CS prop not making rpms

[Jetdriver]

Hello all

 

I own a 75F and at takeoff with prop full forward and leaned for max power I am still only getting 2660 (should be 2700).  24-25" mp indicated.  As I climb out that rpm is decaying to where I'm only making 2600 or less.  My home field is at 4500'.  Prop governor is new as is prop cable. Prop has recent overhaul.  Engine only has about 120 since OH. Any thoughts on what this could be?

Edited August 10, 2023 by Jetdriver

[EricJ]

Make sure the cable is properly rigged so that the control arm at the governor reaches full travel when the control is full forward.   2660 at takeoff isn't bad, actually, and within tolerance for a lot of tachs (i.e., it could just be the tach is off a little).

Mine started doing the thing where the rpms dropped during climb and I figured out it was just that the friction lock had come a little loose and the prop control was actually backing off a bit after takeoff.    That was an easy fix and hasn't been a problem since.

If your oil pressure and fuel flow are good it's likely something simple.

 

 

[ArtVandelay]

Isn’t there a way to adjust this (some set screw on prop control)?

Mine is the same, but I don’t worry about as it’s been that way since installation.

[Jetdriver]

The friction lock is good, in climb the rpms are still coming back even with the lever full forward almost like the prop was on the stop and it was running out of power.

[EricJ]

Yes, there's a set screw on the governor to set the high rpm limit, but I'd touch that almost as a last resort.  It would have been set by the manufacturer/overhauler and shouldn't need to change.  If the governor and cable are new-ish I'd still guess it's more likely to be a rigging issue.   Make sure that full travel of the control results in full travel at the governor, with the control arm at the governor hitting the stop at full forward with a little bit of "cushion" left on the propeller control.   If the cable wasn't routed or mounted properly there may be something exerting pressure back on the cable/arm/whatever that results in the drop.

Also check there aren't any oil leaks around the governor or the hard line (pipe) that runs from the governor to the front of the engine.

If you have an engine monitor, check that manifold pressure, fuel pressure, fuel flow, etc., all look reasonable and aren't potential causes for an rpm drop.   Obviously oil pressure will be important as well.

Does this happen only on initial climbout or will it do it if you do touch-and-goes or a stop-and-go or something?

[A64Pilot]

Sometimes and I know this may sound stupid but on the ground the governor hits the stop, but the engine being rubber mounted will pull a little forward under power and pull it off the stops, hence the requirement of cushion as was mentioned, cushion is nothing more than maybe 1/4 of cable movement left when the throttle, mix or prop hit the stops.

As an A&P I’m not aware of any adjustment I’m allowed to make to a governor except of course adjusting the arm stop screw.

Loss of oil pressure should make the prop go to high RPM, unless it’s a feathering prop or aerobatic prop.

[Pinecone]

First thing is to check the tach to see what the actual RPM is.

Best would be an optical tach.  You can pick up one for model airplanes pretty cheap.  There are also smart phone apps.

[A64Pilot]

The model airplane ones are very accurate.

You can check them for accuracy by selecting two blade prop and pointing it at a florescent light, it should say 3600 RPM, because the light is flickering 60 times per sec, and 60 x 60 = 3600 

[Shadrach]

3 hours ago, Jetdriver said:

Hello all

 

I own a 75F and at takeoff with prop full forward and leaned for max power I am still only getting 2660 (should be 2700).  24-25" mp indicated.  As I climb out that rpm is decaying to where I'm only making 2600 or less.  My home field is at 4500'.  Prop governor is new as is prop cable. Prop has recent overhaul.  Engine only has about 120 since OH. Any thoughts on what this could be?

What kind of Governor?  I have a similar issue sans the continuing degradation.  It started after a PCU5000 installation. In my case the cable and linkage have moved as far as they can I need to.reclock the arm that connects governor.

[Shadrach]

3 hours ago, A64Pilot said:

Sometimes and I know this may sound stupid but on the ground the governor hits the stop, but the engine being rubber mounted will pull a little forward under power and pull it off the stops, hence the requirement of cushion as was mentioned, cushion is nothing more than maybe 1/4 of cable movement left when the throttle, mix or prop hit the stops.

As an A&P I’m not aware of any adjustment I’m allowed to make to a governor except of course adjusting the arm stop screw.

Loss of oil pressure should make the prop go to high RPM, unless it’s a feathering prop or aerobatic prop.

Do you think it would be a no no to reclock the arm on its shaft?

[Shadrach]

1 hour ago, Pinecone said:

First thing is to check the tach to see what the actual RPM is.

Best would be an optical tach.  You can pick up one for model airplanes pretty cheap.  There are also smart phone apps.

I have found the apps to be hit and miss. The Hangar 9 optical tach works well even though it feel a bit cheap. Low risk investment at $36.00.

 

[EricJ]

22 minutes ago, Shadrach said:

Do you think it would be a no no to reclock the arm on its shaft?

That's usually allowed.   If you have the manual for the governor it may say one way or other.

[47U]

22 minutes ago, Shadrach said:

Do you think it would be a no no to reclock the arm on its shaft?

Yes, EricJ is correct.  If you’ve run out of travel on the prop cable and the governor arm isn’t hitting the high RPM stop, you can loosen the six(?) case screws and rotate the governor inside its case to adjust the prop cable travel.  Pretty low torque for tightening the screws, 10 lbs, I think.  Reference your ATH-1 installation instructions for details. 

[N201MKTurbo]

29 minutes ago, 47U said:

Yes, EricJ is correct.  If you’ve run out of travel on the prop cable and the governor arm isn’t hitting the high RPM stop, you can loosen the six(?) case screws and rotate the governor inside its case to adjust the prop cable travel.  Pretty low torque for tightening the screws, 10 lbs, I think.  Reference your ATH-1 installation instructions for details. 

That’s what the governor guy told me. I took out all the screws and rotated it very carefully and put the screws back in very carefully. It leaked like a sieve. I sent it back to him and asked him to fix the leak. I told him I owed him for fixing the leak, but he insisted on doing it under warrantee. It came back like a complete overhaul. Now my engine goes to 2700. A bit sportier on takeoff. 

[Shadrach]

2 hours ago, EricJ said:

That's usually allowed.   If you have the manual for the governor it may say one way or other.

 

1 hour ago, 47U said:

Yes, EricJ is correct.  If you’ve run out of travel on the prop cable and the governor arm isn’t hitting the high RPM stop, you can loosen the six(?) case screws and rotate the governor inside its case to adjust the prop cable travel.  Pretty low torque for tightening the screws, 10 lbs, I think.  Reference your ATH-1 installation instructions for details. 

I have a PCU5000. I’ll checkout the manual. Sure seems like it would be easier to just re-index the arm over one tooth.

 

[EricJ]

13 minutes ago, Shadrach said:

 

I have a PCU5000. I’ll checkout the manual. Sure seems like it would be easier to just reminded the arm over one tooth.

 

For the PCU5000 that might be the preferred method.   The manual should say.   It is something that is expected to be done sometimes.

[47U]

1 hour ago, Shadrach said:

Sure seems like it would be easier to just re-index the arm over one tooth.

If you did that, how will you practice your safety wiring skills?  Six screws vs. only one?  That’s no fun…

[Shadrach]

6 hours ago, 47U said:

If you did that, how will you practice your safety wiring skills?  Six screws vs. only one?  That’s no fun…

My safety wire skills have peaked at mediocre but adequate.  Bragging rights over the skill to remove and reclock that arm without removing the governor is the kind of thing you can really bore your friends with. 

[Pinecone]

12 hours ago, A64Pilot said:

The model airplane ones are very accurate.

You can check them for accuracy by selecting two blade prop and pointing it at a florescent light, it should say 3600 RPM, because the light is flickering 60 times per sec, and 60 x 60 = 3600 

Yes,  I have had one of these for decades and it works well.  I even have a HeathKit one I built in the 60s that still works.

 

[A64Pilot]

@Shadrach Re-clocking the arm would I think increase or decrease RPM depending on which way you went because you’re changing the relationship of the shaft to the set screw. I guess if there is enough adjustment in the set screw to return it to 2700 then you would be OK.

But rotating the assy I think just moves the assy leaving the RPM set point the same and I think would allow much more adjustment.

Can you call the manufacturer and ask?

[A64Pilot]

@Pinecone I’ll take your antique prop tach and raise you mine.

It was originally used by Aero-Commander back in the 60’s but still works well.

 

[A64Pilot]

11 hours ago, Shadrach said:

I have found the apps to be hit and miss. 

 

That’s my experience with Apps in general.

There used to be and may still be Artificial Horizon Apps, and they appear to work, some of the Ag guys would velcro their Iphone on the panel just in case.

But Iphones don’t have gyros, just accelerometers so if you put the phone in a bank and hold it there for a short time because the phone isn’t sensing acceleration, the Horizon will return to level, but your still in the turn

That thing can get you killed

[47U]

1 hour ago, A64Pilot said:

@Shadrach Re-clocking the arm would I think increase or decrease RPM depending on which way you went because you’re changing the relationship of the shaft to the set screw. I guess if there is enough adjustment in the set screw to return it to 2700 then you would be OK.

But rotating the assy I think just moves the assy leaving the RPM set point the same and I think would allow much more adjustment.

Can you call the manufacturer and ask?

Just to be clear, reindexing the control arm on the shaft is NOT permitted.  Loosening the six screws (they don’t have to be removed) and rotating the internal assembly in the outer housing gives you all the adjustment you need to fine-tune the desired cushion on the prop control cable.

[Shadrach]

1 hour ago, 47U said:

Just to be clear, reindexing the control arm on the shaft is NOT permitted.  Loosening the six screws (they don’t have to be removed) and rotating the internal assembly in the outer housing gives you all the adjustment you need to fine-tune the desired cushion on the prop control cable.

Well that sucks. That means taking the governor off. With 1000+hours and 15 years. I’d be tempted to overhaul it if I removed it.

[PT20J]

On 8/10/2023 at 6:20 PM, N201MKTurbo said:

That’s what the governor guy told me. I took out all the screws and rotated it very carefully and put the screws back in very carefully. It leaked like a sieve. I sent it back to him and asked him to fix the leak. I told him I owed him for fixing the leak, but he insisted on doing it under warrantee. It came back like a complete overhaul. Now my engine goes to 2700. A bit sportier on takeoff. 

Don't take out the screws -- just loosen them and rotate the mechanism. Dan Long at West Coast Governor Service warned me not to take them completely out as there is a spring in there that will unseat the mechanism inside.