Propeller Controls, II
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 pitch can be set separately from the throttle and 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 nasty results.
Constant speed propellers are driven by oil pressure or electric power and need a control unit (governor) to maintain the preset RPM by varying the blade angle. Both types will be discussed, but you will encounter normallly during training for a CPL or higher license.
To sense the engine RPM a mechanical or electrical device is used. In todays aviation this usually an oil or electrical operated governor. Some models are able to set the propeller blades into feather or even in a reverse thrust position (but not during flight).
Hydraulic propeller governors
Also called constant speed unit, it consists of a governor which uses engine oil pressure to control a hydraulic operated piston (in the propeller) and changes the propeller blade angle in order to keep RPM constant. The governor contains flyweights which senses engine RPM and change position in relation to a change in RPM, thus changing oil pressure to the propeller and moving the blade pitch (increasing or decreasing) and keeping that preset RPM constant.
The blades themselves contain flyweights which compensate aerodynamic forces so that the oil pressure only needs a little bit of effort to change the blade angle. This means that oil pressure will remain almost constant and engine lubrication is guaranteed during blade angle movement.
When installing a propeller governor on a Rotax 912 or 914 engine you can choose between an electric or a hydraulic model. I have flown with both models on Rotax engines and, although more expensive, the hydraulic constant speed seems to respond better to changes in RPM.
Some cheap electric driven propeller models were worn out after about 100 hours of time in service. The NZ Airmaster Propeller and German MT Propeller (electric) seemed to outperform all other electric types in terms of durability and have much lower maintenance issues.
Normal blade angle governing range is between 11,5° and 40° and the higher the speed of the aircraft the higher the course blade angle will be, even up to 50° for large turbine transport aircraft (ATR-72, Bombardier Q400 etc).
Electric propeller governors
Detection of propeller RPM can be done in several ways: by optical (light), magnetic fields (flux) or sensing the engine RPM by connecting into the ignition or RPM indicator (Rotax). This signal is then fed to a controller which calculates any change in RPM and drives an electric motor inside the propeller to keep the preset RPM constant.
The biggest advantage is that with a flick of a switch the propeller can be set to any RPM, feather position or even operate as a fixed pitch and all of this is done with sophisticated, sometimes even microprocessor controlled electronics (Airmaster Propeller, NZ).
Most modern constant speed propellers (hydraulically or electrical) have an accurate and responsive operation. Any change or tendency to change the RPM, by the pilot, turbulence or change in aircraft attitude, is sensed quickly so blade angle is changed accordingly to keep the preset RPM constant and this occurs almost unnoticeable by the pilot or passengers.
With a hydraulic constant speed propeller you must verify that no oil leaks exist either at the propeller or near the governor. Any leak is reason not to fly as engine lubrication is impaired and failure or seizure of the engine could result due to total loss of oil.
Moving blades need some form of lubricant and a seal to keep the lubricant inside the hub. Check the blades for any leakage of this lubricant as this may result in one or more blades getting stuck in one position or damage of the blade control mechanism.
Hydraulic governors need to be cycled at least three times to pump the oil to the engine and test governor action, do this at around 1700 RPM or follow the checklist or by engine / propeller manufacturer recommendation.
Electric governors do not use engine oil to move the blades but you need to test the electronics for proper operation. Follow the operation manual from the manufacturer for the proper procedure, and any fail means you are grounded.
After succesfully completing these checks the propeller is ready for the flight, but always keep an eye out for proper indication of RPM and oil pressure.