Chapter 17 – Trim System

This chapter deals primarily with the trim system and the optional landing light installation. It also covers the installation of the center console and throttle quadrant.

For the trim systems, I am going to use a trim set up suggested by other builders. I'm going to use the Hanka Roll Trim System for role trim and Alex Strong’s system for the canard pitch trim. The plans system is simple and it works well, but I don't particularly care for those high-tensioned springs. If one spring breaks, you immediately get full and opposite control input. The Hanka trim system has a benign failure mode. If the Hanka trim spring breaks, it essentially disconnects itself and doesn't cause any sudden control inputs. I will install the role system on the firewall attached to the connecting rod for both side sticks and operate it with a MAC servo. I can’t do this step yet because I have not mounted the main spar at this time so this step is put on hold.

I started fabricating the landing light and opted for the stock location. Many builders are putting the lights in the nose, but with front seat weight not being a problem in my case, I opted to have the weight further back for cg considerations. Here are a couple of shots of the landing light installation. I just used a fog light I found at Pep Boys that closely matched the measurements in the plans. I could not find the exact light called out by part # in the plans, but there are many others that you can make work…….

The picture on the right shows the cover that I made to go over the light to keep air from entering the cockpit. The cover was made by shaping foam and covering with box sealing tape and laying up a couple of layers of bid over the top as shown below.

The last shot shows the light installed in the down position. I just put duct tape over the lens to protect it from scratches etc.

Next, I started working on the center console and throttle quadrant. I ordered the two lever internal reversing (remember the engine is in the back so the cable has to “pull” the throttle open instead of “push” like on a standard engine up front) throttle quadrant from Wicks. I opted for the two lever instead of the three lever because I will be installing a Lycoming 0-360 with fuel injection so I won’t need a third lever for carb heat.

The center console goes together just like the rear heat duct in Ch. 8. Just cut out foam, glass the inside pieces first, assemble and glass the outside. The picture on the left shows me glassing the outside and the right picture shows the console and quadrant installed.

Update 3/23/05

Again, looking for little projects to do during the winter months, I fabricated the horseshoe trim piece that I will use for the aileron trim. As mentioned before it is safer because if it breaks, it will not go to hard right or left trim like the plans set up would if a spring were to break. It is also lighter than the plans set up.

The Hanka plans call for a 4” diameter hoop with attachment ends for the servo and the clamp for the aileron tube. I purchased some 1/8” aluminum tubing and just rolled the fiberglass around the tubing. It was a little difficult to get the glass to stay attached to the small tubing. I just continued to check the joint while the layup cured. It eventually stayed in place and everything came out fine.

The hard part was finding a 4” diameter shape to lay everything up around. I finally found a cardboard container for pancake mix in the kitchen cupboard……exactly 4”! I wrapped duct tape around it for a mold release after the glass cured. Everything worked great until I went to take the tape back off the pancake mix. You guessed it, it tore the label of with the tape. I didn’t think too much about it until my wife went to make pancakes a few days later and wondered what happened to the measurements on the side of the container. Well, now I have a permanent mold in case I want to make another horseshoe…..

Here are a few shots of the process……The one on the right shows the horseshoe layed up over the tube, and the one on the left shows the part before final trimming. 3.5 hrs.

Update 7/12/08

Once the horseshoe was trimmed, I mounted a Ray Allen servo onto the right side of the main spar and hooked the assembly to the aileron torque tube as seen in the picture below.

If the trim isn’t strong enough, I will just need to add a couple of layers of glass to the circumference of the horseshoe to stiffen it up.

Once this was done, I had to fabricate trim for the elevators. The plans call for a manual system with metal springs. I opted to go the electric route with other builders having good experience using various forms of linear actuators. I have read many were having issues with trimming in that the actuator would move too quickly for accurate trim during cruise. Most builders are using actuators that move about 1” per second. Doesn’t sound like a lot, but these planes don’t require a lot of movement of the control surfaces to be effective. I also noted that some builders were using actuators that had 6” of travel. With the Davenport spring I intended on using, my total travel from “full up” to “full down” elevator was only 4.5”. My concern was if you had a run away trim in either direction, it would put a lot of force on the spring, and would be more difficult to override manually. I opted for a 4” travel with a movement of .33” per second. I centered the spring on the elevator so that either “full up” or “full down” only requires a .25” deflection of manual effort against the actuator to attain full travel. I purchased the actuator online from http://www.firgelliauto.com/

I then fabricated the Davenport spring. This is just a layup that wraps around the elevator torque tube that is thicker (more layers of glass) the closer it is to the tube. Less layers of glass are tapered to the end where the actuator attaches to the spring. A neat little jig is fabricated first that shows you how many layups are needed at each point along the length of the spring. Here is a picture of the jig with the glass laid up. Clamps are used to hold the layup together until it cures.

The tube in the picture is used to simulate the elevator tube. I attached a piece of plywood to the back of the instrument panel and attached the front end of the plywood to the top of the front wheel cover. This served as the mounting point for the actuator. The picture below shows the installation. Yes, I realize the actuator is not attached to the spring in the photo. It is now……..

This ends Chapter 17.