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  #1  
Old 07-25-2018, 08:10 AM
mulde35d's Avatar
mulde35d mulde35d is offline
 
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Location: Williamsburg, VA
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Default A/P Servos vs Trim

Before I get into the tailcone section I want to ensure I can build in the necessary trays and mounting points for all the equipment I plan to install without having to drill out rivets later (at least that is the intent).

Unfortunately I can't seem to wrap my head around the differences between autopilot servos like garmin or dynon and the aircraft trim system. Specifically the GSA 28 yaw dampener. Does the GSA 28 hooked to the rudder also allow for electric trim of the rudder?

The Garmin sight advertising states:

"When the autopilot is off, the GSA 28 servo provides speed scheduling for manual trim commands. Likewise, when the autopilot is on, the servo adjusts to constantly keep the aircraft trimmed. When servo input is needed to keep pressure on the controls due to an out-of-trim condition, the system will direct the servo to drive the trim until the control pressure is relieved."

But in a separate paragraph it also states:
"the Garmin servo provides a built-in interface to drive a customer-supplied trim servo at no extra cost. "

Now why would you need it to drive a customer supplied trim servo unless the GSA 28 doesn't trim and in fact needs a ray allen servo down the line to perform electric trim functions.

Maybe I am just reading too much into it, but I am hoping someone who is using the system can set me straight faster than I can research the question.

Any examples of how you have done this install would be appreciated. Thanks
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  #2  
Old 07-25-2018, 09:49 AM
rocketman1988 rocketman1988 is offline
 
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Default Difference

The servo move the attached control surface to a position. The trim servo removes the holding force required. It is just like when you are hand flying. You position the flight control to make the aircraft respond. You then trim it to remive the force required to hold the flight control in the desired position. The garmin can accomplish this by wiring the trim servo with the flight control servo. The gsa28 moves the surface then the rac servo trims the pressure off.
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  #3  
Old 07-25-2018, 10:04 AM
rocketman1988 rocketman1988 is offline
 
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Default Also...

If you read the garmin manual, it states thatís the gsa28 is not recommended for rudder trim. Has it been used as a YD and trim? Yes, and it apparently works but garmin does not recommend it, and states that in the manual...
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  #4  
Old 07-25-2018, 03:39 PM
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Larry DeCamp Larry DeCamp is offline
 
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Default Also Confused

I am taking away from this discussion that the Garmin servo can function as the trim OR communicate with a Ray Allen , YES...NO ?
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  #5  
Old 07-25-2018, 04:30 PM
Bavafa Bavafa is offline
 
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Default

Quote:
Originally Posted by mulde35d View Post
Now why would you need it to drive a customer supplied trim servo unless the GSA 28 doesn't trim and in fact needs a ray allen servo down the line to perform electric trim functions.

Thanks
Garmin servo only sends the signal (thru +12v) to the customer provided trim servo (i.e. Ray Allen) to trim the system and it can not do it on its own.

You will need a trim servo regardless whether Garmin servo will control it when it is engaged or you will control it thru your buttons on your stick when the servo is turn off or not engaged.

The yaw dampener is not the same as trim, it just help in turbulence to cushion the effect of turbulence.

Hope this helps.

Hope this make sense
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  #6  
Old 07-25-2018, 04:32 PM
Bavafa Bavafa is offline
 
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Quote:
Originally Posted by Larry DeCamp View Post
I am taking away from this discussion that the Garmin servo can function as the trim OR communicate with a Ray Allen , YES...NO ?
No....
The Garmin servo only sends the signal and can not do any trimming on its own. Ray Allen is what is physically connected to the trim tab and can move it up or down.
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  #7  
Old 07-25-2018, 07:20 PM
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mulde35d mulde35d is offline
 
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Default

Now you all see why I am a bit confused. What I am taking away from the discussion so far is that if I want both autopilot and trim in all 3 axis (pitch, Roll, & Yaw) then I need to install three GSA 28 servos (pitch, roll & yaw) and 3 Ray Allen trim servos. I will also need to build a movable trim tab on the rudder since one does not exist. The Ray Allen trim servos would then each be connected electrically to the GSA 28 Autopilot servos for their use as well as the manual electric actuator buttons / knobs. Does that properly summarize the setup? Has anyone done this and can state that it was effective?

I do appreciate the discussion.
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  #8  
Old 07-25-2018, 07:37 PM
rocketman1988 rocketman1988 is offline
 
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Default Yes

Yes, three of each plus the install kits
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  #9  
Old 07-25-2018, 07:48 PM
PilotjohnS PilotjohnS is offline
 
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Default Important point

The use of an autopilot servo does not negate the need for trim. The trim can also be manual or electric ( Ray Allen). Without trim the autopilot servo would need to hold a constant force to maintain direction. The trim ( manual or electric) can either move a trim tab (elevator) or bias a spring ( aileron and rudder typically) or tabs can be put on all 3 controls. The trim mechanism is meant to exert a constant force to bias the surface into proper position, the autopilot servos move the surface to change direction, etc.

The use of electric trim servos allows the Garmin G3x to auto trim. In other words, if the Autopilot senses that it is constantly having to hold left aileron, for example, the autopilot would automatically run the trim servo until the autopilot servo no longer has to hold the aileron to one side, in this example.
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  #10  
Old 07-25-2018, 08:13 PM
mturnerb mturnerb is offline
 
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Default

Quote:
Originally Posted by mulde35d View Post
Now you all see why I am a bit confused. What I am taking away from the discussion so far is that if I want both autopilot and trim in all 3 axis (pitch, Roll, & Yaw) then I need to install three GSA 28 servos (pitch, roll & yaw) and 3 Ray Allen trim servos. I will also need to build a movable trim tab on the rudder since one does not exist. The Ray Allen trim servos would then each be connected electrically to the GSA 28 Autopilot servos for their use as well as the manual electric actuator buttons / knobs. Does that properly summarize the setup? Has anyone done this and can state that it was effective?

I do appreciate the discussion.
I'm installing yaw servo. My plan is to install permanent tab as needed for cruise trim and use the old fashioned "right foot" for climb, using the yaw servo for it's yaw damper functionality as well as helping keep turns coordinated. Leaving the option open to install (manual) rudder trim at a later date.

I am installing the standard elevator trim and optional aileron trim which will be connected to their respective servos.
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