Electric Trim and autopilot

Background:
I'm considering installing a two servo autopilot later in my build. I did not select the electric trim when I purchased my empennage kit. I'm about ready to begin riveting the skins on the HS and VS. I will soon be ordering the wings. All of my hours are in our Aeronca Champ with no electrical system so manual trim doesn't bother me. My question is more about autopilot/electric trim compatibility and necessity.

Questions:
Is there any advantage to installing the electric trim if I plan on using the autopilot in cruise?

Can I purchase and install the electric trim at this stage of my empennage build or am I committed to manual trim?

Thanks,
Michael

Electric trim

Michael,

I use a Trio autopilot (with the auto trim function) with my electric trim. The advantage of combining the autopilot with an electric trim (in Trio's case) is that you can have "auto trim" added to the autopilot. What this does is automatically keeps the airplane in trim at all times so that when you disconnect the autopilot you don't have a lot of pitch force on the stick. If you don't have any type of electric trim (at least with the Trio) you will have a lot of pressure on the stick, depending on your flight conditions, and could be in for quite a surprise when you click off the autopilot.

So that's the way I understand how the system works. Others may have different or better opinions, but like my signature block says- I have smart friends.

Oly

Early on in my autopilot use a couple of years ago, I had the A/P kick out once because of improper manual trim setting. Disconcerting for sure. Of course, it was during an IPC. Embarrasing.

A few habit changes and the issue disappeared. One was to be more dilligent about trimming for configuration prior to A/P engagement and also to pay attention to the trim warnings coming from the A/P (I had a TruTrak Digiflight IIVS and now use GRT which both give out of trim indications). One can trim manually using the out of trim indications and get it close, certainly close enough to keep the A/P from complaining. If a warning occurs, its easy to center the trim between up and down warnings without turning the A/P off.

If the A/P allows separate engagement and disengagement of the individual servos (Ie GRT) then it is even easier to check pitch trim by hand.

olyolson said:

Michael,

I use a Trio autopilot (with the auto trim function) with my electric trim. The advantage of combining the autopilot with an electric trim (in Trio's case) is that you can have "auto trim" added to the autopilot. What this does is automatically keeps the airplane in trim at all times so that when you disconnect the autopilot you don't have a lot of pitch force on the stick. If you don't have any type of electric trim (at least with the Trio) you will have a lot of pressure on the stick, depending on your flight conditions, and could be in for quite a surprise when you click off the autopilot.

So that's the way I understand how the system works. Others may have different or better opinions, but like my signature block says- I have smart friends.

Oly

Click to expand...

I fly the Trio Pro Pilot with manual trim in the RV-6. I like having the plane in trim at all times so am careful to set trim in cruise prior to engaging the Pro. This means there are no interesting moments when the autopilot is disegaged in cruise.

However, where auto trim is really nice is instances where airspeed changes significantly while the autopilot is engaged. This can happen during a high speed descent from cruise that is being flown by the autopilot. As speed picks up going downhill, trim pressure can become high enough to cause the safety overrides in the Pro to disengage the servo. With auto trim, the system will adjust trim to prevent the servo from being overloaded enough to disengage.

This scenario can be avoided with manual trim by tweaking it a bit as the descent begins, but auto trim makes this a hands-off operation.

Not going to get into pros and cons but just say you can easily add electric trim after the fact if you want to.

Easily is qualified by saying if you mount the trim servo in the space covered by the stabilizer/rudder fairing and use a short actuator cable (similar to the manual cable but maybe 8-10" long) it is doable to add without rebuilding elevator, etc.

Have this setup on my -4 and like the fact added *** of servo isn't flopping around in my elevator.

rbibb said:

Have this setup on my -4 and like the fact added *** of servo isn't flopping around in my elevator.

Click to expand...

Have no idea what transformed "mass" into *** but you can guess what a typo can end up ***.

rbibb said:

Not going to get into pros and cons but just say you can easily add electric trim after the fact if you want to.

Easily is qualified by saying if you mount the trim servo in the space covered by the stabilizer/rudder fairing and use a short actuator cable (similar to the manual cable but maybe 8-10" long) it is doable to add without rebuilding elevator, etc.

Have this setup on my -4 and like the fact added *** of servo isn't flopping around in my elevator.

Click to expand...

I preferred servos in my elevator, as well as aileron. Didn't flop, and no cable "slop"....

RV6A

L.Adamson said:

I preferred servos in my elevator, as well as aileron. Didn't flop, and no cable "slop"....

RV6A

Click to expand...

Just to clarify what I meant by"flopping around" was not that the servos actually "flop around" in the elevator because they would, in fact, be securely mounted but that the servos do not move in conjunction with the elevator movement (other than their movement as part of the overall airframe movement). To me this lessoning of the movement of inertia of the elevator structure seems desirable but obviously is not critical given the large numbers of RVs flying with servos in the elevators.


The larger point was that such an installation is probably easier to retrofit to an airplane with an existing manual trim.

Thanks for the info gentlemen! One more question while I have everyone's attention.

At what point during my build should I purchase and install my A/P? When is it easiest to do?

I am leaning towards the tru trak but I'm going to hold off on any decision until after Oshkosh when I can look over all my options in person.

michael1890 said:

Thanks for the info gentlemen! One more question while I have everyone's attention.

At what point during my build should I purchase and install my A/P? When is it easiest to do?

I am leaning towards the tru trak but I'm going to hold off on any decision until after Oshkosh when I can look over all my options in person.

Click to expand...

Michael, I suggest you not install any autopilot stuff until you are ready to finalize your panel. Things change quickly in the avionics universe and servo mounts can be added to built-up structure without a lot of hassle.

After tire-kicking at OSH you may decide to integrate the AP with an EFIS and this may point you toward a vendor you haven't considered.

No need to lock in to a particular system at this point.

At least with the Dynon AP system, the desire to have you trim up or down is annunciatied on the screen. So if you change something about the plane and need different trim, it doesn't care if you move a manual wheel or an electric trim to make it so. As long as you trim until the indication goes away, it will be in trim when you turn off the AP.

It is good to know that Dynon has the same features as everybody else.

Both of Gill's posts are right on. If you are only going to use the autopilot for cruise, with no speed changes, it's not needed. If you're going to use it for cruise, slow to approach speed, deploy flaps, etc., then it's a 'nice to have' feature.
One thing not mentioned - if you go the Trio Pro route with auto trim, you can use it to adjust the electric trim even when hand-flying, and the trim motor speed is automatically adjusted for airspeed (fast trim at low airspeeds, slow trim at cruise speed). I use this feature all the time.

Thinking about avionics and autopilots prior to finishing wings, fuselage, canopy, engine, prop, and wheel pants is wasting time. By the time you get to the point where you need to make those decisions, everything you decide on today will be outdated ancient technology. Focus your energy on completing the tasks at hand and waste very little time on thinking that far down the road.

Electric trim and autopilot servos can be added at any time later with ease. There are a lot of rivets between you and those decisions. Build on.