Originally Posted by
AC103
I would also have thought propeller efficiency would be reducing as its mach number increases at the tip particularly for a prop the was originally selected for a NA IO-360.
So it is hard to see how the power available curve becomes more slanted toward a higher airspeed if that is what is causing this Vy move in the opposite direction to what we expect.
But that's the thing, as long as IAS stays the same, the aerodynamics shouldn't change. Until you reach compressibility issues.
The speed of sound at the ground is, in the ISA, 340 metre per second, or about 661 knots, and at 20000 FT, 316 metres per second, about 614 knots.
Your Mach is the relation between TAS and speed of sound, so at the ground you'll be climbing Vy, 89 knots, at Mach 0.135, where as in 20000 FT you'll be at, with the adjusted Vy at 93 knots, Mach 0.223.... I don't even think that does change anything, even with the accelerated velocity over the wings and fuselage.... but it's fun to do a little calculus from time to time.
Even the propeller is not "rotating" any faster, since the RPM is the same all the way up.... and if I read the settings correct, the power output is the same?
The propeller angle would be more coarse.... but I'm at a loss if that also means the angle of attack on the propeller is changed? The aircraft is moving through the same "amount of air" up there over time as close to the gound, so the AoA of the propeller is probably not changed.... But I could be wrong.
On the other hand, you have cowling flaps on the Mooney 20J, right? Wouldn't they need to be retracted the higher you go, since the air gets cooler? Once you retract them, less drag, more speed at the same power setting....