Pitch Trim Unit

[lpaaruule]

I'm curious about what it's like flying with the Pitch Trim unit vs the standard trim tab in a Sonex.

On the Sonex website, it says that 1 full turn of the Pitch Trim unit equal approx 2-3mph speed change. This sounds good for cruising, but what about tough and goes? It seems like a lot of knob turning for pattern work.

The Pitch Trim unit used springs to control stick pressure. I'm not entirely sure how it works, but I'm wondering if the springs are constantly pulling on each other (one up elevator, one down elevator), would pitch stick force = elevator air force + spring force at full forward or aft stick?

If anyone has flown with both systems, which would you prefer? Is it a "what's your mission" consideration (cruising vs aerobatics)?

I'm familiar with a standard type trim lever from flying a Citabria, so I assume the Sonex trim lever would be equivalent?

[gammaxy]

I have only flown the Dial-a-Speed style in the Sonex. I installed it because it seemed like less work, but I now suspect it's probably about the same either way. I also had someone with the standard style mention he had trouble precisely adjusting the trim and wishes he had the Dial-a-Speed, but I've heard some good reports about the standard style recently on this forum.

The stick forces are so light and my time in the plane is so small that I don't think I ever keep it perfectly in trim. I set it for takeoff and make a few spin adjustment for cruise and a few spins on approach. It doesn't seem to change stick forces as much as I'm used to when flying a Citabria, so when doing touch-and-goes I don't think I ever touch it. In the citabria I set it on climb out, downwind, and abeam the numbers.

There's a chance I would use the knob more if I had this modification:
https://www.youtube.com/watch?v=_UwjKmpB2IA

The springs do pull against each other. One spring is between the sticks and a solid mount and the other is between the sticks and the trim knob. I think the answer to your question (if I understand it correctly) "would pitch stick force = elevator air force + spring force" is yes. You can easily move the elevator to both stops while parked on the ground, but it will be obvious that there is a spring involved.

[daleandee]

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[mike.smith]

I agree with Dale. I love my stock trim.

[peter anson]

I have the stock trim and mostly agree with Dale. It's easy to build, intuitive and quick to use. The sensitivity is fine at low speeds but I think it's way too sensitive at high speeds. I built mine with a longer lever but it would need to be much longer. On one occasion I managed to (rather clumsily) knock the trim lever while reaching forward to adjust an air vent. The effect was like hitting an enormous pothole in an otherwise smooth road. For a few seconds I thought I had had a mid-air collision.

5) One if the greatest features of the traditional tab is in it's safety. With the Dial-A-Trim system if you should lose the connection at the elevator, it won't help you. With the traditional tab you still have some control over the elevator.

I have tried flying my Sonex using the trim lever but resultant oscillations didn't give me too much confidence I'd be able to control it in an emergency.

Peter Anson
Sonex 894
Jab 3300
185 hours

[gammaxy]

3) By looking at it I can tell where it is set ... can't do that with the dial a trim.

Dale has some good points. Thought I'd mention that you can easily tell how the dial a trim is set by reaching under the dial-a-trim and feeling the position of the cable mounting block.

[lpaaruule]

Thanks everyone for the input.

It seems to come down to user preference. I like the the quickness of the standard trim, but the accuracy of the dial a trim system is nice.

It think the dial a trim modification shown in the link Chris posted above may have leaned me that way. The only time I really want to know where the trim is set is on takeoff, every other trim setting is just to relieve pressure from the stick. So maybe a combination between feeling the position of the cable mounting block noting the orientation of the dial would work well.

I still have plenty of time to decide (unfortunately). So I'd better get back to working on my upper firewall former. :-)

[DCASonex]

One more strongly agreeing with dale.

I have flown in Sonex with dial-a-trim where the pilot does not want to take the time to change when landing and then reset it for takeoff, preferring to fight heavy stick, and mine with the lever just below the throttle on left corner of panel. With electric flaps, one hand stays on stick, the other on throttle, two fingers of throttle hand are all that is needed to reach down and flip that lever to full nose up after 10 degrees of flap are added, then leave there for balance of landing, return to center for take-off. Adjustment in flight is not the least bit tricky and very quick to find neutral balance point, both in climb and cruise, and adjust same as fuel burned off during flight. Dial-a-trim looks neat, but that is about all.

David A.

[Sonerai13]

Knowing where trim is set with the Dial-A-Speed trim is simple. All you do is reach down and feel where the trim block (the part that moves back and forth when the knob is turned) is. You can instantly tell by feel whether the trim is nose-up, nose-down, or neutral. Piece of cake!

[Fastcapy]

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