Today I tested higher altitude Turbo operation.
After a lengthy traditie, I departed runway 19 to the east. #5 CHT was highest before T/O roll at 389F. Climbing at 900 ft/min, at 118 Kias.
I departed southeast and climbed to 5500 ft msl, and made some measurements. During the climb, I noticed less MP oscillation amplitude, but still present. Once at cruise, I sped up but noticed that the MP still had dropped a little more than 1,5”, but the speed increased somewhat better than normal, now indicating 149ktas. I stabilized the flight and with the help of the autopilot I noticed very small MP oscillations while level straight and level. The RAM Air inlet does seem to build some higher pressure, and some more laminair flow to the turbo compressor side, making for better performance even if a little better efficiency.
After a climb to 6500ft msl, I noticed that the MP dropped to 26.0” and remained steady within 0.2”. Very stable air made for a very smooth flight, which may result in a steady MP as well, but I believe that the turbo improvement has effect.
I descended and landed at Murfreesboro TN MBT, using 3/4 of the 4K ft long runway, and visited a friend who was holding a Young Eagles flight day. After an hour plus, I restarted the plane without any problems and taxied to runway 18. Runup was completed without items, and I departed south. At wheels up, the engine seemed to momentarily overspeed, but rpm remained at 2680, and no changes in any other engine indications. I wonder if the turbo kicked in for a short boost, but can’t be certain.
I did climb with 1300 ft/min, at 108kias, which seemed more than usual. I leveled at 2500 ft and turned northwest towards Smyrna, holding 28.0” MP and 2500rpm giving me about 145ktas.
I did get a new problem I had not seen before. The G3X noted a HDG comparison fault. The copilots hdg was 30 degrees off from the rest, and remained like that. Later I also noticed that the attitude was not correct. Next flight I will have to check if this continues.
after a short hold I entered the class D and setup for LH downwind 19. I landed and could have taken runway 32 for taxi.. my landing technique is improving, and I will start to slow another 5 knots to see if I can reduce landing roll even more while still have good controllability during short final.
In conclusion, after three flights with the new setup, is that performance is enhanced but not quite where it should be. The next step might be to replace the turbo controller and check the electrical turbo system once more, to double check its working as it should.
But then I dove back into the Piper Aztec engine maintenance manual. One item that I had not paid much attention to, because I clearly didn’t understand the workings of it, was the sense valve that is operated by the throttle in max setting. I copied the following passage :
Reading this, seems to indicate that the valve needs to operate in order to get the 28.5” MP. If it doesn’t, I seem to understand that the controller doesn’t sense enough and uses ambient air pressure, which drops the manifold pressure even if the turbo is operating. This seems to match what I am seeing, some higher than ambient Manifold Pressure, but not the 28.5” it should hold until critical altitude.
Later, it also describes how it creates a lower critical altitude and power and fuel efficiency. This all seems to match what I am experiencing. Also the manual warns against replacing controllers before adjusting this switch.
Sieders Family Velocity Aircraft 