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Re: conclusion to weight and balance discussion
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Rob Brooke
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Oct 29, 2009 09:37 PST
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Hi again, Bob,
Just for grins, I ran some numbers based on some quick and dirty
measurements of my plane. Using 30% MAC, I wound up with the following
measurements based on distance from the firewall:
Leading edge of upper wing - 4"
Leading edge of the lower wing - 29.25"
Avg leading edge ((LE lower-LE upper)÷2) at root - 12.625"
30% MAC from leading edge of wing chord - 13"
set-back of leading edge at wing mid-point at 6° sweepback - 6.898"
12.625"+13"+6.898" = approx 32.5"
This does not take into account the difference in incidence of the two
wings (upper wing 2° greater), the lift provided by the center section,
the interaction of the two wings nor the effect of the horizontal
stabilizer/elevator. However, it does provide some comfort that the CG
location provided by FFP is rational. I have no views as to the 8" range
of the CG being too large. Since my very loose estimate of the CG based
on MAC puts the CG nearer the forward edge of FFP's range than the aft
limit, I would be happy to make a test flight if 32.5" aft of the
firewall was the result of my W&B. Fuel burn will move the CG aft
somewhat, but both tanks are sufficiently close to the CG that the
change would be very modest.
I thank you for suggesting that we look into establishing the CG through
the use of MAC calculations. I don't think that it is an inconsequential
task to get a totally accurate CG that way for a biplane, the effects of
decalage and stagger being really imponderable, but at least I have been
able to reassure myself that FFP's CG range is not completely off the wall.
Rob Brooke
Tiger Moth #45
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