This post documents a “gotcha” - hopefully some other folks will find this and avoid having to spend as much time troubleshooting as I did.

When installing my new Aero Technologies PCU-5000X propeller governor, I found that the governor control arm travel was large enough such that the prop control in the cockpit tended to hit one of its stops before the governor control arm hit its stop at the governor. I finally managed to get things adjusted so the governor arm would hit both the high and low rpm stops just before the prop control in the cockpit hit its stops. But there wasn’t much margin between the cockpit control and its stops.

When I got the aircraft flying, I noted that as the engine warmed up, the maximum selectable rpm would drop by more than 100 rpm. I pulled the cowlings, and confirmed that the governor control arm was hitting its max rpm stop before the cockpit control hit its stop. I discussed the problem with Aero Technologies, and they suggested that perhaps there was an internal oil leak between the governor and the prop. As the oil warmed up, the viscosity would decrease, and more oil would leak, which could impair the ability of the governor to control the prop. I’ve got an aerobatic prop, which defaults to coarse pitch (low rpm), and the governor sends oil pressure to the prop to force it to low pitch (high rpm). Thus an oil leak in the prop control path could cause the rpm to be lower than selected.

I used the procedure in Lycoming Service Instruction 1462A to confirm the oil leakage at the front of the engine was acceptable, but I couldn’t check the oil leakage at the back of the engine without pulling the prop governor. The access to the governor is poor, as it is on the back of the engine, surrounded by hoses, wire bundles, etc. So I put that task off while I did more thinking.

One day, in flight, I did some more experimenting. I recorded the max selectable rpm and the oil temperatures as the engine warmed up. Then, later in the flight, I fully opened the oil cooler door, and cooled the oil down, then checked the max rpm again. I found that the max rpm was lower than it had been earlier in the flight at the same oil temperature. There had to be another variable. Hmm.

I then hypothesized that perhaps the problem was due to differences between the coefficient of thermal expansion of the prop control cable and that of its sheaf. If the external sheaf expands more than the steel control cable, that would tend to reduce the travel of the cable end, and the cockpit control could then hit its stop before the control arm hit the stop at the governor. As a test, I adjusted the cable at the governor bracket to give more margin to the stop at the cockpit control when the governor control was at the max rpm stop. This meant that the low rpm stop could no longer be reached, as the cockpit control would hit its stop first. But, the low rpm stop is much less important than the high rpm stop.

On the next flight, I found that now the rpm only decreased 40 to 50 rpm as the engine warmed up. This is in the range reported by another local pilot who has three Pitts Specials with MT aerobatic props, so I think I have solved my problem.

Lesson Learned - Be sure to leave adequate clearance between the cockpit controls and the full throttle and max rpm stops. Otherwise you might be sacrificing some throttle or prop control travel after things warm up.