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[HazelNuts39]

Mach, CL and AoA

BOAC;
Thank you for reminding me of the Prandtl-Glauert transformation:

Originally Posted by BOAC; #636

CL = CL(M=0) / (1 - M2)1/2 where M=Mach number

I must shamefully admit to having neglected this sofar. All AoA’s I gave are based on the cL-AoA relation established from a M=0.815 maneuver, without correcting for Mach change. Although there is some doubt as to applying P.-G. in the transsonic flow likely to exist at M=0.815 (see Wikipedia quotation below), if I apply that correction, the numbers I gave earlier change as follows:

Originally Posted by HazelNuts39; #611

If we assume that the CBT’s VLS is defined by the stall, but alphaProt and alphaMax are somehow ‘adjusted’ for buffet boundary, we could estimate alphaMax(stall) as either:
VLS/1.18 = 181kt; alpha = 7.7 degrees, or –
VLS/1.23 = 174kt; alpha = 8.5 degrees.

The revised numbers are:
VLS/1.18 = 181kt; M = 0.55; alpha = 11.5 degrees, or –
VLS/1.23 = 174kt; M = 0.53; alpha = 12.7 degrees.

Originally Posted by PJ2; #510

For 212k kg, FL350 the pitch attitude at AlphaProt is almost 9deg;

The PFD shows 204 kt CAS; pitch attitude 8.3 degrees, while the QF72 data after applying the P.-G. transformation result in 8.4 degrees

From Wikipedia; Prandtl-Glauert transformation:

This correction factor works well for all Mach numbers 0.3 < M < 0.7
It should be noted that since this correction factor is derived from linearized equations, the pressures calculated is always less in magnitude than the actual pressures within the fluid.

regards,
HN39