I got back home Thursday evening from three weeks of flight testing in Wichita. It was a very busy test program, working long hours, six days a week to hold schedule. But, we got it all done, and now I get a breather at home for a week or so before heading on the road again.
The Ottawa forecast was for good weather on Saturday, and it certainly looked good at home, so I headed to Smiths Falls. I was dismayed to see the cloud get lower the closer I got to the airport, and it looked to be very marginal VFR by the time I got there. I have up any thought of heading to Maurice Prudhomme's Fly-In on the Ottawa River. I didn't make it, but RV-9A builder Alfio did, and his photos show the weather was much nicer there.
I removed the spinner and adjusted the coarse pitch stop on the prop while I was waiting for the cloud to lift. I've known for quite a while that the coarse pitch stop was set much coarser than needed. This is an aerobatic prop, which means it goes to the coarse pitch stop if the governor stops sending oil pressure to the prop dome. This ensures the rpm does not increase if oil pressure is momentarily lost while doing aerobatics, but it also means a prop governor failure will cause the prop to go to maximum pitch. I did some testing a while ago, and determined that the pitch would go so coarse that the engine would be drug down to about 1700 rpm, which greatly reduced the power available. The rpm would fall even further as the airspeed decreased, and it looked questionable whether the aircraft could maintain level flight.
I thought I would need to have a prop shop adjust the coarse pitch stop, but I learned a few days ago that this was actually adjustable with the prop installed on the aircraft. There is a threaded rod with two jam nuts extending from the front of the prop dome. Of course it was bigger than any wrench I had, so I zipped into town and bought a new tool. I loosened the jam nut, rotated the main nut three turns CW, then tightened and safety wired the jam nut.
The cloud was breaking up by the time I finished lunch, so I went for a short test flight (brake release to 8000 ft in 4 minutes and 12 seconds - not bad). I discovered that the rpm with full throttle and prop control full aft was now about 2025, and I could maintain level flight, but I wouldn't want to subject the engine to full throttle at that low an rpm for a long period. There was still quite a bit of rpm control in hand at VNE, with an rpm of 2475 with prop control full aft at VNE, so I decided to move coarse pitch stop a bit more and try again. I adjusted the coarse pitch stop another 1.5 turns after landing.
Ottawa Terminal didn't respond to any of my radio calls during this flight, and another local pilot told me after landing that my radio transmissions were carrier only, with no audio modulation. Drat. I know it was working fine earlier in the month, so this is a very recent problem.
I did some quick radio/intercom/headset troubleshooting, but found no obvious cause of the issue. Unfortunately all my avionics manuals and wiring diagrams were at home, so I couldn't really dig too deep into things. After studying the docs later I have come up with a long list of things to check, but the first order of business is to properly identify the problem. I'll get someone to man the Flying Club radio as I try front seat transmissions on Com 1, Com 2 and also Emergency Mode on the Intercom (which supposedly bypasses the intercom, tying the front seat headset directly to Com 1). Then I'll try rear seat transmissions on Com 1 and Com 2, then change headsets and repeat it all again. The results of that matrix should help me narrow down the list of things to check next. Hopefully I can easily find and fix the problem, as I won't be able to do much flying until I sort this out.