hawk37
As you say, HN39's chart is type specific, and I am not sure the small increase in L/D between 0.2M and 0.5 is at all typical - certainly for the aircraft with which I am familiar the drag and L/D are virtually constant up to about 0.6M where compressibility starts to kick in.
I can offer an explanation for the AoA effect though. When compressibility starts to be significant it increases the effective lift dependent drag contribution so the profile/lift dependent drag balance is shifted, pushing the lift coefficient for L/Dmax down to a slightly lower value - say from 0.65 down to about 0.6. This by itself cannot explain the reduction in AoA for best L/D.
The main reason for the AoA reduction is that the lift curve slope of the wing increases dramatically with increasing Mach No. - typically from about 5.0 at 0.2M up to 8.0 or higher at 0.8M. The lift coefficient at zero AoA will be about 0.15, so the AoA for L/Dmax would typically be about 5.7deg at 0.2M falling to about 3.2deg at 0.8M. I'm using a different aircraft to HN39 of course, but the numbers are not very different.
Last edited by Owain Glyndwr; 19th August 2013 at 07:25.