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electric trim question.

swift12

Well Known Member
so Im installing an electric trim in the RV7. id like a switch on the control stick rather than the rocker switch supplied. There is very little information on the Ray Allen electric trim I have included. Does it protect from a runaway trim condition...seems not if you have a short to ground...if a runaway trim occurs is there sufficient elevator authority to overcome it at full trim? from the Ray Allen FAQ see below.


"Can a RAC actuator "runaway" and travel uncommanded?

No. Although RAC actuator have position sensors that feedback information to our indicators, there is no electronic connection between the sensor and the electric motor. The only way to get movement out of a Ray Allen (or a MAC) actuator is for the pilot to manually switch on power. Our REL-1 or REL-2 relay failure will not cause a runaway trim.
Some cautions about using any relays (including our REL-1 and REL-2) to control our actuators. First is that the relay, not the pilot, is controlling yor trim actuator. Second is you are controlling the relay. These relays are activated by a ground signal so a short to ground will lead to actuator movement. Make sure the wire leading from your switch to your relay is not shorting to ground!"

anyone had one stuck at full trim and has anything to comment on? cheers
 
From what you've posted, this sounds like a terminology issue. The RA servo won't "runaway" in the sense that it has a failsafe design so that sensor power can't drive the servo to move in the case of an internal failure. That doesn't mean it's being protected from you commanding it to full deflection, which would be the case in a ground short on your switch. If you want to sanitize your inputs so that a stuck switch or ground fault won't command full trim deflection, then look into grabbing a TCW safety-trim controller. Garmin, dynon, and VPX have built in trim controllers as well that you can set a maximum runtime for trim power in case the switch gets stuck.
 
Ditto

Tom Berge gave me my 7A transition training and one of the things he had me do is experience full elevator trim deflection. it takes a firm grip on the stick, but the plane is definitely controllable.
Note that there is a spec for pitch trim max up/down travel.
Spring 2021, I installed a Dynon HDX panel in my 7A, including the Dynon AP control head. The trim input and output is connected to the AP control head for manual trim commands and auto trim. It only allows x seconds of manual trim command before you have to release the button and depress again (runaway trim protection).
 
I just removed a TCW elevator safety-trim module because of a Dynon AP install. If you research it and think it would work for you PM me and I'll send pics and info. I would sell it for a readable price. If you are going to install Dynon or AFS AP they would have trim protection with the install of the external controler as well.
 
All great replies and thank you very much.
Yes I’m going dynon so that’s great news. Nice to know full trim is controllable. I thought it would be but nice to know real world experience.
A lot of jets I’ve flown have stab cut out switches…aka Boeing. The bus has all its electrickery going on in the back ground with ELAC AND SEC computers and if that doesn’t work it stops the movement and then a jammed stab procedure with manual trim and if that doesn’t work you can still fly it home.

If the trim started to runaway with a ground short issue and is electrically cutout by the dynon system at least it won’t run to full deflection.

I did a few searches but like all these darn searches I didn’t use enough “buzz” words to find my answer.

Now I know there is a good way to add some protection other than a standalone cutout switch.
Cheers.
 
The Dynon SkyView AP system has a 5 second trim control time-out feature. 5 seconds is rather long in this situation. I'd rather have a 3 second time out.

Ref: Page 18-5, SkyView System Installation Guide – Revision AJ.

"Trim Motor Control Safety Features
...
• If a trim control button is pushed for more than five seconds (or longer, such as a stuck button), trim control on that axis will be temporarily inactivated until the button is first released.
..."


Note that the Trim Motor Control Safety Features, including the trim control deactivation after 5 seconds, is only available if the SV-AP-PANEL is installed.
 
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Another possible solution is a trim power switch. In the T-6B Texan II, there was a button on the stick that when pressed and held would remove power from the trim motor. Then with that switch depressed, the trim circuit breaker could be located and pulled. Decent solution for an RV that doesn’t add too much complexity.

We would test the switch on the post landing checklist every flight.
 
If you use a relay deck like a PH Aviation unit applying opposite trim will stop the motor.
 
If you use a relay deck like a PH Aviation unit applying opposite trim will stop the motor.

I like that logic better than the timeout. It’s instinctive and simple. I designed an auto-trim controller some years ago that implemented it. The problem with a timeout is that in a go-around you may need to depress the trim switch multiple times… one more thing to remember during a busy time.

V
 
So “full trim” is controllable IF you limit your trim travel to what is required or recommended (not sure which). So that’s a good argument for finding out how juch you need and then adjusting your trim tab linkage so that you don’t have a ton of extra deflection that you don’t need.
 
Good discussion thanks for helping. A friend has said the ray allen 12A is too long a travel for an RV and the 7A a much better servo for the rv. This is the one vans specified and sent me with the empennage kit. Comments?
 
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So “full trim” is controllable IF you limit your trim travel to what is required or recommended (not sure which). So that’s a good argument for finding out how juch you need and then adjusting your trim tab linkage so that you don’t have a ton of extra deflection that you don’t need.

The reason I think I don’t need all this servo travel. Might look at the smaller travel servo
 
There is a reason that on airliners the stab trim disconnect switch is in a highly visible location. It kills power to the system completely. Too bad the Lion Air guys weren't aware of it's function.
 
There is a reason that on airliners the stab trim disconnect switch is in a highly visible location. It kills power to the system completely. Too bad the Lion Air guys weren't aware of it's function.

Have you tried that profile in a full motion sim? I have...your comment would indicate that simply selecting the stab trim cutout switches to off would have solved the problem. Given their situation, you would be wrong.
 
Before I Installed electric trim on my RV7A I twice experienced situations where the manual elevator trim knob became disconnected from the elevator trim cable. Flying the airplane (if I’m remembering correctly) required holding significant forward pressure on the stick. When landing I had to release this pressure to “flair.” Very uncomfortable feeling - but at all times there was sufficient elevator authority to control the aircraft.

Full nose down or nose up trim cannot exhibit greater control forces than a disconnected manual trim tab. Don’t worry about a runaway electric trim condition. There is plenty of elevator authority to handle the problem.
 
…..Don’t worry about a runaway electric trim condition. There is plenty of elevator authority to handle the problem.

I don’t think anyone believes otherwise. If you think about it, trimming ‘the wrong direction’ actually increases elevator authority. The only question is, Is the pilot strong enough to use it?
The answer for the average pilot in an RV seems to be yes. But I have certainly seen pilots in other aircraft - e.g., a 182 - who could not physically flare the airplane if the trim was way off.
 
You can configure the trim run time in the Garmin G3X menu. The trim motor will only run for the time you set even if the trim button is held down. You will have to release the button and then press again to get more trim. The idea is to prevent trim runaway should your trim switch stick or short to ground. I set mine to run no longer than 3 seconds at a time. Never found an instance where I needed much more than 3 seconds of trim movement and it's a nice peace of mind safety fix that's easy to implement.
 
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