Mike Stirewalt wrote:
> This "aft CG" question can be a force for good or evil. Take it up and
> see how it flies before you make any drastic decisions.
I'm sure Mike means "within reason". I can tell you that 2" aft CG can
easily be fatal, especially if there's nothing you can move to improve
it before landing.
When I picked up Jim Faughn's KR from Steve Bennett, the first thing I
noticed (once I got to altitude and could test it) was that when I let
go of the stick it headed dramatically down and to the left, really
shocking me. The reason I did the test is because of all the back and
right stick force that I had to apply to keep it on heading and
altitude. It was probably more manageable in pitch when Jim flew it,
because he apparently flew with some up pitch applied by elevator trim
tab. When I went to pick it up, Steve and I were looking it over (it'd
been a long time since he'd flown it) and the trim tab was just flopping
around free....the "nyrod" housing had crumbled, so we went to the
hardware store and bought some long #2 screws and big washers, and I
installed those in the joints to force the tab into the same plane as
the rest of the elevator. This was done because I didn't like the
thought of elevator-induced flutter!
When I got home I replaced the nyrod trim cable housing, and now I fly
with up trim back there to make the plane hands-off in pitch. Weight
and balance showed the plane was very forward CG, as Jim had set it up
right at the front with only him in it. I'm even lighter than Jim, so
when I fly it with nothing else in the plane, I'm flying with a forward
CG, and I'm fine with that. No drama with a forward CG, and I decided
it was just fine like it was! I also added a "fixed" trim tab to the
offending aileron, and that fixed the tendency to bank sharply when the
stick is released, and now the "neutral" nyrod trim tab is really
neutral. No doubt, a bit slower, but now I have a lot of tolerance for
adding weight aft, if ever needed. And I'm just too lazy to "fix" it by
moving the engine aft. Besides, the next owner may be be perfectly
centered.
I realize that you can find all kinds of stuff on the web (to
corroborate any point of view), but below is the first thing I saw
regarding pros and cons of forward and aft CGs, at
https://studyflying.com/forward-cg-load-forward-v-s-aftward-cg-load-aftward/
Forward CG
(Pros) positive directional stability – when the side surface greater
aft than ahead of the CG
(Pros) Higher longitudinal stability and better in stall recovery – an
aircraft becomes less controllable, especially at slow flight speeds, as
the CG is moved aft. The recovery from a stall in any aircraft becomes
progressively more difficult as its CG moves aft. Failure in stall
recovery can lead to spin. Indeed, the most critical CG violation would
occur when operating with a CG that exceeds the rear limit.
(Cons) Higher stall speed – stalling AOA is reached at a higher speed
due to increased wing loading.
(Cons) need for greater back elevator pressure – the elevator may no
longer be able to oppose any increase in nose-down pitching.
Aft CG
(Pros) Faster cruising speed – produces the most efficient overall
performance (but it would also result in instability)
(Pros) Lower Stalling speed – “nose-up” trim is required less to
maintain level cruising flight. This means produce a lighter wing
loading. Also, this requires a lower AOA of the wing, which results in
less drag
(Cons) Poor in stall recovery
(Cons) Poor response in sideward force (Such as Cross wind)
Keep in mind that my previous story about my real-life (near death)
experience with aft CG was with the CG only 2" aft of N56ML's aft limit!
Mark Langford
[email protected]
http://www.n56ml.com
Huntsville, AL
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