Did a little “social distancing” on Sunday to test my newly installed elevator pitch trim system in flight.
I have limited vertical clearance under my oversized header tank, so the plans version of elevator trim was not an option. As you have all come to expect from me, I designed my own version.
General description: it consists of two stacked rail systems. The lower rail has a carriage that slides fore and aft in a steel track that is anchored to the top skin of the canard and is driven by a linear servo which is anchored to the firewall. Attached “piggy back” on the top of that carriage is another rail with its own a carriage that is centered/floating between two fixed springs. This upper carriage has a rod-end/push-rod assembly that reaches back to the ear on the elevator torque tube assembly, attaching just above the push-rod coming forward from the stick.
Clear as mud? Here are a couple of pictures of the assembly (not installed in the plane yet). The yellow nylon carriage slides into the rail mounted to the canard. The servo attaches into the hole left of center in the aluminum fixture on top of the yellow carriage. The bottom rail has already been installed in the aircraft at this point. Next, the upper carriage assembly “floats” between the two springs and slides along the aluminum track that is rivetted to the upper C-rail. The lower carriage, driven by the servo, determines how far fore or aft the floating spring assembly is to the elevator torque ear.
After much ground testing followed by test flying, I can report that it works as designed. The springs may be a little on the weak side, but I can change those out or augment them later. The design allows me to swap them out easily. For now, I will fly it as is. By putting in full nose-up setting on this elevator trim, I get about 3/8” down elevator and use neutral reflexor for a 90 mph final approach with neutral stick force. Using this setting visibility over the nose is much improved. Previously I had quite a bit of aileron reflexed up (nose-up) to get the same approach speed and I had noticeably higher in pitch and reduce visibility.
I take off with a little bit of aileron reflexed up and this new elevator trim set to neutral. I will be experimenting some more to see what combinations are required for various cg’s. Overall, I am pleased with this addition to my control system.