The most important point to remember for autopilot altitude
functions is that there be no “lag” in the static input. Any lag
will cause porposing, and the STECs are especially sensitive to this problem.
The STC’d install for STECs always include a dedicated static port.
If you have too many instruments hooked up to a single static
port (especially those pitot-static combined probes with the tiny holes) there
will be lag as the pressure changes. This lag will cause classic oscillations
in the control system math.
One way to address this is to just open up the autopilot static
input to cabin pressure, but it will (as has been noted) make it sensitive to
vent and cabin heat changes, and also have different errors at various
altitudes and airspeeds. The better way is to have a dedicated static port for
the autopilot.
John Huft
RV Spy
2 years part-time spent writing code for TruTrak
From: Lancair Mailing
List [mailto:lml@lancaironline.net] On Behalf Of Sky2high@aol.com
Sent: Wednesday, April 07, 2010 8:01 AM
To: lml@lancaironline.net
Subject: [LML] Re: fixes for S-TEC 50 alt hold
The important point to remember for those components that use
static pressure data is that it be "quiet". No airflow burbles
to upset what should be a solid reference point.
In a message dated 4/7/2010 8:41:42 A.M. Central Daylight Time,
colyncase@earthlink.net writes:
one other comment on the static source: the a/p
doesn't care how accurate the static source is, only that it changes with
altitude. Therefore some people have found that (on spam cans)
putting a dedicated static port in just for the pressure transducer works well.
...or you can just use the vents to fine tune your altitude....