Jump to content

RPM - not getting 2700 reliably


Recommended Posts

I've had kind of an ongoing problem with my M20E where it doesn't give the full 2700 RPM with the prop control full forward. I'm scratching my head over this since it's been in the shop twice for it and it seems to be hard to pin down.

Here's the story: I noticed for a while that the RPM on takeoff, climb and cruise would never go any higher than about 2600. I check with an optical tach and it agreed. I had the airplane in a shop for other work and had them look into the RPM issue. They adjusted the set screw on the governor control. I flew the airplane and for a couple flights it made 2700 RPM. On a subsequent flight it made 2600 initially but at cruise it eventually crept up to 2700 with the prop control full forward.

At this point I took the airplane to another shop and described the symptoms.  They ended up pulling the governor and sending it for overhaul (It's the original H-1 Hartzell). I probably should have just put a new governor on it because it would have been about the same price, but I didn't.

Now the airplane got full RPM again for a few flights, but then it was back to 2600.

Fast forward 9 months: Now the RPM normally is steady around 2600 with the prop control full forward, but every now and then I will get 2700 out of it for a minute or so. I have a JPI 900 now, so I think the RPM reading is accurate. Cruise speed is normal (145-150 knots) and the RPM does not change with airspeed. It seems to be governed at 2600 RPM most of the time. I checked the prop control linkage and it is all tight with no slippage and the arm moves all the way to the stop. The set screw is securely safety wired as well and I can detect no interference.

The only loose correlation I can find for why I sometimes get 2700 but usually not is that if I see 2700 it is usually on the 2nd flight of the day after a fuel stop. Could be total coincidence so don't read too much into that.

I'm at a loss for what to look into next. Once again the engine seems to make full power, the RPM is independent of airspeed and the inflight mag check is good. Just inconsistent and usually low top RPM.

Any ideas?

Link to comment
Share on other sites

If you have the cable rigged so it hits the stop before the knob is all the way in it will cause cable housing to slip if you push the control all the way in. This will loosen the housing in the clamp at the governor and it will not get full travel anymore.  I know because I use to have a partner who thought that the plane would go faster if he shoved the prop control in as hard as he could.

  • Like 2
Link to comment
Share on other sites

13 hours ago, N201MKTurbo said:

If you have the cable rigged so it hits the stop before the knob is all the way in it will cause cable housing to slip if you push the control all the way in. This will loosen the housing in the clamp at the governor and it will not get full travel anymore.  I know because I use to have a partner who thought that the plane would go faster if he shoved the prop control in as hard as he could.

The prop control does hit the stop before the knob is all the way in and I wondered if it was slipping, but I still can't explain why it will sometimes briefly get 2700 RPM during a flight then slowly back off to 2600 or so as the flight progresses. Here's a screen shot of an example where it started at about 2695 RPM and slowly went down to 2610 with the prop control knob fixed:

flight data.PNG

Link to comment
Share on other sites

2 hours ago, Frozen Flying said:

The prop control does hit the stop before the knob is all the way in and I wondered if it was slipping, but I still can't explain why it will sometimes briefly get 2700 RPM during a flight then slowly back off to 2600 or so as the flight progresses. Here's a screen shot of an example where it started at about 2695 RPM and slowly went down to 2610 with the prop control knob fixed:

flight data.PNG

Have you tried cycling the prop a few hundred RPM to see if it comes back up? Are you cruising at 2700 RPM?

Link to comment
Share on other sites

3 hours ago, Yetti said:

Tell us about your prop

The prop is an HC-C2YK-1BF with the B hub. Last overhaul 2011.

2 hours ago, Andy95W said:

And the last time your prop governor was overhauled.

The prop governor was overhauled in Jan 2017. I just got off the phone with the shop that overhauled it but their expert is out for the holiday.

1 hour ago, N201MKTurbo said:

Have you tried cycling the prop a few hundred RPM to see if it comes back up? Are you cruising at 2700 RPM?

I have tried cycling the RPM a few hundred but it doesn't change the top end. I don't cruise at 2700. Usually 2500-2550. In flight from the screen shot I left the prop control full forward to see what it would do (it slowly came back from 2695 to ~2600).

I'm suspecting the max RPM set screw and the post that it contacts might be out of alignment or there might be some lateral slop allowing the two to occasionally miss direct contact. I'm going to have a look at it with a camera while we move the control and see if there is a case where the contact isn't full or direct.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.