Quote:
Originally Posted by Graham Boak
You want a more academic source. I offer you "Design for Air Combat", by Ray Whitford C.Eng MRAeS. Page 13, "Trailing vortex drag typically represents 75% of the total drag in maximum sustained manoeuvre flight.........only 5 to 10% in the low altitude high speed flight." Trailing vortex drag is his term for induced drag. This value is for modern combat jets: for WW2 fighters at about half the speed the induced drag proportion will be higher, about 10-20%.
|
Ah, this is where the things get complicated. Generally the polar of the aircraft stays pretty unchanged up to mach 0,4-0,5. And at low altitude WWII fighters reached this kind of speeds, 600km/h being slightly below mach 0,5 at sea level at standard atmosphere. So simple relations based on assumption of constant polar work pretty well for this kind of analysis.
However, around Mach 0,5 the polar shape as well as the zero lift drag coefficient start to change so the formulas intended for slow speeds and constant shape of the polar work poorly for the jets which can do around Mach 1 at sea level. In other words, induced drag proportion of the jet aircraft drag does not prove much here because these fly at speeds where the drag and lift relations are very different and not constant.
IMHO the best way to show the limitations of constant AoA analysis is to rewrite L/D ratio as:
L/D = Cl/Cd = Cl/(Cd+Cdi)
In other words there is basic polar analysis hiding behind, the Cl and Cdi are just assumed to be constant. And that is a wrong assumption for this particular case.