Rudder Trim/Alodyne

[Reed, Britt/FSC Salt Lake]

I am working on my first piece of my 3500 - the rudder - I chose to skip the stab/elev in case there are some mods to address the rear spar rienforce issues. Meanwhile there is not much archived on this except the pictures of Al's trim .

I just got my Ray Allen trim servo (one size down from the elev trim servo w/o the jack screw) and was planning on putting it in the second bay up and adding a rib to stiffen it up. Initially I planned on 3 rivets in from the trailing edge and the full bay for about a 2.5" x 12" trim tab. I had a nice talk yesterday with whoever answered the phone at the Ray Allen Co. (Ray?) he thought that 2"-3" by 6"-8" would be plenty and apparently has an ongoing joke about the size of the Murphy SR elev trim tab. On the rudder - much bigger than the 3"x8" and he thought you would hit your head on the side window when you touch the rudder trim button.

The Ray Allen website also has a nice design nomograph for selecting the throw vs horn size.

My new plan is to go with a 6"by 2.5" and use the stiffening rib as the bottom of the trim tab and building up a mounting base sorta like the RV guys use.

Meanwhile I noticed that the servo fits nicely between the ribs that bracket the rudder hinge. If I had not already drilled for below the lower rib of the middle hinge I might rethink the position.


The guy from the Ray Allen Co also thought that it is better to mount the elev trim to the upper elevator surface and use the inspection plate (on the lower surface) to get at the servo and inspect and adjust it rather than mounting the servo on the inspection plate requiring removal for inspection and adjustment. With the servo on the upper surface and the horn on the lower surface of the trim tab. I have read the instructions but not yet started work on the elev so I am not sure exactly how well it is configured.

Also I decided to go with the Alumaprep/Alodyne process to pretreat for corrosion protection. My friend is an A&P and recommended the process as not that much of a pain. Brian at MAM agreed that it is industry standard if maybe overkill. Used a 10' plastic rain gutter with a swamp cooler thru hull fitting and a PVC ballcock to drain it for reuse. The spars and ribs were easy and came out great because they are fully covered with the chemicals. The stuff is standard for use in the automotive world and the local sales guy suggested spraying it with an inexpensive hand squirt bottle. I tried to do the rudder skin with it sitting vertically in the gutter and a plastic sheet draining the overspray into the gutter. It did not work very good - an uneven treatment with the areas where the drips starting getting the best treatment. I am going to redo it by making a sheet plastic bed sized for the application (rudder) and immerse one side at a time and see how that comes out. It will probably be too much of a hastle for the wing skins.

Britt Reed - Salt Lake City - SR194

[S & L Aldrich]

Hi Britt. Just got your message from yesterday. I'm in the middle of playing in the black tank goo.

As you know I am going to downsize my rudder trim. I am thinking about remounting my servo up under the fin like Al did. I think you have seen his pictures in the archives. Not 100% sure on that yet though. May leave as is but change later when my rudder may change. I don't suspect the weight of the servo behind the hinge line is very significant but is something to think about.

I'll call you after this next batch of sealant gets spread all over the garage.

Scott