Max RPM Problem - Looking for Suggestions

drill_and_buck · #1 12-11-2011, 01:00 PM

My hangar mate has this nagging RPM issue. We would appreciate any suggestions on how best to troubleshoot and fix.

The plane is an RV-8, 0360, Constant Speed.

The problem: During the first takeoff of the day, max rpm of 2700 can be reached.

On a few occasions RPM has surged to 2800 on takeoff.

On subsequent takeoffs you won't see any more than 2400 max RPM.

This problem has persisted across and engine removal/rebuild, a new Hartzell BA prop and a new governor. The A&P/AI on the field has looked at it a few times and we are all stumped. Tach (mechanical) has also been checked.

Could this be linkage related?

How would you troubleshoot this problem?

Suggestions appreciated.

Mike

flyinga · #2 12-11-2011, 01:05 PM

You could remove the prop control cable at the governer then safety wire the governor at the max rpm position. take a test flight and see how it goes. Running at 2700 rpm for several minutes will not hurt the engine.

Noah · #3 12-11-2011, 09:31 PM

Hi Mike, It seems like it MUST be linkage related, all other parts have been overhauled!

I wonder if max cable travel might be limited by a differential thermal expansion? Something is heating up, expanding, limiting travel of the governor linkage?

I sometimes see rpm go over 2700 for a couple of seconds until the governor gets the blades twisted to reduce the speed - especially if power is applied rapidly, so this does not seem unusual.

rocketbob · #4 12-11-2011, 10:11 PM

Tachs can behave screwy. I would swap with another known good one. The tach cable could also have a break in the wire (shaft) that could potentially cause erratic behavior.

Other thought is the governor could have some junk in it.

rv9aviator · #5 12-12-2011, 06:08 AM

A $30.00 digital tack from your local RC hobby shop works great to check the tach. Just aim it through the windscreen to use.

mahlon_r · #6 12-12-2011, 06:29 AM

My though would be that you might be getting full RPM of the first take off because the oil is thick and viscous and it is easy to make up for clearance issues or leaks in the prop control circuit. Once the engine has been warmed up thoroughly, the oil thins out and you can't get more RPM because this oil allows less governor oil pressure then the thick colder oil did. So, I think there could be a problem with the governor that it can't make proper oil pressure to the prop with warm thin oil or there may be a clearance or leak issue within the prop control circuit. Lycoming has an SB to do a leak down across the engine plumbing and nose bearing to the prop to test for leaks. Basically use a leak down guage and do a leak down of the entire engine /prop control circuit. If the engine passes that test, I think I would try a physically different governor.
The service instruction is SI 1462. In addition to describing how to do the test, it details how to make a plate for the governor pad to do the test but you really only need to install a fitting and hose on your leak down guage and hook it directly to the prop governor line coming off the governor drive or even use a rubber tip blow gun on the leak down hose and hold the tip directly into the port on the governor drive parting surface with the governor removed from the drive.making the plate is a big deal and not really necessary for a one time use.
Another note is that when removing the hose or blow gun from the engine cover the connection with a rag..it can spit back out of the connection when being disconnected and splatter you and all around with oil.
Good Luck,
Mahlon
"The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."

Walt · #7 12-12-2011, 06:53 AM

Correct me if I'm wrong, but with low on no oil pressure from the Gov the prop would be at flat pitch which means speed is controlled only by the prop pitch stop/throttle. Lack of oil pressure from the Gov or a system circuit leak would cause you not to be able to increase pitch (reduce RPM).

n721dp · #8 12-12-2011, 07:37 AM

Is your governor a Hartzell governor?

Had a similar problem on my -6A where the first takeoff was ok rpm-wise, then subsequent takeoff's had lower rpm's wide open, low pitch.

Turns our Hartzell, as a cost saving measure, changed the governor housing from metal to plastic. Heat distorts the plastic, giving non-repeatable results once everything is heat soaked. Especially on rear-mounted installations.

I had a authorized Hartzell repair station in Florida change the housing to metal - no charge, warranty repair.

drill_and_buck · #9 12-12-2011, 12:13 PM

All great leads, thank you! The knowledge and insight on this list is priceless and well worth $25 annual donation.

The RV-8 is going in for its annual in a few weeks. I'll report back on what was tried and what was found.

Mike

rocketbob · #10 12-12-2011, 12:36 PM

Quote:

Originally Posted by n721dp

Is your governor a Hartzell governor?

Had a similar problem on my -6A where the first takeoff was ok rpm-wise, then subsequent takeoff's had lower rpm's wide open, low pitch.

Turns our Hartzell, as a cost saving measure, changed the governor housing from metal to plastic. Heat distorts the plastic, giving non-repeatable results once everything is heat soaked. Especially on rear-mounted installations.

I had a authorized Hartzell repair station in Florida change the housing to metal - no charge, warranty repair.

Wow, thats kinda crazy!