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Re: conclusion to weight and balance discussion
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Rob Brooke
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Oct 28, 2009 06:04 PST
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Hello, Cookie,
The wing chord as measured on my rib pattern is 43 1/2 inches (3' 7
1/2"). The wings, both upper and lower, are swept back 6°. As it turns
out, the ranges fore and aft of the loaded CG _are_ contained in the
Tiger Moth construction manual, but in disguised form. Whoever wrote the
manual ran two illustrative W&B calculations, one with a 150 lb. pilot
(rear seat) and one with a 240 lb. pilot. The front seat is empty and
there is a full fuel load in both upper and main tanks. The CG for the
150 lb. pilot is the magic 29.93" and for the 240 lb. pilot, the magic
38.69". nowhere is it stated that these are the CG limits for the
airplane but, according to Gene Hanson, they are. Kinda lets the builder
know he has to gain or lose weight, but only if he knew these were the
limits, which he doesn't from the manual alone.
I understand your comments about the datum and how using the firewall is
perhaps unusual. However, in the case of a plane with swept back wings,
it does not serve simply to say, "Use the leading edge of the wing as a
datum.". The firewall should work fine. A vertical line dropped to the
main LG hubs would work fine. Anything works fine if you know how to do
the calculation.
As to the specification of the CG range in hundredths of an inch, the
real problem is not the hundredths of an inch, it is that the CG range
(loaded) was not really specified as such, only a couple of example W&B
calculations. The hundredths of an inch are the outcome of the example
calculations. If you wish to take issue, take issue with the fact that
the CG range is _not specified_ in the construction manual!
About little Johnny and his 50,000,002 years, I think that little Johnny
is one bright kid and should be praised for his acumen rather than
scorned for superfluous accuracy.
Rob Brooke
Tiger Moth #45
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