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I have a 1965 comanche 260. I have been chasing an engine surge/RPM wavering issue for a few months. Run up and prop exercise go fine. Full power for takeoff and it goes down the runway with normal performance and engine indications. Once I come off the ground, maybe 1 second afterwards, my engine starts surging/sounding like its increasing and decreasing power. My RPM's used to follow the engine surge. This would last for about 3-4 seconds, then everything would settle out. One time i felt the airplane slow down so decided not to fly until I get this figured out. The rest of the climb at full power is fine. RPM/MAP/Performance all great. I sent the governor off to get looked at. Maxwell Prop said the settings looked out of wack so they reset everything back to what they said was proper for my engine/Prop (3 blade Hartzell). Installed the governor and now the RPMs stay solid, but the engine still surges very dramatically under the same conditions. Right after takeoff and for maybe just a little longer (5 ish seconds) then it stabilizes and everything runs fine.
Something else, the prop shop had us block off the governor mount and run the engine to see where the RPM would go. When we did this, the Prop very slowly went to high pitch/low RPM. An engine shop said this is nothing to worry about since without the governor installed, engine oil pressure can slowly feed back into the governor and change the pitch. We ran the oil pressure differential test to test the engine bearing and it came out at 20 psi which i was told is spot on.
Surging on an injected engine can be caused by a number of factors. Typically, the fuel servo is the cause, but it can also be from an air leak into the system, caused by porous hoses. At high RPM your engine fuel pump is pulling fuel really hard, and if you have a point of air induction the fuel becomes aerated, resulting in a lean mixture. I've experienced this on several twins where the rubber fuel hose from the fuel servo to the fuel distributor (aka "spider" on the top of the engine) had developed pinholes due to age and heat. When the plane was at full power the surging was caused by the lean mixture. When the engine warmed up the heat caused the fuel hose to expand and pinched off the teeny holes and the engine smoothed out.
You don't explain if you hear and observe the surging, or if you are relying on your tachometer. In other words, are you sure that you are getting surging or deciding that it is surging based on your tachometer?
I encourage you to call Lycoming tech support. A surging engine is a Lycoming issue, not exclusively a Comanche issue. 1-877-839-7878
Pat Barry
Thank you Pat. I appreciate your insight. When this all started, I would hear a change in tone or pitch and sometimes see the tach needle move up and down 50 or so rpm. But since this is happening immediately after takeoff and only for about 3-4 seconds, I didnt typically have my eyes on the tach. My dad flew with me once and he said the tach needle didnt move, but I didnt really hear much change in engine sound on that flight either. I have flown it once since the governor was adjusted. I paid as close attention as I could to the rpm and manifold pressure and did not see either of them move this time. the engine still surged dramatically though. My manifold guage move kinda slow and I dont think it would be able to move quick enough to reflect any change if its happening. The fuel flow also seems to be stable.
I'm not sure if "surging" is a technical term. the engine makes a sound (very attention getting) like its revving up and down. it only last for about 3-5 seconds and only right after I lift off. the rest of the climb out at full power is fine. I only leave it at full power till about 5-600 feet AGL then pull back to 25-25. thats plenty of power for climbs.
I'll give lycoming a call.
Thank you
I separate functions. In the case of an engine, the changes are due to either fuel, electrical, or a mchanical issue of some kind.
The tricky part here is that you are describing an issue that occurs on rotation, lasts 3 to 5 seconds and does not repeat. That rules out electrical and mechanical failure, so concentrate on fuel and propeller. I'd be checking all hoses and connectors. The McCauley three blade has a piston in one blade and that can wear eccentrically and cause a vibration issue, but I might be connected with your concern.
How about the seller. You just bought this Comanche 260. Have you interviewed the seller to seek information? Perhaps he has knowledge of it?
Yes, run this by Lycoming. It's difficult not being able to go through this with you in the aircraft.
Talked to Lycoming. They didnt really have anything to offer. They are thinking that ground effect has something to do with it.
