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  #1  
Old 07-28-2022, 12:17 AM
swift12 swift12 is offline
 
Join Date: May 2021
Location: Palmerston North
Posts: 167
Default electric trim question.

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
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  #2  
Old 07-28-2022, 08:40 AM
agent4573 agent4573 is offline
 
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Location: Mountain view
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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.
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  #3  
Old 07-28-2022, 08:59 AM
Mikeyb Mikeyb is online now
 
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There has been lots of discussion on runaway trim. Here is a recent one.

https://vansairforce.net/community/s...t=Runaway+trim

In the first few hours of flight I flew my 8 at cruise and added full up/down trim and simulated a return to landing.
It’s a handful but doable. You want to slow down ASAP.
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  #4  
Old 07-28-2022, 11:03 AM
tthrew tthrew is offline
 
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Default 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).
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  #5  
Old 07-28-2022, 11:43 AM
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Larco Larco is offline
 
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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.
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  #6  
Old 07-28-2022, 03:49 PM
swift12 swift12 is offline
 
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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.
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  #7  
Old 07-28-2022, 03:55 PM
RV8JD RV8JD is offline
 
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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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Last edited by RV8JD : 07-28-2022 at 03:58 PM.
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  #8  
Old 07-28-2022, 04:54 PM
RhinoDrvr RhinoDrvr is offline
 
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Location: Lemoore (Fresno), CA
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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.
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  #9  
Old 07-28-2022, 05:24 PM
Mikeyb Mikeyb is online now
 
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If you use a relay deck like a PH Aviation unit applying opposite trim will stop the motor.
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  #10  
Old 07-28-2022, 07:25 PM
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vlittle vlittle is offline
 
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Quote:
Originally Posted by Mikeyb View Post
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.

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