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Electric Trim Question

rocketman1988

Well Known Member
I'm currently flying in Phase 1, and I have seen that the trim is very sensitive at higher airspeeds. I have a VPX, and have activated the speed trim function, and set it for 70% at 120 its or above. It works as advertised ON THE GROUND in simulated airspeed mode.

In the air, however, the trim ceases to function at 120 or above. The indicator shows it is getting a command but the trim doesn't move. I then increased it to 85% and it works but it is still very sensitive.

My question to the group is if anyone has experienced this issue or is it possible that I have a faulty RAC servo? It apparently does not have the necessary power to drive the trim at a reduced voltage in my case...

Thoughts?
 
trim

Adjusting it to 100% is the problem. At speeds above around 120, the trim is too fast and needs to be slowed down. The problem is, at 70%, it appears that the RAC servo does not have enough power to move the trim tabs.

My question is if anyone else has seen this with their -10 trim. It not, I wonder if my RAC servo is defective...
 
70% is apparently not be enough power to move it in flight. My Dynon pitch trim control is set to 50% and works well above 100 KIAS to slow the trim movement.
 
Adjusting it to 100% is the problem. At speeds above around 120, the trim is too fast and needs to be slowed down. The problem is, at 70%, it appears that the RAC servo does not have enough power to move the trim tabs.

My question is if anyone else has seen this with their -10 trim. It not, I wonder if my RAC servo is defective...

I would first suspect the VPX. Do a run at 100% to find out.

Side note - I ran the trim servo at 100% in my RV-10. I just got use to doing just a bump at speed.

Carl
 
I don’t know how the VPX works. Does it send a reduced dc voltage to the trim motor? If so, perhaps there is insufficient torque to get it going. I use the Trio Pro autopilot to send pulse width modulated voltage pulse to the servo. Since each pulse is 12 volts it never hangs up, even at the slowest speeds. It may also be that there is an unusually large mechanical resistance or binding in the physical mechanism.
 
vpx

Why would you immediately suspect the vpx? As posted, it works perfectly at 100%; when the vpx reduces the voltage to 70%, it will not move the trim tabs above about 100 Knots. The vpx has a simulate mode so that you can test it on the ground at the transition airspeed...it transitions correctly at the set speed and the tabs move at a reduced rate as you would expect. In the air, however, I am getting an indication on the trim indicator that there is a command being sent but the trim doesn't move. Given these facts, it is highly unlikely that the vpx is the problem, other than reducing the voltage instead of using PWM duty cycle.

This is why I am asking folks if they are using the RAC servo at a reduced voltage and getting the same or different results.

If people are using the RAC servo on their -10 with reduced voltage and it is successful, that would indicate a problem with my RAC servo, or the linkage between the servo and tabs. I have checked the cam linkage and found it to be free...

I wish the VPX used PWM; that would likely solve the problem...
 
Why would you immediately suspect the vpx? .........
....I wish the VPX used PWM; that would likely solve the problem...

Didn't you answer your own question?
On the ground, there is only friction keeping the trim motor from moving the tab; in flight, there is a significant force from the air loads that needs to be overcome. The faster you go, the worse this gets.
 
Bob,

Have you reached out to Chad at VP support?

There are thousands of VP-Xs operating and there's plenty of "power" to move the trim motor on an RV-10 in cruise flight.

This one is a head-scratcher though. :eek:

IMO the most likely cause is the mechanical thingy - see if you can swap out the trim motor.
 
trim

I do not think the VPX is the problem as it does exactly what it is supposed to be doing; ate the selected speed, it reduces the voltage to the RAC trim servo to "slow" it's operation. It works perfectly above 85% but when selected below that, the trim motor won't move the tabs.

That is why I posted the question. I am trying to find out if anyone has reduced the voltage on the RAC servo to below 85% and still have the RAC servo be able to move the tabs. It doesn't matter what speed trim system is being used, only that is it reducing the voltage to slow the motor.

Many people use the speed trim function available from several different vendors. If no-one else is having this issue then it would seem that my RAC servo is not performing to standard, ie insufficient power to move the tabs at reduced voltage.

Thanks for the input...
 
I do not think the VPX is the problem as it does exactly what it is supposed to be doing; ate the selected speed, it reduces the voltage to the RAC trim servo to "slow" it's operation. It works perfectly above 85% but when selected below that, the trim motor won't move the tabs.

That is why I posted the question. I am trying to find out if anyone has reduced the voltage on the RAC servo to below 85% and still have the RAC servo be able to move the tabs. It doesn't matter what speed trim system is being used, only that is it reducing the voltage to slow the motor.

Many people use the speed trim function available from several different vendors. If no-one else is having this issue then it would seem that my RAC servo is not performing to standard, ie insufficient power to move the tabs at reduced voltage.

Thanks for the input...

That's why most speed controllers use PWM and don't just reduce the voltage.
 
RV10 trim

Bob, I have seen 1 x trim issue with the 10 caused by an incorrect travel adjustment not enabling one of the limit switches in the Ray Allen motor to do its job resulting in a stripped internal gear. Maybe something similar is going on with yours - just saying....
I’m with MCA on this as I’ve never, except as above, experienced a trim issue in in all of the aircraft I’ve wired (60+) to date.
 
I had an issue with the RAC trim servo after about 40 hours the plastic gear wore a flat spot and the trim would not work at high speed. I have VPX and it set for 50% at 120knots. I talked to Ray Allen and they have a better servo made with all bass gears. Have 250 hours on it and works great. have to modify the bass end to fit the trim attachment just like you did with the plastic end. Give Ray Allen a call.
 
Have no idea how the VPX works, but can report that voltage reduction does work on the RAC unit. I made a little LM317 based voltage reducer to slow down the trim on my 6. I dialed it down to 8.5 or 9 volts and really slowed down the trim to a perfect speed (about half speed, same setting on use on the 10 above 140 kts). The 10 needs quite a bit more torque/power, with probably 3 times the trim surface area, so can't say for sure that would work on the 10.

Might want to find out what the %age means, as 75% of the voltage seems about right and not sure that I would expect the servo to work at 50% voltage. At some point you're going to run out of torque for those big trim pieces. Working on the ground but not in the air would indicate inadequate torque.
 
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I have not noticed any problems with my trim (VPX passed through the GSA-28s) but I am still in Phase I and only have 5 hours so take that with a grain of salt. That said, this thread reminds me of transition training a couple years back with Seager. At one point on the first day he caught me trying to trim adjust an altitude change. His comment was pitch it first and then trim to release the control pressure. I say that more as a question for more information on your issue with the trim. What are you doing when you hit the trim switch?
 
trim

When trimming, the aircraft is flown to the desired attitude and then trimmed. My issue is that at higher airspeeds, anything more than a quick bump on the trim will yield an undesirable pitch change, hence the reduction in trim speed at those airspeeds.

Have you activated your speed trim in the vpx?
 
UPDATE: Don't listen to me on this one. I need to verify my settings. I think I updated after this backup because of the note in the manual about autotrim on the GSA-28s. I will verify this weekend.

I am not at my plane and my settings may or may not match the backup I took in December (I can verify this weekend), but it looks like my XML file is configured as follows:

PitchTrim
Power - 60
Speed - 140

RollTrim
Power - 0
Speed - 0

I have no rhyme or reason I put those numbers in to start with (before first flight), but in all honesty they seem to be working. I should note that I am not a hold the trim button type of person and I tend to bump the switch multiple times in succession to move it. So maybe I am just not noticing that my pitch trim is sensitive.
 
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thanks

I set my pitch trim to 70% and 120 and the trim motor would not move the trim even if I held the button down. So if yours is working at 60% then my RAC servo may be defective...

Thanks
 
Same issue

I have the same issue with my 10.

It is a little odd in that the issue in my 10 is only with the roll trim, not the pitch trim. Above about 120 knots no roll trim movement although the indicator says it is activated. Last long flight I held the trim off and on for a long time and saw some apparent movement on the indicator. I'm not sure. I have my trim motors run through both the G3X which has trim speed scheduling as well as auto trim and the VPX.

I have (perhaps wrongly) attributed the issue to compounding of the VPX and G3X at speed, but have yet to dig into the adjustments on each. The plane flies well with the roll trim static so it hasn't been a priority to sort out. I manage the fuel load to not have more than about 5 gal difference. I also have electric rudder trim I use to center the ball which is generally all that is needed. (I think I get a slightly variable nose wheel fairing difference since rudder trim is not always associated with power settings)

I fly with the autopilot all the time (VFR) with no issues other than a pitch trim warning when I encounter a large sink or lift. Otherwise no issues tracking and holding altitude sets. I am getting an auto trim fail that I also need to track down with Garmin this spring. I have just put up with it since I always trim prior to turning on the auto pilot and it comes out of autopilot the same way it went in.
 
I have a related question: I am installing the VPX and G3X with GMC 507 AP equipment and see that VPX has inputs for Pitch and Roll trim but nothing for yaw. I have installed electronic rudder trim and am wondering how it will intergrade into this configuration?
 
If you are using both the G3X and VPX, you should only wire the trim motors to one or the other system, not both. If you are also using the yaw axis for the autopilot system, the G3X may be the best to wire to, as it includes an output for yaw trim.

The G3X auto pilot uses PWM for the trim output to the trim servo, with the trim output coming from the GSA 28 auto pilot servo. I have my trim speed reduced to about 50% on the pitch trim at cruise speed, and it works very well. And even with the auto pilot off, the G3X will still reduce the manual trim speed.

Regards
 
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Bob, I had the same problem with my pitch trim servo. When I used the VPX to schedule the trim movement at speed, no movement.
In my case the G3X has a speed scheduling function that allows a gradual decline in trim function with speed that I enabled. That corrected the issue.
 
I have a RV10 with a Vertical Power Pro. I reduced trim speed to 40% above 120 and it works just fine. I seems unlikely the problem is in the Vertical Power.
 
I have a RV10 with a Vertical Power Pro. I reduced trim speed to 40% above 120 and it works just fine. I seems unlikely the problem is in the Vertical Power.

I’m wondering if my RAC servo is weak; when I set the vpx for 50% at 120, the trim would not work. Decrease speed below 120 and it works fine. I tried increasing the % in 5point steps and it started working at 85%.

Might have to try a new servo…
 
I just finished flying off the phase I hours on my RV10 and I'm experiencing the same problem (I also have a complete Garmin system and a VPX pro). Have you tried a new Ray Allen pitch trim servo and did it solve the problem?
I called Ray Alan support and was told that the problem was that Garmin uses a PWM that is not compatable with their servo. However, I built an RV14A with the same system and it works without any problems.
 
I’ve had the VPX Pro set to drive the elevator trim servo at 50% above 100 knots in our RV10 and that has worked very well over 700 hours, so maybe as Mark has said it could be a mechanical issue.
 
Maybe a servo issue

Looks like I have something else to check now. Seems as if many have a working system with the VPX below 80%, so I am guessing my servo is not performing to spec.

For those running VPX speed trim with a Garmin suite, do you have the RAC servo being driven from the GSA28 pitch servo for auto trim?
 
Interesting timing in this old thread popping up again. I’ve got a VPX Pro as well. My roll trim has never really been effective. It didn’t bother me, as Lenny mentioned, there are other methods to compensate.

I’m in the process of doing a panel upgrade. I’m replacing my Trutrak Vizion and servos with Dynon servos. I was advised by Rob to take the trim servos off the VPX and use the Dynon AP Panel to control trim.

I just added some more tests to my upgrade. It raises the question about potentially having a bad roll trim servo or an issue with VPX.
 
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