Post Number 3
Rudder trim is an often mentioned subject.
Long before the start of Model IV #2, I had fantasized on the “Wright” technique of wing warping and its possible use for rudder trim. I needed to at least try it. The plan was to make two bottom ribs out of a flexible material, tying them together and having something move the two piece bundle right or left to create an airfoil shape in the desired lift side.
One of my favorite structural materials – Carbon Fiber arrow blanks were used for the ribs. They were tied together at the cam actuators by drilling adjacent holes and using Hysol to glue in a short length of control cable across the gap. To keep the ends in place against the steel structure, I welded projections on to the tubing and threaded the ends of the carbon fiber tubes on the nubs. I also fabricated thin aluminum U channels to be glued on the outside of the tubular ribs to give a flat surface for the fabric to be glued to.
Carbon fiber tubes were also used to connect the cams needed to control the two ribs to conserve weight along with welded steel bell cranks as needed. The servo was attached in a fashion similar as that used on the elevator trim. I decided to rib lace to help support the tubular ribs as they moved against each other. For continuity, I rib laced the entire empennage.
The system works! And to answer the question, “Was it worth the effort?” I have to say yes from the tinkerer’s point of view. The idea has been put to the test and essentially put to rest. However as Dave and others will likely suggest, movable rudder trim is pretty much a nonessential item with our airplanes – sort of a set it and forget it item. Thinking about it, if I were to do the rib warp again, I would have a little hole, probably in an extension of the leading edge fiberglass, that I could insert an Allen wrench into to make fine adjustments and save the weight and complexity of the electric servo.
The last two pictures show the relative curvatures of the two sides needed for the same result Dave and many others have gotten with a simple tab. I do like the clean, though
Post Number 4
A buddies 604 lb. empty weight Model IV was in the back of my mind during my entire build. With all the gadgets I was putting on my airframe, I wanted to subtract some weight. I had heard that the extruded PVC lift strut fairings could add up to 14 lbs. so thought of ways to possibly reduce that. At one of the factory fly-ins, I saw Dan Denney’s highly modified Kitfox and noticed that he had aluminum fairings, so that became my challenge. Then watching a neighbor having a new roof put on with the gutters rolled from a coil of sheet aluminum established my goal. I started working on a rolling mill.
My first goal was to roll a sharp angle at the trailing edge to capture the free trailing edge to avoid what happens sometimes with the aluminum flaperon. Sadly, that is about as far as I got as personal pressures wanted the airplane flying. After rolling a section I used a brake to fold an angle mid span on the aluminum and after folding, that became my airfoil. My shear and brake only had a 30 inch capacity, so there needed to be four lengths of fairing for each section of lift strut – sixteen in all. One thing became clear early that presented an unexpected challenge. When a section of the fairing was folded, being held together with the angled trailing edge, it was not stable. It could be twisted into a near helix just by rotating leading edge against trailing edge. If not careful, this could result in a twist on the fairings. The PVC sections being long enough to cover the entire span avoided this problem, but it still was a slight possibility if not checked carefully. The solution was to glue one length together with great care, ensuring it being perfectly straight. Then an assembly jig was made from that section. This also provided an unexpected benefit – mentioned later. To hold the fairing in place on the lift strut foam spacers were cut using a home made punch.
The lift strut was supported vertically, Wing end down. A section of aluminum fairing was placed over the lift strut and held in place with a complete section of foam at the bottom end, then the clamping fixture was placed and clamped to secure the partial foam section against the strut on the upper end. A 1 oz. mix of twopart Urethane foam was then poured into the fairing shape and allowed to cure. The clamping fixture was then moved to the next location and the process was repeated. I had learned that the Urethane foam was very adhesive and found no need to add adhesive to the aft joint. The benefit of using the clamping jig was when I made sure that each joint between fairing sections had the clamping fixture overlapping to help form a smooth joint. Once complete, the trim to allow for wing folding was accomplished and the V connection covered with a small fiberglass section faired with Micro. The final finishing touch was to form and glue on thin sections of aluminum to cover the joints between sections. The aluminum I used was .010 thick. Early I felt the need to placard with a NO PUSH decal, but decided it was not necessary as the Urethane foam gives a really firm feel to the fairings. Before and after weights indicated that the fairings were very close to 5lbs. total.
Re: A tinkerers Toys
Posting Permissions
Reply With Quote