RE: follow up/Re: Grain Direction in Moulded Prop Blades

From: John Barker <john.barker783_at_ntlworld.com>
Date: Mon, 2 Jun 2008 14:03:37 +0100

Thanks for the enquiry and further advice on the props. I will first
explain what they are for. Our Indoor Tech Committee is trying to attract
more modellers to indoor flying and to this end have set up a Challenge to
improve a beginner’s model, called the ‘Gyminnie Cricket’, and to have a
competition for the improved models towards the end of the year. The basic
GC is a heavily built indoor model of 15” span with a motor stick and a tail
boom and one of those horrible plastic prop assemblies as used on ‘Sleek
Streaks’. The wing area is the same as an EZB but the tailplane area is
only 25%. The improved GCs must retain the same wing and tail outlines and
the same motor stick and boom length but can be lightened and can use any
propeller. The tricky bit is the small tailplane. If you make a very light
propeller you may have to carry ballast in the nose because moving the wing
backwards too far creates stability problems.

I started with the 10” diameter prop on the right of the photo. I needed
about 0.5 g to keep the correct cg position so I only thinned the blade wood
to about 0.027 and moulded it with the grain in line with the spar.
Although the face of the forme (as on the LH side of the photo) was flat the
blade still finished with about 8.5% camber at the widest part of the blade.
Too much in my opinion. In use the prop seems to work fine in the limited
hall time that I have had.

I decided to try a larger, 12” dia, propeller, and that was when I posted
the original query. The blades were sanded a little thinner, because of the
increased area, and both propellers finished at 0.49g. This time the grain
was skewed and the blades gained no camber! I hoped for and expected some.
Also, as the blades are still quite thick, I only carried the spar part way
up the blade; this was probably a mistake because the blades are now gaining
some elliptical dihedral at the tips!

I think the propellers will work quite well – after all I built an A6
recently and they fly very well even though the propellers appear to be an
abomination. However I am not happy because I don’t feel in control. The
blade angles come off the forme as intended but the camber seems to be a
lottery.
John



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Received on Mon Jun 02 2008 - 06:04:12 CEST