So much theory. Scenario originally given was running a tank dry, not initially changing throttle. With a DC3 THIS is what happens. At 150 knots TAS, there is no change in RPM or MAP. At 90 knots, there is some decrease in RPM and because the supercharger RPM drops with it, some drop in MAP. All of course accompanied by (expletive deleted) as throttle is hastily reduced and fuel tanks changed over.
John Tullamarine is usually right on these matters. It's all about the point at which the prop hits the fine pitch stops when there is no power coming out of the engine. At low airspeeds it must go full fine then it behaves as a fixed pitch prop if speed is further reduced. Also less RPM = less MAP. At higher airspeeds, because the engine is turning and producing oil pressure, the governor/prop combo MAY be capable of constant speed control, depending on TAS.
As an aside, I once had a prop run away to just over 2700 RPM on approach but we got slowed down and found that at 90 knots the RPM was quite controllable with throttle, so we kept it running rather than feather it.
Can't speak for other supercharged types.
Heron with the old Gypsy Queen engines, which were not supercharged (as someone recently tried to tell me - he was confused with the DH Dove): Similar reaction if you switched off the ignition in the cruise. Nothing to see except a gentle yaw. When airspeed reduced the RPM decayed, but now (because it was not supercharged), there was no visible change on the manifold gauge. From memory manifold indication was meaningless on those engines anyway as they had an ingenious single lever control of throttle and RPM - for once the Poms built something that was simple for pilots, if not the engineers. Basically if you could hear noise, the engines were working, and if one failed you knew because it got noisier as they went out of synch.
Last edited by Mach E Avelli; 25th Jun 2010 at 06:00.