Further Thoughts on Propellers
A short while ago, I discussed propeller blade failures, particularly those relating to metal propeller blades, the most common type of propeller encountered on the average general aviation aircraft. However many older aircraft, and most ultra-light aircraft, are fitted with wooden propellers. Additionally ‘composite’ propellers can now be found on an increasing number of larger turbo-prop aircraft and also on many ultra-lights.
So what is so different about these propellers compared to metal propellers? Well, simplistically, the basic structure of a metal propeller blade is homogeneous, whereas both wooden and composite propeller blades consist of multiple individual fibres. As I explained last month, when minor damage such as a nick occurs to a metal propeller blade it creates a ‘stress raiser’ in the metal at the base of the nick. The increased tensile stress (by a factor of five or more) at this point can, and often does, lead to the development of a fatigue crack in the metal which slowly progresses until blade failure occurs. The situation with both wooden and composite blades is subtly different. The basic internal structure of these blades is made up of multiple individual fibres oriented along the blade axis (not to be confused with outer surface/torsional layers which are often oriented at 45? to the blade axis). If the blade suffers a nick and some longitudinal fibres are cut, the adjacent fibres take the additional load but significantly, unlike metal, the stress raiser factor does not occur – effectively the load increases in direct proportion to the reduction in cross-sectional area due to the damage.
Fatigue, as such, is not a real failure mode with wooden or composite propellers.
None of the above should be construed to mean that minor damage to a wooden or composite propeller blade is unimportant – it is, but for different reasons.
Firstly, wooden blades. By its nature, wood absorbs moisture. The ingress of moisture is normally prevented by the surface coating, usually paint, clear urethane type coatings or fibreglass. Unfortunately damage to this surface protection will allow water or other contaminants (often engine oil) to get into the underlying wooden structure and weaken it. This can be serious. When minor damage occurs the tensile stress on adjacent wood fibres may not be increased significantly but what may occur, particularly following moisture ingress, is that the bond between adjacent wood fibres is reduced and longitudinal splitting of the wood may result. This in itself may cause catastrophic blade failure. Propellers that utilise fibreglass layers as the protective coating for the underlying wooden structure certainly have an increased resistance to stone damage and have the added benefit of a ‘binding’ that increases torsional rigidity and helps to resist splitting. The downside is increased weight, cost, the labour involved during manufacture, and the thicker (less aerodynamically efficient) blade that results.
Composite blades – usually fibre-glass, carbon fibre or Kevlar – are generally more tolerant of stone damage or nicks. As their structural make-up and damage tolerance varies greatly between manufacturers, it is difficult to generalise, so with regard to damage acceptability and repair, the advice of the manufacturer should always be followed.
Regardless of the type of propeller blade fitted to your aircraft, minor stone nicks and damage are almost inevitable. The secret to long propeller life is fundamentally related to good airmanship. Have any damage that does occur repaired as soon as possible.
It must be remembered that propellers act like giant vacuum cleaners – they suck up stones like there is no tomorrow! Always consider stone damage as a possibility and try to minimise it. Consider carefully the following:
• If the tarmac area where the aircraft is parked has a loose surface, or is covered in stones, it is a good idea to try to push the aircraft to a smooth hard surface for the engine start. During the starting process the RPM may increase significantly and suck stones up off the surface.
• Never, never, never do an engine run-up on loose stones – to do so could virtually destroy your propeller within seconds.
• Do not hold the aircraft on the brakes as you increase power for take-off if the runway surface is loose or covered in stones – keep it rolling.
• Try to always keep the aircraft moving when on loose surfaces and minimise use of the throttle. If you stop, then have to start moving again, more engine power will be required than if the aircraft had been kept rolling. Always taxi with minimal use of power and brakes consistent with good airmanship.
• Aircraft with pusher propellers such as the Cessna 337 or the Rutan Long-Ez are particularly vulnerable to propeller damage from loose stones. In addition to the inherent ‘suction’ caused by the propeller, the nose wheel flicks up stones directly into the propeller disc. Extra care is essential with these aircraft types – avoiding loose surfaces altogether is the best, if not always practical, solution.
Take care of your propeller and it will take care of you!