SonexBuilders.net

For some propellers, the new Sonex spinner may not have adequate surface area because of its hollowed out feature. Why do I say this?.................I have a Jabiru 3300 on my Sonex along with an all wood Sensenich prop and it has always had an issue of losing prop bolt torque. In my conversations with a representative of Sensenich, he believes the problem to be a lack of sufficient surface area on the crankshaft flange and that the solution would be to place a crush plate between the flange and the propeller. I believe that he is correct.

If my calculations are correct, the crankshaft flange has a surface area of 14.6 sq. Inches (not enough). The Sonex solid spinner has a surface area of 19.9 sq. inches and the new hollowed out spinner has a surface area of 11.2 sq. inches, clearly not enough for my propeller as its surface area is considerably less than even the flange.

Perhaps my particular prop contains a "softer"wood than other Sensenich all wood props as not all other users of this prop have reported the same bolt torque loss, but some have. I also, at times, use a Prince carbon graphite coated wood propeller on my Sonex and it does not experience the bolt torque loss, with the exception of change of seasons.

YMMV, Art................Sonex taildragger #95............Jab3300 #261

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Have you tried using the Bellville washers on the prop bolts that Jabiru recommend. Its supposed to help that very problem. I was also surprised Sonex started hollowing out those spinners without at least supplying them with a solid backing plate.