Jim, etal
I’m presuming that you suspect that the
nose gear shimmy might (perhaps) be induced by the behavior of the main gear. I
believe others have hypothesized this before also.
I do know that the gear legs on my high-wing
C172 can/do rapidly oscillate for-aft much more than one would normally imagine
during landing/braking (sometimes as much as 4-6 inches, or more) and the gear
legs are solidly mounted to the fuselage.
A video of an ES main gear (during
breaking/shimmy) would be very interesting. Perhaps you might be able to
just try a small/portable usb PC/Laptop camera taped to the ES step (and wired to
a laptop in the cabin). They are pretty small and relatively
in-expensive. Duct tape and a long USB cable would probably do the trick.
Recording audio from inside the cabin and calling-out deceleration speeds/events
would help correlate the two. You could also re-aim it and video the
front pant from the step on subsequent landings.
Has anyone with relatively high ES hours/landings
also closely checked the main gear receptacles for signs of fatigue/fracture in
the for/aft direction? It seems/appears the design is very stiff/strong
in that orientation and thus I don’t suspect much would happen to it –
except to transfer the energy to the spar bulkheads and fuselage. Ideally
most of it would get damped by the composite fuselage – but perhaps
not. If the energy/excitation were resonant with the nose pant’s Fn
it wouldn’t take much to get the nose pant/wheel going. Then, if
the front strut’s damping where degraded in any way (heat, seals, etc),
hold on.
FYI, I believe the Columbia main gear is quite different than an
ES. The Columbia gear mounts to the fuselage “behind” the
wing, since the Columbia wing is one continuous piece (tip-2-tip) and is mated to
the fuselage from the bottom (versus two ES wing halves mated into the sides).
I have never seen a Columbia
gear without the leg fairings. I also don’t know if the Columbia brakes are the
same/similar (I assume so)?
Also note that the factory ES (407L) has
an earlier/preliminary gear leg design. It’s gear sits much lower
than all of ours. Tim, would know if it’s legs are made from a metal
bar/plate (versus tapered tubes) and/or all composite (like my Corvette
suspension springs).
Rick
From: Lancair Mailing List [mailto:lml@lancaironline.net]
On Behalf Of Jim Scales
Sent:
Friday, August 24, 2007 4:53 PM
To:
lml@lancaironline.net
Subject:
[LML] ES Strut issues
I have just finished
the annual on my Super ES.
I discovered a cracked
engine mount within ten minutes of starting the inspection.
I have experienced the
"ES Shake" since the plane was new. It now has about 860 hours
on it. I have done various things to the plane to address the problem
through the years I have been flying it. Some things seem to improve it,
some don't make a difference. The plane has never been without the shake
completely. The shake occurs at the same speed (30-32 knots) on rollout
after landing and while on the brakes. Releasing brake pressure makes the
shake go away.
The following is a
list of what I did the the plane during this annual, and had done
previously, to help or eliminate the shake:
-Strut rebuild and
update by Lancair (second time)
-Engine mount repair,
update, reinforcement, inspection by Lancair
-New Cleveland brake discs
-Balance all tires and
wheels
-Precisely align main
gear both in toe and camber.
-Shim main gear legs
to reduce "play" between leg and mount.
-Shim spindles to fit
tightly to the gear legs
Test flight showed no
noticeable improvement in the shake.
My next step is to
mount a camcorder on the plane so that I can see the landing gear and attempt
to determine if the origin of the problem is the nose wheel or the main
gear. I am inclined to believe it is the main gear.
I don’t remember
hearing anyone describe how the shake affects the controls. The shake in
my plane causes the stick to move front to rear in rapid cycles. The
plane feels like the tail is jumping up and down. It would seem that a
nose wheel shimmy would cause the plane to move side to side. I
don’t think that is happening with mine.
I will post whatever I
learn from the camcorder exercise.
In a related area, I
don’t recall hearing of or reading about what Lancair has done to help
with this situation. It is certainly possible that I just missed that
part.
It seems that, with a
large percentage of the ES fleet exhibiting the same issues under the same
conditions, whatever they may be recommending is not working.
Perhaps they should do some more detective work (possibly check out some
of the planes that show the problems), and help with a fix. As I said,
maybe I missed the part where this had been done, but I don’t recall
anyone ever mentioning to me that Lancair had discovered the source of the
problem and determined the things that need to be done to eliminate
it. It would be impossible for me to believe the Columbia has this same problem. Wonder
what is different.
I know that I have put
in a great amount of time and no small amount of expense to correct it and it
ain't happened. I gather from my conversations with other builders that I
am not alone in those categories.
Just thought I would
share.............