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07-28-2009, 04:32 PM

#34

3664shaken

On Reserve

Joined APC: Nov 2006

Posts: 15

Cubdriver,

You are confusing and conflating several issues, trying to use them to back up the false belief that lift decreases in ground effect. (I’ll address the secondary issues in my next post, but let’s hit the main one here.)

Your second replies seems to indicate that you wish to dispute sources and argue from what you feel is an authoritative source. No problem there, I only had a netbook and the internet but a full Google search will reveal the numerous other cites that you can visit that agree with me on this. If that is not authoritative enough, I am home now and I looked up some more sources for you.

I suggest the following books that all explain why lift increases in ground effect.

Fundamentals of Aerodynamics – John Anderson
The Illustrated guide to Aerodynamics – H. C. “Skip” Smith
Low Speed Aerodynamics – Joesph Katz & Allen Plotkin
An Introduction to Aircraft Performance – Mario Asselin
Theory of Wing Section – Ira Abbott & A. E. von Doenhoff

You suggested Aerodynamics for Naval Aviators

An excellent book that also agrees that lift increases in ground effect.

If you still don’t believe that the Navy knows then read the NASA report about F-15’s and increased lift in ground effect.

http://www.nasa.gov/centers/dryden/pdf/88337main_H-1999.pdf

Now it is true that NASA hasn’t tested pipers or Cessna, but they did test low-wing transport jets and they also saw an increase in lift in ground effect.

http://www.nasa.gov/centers/dryden/pdf/88010main_H-1273.pdf

Of course that is what the physics and computer models all predicted and has been known for quite some time as the reports allude to.

This is also a good link that explains it very well with some good diagrams and pictures.

http://www.se-technology.com/wig/html/main.php?open=aero

From An Illustrated Guide to Aerodynamics

“Not all of the floating tendency can be attributed to drag. A reduction in downwash also changes the orientation of the relative airstream so that the effective angle of attack increases. This is an increase in angle of attack resulting from changing the direction of the airstream rather than the direction of the chordline, as we normally think of it. The effect is the same because angle of attack is the angle between the airstream and the chordline. Increased angle of attack leads to increased lift; therefore, even thought it is slight, an additional amount of lift is experienced in ground effect as well as decreased drag. Both effects, of course, act to keep the airplane flying.”

From Gliem CFI Toolbox:
http://www.cfitoolbox.com/cfitoolbox_pp_example.pps#10

“The reduction of the wingtip vortices alters the spanwise lift distribution and reduces the induced angle of attack and induced drag. Thus the wing will require a lower angle of attack in ground effect to produce the same lift coefficient, or, if a constant angle of attack is maintained, an increase in the lift coefficient will result.”

(wow even Gliem got it right!)

From An Introduction to Aircraft Performance

“As the aircraft flies closer and closer to the ground, the lift coefficient increases for a given AoA. . .
Another important effect for an aircraft flying IGE is a large reduction of induced drag. The increase in the lift coefficient and the decrease in drag coefficient, for a given AoA, translate into an increase in lift-to-drag ration. This drag reduction will reduce the aircraft’s rate of deceleration (and maybe even increase the velocity if the drag becomes smaller then the thrust available). Combined with the increased lift coefficient, the aircraft will tend to float and refuse to touchdown. . . .”

From the NASA report:

For the F-15 aircraft, the change in the lift coefficient because of ground effect doubled from approximately 0.05 to 0.10 as the sink rate decreased from approximately 6.5 to 0.7 ft/sec.”

As far as John Denker goes, he is highly controversial and I would not use him as a primary source. I think the vast amount of literature by noted authors, educators, scientists, NASA, and a plethora of other sources in aerodynamics is overwhelming in this case.

Could they all be wrong? Maybe but the empirical data that NASA has along with all of the computer models and wind tunnel tests all seem to say the same thing. I’ll put my money and my beliefs behind those guys instead of John Denker.

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