Kitfox Aircraft Stick and Rudder Stein Air Grove Aircraft TCW Technologies Dynon Avionics AeroLED MGL Avionics Leading Edge Airfoils Desser EarthX Batteries Garmin G3X Touch
Page 4 of 4 FirstFirst 1234
Results 31 to 37 of 37

Thread: Trim with the Garmin G3x

  1. #31
    Senior Member
    Join Date
    Dec 2017
    Location
    Mapleton,UT
    Posts
    1,224

    Default Re: Trim with the Garmin G3x

    (B) . I am using the booster and G3x and VPX. Yes the speed of the trim is dependent on aircraft speed but this is done via g3x. The g3x has a page that you setup with trim motor speed percent and IAS. I have no idea how g3x does this but guessing it’s sending intermittent signal? Makes trim really smooth when going “fast” The booster is also suppose to have stuck trim feature? I don’t recall them being that expensive and seems worth it for less brain damage.
    Dustin Dickerson

    Building 7ss STI x 2
    Oratex
    29" shock monster
    EP912STI 155hp
    Garmin
    N33TF......FLYING!
    N53TF......FLYING!

  2. #32
    Senior Member efwd's Avatar
    Join Date
    Jun 2015
    Location
    Yorba Linda, CA
    Posts
    2,817

    Default Re: Trim with the Garmin G3x

    Dustin, Im using the booster and VPX and G3X. I find I am excercising the trim runaway feature nearly every time I land. That feature limits travel based on time (3 sec). I would say it must take about 8 seconds for full travel. If I do pattern work I am always needing to trim for landing and have to push the trim for three seconds. It doesn't reach full nose up for landing on the first push. I have to release the button and push it a second time in order to reach full nose up. I am a bit annoyed at this but I have the travel speed set for 100% at speeds under 85 mph. The fact that you may not be experiencing this has me wondering how that is.
    Eddie Forward
    Flying
    SS7, 912iS, Garmin G3X

  3. #33
    Senior Member
    Join Date
    Dec 2017
    Location
    Mapleton,UT
    Posts
    1,224

    Default Re: Trim with the Garmin G3x

    Eddie that would be annoying! Let’s figure this out. I can start another thread if we need to?
    Dustin Dickerson

    Building 7ss STI x 2
    Oratex
    29" shock monster
    EP912STI 155hp
    Garmin
    N33TF......FLYING!
    N53TF......FLYING!

  4. #34
    Senior Member
    Join Date
    Feb 2020
    Location
    Steilacoom, WA
    Posts
    729

    Default Re: Trim with the Garmin G3x

    I had to go back and read the difference between the Safety Trim which I'll use and the Booster. Obviously the booster is for you autopilot folks, which is something I won't have.

    Both units have that 3 second timer at which point it assumes there is a trim runaway. As mentioned by others, given the comparatively light/slow nature of the Kitfox that feature is sort of in the way. It doesn't seem like there's a way to suppress it or change the 3 seconds to something longer.
    Kitfox 5 (under construction)
    Commercial SE/ME, CFII

  5. #35
    Senior Member
    Join Date
    Dec 2017
    Location
    Mapleton,UT
    Posts
    1,224

    Default Re: Trim with the Garmin G3x

    There is definitely a way to suppress this feature I just need to figure out how I am doing it. That feature is not active on either of our aircraft. When landing you want to press and hold nose up trim to the stop so I agree it would be in the way. I think you would want it to come into play if you had input short for some reason but really not a useful feature on the kitfox imo.
    Dustin Dickerson

    Building 7ss STI x 2
    Oratex
    29" shock monster
    EP912STI 155hp
    Garmin
    N33TF......FLYING!
    N53TF......FLYING!

  6. #36
    Senior Member Eric Page's Avatar
    Join Date
    Jun 2020
    Location
    Toledo, WA
    Posts
    863

    Default Re: Trim with the Garmin G3x

    Quote Originally Posted by Doggitz View Post
    5) I am not an engineer, but believe that there is a simple and inexpensive way to get the required current to the KF trim, and that is through the use of a MOSFET. MOSFETS are voltage devices and will conduct when voltage is applied to the gate. Depending on the MOSFET chosen, you can supply very high currents with a small voltage (from the VPX) applied.

    6) So far so good, since MOSFETS are only a few dollars, way cheaper than the booster. You would need one MOSFET for trim up, and one for trim down.
    Unfortunately, it's not quite that easy. Reversing a DC motor requires that you have the ability to invert the polarity of power to the motor. You can't do that with a single MOSFET in each direction.

    There are two wires to the motor. Each can be positive or negative, depending on desired direction of rotation. This means that to rotate clockwise, wire "A" should be positive (connected to 12V) and wire "B" should be negative (connected to ground). To rotate counterclockwise, "B" is positive and "A" is negative.

    Doing this requires four MOSFETs in an H-bridge configuration. The two transistors connecting to 12V must be P-channel, and the two connecting to ground must be N-channel. Controlling them takes some finesse, as accidentally turning on both FETs on the same side of the bridge results in a direct short from 12V to ground (called shoot-through).

    There are dedicated ICs for this control task, or a carefully coded microcontroller could be used. There are also dedicated half-bridge ICs that perform -- as the name suggests -- half of the H-bridge function. They just require a signal telling them which of their internal FETs to turn on; helpfully, their internal logic enforces dead-time to prevent shoot-through.

    7) I spoke with a tech guy at VPX today who told me that the way the VPX regulates the speed of the trim motor is by varying the voltage sent to the trim motor. That confounds things, because if you do use a MOSFET, it will turn on hard with voltage applied to the gate and completely conduct above some minimum input voltage. That effectively does away with the speed dependent function.
    If what the VP-X tech told you is true, it's worrying. Controlling the speed of a DC motor with varying voltage is a poor technique for a couple of reasons. First, low voltage does make a motor turn slowly, but it also proportionally reduces torque. Second, as you pointed out, using variable voltage makes it very difficult to buffer the output to accommodate motors with higher current draw.

    Both of these problems are overcome if you control motor speed via pulse width modulation (PWM). Essentially, you turn the motor on and off at high frequency, applying full voltage in variable-length short bursts. This controls motor speed while maintaining torque and permits easy buffering by interposing an H-bridge between the control electronics and the motor.

    If you look at the specifications tab for the Safety Trim Booster on TCW's website, you'll see this: "PWM follows autopilot signals." So, TCW's product is designed to pass PWM control to the motor. Perhaps the VP-X guy was "dumbing it down," but I would be concerned if he wasn't.

    B) For those of you who are using the booster, are you sure that the speed of the trim motor is really dependent on airspeed? If so, how have you determined that?
    A booster has nothing to do with controlling direction or speed of the trim motor. All it does is apply power to the motor in the direction indicated by the HAT switch, autopilot or EFIS, for as long as it's told to do so. Essentially, the TCW Safety Trim Booster is a smarty-pants H-bridge-in-a-box. If the pilot sends a constant-on nose-up trim command from the HAT switch, that's what the booster sends to the motor. If the EFIS or autopilot send a half-speed nose-down command via PWM, the booster switches the motor on/off at the frequency and pulse width of the drive signal. All subject to run-away trim time limits, of course.
    Eric Page
    Building: Kitfox 5 Safari | Rotax 912iS | Dynon HDX
    Member: EAA Lifetime, AOPA, ALPA
    ATP: AMEL | Comm: ASEL, Glider | ATCS: CTO
    Map of Landings

  7. #37
    Senior Member
    Join Date
    Dec 2017
    Location
    Mapleton,UT
    Posts
    1,224

    Default Re: Trim with the Garmin G3x

    "A booster has nothing to do with controlling direction or speed of the trim motor. All it does is apply power to the motor in the direction indicated by the HAT switch, autopilot or EFIS, for as long as it's told to do so. Essentially, the TCW Safety Trim Booster is a smarty-pants H-bridge-in-a-box. If the pilot sends a constant-on nose-up trim command from the HAT switch, that's what the booster sends to the motor. If the EFIS or autopilot send a half-speed nose-down command via PWM, the booster switches the motor on/off at the frequency and pulse width of the drive signal. All subject to run-away trim time limits, of course."

    Eddie I think this is your answer. I will look at my settings but I think the G3X is turning on and off the trim motor so that it is resetting the 3 second timer. You just need to change the speed of the motor slightly so that the 3 second time is reset if that makes sense?
    Dustin Dickerson

    Building 7ss STI x 2
    Oratex
    29" shock monster
    EP912STI 155hp
    Garmin
    N33TF......FLYING!
    N53TF......FLYING!

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •