109 prop pitch (rpm) and the supercharger

[Crumpp]

Can we all specify exactly when we are referring to prop rpm, when we a re referring to engine rpm, and when we are referring to both?

What is referred to as propeller rpm is actually measuring engine rpm but you use the propeller governor to control it in a constant speed propeller. In a direct drive, meaning the propeller is bolted right to the crankshaft, fixed pitch propeller the engine control is just the throttle which sets the rpm.

In a variable pitch propeller, the throttle controls manifold pressure and the propeller controls are used to set the engine rpm. What is confusing people is at the limits of the propeller blade movement, it will change rpm. So if you pull the power to idle from cruise, your rpm will change too. You are not controlling rpm with manifold pressure though, it is just an effect because they are dependant upon each other.

The rpm and manifold pressure settings found in the Operating Notes correspond to a specific point of maximum efficiency. This point is based on the speed of the aircraft.

First of all, what is a propeller?

It is just a wing that moves in a circle. Like any wing, it has limited range of angle of attack from zero lift to maximum lift coefficient. It is also subject to critical mach effects like any wing.

All propellers reach peak efficiency for a give pitch angle at a specific velocity. Just like the wings on the airplane have a point of highest lift to drag ratio, a propeller is no different.

That is why fixed pitch propellers come in "cruise" and "climb" propellers.

A variable pitch gives the pilot the advantage of being able to use a wide range of peak efficiencies by adjusting the pitch angle of the blades and the speed at which the propeller rotates. It allows us to adjust the angle of attack and dynamic pressure our rotating wing requires to keep it at the point it can transfer the most thrust force to the air.

The "popping the clutch" analogy was very good for what the German pilots were doing. The engine has a finite amount of power and thrust it can generate. The point of maximum thrust production is defined by a velocity and the most efficient engine setting. We cannot produce extra power from nothing and just changing blade angle is NOT going to change that point of maximum efficiency or necessarily get us there quicker.

Popping the clutch in your car does not increase the cars power or speed it can attain. Done correctly, you gain an increase in acceleration to the maximum point of efficiency. Done poorly, you loose traction and it takes longer to reach the point of maximum efficiency.