MERGED: Engineer debunks theory of flight
Man Bilong Balus long PNG
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Firing The Discombobulator Steam punk Rifle
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We knew this...
Aeroplanes fly because their wings cause the air pressure underneath to be greater than that above, lifting them into the air. For years engineers have been frustrated by a theory that wrongly explained the change in pressure.
Rattly.
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Goblin
[/quote]After playing aerofoils with my hand out of the car window all those years as a kid, you can feel lift. It's almost as if you a compressing the molecules and creating a solid which supports more weight the faster you go, or the more you collect.[/quote]
I actually did this as an adult, and still enjoy it as a passenger. One thing I noted and played around with by changing my hand (airfoil shape) was how the hairs on the back of my hand reacted.
Whilst i can't agree with our learned 'professor', that bernoulli's theorem plays no part, I've always been one to believe several different elements of physics were at work.
We can't change the physics, (nature dictates them to us) merely continue our education as to what nature is doing.
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Watching the video demonstration, whilst I subscribe to the downwash theory (action/reaction theory) I was surprised at how little of the smoke actually struck the underside of the wing. If you watch it again, only one of the smoke runs strikes the wing, and there is little interference to the lower smoke streams as compared to the changes above the wing. I'd actually love to see the experiment redone with 2 changes, 1) more and much finer smoke jets. 2) a lower angle of attack.
Bernoulli's theory is only part of the equation. The belief that the air travelling over the top of the wing should arrive at the trailing edge at the same time as the air over the top, is fundamentally wrong in itself if you understand bernoullis theorem. In most discussions, people really only consider the air passing over the top, creating acceleration and lowering of pressure. If you open your mind, and consider what is happening below, and apply that to what you know about bernoulli's.........
over to you guys
I actually did this as an adult, and still enjoy it as a passenger. One thing I noted and played around with by changing my hand (airfoil shape) was how the hairs on the back of my hand reacted.
Whilst i can't agree with our learned 'professor', that bernoulli's theorem plays no part, I've always been one to believe several different elements of physics were at work.
We can't change the physics, (nature dictates them to us) merely continue our education as to what nature is doing.
--------------------------------------------------------------------
Watching the video demonstration, whilst I subscribe to the downwash theory (action/reaction theory) I was surprised at how little of the smoke actually struck the underside of the wing. If you watch it again, only one of the smoke runs strikes the wing, and there is little interference to the lower smoke streams as compared to the changes above the wing. I'd actually love to see the experiment redone with 2 changes, 1) more and much finer smoke jets. 2) a lower angle of attack.
Bernoulli's theory is only part of the equation. The belief that the air travelling over the top of the wing should arrive at the trailing edge at the same time as the air over the top, is fundamentally wrong in itself if you understand bernoullis theorem. In most discussions, people really only consider the air passing over the top, creating acceleration and lowering of pressure. If you open your mind, and consider what is happening below, and apply that to what you know about bernoulli's.........
over to you guys
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I actually did this as an adult, and still enjoy it as a passenger. One thing I noted and played around with by changing my hand (airfoil shape) was how the hairs on the back of my hand reacted.
Whilst i can't agree with our learned 'professor', that bernoulli's theorem plays no part, I've always been one to believe several different elements of physics were at work.
Whilst i can't agree with our learned 'professor', that bernoulli's theorem plays no part, I've always been one to believe several different elements of physics were at work.
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Maybe I didn't understand it fully, but I always found the "air moves faster over the top because it has further to travel" explanation to be odd. Why would air move faster simply because it has "further to travel" (how does it know?). To my mind the curved surface deflects the air "up", the deflected air is pushed away from the wing surface and takes the path of least resistance, which is to continue with the airstream once it detaches from the wing. This reduces the pressure on top of it, causing the pressure differential that creates lift.
That was always my take on it, simple as it is, but I ain't no aerodynamicist.
That was always my take on it, simple as it is, but I ain't no aerodynamicist.
I believe there are many different factors and theories at play with regards to how lift is created.
I have always rationalized this by thinking of how the airfoil moves through the air rather than how the air moves over the airfoil (as demonstrated in wind tunnels). Picture two air molecules stationary in the atmosphere, one sitting on top of the other. All of a sudden, along comes an airfoil and passes between them forcing them to part. The top one is displaced further than the bottom one as it travels up over the "hump" and then drawn back down. The bottom one is only moved downwards. When the airfoil departs the area, they meet up again and are both moving downwards (downwash). The extra VERTICAL distance covered by the upper portion results in greater velocity and therefore, according to Bernouli, reduced pressure.
but I always found the "air moves faster over the top because it has further to travel" explanation to be odd. Why would air move faster simply because it has "further to travel" (how does it know?)
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I have always rationalized this by thinking of how the airfoil moves through the air rather than how the air moves over the airfoil (as demonstrated in wind tunnels). Picture two air molecules stationary in the atmosphere, one sitting on top of the other. All of a sudden, along comes an airfoil and passes between them forcing them to part. The top one is displaced further than the bottom one as it travels up over the "hump" and then drawn back down. The bottom one is only moved downwards. When the airfoil departs the area, they meet up again and are both moving downwards (downwash). The extra VERTICAL distance covered by the upper portion results in greater velocity and therefore, according to Bernouli, reduced pressure.
Personally I believe it's a combination of Bernoulli's theorem and downwash/action-reaction theory. The airflow over the top of the aerofoil increases its velocity due to camber and in doing so its static pressure falls, causing a pressure differential between the upper and lower surfaces. At the same time, as the lower pressure airflow deflects downwards past the separation point at the trailing edge of the wing (downwash), you get the equal and opposite reaction assisting in forcing the wing upwards. Everyone wins
Man Bilong Balus long PNG
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It bemuses me somewhat that even now, after over a century of powered flight, that whilst the general theory of how an aerofoil generates lift;
(Thanks b_sta)
is known, it seemingly cannot be explained as to why is this so?
All I feel that I really need to know is that if I maintain sufficient airspeed the ground will not arise and smite me!
The airflow over the top of the aerofoil increases its velocity due to camber and in doing so its static pressure falls, causing a pressure differential between the upper and lower surfaces.
is known, it seemingly cannot be explained as to why is this so?
All I feel that I really need to know is that if I maintain sufficient airspeed the ground will not arise and smite me!
Moderator
The airfoil myth
http://www.df.uba.ar/users/sgil/phys..._effect_94.pdf
I remember one of my instructors not understanding why I was laughing when he told me that aeroplanes flew "because of Bernouiili's effect" I really thought he was joking. I asked how you could force the mass flow of gasses through half a hole, the other half being infinity. And if this was the case then why did so many aircraft have symmetrical aerofoils and were able to fly upside down?
Beware the smart-arse students with a background in physics. Isaac Newton got it right.
http://www.df.uba.ar/users/sgil/phys..._effect_94.pdf
I remember one of my instructors not understanding why I was laughing when he told me that aeroplanes flew "because of Bernouiili's effect" I really thought he was joking. I asked how you could force the mass flow of gasses through half a hole, the other half being infinity. And if this was the case then why did so many aircraft have symmetrical aerofoils and were able to fly upside down?
Beware the smart-arse students with a background in physics.
Isaac Newton got it right.
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Wally, my old Dad did the same thing, stick yer hand out the window son..... it is amazing that the roads were not littered with kids hands as trucks went passed?
Whats even more scarey is my old man was a TV repairman too, geez you might be my half brother Wally.
Seriously though, when instructing, I had many students look at me with some doubt when trying to explain this doubtful principal.
One of my wise instructor collegues, John Hewit desribed his theory that it was a bit each way, you could put a piece of flat Masonite out the window of a moving vehicle and get signifcant lift but a curved would just make it more efficient. Low powered aircraft of the 20's and 30's needed lots of curve to produce lift whereas fast aircraft with large amounts of thrust could get away with thin symetrical wing sections?
"As long as they stay in the air" is the most imortant principal.
Whats even more scarey is my old man was a TV repairman too, geez you might be my half brother Wally.
Seriously though, when instructing, I had many students look at me with some doubt when trying to explain this doubtful principal.
One of my wise instructor collegues, John Hewit desribed his theory that it was a bit each way, you could put a piece of flat Masonite out the window of a moving vehicle and get signifcant lift but a curved would just make it more efficient. Low powered aircraft of the 20's and 30's needed lots of curve to produce lift whereas fast aircraft with large amounts of thrust could get away with thin symetrical wing sections?
"As long as they stay in the air" is the most imortant principal.
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Even this "debunking" seems wrong...
Newtons 3rd law is what gives lift - high kinetic-energy air particles hitting the bottom of the wing, and pushing up. Simple as that. The pressure differential is a byproduct of this, but nowhere near strong enough to "suck" the aircraft up against gravity.
Check this out:
Newtons 3rd law is what gives lift - high kinetic-energy air particles hitting the bottom of the wing, and pushing up. Simple as that. The pressure differential is a byproduct of this, but nowhere near strong enough to "suck" the aircraft up against gravity.
Check this out:
It really seems as if a mix of the theories is probably closer to the reality than any one alone.. I love it how there is plenty of experience talking about this, even here on pprune and there is no definitive answer.
It seems that although they certainly fly, symmetric aerfoils are much more unstable, and therefore useful for aeros and high speed ac flown on AoA , but the fat old planks used for trainers and the like seem to give much more stability due to the more stubborn, yet benign movement of the centre of pressure.
I've always been troubled by bernoullis theorem on its own, but have no physics background to argue with myself properly about it!!
It seems that although they certainly fly, symmetric aerfoils are much more unstable, and therefore useful for aeros and high speed ac flown on AoA , but the fat old planks used for trainers and the like seem to give much more stability due to the more stubborn, yet benign movement of the centre of pressure.
I've always been troubled by bernoullis theorem on its own, but have no physics background to argue with myself properly about it!!
Last edited by Homesick-Angel; 30th Jan 2012 at 13:12.
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Happily I have been teaching my grandaughter the Avgas therom, ie: pull stick back plane goes up. Later (she is only 2 1/2) when I feel she is ready for it, I will add the rest ie: pull stick back further plane comes down again. 
Last edited by Avgas172; 30th Jan 2012 at 16:19. Reason: early morning typo
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"It seems the good professor has simply managed to demonstrate that he agrees with NASA. This is hardly news.
"
One of the things I have to do for my exam aerodynamics next friday is go to the NASA website and read the stuff about lift and drag and tell the professor why the way NASA explains it and writes it down is wrong.
"
One of the things I have to do for my exam aerodynamics next friday is go to the NASA website and read the stuff about lift and drag and tell the professor why the way NASA explains it and writes it down is wrong.
I recall a classic quote from one of the many previous occasions that this subject has come up. I wish I could claim credit for it.
Bernoulli "This apple is round"
Newton "No this apple is green"
Bernoulli "This apple is round"
Newton "No this apple is green"
