Rudder skin Repair

[1980Mooney]

A question for those with IA knowledge out there.  There seems to be a lot of different opinions on the "acceptable" way to repair a rudder skin.  Is it possible for an IA in the future to declare a modification or repair previously signed off by a different IA as "Airworthy" at the time of the repair/mod to be "unairworthy" today? (i.e. declaring that it should have never been approved)

[EricJ]

52 minutes ago, 1980Mooney said:

A question for those with IA knowledge out there.  There seems to be a lot of different opinions on the "acceptable" way to repair a rudder skin.  Is it possible for an IA in the future to declare a modification or repair previously signed off by a different IA as "Airworthy" at the time of the repair/mod to be "unairworthy" today? (i.e. declaring that it should have never been approved)

Happens all the time.    Another common instance is an IA disapproving parts that were previously installed as 'not approved', which is why the VARMA program was created, so that an owner can have an actuall letter from the FAA saying it is okay.

[Grant_Waite]

6 hours ago, Fly Boomer said:

Were you able to acquire a new servo, or get the old one rebuilt?

Actually the previous owner had bought a servo a couple months before I bought it so he gave it to me. I’ve just had it sitting at home. I had a highly qualified avionics guy from Sarasota avionics come to me and he diagnosed what made it stopped working in the first place. The in line fuse had blown, from what I’m not sure but most likely because one of the wires was frayed almost in half. Not long before he came I was just using a 12v battery to move the old servo to test it and it shit the bed after a time or 2. The gears inside had all been stripped. See picture attached of the new servo. Old one is identical inside.

[EricJ]

That's one high-tech jack screw right there. 

 

[Grant_Waite]

10 hours ago, EricJ said:

That's one high-tech jack screw right there. 

 

Yeah it’s not the most confident inspiring design by any means. But when it worked it worked. 

[A64Pilot]

Many Experimental’s use Model airplane Servos for Autopilot and or trim

[Slick Nick]

13 hours ago, Grant_Waite said:

Yeah it’s not the most confident inspiring design by any means. But when it worked it worked. 

...Until it didn't.

[Grant_Waite]

4 hours ago, Slick Nick said:

...Until it didn't.

Kinda crazy they still sell them for 2500 bucks. 

[cliffy]

Please report back if you recheck the rudder balance and how it turned out.

[A64Pilot]

2 hours ago, cliffy said:

Please report back if you recheck the rudder balance and how it turned out.

IF it were rebalanced when the servo was installed it should be a little nose heavy, which is conservative for flutter

[PT20J]

I believe that the M20J SMM specifies control surfaces to be balanced trailing edge heavy. 

[N201MKTurbo]

7 hours ago, PT20J said:

I believe that the M20J SMM specifies control surfaces to be balanced trailing edge heavy. 

So, for the rudder, is that the left or right trailing edge?

[cliffy]

Mooney refers to "underbalance" as trailing edge heavy  In the case of the J  it shows in the manual 17.4 to 20.8 ounces at a point of  12.87 inches aft of the hinge line. BEAWARE THE WEIGHT AND THE DISTANCE VARY BY MODEL AND S/N !

Its very specific and detailed in the manual. 

AND its for PAINTED surfaces only. 

We have a 3.4  oz. spread or window for balance on this model.

This is taken from the M20 Service and Maintenance manual Rev D 8/1/81

[Slick Nick]

I've always wondered, how is balance changed? By using weights on the horn? Are they interchangeable, or is the practice to modify the existing weights somehow?

[Grant_Waite]

I finally got the rudder trim back installed Monday and it’s been great ever since. I can make it fly coordinated at any phase with no pressure on the pedals. I haven’t had the rudder rigging or a balance check done. I put everything back the way it was for the last 14 years so I don’t see a need to check anything. It was painted in 2010 with the rudder trim attached so it should have been balance checked then. It’s nice to finally have the damn thing working after over a year. It helped having the avionics guy be mobile because I was dreading having to take it to a shop and pay that premium. Having the ability to trim the rudder and then not, really makes you never want to fly without it again. 

[Fly Boomer]

4 minutes ago, Grant_Waite said:

I finally got the rudder trim back installed Monday and it’s been great ever since.

I was following this but lost track of the final fix.  Did you install the "new" one that came with the airplane when you bought it?

[Grant_Waite]

2 minutes ago, Fly Boomer said:

I was following this but lost track of the final fix.  Did you install the "new" one that came with the airplane when you bought it?

Yes, I did with the original head unit that controls it. The servo runs about 800bucks which is ridiculous considering how simple and cheap the parts inside are. Luckily I didn’t have to buy a new one since I already had it. It was still way cheaper than possibly having to buy a replacement rudder. And now I get the benefit of the trim working. Just hope it stays working for awhile. 

[Fly Boomer]

2 hours ago, Grant_Waite said:

Yes, I did with the original head unit that controls it. The servo runs about 800bucks which is ridiculous considering how simple and cheap the parts inside are. Luckily I didn’t have to buy a new one since I already had it. It was still way cheaper than possibly having to buy a replacement rudder. And now I get the benefit of the trim working. Just hope it stays working for awhile. 

This is only a guess, but I suspect the demise of the servo may be hastened by exposure to TKS fluid blowing around inside the tail.  No way to prove it, and no idea how to mitigate the risk.

[Grant_Waite]

5 hours ago, Fly Boomer said:

This is only a guess, but I suspect the demise of the servo may be hastened by exposure to TKS fluid blowing around inside the tail.  No way to prove it, and no idea how to mitigate the risk.

Probably so but my plane doesn’t have tks anyways. I’m too chicken to even get close to ice, not sure having tks would make me feel otherwise. Luckily ice isn’t a big deal down south. 

[A64Pilot]

On 2/22/2025 at 8:25 PM, PT20J said:

I believe that the M20J SMM specifies control surfaces to be balanced trailing edge heavy. 

You keep saying that, but don’t understand what I am saying.

I’m saying that there is a range of say 1 to 5 oz weight measured at the trailing edge. If your at one ounce then your biased towards nose heavy, 5 oz and your biased trailing edge heavy. That weight is just numbers, I have no idea what a Mooney’s weight is but strongly suspect it’s different by model based on VNE, but haven’t looked.

Not that I’ve seen that many flight controls balance specs, but I’ve never seen one that you weigh the nose, always the trailing edge, so they are all trailing edge heavy

The only point is that if you’re towards max trailing edge heavy, it reduces your flutter margin, meaning flutter is more likely than if you’re at the light end of the range “nose heavy”.

A catch 22 that I’d expect to see if weight is added aft of the hinge line is that it takes more weight to return the flight control to acceptable limits, the problem is that your only allowed a limited amount of weight that you can add, you may be at max weight and barely in the range.

But if your having to change weight, I recommend balancing to bias nose heavy, within limits of course, the reason is as an aircraft ages the flight controls weight changes to bias towards tail heavy from any repairs, paint touch ups or just dirt accumulating over the years. It does that because almost all of the area of the control is aft of the hinge line. I have never seen one’s CG move toward the nose.

[PT20J]

Control surface static (mass) balance can be designed in for reasons other than flutter. Balance affects control forces and stability. When done for flutter reasons, the idea is to move the surface CG at (balanced) or ahead (overbalance) of the hinge line. This causes the inertial reaction to a gust (or other pressure disturbance) to create a moment that opposes rather than reinforces the motion. The susceptibility of a control surface to flutter depends on a number of things in addition to mass balance such as structural rigidity, control surface size and shape (large surfaces and especially surfaces with wide chord are more subject to flutter than small, narrow surfaces), and control system rigidity and friction.

If balance (or overbalance) is not needed for flutter prevention, then balancing can be used for stability and control force tailoring. A underbalanced elevator will increase responsiveness to initial pitch changes. However, with a constant pitch rate (as in a level 2g turn, for instance) it acts as a bobweight and will increase stick force.

[cliffy]

Grant-

You say it was done in 2010 and "should have been checked then" -

Just for your own peace of mind, go back in your log book and find the entry

and see if a balance check was actually signed off then.  

[Grant_Waite]

16 hours ago, cliffy said:

Grant-

You say it was done in 2010 and "should have been checked then" -

Just for your own peace of mind, go back in your log book and find the entry

and see if a balance check was actually signed off then.  

says it was done so I believe it. Hasn’t been repainted since 

[cliffy]

Good to go Don't change a thing  Enjoy it!