Thorp Air Command - T18.net

Thanks Lee.

This is making more sense now. And I've found an article on Kitplanes that spells it out well: https://www.kitplanes.com/static-port-l ... libration/

That article describes a Wheeler Express that was built with a Piper pitot/static mast. All was fine until years later when a Garmin glass panel / autopilot update was added. The autopilot would continually hunt in pitch. Disconnecting the static line at the ADAHARS stopped the hunting, and the autopilot worked perfectly. But the airspeed and altitude were off. So the author describes the process for determining the optimal location for static ports on the fuselage sides. It's a lot of work.

Seem the Piper pitot/static mast is not well suited, at least in some airplanes, for the static source when a sensitive ADAHARS unit is controlling the autopilot.

I'll explore some options- but it would seem like cabin pressure would fluctuate with vent changes, and who knows what else. Guess I'll experiment some with other static sources....

Thanks again,

Peter

On a side note, my TruTrac autopilot would climb on altitude hold every time I keyed the comm radio. Finally moved the antenna to the bottom of the airplane and solved the problem.

_________________
Bob Highley
N711SH
SN 835
KLAL

I've flown a few flights now with the static sourced from cabin air, under the pilot's seat. The Garmin autopilot is rock-solid now- the altitude is captured / held and doesn't appear to vary at all. It's working really well, in any case. But there are the expected errors in indicated airspeed and altitude.

I flew along side a buddy in his Grumman Tiger so I could check for the error. We both flew at an indicated 4500 feet. My cabin-air-static-sourced Thorp was about 150 feet lower than the Tiger's, and indicated just a few knots fast. But, slowing into the pattern, I had the feeling that the indicated airspeed error was growing. I really should have taken it up and stalled it- but I landed with some extra indicated airspeed. I think the increased angle of attack at slower speeds might be lowering the cabin air pressure somewhat. I'll modify (plug the rear hole) the current Piper mast so I can use the static side as an AOA source, since The Garmin G3X system has AOA built in- I just need to plumb it.

I suppose it's not that big of a deal, but I'd prefer to have airspeed and altimeter values that don't vary (much) from reality depending on speed and configuration.

I may fab up a static tube (closed and rounded at the front, with small holes drilled through the tube) and mount that so that it is below, and in front of the existing Piper pitot-static mast. And when that idea fails, I guess I'll put the ports on the side of the fuselage. Maybe. At least then when getting a pitot-static certification every couple of years, they know where to connect the test equipment.

Sorry for the dissertation....!

Peter

This is all interesting reading to me. Please keep it up.

FWIW when I installed my older model TruTrac autopilot the installation instructions recommended installing a dedicated static source to eliminate any possible pitch problems in "altitude hold. I installed a dedicated static port on the side of the A/C . To date it holds altitude perfectly !

RB
NX115RX

Thanks for the info, Rich. It's another data point the suggests I need a new static source.

Until I figure out that new static port location, I'll use cabin air. Autopilot is solid with that setup. Airspeed and Altimeter, no so much... but maybe livable.

I've also begun the setup / calibration of the AOA built into the G3X. I've modified the existing Piper pitot-static mast by plugging the tiny hole on the back of the mast, and then used the static line from that mast to the GSU 25 (AHRS) AOA. That appears to work well.

Thanks again,

Peter

So,

As suggested in the kitplanes article, I fabbed up a "movable static port," and positioned that on the fuselage side per the plans indicated location. And then flew the airplane.

That location, on my airplane, appears to be a low pressure area- the airspeed was reading high. And the autopilot pitch hunting was back. Ugh! But I was able to calibrate the built in AOA indicator, (using the modified Piper pitot static mast) so maybe I can develop some trust in that display.

I think I'll do as others have, and just leave the Garmin GSU 25 (ADAHRS) static port exposed to cabin air. The airspeed indicator and altimeter will be inaccurate, but possibly no worse than some other airplanes.

Thanks,

Peter

Peter,

I have been noodling on this and it occurred to me that a simple orifice in the static line is all you need. The orifice will act as a damper for the airflow the oscillates back and forth within the static line.

Regards,

Jim

Thanks for the input, Jim.

I did, finally, put static ports on the fuselage sides, per the newsletter recommendation. And I plumbed in an "alternate" static source switch- venting the static to the cabin air. The autopilot porpoise is nearly gone, but still perceptible- regardless of static source- the fuselage sides, or the alternate static. The best result, in terms of autopilot pitch / altitude hold was with nothing connected to the Garmin GSU 25 ADAHRS.

If your suggestion is to add a small hole in the static line- near the ADAHRS unit, I'd be up for giving that a try. I'll do it and will report back.

Thanks again,

Peter

I have a pitch oscillation in altitiude hold mode particularly with turbulence. I tried it with the alternate static port open (it's behind the panel) it didn't make any difference. At Oshkosh I talked to the Garmin folks and they suggested to redo the pitch gains. On the way home I tweeked the pitch gain a bit, went from 1.5 to 0.8 and the oscillation was reduced so that I didn't really notice it. When I get some time this week (hopefully) I'm going to redo my pitch gains.

_________________
Bruce Finney
N18JF T-18C #262
Yelm, WA USA

I'll be interested to hear what ultimately cures your pitch oscillations, Bruce.

Garmin was pretty adamant that my porpoising could be cured by the pitch gain settings. But I had already exhausted that avenue before opening my ticket with their G3X team. Still, I tried again to fine tune those settings. At best, I could minimize the porpoise; I could not eliminate it.

But with the static line disconnected, the altitude hold (and the other vertical modes) were rock solid. I flew from California to Oshkosh with that configuration, and the autopilot performed perfectly. This morning I did an IPC flight that included two GPS RNAV approaches to 200' All I did was manage power. This autopilot really performs- it's precise and smooth! I'm very happy!

Please share what you find, Bruce.

Thanks,


Peter