Hey Scott, I can relate to this one as, same thing happened to me many years
ago when the cable attach at the control arm broke due to being rusted away
on a Fly Baby I was flying. Glad I was on down wind when it happened and was
a handful but, made it down OK.Sparky Sparks
-Original Message-
From: Jeff Scott via KRnet
Sent: Thursday, November 06, 2014 3:19 PM
To: krnet
Subject: KR> Part 1 - Flutter
Things have been a bit quiet on here for a while, so I'll relate a couple of
stories I was involved in a couple of weeks ago.
Part 1 - Flutter
I flew my SuperCub to Casa Grande, AZ for the Copperstate Fly In a couple of
weeks ago. Note that my SuperCub is amateur built and has a trim tab on the
elevator (like our KRs) rather than a jackscrew for the horizontal
stabilizer like a Piper Cub. I came as close to having to make a forced
landing as I ever have in 38 years and several thousand hours. I was doing a
nice down hill descent towards St Johns, AZ with lots of nose down trim when
suddenly the elevator went into a violent flutter. At first I thought I has
thrown a prop tip. Then I noticed the engine seemed to be OK, but all the
violent action was happening in the tail and was being transmitter through
the control stick. I slowed to MCA (which is pretty darned slow in a Cub),
but that didn't seem to help. I had concluded a failure in the tail was
imminent and was setting up to make a forced landing. My non-pilot
passenger in the rear seat suggested I try pulling in the flaps to see if
that would make any difference in the ongoing flutter. I pulled in full
flaps, which seemed to reduce the frequency and amplitude of the flutter,
although it was still quite violent. I decided to press on another 6 miles
to the airport flying at fence top level. Short version of the story is
that I landed successfully at the airport.
So what happened? My elevator trim tab was designed using a single
push/pull bowden cable connected to an overhead lever in the cockpit, much
like the Aeroncas. When flying with nose down trim, the trim tab is pushed
up a bit more than the elevator. I remember feeling an odd back and forth
motion in the stick which turned out to be the trim tab working up and down.
The bowden cable that was pushing on the trim tab eventually work hardened
and broke at the trim tab. This allowed the trim tab to go into flutter.
The trim tab flutter acted like a servo and drove the elevator into a
sympathetic flutter. It was violent enough that you could see the shock
waves from the flutter propagating down the length of the lift struts and
wings. Half of the rivets holding the trim tab to the elevator had also
failed while the tab was fluttering. I was able to locate a new core for
the bowden cable and the rivets to re-attach the trim tab in a town about 30
miles away. I send a friend to pick up parts for me in his speedy RV-8
while I inspected the Cub for cracks and damage.
I found no cracks or damage, and my friend in his speedy RV was back in a
flash with parts in hand. An hour and a half later, the Cub was repaired, a
test flight was performed, and I was on my way. Then more issues arose that
I'll write about in part 2.
Lesson learned. I looked at several Aeroncas while at Copperstate. They
didn't use a push/pull bowden cable like I thought. They used two lighter
cables in a pull/pull arrangement for trim. In fact walking around the ramp
at the fly-in, I found no planes with a push/pull cable, although a number
have a push pull trim with a Ray Allen servo and threaded rod (like I have
in my KR). The trim cable will be removed and electric trim servo installed
in the near future. In the mean time, I don't trim the Cub nose down for a
speedy descent. Speedy in the Cub is still painfully slow anyway. :o)
Part 2 coming up this evening...
-Jeff Scott
Los Alamos, NM
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