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  #1  
Old 05-09-2021, 06:15 AM
ctennis ctennis is offline
 
Join Date: Aug 2014
Location: Columbus, IN
Posts: 135
Default G3X/VPX trim - first flight eye opener

I had my RV-10's first flight yesterday, and thought I'd share something interesting that occurred, mostly as a discussion point: My trim did not work in flight.

System: G3X controlled AP, fronted by a VPX Pro, wiring setup as per them and Garmin.

I tested the system on the ground multiple times, and also had another person sanity check my results. This is my first "electric trim" aircraft, having flown a 182 for the past decade and having built an RV-8 with manual trim. A few days ago I realized I had an error: I had the up and down trim buttons on the control sticks reversed - the more forward button should be "down" trim, I had it as "up" trim.

Not a problem, the VPX has a control for both "normal" and "inverse". As well, the G3x configuration mode system also lets you change between a normal and reverse setting. Whew, I can just change this setting and I don't have to fix my wiring, great.

Swapping the VPX value, I checked the system again, and it functioned as expected.

First flight is here, and one of my sage pilot friends suggests I have a little nose-up trim for takeoff since I'm fairly forward CG on the flight, it'll help a bit with getting off the ground. Ok, on my checklist I go ahead and take his advice and do that - trim system move and responds as expected. Trim is now slightly nose up.

Takeoff, everything is great, but man that climb is a bit aggressive - need a lot of forward stick to overcome that trim. No problem, attempt to trim nose down - and nothing happens. Multiple attempts, no movement. I can see the g3x screen indicator for trim turn from white to blue every time I push the trim button, but it is most definitely not moving. This is becoming a problem, I'm pushing really hard to keep relatively level if not modest climb. Nothing is working though. Luckily I had a chase plane to help with traffic and tower while I debugging. Tried the copilot stick - same results. Went into the VPX screen on my MFD and attempted to the push trim up/down from there, same results. It was getting the signal, but not moving the servo. Meanwhile I'm pushing extremely hard with my left arm.

Ended up circling back and coming in - luckily with less power and after applying flaps, the nose up trim was in a pretty good spot to make a nice landing. Luckily I didn't have any other major issues on that flight to deal with, as my attention was 100% focused on this problem.

Taxied back, and while doing so exercised the trim - and it worked fine!

First thought was maybe we had it on a really slow movement setting at airspeed, but checked the g3x config settings against my chase plane's equivalent, and they were basically the same. Also thought maybe something was just binding it up and airspeed pressure was just too strong to overcome, but putting pressure on it manually with our hands, it would still move fine.

I ended up going into the VPX configuration and changing normal back to inverse as it was before. Flew again, and this time it worked fine (albeit, the buttons on the stick were backwards). Used that as an opportunity to just rewire the sticks the correct way, and on a later flight verified it's working as expected.

Why this was an issue: I dunno! Why it worked on the ground but not in the air: I dunno! I suspect there is some kind of vpx/g3x logic that kicked in with airspeed at which they were fighting with each other and neutraling out the command, but that's just a pure guess.

Food for thought.
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N955CT - RV10 completed 2021
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  #2  
Old 05-09-2021, 06:38 AM
Mark Dickens's Avatar
Mark Dickens Mark Dickens is offline
 
Join Date: Apr 2012
Location: Collierville, TN (KFYE)
Posts: 1,471
Default

Memories fade over time, but I had somewhat the same issue a while ago after getting the plane painted and re-connecting the pitch trim servo backwards. I tried inverting the control but that didn't work right either. The only thing I could do was re-wire and it worked fine after that. I have a Dynon set up so there may not be a 1 to 1 correspondence.
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RV-8 #81077 Super Slow Build
Dynon Skyview HDX, Titan IOX-370, Dual P-Mags, AFP FM200A FI, Whirlwind 200RV CS Prop
First Flight 11/20/2016
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  #3  
Old 05-09-2021, 06:47 AM
Walt's Avatar
Walt Walt is offline
 
Join Date: Aug 2005
Location: Dallas/Ft Worth, TX
Posts: 6,283
Default

Why does trim need to go thru the VPX in the first place?
Seems like you've added another level of logic/complexity/failure mode to what is normally a pretty simple system.
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Walt Aronow, DFW, TX (52F)

EXP Aircraft Services LLC
Specializing in RV Condition Inspections, Maintenance, Avionics Upgrades
Dynamic Prop Balancing, Pitot-Static Altmeter/Transponder Certification
FAA Certified Repair Station, AP/IA/FCC GROL, EAA Technical Counselor
Authorized Garmin G3X Dealer/Installer
RV7A built 2004, 2000+ hrs, New Titan IO-370, Bendix Mags
Website: ExpAircraft.com, Email: walt@expaircraft.com, Cell: 972-746-5154
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  #4  
Old 05-09-2021, 07:09 AM
ctennis ctennis is offline
 
Join Date: Aug 2014
Location: Columbus, IN
Posts: 135
Default

Quote:
Originally Posted by Walt View Post
Why does trim need to go thru the VPX in the first place?
Seems like you've added another level of logic/complexity/failure mode to what is normally a pretty simple system.
I wondered how many responses it would be before someone questioned "why did you do it that way?". In the handful of posts I've put on here in the past 7 years, I don't think a single one has gone without someone making a comment like this.

Because it's my experimental aircraft, that's why.
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  #5  
Old 05-09-2021, 07:27 AM
Walt's Avatar
Walt Walt is offline
 
Join Date: Aug 2005
Location: Dallas/Ft Worth, TX
Posts: 6,283
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Quote:
Originally Posted by ctennis View Post
I wondered how many responses it would be before someone questioned "why did you do it that way?". In the handful of posts I've put on here in the past 7 years, I don't think a single one has gone without someone making a comment like this.

Because it's my experimental aircraft, that's why.
OK to put it another way, what is the benefit of integrating the VPX into the trim system? You should know this because you built it...

PS: you're the one that stated "I dunno why", I would think you would want to know the technical reason for the trim behavior rather than taking a defensive attitude to my question. Your I dunno reminds me of my favorite pilot saying when using the autopilot: "what's it doing now?"
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Walt Aronow, DFW, TX (52F)

EXP Aircraft Services LLC
Specializing in RV Condition Inspections, Maintenance, Avionics Upgrades
Dynamic Prop Balancing, Pitot-Static Altmeter/Transponder Certification
FAA Certified Repair Station, AP/IA/FCC GROL, EAA Technical Counselor
Authorized Garmin G3X Dealer/Installer
RV7A built 2004, 2000+ hrs, New Titan IO-370, Bendix Mags
Website: ExpAircraft.com, Email: walt@expaircraft.com, Cell: 972-746-5154

Last edited by Walt : 05-09-2021 at 07:48 AM.
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  #6  
Old 05-09-2021, 08:27 AM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 6,548
Default

Quote:
Originally Posted by ctennis View Post
I wondered how many responses it would be before someone questioned "why did you do it that way?". In the handful of posts I've put on here in the past 7 years, I don't think a single one has gone without someone making a comment like this.

Because it's my experimental aircraft, that's why.
You designed a system for which you clearly don't understand fully how it operates under ALL conditions. That is your right and no one is suggesting it is not. However, it would seem that those questioning that approach and suggesting a move to a simpler system where you CAN fully understand what it is doing in ALL circumstances is good advice, even if it seems unreasonable to you. This does beg the question of why did you post this? If you weren't looking for advice, it might have been better to state that your intention was only to warn others and that you didn't want guidance.

Safety often comes from fully understand how your system will operate in ALL conditions. Consider the fact that you still don't know why the trim didn't work when it should have. Are there other things that the system is going to do under other unique circumstances that will create problems? Probably a good thing to know.

Larry
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N11LR - RV-10, Flying as of 12/2019

Last edited by lr172 : 05-09-2021 at 08:34 AM.
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  #7  
Old 05-09-2021, 09:14 AM
rocketman1988 rocketman1988 is offline
 
Join Date: Jun 2010
Location: Sunman, IN
Posts: 2,677
Default Trim

Well, integrating the vpx with the trim allows for speed trim, much like every other trim controller out there for the Rv-10.

...and I agree with the OPs post about “why did you do it that way” comment.

Anyway, I had a problem with the VPX trim on my -10, too. Check to see if you have the speed trim set up in the VPX config page. You can set it to slow the speed of the servo above a specified speed. The problem comes when you set the % servo value. I found that anything below about 80% for the servo would cause the servo to not move while in flight. The VPX reduces the voltage to the servo to slow it down and by doing so, reduces its ability to move the trim tab. With flight loads and less than 80% power my tab did not move but the indicator would change from white to blue with every trim input. Slowing the aircraft to below the specified speed would immediately restore trim function... it was easily repeatable and I even tried it at multiple specified speeds and power %s.

So check your speed trim settings in the vpx.

Also, knowing what I do now about it, I would likely use the garmin trim functions if I were to do it again. I do not think I will rewire the harness to change it, though...
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  #8  
Old 05-09-2021, 09:24 AM
RV8iator's Avatar
RV8iator RV8iator is offline
 
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Location: Saint Simons Island , GA
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I would also surmise that if you had to push so hard with your left hand you could have easily reduced that push by pulling back with your right hand.
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  #9  
Old 05-09-2021, 09:30 AM
Mark Dickens's Avatar
Mark Dickens Mark Dickens is offline
 
Join Date: Apr 2012
Location: Collierville, TN (KFYE)
Posts: 1,471
Default

If you have decided to use a VPX, as I did, why wouldn't you use the built in trim controller? Seems pretty moronic to ignore the built in runaway trim detection, and electronic CB protection. Of course, this just tees up the never ending argument about VPX vs CBs vs fuses, etc, etc. Personally speaking I'm happy with my VPX and would definitely do it again. Signing off, argue amongst yourselves!
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RV-8 #81077 Super Slow Build
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First Flight 11/20/2016
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  #10  
Old 05-09-2021, 09:49 AM
rsultzbach rsultzbach is offline
 
Join Date: Nov 2012
Location: Memphis, TN
Posts: 23
Default VPX trim versus older technology

I think the actual question here is why would you use old technology with relays that can weld shut causing runaway trim when you have solid state control available through the VPX? How many of our group have replaced their tried and true magnetos for electronic ignition? This could go on and on!
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