Positive control doesn’t like cold weather

[Eric fddt]

My positive control works fine unless the temperature doesn’t go 30F or lower ! When cold outside, the plane is turning right.

My mechanic thinks that one of the servo is stiffer than the other and doesn’t work good in cold temperature.

Is this possible and can I have it redone ? I bought 2 used servos that were suppose to be in good shape but my mechanic did not want to install it cause he says they are not better than mines. 

[0TreeLemur]

I'm skeptical that your problem is due to a temperature-related difference in stiffness between the two servo membranes.  Their material properties of the rubber don't matter that much as long as they are intact.  All they do is pull when vacuum is applied.  What your mechanic means is that one of them becomes so stiff when cold that the vacuum can't retract one of the membranes.   That sounds unlikely if your vacuum is within spec.  Those membranes are not that stiff, even when cold.  More likely to have a small rip in one of the membranes that allows more leakage at cold temperatures.

Do you have a vacuum gauge?  What is the vacuum when cold?   Troubleshooting required.  See section 6 of the Mooney M20 Service & Maint. Manual, Table 2.  Positive Control System Trouble Shooting.

[takair]

I agree with Fred.  Not likely a membrane stiffness.  Which gyro do you have?  Is it the remote or panel mount?  It is possible that the mechanism gets stiff in the cold.

[Utah20Gflyer]

Maybe you have a crack in the system and when it gets cold the material shrinks and opens up the crack to the point the vacuum gets dumped?   I'm far from a PC expert and actually I'm working on troubleshooting my system as we speak, but I would look for leaks in the lines or vacuum servos first, and then go from there.  Preferably while it's cold since that is when the problem exhibits.  

[BobbyH]

Eric, I agree with Fred also, doesn't sound like the servos.  Their properties are usually pretty stable with temperatures.  My 2 cents would be spent looking at the gyro.  Is it getting enough vacuum and not hanging up on you.  If you haven't seen it yet, SW1024 has links to a number of PC Operation and Service files.  I found them quite helpful.  Another reason I would suspect the gyro controller is if the plane is turning, it should read that and readjust to maintain stable flight.  As mentioned, there could be a vacuum leak that isn't allowing the servo to get the message from the controller, which may be a little easier to verify with a separate vacuum source and gauge. 

Bob

[Mooneymite]

Let me think....  Right turn:  check the green lines  Left turn, check the red?

Is that correct?

 

[0TreeLemur]

4 hours ago, Mooneymite said:

Let me think....  Right turn:  check the green lines  Left turn, check the red?

Is that correct?

 

Green line connects to right aileron and aft rudder servo, red to left and foreward.   I would think that when the servos pull on the aileron they provide downward deflection?  If so, then what you wrote is correct.  Anyone know for sure?  I just looked at the S&MM and the parts manual, they don't give enough detail to tell.

[carusoam]

1965 short bodies had the gyro in the tail….

A pretty cold place to operate for 50years…

It may be asking for a service call….?

Best regards,

-a-

[David Lloyd]

I had a stuck pilot valve on my ‘65 C. Easy to envision one being sticky in cold weather.