Some folks may wonder how I can get away with 245 mph IAS in N56ML. My
ailerons are hinged a few inches back from the nose with separate 1/8"
double-shear brackets. The counterweight is a lead-filled piece of
steel tubing epoxied into a hole cut in the foam in the forward inside
edge of the nose. They are about 18" long, roughly, so it's evenly
distributed, and is quite well retained. They are a little heavier than
the plans version due to the shorter moment arm, but the location means
no draggy counterweight out in the breeze. The elevator is
counterweighted also. Having said that, Troy Petteway has both AS5046
wing and standard type aileron counterweights, and I know he's done 240
mph or better, probably quite often.
http://www.n56ml.com/99101051.jpg shows N56ML's aileron nose and hinge
setup (upside down), but this is before I added the counterweight
(inboard of the outer hinge). Apparently I never took a picture of the
counterweight process. For more on the wing construction, see
http://www.n56ml.com/owings.html .
Although this is a great wake up call to go check your counterweight
mounts for cracks, and for new builders to make them out of steel, I
don't think we should all worry a lot about this when flying our KRs at
reasonable speeds. Almost certainly, the vibration vibration Robert's
engine is what set it off, although it could be argued that it might
have happened anyway eventually, given that same RPM and some slop in
the control loop. However, I will say that we all owe it to ourselves
and our families to replace aluminum brackets with steel, at the
earliest convenience.
Since I'm now flying the same plane that lost its aileron balance in
2003, this incident is a reminder to me to replace those with steel. I
talked to Jim Faughn yesterday and he replayed the story....Steve
Alderman was flying formation with him at the Red Oak Gathering and saw
the weight fall off. There was no flutter, it was just fatigued, so
Mike Garbez and Jim made a new counterweight out of melted-down tire
weights and reinstalled it in place. He wasn't entirely sure if the new
bracket was aluminum or steel (since Mike made it), but that's even more
incentive for me to go check those and replace with steel, if needed. I
know the opposite side is 1/8" aluminum, per the plans, so my work is
cut out for me.
Aluminum is not the best choice for those brackets, given the long
moment arm between the weight and the bend. And given the lack of
detail in the plans, there's no telling what material they are made of
our how they were bent...."Attach .125 inch aluminum counter weight arm
with two 8/32" bolts and washers. Check that it does not interfere with
aft wing spar. Attach point should be approximately mid span of
aileron." That's it. I didn't see anything other than "Drawing 72",
which looks like that bracket may be 4" or more (no dimension given).
The drawing shows a sharp bend around the bottom of the aft spar. Of
course even if it was 6061, a guy with a vise and a hammer would break
it way before he hit 90 degrees, so hopefully it was at least a cutoff
of the firewall angle, which is something like 2" x 4" 1/8" thick
aluminum, but also with an unfortunately small inside radius, if I
recall correctly.
Hopefully the bracket is at least extruded 6061 with a radius, rather
than with a corner. I have seen 6063 angle that has no radius at
all....just a sharp corner, and the reason I notice it is usually
because it's already cracked! 6063 is called "architectural"
aluminum....nice and shiny, corrosion resistant, but not as strong as
6061. And since it's architectural, it's often made with sharp inside
radii. This is the kind of thing you buy at the big box homebuilding
store. I can tell the difference between 6063 because 6063 cuts like
butter when sawing or drilling in it, when compared to 6061. It's that
big a difference.
Folks, we need to share more of these kinds of stories to raise
awareness and spark more useful discussions that can improve the safety
of the KR line of aircraft. Missing balance weights are rare, and so is
flutter, but it happens, and if being aware of the cause and fixing it
before it happens saves just one guy and his plane, it's worth the time
spent to post that message.....
Mark Langford
[email protected]
http://www.n56ml.com
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