Bits and Pieces
When You Bend a Propeller!
Would you think this can be fixed?
Newly Straightened Prop Being Checked for Cracks.
By Ian Brown, Editor - Bits and Pieces, EAA 657159
Breaking down my RV-9A flip-over into manageable pieces, starting at the front, I looked at the propeller and thought, "Well, that's toast." To my surprise, someone walked over, looked at it, and said, "I think it's repairable."
Apparently for fixed-pitch aluminium propellers, there are repair standards from folks like Sensenich that spell out exactly how much damage can be repaired safely. Among the obvious limits is if there's too much deformity at the tips of the prop such as what you might see with a severe prop strike where the undercarriage was still retracted on landing. In my prop's case, the magic number was 8.5 degrees. That is the maximum amount of bend allowed in the prop at the tightest part of the bend, and it's measured over a distance of two inches. If the angle changes by more than 8.5 degrees over that part of the prop, then it's irreparable. The service technician mounted the propeller and carefully marked the spot where he could see the tightest part of the bend in the prop. Then he marked two stations, each an inch either side of the first mark. He then measured the angle at both points and checked for the difference between them. By this time I was holding my breath.
You may be aware that Van's Aircraft prices for Sensenich props are fairly competitive, and Van's generally gets his clients great prices from third-party companies, but my prop mechanic said, "I think they're a lot more expensive if you have to buy a second one." I was starting to feel dizzy, and thinking about the mounting total cost of this accident. Still holding my breath, I glanced at his eyes as he turned towards me in what seemed like a slow-motion "No" sign in his eyes, but he spoke the magic words, "Seven degrees. You're lucky!"
Apparently there are two main ways of non-destructive testing of aircraft parts. One is known as magnetic particle inspection (Magnafluxing) and the other, especially for non-ferrous parts like aluminium propellers, is dye penetrant inspection. The propeller is cleaned of all paint and any dirt, and then soaked in a penetrating dye. It is then illuminated with a specific wavelength of light and inspected for cracks which would become very visible.
I got to pick the paint colours for the newly repaired prop. One of the reasons I've heard for people buying a wooden prop is to avoid damage to the engine during a prop strike. Having gone through this, I can understand the rationale, and maybe the engine mount might have survived if the prop had absorbed more of the energy of the impact, but as far as the engine is concerned, I think I would still have had to have it torn down and inspected for peace of mind, if for no other reason.
Next month, let's see if there is any news about the engine.