Electric trim in a QB kit

[Battson]

I had a question about how I did my electric trim, so though I’d post the answer/pictures to make use of the new forum.

My key design parameter was to preclude welding or other mods to the frame.

I have an electric servo driving a pushrod to the stock-standard trim horn in the tail cone. I have no additional welding, you can see here how I attached the servo with a bolted arrangement:
20120916_105554.jpg

You will see I kept a service loop in the wire, and installed a plug which mounts on a bracket to avoid vibration loosening of the terminals. This is to allow removal of the servo if necessary for maintenance.
You can also see the aft end of my baggage tube (very heavy design, to be redone in carbon fibre), and the profiled horisontal stabiliser rib.

In this picture, you can see the inspection ports I installed to allow generous access to the tail area and trim arrangement - I have been endlessly thankful for these since covering the fuselage.
20130307_111404.jpg



The end of the pushrod can be seen here, note the bolts are overlength - temporary install only:
20120916_105853.jpg

This servo is controlled by dual stick grip buttons, which activates a power relay, drawing directly off the master bus. All RAC components.

I am going to add a DPDT safety switch which open circuits the stick grip switches and transfers control to a manual trim switch in the panel. This is because an earth fault in either stick grip or any wiring results in trim runaway. The safety switch provides n-1 redundancy in the control circuits in the event.

Edit: Upon flight testing the system, we found trim runaway was not uncontrollable. It made for more work at 100kts + but slowing the aircraft would obviously allow for a safe precautionary landing.

Another edit: The question of trim travel comes up a lot, here are some photos of the trim at both limits.

NOTE these photos are taken with the pushrod on the lowest hole on the trim tab, which maximises servo action but minimises trim tab travel. In practice I've moved the pushrod to the top hole for the opposite effect, minimum servo action and maximum tab travel.
20120916_105619.jpg


20120916_105714.jpg

[Mark Goldberg]

Jonathan - I don't know much about electric trim in Bearhawks. But any and all precautions possible to prevent run away trim should be done. The trim is super powerful on stick forces, and over coming a runaway trim in either direction that had gone to full would be an event that I hope you would live to remember. Mark

[Battson]

It makes a good case for having trim stops set as tight as practical. This is something we have programmed in for testing during phase 1.

[mikeno]

Did you use the Ray Allen servos and have you had any trouble with servo jitter caused by radio or transponder?

[Battson]

I did use a Ray Allen Company (RAC) servo, and no I have had no trouble.

It seems to me it would take a pretty 'unique' wiring arrangement to experience such a problem? Do you know of a case where this happened? They draw quite a bit of power, more than could be induced unintentionally, and the control wires need to be closed to earth to activate the relay. Perhaps if the relay were right beside a powerful radio stack??

[mikeno]

The RAC servo is remote but the LED indicator and trim switch are above the radio stack. The display sequences continuously even when the radio is not transmitting though there is no servo jitter and it works as it should. I've got some background working with EMI shielding and tried all the usual grounding and shielding tricks without success. Right now it's one of those issues I've put on the back burner while I ruminate on it and bend metal for the BH.

[Battson]

I see your issue Mike. My trim indicator is built into the Dynon panels, so fortunately I didn't experience that issue myself. Let us know if you get a fix, I am sure others would be interested.

[Enewton57]

I too was worried about a runaway trim system, which in a Bearhawk can be devastating. I used a runaway trim prevention system mounted right under the pilot's seat and wired into the stick grips. Here is a link to the system I used: http://www.tcwtech.com/safety_trim.html The system has a 3 second time limit. If your system were to somehow get a short from a frayed wire or something, the system will only run for 3 seconds and then automatically stop. It also has a speed controller so you can adjust the trim speed. I actually had an incident where the safety trim system came in handy. When my wife and I were flying up to Oshkosh in 2009, we were cruising comfortably at 9500 feet in smooth cool air. After about 2 hours, of smooth calm air, she was reading a book and I was sitting there dozing off, when suddenly the airplane pitched nose-up unexpectantly, I had to push fairly hard on the stick to keep the airplane from going vertical. While I was saying what the f%^# just happened and still pushing hard on the stick to keep the nose level, I pulled the throttle back a bit to lower the stick back pressure, and looked around the cockpit trying figure out what was going on. Finally my eyes landed on the right control stick, where my wife had propped her book up on top of the control stick where it was pushing the electric trim button. Luckly, the trim had only ran for 3 seconds and the limiter had stopped it. I highly recommend you buy and install a trim limiter if you are going to install an electric trim system in a Bearhawk. The trim system is too powerful to risk a full travel runaway trim.

[fly_n_low]

I completely agree with Mark and Eric. The trim is so sensitive all you're going to be doing is bumping the switch for anything other than take-off or landing.

Regards,

Ted

[Bradrock]

In the PAC 750 we have an electric trim runaway switch for exactly that scenario. It is frightening how quickly you can lose it when the trim runs away. I don't know anything about the specifics behind it, but would a simple kill switch on the control panel be sufficient for you?

[Enewton57]

In my opinion, it happens too quickly to react. By the time your brain registers what is happening and you have both hands on the stick pushing forwad to maintain control, and your third hand is trying to pull back the throttle, its too late to even think about finding and hitting a switch on the panel.

[Dan R.]

Having a run a way trim safety switch that would remove power to the trim servo can be built for a couple dollars with a 555 timing IC chip and a solid state relay and a few components.

This is where I bought my electric trim servo that I use on my single seat Super Cub. They make servos big enough to RC a Bearhawk or auto, etc.
Paid $80 for the servo + $7 for a DPDT rocker switch. Works great running the elevator trim tab. They are also used on the RV's. Look at a past article in Kitplanes.

http://www.firgelli.com/

Dan R.

[Zzz]

I too was worried about a runaway trim system, which in a Bearhawk can be devastating. I used a runaway trim prevention system mounted right under the pilot's seat and wired into the stick grips. Here is a link to the system I used: http://www.tcwtech.com/safety_trim.html The system has a 3 second time limit. If your system were to somehow get a short from a frayed wire or something, the system will only run for 3 seconds and then automatically stop. It also has a speed controller so you can adjust the trim speed.

Eric, great find. I'm starting to think about rigging my electric trim and this looks like a no brainer.

[Battson]

Just to close out this conversation about trim runaway:

We tested this scenario as part of our phase 1 flight testing regime.
First off, stick forces were high as the trim reached it's greatest deflection. It takes quite a long time for this to happen. One hand on the stick proved adequate to control the aircraft in the event of a full trim runaway at cruise airspeed. You do certainly have to push hard to keep it level. There wasn't any "zoom" upwards, or struggle to control the plane, it was certainly feasible to keep the aircraft straight and level for as long as it takes. Of course you can slow the aircraft down and stick forces become manageable.

[Battson]

Here is a photo from another Bearhawk builder in New Zealand.

He has designed and installed a very clever modification to the basic Bearhawk trim design, to allow him to have both electric and manual trim combined into the same system.

It is a Garmin trim servo with a clutch, and an off the shelf trim controller unit. I understand the controller operates proportional to airspeed, so the trim is more fast-acting at low speed and more sensitive at cruise - but of course the point of this system is to have manual trim available as well as electric, so you can get it "exactly" right for the cruise speed you want, or to trim quickly in a go-around, and to have a back-up should the servo fail.

Of course this is a little more expensive, but offers increased functionality to match.


I am impressed.

[Zzz]

That's cool. I have thought about this possibility, and that special servo with clutch is the key. I guess it's nice to have the redundant control, but it does look heavier. Though in my experience flying the CubCrafters LSA's I've never wished for a manual trim control. In fact, on the one BH I did fly, I found the wheel to be too sensitive, so I'm hoping the slow movement of the BristolSG/RAC servo allows for finer trim adjustment.

Out of curiosity, how does the servo get airspeed information? Is it measured from resistance against the motor? Or an actual measured value from the pitot-static system?

[Battson]

The servo's controller unit it mounted behind the panel with a network connection directly to Garmin avionics (or other avionics if you're smart with that kind of thing) which feed it the airspeed information, so it can do a proportional control calculation.

My servo gives such fine adjustment that I can always set it right, if I am patient enough! The backlash at the trim tab (caused by the external bolted connections with the tab-pushrod) is the biggest issue. There is a null zone, which is annoying, albeit very small. If I am in the null zone then I can't do anything to improve the trim - but this problem is independent of the servo vs manual trim question.

[Zzz]

Jon, I notice in your updated photo that you used the inner hole on the trim control horn. How have you found the leverage/adjustment speed to be in flight? I was planning on using the outermost one to try to slow response and increase resolution of adjustment.