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By Supercat
#1901566
I may be about to announce to the world that I'm thick, but here goes :D
When I was a kid in the back of the car I used to hold my hand out the window and see that when I lifted my hand at the wrist my hand shot up in the wind and the same in reverse.
So this is how I imagined aircraft flew.
Then I read all the books to get my PPL and was taught that that wasn't it, Bernoulli's theory was in use here and the lower pressure above the wing caused it to get sucked up, not pushed up. This didn't feel right but I accepted it.
Now I'm reading 'Stick and Rudder' by Wolfgang Langewiesche which was recommended to read and he explains it the way I used to imagine it.
My question is:
How do you all deep down feel that it's happening?

They can't both be in action, one is and one isn't surely?

(I also appreciate that it kinda doesn't matter either way as long as you understand some principle of how a wing flies then you are good.)
By TopCat
FLYER Club Member  FLYER Club Member
#1901567
The Bernoulli effect is a thing, but the upwards force from the reduced pressure over the wing isn't anywhere near enough to provide the lift required.

The wing pushes down on the air and deflects it downwards. The reaction from the air pushing back on the wing provides the lift.

The kid in the back of the car was right.
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By TopCat
FLYER Club Member  FLYER Club Member
#1901568
... mind you, 'feeling deep down' isn't very often a good way of knowing.

I look out of the window when I fly, and deep down, I feel the earth is flat. But I know it isn't :)
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By T67M
FLYER Club Member  FLYER Club Member
#1901577
In a way, both are correct - and both are wrong. They are both simplifications of the way a wing truly works which enable simple calculations to get to approximately the correct answer without doing incredibly complex calculations to get to exactly the correct answer, and both have limitations beyond which they give the wrong answer. Another simplification is that the lift from a wing is produced by the circulating airflow around the wing (see http://www.onemetre.net/design/downwash/Circul/Circul.htm), however this is also just a more complicated simplification of a very, very complicated phenomenon.

As a pilot, you only need to know approximately the correct answer - basically just enough to understand how the wing will stall and how to recover a wing which has stalled. An aircraft engineer or designer will know a little bit more, but for most things, they can still use the Bernoulli and Newtonian simplifications. Even the designers of the Concorde or Blackbird wings which exploit vortex lift would have mostly used these simplifications and approximations. There is a reason that many good aerodynamicists spend and entire lifetime studying the subject and have post-doctoral degrees!
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By Supercat
#1901578
Thanks for the reply TopCat, that's what I had been thinking, I do know Bernoulli was correct as carbs wouldn't work without the venturi, I just felt like it wasn't going to be enough on it's own and the book I read for my exams kinda poo-poo'd all else.
Not knowing any other pilot I just thought I'd ask here as my Mrs get fed up of me positing theories while Tipping Point is on :roll: :wink:
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By T67M
FLYER Club Member  FLYER Club Member
#1901725
T67M wrote:There is a reason that many good aerodynamicists spend and entire lifetime studying the subject and have post-doctoral degrees!


I accidentally forgot to mention the Lift Demon and Thrust Pixie simplification which, for many people, is actually the most accurate model of how a wing flies. :wink:
By rdfb
#1901727
If you want to be really pedantic about it, low pressure cannot suck. Just like cold is the absence of heat, low pressure is the absence of higher pressure. So I can't speak for the aerodynamics side of it, but if you take the pedantry through to basic physics, there is no sucking going on.
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By Mz Hedy
#1901741
Yes but... what the wing reacts to is the difference between the pressure above the wing and below it.

My vacuum cleaner reduces the pressure in the hose to below that in the room with the resultant effect that it sucks up the dust. I suppose I could arrange somehow to increase the pressure beneath the carpet in order to blow the dust into the vacuum cleaner but I suspect it would be less effective.
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By lobstaboy
#1901753
'Stick and Rudder' explains it in a practical way that works for pilots. That's all we need.
In fact Langeweische goes on at some length to emphasise that there is lots of complex theory that the engineers and designers know but that is irrelevant to the skill needed to fly the ship (as he would say).
The wing pushes the air down and the reaction to this is the lift we need.
(If it was solely Bernoulli then how do aeroplanes fly upside down?)
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By Charles Hunt
#1901757
Way back when, if lift is only produced by the aerofoil section, then why did control line stunt or 'combat wing' aircraft have symmetric profiles allowing them to fly both right way up and inverted, I puzzled.

Back then no-one I found spoke about angle of attack, stick your hand out of the car window, angle the leading edge up, and up goes the hand.

To my simplistic mind with a low angle of attack the speeding wing passing through stationary air is pushed upward. The aerofoil camber is a separate effect that reduces the pressure above so the wing is sucked from above or pushed up from below depending on how you want to think about it.
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By Trent772
#1901769
rdfb wrote:If you want to be really pedantic about it, low pressure cannot suck. Just like cold is the absence of heat, low pressure is the absence of higher pressure. So I can't speak for the aerodynamics side of it, but if you take the pedantry through to basic physics, there is no sucking going on.



Married 37 years.... Agreed
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By T6Harvard
FLYER Club Member  FLYER Club Member
#1901792
T67M wrote:
T67M wrote:There is a reason that many good aerodynamicists spend and entire lifetime studying the subject and have post-doctoral degrees!


I accidentally forgot to mention the Lift Demon and Thrust Pixie simplification which, for many people, is actually the most accurate model of how a wing flies. :wink:


Brilliant link! Thanks, I really enjoyed it and have shared.
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By Rob L
#1901912
rdfb wrote:If you want to be really pedantic about it, low pressure cannot suck. Just like cold is the absence of heat, low pressure is the absence of higher pressure. So I can't speak for the aerodynamics side of it, but if you take the pedantry through to basic physics, there is no sucking going on.


In a similar vein, a Stirling Engine is one that works on differential heat. I have one that can run on the heat from the palm of my hand.

Showing folk that I can run that same engine using an ice cube on top usually makes them think!

Physics & thermodynamics :D
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