A conversation with S-Tec while sitting in the plane at the hangar will
result in my performing some ground tests - but, note this -
Engaging hold on the ground sometimes resulted the stick moving forward.
While talking, I saw the trim lights (randomly) come on and a stick bump would
turn them off.
Sucking and blowing on the sensor generally resulted in down elevator
movement???????????
More tests today.
I will mention the "sensitivity" fix to S-Tec tech. This is probably
a good solution to alt hold oscillations, but there is something seriously wrong
with my A/P even though the pitch servo motor is new, the sensor is new and my
computer was checked by S-Tec within the last 30 days.
>>>>>>>>>>>>>>>>> (the
originating message is at the end of this missive)
More importantly, the sucking and blowing produced voltages of the same
sign at the servo. So.........................
With agreement from S-Tec, I sent my AP computer in once again.
After 10 days and two calls later, I was informed that S-Tec tests showed
the computer to be operating properly. In a state of exasperation, voice
moving up an octave and several Db in volume, I went thru all of the problems
over the last few years, including the prior $200 S-Tec ck that said it was OK
and our recent tests while the tech was on the phone with me (it is
important to stay with the same person). I said, "don't bother to send it
back without it being fixed," as I reminded him of his participation in the
tests with me. After having his memory jogged, he said that he would have
the pitch control boards replaced and would do so under warranty.
Yesterday I had a chance to install and test fly the AP with the new
pitch boards. After ensuring I had fine tuned the roll computer, I was off
into the mild blue yonder. All navigation functions were first
checked and then Alt Hold was engaged. Remember that elevator neutral now
had the servo set with the push-rod connector at 12 o'clock - max control
sensitivity. One minor oscillation and the altitude was held within 50
feet. The stick was bumped, pushed/pulled and the proper trim lights came
on, afterwards always returning to the "held" altitude. I even mis-trimmed
the elevator and engaged hold resulting in the AP settling down after a few
minor oscillations to the desired altitude. This is the best
that altitude hold has ever worked. I will need no adjustments to the
"sensitivity" unless the AP gets weird in turbulence, a function yet to be
checked.
Conclusions:
1. There are serious problems with S-Tec's ability to check the
proper performance of their autopilots. They have been informed of this
and whether anything comes of it remains to be seen. In this case, they
were unable to detect or note the sign (controls motor direction) of the voltage
sent to the pitch servo!
2. I believe that the pitch function was deteriorating over time -
eventually destroying the original servo motor ($600 to replace). This was
probably caused by randomly signed short burst low voltages being sent to the
motor even though "hold" was not engaged. Note that the motor can be
operated independently of hold having been selected. Heaven knows
what the computer was sending during the periods when hold
was activated but controlled the airplane poorly. Hmmmmm, now that I
think about it, maybe the motor wasn't destroyed -- If only the boards had been
fixed before I replaced the motor. Of course, I had to disassemble the
motor and gears - a curious mind must sometimes de-construct to learn.
3. The old board (circa 1992) may have had inherent pitch sensitivity
problems. S-Tec noted that there have been several re-designs of that
function.
4. Persistence pays off (to whom I don't yet know). $1600, 3 months,
numerous test flights, many "service" hours in the hangar, numerous phone calls,
special test equipment and an understanding tech finally got this thing
operational. Or, at least ready for a sale so I can get a Tru-Trak.
5. Note that I have a amateur-built break-out box suitable for testing
50-pin D-sub connections (9, 15 and 25 too). Fly-in sometime and we can
check your broken electrical stuff.
Scott Krueger
AKA Grayhawk
N92EX IO320 Aurora, IL (KARR)
Some Assembly Required
Using Common Hand Tools
UPS and a Telephone.
<<<<<<<<<<<<<<Original message
10-19-04
I have an S-Tec 50 (circa 1992) that has been updated over time and
utilizes GPSS via a G430. I have been having altitude hold problems all
summer and so far this fall.
But first, the main connector to the computer is locked together with a
spring-loaded latching mechanism. I have had connectors apart many time
and have installed my breakout box between the cable and the computer and I have
not seen a problem on the ground. Colyn and Gary, I hope that the
connectors are not the problem as other A/P functions are fine.
Here's where I am at:
1. The first thing I did was to clean the brushes and armature on the
pitch servo motor. It ran fine on the ground and in place although it
sounded different when run in opposite directions. It could be heard
running from about 3 to 36 volts. Higher voltage (up to 40 volts) is used
by the local avionics shop (JA) to clean the buildup of carbon and I have been
told this is good for about a year. Physically cleaning is a longer term
solution although I was recently told by S-Tec that the FAA does not allow S-Tec
to "overhaul" or service these motors - they must replace them with new because
they have been known to jam from a brush blocking the armature or a short across
armature segments. The motor should turn slowly at very low
voltages. The tech also told me that the brushes can be "hardened" against
chipping by heating them with a hair dryer until they smoke a bit, thus getting
all the moisture out. (I have not done this because ????). Anyway, the
next flight test provided no control of altitude - that is, hand trimming the
elevator was the only way to obtain relatively level flight.
2. There is no way to fully check out the system on the ground
because the accelerometer affects elevator management also. Thus, I
suspected the pressure sensor. Previously I had removed the pressure
sensor from the static system thinking that might be a problem. Using
cabin pressure made no difference. Note that I have carried a spare
altimeter aloft using the cabin air pressure which is generally .2 less
(200 feet higher than the external static source. OK, I bought the new,
improved sensor for about $800 and temporarily mounted it in the horizontal
orientation in a relatively vibration damped area using cabin pressure.
Drat, no difference!
3. S-Tec tech suggested there might be a problem with the computer -
so I sent it in. Returned with a clean bill of health for a mere
$215!
4. I cleaned and tested the motor again - this time after completely
removing the servo located in the battery compartment behind the co-pilot seat
and requiring that the battery be removed also. BTW, I am now utilizing
more nutplates on the battery hold-down and certain A/P servo mounting
brackets. After another failed test flight, I found that low voltage would
not turn the motor. So.... I ordered a new one (note that S-Tec will only
sell thru a distributor) for about a mere $600! The new motor will turn
even though I used a single AA battery.
5. Let's see, I have now invested about $1600. Why don't I just
switch to a Tru-Trak? I said to myself, "Self, you have a beautiful
fiberglass panel with the three large pilot side instrument holes (TC, DG and
AP) that would be replaced by a little airplane and a knob. What would I
do with those other big holes?"
6. I reconnected the original pressure sensor and went for a
flight. The first engagement of the ALt Hold function caused a rather
large climb and dive. The second engagement got me in a climb angle I
didn't like. OK, OK I did change something else too. I noted that at
elevator neutral, the connecting rod end was positioned at about 10 o'clock -
towards the nose. Thus, "large" movements of the servo would result in
small fwd-aft movement of the elevator pushrod. Hmmmm..... pretty much
de-sensitized. Thus, I repositioned the pushrod so that it was at the 12
o'clock at elevator neutral (having forgotten why I had set it up the way it
was).
7. My next two flights will be such that first the new expensive
(Gary?) pressure sensor will be checked and second (if necessary) the pushrod
will be returned to its' original postion. I will report the results and
any further conversations with S-Tec.
>>>>>>>>>>>>>>>>>>>>>>>>>