I used the Nyrod trim tab system per the plans. On the take-off run on my
first flight the system failed and put in full nose up trim just as I nudged
the stick to lift the nose. Estimate the speed was about 65 knots at that
point. The aircraft immediately pitched vertical. I over corrected with
forward stick and the aircraft pitched straight down. I recovered from that
dive pulling 2.6 g. Ground observers said I cleared the ground by about six
feet. I had to hold about ten pounds forward stick to maintain steady climb
and level flight. Trim tab control in the cockpit had zero effect on the
trim. After landing found the Nyrod sheath had broken at the forward part
of the horizontal stabilizer. The slightest aft stick movement would move
the trim tab to full nose up and would stay there. Fixed that with two Adel
clamps at the aft end of the sheath.
Moral of the story: Bowden cable sheath must be firmly attached at both
ends of the sheath.
Sid Wood
Tri-gear KR-2 N6242
California, MD, USA
----------------------------------------------------------------------------
I would avoid trying to control a trim tab with a single bowden cable. I
did that on my SuperCub Clone per the Wag Aero plans. When I trimmed nose
down, the cable was pushing the trim tab up. The bowden cable started
flexing, then broke, leaving the trim tab flapping free in the breeze which
brought on quite a case of tale flutter. Slowing to minimum controllable
air speed did not moderate the flutter, which was quite violent. The Cub
has a braced tail structure, but even with the bracing, the tips of the
elevator were moving roughly 6" vertically while the trim tab was in
flutter. You could both feel and see the shock from each flutter cycle
propagate through the air frame and out the wings. You could quite
literally see the shock waves moving out the length of the wing from the
violent shaking in the tail. Pulling in full flaps changed the air flow
over the tail enough to moderate the flutter enough that I was able to make
my way into the nearest airport where I mad!
e temporary repairs. After I returned home, I stripped the tail and
installed a Ray Allen trim servo. I also found a few cracks in the
elevator, likely a result of having flown for several minutes with the trim
tab in full blown flutter.
I know of numerous other failures of a single bowden cable on the trim tab
with similar results. I recommend avoiding that type of configuration.
-Jeff Scott
Cherokee Village, AR
------------------------------
I?m adding an autopilot and will need trim availability. I?m thinking
Bowden cable and trim lever.
A Bowden cable (manual push-pull arrangement) is exactly what's called
out in the plans. This was sold by Rand Robinson in the form of a
"ny-rod" plastic pushrod in a plastic housing, straight from your local
RC model store. This is a somewhat low-friction design, but it quite
springy due to the low stiffness of the housing and flexible pushrod.
This leads to a very springy trim tab, which as Jeff pointed out, is not
a good thing. And after 20-30 years, the housing cracks and you have
nothing at all!
I second the Ray Allen trim system. It's quite stiff, by comparison.
There are some details on my trim tab installation at
http://www.n56ml.com/trimtab/index.html. This setup easily handles all
of the CG changes that I ever need on N56ML, with probably half the
range unused. I'm happy to keep that in my back pocket though, as you
never know when landing without an elevator or something similar will
make me thankful that I have the capability.
One point to make on the Ray Allen system...use shielded cable to run
the wires from the switch and display units back to the servo, and
ground one end of the shield, and avoid close proximity to the radio and
transponder antenna cables. When keying the mic, the trim position
display LEDs goes bright and dim, and eventually it fries them! My
middle three LEDs are cooked, and it's because those three LEDs have
been lit while the transmitter was broadcasting. The same thing
happened on N891JF, and I've heard several other folks have had the same
problem. Keep those wires shielded and isolated from RFI (radio
frequency interference).
It's not a big deal to not know exactly where the trim tab is anyway, as
it's so easy to overcome by stick pressure anyway. It is kind of nice
to set it neutral on takeoff, as the plane will takeoff by itself with
no stick pressure from the pilot. That's good advice that Troy Petteway
gave me for my first flight...."just keep it centered on the runway and
when it's ready to fly it will lift off and climb. You shouldn't have
to pull back on the stick to make it climb...it will do that all by
itself".
See http://www.n56ml.com/trimtab/index.html ....
Mark Langford, Harvest, AL
ML "at" N56ML.com
www.N56ML.com
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