Electric trim in a QB kit

[Battson]

Hi Z,
It is sensitive, but more importantly I find it almost exactly right in terms of deflection.
When I am light with a forward CG I usually need most of the up-trim available to get it balanced on a slow approach to land. Conversely, I need to trim nose down on approach with an aft CG, but not much trim by comparison.

Another important note - my trim install is not centred about neutral trim, it's biased so I have about 20%-30% more nose up trim, and of course less nose down trim. I built in the bias with the length of the pushrod.

The faster you set your trim, the quicker resetting trim after a go-around (holding large stick forces at bay) will be, but at first it's fiddly for trimming into a long steady cruise - but I soon became accustomed to just holding the switch for a quarter second at a time, or less. The resolution of these jack-screw servos is great. It takes maybe 6-8 seconds to trim from centre to full trim up, with the servo I have and the offset.

If you set your mechanical arrangement to get the correct deflection, you can add a trim speed controller into the circuit (at least for the RAC servo / relay combination) which allows you to adjust the speed. I decided not to do that, because it's easier to just push the button quickly than to fiddle with a knob on the panel before trimming.

[Gerhard Rieger]

Hi, looking good, what servo did you use? can you let me know please. is it RAC

[Battson]

Hi Gerhard,
Yes it was the largest of the three RAC servos. The T3-12A Servo.

http://www.rayallencompany.com/products/servos.html

[Zzz]

Yeah, the CubCrafters trim is that kind of operation: Hold the rocker until it's in the ballpark. Then tap tap tap to get it just right.

[whee]

I'm considering electric trim. You guys that have it still happy with it? Anything you would change? Battson, you still comfortable with no runaway trim protection?

[Battson]

I am naturally going to say so, but yes I am really enjoying it.

Combined with the AOA audible stall warning, it means I can remain fully focussed on the threshold and surroundings when I am doing short strip landings, and I can make many, many instant trim adjustments to ensure I maintain a stabilised approach profile, as I bleed the speed off, culminating in a very low speed short final "over the fence" - although there is usually no fence This means I can be more precise, reduces my workload, and ultimately means I can complete more difficult approaches more safely than I otherwise could. Sometimes getting closer to obstacles means a better landing, be they trees or the mountainside or boulder under the wheels just before the threshold, but you need to stay focussed on them to maintain a margin.

Drifting off & on topic.....
I wish I would remember to video some of my more difficult landings to showcase what the plane can do. The Bearhawk really shows it's best colours in those tough situations. I am thinking of our last hunting trip, landing on a short undefined airstrip in poor weather and low light, with obstructed visibility dog-leg approach requiring full flap and a well timed slip to get down, between trees, low to mid 30's ground speed on short final and a crosswind landing all at once. I feel like I am only now unlocking the real capability of the aircraft - recently we've been touching the edges of the envelope more than ever before. So my point is, I think the extra features like electric trim and AOA alarms, which reduce workload, as well as existing features like the manual flaps which increase control, make it easier to fly the plane well and safely - and to learn these skills more quickly and with less risk.

Trim-override: yes I am still happy. Seems like electrical cable is pretty reliable stuff, and it's not exposed to wear. Besides the elevator can overpower it in an emergency.

[whee]

Thanks Battson...you may have just pushed me over the edge.

[Battson]

Don't worry - someone will push you back the other way
Best to decide what you like the most, some guys who've flown both will say they can't stand electric. All personal preference, which is the beauty of homebuilding.

[Zzz]

Here's where mine ended up, though this is an older photo. I've been tossing around the idea of inverting the servo to make inspection/repair access better.



What switch control did you use Battson? I love the rocker on the CC stick but the 4-button might be a good choice too as I'm not wild about the PTT on the front of the stick.

[Battson]

Yeah I reckon inverting it would be a good idea. I would have a hard time getting access to mine.

I used the Ray Allen control stick with the PPT on the front and 4 buttons on top. It can be hard to reach the trim down button when you're resetting for a touch and go....

[Zzz]

Update, I've inverted it so as to make easier access to the servo from the side inspection hole on the tail.

[Bradrock]

Jono, I can't remember if you installed roll trim as well? If so, where/how did you install the servo?
Anyone else care to chime in on roll servo please?

[Battson]

I didn't need roll trim, and continue to see no need for it. This is something you fix once and leave, the lateral movement of the CG is insignificant.

Rudder trim on the other hand, that would be worth having IMHO.

[mswain]

Battson ,Do you have any more details on the NZ BH trim system you posted about earlier? ie., Is he using a Garmin autopilot servo (GSA 28) as a trim servo? What is the "Servo Controller"? I am interested in a similar setup, ie. have both manual and electric trim.

[Battson]

Battson ,Do you have any more details on the NZ BH trim system you posted about earlier? ie., Is he using a Garmin autopilot servo (GSA 28) as a trim servo? What is the "Servo Controller"? I am interested in a similar setup, ie. have both manual and electric trim.

He has a Garmin capstan servo with a clutch.

Here is a picture of a capstan servo for anyone unfamiliar with them


The servo is mounted in the tail section of the fuselage, and the trim cable is wound around the capstan winch. The winch freewheels when the clutch is disengaged, allowing trim adjustment with the manual wheel over the pilot's head. When the pilot presses the stick grip buttons, the clutch engages and the servomotor turns the capstan, which moves the trim system (tabs, cable, and overhead wheel).

The trick to making this system work, is deciding how you want to use it before you buy the parts. You need to match the design to your needs, else it wont suit.
For isntance, suppose are you going to use the overhead wheel for fine adjustments in cruise, and use the servo for fast corrections during a final approach or go-around. In that case, you want a fast moving servo. This is how Lars set up his Bearhawk (the one I refer to earlier), and it works nicely when you use it that way. But in his setup, if you try to use the electric trim for fine tuning the trim during cruise, then you are not going to have any success. It moves too fast. Also, the clutch has a delay before it kicks in, so there is a slight delay between button press and trim movement. That makes fine adjustment even more challenging. But used the way it was designed, it works really well.

[mswain]

Ok, thanks for the info. In your earlier picture in this thread, it looked like the servo was connected via a pushrod. Maybe he changed to a capstan setup later? Or maybe I am misinterpreting the photo.

[Battson]

Ok, thanks for the info. In your earlier picture in this thread, it looked like the servo was connected via a pushrod. Maybe he changed to a capstan setup later? Or maybe I am misinterpreting the photo.

We are talking about two different aircraft, both are in NZ. They are actually consecutive kitsets too, and came over in the same shipping container.
My Bearhawk uses a pushrod design with electric trim only.
The other machine has electric trim via capstan winch and manual trim as well.

[mswain]

Ok, maybe I was getting the planes mixed up. I was specifically curious which servo is in this picture:



Looks like it is connected into the trim horn?

[Battson]

Sorry my bad - I think I've made this more confusing than it needed to be.

Let me explain what I can - I think he started out with this arrangement, and this was years before he flew. Subsequently, I think he decided that he preferred the capstan winch design to this design, something about the backlash if I recall correctly, and so he changed the setup to a capstan winch.

BUT - I never saw the finished setup before he covered it. It's entirely possible I have mixed up exactly what happened.