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"Nigel Gray" > Say you want to decrease speed whilst remaining straight and level. I > know that in the PA23 I select 18"/2300 (down from 22"/2300) and > maintain altitude by pitching up. I know that because Ive been > taught it by the venerable George Orive, CFI. But why do I do that? > Whats the theory behind the reduction in power? The prop control is essentially the same as the gearshift in a car, except that 1) it works backwards from how a car works, and 2) its continuously variable, whereas a car typically has 3, 4, or 5 discrete gear ratios that you can pick from. In a car, you have two controls; the gas pedal and the gearshift. The gas pedal is exactly the same as the throttle on a plane; it simply is connected to the throttle plate in the carburator by linkages. Push down on the gas pedal, or push forward on the throttle, and the plate opens up more, allowing in more fuel/air mixture (minor handwave here when some annoying twerp in the studio audience asks about fuel injection). In both the car and the plane, the engine is most efficient when its running in a certain RPM range. To keep the RPMs within that narrow range, you need to adjust the transfer ratio in either case. In the car, you do with with the gearshift. You watch the tachometer (assuming you have one) and shift up when the RPMs get too high as your speed climbs. This imposes a higher gear ratio between the crankshaft and the wheels, letting the engine run back down at a reasonable speed. In the plane, the difference is that the pitch of the prop blades is continuously variable, instead of stepwise variable like in a car. Also, built into the CS prop hub is a mechanical governor, which keeps changing the pitch to get the RPMs to be whatever you set with the blue knob. -- Roy Smith, CFI-ASE-IA |
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