Quote:
Originally Posted by TomcatViP
No Mig Sry but Mach nbr is the local mach, not the plane frwd speed.
At 15% thickness, the speed is in high subsonic when the plane is flying around 600kph.
At this local speed, drag effects are not linear and raise sharply.
Moreover, I think I was one of the first to put the dK/dt=SIGMA(P) eq around on forums. So don't nurse me with it. Thx in advance.
Remind simply that this give only the max speed any increase of pow will give to a plane. At high speed, this is not linear.
Generaly speeking:
Incompressible theo apply only for M<0.3
Btw M0.3 to 0.8 effecst are sharp and results vary btw 6% to 100%
Over 0.8, you hve shock waves and you need to apply according theo.
Of course many guys rely only on incompressible, simply because they never really to get into the others case  |
Hoerner gives 10% rise on Cd at mach 0.55 in his analysis on the Bf 109G, steep rise starts around mach 0,6. We are talking here about 15kmh speed differences around mach 0.4 (460-500kmh at sea level) so we can safely assume that there is less than 2% difference in the Cd due to compressibility and that causes much less than 1kmh error in the calculation.
I don't think that anyone is relying on incompressible theory here, just giving a quick estimate of speed change due to power change at good enough accuracy. You can, of course, point out that compressibility is not accounted (nor Cl, prop efficiency etc.) but can you prove that there are significant errors?
BTW that calculation method has been criticized earlier in this thread, actually well before Holtzauge used it and you posted your stuff.
10-18-2012, 09:30 PM
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