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In a message dated 1/30/2006 12:17:05 A.M. Central Standard Time,
rpastusek@htii.com writes:
First, with about 400
hours on the aircraft, he has considerable wear on the right aileron and right
elevator hinges, as well as some play in the right aileron linkage. The
combination allows visible flutter of the right aileron during high speed
flight. He has not yet opened the linkage access panel to see what's worn in
side, but will do so this week. When discussing hinge replacement, I advised
him that some folks installed Teflon-lined hinges that were supposed to reduce
wear. Could you provide recommendation as to the effectiveness of this, and a
source for this material?
Bob,
The common material for "teflon" pins (ss welding rod and teflon spagetti)
is slightly smaller than ordinary hinge pins. The looseness is actually
worse as the teflon can flow a bit over time. Chris Zavatson and I use SS
welding rod that is slightly larger than ordinary hinge pins and find no loss of
smoothness or increased friction (3/32 dia, I think. Note that a ream was
used to clean up the hinge). This will greatly reduce system lash.
Additionally, I rig my ailerons slightly up for a small air load so that
they do not bounce around in the very low force dead band at neutral.
Even with the larger pins, I still see wear on the right elevator hinge - I am
investigating airflow and airframe vibration harmonics (plus a too wiggly tail)
to try to resolve the elevator condition.
Second, the airplane can
not be trimmed to level flight at high speed. With the trim full down, the
stick must still be held forward to keep the nose on the horizon. We discussed
installing a stronger spring on the trim system, but I told him we should ask
the experts about this first. What's the best way to fix
this?
Did he shorten the elevator bell crank by 1 inch (a common mod)? If
so, forces are raised (throw reduced) and a stiffer spring is required.
Also, the spring stops can be moved closer together to increase force although
the trim could be maxed out anyway. Is he using the cruciform trim
mechanism that relies on friction to hold the arm position? If so, the
stiffer spring requires a greater friction and more effort to move the trim
arm. Also, the trim grip on the elevator push rod could be moved slightly,
but one must carefully check that there is plenty of up elevator for the
so-called flare.
Another aspect is where the flaps are reflexed to during cruise. If
set too high, the nose pitch up forces are very pronounced. The flaps will
move higher in flight than the angle set on the ground -- just pick them up by
hand after they are rigged and that is where they will be during flight.
He could try to slightly extend the flaps (reduce the reflex) and check the
effect on speed and trim.
Scott Krueger
AKA Grayhawk
Lancair N92EX IO320 SB 89/96
Aurora, IL
(KARR)
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