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Not reaching 2700 RPM?

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SpamPilot

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My new M20F flies well and appears to be putting out full rated power.  But I have yet to see an honest 2700 RPM on the tach.  2625 is the most I can get in cruise, even at low altitude.  It's a digital tach so I doubt this is an issue with tach error.

 

I don't intend to do a lot of cruising at 2700, so it's not a big deal, but I'd still like to know why I can't get that last 75 RPM.  Any thoughts?  Maybe a cable or prop governor adjustment?

Riq Contributor

Riq

Posted
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I had the same "problem" on my new to me plane. The dealer and I adjusted it ourselves. I wouldnt recommend this unless youve done it before, as its a pita.

 

My plane had just had a new tach put in so we assumed it was ok, and the gov  just wouldnt give over 2600 when warm.

 

Well, after cruising around the country for 100hrs I decided to use a rc airplane tach and sure enough, my gauge was off! I was burning up the sky at ~2650 all over.

 

Make sure your gauge is accurate.

silent101 Apprentice

silent101

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I have the same issue in my 67F but its more like 2550 is my max RPM. I haven't adjusted it because I figure the stock tach could be off. Needless to say I have flown 120hrs like this but my annual is this Friday so I will let you guys know the outcome.

Marauder Grand Master

Marauder

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Having just gone through a tach change from mechanical to electrical, I second or third or fourth the recommendations of verifying the actual prop speed. My mechanical tach was showing 2600ish and the new electronic one shows 2690, verified with a secondary rpm gauge. At what I thought was my cruise setting of 2500, I was actually running around 2400.

gregwatts Rising Star

gregwatts

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Tachs are not perfect. Before adjusting anything you should verify the accuracy of the tachometer. I went thru the same thing to find out that my tach reads 80 rpms low.......from 2300 to 2700 rpm! You can buy a new tach to correct the error,........but I just set the rpms 80 less than where I want it to be.

kmyfm20s Veteran

kmyfm20s

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My new M20F flies well and appears to be putting out full rated power.  But I have yet to see an honest 2700 RPM on the tach.  2625 is the most I can get in cruise, even at low altitude.  It's a digital tach so I doubt this is an issue with tach error.

 

I don't intend to do a lot of cruising at 2700, so it's not a big deal, but I'd still like to know why I can't get that last 75 RPM.  Any thoughts?  Maybe a cable or prop governor adjustment?

It is for take off that you really want all 2700 rpm. At 2625 you might be missing out on 10 to 15 Hp.

jetdriven Grand Master

jetdriven

Posted
Posted

There is a stop screw on the governor that sets the max RPM. It is on the bell crank where the cable attaches.

I have adjusted ours three times. Its 25 RPM per full turn on that stop screw on the McCauley governor. First make sure that the prop is digitally verified to be low on RPM. Then make sure the arm is hitting the stop screw.

 

You first adjust the stop screw, then, adjust the rod end on the prop cable so the arm hits the stop screw full forward on the prop control. Make sure that the knob or lever has ever so slightly more travel than required to make the governor arm hit the stop. 1/2 turn on the rod end beyond what is required to make it hit the stop on the governor is about right.  The cable shouldn't limit the travel on the RPM, the stop screw should.

 

Go fly it 5 minutes with the prop control full forward and at least 75% power. Verify that you are getting 2700 RPM. If it is not 2700 (I would use +15 RPM, -25 RPM), readjust then re-test.

 

Note, the A3B6D configuration is easy to adjust. The A3B6 is a real #$%@**$^)%

  • Like 1
Cody Stallings Experienced

Cody Stallings

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If its not an indication or gov problem, it can be a buildup of sludge in the piston assembly of your propeller. This buildup will restrict the movement of the pitch change rod, an will not allow the piston to come all the way back to the full low pitch setting. I have seen this problem on several pipers, mooneys an cessna..... It's a relatively easy fix. What part of the country are you in?

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  • 2 months later...
Mooneymite Grand Master

Mooneymite

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Isn't here also a stop on the propeller hub which can limit rpm? If you can't get max rated rpm using the governor, the low-pitch stop on the prop hub may need to be tweaked.

.

jetdriven Grand Master

jetdriven

Posted
Posted

Thats why the service manual says to fly it at a high power setting and high speed, then the prop is governing. Below around 120 MPH, the prop can be sitting on the low pitch stop.

  • Like 1
flyboy0681 Grand Master

flyboy0681

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Same situation here. After reporting to my A&P that I'm not seeing any more than 2600 he gave me his portable Tru Tach to use in flight. The mechanical tach agreed with the optical one and I'm waiting for him to come by and adjust the governor.

Bob_Belville Grand Master

Bob_Belville

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I see RPM as high as 2750 right at take off. It's a busy moment with manual gear and flaps to retract but I twist the prop knob just a little and RPM drops 100 RPM. I think it would probably stabilize but I'd rather not leave it in overspeed.  This is a Hartzell Top Prop (Scimitar) on an IO360A1A. 

 

Should I get governor adjusted?

jetdriven Grand Master

jetdriven

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Bob, I'd follow the service manual. Get an optical tach, takeoff and fly with the prop control full forward at 75% power or higher. Determine actual RPM. If it is over 2700, limit RPM to 2700 and leave the prop contol where it is. Then adjust after you land. 25 RPM per full turn of the stop screw.

Bob_Belville Grand Master

Bob_Belville

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Byron, I think the high speed might be a function of oil temp (& viscosity) I pulled up a December flight and note that rotation rpm was 2644. oil was 166f. Compared to 2750 RPM with 218 F oil temp. Is that possible?

Marauder Grand Master

Marauder

Posted
Posted

BTW -- your TSOd direct factory replacement 930 should be accurate, but so was the factory tach. The 930 runs off of the mag and our planes originally had mechanical Tachs. Even my EI tach (direct replacement) and JPI 830 (secondary instrument) do not agree exactly even though they are getting the same information from the same source. At least I know they are within acceptable error of each other.

Bob_Belville Grand Master

Bob_Belville

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Byron, I went back to the data from the JPI 930 and noted the max rpm. On take off. It definitely correlates closely to oil temp.

 

The max rpm looks fine as long as oil temp is below 195. I would not think I would want to move the stop which would cost me power on most take offs. I could back off prop knob position when Oil Temp is over 195F. This would only be when taking off when the engine is already hot. 

 

But perhaps there's an underlying problem?

 

Flt# date max rpm oil temp
18 28-Dec 2641 145
37 2-Feb   2655 150
16 28-Dec 2644 152
21 4-Jan    2648 157
41 15-Feb 2656 163
7 19-Dec   2650 168
52 18-Feb 2662 170
70 11-Apr  2655 173
35 29-Jan  2653 174
72 28-May 2654 176
50 15-Feb 2659 176
12 23-Dec 2695 176
39 14-Feb 2652 177
69 10-Apr 2657 181
63 1-Apr   2667 181
48 15-Feb 2652 184
31 18-Jan 2686 186
25 4-Jan   2681 187
76 31-May  2661 188
60 24-Nov  2666 188
27 4-Jan    2691 189
44 15-Feb  2691 193
46 15-Feb  2687 195
58 18-Feb  2701 198
81 16-Jul    2724 199
54 18-Feb  2714 200
23 4-Jan     2724 201
83 16-Jul    2751 218
78 31-May  2725 224
74 28-May  2726 228
Marauder Grand Master

Marauder

Posted
Posted
Looking at the data from 30 take offs, I'm thinking it is unlikely the tach is the issue.
Bob -- I will graph out my rpm/oil temp as well as see how mine compares. Should be interesting to see if they correlate.
  • Like 1
Bob_Belville Grand Master

Bob_Belville

Posted
Posted

Perhaps I should I talk to Hartzell about the max rpm/oil temp correlation?

 

Even if my tach is off, which I'm doubting, the correlation exists.

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