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From: Robert R Pastusek <rpastusek@htii.com>
To: Lancair Mailing List <lml@lancaironline.net>
Subject: RE: [LML] Re: Elevator trim control
Thread-Topic: [LML] Re: Elevator trim control
Thread-Index: AQHOK72wwHBkYdRtQk6ICkZ73aw8gpi7PU9g
Date: Thu, 28 Mar 2013 16:12:49 +0000
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Angier wrote:

My elevator trim on at pattern speeds works well but is way too sensitive a=
t high speed. Ray Allen makes a speed control and I'm curious to know if an=
y of you have tried it. Any other solutions out there?

Angier,
There are many ways to control the trim speed, but two simple ones are to r=
egulate the voltage applied to the trim motor or to "pulse" the application=
 of power to the motor. Have not checked for several years, but the Ray All=
en trim control I used originally was a voltage controller. The problem wit=
h these devices is that they can control the speed over a rather small rang=
e because at some point as the voltage is reduced, the trim motor won't sta=
rt running, depending on the starting load and other factors. Any electroni=
cs hobbyist reference will have a simple circuit you can build to control b=
oth the pulse timing and duration. These are very effective in controlling =
trim "speed" over a rather large range, but electric motors don't like to s=
tart/stop rapidly, so this type circuit is rather more taxing on the trim m=
otor.  On balance, I'd recommend the latter if you really want one to do th=
is... Now for the bottom line:

I configured the pitch trim circuit in my IV-P to run at normal speed with =
the flaps in any position other than full up, and to switch to low speed wh=
en the flaps were fully retracted. I used a Ray Allen speed control in the =
slow side of the circuit. It was effective in slowing the trim to a point, =
but during flight test tweaking, I reduced the voltage too much and the tri=
m would not function in the "default" flight configuration (flaps full up) =
mode, so I disabled it. I then built a pulsing circuit to replace the volta=
ge controller. As I was still in flight test at the time, and much more int=
erested in flying than building, I finished flight testing before I had the=
 pulse circuit ready to install. ...so it was off to Oshkosh with my new bi=
rd (2008) and so on...

Although the pulse circuit worked exactly as designed on the bench, I thoug=
ht I needed to also add a good "bypass" capability. This was getting comple=
x... And by that time (50+ hours),  I'd gotten used to the pitch trim sensi=
tivity. In the end, I elected not to install it. I've been flying for 900+ =
hours/almost six years now and have never had a problem with the trim syste=
m. I do have to remember to caution folks to "blip" the pitch trim when I l=
et them fly, but other than the occasional pitch excursion when they try tr=
imming for the first time, I've had no problems. The fast response is parti=
cularly useful during configuration changes and I wouldn't change that at a=
ll, so the "light touch" on the switch seems a small price to pay for a sim=
ple and reliable system.

The only other consideration I've had is for run-away trim. I've never had =
this; the more likely fault is to the trim inoperative mode, but I've heard=
 of these failure modes. I installed a toggle switch on my panel that switc=
hes trim control from the left to the right side stick, with a "center/off"=
 position. This removes power from all trim circuits in an emergency. I've =
never tested whether or not I could recognize a run-away trim problem and g=
et it turned off before the pitch went to full down or full up...  I did te=
st my airplane to be sure I could control it with pitch full up and also fu=
ll down during flight test, and I strongly recommend you do this. It's an i=
nteresting experience, and if your airplane is like mine, you wouldn't be a=
ble to fly it very long in that condition...but it is controllable...

Hope this helps.

Bob