On 7/20/2022 7:34 PM, Jamie Lockhart via KRnet wrote:
Therefore I ask how the "correct empty C of G position" is readily
established in a modified aeroplane?Generally speaking, the aft CG
limit is driven by static stability requirements - such as stall/spin
recovery - and the forward limit by pitch authority considerations,
such as the landing flare.The Rand Robinson plans I presume specified
a CG range for the MAUW KR2 (900lb).
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The simplest way to determine correct empty CG location would be to use
a bit of reverse engineering. Build the airplane complete to the
firewall and then determine the desired location of the engine that will
give you the most aft CG location when considering arm and weight of all
expected loading. Make sure you stay in range with the extreme opposite
loading also.
The plans, at least my set from 1990, list only the CG range with no
regard to weight. They state the "structural design" is rated at 7 G's
at 800 pounds. The 900 pound gross, to my knowledge, is a limit imposed
by the authorities in the U.K. or somewhere in Europe. Willie Wilson
use to mumble about that a lot. The FAA has no similar restrictions. I
licensed my KR at 1350 pounds gross which gives it a "design" limit of
4.15 G's. I picked that number in case I was flying with a 200 pound
passenger and full fuel (25 gallon) and pull off a wing I would not be
over gross and violating any regulations. I considered it a CYA issue.
At that loading any baggage at all will put me beyond the aft limit.
In the case of the KR being as strong at it is, the critical issue is
not so much the weight but where it is located. 95% of my flights are
made at 1150 pounds with the CG at the center of the range. My KR came
in at 765 pounds empty 20 years ago, I now weigh in at 220 pounds, and I
normally fly (take off) with 15 to 18 gallon of fuel. My CG moves
forward with fuel burn, a maximum of 1 inch from full (25 gallon) to
empty. Even with the 24 inch stretch on my fuselage I can tell the
difference in pitch sensitivity with added passenger weight. My KR is
an AWESOME flyer when solo and the power of the 0-200 helps compensate
for the extra weight. Extra weight with less horsepower can be a real
performance killer.
Every KR is different in some way so don't rely on the performance or
the numbers achieved by other KR fliers. Make sure YOURS is correct and
test to determine what you can and can't do in your airplane.
Good luck to all.......
Larry Flesner
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