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Electric trim - how to slow?

RV7A Flyer

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I've always found the electric trim to be just a tad too sensitive in cruise. I'm sure you all know what I'm talking about...trimming to hands-off can require the absolute shortest, quickest of taps to get it set, oftentimes with a single tap giving it just a bit too much, requiring another in the opposite direction, etc.

Someone mentioned putting a resistor in line to slow it down a bit. Anybody familiar with this? I wouldn't mind if it's a bit slower while on the ground or in the climb/descent. No big deal there.

And before someone says it...I am not interested in tearing into things to add a speed controller based on airspeed, etc.

Is this a thing, or where they misinformed? Enquiring minds want to know. :)
 
Trade Offs

Putting the resistor in line will reduce the voltage seen at the trim motor which should slow it down. However, I suspect the torque will drop too. This could cause a reduction in end to end travel of the trim tab.

A simple speed controller as pointed to by DaleB is a better solution.
 
Controlling the speed by lowering the voltage (the byproduct of adding a resistor) is less than desirable because it also reduces the torque of the motor.

This can cause the motor to not run when it has higher loads on it, depending on how slow you make it.

If you would be happy with it slowed very slightly, it might work fine, but the best way is to use a PWM speed controller.

I think the one sold by Matronics is a PWM style controller. My understanding is that the one sold by Ray Allen is just a variable voltage type, but maybe it is PWM as well.
 
*as I said*, I'm not interested in installing one that uses pitot/static.

Safety Trim is available in a number of configurations. They make one that does not tap into the pitot as I recall.

Or, you can just use a switch on the dash instead of a pitot controlled switch.

Also, it can reverse the servo if you get a runaway.
 
If you just want a panel-mounted knob that adjusts the trim speed via pulse width modulation, I'm sure someone here can draw you a schematic that uses $5 of electronic parts.
 
Trim speed

The Garmin G3X will adjust the trim speed reference (slow it down) as the a/c speed increases.
 
I have designed a small PWM trim speed controller that will adjust the speed from about 10% to about 90% It does not have pitot inputs.

In order to test it properly I have even connected it to a standard flap actuator and tried to hold it down. It can handle that as well.

I have e few built up units floating around. Send me an email and I am sure we can work something out to get it to you. Think I can get it to you for about $35 for the circuit board including shipping.
 
I used to use a small pressure switch tapped on the pitot line (set to about 180mph) that turned on a variable 12 volt regulator which slowed the RAC servo down for high speed cruise, worked great for slower speeds and high speeds. Was cheap to make. When I installed a VPX it took the speed controller job. You don't need much torque for cruise speed trim since it's always close to neutral, you only need more torque and faster trim at slower speeds like landing. It's beneficial to have a two speed trim.
 
OR, temporarily install a potentiometer in series with the motor, adjust/play with it's setting until desired speed is reached, remove potentiometer, measure its resistance, install like resistor in it's place. Yes it will reduce torque, so there is a compromise, but it's easy to accomplish and slows the trim tab down enough so tapping on the switch doesn't over shoot. Did this on my Harmon Rocket and it worked fine.
 
Controlling the speed by lowering the voltage (the byproduct of adding a resistor) is less than desirable because it also reduces the torque of the motor.

This can cause the motor to not run when it has higher loads on it, depending on how slow you make it.

If you would be happy with it slowed very slightly, it might work fine, but the best way is to use a PWM speed controller.

I think the one sold by Matronics is a PWM style controller. My understanding is that the one sold by Ray Allen is just a variable voltage type, but maybe it is PWM as well.

I believe the Ray Allen one works via voltage reduction, not PWM. I followed there lead and built a little LM317 based voltage reducer. It has a trimmer on it to adjust the voltage. It was $3 in parts and works very well for me. I have it set for 8.5-9 volts, if I remember correctly and that slows it down enough to be functional in cruise. I never experienced a case where I pushed the switch and the trim didn't move. I run the panel trim switch through this regulator and the stick-based switch does not. I only use the reduced voltage version in cruise, where it is otherwise difficult to trim.

I don't believe a simple resistor in series is an elegant way of doing this.

Larry
 
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Voltage control

I also have a home made voltage control speed reduction like Larry, along with a switch to alternatively use full power, primarily to cover the case of the voltage regulator failing, though unlikely. And it works just fine, no issue with torque, range, start up whatsoever. For that matter, the trim rate does not seem to be sensitive to flying speed from standstill to cruise. I think this is the best solution, better than sending noise producing PWM down wires running the length of the plane. Agree a resistor is a bad idea - it will limit torque because it limits current, unlike the voltage regulator.

Reinhard Metz
 
For more of a "homebuilt style" option, do an internet search for "40A DC Motor Speed Control Reversible PWM-12V" and you can install one of these to control the speed of your trim motor without loosing as much motor torque as a resistor option. Needs to be the reversible version... replace the little toggle switch with your existing trim switch. You can mount the knob on your panel or hide out of site since it doesn't get used much.

They sell for $10 to $20.
 
Do the PWM speed control options create a noisy system?

All of the popular EFIS systems that have the option to control trim speed, do it via pwm. The trim speed control system in the RV-12 is via pwm. Many of the stand alone speed controls are as well, so I would say the answer is no. There might be some that do but I haven?t seen any.
 
All of the popular EFIS systems that have the option to control trim speed, do it via pwm. The trim speed control system in the RV-12 is via pwm. Many of the stand alone speed controls are as well, so I would say the answer is no. There might be some that do but I haven’t seen any.

PWM is more sophisticated, using high frequency switching and requires good design principles to elimnate noise. No surprise that Garmin and others are very quiet. However, this will not necessarily translate to simple, stand alone PWM controllers. Noise suppression with high frequency switching is not simple and requires effort.

An example of problems with high frequency switching is found in USB power supplies. I am sure you have seen the numerous posts discussing the noise produced by USB power outlets. Some are quiet, some are noisy; Just depends on how they were designed.

Larry
 
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PWM is The yt5et more sophisticated, using high frequency switching and requires good design principles to elimnate noise. No surprise that Garmin and others are very quiet. However, this will not necessarily translate to simple, stand alone PWM controllers. Noise suppression with high frequency switching is not simple and requires effort.

An example of problems with high frequency switching is found in USB power supplies. I am sure you have seen the numerous posts discussing the noise produced by USB power outlets. Some are quiet, some are noisy; Just depends on how they were designed.

Larry
The stand alone one that was popular for many years (Matronics) is noise free.
 
The arduino has PWM output, and you can buy tiny versions of it for around 15 dollars. In fact, I already have one setup to flash my landing lights. Just a little more code and it could also control the servo speed.
 
Dynon Skyview has it too.

The Garmin G3X will adjust the trim speed reference (slow it down) as the a/c speed increases.

The SkyView has fast/slow if you use the autopilot panel. It even has auto trim.

What more do you need.

Brian
 
All hail the 555 timer and NPN (or was it PNP?) circuit. Doesn?t get much simpler than that.
Could repurpose a PWM PC fan speed controller too. They are cheap and all over the web.
 
There's theoretical and then there's practical.

I chose to mount a momentary pushbutton next to my trim rocker. I hold the button to engage a resistor and then use the rocker. Easy, one handed operation since this is at cruize. Fast trim speed in pattern, no button, just rocker.

I could have built an adjustible PWM controller. Did not feel it was necessary or warranted. No problems @ 800hrs.
 
Caveman method...

I had the same problem. After looking at it a bit, I decided to move the end of the the servo shaft farther out on the control horn. It worked great at cruise, but noticeably slower at lower speeds, wasn't a problem though, just hold the switch longer.
 
I was in the same boat (or airplane) and wanted to be able to slow my trim down as it's just too sensitive in cruise just like RV7A Flyer. The Ray Allen controllers seem to work but you need so many seperate boxes for a simple 2 trim servo installation. So I opted to build my own.

trim_controller.jpg


This is a dual trim motor controller I built with PWM speed control via the two trim pots. It can supply 1.5 amps of current per motor and the Ray Allen motors all use under 1 amp of current. I have the Ray Allen T2-7A trim motors and so far so good. This is my simple all in one solution to slow down my trim motors. There are more sophisticated trim controllers but I needed something simple that combined everything in one tiny enclosure. It measures 2.5" x 4.5" and weighs 4.5 ounces.
 
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I had the same problem. After looking at it a bit, I decided to move the end of the the servo shaft farther out on the control horn. It worked great at cruise, but noticeably slower at lower speeds, wasn't a problem though, just hold the switch longer.

Adjusting the speed by changing the mechanical ratio is another way to do it but it also reduces the amount of overall trim tab travel. In most RV?s that will result in not enough.....
 
And is working very well in many many RVs!

I just ordered a Matronics governor today.

An additional benefit of slowing the trim servo speed down is fewer actuations of the trim control switches...with resulting longer switch life.

My elevator up trim switch on my Ray Allen grip recently failed and shorted...it's only been in service perhaps 18 months....so its a matter near and dear to my heart. New Infinity grip also ordered today.
 
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