On 10/16/2019 9:56 AM, n357cj via KRnet wrote:
I have been on this net for at least 20 years now and I can't even think of how many times
lengthing has been discussed. So many times the answer was "add a bay". I really thought
this is heading for trouble for someone someday. adding a 14" bay to the rear certainly
changes the W&B. lacking a common reference point it just is unusable information.
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Adding length to the tail could certainly change the W&B but that can be
negated by other changes made to the forward end of the fuselage /
engine area. That being said there is a "common reference point" and
that is the C.G. range stated in the plans. The plans call for a C.G.
range on the standard KR2 as being 15-35% of the wing cord or 8"-16" aft
of the inboard wing leading edge. That's based on the size of the
horizontal stabilizer and elevator , the elevator deflection angle, and
the distance of the two aft of the C of L (center of lift). It doesn't
matter what datum you use to measure from - the C.G. must fall in that
range for a controllable aircraft. That range is pretty standard for
similar shaped wings. I've not heard or read the range of the new wing
but it was assumed to be similar and can be verified with testing. That
goes for the 2s also as I don't recall a new C.G. range was ever stated
for the stretch.
Adding length to the tail (longer arm) or increasing the size of the
horizontal stabilizer / elevator could possibly increase that C.G. range
but I'm not an engineer and not capable of running the numbers to
establish what change in C.G. can be handled safely. Rule of thumb says
that increasing to tail volume or arm should give you more pitch
authority but actual numbers are left to the aircraft designer. We have
over the years gone with that assumption and it seems to have worked out
o.k.
Given all that, it seems that whatever datum you use to calculate your
C.G. , if it falls in the design range you're probably in good shape.
My KR is stretched 24"s with standard KR2 tail feathers. When I fly
solo, with full fuel, my C.G.falls in the exact middle of the range and
it flies great. However, with Mark Langford in the right seat and all
our "stuff" for Oshkosh, it was in the rear area of the C.G. range. It
was controllable but quite pitch sensitive and not a lot of fun to fly.
I'm not sure what that adds to the discussion but that's my story and
I'm sticking to it.
Larry Flesner
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