Prop bolts don't fit

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Prop bolts don't fit

Postby lutorm » Sat Dec 09, 2017 9:36 pm

So I'm trial fitting the props on our new Force One hub (we have a Sensenich and a Sterba one) and it turns out neither fits. They don't fit on the old Aerovee hub either, so it's not the Force One that's wrong. It appears the wood has expanded in the cross-grain direction, because the hole circle has larger diameter in that axis such that the bolts don't go through.

I know humidity affects the thickness, Sensenich says 1% humidity difference changes the thickness by 0.01" or 0.25mm, but it seems surprising it would make enough of a difference to make the bolts not fit. It makes sense though, they are maybe 1mm off and since the radius of the prop circle is roughly the same as the thickness of the hub that would only require a ~4% change in humidity. I could easily imagine such a change between the manufacturing location and here in Hawaii.

Has anyone experienced this? I could put the propellers in a dehumidified room and see if they shrink, but then they will lock the bolts once they get outside on the airplane so you won't be able to torque them easily. It seems reasonable to enlarge the holes enough that they don't bind on the bolts under any humidity condition, since the bolts should never be loaded in shear anyway.

Edit: I just found this study: https://www.fpl.fs.fed.us/documnts/fplrn/fpl_rn309.pdf, which documents exactly such substantial uneven dimensional changes. It also shows the equilibrium moisture content of Sensenich props at different humidity levels, and with our average humidity of about ~80% it would be 16.5%. Just going down to 60% drops this value to 11% and at 40% RH it's 8%....
lutorm
 
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Location: The Island of Hawai

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