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  #11  
Old 09-06-2018, 03:03 PM
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FasGlas FasGlas is offline
 
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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.
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  #12  
Old 09-06-2018, 03:05 PM
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dwrichey dwrichey is offline
 
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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.
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  #13  
Old 09-06-2018, 11:10 PM
lr172 lr172 is offline
 
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Quote:
Originally Posted by rvbuilder2002 View Post
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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Last edited by lr172 : 09-06-2018 at 11:25 PM.
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  #14  
Old 09-07-2018, 08:02 PM
N49ex N49ex is offline
 
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Location: Wheaton, Illinois
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Default 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
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  #15  
Old 09-07-2018, 09:01 PM
Andrew Anunson Andrew Anunson is offline
 
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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.
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  #16  
Old 09-07-2018, 09:03 PM
Andrew Anunson Andrew Anunson is offline
 
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Quote:
Originally Posted by N49ex View Post
better than sending noise producing PWM down wires running the length of the plane.
Do the PWM speed control options create a noisy system?
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  #17  
Old 09-07-2018, 10:07 PM
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rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by Andrew Anunson View Post
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.
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  #18  
Old 09-08-2018, 12:45 AM
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Here's how I added two-speed trim to early model RV-12... http://www.vansairforce.com/communit...&postcount=299
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  #19  
Old 09-08-2018, 08:46 AM
lr172 lr172 is offline
 
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Quote:
Originally Posted by rvbuilder2002 View Post
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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Last edited by lr172 : 09-08-2018 at 08:49 AM.
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  #20  
Old 09-08-2018, 09:24 AM
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rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by lr172 View Post
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.
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