AIRPLANE FLIGHT MANUAL's

[6643]

I can't put my fingers on the proof, but my research led me to believe that there never was a restriction on the C90/McCauley combination. Now, if I could only land a copy of SB12....

[a64pilot]

Prop avoid ranges are often due to a resonance that a particular engine can set up in a prop at certain RPM’s as well as the wind and can be aggravated by the airframe, stiffness of the engine mount etc. It amounts to I believe a vibration that the engine or wind etc. can produce that will correspond to a natural resonance frequency of the prop.

Of course change things and the vibe survey the prop manufacturer did is invalid, I’ve talked to Hartzell Engineers and for example they hate electronic ignitions, as many can vary timing to some extent, and this timing change can and does wreck props, particularly C/S props.
High compression pistons etc can do the same thing.

It’s actually an enormously complex problem, however Hartzell for example has apparently collected so much data that they can most often approve a propellor based on analysis. I’ve gone through several prop Certifications and we never had to do a vibe survey, we had to do a whirl flutter test once, but that was with a big motor and big prop.

This AC can give you an idea just how complex it is.
https://www.faa.gov/documentLibrary/med ... 20-66B.pdf

Propellor vibration analysis is believe it or not is an on going science, whirl flutter and reactionless mode are fairly recent discoveries, brought about after several fatal accidents.

Reactionless mode for example is a vibration set up that is cancelled out in the prop. in fact a pilot will feel or hear no vibrations at all, but the prop is tearing itself apart.

Sometimes flying an old design isn’t a bad thing as just about everything that can happen, has happened and been dealt with.

The Experimental crowd with swapping props around, changing to high compression pistons and electronic ignition are often playing with fire and most have no idea what could happen, and most don’t even know they should call and ask.