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eyeinthesky
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PostPosted: Fri Apr 21, 2017 2:07 am    Post subject: A theory of how lift is generated by a wing is debunked. Reply with quote #1   
Never quite made sense to me.

http://www.telegraph.co.uk/news/science/science-news/9035708/Cambridge-scientist-debunks-flying-myth.html
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waveview
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PostPosted: Fri Apr 21, 2017 3:33 am    Post subject: Reply with quote #2   
This is the video refered to in the link above:

Link

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peanuts
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PostPosted: Fri Apr 21, 2017 9:48 am    Post subject: Reply with quote #3   
suggested reading: STICK AND RUDDER
by WOLFGANG LANGEWEIS


there's a reason that they're called aero PLANES
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ksglider
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PostPosted: Sat Apr 22, 2017 5:20 am    Post subject: Reply with quote #4   
Hi All;

Here is a very interesting video. It's a little long but well worth watching. If one was ever to listen to anyone about this subject, then this man is the one.

https://youtu.be/QKCK4lJLQHU

Doug
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RobertKesselring
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PostPosted: Sat Apr 22, 2017 6:47 am    Post subject: Reply with quote #5   
ksglider wrote:
Here is a very interesting video. It's a little long but well worth watching.

Ditto
I saw this a few months ago. When I saw this thread, I thought I should try to find it again and post it, but hadn't gotten around to it yet. Thanks ksglider for posting it.

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dbotos
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PostPosted: Sat Apr 22, 2017 6:58 am    Post subject: Reply with quote #6   
Here's a short related video I found interesting:


Link
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hgldr



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PostPosted: Sat Apr 22, 2017 11:25 am    Post subject: Reply with quote #7   
Doug,
That's a good video from Boeing's Doug McClean. I left a note on the youtube comments reflecting the miscperception which we always hear, that speed causes low pressure for some unknown mysterious reason. Simple fact which McClean points out is that differential pressure on opposing sides of each infinitesimally small parcel of air (or of ANYthing for that matter) cause acceleration, and the integral of acceleration is velocity. This is actually what Bernoulli originally wrote but, repeated incorrectly in many books is that, "speed causes pressure".
- Darrell
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adyr
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PostPosted: Sun Apr 23, 2017 3:48 am    Post subject: Reply with quote #8   
Quote:

speed causes low pressure for some unknown mysterious reason

It's not actually 'unknown', it would be the conservation of energy.

Letting aside the gravitational potential energy which for a wing that's quite small and almost horizontally oriented so the differences in height are small enough to be quite safely ignored, you have the (macroscopic) kinetic energy of the gas (/volume) and the pressure, which is potential energy/volume (it translates to microscopic kinetic energy of the molecules but you can ignore that for now).

To conserve the total energy, if one goes down, the other must go up (this has some simplifying assumptions built in, as 'incompressible' fluid flow).
The reason is not so mysterious.

A description of fluid flow by words tends to have some mistake in it, because one tries to explain a complex phenomenon by some simple means. A more correct answer just gives out the Navier-Stokes equations allowing people to stare at them helplessly Smile

And not even those are entirely correct Razz
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waveview
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PostPosted: Sun Apr 23, 2017 4:54 am    Post subject: Reply with quote #9   
It's sounds like a bit of a mistery about what keeps us all up in the air? These hang gliders are even more improbable after listening to the physics involved. Cool
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RobertKesselring
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PostPosted: Sun Apr 23, 2017 4:55 am    Post subject: Reply with quote #10   
Conservation of energy tells you that it must happen but doesn't do a very good job of telling you why it happens (in my opinion).

The most intuitive explanation I've heard goes something like this...

Start with the following...
Definition: Acceleration is any change in speed OR direction of motion of a mass. A curved path is the result of acceleration toward the inside if the curve.
Premise 1: A force is required to cause a mass to accelerate.
Premise 2: Forces come in equal and opposite pairs.

A wing is curved. Air mass flowing under and over the wing follows the same curves. Since the air is flowing a curved path, it is accelerating toward the inside of the curve, and so must be experiencing a force in that direction. That force is matched by an equal and opposite force on the wing.

dbotos's video is a great graphical version of this same explanation.

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SeeMarkFly
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PostPosted: Sun Apr 23, 2017 6:43 am    Post subject: Theory of lift Reply with quote #11   
How about if you suck the air off the top of the wing?



Link

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adyr
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PostPosted: Sun Apr 23, 2017 1:23 pm    Post subject: Reply with quote #12   
Quote:

why it happens

Physics does not answer to the question 'why', it only gives description of 'how'. The 'why' goes to philosophy or religion Smile

Quote:

A wing is curved.

Not necessarily. A flat, plane wing can fly just fine. It's not as good as a profiled one, but still it can fly.

To understand how things go one has to think in terms of the particles involved, about the collisions among them and with the wing...
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TjW
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PostPosted: Sun Apr 23, 2017 9:47 pm    Post subject: Reply with quote #13   
adyr wrote:
Quote:

why it happens

Physics does not answer to the question 'why', it only gives description of 'how'. The 'why' goes to philosophy or religion Smile

Quote:

A wing is curved.

Not necessarily. A flat, plane wing can fly just fine. It's not as good as a profiled one, but still it can fly.

To understand how things go one has to think in terms of the particles involved, about the collisions among them and with the wing...


Worse, a wing that does have camber can still fly when it's upside down.
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CHassan
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PostPosted: Mon Apr 24, 2017 2:18 am    Post subject: Reply with quote #14   
waveview wrote:
It's sounds like a bit of a mistery about what keeps us all up in the air? These hang gliders are even more improbable after listening to the physics involved. Cool


Hang gliders don't fly at all. They always fall.

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RobertKesselring
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PostPosted: Mon Apr 24, 2017 2:38 am    Post subject: Reply with quote #15   
adyr wrote:
A flat, plane wing can fly just fine. It's not as good as a profiled one, but still it can fly.

TjW wrote:
Worse, a wing that does have camber can still fly when it's upside down.

Yes. You can get lift with almost any airfoil if you set the AoA such that the airflow gets curved downward. Using flat or inverted airfoils will incur more drag, but as long as you can maintain airspeed against the increased drag, it will still fly. The important thing is getting that downward curvature in the airflow.

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Nigel Hewitt
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PostPosted: Mon Apr 24, 2017 3:59 am    Post subject: Reply with quote #16   
I always worry that the wind tunnel image, although useful gives a wrong impression. The air isn't flowing over the wing, it was just sitting there, minding its own business when this wing rushed past and threw it about. Yes, we know that on short scales there is no difference between who is moving but lift and drag make more 'sense' when you look at what you leave behind. Air has been accelerated forwards, 'dragged' along by the wing and accelerated downwards 'lifting' the aerofoil. It will all settle down in time and get back to what ever it was doing before but we have extracted some mass times acceleration style forces from it as we passed.

The whole 'how far does the air travel?' question is a fallacy that comes from this image. The air isn't flowing over the wing. It is being pushed out of the way and accelerated. The air that goes over the wing and the air that goes under it were once adjacent but have now been pushed apart and have no interest in getting back together. The air going over the wing is not 'going faster', the air under the wing is just being dragged along more and accelerated.

I'm not sure what is a good image to help understand it. We humans tend to apply our simple world ideas to complicated things like this and even as a time served physicist I catch myself doing it at times. Looking at the pressure tells you where the lift comes from and the pressure tells the air how to move.

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ksglider
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PostPosted: Mon Apr 24, 2017 5:02 am    Post subject: Reply with quote #17   
Nigel;

Nigel Hewitt wrote:
I always worry that the wind tunnel image, although useful gives a wrong impression. The air isn't flowing over the wing, it was just sitting there, minding its own business when this wing rushed past and threw it about. Yes, we know that on short scales there is no difference between who is moving but lift and drag make more 'sense' when you look at what you leave behind. Air has been accelerated forwards, 'dragged' along by the wing and accelerated downwards 'lifting' the aerofoil. It will all settle down in time and get back to what ever it was doing before but we have extracted some mass times acceleration style forces from it as we passed.

The whole 'how far does the air travel?' question is a fallacy that comes from this image. The air isn't flowing over the wing. It is being pushed out of the way and accelerated. The air that goes over the wing and the air that goes under it were once adjacent but have now been pushed apart and have no interest in getting back together. The air going over the wing is not 'going faster', the air under the wing is just being dragged along more and accelerated.

I'm not sure what is a good image to help understand it. We humans tend to apply our simple world ideas to complicated things like this and even as a time served physicist I catch myself doing it at times. Looking at the pressure tells you where the lift comes from and the pressure tells the air how to move.


I like your way of looking at this.

I especially like your last phrase. It's kind of like the really simple explanation of The General Theory of Relativity, "Mass tells space how to 'bend'; 'bent' space tells mass how to move".

Fun stuff to talk about . . .

Doug
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ksglider
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PostPosted: Mon Apr 24, 2017 5:11 am    Post subject: Re: Theory of lift Reply with quote #18   
SeeMarkFly;

SeeMarkFly wrote:
How about if you suck the air off the top of the wing?


Link


Look up the topic of the Custer Channel Wing. Very interesting concepts from the 1950's. As I remember, there was a BIG increase in lift at very low speeds, but LARGE drag at cruise speeds.

Again, fun stuff to talk about . . .

Doug
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aeroexperiments
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PostPosted: Mon Apr 24, 2017 7:56 am    Post subject: momentum? Reply with quote #19   
For an exhaustive on-line discussion questioning the idea that the production of lift inevitably involves giving some momentum to the surrounding airmass, see this topic on a model-airplane forum http://www.rcgroups.com/forums/showthread.php?2805910-Momentum-Survey and other related threads in the "Modeling Science" area.

I haven't followed the whole discussion very closely, but I've noticed that "ShoeDLG"'s posts are usually enlightening.

Steve

Nigel Hewitt wrote:
I always worry that the wind tunnel image, although useful gives a wrong impression. The air isn't flowing over the wing, it was just sitting there, minding its own business when this wing rushed past and threw it about. Yes, we know that on short scales there is no difference between who is moving but lift and drag make more 'sense' when you look at what you leave behind. Air has been accelerated forwards, 'dragged' along by the wing and accelerated downwards 'lifting' the aerofoil. It will all settle down in time and get back to what ever it was doing before but we have extracted some mass times acceleration style forces from it as we passed.
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SeeMarkFly
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PostPosted: Sat Apr 29, 2017 8:52 am    Post subject: Re: Theory of lift Reply with quote #20   
ksglider wrote:
Custer Channel Wing. Very interesting concepts from the 1950's. As I remember, there was a BIG increase in lift at very low speeds, but LARGE drag at cruise speeds.

The Fanwing promised STOL freight carried short distances (no high speed flight needed) but the amount of space needed to "safely" land is still way more land-use that could be used to park trucks (that could drive there in less time it was needed to load a plane, taxi out, takeoff, land, unload, and refuel).
http://www.fanwing.com/

Limitations:

The throttle directly affects the pitch. This means increased throttle can decelerate the aircraft.

Glide-ratio in case of power-failure is low (about 1:3) but if the rotors are allowed to auto-rotate, it can still glide.

Previous attempts using the Coanda/Magnus effect failed because the rotating parts caused gyroscopic effects which impaired maneuverability.



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