The 1/3 coeff i s good (came from dEc/dt=SUM(P)). Seems at least this went trough the mind of the bloggers.
However at high speed (and we will talk abt what is high speed), drag does nit increase linearly, but rather as a square function at the rate of the maximum local speed on the extrados of the wing (I am taking into account wing drag only) which is already significantly higher than the plane frwd speed.
To be rigorous also, at speed higher than Mach0.3, you'll need to make the conversion btw local press, ro and speed. The relation btw the Power and the speed is not true anymore if you don't add a term in ^2 to reflect the wet surface and the viscous drag effect.
So there is no linear relation btw speed and power, hence no guess work on the estimate gain in speed. A close look at a pressure plot of any airfoil will give you a hint. Usually an honest guy will use this equation in reverse, to have an idea of what is the ABSOLUTE MINIMUM of POWER you'll need for any increase of speed.
10-01-2012, 06:25 PM
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