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I've got a bunch of comp time saved up, and my schedule opened up when good weather occurred this week, so I went flying three times during the week. This time of the year you need to take the flying opportunities when you can get them.

Monday I did some more autopilot testing (flt 101), trying to find a combination of settings that gave acceptable altitude hold performance in turns at forward CG. It works marginally well at shallow bank angles, but I prefer to have the autopilot use 25 to 30 degrees of bank. There is typically a 150 ft altitude loss in a turn with that bank angle.

I also did a test to confirm that the autopilot automatically disengages during the takeoff roll if it happened to be engaged. It disconnected at about 40 kt, just as advertised. I set up the minimum and maximum speed protection at 70 kt and 190 kt respectively. I did a low power climb and a high power descent, both in vertical speed mode, and found that the minimum and maximum speed protection worked as expected.

I called Trio Avionics late Monday afternoon to discuss the altitude hold performance with Chuck. He said that there were several other RV-8s flying with their autopilot, and no one else is complaining about the performance. He also said that they design the system to use about 15 degrees of bank during turns, and accept up to 30 ft of altitude loss. I'm seeing 40 to 50 ft of altitude loss at 15 degrees of bank. We discussed the possibility of Trio changing the upper limit on altitude hold gain - I had already tried the current maximum value of 60. Chuck also reported that one user had found poor performance which was eventually blamed on turbulence form the static port. On that aircraft the interconnection between the two static ports was plumbed close to one of the static ports, and the performance improved when the interconnection was moved halfway between the two ports. Chuck hypothesized that different pressures on the two sides of the fuselage caused flow between the two static ports, and this caused turbulence in the line which affected autopilot performance. He suggested taping one port over for a quick test, as this would prevent any cross flow between the two ports. The other option is to move the pushrod pivot on the elevator bellcrank closer to the bellcrank's pivot, which would increase the amount of elevator movement for a given amount of autopilot servo movement.

Thursday I spent the whole day at the airport. First I did a short flight (flt 102) with one static port taped over, and found no appreciable difference in autopilot performance. Next I removed the aft baggage compartment to provide access to the pitch servo and drilled a new hole in the elevator bellcrank. The new hole gives about 50% more elevator movement for the same servo movement. I did another flight (flt 103) after this change, and didn't see much of an improvement.

Thursday I also gathered some cruise performance data with the wheel pants off. The wheel pants have a tendency to fill up with snow when operating from snow covered runways. The heat from the brakes may cause some of the snow to melt, and the water can freeze on the brakes during flight, causing the wheels to be locked on landing. It is a pain to put the wheel pants on and off as the conditions change, so most RVers just leave them off all winter.

Saturday I flew again (flt 104), experimenting to determine the best way to use the autopilot when manoeuvring, given its performance. I'll set up the autopilot to do shallow turns. If I want to make a sharper turn I'll press and hold the autopilot disconnect switch - if I hold it for more than five seconds the autopilot goes into what Trio calls Pilot Controlled Steering mode. It will reengage when I release the switch, and will then hold the current ground track and altitude (assuming the vertical speed is less than 200 ft/mn when I release the switch). This allows me to easily intervene manually to fly the turns, but have the autopilot do the straight and level bits. This allows me to get most of the workload reduction I hoped for when manoeuvring in instrument conditions.

I also gathered some more wheel pants off cruise performance data. I've got some more work to do to finish analyzing this data, but the initial look seems to show the aircraft is 8 kt slower at the same power than it was with wheel pants on.

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I was in Phoenix, AZ for the week, stuck in meetings developing design standards for Synthetic Vision Systems and Enhanced Vision Systems.

I got to the EAA Chapter 538 monthly meeting Tuesday evening, where I ran into Mike P., who has a very nice RV-6 with a turbo-charged O-360. He bought the aircraft, but added the turbo after the original engine had a catastrophic failure. He offered to take me up for a flight, and we did just that late Thursday afternoon. The aircraft performs very well, but he really should get an oxygen system so he can take full advantage of the great tailwinds that are often present between 15,000 ft and 25,000 ft.

Thanks for the flight Mike!

Mike's aircraft has a very nice (and heavy) instrument panel and interior.

Strangely enough, his aircraft also has logo lights in the outboard ends of the horizontal stabilizers, shining on the side of the vertical tail (installed by the original builders, not Mike).

I didn't get flying this weekend, but hope to take a few hours off work Monday afternoon.

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Flt 100 Notes, 09 Jan 2011

Purpose:

#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

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I've got a Trio Pro Pilot autopilot driven by a Garmin GNS 430W.

The autopilot behaviour when there is no GPS flight plan differs from what is described in the Trio docs. There are supposed to be two GPS-related autopilot control head messages. "NO GPS" means that no GPS information is found by the autopilot. This is expected to be shown if the GPS is OFF or has failed, or if the GPS does not have a valid position. "NO FLTPLN" means that valid GPS information is detected, but there is no flight plan or Direct-To, so there is no defined track line to navigate along. In this case CRS mode can be used, but TRK mode is not available. But, on my autopilot, I found that "NO GPS" is shown when the GPS has a valid position, but no flight plan has been entered. The "NO FLTPLN" message is never seen.

I called Trio this evening, and the guy I spoke with said that the autopilot behaviour is different depending on whether the GNS 430 has the WAAS upgrade or not. With the original GNS 430, the autopilot behaves as described in the manual. If no GPS signal is received, you get a "NO GPS" message. If GPS info is received, but the GPS has no flight plan, you get a "NO FLTPLN" message.

But, once the WAAS upgrades hit the field, Trio started getting reports of "NO GPS" messages even if there was a GPS signal, unless there was a GPS flight plan. Garmin claims there were no changes in the info sent by the GPS, but something must be different. Trio says that switching the GPS's RS-232 format to "Aviation No Alt" has been reported to help by some users. I currently have "Aviation" as the RS-232 format. I'll try "Aviation No Alt" next time I go flying.

This bug isn't a significant problem, as the autopilot will most often be used when on a cross country flight, in which case a GPS flight plan will certainly be present. And for a local flight, it only takes a few seconds to do a Direct-To somewhere if I want to use the autopilot. I don't have to actually want to go to where ever the Direct-To is, as I could use CRS mode to fly in any desired direction.

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I got another flight off this morning, this time with a passenger, so I could do some mid-aft CG autopilot testing:

Flt 99 Notes, 03 Jan 2011

Purpose:
#2 Cylinder Break-In
Trio Autopilot checkout at mid aft CG

Ran engine at about 75% power with mixture set for 100 deg ROP at 2500 to 4000 ft.

Had about 191 lb of self-loading ballast (thanks Ian!), and 36.6 USG fuel, for 1800 lb/84.47" wt/CG at start.

I had been concerned that the large difference in longitudinal stick forces between forward CG (solo) and aft CG (with passenger) would mean that no single set of autopilot pitch gains would work for both cases. The results from this flight suggests that it is very likely that this is not a problem.

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 | 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:        AutoVNAV Servo DB:        4

Trio Autopilot Preferences:

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

Trio Autopilot MSN 316VF
Trio Autopilot SW 100612

The following autopilot gain checks were conducted in relatively smooth air:

ALT HLD - 75% power cruise, wings level, and turns of 15 and 25 degrees of bank. The altitude hold performance was very good at all times.

VS - climbs and descents at 500 and 1000 ft/mn were performed. The selected vertical rate was changed during the climb or descent. The autopilot promptly and smoothly captured the target rate with very little overshoot.

ALT SEL - the selected altitude was smoothly captured with minimal overshoot.

CRS - wings level cruise was excellent. Turns only used about 15 deg of bank angle, and the turn rate was further reduced during the last 30 degrees before rolling out. Turn performance should be further assessed with various settings of the "Max Turn Rate" config value.

TRK - track performance was excellent.

GPSS - excellent turns to intercept a flight plan track.

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We spent Christmas in Wisconsin, visiting some of Terry's sisters, and got back home late on the 30th.

Yesterday afternoon the weather was looking pretty good here at home, and not so good later in the weekend, so I decided to try to get up for a quick flight. The ceiling was lower at Smiths Falls, but it looked to be good VFR, so I launched. The cloud bases proved to be about 1900 ft, and it was bumpy underneath with 75% power set for the cylinder break in. The autopilot handled the bumps well, but the ceiling was too low to do the climb and descent testing I had hoped to do, so I landed after 35 minutes.

After landing, given that it was quite warm, I decided to troubleshoot the Narco 122D VOR/ILS which had died sometime two or more flights ago (I noticed it wasn't working two flights ago, but hadn't looked at if for several flights before that, so I don't know when it died). I checked the fuse, and it was OK.

I had been working behind the instrument panel to install the autopilot, so there was a chance I had somehow dislodged the wiring. I dove under the panel, and found the ground wire partially pulled off its connector on the ground bus. I may have caused this when I was messing around with the ground wire for the autopilot. I also found the connector on the back of the 122D was a bit loose. I pushed the ground connector back in place, and reseated the connector on the back of the unit and now the Narco 122D works just fine. The good news is that it was a cheap fix.