M20C Rudder Trim Adjustment

[0TreeLemur]

Hi everyone.  So far I've logged 400 hours flying our '67C since we acquired her 3 years ago.  It's been a blast.

In cruise the "ball" on the Brittain TC100 lies a little-bit off center to the right, but I ignored it, because the TC100 doesn't really seem that sensitive.  However, since I installed the AV-20, when displaying the AI function I noticed that "ball" is about 1/3 out of center to the right in cruise and is quite sensitive.  In cruising level flight trimmed with no rudder input, when I put in enough right rudder to center the ball, she starts to climb.  This tells me that in normal cruising flight with no rudder input, the ship is out of trim.

Looking at the S&MM, I see the procedure to bend the rudder tab to effect rudder trim.   The tab on my aircraft is not bent, so it seems that none of the prior owners took this step.  Looking at the rudder pedals, the right one is much more worn than the left, which causes me to suspect that pilots have kept some right rudder in for thousands of hours.

Has anybody done this?   The S&MM manual gives a maximum deflection of 0.08", but no procedure or steps.   I'd like a procedure.  Should I just try increments?  I'd appreciate hearing from someone who has done this.   Is 0.08"  too much or just enough?   Given the 0.08" limit, I'm thinking about trying 0.02" increments of bend to the left to impart a bit of right rudder force.  Thoughts?

[N201MKTurbo]

Does your plane have a trim tab riveted into the trailing edge of the rudder? If it does don't be afraid to bend it. If it doesn't you can bend the trailing edge rivet line. When you bend the rivet line, it depends on how much of the rivet line you bend.

Expect to make 4 or 5 flights to get it right.

If you are bending the trailing edge of the rudder, make some wood blocks with recesses for the rivet heads, clamp them on with C-Clamps and then bend it with the wood blocks.

[0TreeLemur]

3 minutes ago, N201MKTurbo said:

Does your plane have a trim tab riveted into the trailing edge of the rudder? If it does don't be afraid to bend it. If it doesn't you can bend the trailing edge rivet line. When you bend the rivet line, it depends on how much of the rivet line you bend.

Expect to make 4 or 5 flights to get it right.

If you are bending the trailing edge of the rudder, make some wood blocks with recesses for the rivet heads, clamp them on with C-Clamps and then bend it with the wood blocks.

Thanks.  Yes, it does have the trim tab riveted to the trailing edge of the rudder.

[PT20J]

First thing I always do when checking an instrument indication is to verify that the instrument is accurate. There should be a leveling procedure in the service manual (usually you put a level across the seat rails and let air out of the tire on the high side until it’s level). Once level, adjust the slip indicators if necessary to center.

Skip

[carusoam]

+1 Before getting started... instrument panel and instrument should be level with the plane... shock mounts are key...

+1 Making the adjustment...

-1 for noticing the right rudder pedal being more worn than the other...  we are going to use plenty of right rudder all the time during T/O and climb... unless you have a powered rudder trim...

The left rudder hardly gets used during the descent... you will notice the ball go the other way in smooth air during the descent...

No, I never noticed any of this while flying my M20C...  there is so much going on with how the fuel is loaded... how much the pilot weighs compared to the Co-pilot seat...

It gets more noticeable when you can add a touch of rudder trim... depending on the flight of the day...

So...  don’t expect too much or adjust too much...

The weather will change... the loading will change... something will change...   tune it for your typical loading...

You might experiment with fuel loading first...

The rudder gets used to lift the heavy wing.... when your fuel balance, and passenger balance is a little off to one side... the rudder trim used to lift the heavy wing...

It turns out the adjustable rudder trim in flight makes a lot of sense... not sure if a static trim can do very much beyond get it in the ball park...

PP thoughts only, see if I got any of that correct...

Best regards,

-a-

 

[0TreeLemur]

New panel installed in 2018 with new shock mounts.

Both balls center during straight taxi and 30 psi in all tires.

In cruise, trimmed for level flight with no rudder input, the tendency to climb after centering the ball indicates an increase in energy (speed).  I think it is a rudder trim issue. 

 

[0TreeLemur]

12 hours ago, carusoam said:

You might experiment with fuel loading first...

 

While on a big trip out west in the summer of 2018 the left tank suddenly decided to stop weeping and start leaking.   It would lose over 1 gallon overnight into bucket. For the most of that trip, I would use the left tank in its entirety first, then switch to the right.   I didn't notice a big effect on the behavior of the aircraft in cruise. The PC system helped to compensate, and I was reliant on the relatively hard-to-read TC100 because had not yet installed the AV-20.  I think the PC system masks a lot of behavior.  The sensitivity of the AV-20 is what showed me that there was something amiss. 

[47U]

When was the last time the flight control travel boards have been on the aircraft?  I think I’d start there.  Flaps, ailerons, and rudder.  Then configure fuel/pax for your most common loading and flight test.

[RobertGary1]

9 minutes ago, 47U said:

When was the last time the flight control travel boards have been on the aircraft?  I think I’d start there.  Flaps, ailerons, and rudder.  Then configure fuel/pax for your most common loading and flight test.

They’re a good starting point when installing controls etc but afterwards you’ll still have to fine tune by experiment 
 

-Robert

[0TreeLemur]

1 hour ago, 47U said:

When was the last time the flight control travel boards have been on the aircraft?  I think I’d start there.  Flaps, ailerons, and rudder.  Then configure fuel/pax for your most common loading and flight test.

In every way she flies true, except requiring a bit of right rudder in cruise.   I've checked the ailerons and flaps in accordance with the S&MM.  They are fine.  She's a stock C except for a few speed mods: flap-gap seals, closed bottom-half of guppy mouth and ADF removed.  Still has the Willy's CJ2A jeep-style windshield and original cowl.  All gear doors close nicely.  According the TAS tool in the 430W using POH CAS values, she trues out at 138-143 knots depending on altitude and phase of the moon at 2400 rpm with a 2-blade Hartzell prop.  I find cruising at 2400 rpm quieter and more tranquil than 2500, with no detectable difference in speed.

When I level out and trim w/o right rudder, the ball is slightly deflected to the right.  When I center the ball, she starts to climb.  That tells me that the application of that little bit of right rudder cleans her up just a bit more.

My original question was answered by @N201MKTurbo who wrote above that a bit of a trial and error helps to improve rudder trim.   I'll post my results after I incrementally bend the tab to the left and observe the effect.  At cruise I don't expect it to affect anything other than required rudder input and if it does I'll be really surprised.

Finally, I don't expect that w&b has a significant effect on the yaw axis.  As I wrote before, I have experience flying around with one tank empty and one tank full, and didn't notice anything particularly unusual regarding required rudder input.  Of course the PC system could partially mask that.

 

[N201MKTurbo]

7 minutes ago, 0TreeLemur said:

In every way she flies true, except requiring a bit of right rudder in cruise.   I've checked the ailerons and flaps in accordance with the S&MM.  They are fine.  She's a stock C except for a few speed mods: flap-gap seals, closed bottom-half of guppy mouth and ADF removed.  Still has the Willy's CJ2A jeep-style windshield and original cowl.  All gear doors close nicely.  According the TAS tool in the 430W using POH CAS values, she trues out at 138-143 knots depending on altitude and phase of the moon at 2400 rpm with a 2-blade Hartzell prop.  I find cruising at 2400 rpm quieter and more tranquil than 2500, with no detectable difference in speed.

When I level out and trim w/o right rudder, the ball is slightly deflected to the right.  When I center the ball, she starts to climb.  That tells me that the application of that little bit of right rudder cleans her up just a bit more.

My original question was answered by @N201MKTurbo who wrote above that a bit of a trial and error helps to improve rudder trim.   I'll post my results after I incrementally bend the tab to the left and observe the effect.  At cruise I don't expect it to affect anything other than required rudder input and if it does I'll be really surprised.

Finally, I don't expect that w&b has a significant effect on the yaw axis.  As I wrote before, I have experience flying around with one tank empty and one tank full, and didn't notice anything particularly unusual regarding required rudder input.  Of course the PC system could partially mask that.

 

The CG has a slight effect on the magnitude of rudder trim errors. The airplane yaws around the CG so as the CG moves backwards, the rudder arm is reduced and the rudder is less effective. As the CG moves forward the arm is longer and the rudder is more effective.

Don't worry about the minutia. Just trim it as you fly it and give your foot a rest.

[jaylw314]

35 minutes ago, N201MKTurbo said:

The CG has a slight effect on the magnitude of rudder trim errors. The airplane yaws around the CG so as the CG moves backwards, the rudder arm is reduced and the rudder is less effective. As the CG moves forward the arm is longer and the rudder is more effective.

Don't worry about the minutia. Just trim it as you fly it and give your foot a rest.

I'd kill for rudder trim.  I think my right leg is now twice the size of my left

[Hank]

4 minutes ago, jaylw314 said:

I'd kill for rudder trim.  I think my right leg is now twice the size of my left

You'll need a newer, more powerful Mooney to have rudder trim.

[Andy95W]

23 minutes ago, Hank said:

You'll need a newer, more powerful Mooney to have rudder trim.

Actually, $1400 will get you rudder trim.  STC’ed for all Mooney models except the M18 for almost 30 years.

http://www.aerotriminc.com/id2.html

[jaylw314]

1 hour ago, Andy95W said:

Actually, $1400 will get you rudder trim.  STC’ed for all Mooney models except the M18 for almost 30 years.

http://www.aerotriminc.com/id2.html

Oooh, interesting!

[47U]

2 hours ago, 0TreeLemur said:

In every way she flies true, except requiring a bit of right rudder in cruise.   I've checked the ailerons and flaps in accordance with the S&MM.  They are fine. 

Following the mx manual is a good thing.  I curious if the rudder is centered up with the nose gear when towing/pushing the airplane in a straight line.  Do you have access to the travel board for the rudder?

Your Willis Jeep windshield made me smile.  It’s a comparison I hadn’t heard before, but very true.  

I’m sure you’ll get her pointed in the same direction as she’s flying soon.  

[carusoam]

Note: the Aero-trim is the device mentioned above.   It was produced by the guy called the Mooney Mizer...

Trimming the rudder via the maintenance procedure is a great idea...

Finding somebody with that extra experience would be nice...

Equating the small mechanical change to how much the ball moves probably will have you do this locally... small changes over a bunch of flights...

But if you want to go all out... DMax is probably the best way to get it done...

Having a perfectly trimmed Mooney is wonderful... my ragged M20C was nicely balanced... with no record of it ever seeing travel boards in 50years... 

PP thoughts only, not a mechanic...

Best regards,

-a-

[0TreeLemur]

1 hour ago, 47U said:

Following the mx manual is a good thing.  I curious if the rudder is centered up with the nose gear when towing/pushing the airplane in a straight line.  Do you have access to the travel board for the rudder?

Your Willis Jeep windshield made me smile.  It’s a comparison I hadn’t heard before, but very true.  

I’m sure you’ll get her pointed in the same direction as she’s flying soon.  

No, I don't have any access to travel boards.   I've searched these hallowed pages.  Lots of discussion of travel boards, I remember seeing CADD pages and stories about folks making them.  I came away from my research with the impression that travel boards are good for getting things close as @RobertGary1 wrote above, but actual final adjustment requires flight testing.   While taxiing, I think it is hard to tell with our ankles if the rudder pedals are exactly the same point when traveling in a straight line.  I'll look over at the passengers side when I taxi this weekend.   Interesting idea.

I'm going to make a silly little travel board to fit the now-unbent rudder trim tab so I can measure my deflection as I adjust it.  That way I can measure the change.  I'm really curious to learn how much effect that tiny little strip of aluminum exerts on the rudder position in cruise. I'll file a report.

[Hank]

2 minutes ago, 0TreeLemur said:

I'm going to make a silly little travel board to fit the now-unbent rudder trim tab so I can measure my deflection as I adjust it.  That way I can measure the change.  I'm really curious to learn how much effect that tiny little strip of aluminum exerts on the rudder position in cruise. I'll file a report.

I think you'll be surprised. The airflow at 140 knots / 165 mph is significant!

[PT20J]

Not sure about the short bodies, but on the M20J, the manual calls for the rudder to be offset 1-deg right with the rudder pedals centered and clamped.

Skip

[carusoam]

Expect the deflection to work opposite to what -a- considers logical... 

The trim tab goes one way

The rudder goes opposite....

The plane moves the other direction...

Be sure to know which way the bend is made to keep from overworking the aluminum...

PP confusion at best, let me know the answer if you can...

Best regards,

-a-

[0TreeLemur]

50 minutes ago, PT20J said:

Not sure about the short bodies, but on the M20J, the manual calls for the rudder to be offset 1-deg right with the rudder pedals centered and clamped.

Skip

S&MM says 1-degree right ± 2 degrees right, which I guess means 0-3 degrees right. 

Seems about right. 

[0TreeLemur]

On 11/3/2020 at 2:25 PM, 47U said:

Following the mx manual is a good thing.  I curious if the rudder is centered up with the nose gear when towing/pushing the airplane in a straight line.  Do you have access to the travel board for the rudder?

Your Willis Jeep windshield made me smile.  It’s a comparison I hadn’t heard before, but very true.  

I’m sure you’ll get her pointed in the same direction as she’s flying soon.  

 

[0TreeLemur]

Sunday I used straight-edges, calipers, etc. to adjust the rudder trim tab as described in the S&MM with the objective of adding right rudder trim in cruise.

The tab was bent slightly to the left at the top, and slightly to the right at the bottom.  Using the sheet metal bending tool, I went ahead and to the best of my ability bent it all to the left by the maximum published amount, 0.08".   Then I went flying.

Flight report:  In cruise the ball is maybe slightly more centered.   It helped a little, but not much.   The most obvious difference is that before adjustment, when holding right rudder in to center the ball in cruise, when I would take my foot off the rudder pedal, there would be some tail wagging as it adjusted to the lack of trim.   It doesn't wag as much now, so it seems to help.   I conclude that the maximum 0.08" deflection of the tab provides insufficient trim, but making it more uniform along the tab helped.

[takair]

1 hour ago, 0TreeLemur said:

Sunday I used straight-edges, calipers, etc. to adjust the rudder trim tab as described in the S&MM with the objective of adding right rudder trim in cruise.

The tab was bent slightly to the left at the top, and slightly to the right at the bottom.  Using the sheet metal bending tool, I went ahead and to the best of my ability bent it all to the left by the maximum published amount, 0.08".   Then I went flying.

Flight report:  In cruise the ball is maybe slightly more centered.   It helped a little, but not much.   The most obvious difference is that before adjustment, when holding right rudder in to center the ball in cruise, when I would take my foot off the rudder pedal, there would be some tail wagging as it adjusted to the lack of trim.   It doesn't wag as much now, so it seems to help.   I conclude that the maximum 0.08" deflection of the tab provides insufficient trim, but making it more uniform along the tab helped.

I think that the .08” limit applies to the trailing edge of the rudder, not the trim tab.  Note that the image in the manual depicts the riveted trailing edge and even mentions replacement of rivets if the head pops off in the process.  I think this was the reason for the limit.  The trim tab does not have this problem.  That said, not sure where to find that limit, May have been and SB.  My thinking is that you can go a little more.  Just to confirm, you have a short rudder with the tab at the bottom and the gurney flap (aka pencil holder) mid way up?....or do you have a long rudder?