hgmckay
Joined: 23 May 2006
Posts: 397
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 Posted: Sun Dec 16, 2007 11:55 am Post subject: Electric Flaps - Incident |
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Everyone,
The other day as I and my instructor were flying my Allegro 2000, we had an
incident with the electric flap drive that I felt was worth sharing. Having
made a short flight (10 miles) to a different field, landed, and taken back
off, I had returned to my home field and landed, and taken off twice and was
landing the third time when the plane began to behave very differently on
final. I made the landing alright although it probably was not very pretty.
After we came to a stop my instructor and I looked at each other and we
agreed on "let's try that again".
Well, as I began my pre-takeoff checks and got to the point where I set the
flaps to take off position, I looked out the window as I turned the switch
to position 1, and the flaps did not move. They were in position "0". I
tried again and they did not move. I then noticed the flaps breaker on the
panel was tripped. I reset the breaker and tried again, and the breaker
tripped. Well that ended that days flying.
After returning to the hanger I checked the flap servomotor drive, and limit
switches. As I removed the protective cover I saw the drive was at the top
position, and the spring loaded metal trip plate was at the top micro switch
and slightly to the side. It was enough for the trip plate to miss the
switch. Some how the Potter & Brumfield breaker tripped the circuit before
the drive motor drove the vertical rod any farther. I was able to get the
motor to re-position the drive to position "1". The trip plate was still in
its original position on the worm gear drive unit, and tight. After trying
all three positions again I noticed that each time the flap switch was
activated and the motor torque began to drive the worm gear, the whole worm
gear drive rotated a slight amount. I knew about this earlier when setting
up the flap drive. In fact, I had earlier shimmed (with washers) the upper
forked end of the vertical flap rod where it fits over the bearing of the
flap lever to take the rotational play (there is some unless you shim it)
out of the rod. The shims were still in place and the rod was tight.
However, when I activated the drive I noticed some slight rotation of the
worm gear drive unit (the red cylinder) before the vertical movement began.
This occurred in the up and down direction. This slight rotation combined
with the narrow width of the spring loaded metal trip plate had allowed the
trip plate to rotate enough to slightly miss the micro switch. I don't know
why this rotational movement is occurring in the worm gear drive unit, other
than normal play between the gear drive and the worm gear.
To correct this problem I have fabricated a small metal piece that I have
riveted on the end of the metal trip plate that is wider than the original
trip plate width. This wider piece takes into account the slight rotation
that is occurring and assures that the micro switches do come in contact
with the trip plate, and properly trip the drive unit. Also, if one had to
replace one of these micro switches, does anyone know what brand, type,
model Number, etc., they are? Each of you might want to check your drive
units to see if you are getting this same type rotation (or play), and
determine if it could be a problem in the future. If any one is interested I
can post a photograph of the "fix".
Hugh McKay in NC
Allegro 2000
Rotax 912 UL
N661WW
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