In the old days before computer mixing we used to build out own mechanical mixers. As a mater of fact dubro used to sell one that mounted on top of a servo. These mixers all consist of one servo in a fixed position and another that would either slide or have a device mounted to the servo arm that would allow a forward and reverse movement. I built one for a micro model with Cannon radio. Much easier if you just bite the bullet and buy a computer radio.

As far as designing and deturmining the balance point on the dellta I have a suggestion for you. Build small balsa profile models at maybe 1/4 scale of the projected design size. Use 1/16" balsa and use this model to find the correct CG and explore gliding properties. Use small washers or some weight to add nose weight until the model exibits a smooth gentle glide. This test will get you real close to the proper CG for the full size model. I have used this techneque for many years.

Enjoy!