Tinstaafl
I think you're quite right. The assumption seems to be that fuel burn per unit power is independent of altitude.
Consider a 360 cubic inch engine operating at 2000 RPM throttled to 15". If I've got my sums right, the pumping losses with an ambient pressure of 30" cost about 6 HP. Full throttle at 18,000 ft, you get 6 HP more for your fuel.
[Using a real engine chart I get similar numbers. I only have an O-360 chart to hand but that suggests 18" 2000 RPM 6.4 USgph gives 90 HP at a full throttle altitude of 12800 ft but only 78 HP at sea level.]
That doesn't sound like much, but if you consider a typical aircraft with a 360 cubic inch engine (say a 2740 lb Mooney with a 12:1 L/D max, corresponding to 90 KCAS) the power required at best L/D is only 60 HP anyway. 6 HP makes a huge difference. For 66 HP, you can do more than 110 KCAS at sea level, a little less at altitude.
So who's going to tell Byington that his assumptions are shot to hell?
