sound and forms that cone of condensation along the shockwave... :d (or those Formula 1 cars, when THEY exceed the speed of sound and exhibit the same phenomena.)

(tongue firmly in cheek)

Brian

 

[Ken Stallings]

Not only did you fail to outline how I was wrong, but you're agreeing with him and then telling him he's wrong. Either the plane in his scenario takes off or it doesn't. Going back to my magical machine, it'd be stupid to assume that you can't hold an aircraft on the ground any faster than liftoff speed, and any excess speed would make the flight longer.

Try this on for size, I already know it fits: you're both wrong.

OK, if that makes you feel better, knock yourself out.

But I don't think he implied the pilot had to avoid rotating the aircraft by back elevator forces. On the other hand, while some aircraft have to be rotated to lift off, others do not. You can also simply apply takeoff trim setting to the elevator and have it already set for rotation.

Just curious, did your CFI during your Private Pilot instruction not demo a hands-off-the-yoke takeoff in a Skyhawk? You can do it on a calm day by just using rudder to maintain centerline. Trim the Skyhawk for takeoff, and it will absolutely takeoff without any overt extra elevator inputs.

Cheers,

Ken

 

[Ken Stallings]

Yes, we all know and understand that. Your lack of concern coupled with the fact you know you can get away with it without repercussion here is precisely why you spend your time talking trash, insulting others and loudly offering your opinions on religion, politics and anything else under the sun. It doesn't matter whether you're wrong or right, (such as where you're wrong here and still being arrogant and blunt), you're just here for the game.

No, I'm here for information exchange.

Cheers,

Ken

 

[Sideshow]

Actually, you're wrong. It is quite possible to hold an aircraft at speeds faster than the speed at which it will fly. If you don't rotate a heavy, it'll likely never get airborne in the runway you have.

Haha sorry Tig, my subtle attempt at humour obviously didn't work.

You are correct in saying that I assumed the wing was already at the optimum angle of attack to produce maximum lift. Obviously this isn't normally the case with most aircraft when they are on the ground and your above statement about rotating is perfectly valid.

Like I said, don't mind me. Infact your whole argument till this point has been solid and I actually agree with you.

 

[tigisfat]

Haha sorry Tig, my subtle attempt at humour obviously didn't work.

You are correct in saying that I assumed the wing was already at the optimum angle of attack to produce maximum lift. Obviously this isn't normally the case with most aircraft when they are on the ground and your above statement about rotating is perfectly valid.

Like I said, don't mind me. Infact your whole argument till this point has been solid and I actually agree with you.

And I apologize if I was rude. If I did in fact offend you, I wish to offer you a dancing ninja as a form of apology for my bitterness. This Ninja is valued at +1 internets. You can have the receipt in case you wwant to return it and get something more practical.

 

[tigisfat]

No, I'm here for information exchange.

Cheers,

Ken

And no, I never had that demo'd to me. It may have had something to do with the winds in Abilene.

 

[Sideshow]

No offence taken. And thanks for the Ninja

 

[tigisfat]

No offence taken. And thanks for the Ninja

You're welcome, it was the least I could do.

 

[Wing_Z]

On a really fast, sticky, long, treadmill, an aeroplane would fly off the back and crash... .

Does that count?
:d

 

[AckAck]

I distinctly remember a teacher telling us that if a penny were dropped off the Empire State building, it would easily kill someone. Gimme a break!! If you were to follow the same scientifically bankrupt reasoning, an automobile dropped from a thousand feet would easily beam a hole straight through the planet and come flying out of the ground somewhere in India.

I think it is a completely different scientifically bankrupt reasoning (from the penny dropped from the Empire State Building and killing someone) that would have a car dropped from 1000 ft go completely through the planet and kill someone on the opposite side. That has absolutely nothing to do with the terminal velocity - unless you assume that a penny and a car dropped from the same height (Emipire State Building is around 1000ft, no?) accelerate at different rates, and the car, by virtue of its larger mass, is going a heck of a lot faster than a penny - somewhere near the speed of light, I would guess, to go completely through the planet.

I would, however, stipulate that if you dropped a car from the top of the Empire State Building, you could kill someone on the ground...

Brian

 

[tigisfat]

On a really fast, sticky, long, treadmill, an aeroplane would fly off the back and crash... .

Does that count?
:d

Yes, I believe it does. Especially if the pilot has the parking brake set and he's not paying attention to the airplane being placed on a giant treadmill.

 

[Wing_Z]

Tch...all the things a pilot has to remember...
Now:
If a plane's takeoff speed is 94kts
and there is a train going at 110kts with a treadmill on its roof running backward at 116kts...
pointing at the locomotive end, will the plane take off from the treadmill?

 

[tigisfat]

Tch...all the things a pilot has to remember...
Now:
If a plane's takeoff speed is 94kts
and there is a train going at 110kts with a treadmill on its roof running backward at 116kts...
pointing at the locomotive end, will the plane take off from the treadmill?

Yes, and if the train is long enough, it will take off backwards too. Both are assuming the airplane has the excess thrust required for flight and isn't held to any position on the train. This is also assuming that the plane won't fall off the train and the joints between cars are somehow seemless. I do suspect that you could do an immediate liftoff though.

It'd work just like this:

[YOUTUBE]EIV1ZcIzhLQ[/YOUTUBE]

 

[robcap]

Beaver launching, cool

 

[tigisfat]

now if you put a non-amphibious floatplane on a treadmill...

 

[OBIO]

How come flatulence, which every one knows is faster than the speed of smell, never has those vapor cone thingies? Sure would make it easier to know who floated the air biscuit.

 

[tigisfat]

Theoretically, would this work?

If the tether was taut from the start, and the engine provided enough airflow over the wing surface, would the aircraft lift off?

View attachment 7601

umm,

If that were going to work, it'd have to be on a prop plane with enough lift produced from the prop's accelerated slipstream to make it hover. I may be wrong, but there hasn't ever been one of those. There are a few prop planes that can hover in the vertical, but none that can do what you're offering. That's assuming a zero wind condition.

 

[Quixoticish]

My gripe with the "plane on a treadmill" Mythbusters episode was that testing it goes against the spirit of the problem, their awful grasp of science aside.

It's one of those theoretical physics questions that relies on a lot of assumptions that you can't actually re-create in real life.

When you're answering a physics exam paper there are always certain rules laid out before a question. Ice that is completely frictionless. The force of gravity is always exactly 9.8 ms-2. Air resistance is negligible. Energy lost as thermal energy is negligible. Friction is a constant force. Acceleration is constant and does not fluctuate.

There are a million and one of them and anyone with any qualifications in physics will be well aware of them. And in the case of the plane on the moving runway it relies on one of these assumptions, in this case a conveyor belt that can instantly and precisely match the aircrafts speed. We're not taking into account the effects of friction, wind resistance, pretty much anything. The only fact we are given is that the treadmill instantly matches the aircrafts speed. Of course, I'm sure that since it was released into the wild the question has now become bastardised and shortened, but in its original form that is how it would have been phrased.

Of course in real life we can't manufacture something like that, it is simply impossible, and this leads to two answers. In real life the aircraft will take off, the aircraft will be able to accelerate quicker than the conveyor belt can in the opposite direction. Obviously the net result is a take off.

But on paper it can't, due to these little physics assumptions that we make, and the correct answer is obviously no, it wouldn't take off. This is why it's against the spirit of the question trying to test it. It is a question designed to hone your analytical skills. Carrying out experiments is easy, but picturing obtuse and often illogical concepts and actions in your head can prove exceedingly difficult, and that is exactly what this question is designed to test.

 

[Wing_Z]

Interesting...
What would be the conditions you impose for the plane not to take off, then?