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  #21  
Old 12-16-2020, 12:40 PM
BobTurner BobTurner is offline
 
Join Date: Dec 2011
Location: Livermore, CA
Posts: 7,151
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Quote:
Originally Posted by andrewtac View Post
Every prestart check when I check circuit breakers in I put my fingers on the trim circuit breaker. I might add an switch on my 10 to be able to interrupt up after seeing this. Need to do some research. Thanks for sharing, got me thinking.
I put my trim and autopilot pullable CBs right in front of the pilot, marked with a line around them. (Since the Trio can do auto-trim, in a runaway I need to pull both breakers immediately, then sort out the problem.)
Right seat has its trim switch on the panel. Too many stories about passengers resting a book on their stick trim switch.
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  #22  
Old 12-16-2020, 12:45 PM
SuperCubDriver SuperCubDriver is offline
 
Join Date: Feb 2011
Location: Germany
Posts: 455
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I have the Dynon autopilot panel which also controls the elevator trim. Even when flying manual the trim switch actuates whatever is in the box and so drives the trim motor. There is a safety feature which allows only 3 sec of trim at a time. But I donīt see how this thing is build and thought trim runaway could be possible.
So I installed a guarded switch which selects power to the trim motor from the autopilot/trim panel or - when the guard is open and the switch activated - to a second springloaded alternate trim switch (no electronics inbetween). I believe this reduces the chance of trim runaway considerably.
I also have a collar on the Trim C/B to be able to quickly locate and pull it
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  #23  
Old 12-16-2020, 07:43 PM
gereed75 gereed75 is offline
 
Join Date: Sep 2007
Location: pittsburgh pa
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I am glad to see this issue getting some attention. I am also glad to hear that the OP was able to control his AC in a very extremis situation. I can add nothing to the suggestions already made here regarding fixing his problem. I can add my opinion about the system and what I did to mitigate runaway trim risk

My opinion is that the typical RV trim system in general has more trim authority than necessary. There is no option that I know of to limit the travel of the typical Ray Allen trim servo. During Phase 1 while simulating run away trim, it became very obvious that stick forces could potentially build to disastrous levels pretty quickly in some flight regimes.

My approach was to add a TCW system set to a very short trim cycle (about three seconds) and run the trim at very slow speeds.

For those who are relying on reaching a circuit breaker to stop a runaway, I suggest that you run your trim donward at max cruise speed for about three seconds and then see if you would like to remove one of your two hands that will be fighting the forward stick pressure to pull that breaker. Not saying it is undoable, just probably harder than you imagine.

In addition. I "biased" the trim servo so that it can run the servo in nose down trim only the amount necessary to trim for level flight at max cruise speed. this leaves a lot of trim authority in the nose up direction, but my thinking is that a nose up runaway is much easier to handle than a nose down one. As the nose goes up, with a timely power reduction, airspeed will bleed off quickly to a point where reduced trim tab authority could be overcome by stick pressure and balanced flight (or at least manageable forward stick pressure) could be achieved without disaster.

Looking back at it, I would probably install manual trim. With the inclusion of auto pilots that drive the trim tab, there is a bunch of trust and complexity built into a system that should be kept very simple.

All IMHO of course.
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Last edited by gereed75 : 12-16-2020 at 07:49 PM.
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  #24  
Old 12-17-2020, 06:34 AM
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Walt Walt is offline
 
Join Date: Aug 2005
Location: Dallas/Ft Worth, TX
Posts: 5,839
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I'm pretty sure most RV guys would find this overkill/to costly but this is the 'old' way to avoid trim issues. You have to push both switch sections to activate trim, one switch is for trim and the other is for trim power. So power is shut off to the system unless you operate both the power and trim switch simultaneously. I've considered installing this myself as I don't use aileron trim anyway.

Switch quality is another factor, check out the Otto switches and compare them to what some other grip manufacturers choose to use, you get get what you pay for. Check out the Otto grips vs another popular 'military' grip maker, the price difference is mostly the quality of switches used.

In reality I think good wiring practices along with good hat switches and the Garmin limit function is a pretty safe bet, and no way I'm giving up 'autotrim'.

Slowing down is the first thing to do with out of trim situations, much less force required at slower speeds.



Links to switches and grips:

https://www.aircraftspruce.com/catal...istolgrips.php

https://www.aircraftspruce.com/catal...mSwitches2.php

https://www.aircraftspruce.com/catal...ch11-15410.php

https://www.aircraftspruce.com/catal...ickkey=7529516

https://www.aircraftspruce.com/catal...clickkey=13068
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RV7A built 2004, 1700+ hrs, New Titan IO-370, Bendix Mags
Website: ExpAircraft.com, Email: walt@expaircraft.com, Cell: 972-746-5154

Last edited by Walt : 12-17-2020 at 06:51 AM.
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  #25  
Old 12-17-2020, 07:44 AM
rockbottom rockbottom is offline
 
Join Date: Nov 2009
Location: Fayetteville, GA
Posts: 49
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Dan,
I have the same setup in my 8 and as others have suggested, I would suspect a problem with either the switch or a short somewhere in the wiring. Since you were able to control the trim with the soft keys I would suspect the former. Just curious, but did the VPX present a fault message when the trim ran away? In addition to the advice from the G3X experts, it might be worthwhile to run the VPX configuration routine outlined in the installation manual to verify you have it programmed the way you want it.

Good job getting on the ground safely, especially with this problem on an approach down to minimums.

J. Baker
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  #26  
Old 12-17-2020, 07:55 AM
rapid_ascent rapid_ascent is offline
 
Join Date: Mar 2008
Location: Dublin, CA
Posts: 1,381
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I've seen posts in this thread and others about having circuit breakers on the trim servos so that you can pull them in the event such as this. I'm actually at the point in my panel layout were I'm locating some of the breakers and in fact on this iteration I have breakers for the pitch and roll trim to the GAD27 power inputs. My question is how does this solve the problem if the servo has run to an extreme limit?
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  #27  
Old 12-17-2020, 08:06 AM
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DanH DanH is offline
 
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I realize a trim runaway is disconcerting, but it may not be something worth adding a lot of backup/safeties/support systems, which often bring new issues into play. The aircraft is controllable with trim at the limits. Stuff not installed has a perfect reliability record.
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  #28  
Old 12-17-2020, 08:06 AM
PhatRV PhatRV is offline
 
Join Date: Mar 2018
Location: Buena Park, California
Posts: 380
Default Lesson from the 737Max

I just started building my fuselage when the 737Max double crashes happened. I didn't know these big birds have the build in trim cutout switches to prevent the runaway trim. Unfortunately, the pilots of the doom planes didn't use them.

From that lesson, I now have the trim cutoff switch and the flap cutoff switch mounted directly in the front panel. These two switches will be normally on all the time to provide power, except for the case I think the trim motor is misbehaving. Then it is a simple step to flip the switch to off and cut off power the motor. This action is much faster than pulling circuit breaker or fuse. They can be pulled at anytime afterward.

I have the flap cutoff switch for the same reason. I purchased the Flap Positioning System from Vans, which included a flap controller. I discovered during the installation that if the guide-rod isn't properly aligned, the flap motor won't shut itself off. So that is the reason to have another cutoff switch.
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  #29  
Old 12-17-2020, 08:29 AM
9GT's Avatar
9GT 9GT is offline
 
Join Date: Jan 2010
Location: Southern Michigan
Posts: 2,074
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Quote:
Originally Posted by Walt View Post
I'm pretty sure most RV guys would find this overkill/to costly but this is the 'old' way to avoid trim issues. You have to push both switch sections to activate trim, one switch is for trim and the other is for trim power. So power is shut off to the system unless you operate both the power and trim switch simultaneously. I've considered installing this myself as I don't use aileron trim anyway.

Switch quality is another factor, check out the Otto switches and compare them to what some other grip manufacturers choose to use, you get get what you pay for. Check out the Otto grips vs another popular 'military' grip maker, the price difference is mostly the quality of switches used.

In reality I think good wiring practices along with good hat switches and the Garmin limit function is a pretty safe bet, and no way I'm giving up 'autotrim'.

Slowing down is the first thing to do with out of trim situations, much less force required at slower speeds.



Links to switches and grips:

https://www.aircraftspruce.com/catal...istolgrips.php

https://www.aircraftspruce.com/catal...mSwitches2.php

https://www.aircraftspruce.com/catal...ch11-15410.php

https://www.aircraftspruce.com/catal...ickkey=7529516

https://www.aircraftspruce.com/catal...clickkey=13068
Though not as handy as this high dollar switch, I put a simple trim power toggle switch on the panel. I switch it on and trim with the stick grip hat switch, then turn off the power. Runaway trim scares the heck out of me.
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  #30  
Old 12-17-2020, 09:15 AM
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DanH DanH is offline
 
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Trim runaway cutoff switches? By the time a pilot realizes what is happening and flips the switch, the trim will already be nearing the limits. Even less time if the autopilot was engaged.

As Walt said, the runaway trim response is to simply slow down. Next flight, back it off to 100 knots and run the trim to the limits, because chances are that's where it will be, even with a cutoff switch. KIS.
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