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New Service Instruction for Rudder Slippage


Jeff_S

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I just got a new Mooney service instruction for a maintenance repair to help stop “rudder slippage during extreme deflection” (I paraphrased that a bit). Anyone else seen this? It appears to apply to all long bodies with electric rudder trim. I’ve never heard of this problem and don’t feel like spending the money for my A/P to buy the parts and tools needed to complete the job. Plus, the instruction just came out today and my plane is going into annual next week so I would be a guinea pig on this task most likely...not my favorite thing to be!

Anyway, it’s not an AD or even an SB so I’ll likely just ignore it since I don’t use the electric rudder trim anyway, but I’m curious if anyone else has thoughts or knowledge on this. Thanks!

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Jeff, If you have a copy that I can read....?

Sounds like it is possible to use the electric trim, and drive the rudder to the stops... then something creeps under the force of that... followed by not coming back to Center after the creep.

If this is the case,  you would know something isn’t right because the ball on the TC would be out of whack...

And taxiing in a straight line would become a challenge with the rudder trim indicator centered...

I don’t think I have ever used that much rudder trim... I may come close on elevator trim, I put one hand on the wheel to know if it stops...

Best regards,

-a-

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My MSC had warned me about running the rudder trim hard on the ground due to trim slippage. He preferred to run the trim in on the takeoff roll rather than against the static centering forces.     Luckily my SN is before the affected list, and I've never seen the problems. 

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I experienced the described problem yesterday with an Acclaim after a takeoff in turbulence. The rudder trim stuck in the full right position (=takeoff position).
After two subsequent flights the same day with the left leg continously on the rudder in flight I opended the side panel next to the battery compartment and moved the rudder wire slightly. Funktion of the electrical switch came back to normal after that.

Learned about the SI from my repair shop last night after returning home.

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I have never used the rudder trim, and was similarly wondering if this was a solution in search of a problem. Sounds like Peter found out it is real. I intend to wait till next annual in October and then I'll see what I'm going to do about it.

Robert

PS: welcome on board Peter "L"

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  • 3 weeks later...

I'm getting this issue addressed within the next two weeks at DLK Aviation in Atlanta.  Mooney has the parts in stock.  I'm always using almost full right rudder trim in the climb and during high-speed descents, I'm at full left rudder trim.  It will be covered under warranty, which helps.   I never operate the rudder trim unless the aircraft is in motion on the ground as per my excellent Acclaim Instructor Mr. Mike Elliott.  

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It turns out the rudder trim works as an electric tiller...

Setting the rudder trim in the checklist then taxiing to the runway, it becomes noticeable....  the plane is pulling to the right.

I am just recognizing how much stress that could be while not moving...  :)

PP observations only...

Best regards,

-a-

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On 4/30/2018 at 9:21 PM, Mooney_Allegro said:

I'm getting this issue addressed within the next two weeks at DLK Aviation in Atlanta.  Mooney has the parts in stock.  I'm always using almost full right rudder trim in the climb and during high-speed descents, I'm at full left rudder trim.  It will be covered under warranty, which helps.   I never operate the rudder trim unless the aircraft is in motion on the ground as per my excellent Acclaim Instructor Mr. Mike Elliott.  

Those airline pilots are used to 1000 fpm+ descents! I guess also when you fly your Mooney in the high teens you have to point the nose downhill pretty aggressively sometimes. We normally aspirated folks don't have that problem as much! Seeing as I never use rudder trim, I have to admit I've not gotten much instruction on it. I've never heard that you should only apply it while in "motion on the ground" so I'm curious about that.  Mike, can you elaborate?

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PP observations...

  • The rudder trim is pretty strong, as in it has a strong motor...
  • Some versions don’t have enough strength to resist slipping...
  • I can’t move the rudder very well on the ground, unless I am moving...
  • I have always used the move before steering in the car.... makes sense in the plane too....

As far as setting the trim on the ground... that is what I have always done... may rethink that if I can...

As for using it in flight... keeping the ball centered during the climb and in the descent...  must be worth a couple of KIAS to fly straight through the air... no additional effort needed...

PP thoughts only...

Best regards,

-a-

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I can hold the wheel straight whether the rudder is trimmed for take off or not. Wouldn’t the rudder trim have the same force on it whether I set in motion and held the rudder straight or the rudder is stationary straight and I apply the trim?

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Yeah, well, I checked the POH and there’s nothing in there about not applying rudder trim while stationary on the ground. As to the car allegory, that makes some sense because in a car you’re actually turning the angle of the tire on the ground, but rudder trim doesn’t actually turn the nose wheel...it just deflects the rudder, right? (And Anthony, maybe you need a car with power steering! :))

Anyway, it’s a moot point since I don’t use rudder trim but I am still curious about the recommendation on ground use.

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Nose wheel and rudder are tied together through the rudder pedals...  and springs to the airlerons too...

The IO550 sitting out over the nosewheel adds to the forces required to steer the nose wheel.

The rest is good tire grip.

What generates the large force while not moving is... the power steering is twisting the tire a long distance.  While moving... the tire slips in tiny increments and movement is allowed without a lot of force building up...

the cars in the carusoam stable only have power-assisted steering.  Better road feel, compared to Mom’s station wagon.  One finger turns the wheel while parked on dry pavement.

over powering the tire’s resistance with a motor leads to complaints from the power steering pump... and funny circular scratches on the rubber. The driveway surface isn’t going to like it either... test this on a light colored concrete.  You will see the dark rubber evidence....

CB test...  If you get antsy after your 255/35VR17 tires leave dark marks on the concrete, you know the 0.4 AMU tire just lost some valuable microns of thickness...  And somebody else is going to be dissapointed with the marks left on the driveway....

 

So in the end... Some long bodies are proven to be stronger than others regarding this accidental stress test.  The weak ones are clearly documented by serial number in the service instruction...

The instruction just mentions this weakness but doesn’t add to any limitations.

If you have one of the effected serial numbers, you want to get the issue addressed.  Before something slips and causes a control challenge...

There is a line regarding control during the take-off run that doesn’t sound very friendly.  ...Trim slips during Extreme rudder deflections during the T/O run?...

I haven’t used extreme deflections of the rudder very often....

Using them while on the ground would be a wild ride, I suppose...

 

PP Thinking out loud, not a mechanic...

Best regards,

-a-

 

 

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Okay, I've learned more about this topic then I ever thought possible, so thanks for that! But until such time as I don't have the strength in my calf to hold right rudder on takeoff I'm not going to lose much sleep over this one. It's true that on takeoff the 310hp requires a good deal of rudder, but I rather enjoy the chance to do this and relive the small amount of tailwheel time I've had. (I almost drove a Super-D off the runway on my first takeoff...my first experience with gyroscopic forces.)

Maybe on a super smooth day I'll point the nose downhill and see if I really need left rudder descent. So far I have never noticed the ball outside in this scenario.  Perhaps I'm missing out on that pure Mooney speed!

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Wait Jeff, there is more.... :)

The gyroscopic effect is what occurs with the change of attitude.  When rotating the gyro (engine / spinning parts) a lot, it becomes more noticeable...

Tail wheel plane will get funky when quickly raising the tail...while still on the ground...

Tricycle planes get funky when lifting the nose, but your not on the ground for long after doing it....

These are very dynamic situations.  Feet on the rudder pedals handle this...

 

The rudder trim is less dynamic and more steady state...  this funk is a byproduct of the attitude of the plane and the AOA of the propellor...

With positive attitude the blades of the prop get a different amount of lift depending on if they are moving up or down...

With a negative attitude the AOA effect is reversed...

 

The reason we use a lot of trim in the climb... the attitude is pretty steep.

If we used the same amount of attitude for the descent, we would exceed redline pretty fast.

 

So we use rudder trim, because we have it...  and climbing into the teens takes some time

When in cruise... center the ball using trim... flying straight is fast...

In descent... center the ball trimming the other way... because we can...

 

It really takes stable air to be able to see this effect.  Otherwise, bumping around turbulence masks the whole thing...

 

Helicopter blades really have a strong AOA effect because of the forward moving blade and retreating blade are seeing different relative wind related to the airspeed.   The ‘swash plate’ guides each blade’s AOA, using roller cam followers.... increasing the blades AOA as it retreats, and decreases the AOA as it’s relative wind increases...

Who knew we could get anything useful  out of all those semesters of physics.... :)

 

PP thoughts only, not a physics guy or CFI....

Best regards,

-a-

 

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