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Subject: Re: [LML] Re: Elevator trim control
From: Colyn Case <colyncase@earthlink.net>
In-Reply-To: <list-6162868@logan.com>
Date: Thu, 28 Mar 2013 13:21:33 -0400
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References: <list-6162868@logan.com>
To: "Lancair Mailing List" <lml@lancaironline.net>

To Bob's last point, I think many Lancairs have more pitch trim =
authority than you really want.
Ray Allen sells some limiters that go right in the servo, but you may =
also need to change the lever arm ratios to really reign it in.

"Not very long" on my airplane was not long enough to complete an =
instrument approach when I first flight tested it.

On Mar 28, 2013, at 12:12 PM, Robert R Pastusek wrote:

Angier wrote:

My elevator trim on at pattern speeds works well but is way too =
sensitive at high speed. Ray Allen makes a speed control and I'm curious =
to know if any 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 regulate 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 Allen trim control I used originally was a voltage =
controller. The problem with these devices is that they can control the =
speed over a rather small range because at some point as the voltage is =
reduced, the trim motor won't start running, depending on the starting =
load and other factors. Any electronics hobbyist reference will have a =
simple circuit you can build to control both 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 start/stop =
rapidly, so this type circuit is rather more taxing on the trim motor.  =
On balance, I'd recommend the latter if you really want one to do =
this... 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 when 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 trim 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 voltage controller. As I was still in =
flight test at the time, and much more interested 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 bird (2008) and so =
on...

Although the pulse circuit worked exactly as designed on the bench, I =
thought I needed to also add a good "bypass" capability. This was =
getting complex... And by that time (50+ hours),  I'd gotten used to the =
pitch trim sensitivity. 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 system. I do have to remember to caution folks to =
"blip" the pitch trim when I let them fly, but other than the occasional =
pitch excursion when they try trimming for the first time, I've had no =
problems. The fast response is particularly useful during configuration =
changes and I wouldn't change that at all, so the "light touch" on the =
switch seems a small price to pay for a simple 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 switches 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 get it turned off before the pitch went to =
full down or full up...  I did test my airplane to be sure I could =
control it with pitch full up and also full down during flight test, and =
I strongly recommend you do this. It's an interesting experience, and if =
your airplane is like mine, you wouldn't be able to fly it very long in =
that condition...but it is controllable...

Hope this helps.

Bob


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