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  #1  
Old 04-17-2021, 06:10 PM
ShortSnorter ShortSnorter is offline
 
Join Date: Dec 2013
Location: NOLA
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Exclamation RV-14 Garmin GSA 28 Over Center

I had an interesting experience this afternoon when I was doing some ground testing of a standard install G3X AP system. After I got home I did some "VAF'ing" to see what others experience has been and ran across this thread. Post #33 was exactly the scenario I was testing.

To be as brief as possible what I was doing was verifying that the torque on the GSA 28 was low enough I could over power. Previously I had done the directional control tests ensuring the wiring was correct, but had never done the "over power test." To my surprise, when I tested the right aileron up movement force (trying to over power with left stick), I heard a THUNK. At that point I had no left aileron (jammed controls) but could move center to right freely. I knew the only thing it could be was some sort of linkage issue so I pulled the inspection panel and found the first picture below. I couldn't for the life of me figure out how it went over center as I had check control stops and verified with the servo arm travel positions.

I was able to move the arm back from the over center position and verify the mechanical control stops and the the corresponding position of the servo arm (2nd picture). I'm not sure how, but my theory of what happened was as it reached it maximum limit, the servo arm was JUST far enough to allow it it pivot over center with enough force. I verified this by manipulating the servo arm by hand and pivoted past the mechanical stop to the over center position (1st picture).

(3rd Picture) I moved the push rod linkage to the center hole (from the hole nearest the shaft) and due to the changed geometry, was unable to replicate the over center position by hand or with AP servos active. I am definitely planning on installing the mechanical stops that came with the servo! The only reason I did not install was the instructions say as long as the geometry does not allow over center then the stops were optional.

Please go check your AP Servos!!! Even though though they check out visually through full range of motion you could have a situation like mine. I doubt there would ever be a real world scenario of full AP commanded aileron deflection that could potentially cause this scenario, but its worth a piece of mind to check it out.

**Not sure why the pics are turned?!
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Last edited by ShortSnorter : 04-17-2021 at 06:18 PM.
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  #2  
Old 04-17-2021, 10:02 PM
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fl-mike fl-mike is offline
 
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Default

Why is the stop bracket not installed?
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  #3  
Old 04-18-2021, 04:59 AM
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Plans call for an AN3 bolt for the rod end attachment, why are you using a screw?
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  #4  
Old 04-18-2021, 06:06 AM
ShortSnorter ShortSnorter is offline
 
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Quote:
Originally Posted by rv6rick View Post
Plans call for an AN3 bolt for the rod end attachment, why are you using a screw?
Great question! I used a screw because that's what the plans call for However, it appears I connected the screw from the wrong side.


Appreciate the feedback guys - My point with this post was that I installed per plans and tested range of motion with no obvious issues until the servo was powered and it was able to pivot within the rod end past over center. It took quite a bit of force to replicate by hand which is why I didn't catch it when testing normally.
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Last edited by ShortSnorter : 04-18-2021 at 06:17 AM.
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  #5  
Old 04-18-2021, 06:35 AM
lr172 lr172 is offline
 
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Quote:
Originally Posted by ShortSnorter View Post
Great question! I used a screw because that's what the plans call for However, it appears I connected the screw from the wrong side.

.
The plans also appear to call for the attachment in the lowest hole, not the top hole. They probably specified this for the reason you experienced.
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Old 04-18-2021, 10:08 AM
ShortSnorter ShortSnorter is offline
 
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Quote:
Originally Posted by lr172 View Post
The plans also appear to call for the attachment in the lowest hole, not the top hole. They probably specified this for the reason you experienced.
It's hard to tell in the pictures, but the first two photos are the original install with using the lowest hole in the arm.

The final photo is where I moved it to the middle hole in the servo arm which limits the travel of the arm significantly.
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  #7  
Old 04-18-2021, 10:15 AM
PilotjohnS PilotjohnS is offline
 
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Default Missing stop

I thought the Garmin servo came with a stop/ bracket that attaches to the servo and prevents this? That bracket/ stop appears to be missing in your photos. I think the missing bracket is a safety of flight issue and must be corrected.
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Last edited by PilotjohnS : 04-18-2021 at 01:29 PM.
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  #8  
Old 04-18-2021, 11:23 AM
lr172 lr172 is offline
 
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Quote:
Originally Posted by ShortSnorter View Post
It's hard to tell in the pictures, but the first two photos are the original install with using the lowest hole in the arm.

The final photo is where I moved it to the middle hole in the servo arm which limits the travel of the arm significantly.
See that now. Misunderstood the pics.
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  #9  
Old 04-18-2021, 11:48 AM
molson309 molson309 is offline
 
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Admittedly I am paranoid, but even when using mechanical stops I like to keep away from being close to center at either end of the rotational arc, preferably about the same number of degrees away from center on each end. If I can't manage that I do what you did - move the linkage out another hole on the lever.

The thought of a control linkage going over center while in flight gives me nightmares.
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  #10  
Old 04-18-2021, 02:27 PM
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Walt Walt is offline
 
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You should not move the arm to another hole in the servo arm or you will change the torque, it was spec'd there for a reason.
Adjust the length of the pushrod if you need to change the position of the servo arm. The arm should be approx. perpendicular to the servo when the controls are in the neutral position and swing approx. 45deg each way. I install the Garmin supplied servo stops on all my installs for safety (but have not done a 14 yet so maybe something prevents that?).
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Last edited by Walt : 04-18-2021 at 02:31 PM.
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