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ALTRAK pitch system flight report

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Old September 21st 03, 08:15 PM
optics student
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Default ALTRAK pitch system flight report

I installed the ALTRAK standalone pitch axis autopilot made by TruTrak
Flight Systems in our Glasair 1.
This augments the already installed STEC 30 roll axis autopilot. Making up
the wire harness that connects
the servo to the computer to the aircraft instrument panel is
straightforward and took 2 hours. I am very slow
and carefull doing electrical work. I hate to see smoke when I power up for
the first time.
You only have to mount in the instrument panel a little illuminated push
botton that lights up ALT when engaged.
Actually, this switch was too large to mount conveniently where we wanted it
in our already full panel so
I used a little Switchcraft momentary contact push button and a miniature
LED indicator right above it.
The autopilot consists of two components,
1) A programmer that has inside a little MEMS gyro (all solid
state, no moving parts) and a pressure transducer to read altitude. It is
about the size of a box of playing cards.
2) A servo motor that is attached to a fairly heafty aluminum baseplate that
contains within it the gear train that conveys
stepper motor motion to the actuator arm. A small drive control box is also
attached to the baseplate. These
drive electronics take input from the programmer and translate it into the
proper motor control voltages to take the right
number of steps.
I asked TruTrak to send me an 18" long push-pull rod to connect the servo to
the elevator bellcrank. The 10" one supplied
was too short for my intended mounting location. It arrived free of charge
in about 2 days. Great service. My pitch servo
is mounted at the forward end of the control tunnel, mounted to the tunnel
left side wall. This places it right behind the seat back,
which is what I wanted since the motor weighs a few pounds and I wanted to
get it as far forward in the plane as possible.
It probably would have been MUCH easier to mount it on the other side of the
elevator bellcrank, between the
bellcrank and the tail, but I didn't want it that far back. The control
tunnel on the Glasair contains two rudder cables, two
elevator trim cables, and two elevator push-pull tubes. Along with the
mounting hardward, pulleys, etc, basically it is packed.
Finding a place for the servo that
didn't interfere with anything took some planning. The programmer is
mounted to the seat back close to the servo and
connects with a short length of the harness. The installation must be
planned so that the servo arm does not come close to
over center when the elevator is at the hard stops for both full up and full
down elevator. This is VERY important from a
safety perspective. I don't feel I needed stops at the servo arm for this
The wires going from the components in the back to the instrument panel are
just the 12V and ground wires and the
wire from the engage switch and the wire from the engage light. About 8
Thats why I put the programmer in the back next to the
servo. There are about 10 more wires going from the programmer to the servo
and I didn't wand to run all those
wires back from the instrument panel.
Both the programmer and the stepper servo get 12V, but the servo draws an
amp or so and the programmer very little, so
the servo power wire should be about #20 and all the other wires can be #22
or #24. The ALTRAK wiring diagram shows both the
programmer and the servo getting power from the same circuit breaker, which
could be fed from the avionics master switch.
I.E. when the avionics master is on, the programmer and servo are powered up
and read to go when you push the engage
switch. But I wired it differently. The programmer wants to be in a still,
non-moving condition (no pitch motion)
when first powered up for 10 seconds or so to initialize the gyro. So
states the instruction manual.
That seems to rule out powering up in the air from a cold off state whenever
you want to use it, since the airplane will probably NOT be perfectly still
in pitch. So, I wanted the programmer to get
12V when the avionics master switch is turned on just after engine start on
the ground. While I am tuning the radios and getting
the ATIS, it initializes. It is now ready whenever I need it since it get
power continuously during the flight.
But, I DON'T wand the servo motor to see 12V power until I
want to use it. That way, in a pitch emergency if the programmer is
making the pitch servo do something crazy I can cut power to
it immediately. So, the 12V to the servo motor comes off of the Autopilot
switch in the panel currently feeding the STEC 30. When I
flip that switch, both the STEC and the ALTRAK servo get 12V. Now when I
push the ALTRAK engage button, the servo
motor does it's thing. In reality, I cannot think of a case where I would
want to fly with the pitch servo but not the roll servo.
The other way around, sure, if I am climbing or descending on some heading.
But I don't want the airplane to hold altitude if
I am flying it in roll manually.
After doing the install and I checked everything on the ground (you have to
carefully get the mechanics right so that the servo
moves the elevator in the right direction, up elevator when the aircraft
climbs to counteract it and vice versa).
You can actually use a jumper wire to reverse the motor direction, but
simply turning the servo 180 degrees
accomplishes the same thing. You cannot, however, rotate the programmer,
since this will correct the servo motor motion
for gyro pitch changes, but it will now do the wrong thing for pressure
changes. One side of the programmer comes from the
factory marked "THIS SIDE MUST FACE RIGHT". Remember, the autopilot first
flys the
pitch axis to hold the gyro steady, but uses pressure data to actually hold
altitude. This is the key to its stability.
In control systems lingo, it's called an inner and outer loop.
After climbing to 3000 and trimming for straight and level, I switched on
the STEC roll autopilot first and set the
DG bug to a heading and had that autopilot hold heading. I then punched the
ALTRAK engage button. Nothing
happened. I couldn't feel anything change, didn't see anything change,
didn't smell anything. Trying the move the
stick revealed it to be locked onto the STEC in roll and the ALTRAK in
pitch. So something did happen.
A minute later, nothing changed. I was still
at EXACTLY 3000. Holding altitude in a Glasair is a full time job. Even
with the trim set the best a human could
possibly do it, sitting for a minute with my hands in my lap would find the
Glasair at something a little different than
3000. I put my pinky finger lightly against the stick in the pitch
direction. Motion was just barely detectable. I rotated
the DG heading bug 90 degrees from the present heading. The aircraft banked
into a standard rate turn and lost about
50 feet as the turn got established but the ALTRAK quickly regained the lost
altitude. Rolling out straight and level
the plane climbed maybe 25 feet but ALTRAK corrected downward in short
order. Over 1/2 hour in straight and level
flight and the foot hand on the altimeter never budged more than +/- 10
My final thoughts. This thing is awesome. Works just as advertised.
Absolutely no pitch porposing. There are even
three electrical sensitivity settings. Low, medium, and high servo
activity. Ours is set to medium and I have not experimented
with low or high. Seems to work just fine as is.
Recently, before this thing hit the market, I investigated upgrading our
STEC 30 to the 40 which has a pitch axis servo.
STEC would not sell it to me, stating that they have not been able to fly
the pitch axis of the Glasair without excessive
porposing. I have no complaint with their roll axis, however.
We also the the GPSS module from STEC that connect between the Garmin 430
and STEC autopilot that flys the whole
flight plan, with turn anticipation. Now, I need to do a long cross
country, enter the flight plan in the Garmin and punch
altitude hold and fly by wire the whole way.
I would recommend this ALTRAK to anyone.

Old September 21st 03, 10:09 PM
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"optics student" wrote in message
news:[email protected]
I installed the ALTRAK standalone pitch axis autopilot made by TruTrak
Flight Systems in our Glasair 1.

I would recommend this ALTRAK to anyone.

Jim in NC

Old September 21st 03, 11:49 PM
optics student
external usenet poster
Posts: n/a

Price is on their website at
http://www.trutrakflightsystems.com/...ts.html#Altrak I don't work
for the company
and they don't pay me to be a salesman so I didn't give it out.
This price is $200 more than what we paid. Looks like they raised their
prices in September.
They told me they were going to.

"Morgans" wrote in message

"optics student" wrote in message
news:[email protected]
I installed the ALTRAK standalone pitch axis autopilot made by TruTrak
Flight Systems in our Glasair 1.

I would recommend this ALTRAK to anyone.

Jim in NC


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