Flt 100 Notes, 09 Jan 2011


#2 Cylinder Break-In
Trio Autopilot checkout at fwd CG

27.9 USG of fuel for 1555 lb/80.35" wt/CG at start.

The last few days have been marked by a continual succession of snow flurries, but they finally stopped yesterday evening. This morning the cloud was much higher, there was no snow in the forecast at Ottawa, and the radar didn't show any snow flurries around, so I headed to Smiths Falls. I set up the Garmin Aera 510 in my car, so I could see what the XM weather feed was showing as I drove to the airport.

I was troubled to see the XM weather feed showing an area of "snow" north of the airport, moving slowly towards the airport. The XM weather feed seems to have a finer threshold for snow than the typical aviation radar. I have found many times that areas shown as snow in the XM weather are just thick clouds. Sure enough, I could see an area of cloud not too far from the airport, but there didn't look like there was any snow coming out of the cloud.

The airport was in much better shape than I anticipated. I expected there would be quite a bit of snow to clear, but I found that the snow clearing crew had almost finished by the time I arrived. The airport is owned by the town of Smiths Falls and the Township of Montague and operated for them by the Smiths Falls Flying Club. Volunteers from the Flying Club supply the vast majority of the labour to clear the snow. They have an ancient Sicard snow blower and a large dump truck with a snow plow and wing. The club does an excellent job of clearing snow - they usually have the runway cleared within a few hours of typical storms. The town does help out if there is a really major storm - they send some of their snow clearing equipment over to the airport once they have the town's streets cleared.

I cleared the snow between the hangar and taxiway, then launched for some forward CG autopilot testing.

NO GPS Flt Plan - I changed the RS-232 serial settings in the GPS from "Aviation" to "Aviation No Alt". I found that now the autopilot works as described in the Trio docs. It shows a "NO FPLAN" message if the GNS 430 has a valid position, but no flight plan has been entered. CRS mode is available.

CRS - I changed the Max Turn Rate from "Auto" to "Man 3" (3 degrees/second). The auto setting is claimed to vary the commanded turn rate with speed to achieve a reasonable bank angle. I found that this resulted in too low bank angle at typical speeds. I was getting about 15 degrees of bank at 160 kt TAS, which gave a lower turn rate than I wanted. The "Man 3" setting gives about 25 degrees of bank at 160 kt TAS, which is quite acceptable.

ALT HLD - I have not yet found a mix of settings that gives acceptable altitude hold performance during turns at forward CG. The altitude will decrease about 150 ft during the turn, and recover to the desired altitude after the turn is completed. The Alt Hld gain is set to the maximum, and I have experimented with various VNAV Servo DB values, with no improvement. The performance was much better with a passenger last week. The amount of control input required to hold altitude during a turn is less at aft CG than at forward CG.

I thought that perhaps the control force required to hold altitude during the turn was too high, but an experiment with the aircraft trimmed nose up shows that this is not the problem. The altitude still decreased 150 ft during the turn, even when the trim was such the nose would pitch up if I disconnected the autopilot during the turn. It seems that the altitude hold gain is simply not high enough - i.e. the amount of control input for a given amount of altitude error is not high enough. But, I already have the altitude hold gain set to the maximum setting. I discussed this issue with Trio support last week - they suggested I change the gearing between the servo and flight controls so the flight controls moved further for a given amount of servo rotation. I am reluctant to do that just yet, as I get pretty regular "Trim Up Required", or "Trim Down Required" messages if the speed is changed. These will be even more frequent if the I reduce the leverage between servo and flight controls. I'll see if I can convince Trio to increase the range of allowable altitude hold gains via a software change.

GPS failure - The Trio docs claimed that the autopilot would roll wings level if the GPS signal was lost. But, I wanted to verify that, just in case the autopilot worked differently than this. I did two GNS 430 failure test points, one in a turn in CRS mode and one in a turn in TRK mode. The autopilot commanded a roll out to wings level in both cases, which is quite acceptable.

The low cloud north of the airport moved over it about 30 minutes after take-off, giving a 1500 ft ceiling. I slipped under the cloud and returned to the circuit, and did a bunch of touch and go landings. There was a bit of a right crosswind, with the wind tumbling over the hangars, which made it a bit choppy on final. My first two landings weren't very pretty, but the last two were much better.

GNS 430W RS-232 settings:

+--------+-----------+-----------------+----------------------------------------+|        |   In      |   Out           | Notes                                  ||--------+-----------+-----------------+----------------------------------------+| Chnl 1:| Icarus Alt|   OFF           | From GTX 327 Xponder                   || Chnl 2:|   OFF     |   OFF           |                                        || Chnl 3:|   OFF     | Aviation No Alt | To Trio Autopilot and data recording   || Chnl 4:|   OFF     |   OFF           |                                        |+--------+-----------+-----------------+----------------------------------------+

GNS 430W ARINC 429 settings:

+-------+------------+| IN 1: | OFF        || IN 2: | OFF        || OUT:  | ARINC 429  |+-------+------------+

Trio Autopilot Config Settings:

VNAV Servo Direction: ReverseHNAV Servo Direction: NormalMax Turn Rate:        Man 3VNAV Servo DB:        4

Trio Autopilot Preferences:

TRK Gain:      3CRS Gain:      4PI Gain:       9ALT HLD Gain: 60VS Gain:      50H Servo Gain: 35