MT Governor / Hartzell Prop RPM Loss With Temp

I currently am flying our RV 7A with a Superior XP-360 Carbureted Engine. Full 2700 RPM is developed for takeoff when engine is fairly cool (ie. first take off of the day) After everything warms up to full temperatures, subsequent takeoffs only develop 2550 RPM. Tried adjusting max governor setting one turn, and it increased the first take off RPM a bit, but after everything up to full temp again, only 2550 RPM. Prop is Hartzell Blended Airfoil. Also, RPM will not increase to 2700 in a descent with prop control full forward.

Any ideas anyone! Thanks!

Jim

RPM

Could heat be causing expansion etc so that the governor control lever does not go all the way to the stop - presuming it does when cold? Seems unlikely that MT governor would be so affected by heat. Hope you can cure this soon. Bill

Correction

Jim,

Please forgive me for the reply posted earlier. I had it backwards becasue I was thinking in the feathering/racing prop mode. For the pressure-to-increase pitch props we sell to Vans, the blade remains at low pitch until oil pressure increases blade pitch to maintain RPM. In your case the blades should be on the low pitch stop without any oil pressure from the governor. If RPM droops at elevated temperatures, then it could be caused by heating of the governor or governor controls reducing the high-RPM stop/adjustment. I would speculate that some part of the control cable, adjustment screw or something else is causing the high-RPM adjustment to be reduced a little. The solution may be as simple as increasing the high-RPM stop a few RPM.

Les--------------------------

We hear of this problem occasionally when oil temps are elevated. The governor is an oil pump generating pressure to counteract the forces generated by the propeller blades - both aerodynamic and centrifugal - and the propeller return spring. Low-speed, high-power conditions require the highest pressures to keep the blades at low pitch. When oil temperatures increase and oil thins, leakage increases. The leakage occurs in the governor itself and through the front crankshaft bearings. If the governor does not have enough capacity or high enough relief valve pressure setting, it may not be capable of keeping the propeller on the low pitch stop and your static/low-speed RPM may drop a little.

I don't know much about the MT governor capacity but I haven't heard this to be a systemic problem, so it probably has enough capacity. I suspect the relief valve setting on the governor is a little low. If you can obtain enough info on the governor to adjust the relief valve to a higher pressure, I would try that. If you are unsure or uneasy about doing it yourself, I would recommend you pull the governor and have it bench-checked by a propeller shop. They should also be able to check the capacity against the specs to make sure all is well there. Our props are certified up to 325 psi but you should not need that much.

Let me know what you find out.

Les Doud
Propeller Integration Engineer
Hartzell Propeller Inc.
937-778-4262

superflycfi said:

I currently am flying our RV 7A with a Superior XP-360 Carbureted Engine. Full 2700 RPM is developed for takeoff when engine is fairly cool (ie. first take off of the day) After everything warms up to full temperatures, subsequent takeoffs only develop 2550 RPM. Tried adjusting max governor setting one turn, and it increased the first take off RPM a bit, but after everything up to full temp again, only 2550 RPM. Prop is Hartzell Blended Airfoil. Also, RPM will not increase to 2700 in a descent with prop control full forward.

Any ideas anyone! Thanks!

Jim

Click to expand...

Have you tried checking against an optical tach? It's possible that the drift is occuring in your tachometer as it heats up.

James Freeman

MT Gov. RPM loss

I had an unrelated problem with my MT Gov. Called the Factory Rep. in Fla. who informed that there was a Service Builletin that needed to be done anyway. Shipped the Gov to them. Got it back a week later. Solved the original problem too!

Phone # is at the hangar. E-mail me at----mannanj--at--alltel--dot--net.
I'll try to remember to get it today when I go for grin replenishment this afternoon!

Mannan Thomason
RV-8 N1 61RL