The Paper Clip and the Propeller
By Bernie Willis
How many times can you bend a paperclip before it breaks? Does it matter if it’s a large or small clip? If you bend it fast or slow? What you do know, is that it will break sooner or later.
Most of the materials in our aircraft have a fatigue life. That’s a time limit based upon their exposure to stress. As of this writing the Cirrus line of aircraft are limited to 12,000 airframe hours. I know of some Alaskan Cessna 207’s with 27000 hours and a few Bonanzas with 20,000 plus hours. Our airliners sometimes fly 150,000 hours.
Material science and the subsequent development has benefited all of us. When I was a kid climbing rock faces fascinated me. The lead climber pounded pitons in rock cracks from which we clipped a carabiner and ran a rope that protected them from a long fall. The pitons were used only once and often left in place. Then chromoly pitons became available. They cost more but could be pounded flat after use and driven into a new crack on the next climb. Chromoly steel is the basis of our current tube an fabric aircraft. It is strong but not brittle. Unfortunately, much of our aircraft are not made of this steel. Aluminum is subject to fatigue too. Think about the plastic tie wraps that may be holding wires in place near the engine. What about the delicate electronics in the expensive box that came protected by soft foam. Every part of our aircraft is really similar to the paper clip. It can be broken in time.
Pre-flights and annual/condition inspections should catch the obvious problems before they become dangerous. But what can we do to minimize material fatigue? The air is not always smooth and some landing are, well, unmentionable. But these are usually of short duration. The single biggest cause of stress in aircraft is vibration. At the museum I like to show our visitors the FOD probe on the B-737-200 along with the nose gear “sled” that allowed it to operate on gravel runways. Visitors get a good look at the engine fan blades and are amazed that they turned 27,000 RPM. Any imbalance from rock chips would cause a lot of vibration. The same is true on our piston driven engine propellers. It’s rare that one breaks in flight but even a tip coming off could cause the engine to come off its mounts and if lucky remain in the cowl.
Years ago a friend had a nice PA-12 but it vibrated no matter the RPM. The struts never settled down. The frequency was faster than a vibrating chair, really uncomfortable. I suggested we change the propeller just as an experiment. We traded props. The vibration followed the prop. Now my 12 was horrible to fly. The suspect prop looked OK but when the prop shop examined it, they found that the blades were pitched differently, not much but not the same. The bend was near the hub and unfixable. In the days of wood propellers the cautious flyers placed them horizontally when parked because moisture would remain or become equal on both blades keeping them balanced. When we made our own propellers they were statically balanced on a cone suspended from a string. An extra coat of varnish on the light blade would sometimes bring it into balance. Has a nick been dressed out of one of your blades lately? The manufacturers call for the nick to be removed and then the adjacent material removed at a 10/1 slope so a crack doesn’t develop. That could be a lot of material and maybe weight from only one blade.
While unbalanced props are the usual cause of vibration, engines built up without attention to piston and connecting rod weights can contribute also. Maybe a cylinder needs replacing and a new piston comes with it. How does it compare to the one it replaces? Also inside the engine are the crankshaft counter-weights. On some engines these weights are designed to move slightly to counteract internal vibrations and certain propeller combinations. Some of us have gone to three blade propellers for a smoother ride only to find it doesn’t always work out. Stuck counter-weights maybe the cause.
If your car tires are out of balance you know it and get them balanced. If you put your feet flat on the floor of your airplane for a few moments you can feel the vibration. The machine that measures this vibration translates it to inches per second. On one of my planes after a prop change it was at 1.5 inches per second, but after a couple washers were installed on the spinner backing plate the measurement went to .2 inches per second. I could really feed the difference. The proverbial paper clip was getting a rest for a longer life.