PDA

View Full Version : Rudder Trim



Dave S
10-22-2008, 06:45 PM
Good evening,

I am a little over half way with my phase 1 flight testing on my series 7 W/912 ULS/Warp Drive.

It is becoming obvious that cruise flight requires some constant right rudder. Not a big deal; however, rudder trim would certainly be an improvement.

Has anyone on the list done anything about rudder trim; and, if so, what type of trim system have you developed. I did not make any provisions for rudder trim attachment before covering the rudder.

Thanks for you input.

Sincerely,

Dave S
St Paul, MN
Kitfox 7

RandyL
10-22-2008, 07:00 PM
Dave,

Since you can't build one in at this point I suggest you experiment with small wedges of balsa or plastic just taped on until you get the configuration that works. Then you can make a nice painted one and attach it with double-stick tape. Make sure you put in in a place with good airflow, not right behind the HS for example or it will be partially blanked. You can make your own wedges or buy them commercially, they're available from Avery Tool. You'll probably need to put them on the left side of the rudder since the rotating prop blast favors that side.

You could also design and fabricate some sort of spring bias system that works on the pedals but... no, don't do that.

Lastly, be advised that if you change props your rudder trim will likely change, been there & done that.

wilson1steve
10-22-2008, 07:16 PM
Dave,

I know this advice is late for your benefit but I noticed and took a coupla pics at Kitfox of the rudder trim deflector application. The owner...before covering the rudder...added backing so as to be able to attach the deflector with rivets to secure it. See pics in my "some build photos" album for those of you that haven't covered the rudder yet for an idea.

Randy,
You might be able to tell me how to grab photo off one of my photo albums and be able to paste it here. Thanks.

Mnflyer
10-23-2008, 10:28 AM
Hi Dave, I added a rudder trim tab, and like you the rudder was covered before the tab was added. What I did was to take some 0.040" alum. and bend it around a tube the size of the rudder frame about half way then with another piece about 1" wide X 7" long did the same to make a clamp. drilled for # 6 screws and clamped it to the rudder frame over the fabric ( first piece was about 7" long X 2.75" wide for the tab). It works good I was concerned about drilling into the small diameter frame as there would not be enough material so tried the clamp on and its working.

airlina
10-23-2008, 03:31 PM
The evolution of my rudder trim system is as follows: When I initially built my Series 5 , I incorporated a ground adjustable rudder trim tab on the rudder. While this worked to get me close to a cruise flight coordinated flight condition, I found that my Kitfox rudder does not always center to the same position when deflected then released, so the "ball" never seemed to end up in the same place twice. This could be due to several factors: the Kitfox rudder is not a balanced flight control which may keep the rudder from returning to the same centered position every time, or friction in the system. Also you will find that the Kitfox rudder is very powerful so even a small deflection will get big results. I found this out when I would move the ground adjustable trim tab to correct a ball out of center condition. I would land , move the tab about 1/16" and on the next flight the ball would be out the other way. I finally decided that a inflight rudder trim tab was the answer. Where to find this system? The local hobby shop of course. In a nutshell , I used a high torque model airplane servo buried in the rudder that drives my tab. The servo controller I found on the internet and it is wired to the servo with three wires. I think the photos will tell the story. This system works great and I can trim to pure coordinated flight in cruise and take my feet off the pedals . total cost was about 150 bucks. Bruce Lina N199CL

airlina
10-25-2008, 05:24 PM
I was able to dig up an old pre-cover building photo that shows the attach point for my rudder trim tab. I simply used a piece of leftover angle aluminum and epoxied it (back then we used scotch weld) between the aft rudder tube and two ribs as you can see in the photo. The actual attach points are tinnerman nuts with the rudder tab screwed to these. After the rudder is covered this of course is all hidden. My reasons for using the model airplane servo where many. First size and weight -I had to fit the servo in the rudder and found that at 1.5" long by 3/4" wide it was the perfect fit (weight is less than 2 oz.)The Ray Allen stuff is too big for my aplication. Next torque- remember that we are flying a Kitfox not a Boeing so the 8 lbs of torque the servo puts out is plenty. I get full tab deflection at any speed. Last but not least was price- my servo was $40 and the servo controller was $50 versus $250 or so for the Ray Allen unit. Hope this helps. Bruce