Trim wheel for electric trim?

I've read the discussions on manual vs. electric and decided on electric a long time ago (not looking to start a debate or solicit opinions on the merits of each).

Despite my preference for an electric system, I?m not satisfied with toggle or rocker switch (on/off) control. Using the servo speed switching device would seem to mitigate the problems with on/off control to a certain extent, but this is not an acceptable solution for me.

I like the trim wheel controller that is offered by Perihelion Design (see http://www.periheliondesign.com/moreproducts.htm ). This would behave exactly like the trim wheel in the C-150 that I learned to fly in. This configuration is very intuitive: roll forward, relieves forward pressure ? roll back, relieves back pressure.

I?m curious if anyone in the RV community has actually used one of these and what their impression has been.

Also, any of you electrically inclined types know if it would be possible to use this device in conjunction with a system that would be able to store pre-programmed positions precisely (take off, normal landing stages, etc.)? If the wheel is holding a potentiometer in a fixed position, wouldn?t such a system need to physically displace the wheel to adjust to the pre-programmed positions?

Aaron,

Interesting concept and I don?t see any major technical problem implementing your design for an electric trim system controlled via a wheel of some type. The wheel would be connected to a pulse encoder of some type rather than a potentiometer (like the volume control in a modern car radio) this eliminated the need for maintaining a physical synchronization between the trim setting and the wheel itself. The system would be controlled via a micro controller and the wheel would provide pulses (with direction information) to the micro controller. The controller would simply add or subtract the pulses to a target value which would be compared with the MAC controllers potentiometer voltage output. The controller would simply drive the MAC servo (in the appropriate direction) until the servo gets to the target location determined by the servo potentiometer output. Several buttons could also be read by the controller that would cause the controller to seek a preset trim position for takeoffs, landings, etc.

The only problem I see with the design would be an ergonomic issue. Since you could spin the wheel faster than the MAC servo could position (something that cannot occur in a mechanical system) you would tend to over control the trim system. I think it would be difficult to get used to, you could spin the wheel and it may take the trim servo 5 or 10 (up to 15 seconds) to get caught up with the target location.

Just my thoughts?

J.C.

My personal opinion is that it's a bad idea to have the trim controlled with any sort of closed loop mechanism. The potential for a runaway is just too great. The trim as it is now is very safe because it ONLY moves when you're physically pushing the button to make it move.

For an autopilot, there is presumably logic in there that constantly monitors the health and status of the servo feedback and disengages if something goes funny (someone correct me if I'm wrong here...I would be VERY surprised and a bit concerned if I am).

To do this safely is probably way more trouble than it's worth.

Just my $.02.

looks cool but heavy and $350?

All good comments and like his stuff, but is expensive and heavy. That roller blade wheel and metal frame is way more than a simple plastic toggle.

A cheap little MAC toggle on the panel works just fine. I am not sure if it's a safety thing? I mean you can over power a runaway trim; its not like a B767 stab runaway. I worry more about the plastic clevis coming undone on the trim tab and causing flutter or something. A stretch but, it never hurts to think of the safety aspects of any system and worst case.

Bottom line, light, simple, works and less expensive, I am sticking with the toggle. You learn to jab it to make fine adjustments. I am not trying to emulate a C-150 in any way, not that a mechanical trim wheel is bad. I suggest if you like that mechanical deal you go with vernier mechanical option, it will give you that proportional thing. You can always add a MAC position indicator if you like, if the control knob position is not good enough.

I do like using RC servos and a little PWM control. You can make my own fairly easy. It is small light and could be used to move vents.

a slightly different way

Aaron,

If you want to fabricate your own knob, this might be one way to go

Use the True Servo Conversion for M.A.C.-R.A.C. ?Sort-of-Servos? from Perihelion Design

http://www.periheliondesign.com/moreproductsfiles/ServoController macrac.pdf

And a multi-turn pot. such as this (a mere $3.95.... ...)

http://www.elexp.com/cmp_ptf.htm

And retain which ever of the MAC position indicators you prefer for a position readout.

Then you can make the trim wheel bit any way you want....

gil in Tucson

The beauty of the whole thing is it's your plane! By all means ask for opinions and advice but in the end it's your call, build it the way you like it. I kind of like that razor scooter wheel look. If it was the same wheel without the polyurethane it would be a nice looking design, IMO.

I considered a potentiometer matching feedback control (like I think one would set up with the Perihelion wheel as a cockpit control), until I thought about the runaway conditions mentioned by another post in this thread. In this control configuration, the system tries to match the voltages coming out of the Mac potentiometer and the cockpit control potentiometer by feeding the two voltages through a differential amplifier to obtain an error signal output that causes the mac motor to move.

I believe that if there is either a short or a wire break in the wrong spot,(in the flexing lines passing through the elevator, for example) the trim tab could be "told" to fully deflect in one direction or the other depending on failure mode.

So, for me, the rocker switch looks like the right solution. If I was to do over again, I would choose manual trim.

Two replies in this thread bring up the question of runaway trim conditions and additional failure modes introduced by a smart trim control. The basic MAC servo installation with a single panel mounted rocker switch is fairly immune to a runaway trim failure mode although it is remotely possible. However, many if not most installations of the MAC trim system use trim switches or pushbuttons mounted in the control sticks in conjunction with a relay deck module which allows multiplexing of the trim control function. There are quite a few more failure modes in this type of installation including several that can cause a runaway trim situation, stuck pushbutton, shorted wires coming out of the stick not to mention relay failures.

Anyway, the point I want to make is that practically nothing in experimental aviation is failsafe, that?s one of the reasons they call it experimental. As ?gmcjetpilot? stated, a runaway trim control in an RV should NOT be a fatal condition as it might be in a larger commercial jet. If a runaway trim condition could not be over ridden in an RV then I think the only safe bet would be for everyone to install manual trim controls or perhaps just a simple MAC servo installation with ONLY a single rocker switch on the panel.

Another 2 cents.

J.C.

647jc said:

If a runaway trim condition could not be over ridden in an RV then I think the only safe bet would be for everyone to install manual trim controls or perhaps just a simple MAC servo installation with ONLY a single rocker switch on the panel.

Click to expand...

I have my "trim killer" switch located just to the left of my throttle. It has a red painted U-shape shield around it, to prevent annoyance switching. I made the sheild from square stock, and it doesn't require lifting a hinged gaurd to activate the switch.

Basically, it just kills all trim power, yet elevator & ailerons also have individual panel mounted circuit breakers, should only one servo or associated wires become a problem.

I do use the servo relays with left stick mounted trim, and panel rocker switches for the right seat.

L.Adamson

All valid points to consider. A runaway condition would definately be a distraction, but I suspect it would not be a fatal one if you were paying a reasonable amount of attention. Maybe it would be a good idea to test it out during one's test flight program. I seem to recall hearing that RVs are manageable if you get a runaway trim.

All things considered, I still like the closed loop approach and that wheel does seem pretty cool. I'm still curious if there is anyone out there that has actually used one and how it worked out. Anyone???