How does split flap increase lift?
As Clear As Mud!
I'm not going to disagree with anyone here because frankly I don't know the correct answer, but......
I have a video somewhere what shows a wing in a wind tunnel with smoke being puffed over it. The marker in the smoke that passes over the wing reaches the trailing edge before the smoke that passes under the wing. At least that is how I remember it. That would seem to indicate the old theory about air passing over the top of the wing needing to travel faster and hence creating a low pressure area, etc, etc.
I also seem ro recall seeing a video on Youtube that showed a flat plate in a stream of water from a tap being sucked in to the stream. This again seems to indicate that the high / low pressure theory have some legitimacy.
Edit - here it is YouTube - Lift from water over a spoon
I may be off the reservation here but some of the comments so far seem to indicate that this pressure differential theory is bogus.
Go ahead - shoot me down!!!!
I have a video somewhere what shows a wing in a wind tunnel with smoke being puffed over it. The marker in the smoke that passes over the wing reaches the trailing edge before the smoke that passes under the wing. At least that is how I remember it. That would seem to indicate the old theory about air passing over the top of the wing needing to travel faster and hence creating a low pressure area, etc, etc.
I also seem ro recall seeing a video on Youtube that showed a flat plate in a stream of water from a tap being sucked in to the stream. This again seems to indicate that the high / low pressure theory have some legitimacy.
Edit - here it is YouTube - Lift from water over a spoon
I may be off the reservation here but some of the comments so far seem to indicate that this pressure differential theory is bogus.
Go ahead - shoot me down!!!!
Last edited by Flyer517; 18th Feb 2009 at 11:01. Reason: Adding detail!
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I'm with Andy RR.
The aerofoils most of us here use do not work as a result of the "inclined plane" theory.
The aerofoils we use work in the same manner as a venturi. Albeit one sided.
To say that the upper surface of a wing is shaped as it is to reduce drag is incorrect.
Try flying with this camber disrupted - you will see how critical it is.
The aerofoils most of us here use do not work as a result of the "inclined plane" theory.
The aerofoils we use work in the same manner as a venturi. Albeit one sided.
To say that the upper surface of a wing is shaped as it is to reduce drag is incorrect.
Try flying with this camber disrupted - you will see how critical it is.
Last edited by currawong; 18th Feb 2009 at 11:58.
Moderator
This goes the same for me as trying to explain how those barn door Kruger Flaps actually increase lift???? swinging forward from under the leading edge??? dropping vertically down??? Wouldn't that kill any smooth flow?
It seems that forcing the door open, forces more air to go over the wing in that local area, and more air going over, means more lift, so the left wing lifts more, so the plane turns right. Like a Kruger flap, I'm sure this has it's practical limitations, and probably I reached them doing what I was doing, but it proved a lesson to me that increased lift does not always come from a streamined something altering the airflow.
Pilot DAR
To say that the upper surface of a wing is shaped as it is to reduce drag is incorrect.
Try flying with this camber disrupted - you will see how critical it is.
Try flying with this camber disrupted - you will see how critical it is.
To those who think the Newtonian explanation of lift is wrong, I challenge you to design a working wing that does NOT accelerate a mass of air down in accordance with Newtonian predictions.
To those who think that the Bernoulli explanation is wrong, I challenge you to create a wing that does NOT accelerate a mass of air over the top of the wing and that the subsequent change in air pressure over the top of the wing does NOT fit with Bernoulli's equations.
They're both simplified explanations that focus on different parts of the same process. And they're both equally right (except the part about the air particles having to meet up at the trailing edge, that's just wrong.)
This is from the NASA website (my emphasis.)
Originally Posted by NASA
When a gas flows over an object, or when an object moves through a gas, the molecules of the gas are free to move about the object; they are not closely bound to one another as in a solid. Because the molecules move, there is a velocity associated with the gas. Within the gas, the velocity can have very different values at different places near the object. Bernoulli's equation, which was named for Daniel Bernoulli, relates the pressure in a gas to the local velocity; so as the velocity changes around the object, the pressure changes as well. Adding up (integrating) the pressure variation times the area around the entire body determines the aerodynamic force on the body. The lift is the component of the aerodynamic force which is perpendicular to the original flow direction of the gas. The drag is the component of the aerodynamic force which is parallel to the original flow direction of the gas. Now adding up the velocity variation around the object instead of the pressure variation also determines the aerodynamic force. The integrated velocity variation around the object produces a net turning of the gas flow. From Newton's third law of motion, a turning action of the flow will result in a re-action (aerodynamic force) on the object. So both "Bernoulli" and "Newton" are correct. Integrating the effects of either the pressure or the velocity determines the aerodynamic force on an object. We can use equations developed by each of them to determine the magnitude and direction of the aerodynamic force.
Last edited by AerocatS2A; 18th Feb 2009 at 13:24. Reason: Expansion on the subject, added cite.
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Well please explain to me then Andy_RR how the drivel should be expressed?
CB
Edited to add, I do love it when the Bernoulli lovers say Newton cannot explain lift. EVERYTHING in motion is explained by Newtons three laws. Everything can be derived from what these three laws describe. To say otherwise is denying the laws which govern motion and energy in our universe.
Perhaps I was simplifying a little too much to say lift only comes from the underside of the wing and the camber of the top surface has nothing to do with it. Indeed it does, but again the camber or shape of the top surface acts to accelerate the airflow DOWNWARDS!!!!!!! These issues always arise when you try and dumb down an explanation so that those who are not engineer amongst us and who ask a seemingly simple question can at least partially explain it. Have a look in the wind tunnel videos and you will see this. The opposite reaction is for a force (known as Lift) to be imparted on the wing and thus an aircraft flies. I would dearly love to know how Newton is incorrect! If he is then there are a few things which will be difficult to explain, but like any good scientist and engineer I do have an open mind. And Andy_RR I guess NASA is incorrect as well. As far as I can tell, what I said (in very very simplified terms), is the exact same way that NASA explains how lift is produced?
Anyway I do love a good debate. Chicken or egg next?
CB
Edited to add, I do love it when the Bernoulli lovers say Newton cannot explain lift. EVERYTHING in motion is explained by Newtons three laws. Everything can be derived from what these three laws describe. To say otherwise is denying the laws which govern motion and energy in our universe.
Perhaps I was simplifying a little too much to say lift only comes from the underside of the wing and the camber of the top surface has nothing to do with it. Indeed it does, but again the camber or shape of the top surface acts to accelerate the airflow DOWNWARDS!!!!!!! These issues always arise when you try and dumb down an explanation so that those who are not engineer amongst us and who ask a seemingly simple question can at least partially explain it. Have a look in the wind tunnel videos and you will see this. The opposite reaction is for a force (known as Lift) to be imparted on the wing and thus an aircraft flies. I would dearly love to know how Newton is incorrect! If he is then there are a few things which will be difficult to explain, but like any good scientist and engineer I do have an open mind. And Andy_RR I guess NASA is incorrect as well. As far as I can tell, what I said (in very very simplified terms), is the exact same way that NASA explains how lift is produced?
Anyway I do love a good debate. Chicken or egg next?
Last edited by Cloud Basher; 18th Feb 2009 at 20:03.
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Split Flaps
Don`t you think that we are all wrong, the air does not flow over the wing, it remains stationary relatively speaking and the airfoil just parts it.
The airflow is deflected downwards creating a place for newtons 3rd rule by the down wash and at the same time producing greater profile drag.
The airflow is deflected downwards creating a place for newtons 3rd rule by the down wash and at the same time producing greater profile drag.
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Flyer 517,
Your yourtube video link actually proves what I am trying to say. The water is accelerated over the "aerofoil" (spoon) and is deflected downwards (sideways) you can actually see this towards the end of the video. Thus the equal reaction is for the spoon to move in the opposite direction. As I said earlier, bernoulli's equation puts a numerical value on what you are seeing. There is other forces at play but we will ignore them for the sake of simplicity.
Suffice to say Nature abhors a vacuum and so will try to equalise the lower pressure by pushing (or sucking) the wing up. This comes from the water accelerating downwards and thus Newton states the spoon must move into the water flow in order to react to the water going downwards. So the high/low pressure is spot on. You can even mesure the poressure differential very very accurately. Bernoulli's equation is derived from this. BUT the high.low pressure comes from the fact that the water is being deflected downwards, so the opposite reaction is necessary to equalise the pressure. It is all a big circle and as the NASA quote (and as I believe I stated earlier) they are both correct!
Cheers
CB
Your yourtube video link actually proves what I am trying to say. The water is accelerated over the "aerofoil" (spoon) and is deflected downwards (sideways) you can actually see this towards the end of the video. Thus the equal reaction is for the spoon to move in the opposite direction. As I said earlier, bernoulli's equation puts a numerical value on what you are seeing. There is other forces at play but we will ignore them for the sake of simplicity.
Suffice to say Nature abhors a vacuum and so will try to equalise the lower pressure by pushing (or sucking) the wing up. This comes from the water accelerating downwards and thus Newton states the spoon must move into the water flow in order to react to the water going downwards. So the high/low pressure is spot on. You can even mesure the poressure differential very very accurately. Bernoulli's equation is derived from this. BUT the high.low pressure comes from the fact that the water is being deflected downwards, so the opposite reaction is necessary to equalise the pressure. It is all a big circle and as the NASA quote (and as I believe I stated earlier) they are both correct!
Cheers
CB
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I remember reading another 'discussion' about Bernoulli vs. Newton... and someone pulled out this gem:
One is a Theory... the other is a Law
Having started my flying in the world where it was all about High and Low pressure differentials... and then came back about 7 years later where it was all about displacing air (which I might add is much like how boats, made of heavy objects like steel, float by displacing water)... it was a bit difficult to wrap my head around it...
I guess it is very much like when someone decided the earth was round... took people a while to let go of what they knew to be the "The Truth''(tm)...
Can I have a fiver on Newton?
One is a Theory... the other is a Law
Having started my flying in the world where it was all about High and Low pressure differentials... and then came back about 7 years later where it was all about displacing air (which I might add is much like how boats, made of heavy objects like steel, float by displacing water)... it was a bit difficult to wrap my head around it...
I guess it is very much like when someone decided the earth was round... took people a while to let go of what they knew to be the "The Truth''(tm)...
Can I have a fiver on Newton?
Gravity is a theory as well 
.
There is no difference between a stationary wing with air flowing around it and a wing moving in stationary air. That's why wind tunnels work. Air flow around a wing causes a velocity change (Newton) and a pressure change (Bernoulli.) They are just measuring different things.
To Cloudbasher. You're statement that the upper surface of the wing doesn't matter except for reducing drag is wrong. The shape of the upper surface of the wing has MORE to do with turning the mass of air then the bottom does.
Edit: Ignore that last bit Cloudbasher, I see you've clarified your position.
.
Don`t you think that we are all wrong, the air does not flow over the wing, it remains stationary relatively speaking and the airfoil just parts it.
The airflow is deflected downwards creating a place for newtons 3rd rule by the down wash and at the same time producing greater profile drag.
The airflow is deflected downwards creating a place for newtons 3rd rule by the down wash and at the same time producing greater profile drag.
To Cloudbasher. You're statement that the upper surface of the wing doesn't matter except for reducing drag is wrong. The shape of the upper surface of the wing has MORE to do with turning the mass of air then the bottom does.
Edit: Ignore that last bit Cloudbasher, I see you've clarified your position.
Last edited by AerocatS2A; 18th Feb 2009 at 22:57.
This topic's been debated for years...
Cloud Basher, I think you'll find that the downwash/displacement theory doesn't fully explain all of the lift a wing generates - it is too simplistic. For example, how does this quasi-Newtonian theory explain a positive lift coefficient at zero AoA? Or, a zero CL at a negative AoA? Both of these cases are possible with cambered aerofoils
Getting back to the split flap question, I think deploying a split flap increases both AoA and camber if you consider the cord line to be between the leading edge the average of the (now) two trailing edges. In practice, the flow will be more complex than this, but close enough for descriptive purposes.
Getting back to the split flap question, I think deploying a split flap increases both AoA and camber if you consider the cord line to be between the leading edge the average of the (now) two trailing edges. In practice, the flow will be more complex than this, but close enough for descriptive purposes.
Seemingly, some are saying that it's not the faster airflow over the upper side of the wing (Bernoulli) that's creating the lift, but rather the lower surface pushing the air down.
If so, wings wouldn't need to be shaped as they are, they could simply be constructed as a flat plate !
Non believers of Bernoulli, do this: Go outside into a 15 - 20kt wind with an umbrella. You'd think it'd be pushed down, but feel how much lift it generates !
If so, wings wouldn't need to be shaped as they are, they could simply be constructed as a flat plate !
Non believers of Bernoulli, do this: Go outside into a 15 - 20kt wind with an umbrella. You'd think it'd be pushed down, but feel how much lift it generates !
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Here's my one cent worth (adjusted for the global economic crisis).
I find it interesting that people are mentioning the water-over-the-spoon demonstration, but no-one has yet mentioned the name Coanda. The spoon demo has nothing to do with Bernoulli, it demonstrates the theories of Henri Coanda. Remember, Bernoulli was not an aerodynamicist and never claimed his theories had anything to do with flight; other people decided that they did. Coanda stated that a stream of fluid will cling to the contour of any shaped surface over which it runs (the water & spoon bit), thereby deflecting the fluid downward. This is where Newton comes into it. The reaction to the deflection is what we call lift. If you do the demo at home (don't waste the water, though) you will feel the spoon being drawn into the water stream if you hold it gently between two fingers.
Bernoulli's theories are, I believe, negligible, because he formulated them in a closed system (between two curved surfaces). You cannot get a more open system than the rest of the sky. I'm not saying that there are not differentials in pressure over the upper and lower surfaces, but just that they aren't powerful enough to generate the lift needed to fly.
And there you have what I reckon!
Walrus
I find it interesting that people are mentioning the water-over-the-spoon demonstration, but no-one has yet mentioned the name Coanda. The spoon demo has nothing to do with Bernoulli, it demonstrates the theories of Henri Coanda. Remember, Bernoulli was not an aerodynamicist and never claimed his theories had anything to do with flight; other people decided that they did. Coanda stated that a stream of fluid will cling to the contour of any shaped surface over which it runs (the water & spoon bit), thereby deflecting the fluid downward. This is where Newton comes into it. The reaction to the deflection is what we call lift. If you do the demo at home (don't waste the water, though) you will feel the spoon being drawn into the water stream if you hold it gently between two fingers.
Bernoulli's theories are, I believe, negligible, because he formulated them in a closed system (between two curved surfaces). You cannot get a more open system than the rest of the sky. I'm not saying that there are not differentials in pressure over the upper and lower surfaces, but just that they aren't powerful enough to generate the lift needed to fly.
And there you have what I reckon!
Walrus
Bernoulli's theories are, I believe, negligible, because he formulated them in a closed system (between two curved surfaces). You cannot get a more open system than the rest of the sky. I'm not saying that there are not differentials in pressure over the upper and lower surfaces, but just that they aren't powerful enough to generate the lift needed to fly.
And there you have what I reckon!
And there you have what I reckon!
Once again, read the link (or at the very least read the section I've quoted.) If you think it's wrong, then I'd like you to explain how it is that you are right and the collective aeronautical minds at NASA are wrong.
Bernoulli and Newton
There's the link again so you don't have to search for it in this thread.
Last edited by AerocatS2A; 18th Feb 2009 at 22:44.
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Definitely hard to go past NASA, but the "PFM theory" is starting to emerge as a real possibility. If we consider that the Dept. of Physics at Frankfurt Uni in Germany has some credibility, then we find this statement.
Conclusion
The deliberation of Bernoulli's law in schools and textbooks has serious drawbacks. Unfortunately many applications are erranous and misleading. One source of confusion is the derivation of Bernoulli's law based on the theorem of energy conservation. Bernoulli's law should be derived from the tangential acceleration as a consequence of declining pressure. Another source of difficulties is the fact that many physics textbooks do not mention normal acceleration of flow and the resulting pressure gradients perpendicular to the flow.
Both, Bernoulli's law and the generation of pressure gradients perpendicular to the flow are consequences of Newton’s laws. None of them contradicts those.
Bernoulli's law is insufficient to explain the generation of low pressure. A faster streaming velocity never produces or causes lower pressure. The physical cause of low or high pressure is the forced normal acceleration of streaming air caused by obstacles or curved planes in combination with the Coanda-effect. Pressure gradients generated by the deflection of streaming air can be clearly demonstrated by simple experiments which would substantially improve the discussion of fluid mechanics in schools and textbooks.
You can locate the paper here: Misinterpretations of Bernoulli's Equation.
I love this argument! Lets have a look for what else we can find.
Walrus
Conclusion
The deliberation of Bernoulli's law in schools and textbooks has serious drawbacks. Unfortunately many applications are erranous and misleading. One source of confusion is the derivation of Bernoulli's law based on the theorem of energy conservation. Bernoulli's law should be derived from the tangential acceleration as a consequence of declining pressure. Another source of difficulties is the fact that many physics textbooks do not mention normal acceleration of flow and the resulting pressure gradients perpendicular to the flow.
Both, Bernoulli's law and the generation of pressure gradients perpendicular to the flow are consequences of Newton’s laws. None of them contradicts those.
Bernoulli's law is insufficient to explain the generation of low pressure. A faster streaming velocity never produces or causes lower pressure. The physical cause of low or high pressure is the forced normal acceleration of streaming air caused by obstacles or curved planes in combination with the Coanda-effect. Pressure gradients generated by the deflection of streaming air can be clearly demonstrated by simple experiments which would substantially improve the discussion of fluid mechanics in schools and textbooks.
You can locate the paper here: Misinterpretations of Bernoulli's Equation.
I love this argument! Lets have a look for what else we can find.
Walrus
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Dear posters,
I think this discussion is now getting more to "lift" issue than the "split flap". Let's leave the lift issue whatever it is, because we all know lift by one of these means, and my concern was only with "split flap" and if you still insist that it's the issue; then; those who are claiming that the lift is byproduct of differential pressure tell me how doest split flap can create extra differential of pressure on upper and lower surface of the wing when deflected? and those who are claiming that the lift is byproduct of downwash and split flap increase downwash, tell me how can an aircraft then fly better in "ground effect" where downwas is disrupted by ground interference and the downwas is smoothened or reduced?
I think this discussion is now getting more to "lift" issue than the "split flap". Let's leave the lift issue whatever it is, because we all know lift by one of these means, and my concern was only with "split flap" and if you still insist that it's the issue; then; those who are claiming that the lift is byproduct of differential pressure tell me how doest split flap can create extra differential of pressure on upper and lower surface of the wing when deflected? and those who are claiming that the lift is byproduct of downwash and split flap increase downwash, tell me how can an aircraft then fly better in "ground effect" where downwas is disrupted by ground interference and the downwas is smoothened or reduced?
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To answer your question AvEnthusiast,
the split flap changes the mean chord of the wing (somewhat inefficiently) as some have already pointed out.
Downwash/wing tip vorticies are disrupted in ground effect, leading to a reduction in induced drag.
Try it - a 100 kt aircraft seems to gain 5 - 10 kts with a smaller angle of attack in ground effect.
the split flap changes the mean chord of the wing (somewhat inefficiently) as some have already pointed out.
Downwash/wing tip vorticies are disrupted in ground effect, leading to a reduction in induced drag.
Try it - a 100 kt aircraft seems to gain 5 - 10 kts with a smaller angle of attack in ground effect.