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propeller work


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It's Winter. Having a wooden prop means a re-torque. While taking off the prop I was going to take the opportunity to crop it a bit and reduce the pitch a little......It's 69 x 54 which is too much pitch. I need about 200 more revs. Anyway, take the prop to the workshop. crop the tips by 3/4 inch and compare the cross sections now revealed. Shock/horror. about 5 mm difference in thickness!....If the tips are so different, what about the meaty bit? So I draw "stations" along the length of each blade at 50mm intervals. I compare thicknesses and camber high points. plus the "flatness" of the bottom airfoil......My goodness what a mess! The "flat" lower surface is almost semi symetrical!..The high point of the upper surface wanders from 30% to 55% of the chord. Mr Sweetapple must have had an off day when he made it....After 4 days of sanding. I have depitched the blades somewhat. Got the undersurface reasonably flat. Got the highest point of the airfoil at 38%-42% as far as possible. Now t0 recover the prop with glass. Anybody got an idea what weight of glass to use? I suspect 80g per sq mtr....Does anybody actually know?????



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