Grob Tutor question

[Fg Off Max Stout]

You are thinking of traditional fixed pitch props. The Grob has a constant speed / variable pitch prop. Basically...

The RPM control sets the desired rotational speed of the engine and propellor. The prop is in most cases bolted directly to the engine and hence they turn at the same speed. A constant speed unit (CSU) adjusts the pitch of the propellor blades, changing the thrust produced and torque required, to maintain a constant RPM.

The throttle simply controls the power output of the engine.

So, open the throttle, more power is developed by the engine, the prop RPM will try to increase, but the CSU will coarsen the pitch of the blades, producing more thrust and requiring greater torque absorbing the increased engine power, thus maintaining RPM at the selected value. Consequently in simple terms throttle controls thrust, RPM controls well, er, RPM.

To take it to the next level of geekiness, a variable pitch / constant speed prop is more efficient because it allows the prop blades to operate close to their optimum angle of attack over most of the airspeed range and the engine can be made to run at its most efficient RPM despite changes in aircraft speed and power output. The CSU works mechanically using centrifugal weights opposing oil pressure and spring force. As I said, in most cases the prop is bolted directly to the engine and they turn at the same RPM. In some aircraft the prop may be geared so that its RPM is directly proportional and in a single fixed ratio to that of the engine. You will not find gearing as on cars with several selectable ratios. The CSU / variable pitch prop has a performance effect in some ways equivalent to a car gearbox, but achieves the result in a diferent way, as described above.