Ben Lanterman
Posts: 1286
Joined: 10/27/2002
From: St. Charles, MO, USA
Status: offline
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OK one last try,
Tim, Adam-one, others - Go to
http://www.av8n.com/how/htm/airfoils.html
and read it.
Section 3.1 talks about upwash and downwash, stagnation lines, and some more stuff.
Section 3.2 talks about Pressure Patterns Near a Wing. Good figures. I'm including a good quote -
"There is a lot we can learn from studying this figure. For one thing, we see that the front quarter or so of the wing does half of the lifting. Another thing to notice is that suction acting on the top of the wing is vastly more important than pressure acting on the bottom of the wing. "
Keep in mind that the author is using the aero short cut with the word "suction" as a pressure lower than ambient (Pressurefreestream-Pressureabovewing) and "pressure under the wing" as higher than ambient (Pressureunderwing-Pressurefreestream). This is consistent with the concept that low pressures can't "suck" , they only allow other pressures to "push"
"At this angle of attack, there is almost no high pressure on the bottom of the wing; indeed there is mostly suction there.6 The only reason the wing can support the weight of the airplane is that there is more suction on the top of the wing. (There is a tiny amount of positive pressure on the rear portion of the bottom surface, but the fact remains that suction above the wing does more than 100% of the job of lifting the airplane.)7"
"This pressure pattern would be really hard to explain in terms of bullets bouncing off the wing. Remember, the air is a fluid, as discussed in section 3.6. It has a well-defined pressure everywhere in space. When this pressure field meets the wing, it exerts a force: pressure times area equals force."
Read the whole thing from top to bottom. It will give you the a simplified but accurate picture of the lifting process.
Section 3.15 is called Momentum in the Air. The following quote fits -
"Since force is just momentum per unit time, the same process can be described by a big “closed circuit” of momentum flow. The earth transfers downward momentum to the airplane (by gravity). The airplane transfers downward momentum to the air (by pressure near the wings). The momentum is then transferred from air parcel to air parcel to air parcel. Finally the momentum is transferred back to the earth (by pressure at the surface), completing the cycle. There is no net accumulation of momentum anywhere (in long-term steady flight)."
Finally here is a summary from the article - be sure and read the whole thing.
"3.16 Summary: How a Wing Produces Lift
* The flow pattern created by a wing is the sum of the obstacle effect (which is significant only very near the wing, and is the same whether or not the wing is producing lift) plus the circulation effect (which extends for huge distances above and below the wing, and is proportional to the amount of lift, other things being equal).
* A wing is very effective at changing the speed of the air. The air above is speeded up relative to the corresponding air below. Each air parcel gets a temporary change in speed and a permanent offset in position.
* Bernoulli’s principle asserts that a given parcel of air has high velocity when it has low pressure, and vice versa.
* Below-atmospheric pressure above the wing is much more pronounced than above-atmospheric pressure below the wing.
* There is significant upwash ahead of the wing and even more downwash behind the wing.
* The front stagnation line is well below and behind the leading edge.
* The rear stagnation line is at or very near the trailing edge. The Kutta condition says the air wants to flow cleanly off the sharp trailing edge. This determines the amount of circulation.
* An airfoil does not have to be curved on top and/or flat on the bottom in order to work. A rounded leading edge is a good idea, but even a barn door will fly.
* Air passing above and below the wing does not do so in equal time. When lift is being produced, every air parcel passing above the wing wing arrives substantially early (compared to corresponding parcel below the wing) even though it has a longer path.
* Most of the air above the wing arrives early in absolute terms (compared to undisturbed air), but this is not important, and the exceptions are doubly unimportant.
* Lift is equal to circulation, times airspeed, times density, times wingspan.
* Well below the stalling angle of attack, the coefficient of lift is proportional to the angle of attack; the circulation is proportional to the coefficient of lift times the airspeed.
* Air is a fluid, not a bunch of bullets. The fluid has pressure and velocity everywhere, not just where it meets the surface of the wing.
* There is downward momentum in any air column behind the wing. There is zero momentum in any air column ahead of the wing, outboard of the trailing vortices, or aft of the starting vortex.
* Vortex lines cannot have loose ends; therefore you cannot produce lift without producing wake vortices.
* Induced drag arises when you have low speed and/or short span, because you are visiting a small amount of air and yanking it down violently, producing strong wake vortices. In contrast there is very little induced drag when you have high speed and/or long span, because you are visiting a large amount of air, pulling it down gently, producing weak wake vortices."
So this is a good read, If you are really interested in how the whole thing works with respect to wing and airplane stuff read the whole site at -
http://www.av8n.com/how/#contents
I don't know if I have the strength to draw the correct vector figure..
Tim, Adam-one, read the above site and then come back and talk. Adam-one your concepts are just all wrong. You are saying blow down on the airfoil and it will lift. Not in this universe. Honestly guys read the stuff in the site above, it will give you the start of understanding.
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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 Mag... - 
