Servo trim speed controller installed?

randylervold · #1 05-22-2007, 02:05 PM

Based on the experience of most people with RAC elevator trim servos I installed a RAC SPD-1 speed controller on the trim circuit in my RV-3B. My installation is a bit custom and may have different travel rates due to mechanical linkage differences than the newer RVs. When I powered it up I found that with the speed controller dialed up to its fastest setting it takes 15 seconds for the trim to go from neutral to either full up or full down, and 29 seconds for full up to full down. I understand you don't want it too sensitive in cruise, but this seems a bit slow to me.

I'd like to know how many guys installed the SPD-1 versus not, and if anyone has timed the travel of their systems please post the results for us all to compare.

Thanks!

randylervold · #2 05-22-2007, 03:14 PM

More data...

Voltage to the servo without the speed controller measured 12.05 volts. Voltage with the speed controller in the circuit but turned up to full fast was 10.52 volts meaning that it drops it 2 volts when wide open. The difference in speed was noticable although I forgot to time it for comparison. Turning it down showed progressive voltage reduction until the servo wouldn't move. I think I might need to remove it from the circuit altogether.

You'd think when it was turned up full fast it wouldn't attenuate the voltage but that is not the case. Too bad, what if say 11.0 volts was the perfect setting, you can't get there.

RV7Guy · #3 05-22-2007, 04:54 PM

Hi Randy,

With over 200 hours on my 7 now and most of that Cross Country, I wish I had installed the "rate" switch on the elevator trim servo. At cruise one bump is often too much. The Trutrak trim indicator is somewhat hard to nail with only the fast rate.

My neighbor is building a 7A and I convinced him to do the rate switch. It only added a couple of hours of work to install and I'm sure it will be beneficial.

Mark Chamberlin took another approach and installed a rheostat on his panel for the elevator trim servo. He can set the rheostat which sets the speed when the trim switch is activated. Also a great idea.

kentb · #4 05-22-2007, 05:57 PM

My servo voltage is set at close to 7 volts with flaps up. It works well for my 9A.

I initially had it wired upside down and could not understand why when in cruse flight it was always to sensitive.

Kent

rv8180 · #5 05-22-2007, 11:03 PM

Quote:

Originally Posted by RV7Guy

Mark Chamberlin took another approach and installed a rheostat on his panel for the elevator trim servo. He can set the rheostat which sets the speed when the trim switch is activated. Also a great idea.

Darwin,

Could you get ordering info on the rheostat, or put me in touch?
Thanks!

GAHco · #6 05-22-2007, 11:08 PM

I did not vote, I ordered the speed controllers yesterday and they are coming tomorrow.

I will be installing them based on a large amount of comments on this forum of people who wished they would have.

RV6

dan · #7 05-23-2007, 01:27 AM

Randy, it looks like you installed a 2-axis autopilot. Imho having altitude hold helps rationalize NOT needing the complexity/cost/weight of a speed controller. Just my 2 cents.

db1yg · #8 05-23-2007, 06:10 AM

I have an RV9a with TruTrak a/p with altitude hold. I initially flew my plane without the speed controller and found that the Ray Allen elevator trim was far too sensitive when at cruise speed. You had to learn to "stab" at the trim switch for minimal ele movement--and it was still usually too much. I installed the speed controller (a couple of ounces additional wt) and adjusted the speed down about 1/4 to 1/3 and it is now very easy to obtain good trim in all flight configurations. Please note that I have the TruTrak AlTrak altitude hold which does not correct out of trim conditions when engaged--therefore you must get good trim in cruise before engaging the a/p.

Given my experience, even if I had an a/p with trim adjust I would still install the controller to make it easier to manually adjust when the a/p is not engaged. YMMV.

Cheers,

db

randylervold · #9 05-23-2007, 07:26 AM

Quote:

Originally Posted by dan

Randy, it looks like you installed a 2-axis autopilot. Imho having altitude hold helps rationalize NOT needing the complexity/cost/weight of a speed controller. Just my 2 cents.

Dan, it weighs almost nothing (1-2 oz.?), costs $39, and can easily be eliminated from the circuit the way I wired it so I didn't see much downside.

Interesting that the poll shows more guys do not have one yet all the comments seem to indicate it's necessary. Good info, I'll fly it the way it is and see how it feels. My conern was in doing go-rounds where it might take a long time to get from landing configuration trim to climb. Small plane though, it can't be too tough to just hold some stick pressure while the trim catches up.

Thanks to all for the comments!

BTW, I'm transporting it to the airport this weekend and hope to be airborne during the first half of June. Inspection is scheduled for 6/1 if my prop is returned from WW in time.

f1rocket · #10 05-23-2007, 08:50 AM

I put one in the Rocket and have it set at its slowest speed, and it's still too fast. All it takes is one little bump of the switch to change the trim. The A/P likes the airplane trimmed hands off as it seems to be smoother than when there is pressure on the stick.