Propeller Indexing

This was posted by Mark Langford after he had a broken crankshaft. He suggests that indexing of the propeller is an additional tool or safeguard that can assist in the longevity of the engine. He says that prop indexing to reduce the additive forces of peak combustion pressure and the bending of the crank during climb is worth doing, as are all the other things we do to minimize destructive forces on the crank. After all, it's absolutely free, and requires no more effort than NOT indexing the crank. The advice below that he posts on how to do so comes from Ken Smith of Teledyne Continental engines.

"For your two blade propeller, just index it 60 to 90 degrees from the #6 crankpin. On GA aircraft the prop is indexed in order to minimize blade bending stresses. The prop does not significantly affect crank bending stresses on GA aircraft. However, you will have noted that the front main bearings in a GA engine have a much greater effective length than in the Corvair crankcase. The latter is therefore more susceptible to crank/prop interaction."

More information, this from From the Sacramento Skyranch Engineering Manual and titled Operation, failure and repair of piston aircraft engines Written by John Schwaner:

"With four cylinder engines the prop can be indexed so the blade center lines are contained in the plane of the crank pins. This means that the prop blades are horizontal when any cylinder is at the top of the compression stroke. This is the point of minimum crankshaft rotation inertia. Historically, props are indexed 60 degress ahead of horizontal so the prop is in a convenient position for hand proppping. However, if you do not intend to hand prop the engine, this index configuration will minimize transmitted vibration levels. As the crankshaft rotates the moment of inertia changes. At dead center positions, small torsional oscillations can occur without any movement in the reciprocating mass. At 90 or 270 crank positions, torsional oscillations of the crankshaft result in almost equal movements of the reciprocating mass..... This also holds true for any other horizontally opposed engine with an even number of cylinders".

Note from WW - several certified aircraft and propeller combinations do it completely differently and don't heed the above-mentioned information at all. Additionally, Corvair engines using Warp Drive propellers have a different bolt configuration and don't allow for the approach presented above.