I was solo on this trip, Louise being otherwise occupied on the Gulf Islands National Seashore managing the oil spill cleanup efforts. She was going to join me in Reno (via SWA) at the end of the week, but I figured that since I had already freed up my schedule, I?d go out first. It?s wonderful having her support - she even suggested I go for a day of soaring down near Palomar, something I haven?t done for over a year. It?s been a while since I?ve done a long cross-country alone in the Val, so this trip gave me a chance to investigate a few improvements that have been made in the EFIS and autopilot software in recent months.
With smooth air, no weather and low winds, there wasn?t much to the task of flying, so I let the electronics take us westward. Two things piqued my interest along the way, one that I had noticed briefly before, and another that I hadn?t really thought about much. The first was an interesting pitch behavior that I am noticing now that the GPSS/GPSV gains between the GRT EFIS and the TT autopilot have been worked out. There was a strong tendency toward oscillations in pitch with the previous software, especially at aft CG?s, and that had precluded much evaluation work in that channel. But now that it is working well, I noticed a very subtle behavior which I nicknamed the ?Saw Tooth Cruise?.
Anyone that has done a good set of performance tests in Phase 1 should be familiar with the ?Saw Tooth Climb? ? a method of determining a lot of performance data quickly with a series of climbs and descents that look like a saw blade in profile. Well, the Saw Tooth Cruise is an odd behavior that is easy to miss if you?re not looking for it. Once you are settled down in cruise with GPSV holding altitude, it manifests itself as a very small, slow pitch in one direction, followed by a quick pitch back the same amount, but in the opposite direction. It is like a slow drift, followed by a quick correction ? and we are talking far less than a degree of pitch. In fact, if you have even mildly rough air, it would be completely masked by the chop. After watching this for awhile, I gave a blip of trim, and the behavior settled down. I then played with taking the trim a little farther in that same direction, and managed to get the saw teeth in the opposite direction ? whereas at first, the slow drift was a pitch down, followed by a quick pitch up, now the slow drift was upward in pitch. All the time, the airplane was trimmed close enough to neutral so that the autopilot was not calling for any trim action.
My theory, after watching this awhile, is that if the airplane isn?t perfectly in trim, it will tend to drift within the ?dead band? of the autopilot. Any measurement system has a threshold below which, it doesn?t know anything has happened. I think this is the slow drift. When it finally reaches a point where the system recognizes the deviation, it takes quick action to correct it. Very normal behavior, and a reason to get things in close to perfect trim before turning the flying over to ?George?- or sweetening it up once he?s on the job. That?s my theory ? I am ready for alternative ideas!
The other thing I played with on the trip was climb modes. The GRT EFIS will command the autopilot to climb, and you can have it work in one of two modes ? constant Vertical Speed mode, or Constant Indicated Airspeed mode. Generally speaking, I have always used the vertical velocity ? I can do the math for the distance I have to climb (or descend), and figure out the rate I need to hit my target altitude. Dial that in and watch it go. Jets frequently have an ?airspeed climb? mode, and I have tried to figure out how that might be useful in our applications. Certainly, if you have an airplane in a very long (high) climb in vertical speed mode, it will keep pitching up trying to maintain the rate, and unless it has stall protection (the GRT/TT does), it can get very exciting up high! So climbing on speed is a bit safer in that regard. But today, I found yet another good use for it ? engine cooling.
Climbing out of Casa Grande (south of Phoenix) with ground temps around 100 degrees, and an engine already warm from a long day of flying, there was no way I was going to get away with a 1500 fpm climb. Even 500 fpm was giving me an airspeed that yielded insufficient cooling, judging by the oil temps. But I knew that 130 KIAS seemed to be lowering the temps, so I switched the EFIS to IAS Climb mode, and took whatever rate I got. I had 100 miles to get to altitude for my mountain approach into Big Bear Lake, so I didn?t care how long it took. That worked really well ? the engine was happy, the airplane was stable ? and the pilot was happy because all he had to do was watch!
Smooth air on a long cross-country is rare, and it was good to take advantage of it to learn some more about this constantly-evolving software that we are getting in the Experimental market. I try to thoroughly investigate each new drop to see if they have any bad corners that I?d rather know about in advance. At this point, the GRT/TT combo is working really well ? it will be interesting to see what they come up with next!
Paul
With smooth air, no weather and low winds, there wasn?t much to the task of flying, so I let the electronics take us westward. Two things piqued my interest along the way, one that I had noticed briefly before, and another that I hadn?t really thought about much. The first was an interesting pitch behavior that I am noticing now that the GPSS/GPSV gains between the GRT EFIS and the TT autopilot have been worked out. There was a strong tendency toward oscillations in pitch with the previous software, especially at aft CG?s, and that had precluded much evaluation work in that channel. But now that it is working well, I noticed a very subtle behavior which I nicknamed the ?Saw Tooth Cruise?.
Anyone that has done a good set of performance tests in Phase 1 should be familiar with the ?Saw Tooth Climb? ? a method of determining a lot of performance data quickly with a series of climbs and descents that look like a saw blade in profile. Well, the Saw Tooth Cruise is an odd behavior that is easy to miss if you?re not looking for it. Once you are settled down in cruise with GPSV holding altitude, it manifests itself as a very small, slow pitch in one direction, followed by a quick pitch back the same amount, but in the opposite direction. It is like a slow drift, followed by a quick correction ? and we are talking far less than a degree of pitch. In fact, if you have even mildly rough air, it would be completely masked by the chop. After watching this for awhile, I gave a blip of trim, and the behavior settled down. I then played with taking the trim a little farther in that same direction, and managed to get the saw teeth in the opposite direction ? whereas at first, the slow drift was a pitch down, followed by a quick pitch up, now the slow drift was upward in pitch. All the time, the airplane was trimmed close enough to neutral so that the autopilot was not calling for any trim action.
My theory, after watching this awhile, is that if the airplane isn?t perfectly in trim, it will tend to drift within the ?dead band? of the autopilot. Any measurement system has a threshold below which, it doesn?t know anything has happened. I think this is the slow drift. When it finally reaches a point where the system recognizes the deviation, it takes quick action to correct it. Very normal behavior, and a reason to get things in close to perfect trim before turning the flying over to ?George?- or sweetening it up once he?s on the job. That?s my theory ? I am ready for alternative ideas!
The other thing I played with on the trip was climb modes. The GRT EFIS will command the autopilot to climb, and you can have it work in one of two modes ? constant Vertical Speed mode, or Constant Indicated Airspeed mode. Generally speaking, I have always used the vertical velocity ? I can do the math for the distance I have to climb (or descend), and figure out the rate I need to hit my target altitude. Dial that in and watch it go. Jets frequently have an ?airspeed climb? mode, and I have tried to figure out how that might be useful in our applications. Certainly, if you have an airplane in a very long (high) climb in vertical speed mode, it will keep pitching up trying to maintain the rate, and unless it has stall protection (the GRT/TT does), it can get very exciting up high! So climbing on speed is a bit safer in that regard. But today, I found yet another good use for it ? engine cooling.
Climbing out of Casa Grande (south of Phoenix) with ground temps around 100 degrees, and an engine already warm from a long day of flying, there was no way I was going to get away with a 1500 fpm climb. Even 500 fpm was giving me an airspeed that yielded insufficient cooling, judging by the oil temps. But I knew that 130 KIAS seemed to be lowering the temps, so I switched the EFIS to IAS Climb mode, and took whatever rate I got. I had 100 miles to get to altitude for my mountain approach into Big Bear Lake, so I didn?t care how long it took. That worked really well ? the engine was happy, the airplane was stable ? and the pilot was happy because all he had to do was watch!
Smooth air on a long cross-country is rare, and it was good to take advantage of it to learn some more about this constantly-evolving software that we are getting in the Experimental market. I try to thoroughly investigate each new drop to see if they have any bad corners that I?d rather know about in advance. At this point, the GRT/TT combo is working really well ? it will be interesting to see what they come up with next!
Paul
