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A/P Servos vs Trim

mulde35d

Well Known Member
Friend
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
 
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.
 
Also...

If you read the garmin manual, it states that?s 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...
 
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 ?
 
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
 
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.
 
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.
 
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.
 
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.
 
Mounts

Has anyone come up with mounting hardware kits for the GSA28 and Ray Allen servo's in the RV-14 yet. I see them for the 7 and 10 all over, but hadn't seen an RV-14 specific one yet.
 
Has anyone come up with mounting hardware kits for the GSA28 and Ray Allen servo's in the RV-14 yet. I see them for the 7 and 10 all over, but hadn't seen an RV-14 specific one yet.
You will get the mounting HW with the kit and it is pretty easy setup.

For the rudder, if an electric trim is not a must, you can use a manual one. It is rather easy and inexpensive. RVs require little to no rudder, other than take off or slipping. Once you get it setup for cruise setting, you will find it hardly any need for a trim.
 
Maybe someone reading this at Oshkosh would go to the Vans tent on our behalf and ask -

1. Are yaw dampers required or beneficial on the RV14 and RV10 ?

2. If so, why and why have you not developed a rudder trim system ?

Please report back asap :D:D:D
 
Need for yaw servo?

I'm still looking for anyone to comment on the effectiveness of a yaw servo in a 14? I'm thinking I don't need one.
Any thoughts?
 
Yaw Servo

I had the pleasure of flying the RV-14 demo aircraft with Vic Syracuse near Atlanta which had the Garmin yaw servo installed. Based on that 1 flight I made the following observations.

The RV-14 did not have any adverse yaw or dutch roll characteristics. The aircraft was flown at cruise speeds (120 - 148 KIAS) both with and without the Yaw Dampener engaged. While engaged, the aircraft had a deadbeat response to disturbances about the vertical axis.

Translation: the aircraft is pretty smooth to begin with and does not "need" the Yaw Servo. With that being said, if you regularly fly long cross country flights with a passenger who gets queasy easily, the yaw Dampener will reduce the air sickness in gusty or turbulent conditions by bringing the nose back to center without oscillation after a disturbance.

I chose to install both the Yaw servo and an electric yaw trim since I wanted to play with all the toys, could afford it, and wanted to make sure my wife would "enjoy" flying cross country with me. With that said, you should evaluate your mission, cost position, and comfort needs on this one.
 
I had the pleasure of flying the RV-14 demo aircraft with Vic Syracuse near Atlanta which had the Garmin yaw servo installed. Based on that 1 flight I made the following observations.

The RV-14 did not have any adverse yaw or dutch roll characteristics. The aircraft was flown at cruise speeds (120 - 148 KIAS) both with and without the Yaw Dampener engaged. While engaged, the aircraft had a deadbeat response to disturbances about the vertical axis.

Translation: the aircraft is pretty smooth to begin with and does not "need" the Yaw Servo. With that being said, if you regularly fly long cross country flights with a passenger who gets queasy easily, the yaw Dampener will reduce the air sickness in gusty or turbulent conditions by bringing the nose back to center without oscillation after a disturbance.

I chose to install both the Yaw servo and an electric yaw trim since I wanted to play with all the toys, could afford it, and wanted to make sure my wife would "enjoy" flying cross country with me. With that said, you should evaluate your mission, cost position, and comfort needs on this one.

Same decision here, for the same reasons. I would add that it's a very easy install. Not much downside.
 
When to install?

Waking this thread back up since I'm about to start my empennage build. I definitely want a 3 axis autopilot, and at least 2 axis electric trim. When is the best time to install all of these servos? Assuming I can cobble together my penny jars, will it be significantly easier to install pitch/yaw while building the tail or is the -14 so well designed that the servos are simple to add later on with the avionics?

I'm leaning towards a Dynon or AFS system, but who knows what will be available when I get to avionics in a few years? It seems like it would be easier to add servos now, but a big of a gamble since I might wind up going with a different system. It looks like Dynon has a mounting kit for roll and pitch, but nothing mentioned for yaw.

Also, how did you guys go about adding electric yaw trim to the RV-14?

Thanks and sorry for the noob questions!
Matt
 
You can buy the servos later when you've decided on your avionic package. I did purchase the mounting hardware for the yaw, and pitch servos. Much easier to put them in while you are working on the tail cone.
 
You can buy the servos later when you've decided on your avionic package. I did purchase the mounting hardware for the yaw, and pitch servos. Much easier to put them in while you are working on the tail cone.

Wouldn't it be different mounting hardware for different brand servos or are they common enough to work with either?

Matt
 
Hardware

Definitely easier to install the mounting hardware and wiring during the empennage build, but quite simple to add the expensive servos later on when you get the electronics. I found it beneficial (if you know what you want) to buy the electrical connectors and pre-wire them before closing out the tailcone, but I also did all my own wiring.

In regards to electric yaw trim I fabricated a ray allen servo in the rudder with a piece of piano hinge on the trailing edge. Is it needed in a 14? No. But I wanted it so I built it. It is controlled by a ray allen rocker switch on the center tunnel.

Sorry, not the best pictures of the electric rudder trim, but you get the idea.
7-0-3.jpg

7-12-6.jpg
 
Fair

In case you haven't seen this report:

https://smartpilot.ca/images/TSB/PDF/a10o0018.pdf

Quote:
The addition of weight, however, can decrease flutter speed by 50 knots or greater.
Any imbalance, such as paint and filler, which increases the weight of the rudder aft of the hinge line, has an adverse effect on flutter speed.

So after reading the report my takeaway is that the builder should be sure to balance the rudder/ailerons with the appropriate counterweights after painting or otherwise adding weight to the aerodynamic surface. A good procedure to do no matter what is installed. If done properly there is no effect on the "flutter speed". Sounds like everything on the plane whereas if it is done improperly it may kill you.
 
If done properly there is no effect on the "flutter speed".

Maybe.....

One of the influencers of flutter susceptibility is control surface stiffness.

Not saying that yours is at risk (can't tell that from just looking at photos), just pointing out that balance is not the only thing to consider.

The challenging aspect of doing a mod. like this "properly", is how much balance weight to add.

If I remember correctly, the RV-14 rudder if built to plans is not 100% statically balanced (wasn't needed for adequate flutter margin). That means that at a minimum the balance should be adjusted to get it back to the same point it would have been without the rudder trim system. Going a little further would probably be wise to account for a potential slight degradation in overall stiffness.
The reason for doing this vs just balancing 100% is that I think you will find that to do so would require adding a lot of additional weight which considering the location, can induce other problems.
 
Going a little further would probably be wise to account for a potential slight degradation in overall stiffness.

Scott - there are several different rudder counterweights in the catalog (E-614, E-614-020, and E-714). Can you tell us if they all have the same hole spacing, and if so, what their weights are?
 
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