Why Do Aircraft fy? Flat Plate Lift Vs Bernoulli?
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Why Do Aircraft fy? Flat Plate Lift Vs Bernoulli?
Right, a little shamed to admit this, having done a 3 year aerospace engineering degree. I have tried to figure this out for a long while and am no closer to resolving my brain-fart.
At university, we were taught about Bernoulli and lift generated due to difference in pressures. We were taught about circulation, how lift is proportional to the amount of circulation, about starting vortexes. About without viscosity, we couldnt fly. Reynolds Numbers.
we were also taught however in one lecture (I clearly remember it because what he said confused the hell out of me) that lift cannot be completely be resolved purely as a result of faster flow over the top of the wing. It was mentioned that flat plate lift played a major part as well. I think he was talking hoop, although I now have a doubt in my mind.
Flat plate lift cannot explain symmetrical aerofoil sections at 0 degrees AOA. surely, no lift = aircraft falls out of the sky according to flat plate lift? Yet, I can understand that an aircraft MUST generate lift as well as a result of diverted flow at the trailing edge (for every action reaction blah) .
Which hypothesis is correct?
1) Lift purely due to bernoulli?
2) Lift purely to flat plate?
3) Lift a result of both, which both need to be taken into account exclusively?
4) the sneakiest answer (the one I think may be right) that they are the same thing through some form of coupling, in other words, bernoulli can account for downwash at the trailing edge and flat plate lift can go some way to explain I used to hate aerodynamics, as I felt it was the one topic that was the least well understood by the teaching staff.
The only way I can account for a flat plate producing lift (say paper aeroplane) is that at release, for arguments sake released at 0 degrees AOA, the aircraft falls as there is no lift supporting the weight of the aircraft. This downward velocity, in combination with forward velocity, has thus generated an AOA and thus the flat plate wing of the paper aeroplane now generates lift. Or is this paragraph complete guff?
Its only been 7 years since I finished my degree, think I should resolve this one once and for all. Over to you all!
At university, we were taught about Bernoulli and lift generated due to difference in pressures. We were taught about circulation, how lift is proportional to the amount of circulation, about starting vortexes. About without viscosity, we couldnt fly. Reynolds Numbers.
we were also taught however in one lecture (I clearly remember it because what he said confused the hell out of me) that lift cannot be completely be resolved purely as a result of faster flow over the top of the wing. It was mentioned that flat plate lift played a major part as well. I think he was talking hoop, although I now have a doubt in my mind.
Flat plate lift cannot explain symmetrical aerofoil sections at 0 degrees AOA. surely, no lift = aircraft falls out of the sky according to flat plate lift? Yet, I can understand that an aircraft MUST generate lift as well as a result of diverted flow at the trailing edge (for every action reaction blah) .
Which hypothesis is correct?
1) Lift purely due to bernoulli?
2) Lift purely to flat plate?
3) Lift a result of both, which both need to be taken into account exclusively?
4) the sneakiest answer (the one I think may be right) that they are the same thing through some form of coupling, in other words, bernoulli can account for downwash at the trailing edge and flat plate lift can go some way to explain I used to hate aerodynamics, as I felt it was the one topic that was the least well understood by the teaching staff.
The only way I can account for a flat plate producing lift (say paper aeroplane) is that at release, for arguments sake released at 0 degrees AOA, the aircraft falls as there is no lift supporting the weight of the aircraft. This downward velocity, in combination with forward velocity, has thus generated an AOA and thus the flat plate wing of the paper aeroplane now generates lift. Or is this paragraph complete guff?
Its only been 7 years since I finished my degree, think I should resolve this one once and for all. Over to you all!
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The Tailplane of an aircraft is an inverted aerofoil yet still produces 'lift' but in a downwards direction. I doubt the aircraft would fly with any semblance of control if it were simply a flat plate!
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In their book Aircraft Flight: A Description of the Physical Properties of Aircraft Flight R.H. Barnard and D.R. Philpott dismiss the Bernoulli theory as a simple way of describing lift production which has unforunately stuck.
I think they also state this in the introduction to recent updates of A.C. Kermode's Mechanics of Flight.
I think they also state this in the introduction to recent updates of A.C. Kermode's Mechanics of Flight.
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I think this thread should be renamed "Why do airfraft fly? Bernoulli or Newton"
Bernoulli theory sure does sound like the most exciting theory to tell someone when they ask why an aircraft flys. Its the one theory that you was drilled into you at your flying school because thats what the instructor read in a book. But i dont believe it entirely.
My paper aeroplanes fly, and they dont have any camber.
High speed aircraft fly and they have minimal/no camber to increase Mcrit and delay the onset of drag
Newtons law explains why a wing will fly. "for every action there is an equal and opposite reaction" Deflect the relative air down and you will go up. Increasing AOA, will Increase the lift produced. Simple
And rubik101 you say:
"The Tailplane of an aircraft is an inverted aerofoil yet still produces 'lift' but in a downwards direction"
Not entirely true. The tailplane produces upward OR downward lift to balance the aircraft, which is dependant on the CofG position
Bernoulli theory sure does sound like the most exciting theory to tell someone when they ask why an aircraft flys. Its the one theory that you was drilled into you at your flying school because thats what the instructor read in a book. But i dont believe it entirely.
My paper aeroplanes fly, and they dont have any camber.
High speed aircraft fly and they have minimal/no camber to increase Mcrit and delay the onset of drag
Newtons law explains why a wing will fly. "for every action there is an equal and opposite reaction" Deflect the relative air down and you will go up. Increasing AOA, will Increase the lift produced. Simple
And rubik101 you say:
"The Tailplane of an aircraft is an inverted aerofoil yet still produces 'lift' but in a downwards direction"
Not entirely true. The tailplane produces upward OR downward lift to balance the aircraft, which is dependant on the CofG position
Try Mike Griffith at Oxford Aviation's ground school.
There have been many debates on this subject in Prune over the years , as a search will show. I only wish my late mate Chris Braund had got his flat plate Do28 into the air. Chris also claimed to have invented winglets when he flew a Seafire with the tips turned up, and achieved a 10 knot reduction in Vs.
Look for Oxford at http://ask.oxfordaviation.net/viewforum and make a post there.
There have been many debates on this subject in Prune over the years , as a search will show. I only wish my late mate Chris Braund had got his flat plate Do28 into the air. Chris also claimed to have invented winglets when he flew a Seafire with the tips turned up, and achieved a 10 knot reduction in Vs.
Look for Oxford at http://ask.oxfordaviation.net/viewforum and make a post there.
Aircraft fly because they all accelerate a portion of the air that flows past them downwards, causing a reaction force upwards (at least that's what I've been taught by my aerodynamics teacher).
How this acceleration is achieved is where Bernoulli, Newton and all other theories come into play. I personally think it's a combination of different causes and effects, since one single theory does not cover every aspect.
How this acceleration is achieved is where Bernoulli, Newton and all other theories come into play. I personally think it's a combination of different causes and effects, since one single theory does not cover every aspect.
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I remember the BAe ATP flew only because the engines generated more vibration which forced the air molecules apart above the wing, hence the low pressure and up she went...slowly.
On landing, you closed the throttles and the reduction in vibration caused you to fall out of the sky, hence the landings......
I would have thought three years of aeronautical degree studies would have got you to the point where you could teach others exactly why.
The answer IS Viscosity. Without that, you have a pointy land vehicle with wings.
A flat plate will fly because the viscosity allows it to generate asymetric airflow, the fundemental requirement to allow Mr Benoulli to do his bit.
The viscosity is the property that allows the air to seperate at the trailing edge and not come back up and join the other stream in the opposite place to the separation point at the front. I think....
How the air gets up and around the front of the flat plate is the bit that I can't figure out......we didn't get that far in Principles of Flight lectures, though Mr Alan Smith did correct a few errors in Barnard & Philpott.
It makes my head hurt, but I got 100% and came out early so it must have been good stuff.
On landing, you closed the throttles and the reduction in vibration caused you to fall out of the sky, hence the landings......
I would have thought three years of aeronautical degree studies would have got you to the point where you could teach others exactly why.
The answer IS Viscosity. Without that, you have a pointy land vehicle with wings.
A flat plate will fly because the viscosity allows it to generate asymetric airflow, the fundemental requirement to allow Mr Benoulli to do his bit.
The viscosity is the property that allows the air to seperate at the trailing edge and not come back up and join the other stream in the opposite place to the separation point at the front. I think....
How the air gets up and around the front of the flat plate is the bit that I can't figure out......we didn't get that far in Principles of Flight lectures, though Mr Alan Smith did correct a few errors in Barnard & Philpott.
It makes my head hurt, but I got 100% and came out early so it must have been good stuff.
Having spent 3 years doing an Aerospace course in our final week we were told that all you really needed was a barn door and enough thrust.
I think it's year 4 where you learn about the lift pixies
I think it's year 4 where you learn about the lift pixies
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There was a great (but long) thread almost 3 years ago:
http://www.pprune.org/tech-log/20728...lers-pull.html
The short answer is: Bernoulli and downwash are intrinsic and inseparable in defining and measuring lift. The downwash is most noticeable downstream of a propeller or helo rotor, but it's there behind a fixedwing bird as well.
http://www.pprune.org/tech-log/20728...lers-pull.html
The short answer is: Bernoulli and downwash are intrinsic and inseparable in defining and measuring lift. The downwash is most noticeable downstream of a propeller or helo rotor, but it's there behind a fixedwing bird as well.
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VinRouge
NASA have an interesting take on your discussion at the following if you have not already read it:
What is Lift?
Naturally there are some that dispute their theories, but wont there always be.
Secondly, have you read Aerodyamics for Naval Aviators? I have always thought that it was one of the definitive books on aerodynamics.
(ISBN-10: 156027140X)
Good luck
NASA have an interesting take on your discussion at the following if you have not already read it:
What is Lift?
Naturally there are some that dispute their theories, but wont there always be.
Secondly, have you read Aerodyamics for Naval Aviators? I have always thought that it was one of the definitive books on aerodynamics.
(ISBN-10: 156027140X)
Good luck
Last edited by Che Guevara; 22nd Dec 2008 at 17:20.
Redwine
Q. Which hypothesis is correct? –
Ans. 3) Lift a result of both.
Sticking with Subsonic airflow conditions and not wanting to go into too much detail - which I’ve no doubt you fully understand - a standard high lift camber wing section (Göttingen 387) produces about 70% of its lift on the upper surface and around 30% from the lower surface, so although both sides contribute to total lift, it could be argued that the wing is mainly sucked into the (sky) lower pressure air above the wing.
Upper Surface – Bernoulli’s Principle
Due to the camber - as you point out - the upper air flow has a greater distance to travel and in order for the airstream to meet up again at the trailing edge, the upper airflow must travel faster resulting in a drop in pressure as there are fewer particles of air in any given volume verses the lower air flow - think stretching an elastic band.
Lower Surface – Flat Plate Effect
When mounted onto the airframe, the rigging angle of the wing is set at around 6 degrees relative to the longitudinal axis. The effect of this is that the airflow (relative to the wing angle of attack in straight and level flight) strikes the lower surface and is then deflected downwards - dynamic momentum transfer. Newton's third law results in a force in an upwards direction adding to the vacuum created on the upper surface and more lift is produced.
Q. Which hypothesis is correct? –
Ans. 3) Lift a result of both.
Sticking with Subsonic airflow conditions and not wanting to go into too much detail - which I’ve no doubt you fully understand - a standard high lift camber wing section (Göttingen 387) produces about 70% of its lift on the upper surface and around 30% from the lower surface, so although both sides contribute to total lift, it could be argued that the wing is mainly sucked into the (sky) lower pressure air above the wing.
Upper Surface – Bernoulli’s Principle
Due to the camber - as you point out - the upper air flow has a greater distance to travel and in order for the airstream to meet up again at the trailing edge, the upper airflow must travel faster resulting in a drop in pressure as there are fewer particles of air in any given volume verses the lower air flow - think stretching an elastic band.
Lower Surface – Flat Plate Effect
When mounted onto the airframe, the rigging angle of the wing is set at around 6 degrees relative to the longitudinal axis. The effect of this is that the airflow (relative to the wing angle of attack in straight and level flight) strikes the lower surface and is then deflected downwards - dynamic momentum transfer. Newton's third law results in a force in an upwards direction adding to the vacuum created on the upper surface and more lift is produced.
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I am not a AE but I have found Dr. Denker's paper on the physics of flying interesting. Gave me a 'better' understanding of the concept of Lift....
Airfoils and Airflow [Ch. 3 of See How It Flies]
Enjoy!
Airfoils and Airflow [Ch. 3 of See How It Flies]
Enjoy!
it could be argued that the wing is mainly sucked into the (sky) lower pressure air above the wing.
Is sucking a force that pulls or pushes?
Does a vacuum cleaner suck air? Is the air being pulled in to the end of the hose, or is it being pushed?
Air pressure on a surface can only PUSH on that surface, it can not pull!
A complete vacuum is simply a complete absence of air molecules and therefore a complete absence of this pushing force. A vacuum is not an existence of any pulling force.
A reduction in pressure can only mean a reduction in this pushing force, BUT IT IS STILL PUSHING ON THE SURFACE. It can not pull on that surface.
So when you say the pressure above the wing is sucking the wing in to the sky, the forces on the upper surface of the wing is not contributing to the total lift force by pulling the upper surface up, it is simply reducing the force pushing the upper surface down. But the force on the upper wing is still in the downward direction (perpendicular to the surface)! It's just that it is no longer equal to the opposing pressures being exerted on the underside of the wing.
So the way I see it, when a wing is suspended in still air, the air pressure pushing up on the underside of the wing is countered by the air pressure on the upper side pushing down. The forces cancel each other out and the result is zero force (no lift).
When the air is flowing past the wing (with a positive angle of attack) and colliding with hitting it's underside, compressing together slightly and so the pressure exerted by the air molecules on that side of the wing increases slightly.
On the upper side the air molecules are being pulled apart from each other as the the momentum of each molecule does not allow them to keep up with the upper surface of the wing's trailing surface as it slopes downward and away from the air molecules.
As result of this greater distance between molecules, a vacuum is created, ie the downward force on the upper surface of the wing is less than it would be if the wing was stationary. This reduction in downward pressure is not enough to counter the pressure on the underside of the wing. The resultant force is up.
Anyway that is the way I see lift being created by a wing, or a flat plate.
And to answer the question posed above, air is pushed in to the end of a vacuum cleaner hose not pulled!
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It's quite interesting to look at graphs (polars) of lift vs Angle of attack. Many produce positive lift even at negative AOA..
For NACA 4412 Cl peaks at about 1.4 but notice that CL is still positive right down to an AOA of MINUS 4 degrees. At an AOA of zero CL is just under 0.5 so Bernoulli is clearly contributing a significant percentage.
AOA = 0
AOA = -4
How do I... Read Polar Diagrams?
For NACA 4412 Cl peaks at about 1.4 but notice that CL is still positive right down to an AOA of MINUS 4 degrees. At an AOA of zero CL is just under 0.5 so Bernoulli is clearly contributing a significant percentage.
AOA = 0
AOA = -4
How do I... Read Polar Diagrams?
Last edited by cwatters; 22nd Dec 2008 at 19:39.
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Looking at that pic though, the flow leaving the top surface (if still attached) has a definite downward vector, thus surely contributing to lift due to Newton?
Thanks for everyones' responses so far, as well as the reminder of that other fantastic thread with fantastic links I couldnt Find.
Think I am going to stick with the pixie dust explanation.
Thanks for everyones' responses so far, as well as the reminder of that other fantastic thread with fantastic links I couldnt Find.
Think I am going to stick with the pixie dust explanation.
Blip
Oxford English Dictionary
Suck
• verb 1 draw into the mouth by contracting the lip muscles to make a partial vacuum. 2 hold (something) in the mouth and draw at it by contracting the lip and cheek muscles. 3 draw in a specified direction by creating a vacuum. 4 (suck in/into) involve (someone) in something without their choosing. 5 (suck up to) informal attempt to gain advantage by behaving obsequiously towards. 6 N. Amer. informal be very bad or disagreeable.
The above Oxford English Dictionary does not suggest that Sucking is to push in a specific direction, but to draw. If I refer to a thesaurus I could look under synonyms for DRAW and come across Pull and Suck.
Force. Means anything that pushes and pulls.
Isaac Newton did not note that the apple was pushed to the ground, but pulled. Your argument is that there are only pushing forces and not pulling (Sucking/Drawing) forces.
The only reason that ambient air pressure pushes air up a vacuum hose is that the vacuum (produced within the cleaner) allows ambient air to overcome that caused by the reduction in air pressure within the vacuum cleaner.
With or without a camber on the upper surface of a wing, the pushing force on the underside of a wing is not increased as a result of the lowering of static pressure on the upper surface, but the static force acting downwards on the upper surface of the wing is reduced as a result of reducing static pressure, thereby allowing the wing to be drawn (sucked/pulled/whatever) into the reduced pressure zone.
Oxford English Dictionary
Suck
• verb 1 draw into the mouth by contracting the lip muscles to make a partial vacuum. 2 hold (something) in the mouth and draw at it by contracting the lip and cheek muscles. 3 draw in a specified direction by creating a vacuum. 4 (suck in/into) involve (someone) in something without their choosing. 5 (suck up to) informal attempt to gain advantage by behaving obsequiously towards. 6 N. Amer. informal be very bad or disagreeable.
The above Oxford English Dictionary does not suggest that Sucking is to push in a specific direction, but to draw. If I refer to a thesaurus I could look under synonyms for DRAW and come across Pull and Suck.
Force. Means anything that pushes and pulls.
Isaac Newton did not note that the apple was pushed to the ground, but pulled. Your argument is that there are only pushing forces and not pulling (Sucking/Drawing) forces.
The only reason that ambient air pressure pushes air up a vacuum hose is that the vacuum (produced within the cleaner) allows ambient air to overcome that caused by the reduction in air pressure within the vacuum cleaner.
With or without a camber on the upper surface of a wing, the pushing force on the underside of a wing is not increased as a result of the lowering of static pressure on the upper surface, but the static force acting downwards on the upper surface of the wing is reduced as a result of reducing static pressure, thereby allowing the wing to be drawn (sucked/pulled/whatever) into the reduced pressure zone.
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A distinguishing characteristic of fluids is that they cannot carry tensile forces (i.e. pulling, sucking, dragging...) and can rather only produce compressive and shear forces.
So if an airfoil is regarded as
, I read this as: Of the total wing loading (N/sq.meter or psf), 70% of that is reduced atmospheric pressure on the upper surface, and 30% is increased pressure on the lower surface.
So if an airfoil is regarded as
70% of its lift on the upper surface and around 30% from the lower surface