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[Altaïr]

https://www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/

Surprising... I'm not the best at aerodynamics, but I fail to see where the problem is... I'm fine with Bernouilli's principle and Newton's 3rd law, to me those are 2 different ways to handle the problem... Let's say the debate is too subtle for me...

 

[Roger Jolly]

Surprising... I'm not the best at aerodynamics, but I fail to see where the problem is... I'm fine with Bernouilli's principle and Newton's 3rd law, to me those are 2 different ways to handle the problem... Let's say the debate is too subtle for me...

Newton’s 3rd is fine, but Bernoulli’s just doesn’t do it;
Really just some hand wavey “negative pressure and so lift...yeahhh”, so then why can planes fly upside down?

 

[The Dark in the Light]

Newton’s 3rd is fine, but Bernoulli’s just doesn’t do it;
Really just some hand wavey “negative pressure and so lift...yeahhh”, so then why can planes fly upside down?

Because of science.

 

[Horus Lupercal]

Newton’s 3rd is fine, but Bernoulli’s just doesn’t do it;
Really just some hand wavey “negative pressure and so lift...yeahhh”, so then why can planes fly upside down?

Because you're thinking about it wrong.

 

[Horus Lupercal]

Planes can't fly upside-down.

 

[Tony]

 

[Altaïr]

Newton’s 3rd is fine, but Bernoulli’s just doesn’t do it;
Really just some hand wavey “negative pressure and so lift...yeahhh”, so then why can planes fly upside down?

There's no such thing as "negative pressure that sucks the wing upwards". That's a current misconception. Below is a higher pressure zone, while above is a zone of lower pressure, and the pressure difference generates a force upwards, hence lift. But that's the pressure difference that matters, not only the low pressure zone.

Planes can't fly upside-down.

Generally they can't. But some are able to, see aerobatic aircrafts for example. Symmetrical wing profiles exist, and still they are able to generate lift (though they are not the most efficient). What matters in this case is the angle of attack.

 

[Roger Jolly]

Planes can't fly upside-down.

Their lift may degrade upside down but they don’t fall as if lift has been inverted, would be something like falling at double gravity;
But maybe that’s just Newton saving the day, I’m no engineer, it’s just one of those why don’t we get this by now things

 

[Horus Lupercal]

not only the low pressure zone.

The article was annoying me a little bit because it threw all these references to 'aerodynamic experts' that were all puzzled by a low pressure force sitting on top of the curve of the wing and couldn't explain what it was.
And I'm sat here, a layman by comparison, going 'isn't that just drag being generated by the over-wing airflow because it doesn't move in nice straight lines and will swirl about when told to move. Especially at high angles of attack, which is what eventually causes wing stall.
It's what leading edge slots were designed to combat, but what do I know?'.

Generally they can't. But some are able to, see aerobatic aircrafts for example. Symmetrical wing profiles exist, and still they are able to generate lift (though they are not the most efficient). What matters in this case is the angle of attack.

Ha, I mean properly fly upside down. Most aircraft can operate 'inverted'

And I'd never say otherwise

But as you say, that depends massively on an increased angle of attack than normal and newton rather than bernoulli to stay airborne.

Their lift may degrade upside down but they don’t fall as if lift has been inverted, would be something like falling at double gravity

Upsidedown wings generate downforce, not lift. They do act as if gravity has been doubled (or even increased by a factor of 5). That's how F-1 cars work.
If the article was correct and wings operate the same regardless of orientation, then helicopters could fly upside down without issue as long as you could rotate the blades sufficiently to a positive angle of attack.
Aaaanndd they can't. Yeah, there are helicopters that can go upside-down. Lynx for example is fitted with a rigid rotor hub (no 'blade sail' when maneuvering) which means it's one of the few helicopters that can do rolls and loops etc. It's also the fastest true helicopter (Osprey is not a helicopter) in the world, and will always hold that title cos physics.

But even that doesn't really fly upside-down. The loops are more like stall turns and rolls are done safe in the knowledge that you'll never be at the start altitude when you finish. If it did work, then trust me the pilots would do it. I've been in Lynx many times when it was in service and the pilots are not shy.

Look at any aircraft upside down. The nose is much higher, especially at low speeds. That's because the engines are having to push the aircraft up, rather than the wings lifting the aircraft up. Helicopters generate little to no lift thrust and only fly because of lift generated by the rotors.

Which is why being upside-down in a helicopter for any length of time ends in a crash. Always.

The article goes on to explain that wings without curves are also used to generate lift. Yeah, speak to any pilot who flew the F-104 Starfighter and ask him about the lift generated by the less than a centimetre thick blades attached to the side of his aircraft.
There's a reason why they were nicknamed flying coffin nails.

 

[Pink]

Arguments about how wings work never end will.

How about this instead:
Can a 747, on a giant treadmill that moves backwards at the same speed that the wheels are rolling forwards, take off successfully?

 

[Altaïr]

The article was annoying me a little bit because it threw all these references to 'aerodynamic experts' that were all puzzled by a low pressure force sitting on top of the curve of the wing and couldn't explain what it was.
And I'm sat here, a layman by comparison, going 'isn't that just drag being generated by the over-wing airflow because it doesn't move in nice straight lines and will swirl about when told to move. Especially at high angles of attack, which is what eventually causes wing stall.
It's what leading edge slots were designed to combat, but what do I know?'.

Ah, that may be the difference between theory and practice. In the article, it is assumed that the air flow is perfectly laminar. Turbulences are not considered.

What happens when the angle of attack is increased too much is a boundary layer separation:

Turbulences appear on the upper side, and you lose the benefit of lower pressure (it also comes with increased drag), hence the wing stall.

That's an actual problem indeed, but the article doesn't push the reflexion that far.

By the way I agree about the fact that the article is annoying. It's long, it repeats itself, sometimes it contradicts itself... It's hard to read.

 

[Altaïr]

Arguments about how wings work never end will.

How about this instead:
Can a 747, on a giant treadmill that moves backwards at the same speed that the wheels are rolling forwards, take off successfully?

If the treadmill compensates the speed of wheels, then the plane is motionless, so it shouldn't be able to take-off... Unless it faces a strong enough wind to make it lift

 

[Pink]

If the treadmill compensates the speed of wheels, then the plane is motionless, so it shouldn't be able to take-off... Unless it faces a strong enough wind to make it lift

Aha, but the plane doesn't care what the wheels are doing if the axles have bearings, no? The engine thrust will push the plane forward all the same.

 

[Horus Lupercal]

What happens when the angle of attack is increased too much is a boundary layer separation:

Yeah, I remember reading somewhere (a looooong time ago) that aircraft propellers have a 15 degree maximum angle of attack before they enter stall state and become drag generators rather than thrust.

That's an actual problem indeed, but the article doesn't push the reflexion that far.

Probably cos it's talking shit ha. It complains about low pressure areas on the top of the wing and how no one knows why, ignores drag and stall completely but then says that the principal of lift requires a low pressure area on top of the wing to work and its formed by air moving faster over than under the wing.

I'm like 'yeah, so you've answered your own problem there with a principal that has been common knowledge for several hundred years'.

Arguments about how wings work never end will.

How about this instead:
Can a 747, on a giant treadmill that moves backwards at the same speed that the wheels are rolling forwards, take off successfully?

The major problem with questions like this is it highlights a severe lack of knowledge of how aircraft work.

For a start, 747 wheels aren't powered.

Second, as Altair put it, it's air that lifts an aircraft, not speed. Put sufficient airflow at a wing and it will generate lift, that principle is independent of the forward ground speed of the aircraft.
It's how Storchs worked. They had incredible wing loading and a very low stall speed and could, in a strong headwind, take off almost vertically.

Aha, but the plane doesn't care what the wheels are doing if the axles have bearings, no? The engine thrust will push the plane forward all the same.

Mythbusters actually did this one. The aircraft took off as normal regardless of which way it was on the conveyer belt

 

[Altaïr]

Aha, but the plane doesn't care what the wheels are doing if the axles have bearings, no? The engine thrust will push the plane forward all the same.

Oh great, I'm puzzled now. I should have suspected a trap...

On the other side if the plane starts moving forward despite the treadmill (which is possible indeed), your first statement:

... a giant treadmill that moves backwards at the same speed that the wheels are rolling forwards...

is no longer verified. The wheels will necesarily go faster than the treadmill if the plane starts moving. In the end the paradox comes from a logical flaw.

 

[Roger Jolly]

The wheels are basically lubrication and the 747 doesn’t care if or how the ground moves it would just rather not drag on it

 

[Pink]

Oh great, I'm puzzled now. I should have suspected a trap...

On the other side if the plane starts moving forward despite the treadmill (which is possible indeed), your first statement:

is no longer verified. The wheels will necesarily go faster than the treadmill if the plane starts moving. In the end the paradox comes from a logical flaw.

That is a flaw with my English!
How about: ... a giant treadmill that moves backwards at the same speed that the plane would roll forwards due to engine thrust...

 

[Roger Jolly]

That is a flaw with my English!
How about: ... a giant treadmill that moves backwards at the same speed that the plane would roll forwards due to engine thrust...

The wheels would roll twice as fast

 

[Horus Lupercal]

On the other side if the plane starts moving forward despite the treadmill (which is possible indeed), your first statement

It will do. The problem with the question is the ridiculous belief that an aircraft is moved by its wheels and not on its wheels. The wheels are its attachment to the ground not the method of propulsion.

That is a flaw with my English!
How about: ... a giant treadmill that moves backwards at the same speed that plane wants to roll forwards...

As soon as you turn the engines on, the aircraft will move forwards, regardless of how fast the conveyor belt is moving as planes move by dragging air from one place to another, not moving ground.
I mean yeah, you could add some resistance from the tyres, bearings etc but that's negligible caused by from mechanical inefficiencies.

You need to stop thinking of aircraft on treadmills in the same way as a runner on a treadmill or a car on a dyno. It doesn't work like that.

 

[Roger Jolly]

If we did have a wheel powered 747 on a treadmill it wouldn’t fly, it would tread

 

[Pink]

You need to stop thinking of aircraft on treadmills in the same way as a runner on a treadmill or a car on a dyno. It doesn't work like that.

I don't, I know that airplane wheels are a good approximation of one of those frictionless things that school physics problems love.
(I'd be a poor mechanical engineer if I thought otherwise)

Great write-up of the problem:
blog.xkcd.com/2008/09/09/the-goddamn-airplane-on-the-goddamn-treadmill/

 

[Roger Jolly]

Can a seaplane land on a waterfall?

 

[Pink]

Can a seaplane land on a waterfall?

Depends on what you mean by "land". Very stationary compared to the falling water? Sure, for a short while.

 

[Horus Lupercal]

I don't, I know that airplane wheels are a good approximation of one of those frictionless things that school physics problems love.
(I'd be a poor mechanical engineer if I thought otherwise)

I wouldn't say they're frictionless and have mentioned things that would create a tiny amount of resistance to wheel roll. But the friction generated is tiny. If you dialed up the engines to enough thrust to overcome that rolling resistance and then started your conveyor belt up, no matter how fast you rolled the belt, you'd not need to apply any more than the most minimal thrust to overcome the additional rolling resistance.

Can a seaplane land on a waterfall?

No. Cos seaplane floats generate an enormous amount of drag on the water and as soon as you throttled down (not withstanding that you'd be in a vertical climb at this point) the water would drag onto the floats and push you backwards.

 

[Pink]

No. Cos seaplane floats generate an enormous amount of drag on the water and as soon as you throttled down (not withstanding that you'd be in a vertical climb at this point) the water would drag onto the floats and push you backwards.

Wrong way up, we're landing downwards!