Ben Lanterman
Posts: 1284
Joined: 10/27/2002
From: St. Charles, MO, USA
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"As a wing moves at an angle through the air, the air above tries to move into the space vacated above the wing, however since it has mass, it doesn’t move instantaneously, but is accelerated toward the space. Two things happen. An area of reduced pressure is formed above the wing (Boyle’s law), and the affected mass of air acquires a downward motion. These things must happen together. The actions are inseparable. They are essential parts of the same action. One simply can’t exist without the other. One is not merely the by-product of the other but they are both necessary parts of the whole."
Lou are we talking about the same thing. I believe we have been using the terms like "deflected" to mean the downwash effects - as in the air above the wing is deflected into the low pressure area being developed. Yes the air flows over the wing, is accelerated, increases in velocity and the pressure above the wing is lowered. Then the air above is accelerated into the low pressure just as the wing is accelerated toward the low pressure. We aren't saying that one can't exist without the other. The fact is indeed that the downwash is a by-product of lift, it reaches it's maximum when the pressure field on the wing (and lift) is at a minimum. Because they both happen does not mean that the downwash is accelerating the CG of the wing. The fact is also that the air flowing around the leading edge of the wing, that is accelerated is at a maximum velocity when the low pressure is at a maximum.
Lou - what is accelerating the CG of the wing? It's F=ma. simple and direct. Where did the force come from? The sum of the forces on the wing must come from the OPPOSITE direction that the wing moves isn't it? The wing moves up or at least stays level with respect to 1g gravity acceleration. Which direction and what magnitude must the force on it come from? It is simple vector math. It must come from the bottom. There is no downwash on the bottom causing an up force is there? What is there to cause the up force - the answer is pressure.
You might think of it as details but I tend to think of it as the accurate statement of what is happening and everyone can understand it until someone comes by like Jeff and says something that is wrong, not a way of looking at it but in the most basic sense of everything you or I have studies, is wrong.
"The fact that engineers tend to deal with the part that is easier to measure and calculate, and ignore the rest, doesn’t make the rest any less true."
That is meaningless with respect to what we are discussing.
"Without the deflection (acceleration) of air there can be no pressure field developed therefore no lift. It’s really that simple."
If you are talking about the air going over the leading edge of the wing, following the countours of the wing, being accelerated and increasing in speed causing a low pressure area and calling that process "deflection" then it's true. I think from the first paragraph in this discussion we have all agreed with that.
Unfortunately some readers will think the deflection you are talking about is the downwash term. Look back at the drawing I made. What isn't clear about it? I have always differentiated the two processes at least by terminology. Don't confuse Leszek_K any more than necessary without defining your terms a little better.
< Message edited by Ben Lanterman -- 2/17/2005 7:21:34 PM >
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Ben Lanterman
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All Forums >> RC Airplanes >> Aerodynamics >> RE: Jef Raskin article on aerodynamics in Fly RC Magazine 
RE: Jef Raskin article on aerodynamics in Fly RC Magazine - 
