Propeller Controls, I
Fixed pitch propellers are operated by changing the throttle only, thereby changing thrust and RPM at the same time. Constant speed propellers are more complicated because the blade pitch can be set separately from the throttle and has a range from full fine to coarse during flight and into feather and reverse for certain operations as emergency and ground maneuvers.
A full understanding of operating propellers is required of the pilot as overspeeding the engine or setting reverse in flight or on finals would be nothing less than a real emergency with possible less desirable results.
This page focuses on preflight checks applicable for any propeller (fixed or controllable) and the in-flight checks for constant speed propellers as the fixed pitch types usually do not have controls in the cockpit to manipulate them during flight.
A substantial amount of aircraft have variable pitch propellers of some sort in use today, as such it is important that the pilot is very familiar with the operation of the propeller in normal and emergency situations. Even the type of engine (gasoline, diesel or turboprop) will change operating procedures for the propeller. Limitations on certain combinations of RPM and manifold pressures are not uncommon due to frequency resonations with the engine/propeller combination.
Check the propeller blades for damage, nicks and any oil leaks under the spinner. This would indicate an non-serviceable aircraft as with some propellers engine oil is used to change the pitch of the blades. For optimum propeller performance do clean the blades after every flight, this also gives you the chance to check the blades properly for any damage.
Set the propeller control in the forward (full fine) position for piston engines, this reduces the load or drag of the propeller. Free turbine turboprop aircraft usually leave the propeller feathered until the engine is fired up and N1 speed is rising, it then will automatically start to come out of feather and go to fine pitch.
During the runup, hydraulic operated propellers are verified for proper operation by changing the control from full fine to coarse (or a set RPM drop) two or three times. This makes sure that any cold oil left in the propeller is run through the engine for warm-up so that propeller operation and blade movement will not be sluggish and can not overspeed the engine when full power is applied on take-off.
During cruise operate the engine RPM and manifold pressure according to the aircraft flight manual (read it thoroughly). Avoid over-boosting by more than the recommended values. Any change in power settings should be done per procedure described below.
Excessive manifold pressures with low RPMs will raise the cylinder compression pressure and may cause stress and permanent damage in the engine, possibly resulting in an in-flight failure.
Power change procedure
To increase power for a higher RPM:
- Increase RPM with the PROPELLER control
- Increase MAP with the THROTTLE control
To decrease power for a lower RPM:
- Reduce MAP with the THROTTLE control
- Reduce RPM with the PROPELLER control
Modern aircraft nowadays have a controller which takes care of these old school techniques for you. They have a single lever power control where the pilot selects % power and the engine controller takes care of RPM and Pitch.
The procedure shown above discusses the correct way of changing power with a constant speed propeller without the possibility of overloading the engine.
It is perfectly fine to operate the engine in a so called oversquare condition where the RPM divided by 100 is lower than MAP, for example: 2200 RPM and 25" MAP, read here about running oversquare and in the aircraft/engine manuals. This will lower fuel consumption (better SFC) and increase the range of the aircraft.
In case of engine failure position the constant speed propeller in the full coarse pitch (by pulling the blue lever fully backward) as to reduce the drag from it and increase glide range. Some electrical constant speed models have a separate switch or control for that.
During landing on final approach and go-around the propeller should be in the full fine position to assure that maximum power is available should this be needed. Short final is the place where this check can be done together with setting carburetor heat set to the cold position. Again: this assures maximum power and keeps the dirty low level air out of the engine as carb heat is unfiltered air. Should a go-around become necessary.