Quote:
Originally Posted by Sidious  What do you mean by back-of-the-envelope? |
Rough calculations. Figure out what the change in velocity of the prop tip is 2700 vs 2400 rpm and compare that figure to the forward airspeed. A little trig will tell you what the change in AOA is.
Let's see, 6 ft prop means that the propeller arc is pi * 6 = 19 ft. And the prop tips covers that distance in either 2700 or 2400 RPM. Converting all that to ft/s gives 954 ft/s for 2700 RPM and 848 ft/s for 2400 RPM. A cruise speed of 140 knots is only 236 ft/s.
Using tan(theta) = 236/954 vs tan(theta) 236/848 is 15.5 degrees vs 13.89 degrees. So the relative wind changes angle 1.6 degrees. I can't say exactly how many degrees the prop pitch changes to slow to 2400 RPM, but I think you'll agree that it's intuitively more than 1.6 degrees.
Check my math.