Selecting the correct propeller for your aircraft depends on a number of factors: manufacturer recommendations, engine type and power, RPM range, cruise speed, ground clearance, noise abatement issues and last but not least: personal preferences.
The type of aircraft might also be limiting for the propeller as, for example, the Vari and Long EZ are limited to rather short propellers due to the pusher design. As far as the engine is concerned, the weight of the propeller plays a role too with its inertia.
Other factors include composites, wooden or metal type propellers. Also other pilots or aircraft owners might prefer a controllable propeller of some kind to maximize the performance during all flight regimes. We will investigate this too in our article.
As already mentioned in our series of articles on aircraft propellers there are different kinds of propellers which can be used on your aircraft:
Usually made out of one piece and the blade angle can not be changed by the pilot during flight, this is set by the manufacturer. With some models this is adjustable on the ground. They can be two or more blades and are made from different type of materials: wood, metal or composite.
With these propellers the blade angle can be preset (ground adjustable) or set during flight by the pilot with the blue RPM knob in the cockpit. The set RPM is either held constant by an electric motor and some smart piece of electronics or hydraulic oil pressure regulated by a propeller governor.
Fixed vs Controllable.
Cost wise the fixed pitch propeller clearly wins, but performance wise the winner is the propeller which can be controlled. With this type the maximum performance for the engine / propeller / airspeed combination can be obtained for any airspeed and this results in higher climb and forward speeds and a lower fuel consumption.
Metal vs Wood vs Composite
Metal props are heavier than the other two types. Wooden and composite propellers dampen the engine vibration much better which may lead to reduced blade failures. On the contrary, metal can handle rain and the odd stone better.
Wooden propellers are probably the cheapest, composites can be more expensive than wood as layers of carbon or glass and epoxy are used to re-enforce the wood inside of the blades.
A number of factors must be taken into account when you are going out to buy that all time super performing propeller for your aircraft.
With propellers size does matter. With a larger disc area, more thrust can be generated by the propeller, engine power permitting of course. But with longer blades come another set of problems, ground clearance and noise are one of them. Propeller aerodynamics shows us the propeller tip speed and if it will break the sound barrier an increase of noise level during takeoff may be expected.
Engine & aircraft
With taildraggers one might select a larger propeller if the engine can handle it. With tri-cycle aircraft ground clearance usually becomes an issue. These must take a flat nose tire and strut into account so that the propeller does not hit the ground in those situations.
A good test to see if the engine can handle the prop is the static RPM test. If that can be reached one can assume that the engine will perform as required and you can fly the aircraft with this propeller.
You can also check to see what other builders are using with the same type of aircraft and engine, this will give a good reference of the expected performance of your preferred combination.
Weight & Balance
Metal is heavier and installing that lump of metal on the nose of the aircraft will definitely change the weight and balance. The center of gravity will go forward so a new basic calculation must be done. A composite propeller might help here should the C of G be to far forward.
Simple choice. If you have found a damaged propeller and want to use it on your aircraft, then it have it repaired by the propeller manufacturer or a certified repair station. Some will try to chop off the damaged section leaving the propeller a bit shorter and with possible stress damage left inside. Do not take any chance by playing with your life and that of others.
The propeller has a certain amount of mass and this should be evenly arranged among all blades. If this is not the case the propeller is said to be out of balance. This can be easily shown by taking the propeller off the airplane and balancing it on the mounting holes. The heaviest blade will eventually rotate to the lowest position.
This statically imbalance should be taken of by the manufacturer. Balancing the propeller on an engine is called dynamically balancing and is normally done on the aircraft in question with a running engine on different RPM settings. Small counter weights are used to correct any imbalance.
When its done properly, the engine will run silky smooth and less vibration means less damage caused by fatigue for the aircraft and its occupants.