Wayne and John,
My many years of experience with the S-Tec 50 as reflected in a file that
is over 1.5" thick, provides these observations:
1. The pitch board (regardless of version) is a dangerous, but
interesting, device:
a. One type of pitch board failure results in the
TC tach test pin to always "true" even if the TC has failed or is
electrically off. A possible preflight Pilot checklist test should
be to pull the TC circuit breaker and reset the AP. If it becomes
operational (green ready light), the board is bad. Note, S-Tec test
procedures will not detect this failure. If the AP is used during
this failed condition whilst in flight, the resulting death spiral is
interesting and demonstrates the usefulness of an AP disconnect
switch.
b. I know nothing about static lag, but a bad
static hookup can be avoided by leaving the sensor open to the cockpit
pressure (if the cabin is not pressurized). Since altitude hold is used
in stable cruise flight, it is responsive to the consistent cabin pressure,
merely changing no more than 50 feet when the cabin air vents are opened or
closed (thus demonstrating lack of "lag").
c. In my thick file is a page describing 34
different component configurations on the pitch board for modifying pitch
sensitivity. That is, simultaneous modifications to 7 or eight
components, not just a single resistance.
2. The S-Tec 50 occupies 3 valuable 3" holes in the panel (TC, DG and AP
Brain).
3. The TC gyro is tilted (as they are in all TCs) and turbulence
results in AP turns - sometimes as much as 5 degrees off the course.
4. GPSS (ARINC Roll Steering) requires the addition of another box.
Otherwise, the thing tracks the necessary 430W over-the-ground
course line like a pig after a truffle. Too much work to switch to
something else now.
Scott
Krueger AKA Grayhawk
Lancair N92EX IO320 SB 89/96
Aurora, IL
(KARR)
Pilot not TSO'd, Certificated score only >
70%.
In a message dated 10/17/2008 1:26:25 A.M. Central Daylight Time,
aflyer@lazy8.net writes:
One
thing I do know about the S-Tec autopilot is that the pitch algorithms are
extremely sensitive to any lag or delay in the static system. On certified
installations the “brain box” is required to be mounted very close (3
feet or less) to the static ports, and the autopilot must have its own
static ports, and not share them with any other instrument. Also not allowed
are the little bottles that are supposed to catch moisture in the
tubing.
Pitch
oscillations are the main symptoms of too much static
lag.
John
Huft
RV
spy
From: Lancair Mailing
List [mailto:lml@lancaironline.net] On Behalf Of Wayne
Bailey
Sent: Sunday, October 12, 2008 6:49 PM
To:
lml@lancaironline.net
Subject: [LML] Blue Mountain Avionics
Autopilot
Small tail 360 with Stec50 autopilot.
AP works OK to maintaining heading - but
pitch enters oscillations eventually becoming so severe
requires disconnection of AP after about 30 - 40
seconds. Incredibly there is no 'gain' adjustment in the
pitch axis of the STEC 50. I have read about replacing resistor
on board to modify gain - but a little uncomfortable with
this option. No slop in the linkage.
Have not dug into the servos yet - but
hoping there is a way to mechanically reduce the 'gain'
by extending the servo attachment point radially outward on bellcrank
- or ??
Alternatively thinking about installing BMA
EFIS Gen 4 system with new servo actuators. Anyone have any
experience with BMA gen 4 EFIS on a close coupled LNC2? I do know
there is gain on both axis.
Thanks again for the
help.
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