Trim with the Garmin G3x

I'm still trying to fully figure this one out. Everytime I think I have it figured more information comes to me that changes everything.

The G3X can control trim. But it's not as easy as connecting it up I thought.

As it stands my understanding is that it would need something like the safteytrim system to correctly control the actuator due to amperage. Now, what I'm still not sure of is how does the G3X know the current position? Is that baked into the safety trim? Do i need another position sensor? The actuator is only 2 wire so I can't see it returning any analog info on it's position. And so unless the safety trim using timing to guesstimate, I don't see how it would either. And how much does it really matter if I know the exact trim position? I'm going to know by the feel of the controls. Or would i in KF?

I believe the servos have the means of detecting the pressures and energy being applied to the controls.

I'm not familiar with the G3X, so I don't know anything about its integrated trim controller. However, for smart trim control, you don't really need position feedback. It will adjust the rate based on your airspeed.

If you want trim position indication on your G3X, you will need to install a trim position sensor. Kitfox sells a kit for this. It's easy to install. I installed it because I wanted to be able to set takeoff trim easily.

I think what Eddie is talking about is letting the Autopilot auto-trim. For this, he's right, the autopilot servos do have torque feedback, and can trim control forces out so that the autopilot servos aren't loaded too heavily and so that when you turn the autopilot off, the airplane is already trimmed. I don't know if your G3X has this built in. Dynon Skyview can do this if you have the optional AP panel.

Thanks Brian. I think I prematurely jumped to the AP functioning and trim with the G3X. Since I have the professionally assembled harnesses I do have wires to hook up a trim position sensor installed. I even have the trim LED bar from Ray Allen but I would need a sensor. I don't aim to install it. Trim is felt in your hand with the exception of setting trim for departure. I can count to three or whatever it takes to set the trim just as I did during the transition training at Stick and Rudder. While speaking of the Safety Trim, be sure you are getting the "Safety Trim Booster". I got mine from Kitfox. I have looked at the TCW website after the fact and note that the Safety Trim "Booster" is not listed on their site. They do have a Safety Trim. I have no idea what the difference is but I have had John McBean specifically ask if I had the "Booster" back when I was having auto pilot servo trouble.

Hmm, booster eh... i'll have to find out about that.

I've decided to pass on the AP right now.

But sounds like a trim position sensor - if not too expensive - is what's needed here too. So many things.

Just kinda reviewing this post.
I see we have two aspects that we are discussing. The G3X will adjust the rate at which the trim will move and its based on the airspeeds that you program in as when to move fast and when to move slow when you hit the switch. The G3X wont actually auto trim unless you have the AP as I believe Brian states how Dynon works. What the Garmin lacks is the amperage needed to drive our Pitch servo motor hence the Safety Trim Booster.
FWIW the Vertical Power unit also has the "trim rate" function for low and fast air speeds.

Quote:

---

Originally Posted by efwd

The G3X wont actually auto trim unless you have the AP as I believe Brian states how Dynon works.

---

"Auto trim" is a deceiving term. One might think that the airplane will keep itself in trim while you're flying it. It won't. It allows the autopilot to operate the trim to alleviate the load on the AP pitch servo, just like the pilot would.

Quote:

---

Originally Posted by efwd

What the Garmin lacks is the amperage needed to drive our Pitch servo motor hence the Safety Trim Booster.

---

Just to clarify, by Pitch servo motor, you mean the trim motor, not autopilot servo correct? The Vertical Power and Advanced Control Module both have "smart trim" built in, and can supply the trim motor with adequate current. It might be something to consider when deciding to spend the $$$ on one of these systems. They incorporate a lot of functionality into a single red box.

Quote:

---

Originally Posted by efwd

Just kinda reviewing this post.
I see we have two aspects that we are discussing. The G3X will adjust the rate at which the trim will move and its based on the airspeeds that you program in as when to move fast and when to move slow when you hit the switch. The G3X wont actually auto trim unless you have the AP as I believe Brian states how Dynon works. What the Garmin lacks is the amperage needed to drive our Pitch servo motor hence the Safety Trim Booster.
FWIW the Vertical Power unit also has the "trim rate" function for low and fast air speeds.

---

Right. I was just reviewing the VPX docs as I am planning on using it. The trim is locked at 1 amp, so no good either.

yes, pitch trim motor not the AP servo.

Sorry to bring back an old thread. Been searching on the trim motor issue.
Is the current solution still to install the booster module?
Currently drafting wiring diagrams for VPX GSA28 integration and it looks like regardless of configuration the GSA will supply the trim motor and require a boost.
Confirmation would be excellent.
TIA!
Pat.

Pat,

Think the booster is the only way. The trim motor wants too much current for the GSA to supply. I am in the process of doing some of my wiring and this is the schematic I came up with. NOTE that I have not tested this so can't tell you it's right with 100% confidence but think it's in the right Zip code. Also I am using a trim switch on the stick and not the center console so you won't see the switch wiring that Kitfox uses. I think the VPX will swap the power and ground when you command nose up or down.

Hope this helps

Gary

Attachment 30029

Yes you need the safety trim from TWC. It provides power to the trim motor.

One other factor to note is that the trim switch provided by KF can handle the current required to activate the trim actuator. If you
plan on using trim buttons on the control stick, though, they typically don't have the capacity for higher current. If you are using trim
buttons on the control stick you'll need to use the TCW Safety Trim Booster which can use low-current buttons to control the
trim and can provide sufficient current to drive the trim actuator. The Booster is also connected to pitch servo so that it can be
controlled by the AP (Garmin).

You'll find mention of the Booster here: https://www.tcwtech.com/product/safe...vo-controller/

The Booster also has a pitot input for controlling trim actuator sensitivity based on airspeed when you're using
manual trim control.

Victor

Quote:

---

Originally Posted by VictorV

One other factor to note is that the trim switch provided by KF can handle the current required to activate the trim actuator. ..........Victor

---

In this case the OP is wanting to use trim that routes through the Garmin autopilot pitch servo and it is the limiting factor for current and not the switch itself. The servo won't carry the current that the trim motor is looking for.

Gary

Quote:

---

Originally Posted by Geek

In this case the OP is wanting to use trim that routes through the Garmin autopilot pitch servo and it is the limiting factor for current and not the switch itself. The servo won't carry the current that the trim motor is looking for.

Gary

---

The current does not flow through the AP servo. Only the control signal goes through the servo. The actuator current comes directly from the TCW Booster.

With regard to the VPX and whether or not it swaps polarity on trim output pins (J1 pins 11 and 12). I emailed VPX the drawing and asked the question because if it didn't, I was going to have to replace the VPX in my trim system with two relays (not hard but just another component in the system). Really happy with the response time from VPX- great support. Turns out the VPX does swap the polarity on those two pins so it puts out both the 12VDC and ground needed to correctly send signals to the auto pilot that then passes the commands to the trim booster. You still need the trim booster to power the trim motor since both the Garmin auto pilot and the VPX are limited to provide a max of only 1Amp and I've heard that the trim motor requires something along the lines of 7 amps. Only thing on the drawing that VPX noted was that I show power on J2 and there is no power available on J2. That is there only as a reminder to me because I am still laying out my power distribution (adding an external alternator). Hope this helps

Quote:

---

Originally Posted by Geek

Pat,

Think the booster is the only way. The trim motor wants too much current for the GSA to supply. I am in the process of doing some of my wiring and this is the schematic I came up with. NOTE that I have not tested this so can't tell you it's right with 100% confidence but think it's in the right Zip code. Also I am using a trim switch on the stick and not the center console so you won't see the switch wiring that Kitfox uses. I think the VPX will swap the power and ground when you command nose up or down.

Hope this helps

Gary

Attachment 30029

---

Hello

Thanks for your input.
I was struggling to decide why I would complicate things by bringing the VPX into the trim system.
What I really want is EFIS indication and VPX says it can't be done without a RAC T2.
From what I can see there is no difference in indicators between T2 internals and POS sensor pot feedback. (Both 0-5K)
The RAC RP4/POS combo uses +5V instead of the VPX +2.5 for some reason but the premise is the same and should scale.

I think I am going to wire the J2-18/19 switches into the VPX for trim. The POS to J1-8/9/10 and then run the J1-11/12 outputs to the GSA and onto the BSTR.
This means the BSTR switch output is not being used on #2 therefore cut/cap.

I have to admit. I didnt think my wiring diagram would be 12 pages but here we are.


Attachment 30231
Attachment 30232

English?:) Are you trying run trim power from the vpx?

Quote:

---

Originally Posted by Shadowrider

English?:) Are you trying run trim power from the vpx?

---

I'm not. VPX won't do it on its own - not enough poop. In English and in my case, I am not using the switch on the console but rather a hat switch on the stick. That console switch swaps the 12V and ground between the two wires going directly to the trim motor to make it change direction. The stick switch is just gives a ground. So the solution is to either make the hat switch command two relays that will do the swap for you when commanded or use the hat switch to command the VPX. What I had confirmed with the VPX guys was verifying the output from the VPX was indeed that the 12V and ground were swapping depending on which way you commanded the trim. I didn't find anything in the manual that told me specifically that it did that. I saw a drawing that sort of said that but in my tiny mind I was still making an assumption.

As for the trim indicator that Patrick is talking to - I'm not doing one but think he is talking about using the VPX to put the position up on the EFIS and the acronyms might be what the VPX is using. Since I'm not using one, I am staying away from that rabbit hole. First plane I ever flew in the Navy didn't have a trim position indicator in the cockpit. It was a black line painted on the fuselage that the horizontal lined up with - the KISS principle. It worked so that's my trim indicator.

Geek

Ah makes sense now what your doing. As for trim indicator yeah it’s not really needed. With the 915 I will give you the gouge. All the way nose up for landing, and almost all the way nose down for takeoff. Its not like a turbine aircraft, if it’s trimmed wrong it’s not going to kill you.

Quote:

---

Originally Posted by Shadowrider

Ah makes sense now what your doing. As for trim indicator yeah it’s not really needed. With the 915 I will give you the gouge. All the way nose up for landing, and almost all the way nose down for takeoff. Its not like a turbine aircraft, if it’s trimmed wrong it’s not going to kill you.

---

I really like having the trim indicator. Initially I had the cable & coiled spring mechanical indicator, then I put a RayAllen linear rheostat on the horizontal stab that shows on the G3X screen. With that said it has been inop a couple times and I found I could count to 7 to retrim from full nose up after landing and it would be perfect for takeoff.

While I agree it's not going to kill you I would highly suggest practicing some full power go arounds at altitude with the trim in the full nose up/landing position. This might be a bigger issue for planes without the trim assist spring. There is a big pitch change that happens when power is applied that takes a fair amount of force on the stick to counter.

I just wrapped up my condition inspection yesterday where installed the trim assist spring kit and the TCW Safety Trim Booster. I hope I have time to do the post inspection flight today before traveling on business tomorrow.

The trim indicator is nice and I like as many gadgets on the g3x as possible. I setup my trim indicator with a green takeoff section and trim direction to match the corporate aircraft that I fly. I prefer No trim assist because I prefer heavier controls also to help with transfer from the corporate aircraft I fly. Yes it takes a lot of input when you are trimmed full nose up for landing and you do a go around, and also removing flaps, but the aircraft would not be safe if it was not possible to do go around safely as every landing has a possibility of a go around. I have done too many to count as it’s IMO something you should do when landing in the backcountry in a new off-airport spot. I know you are not arguing against this and I agree with you, you want to practice a go around so you know what you can expect. There are some aircraft that if you where to attempt a takeoff with the trim in the wrong position you literally will not be able to takeoff, ie wreck and possibly die.

I know an operator that attempted to take off in a falcon 900 with the recommend trim position and was not able to rotate. They did a high speed abort past V1 and departed the end of the runway into a burm and the aircraft was destroyed and they suffered injuries. Well, the manufacturer discovered recommend trim settings in the manual where incorrect and the operator was awarded a new aircraft. Point is, some aircraft will not fly with trim in the wrong position, but not the kitfox. I think if I didn’t have an indicator I would just trim full nose down for takeoff and turn around and glance at the stab and verify. I like the trim indicator but if you don’t install one it’s not that big of deal IMO.

The safety trim is nice and I also like the feature on the G3x that you can slow down the trim with speed. Makes smooth trimming at "high speed" :]!

I just started doing my wiring diagrams this past weekend. I've always had it in my mind that I was going to use the Safety Trim unit because I want to control stab trim through the rocker switch on my center console and the hat switches on the sticks. From reading on this forum I understand that the rocker switch has a hard life because of the current going through it, and I'm quite sure the hat switches would be happier working through relays.

And this will sound funny but I wanted a Ray Allen LED trim position indicator because the hole is already cut on my center console for an indicator there. A few months ago a guy popped up on the EAA forum with a Ray Allen trim indicator and the Ray Allen rocker switch for cheap, so I bought them. I have the ability to put a trim indicator on my MGL displays too (and I may do that just for laughs). The web site for the Safety Trim has a diagram for the hat switch/RA rocker switch combo so I incorporated that into my design.

Attachment 30248
Per the instructions I also incorporated a stab trim cutout switch, which I need for sure now that I went away from Klixon type breakers to an hidden fuse block. My kit came with the stab trim position sensor (it's basically a rheostat, right?) to go with my LED indicator. I'm going to bench test that setup to make sure the pieces play nicely together.

One of those things I hadn't thought of is that the trim position portion of the system doesn't share a single wire with the stab trim motor, switches, etc. I mean it makes sense, I just hadn't put any neurons to work over it until now.

Interesting thoughts on not using the trim assist kit. It's one of those things that is easy to retrofit. I have not yet put the airspeed switch in the diagram which gives you the two speed trim motion. Still considering it.

Alex, I gotta ask......what are the diodes for? The drawing was hard to read on my computer so I can't see how they are connected to the Safety Trim and I can't see how your panel switch is wired. I am assuming it's like the switch in the Safety Trim instructions.

Gary

Quote:

---

Originally Posted by Geek

Alex, I gotta ask......what are the diodes for? The drawing was hard to read on my computer so I can't see how they are connected to the Safety Trim and I can't see how your panel switch is wired. I am assuming it's like the switch in the Safety Trim instructions.

Gary

---

Yeah that image didn't work out very well. I'll figure out a better way to share it so it can be viewed clearly.

I pirated that section of the diagram from the Safety Trim instructions, which fortunately covers my desire to have the hat switches and rocker switch enabled. It calls for the specific Ray Allen rocker switch I have in my possession and says something about it being internally shorted. That and "don't pee on a horse fence" are about all I know when it comes to electricity.

Quote:

---

Originally Posted by alexM

......I pirated that section of the diagram from the Safety Trim instructions, which fortunately covers my desire to have the hat switches and rocker switch enabled. It calls for the specific Ray Allen rocker switch I have in my possession and says something about it being internally shorted. That and "don't pee on a horse fence" are about all I know when it comes to electricity.

---

If you have the time, is there a link to that diagram you could post? My instructions don't have any of that in it and shows the rocker switch as wired like the Kitfox instructions (which is why I was asking about the diodes). Peeing on the horse fence is indeed one of those important life lessons that thankfully I did learn early on working on my uncle's farm. When I went to nerd school they left that lesson out of the curriculum - which may go a long way to explaining some things.

Thanks

Gary

Gary,
Here's the link
https://www.tcwtech.com/wp-content/u...t-Switches.pdf

In case that link doesn't work, on the TCW web site if you click on the documentation tab, look in the Safety Trim section, the second from the bottom is the link for the above document.

Quote:

---

Originally Posted by alexM

...Safety Trim instructions, which fortunately covers my desire to have the hat switches and rocker switch enabled. It calls for the specific Ray Allen rocker switch I have in my possession and says something about it being internally shorted.

---

That's the only explanation that makes any sense WRT the diodes. If the "up" and "down" contacts of the RS2 switch are shorted internally when the rocker is centered, then pins 5 and 6 on the Safety-Trim would be shorted together. The diodes prevent that. Pin 5 can't push current through the left diode to pin 6 and pin 6 can't push current through the right diode to pin 5, but the switch can pull either of those pins to ground through their respective diode.

Quote:

---

Originally Posted by alexM

Gary,
Here's the link
https://www.tcwtech.com/wp-content/u...t-Switches.pdf

In case that link doesn't work, on the TCW web site if you click on the documentation tab, look in the Safety Trim section, the second from the bottom is the link for the above document.

---

Nope that worked. I see where my corn-fusion was - It is hooking up to a Safety Trim Controller and not the Safety Trim Booster that I am using. Different animal. What has to happen with the booster is that the Up/Down commands to the booster need to swap polarities depending on which way you want the motor to go. I couldn't see how that worked with the hook up you had. I didn't look closely at the controller but if I read the description right (albeit quickly) it takes care of the swap internally for up to an 10A draw from the motor. Still not sure about the diodes but hey, you're right - it's in the instructions that are written by the guys a lot smarter about this than me.

Thanks for the clarification.

Gary

Just read Eric's post and that makes sense cause there would be 12V from the controller floating out there waiting for a ground command. - Thanks Eric.

I have been following this thread with great interest. I have a few thoughts and questions

1) I am using GRT avionics and VPX. I plan to control the trim motor through HAT switches on the stick that are connected to the VPX. The KF option trim position sensor (really just a voltage divider) will feed the VPX and get sent to the EFIS through the serial bus.

2) I measured the actual current draw of the KITFOX trim motor and found it to be about 1.5-2.0 amps (under load). I am unclear why KF calls for a 7A breaker, but so be it. The issue arises when trying to control the KF pitch motor with the VPX, which has only a 1A output.

4) Apparently, some have used the TCW booster to increase the current to that required by the KF trim motor. There has also been discussion about the virtue of variable speed trim, depending on the speed of the aircraft. Sounds really keen.

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.

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

8) I do not know how the BOOSTER works, but I am guessing that it uses a circuit similar to the design I have in mind. If that is true, the booster will not control the trim motor speed at varying air speeds.

So after all that, here are my questions:

A) Has anyone tried a MOSFET control of the trim motor?

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?

I appreciate your thoughts

Fred
7SS, 915is
In the process of covering and playing with avionics

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

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.

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.

Quote:

---

Originally Posted by Doggitz

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.

Quote:

---

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.

Quote:

---

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.

"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?