When Sensenich began the project of developing a metal propeller for the RVs back in 1989, we did not start out planning on a placarded 2600 RPM range. We developed four (4) different designs and found that each had an area of high blade stresses due to the crossing of a propeller natural frequency and engine output frequency.
In the end, and after extensive inflight vibration testing of all designs, we opted to market the 70CM because we found that even with the placard, the 70CM gave excellent performance even at the lower RPMs, thus better fuel economy. This has been validated by many builders who have purchased the propeller, and also by the increase in demand for the propeller in the last year.
Since the propeller was developed in 1992, we have tried three additional designs to remedy the placarded range. With all three we found that the blade section thickness changes needed to "retune" the propellers natural frequencies to remove the placard, resulted in propellers with lower performance. Vans helped us to test each of these and we decided not to continue with any of them.
Sensenich would like to remove the placard and will continue to investigate a a modification that could remove the placard. We are also planning a new vibration test of the 70CM on both hollow and solid crankshaft O-320s using a new telemetry system for the inflight vibration testing. This system combined with a new computer logging system will give us the best data on the vibration stresses. Our hopes are to more accurately define the high stress area and stress levels, which could result in the removal of the placard without modification to the current design.
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