Nick
Re NACA Report;
In no way can I see how this 1928 report arrives at its conclusion that "The induced drag of an airplane is reduced upon approaching the ground ...'.
Only the [propeller RPM] and the [forward velocity] were used as inputs to this report. These two variables, plus a homemade 'fudge factor', the [influence coefficient], are the constructs of the lift/drag graph. I would suggest that the only conclusion, which can be drawn from this report, is that the ground improves the lift/drag ratio.
[Angle of attack] also improves the lift/drag ratio, but the report does not even consider the [angle of attack]. I believe that a good argument can be put forth that states 'The angle of attack is increased in ground effect. The drag is increased in ground effect, also; but not to the extent that the lift/drag ratio is increased.'
Re NASA Technical Memorandum 4604:
"Ground effects generally caused an increase in the lift, drag and nose down pitching moment coefficients". This appears to support the above argument.
The reduction of tip vortices obviously improves lift and the ground may well reduce these vortices. This is may be especially beneficial for aircraft with highly swept wings, where the lift is "near the wingtips of a swept wing".
Vortices reduction is probably only one component of ground effect. Perhaps at 150-170 knots the F-15 is out in front of other ground effects.
Dave J.
Last edited by Dave Jackson; 21st Nov 2002 at 05:24.