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STRONG PITCH TRIM PWM |
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After being in production for 12 years with 282 units flying in many parts of the world, Pilot reports on
the Strong Electric Pitch Trim has been positive. I used to say at COZY gatherings " I must be doing alright since once I send you the units I do not hear from
you again". Lately I have been getting reports of Pitch Trim sensitivity particularly on aircraft with bigger engines
and faster speeds. In the past such comments were usually followed by " I can live with it" Recently aircraft with Ly-0540 engines that fly in excess of 240 KT. have brought the Pitch sensitivity
issue to the front. I have been experimenting with different methods of speed control. 1- Finer pitch lead screw which would increase cost substantially. 2- Redesigning the geometry of the control linkage. This involved somebody else's design which I will
not do without prior permission. 3- This I believe is the answer, Pulse Width Modulation ( PWM) The ability to control DC motor
speed without losing torque. The unit I am using is modified and adopted for our use (Aircraft) Definition: PWM Drive & How it works. "A drive is the power electronics board which is used to control the speed of a motor. Drives may use various techniques to control speed. A moderately low-cost form of drive for controlling DC motors makes use of one of several types of power transistors. These transistors are switched rapidly on and off in a technique known as Pulse Width Modulation (PWM). Hence the term PWM drive . A transistor acts as a gate to allow a precise amount of current to flow to the motor. As this gate is rapidly switched on and off, the amount of current which flows is dependent upon the ratio between On time and Off time. The larger the ontime is, the more current will flow the faster the speed. The lower the ontime, the less current will flow the slower the speed. Since the transistor is pulsed, the technique is known as pulse width modulation. Fig. 10 ( See Logo at top of the page) Through feedback systems, this PWm technique can precisely control the voltage allowed to generate the armature field of the motor, and thus the speed is controlled. PWM drives are most commonly used to control AC motors, but the system is also used in many newer, more precise, DC drives controlling DC motors. Pilot Reports; "James was doing formation flying for his video. when he sent me this report. Alex" "However, none of it was long enough in cruise to get things trimmed out and stable...so I didn`t get to truly eval. the PWM unit. However, I did use it quite a bit in the formation work....and it worked perfectly right up to 200KT. Now I need to get some max cruise work done and check the 200+ KT usage. Looks good so far!" James Redmon wrote; Aug. 9, 2005 After the Airventure Race. "I was really looking forward to giving the unit a through test during the Jessie Huerta wrote; Friday, September 09, 2005 5:11 PM Alex,
Just wanted to give you an update. I finally got the airplane flying again after its annual and
I checked out the system. The PWM appears to be working as advertised. I had no trouble finding the correct trim
position and I did not have to chase around to get the right trim. I found it especially helpful at the upper end of
the speed range. I did not take further voltage readings and I would like to try to do that at some point just to double
check everything, but for now, I'm happy with where it is. Thanks for all your help. If I run across anything
of interest or have any problems, I'll drop you a line.
Jesse,
"I normally set the elevator at max up when taxiing or parked to minimize the possibility of the canard lifting in windy conditions. When ready to take-off I set the Trim speed at max to quickly move the elevator to take-off position, then rotate the pot CCW to the stop. What to expect, At the stop setting you will have to hold the Hat switch for about 2 seconds before you
will notice the nose respond. However by turning I now have 10hrs. on the PWM." Alex Strong COZYIII N306AN The Kit The kit consists of a PWM module with a remote POT attached. A Switch Plate to limit lead screw stroke to prevent locking of the lead screw in either direction. The gear box has a slip clutch designed to release at the end of of the stroke in both directions and reguires full speed to release, However since the PWM enables slower speed limit switches are needed. An operator and clamp which attaches to the over ride tube and trips the limit switch in either directions is provided. FAG Q- Why can I only get motor action on 2/3 of the pot? A- The pot is a 3/4 turn pot.
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