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Thread: Dynon Users: Autopilot Trim Control?

  1. #1
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    Default Dynon Users: Autopilot Trim Control?

    I've been reading through the Dynon manuals and came across a 2 amp limitation on servo current for their SV-AP-PANEL auto-trim output. I seem to recall reading somewhere that the brushed DC motor driving the linear actuator on Kitfox stab trim can pull more than this.

    So, I tested my trim actuator. I pushed as hard as I dared (50-70 lbs?) and wasn't able to stall the motor, but I did get it to pull a tad over 3 amps, very briefly. However, I'd be surprised and alarmed if there was anywhere near that much force on it in flight. With more reasonable force against it (~20 lbs), current draw was around 1 amp.

    Has the 2 amp current limitation been an issue for anyone using a Dynon SV-AP-PANEL to drive stab trim?
    Eric Page
    Building Kitfox 5 Safari
    Member: EAA, AOPA, ALPA
    ATP: MEL / Comm: SEL, Glider / ATCS: CTO
    Map of Landings

  2. #2

    Default Re: Dynon Users: Autopilot Trim Control?

    Eric
    I have Dynon 42L servos for pitch and roll powered through the ACM. They work just fine however I installed Ray Allen G405 grips and use the upper and lower buttons to actuate the horizontal actuator. Initially it was hooked directly through the ACM. I had two problems with this setup. First if I was adjusting the trim from stop to stop (on the ground, not in flight) the actuator would stop about half way through the cycle. It did not trip the breaker, it would just stop and I would release the button then hit it again to complete the cycle. Second problem occurred intermittently when executing a go-a-round, with the horizontal trimmed all the way down for landing, I would go to full throttle to climb out and trim accordingly. But sometimes the horizontal actuator would not work at all. That made it pretty hard to keep from stalling. When I got to pattern altitude on the down wind I would throttle back and found that I was able to trim.
    I contacted Rob Hickman, president of Advanced Flight Systems, and told him about the trouble I was experiencing. The solution was to supply more amperage to the actuator using a TCW amp booster. He made up a three way harness that hooked up to the ACM, that way the G405 grips could still be used, then to the booster, and to the wiring for the actuator. It works great. The TCW product is called a Safety-Trim Intelligent Servo Controller Model #. ST-1-12v-10a-BSTR. Hope this helps.
    Joe Gallagher
    Flying 7SS

    http://frontierfox.home.blog

  3. #3
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    Default Re: Dynon Users: Autopilot Trim Control?

    Thanks, Joe. Two questions:

    1. What's an ACM?

    2. Do you have the Skyview system with the SV-AP-PANEL autopilot control panel?
    Eric Page
    Building Kitfox 5 Safari
    Member: EAA, AOPA, ALPA
    ATP: MEL / Comm: SEL, Glider / ATCS: CTO
    Map of Landings

  4. #4
    Senior Member efwd's Avatar
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    Default Re: Dynon Users: Autopilot Trim Control?

    I didn't comment previously because I have a Garmin G3X. In my system, the TCW boost is required because the G3X puts out too few amps when the auto pilot attempts to trim. I believe this motor on the SS7 is a 5 amp motor. What FrontierFox describes, with regard to trim stopping mid travel, the G3X limits travel to 3 sec max. This is a feature that is aimed at limiting travel if you get a run away trim motor (stuck switch). Stop to stop I have to reengage the switch twice while on the ground. If I only have the master power on, (only PFD and the radio on) my motor works stop to stop. Turn on the avionics and these travel limits become enabled. Admittedly, this feature can be annoying while flying. With Garmin, I have my trim motor speed at full speed while below 80mph and above 80 I have the motor speed at about 75% when I push the trim button. I suspect that the trim motor speed would be hindered if the output from the G3X were less than the amps required to run the motor at full speed, hence the TCW boost.
    Eddie Forward
    Flying
    SS7, 912iS, Garmin G3X

  5. #5
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    Default Re: Dynon Users: Autopilot Trim Control?

    Quote Originally Posted by efwd View Post
    In my system, the TCW boost is required because the G3X puts out too few amps when the auto pilot attempts to trim. I believe this motor on the SS7 is a 5 amp motor.
    The 5 amp number is what I remember reading on there somewhere. That said, 5 amps will be the current when the motor stalls, and we shouldn't be anywhere near that much force. I could barely get it above 3 amps resisting it by hand.

    What FrontierFox describes, with regard to trim stopping mid travel, the G3X limits travel to 3 sec max. This is a feature that is aimed at limiting travel if you get a run away trim motor (stuck switch).
    Dynon does the same thing. I suspect that's what FrontierFox experienced, but I'm not sure that he has the setup I'm asking about.

    With Garmin, I have my trim motor speed at full speed while below 80mph and above 80 I have the motor speed at about 75% when I push the trim button.
    Dynon allows the same settings. That's another can of worms, as Dynon (and likely Garmin) uses pulse width modulation to control motor speed. Whatever you use to "amplify" current from the autopilot's trim controller needs to be able to switch cleanly and stably at the PWM frequency.

    I'm not familiar with how the Garmin system works. In the Dynon system, the trim switches are wired to the autopilot control panel, which has outputs to drive the pitch trim motor (up to 2A). When the autopilot is engaged, it controls trim. Trim motor speed is programmable with airspeed and controlled via PWM. Dynon has trim safety cutouts very similar to the TCW products, so I don't need all of the TCW's features; at most I just need a current amplifier. That's fairly trivial, but it would be nice to know if it's even necessary, and if so, what the PWM frequency is.

    Thanks!
    Eric Page
    Building Kitfox 5 Safari
    Member: EAA, AOPA, ALPA
    ATP: MEL / Comm: SEL, Glider / ATCS: CTO
    Map of Landings

  6. #6

    Default Re: Dynon Users: Autopilot Trim Control?

    Eric

    An ACM (Automatic Control Module)is much like what Vertical Power does. I have an entire Dynon Skyview system. 5600 EFIS, autopilot, servos, remote AP etc. Everything is plugged into the ACM it supplies power to all components including nav lights heater fan USB port etc. and is programmable, meaning I can adjust the amperage to any component within limits. There are no external breakers. The problem, as I see it and I reserve the right to be wrong, is that the horizontal actuator supplied by Kitfox is old school, it needs 5a minimum. Iím not saying that it is a poorly designed or antiquated component, but when installing new age components one needs to keep in mind older style components. Had I not used the Ray Allen grips for my trim control I would have had to install a breaker or fuse for the actuator. But I sure do like the trim being on the grips, no searching for the rocker switch when I would rather keep my attention outside the cockpit or quick glances at the airspeed indicator.
    Joe Gallagher
    Flying 7SS

    http://frontierfox.home.blog

  7. #7
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    Default Re: Dynon Users: Autopilot Trim Control?

    Thanks, Joe. That all makes sense now.
    Eric Page
    Building Kitfox 5 Safari
    Member: EAA, AOPA, ALPA
    ATP: MEL / Comm: SEL, Glider / ATCS: CTO
    Map of Landings

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