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Extending Aircraft Propeller Life

Dynamic balancing the propeller has little effect on propeller stress and therefore propeller service life. The little balance weights added to the spinner backplate don't change the bending stresses created as the blades pull the aircraft thru the air; neither do the weights change the centrifugal forces created by the whirling mass. Dynamic balancing places the propeller's mass center at the axis of rotation so that inertia forces aren't created by accelerating the mass. 

For example, a one ounce imbalance 2 inches from the rotation axis causing a force of 21 lbs. But a Sensenich fixed pitch propeller on a small aircraft, such as the Piper Pacer, generates 13 tons of centrifugal stress per blade at its rated rpm of 2600. Vibrations caused by imbalance stress the propeller attachment and everything downstream including the passengers, but not the blades.  The type of stress that damages propellers is vibratory stress induced on the blade by the engine's power pulses. Piston engines produce pulses of power. The larger the cylinder bore, the more torque that socks the propeller at each power stroke. A propeller can act as a tuning fork such that if the frequency of the power pulses are the same as  the natural frequency of the propeller, the propeller vibrates with increased amplitude. Four methods are used to control resonate vibrations in the propeller:

• installation of tuned bifler absorbers (engine counterweights)
• limiting or restricted engine rpm range
• selecting a propeller who's natural frequency is above or below the usable rpm range of the engine. 
• slight and random changes in engine timing.

The latter method, slight and random changes in engine timing, was used deliberately with some early aircraft engines. Our magneto ignition timing also produces slight and random changes in engine timing, although not deliberately, because of gear train clearances and other characteristics of a mechanical timed system. An interesting illustration of how the magneto can change propeller stress is illustrated in my Magneto Ignition System book where two propeller stress surveys where performed on the same engine/aircraft but with the only difference being the brand of magneto. There was a significant difference in propeller stress between one magneto brand and another.

The existing system of engine/propeller combinations that have served well with traditional magneto ignition systems may not work as very accurate electronic ignition timing replaces magneto timing. New stress surveys of propeller/engine combinations are required.

Fatigue, Nicks, and Cyclic Stress

You cannot break a metal paper clip by pulling on it with our hands because the tensile strength of the metal exceeds the pulling strength in your arms. The propeller's tensile strength also easily withstands 13 tons of centrifugal force. Yet there are two ways that you can break the paper clip and the propeller without exceeding the tensile strength of either.

You can easily break a paper clip by bending it. Bending creates alternating stress cycles which breaks the paper clip well below its tensile strength. If we want the paper clip to break with fewer stress cycles, we can place a notch in the metal to increase local stress. Notches are used to cut glass and is how potato chip bags are made easy to open.

Propeller blades can be broken with alternating stress cycles and nicks and scratches create stress risers that lowers the number of stress cycles before failure. This type of failure is called "fatigue" failure.

Vibratory stress is just like the bending stress on the paper clip. With a small amount of stress the paper clip or propeller will never break. Increase the stress, and at some number of stress cycles, the metal breaks. Each propeller has a stress-strain curve (S/N) showing how many stress cycles until fatigue failure at each level of stress. This endurance strength of the propeller is a property of the aluminum forging and is the amount of stress the material will withstand in repeated flexing. Ideally, your propeller for your engine is designed so that the propeller can have an infinite number of stress cycles before its endurance limit is reached.

This design consideration assumes that the stress produced by your propeller and engine is comparable to the engine and propeller used to establish the stress levels for the engineer. Modify the engine or propeller and you don't know where your propeller's stress is on the stress-strain curve.  An accidental  modification, such as  a scratch, nick, or corrosion pit reduces the propeller's endurance limit such that failure may occur in a finite number of stress cycles. The propeller may last an hour or it could last two years until reaching its endurance limit and fatigue failure. Another method of reducing the endurance limit is by reducing the diameter of a blade which also changes the propeller's natural frequency.  If the natural frequency of the propeller and the engine coincide, a destructive resonate vibration may occur.

Metallurgically aluminum is just frozen slush--left alone it falls apart and returns to its natural state. The best thing you can do to extend propeller life is to take an active role in inspecting, dressing out nicks and scratches, and painting to prevent corrosion. Changes in engine performance should always include a new stress survey of the propeller. Periodic overhaul according to the manufacturer's recommendations is a time proven method of extending propeller life and safety. 

Now before you take-off take stock of yourself and see whether you are satisfied with your own mental equipment.  Patrol Sense US NAVY 1944

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Disclaimer: sacskyranch.com contains abundant information relating to aircraft maintenance. The information provided  is not intended to supercede or supplement the F.A.A. approved  maintenance and/or operator’s manuals. Those F.A.A. approved manuals must be utilized when performing maintenance and/or operating aircraft.