Direct Drive uses the full throw of the servo, which is about 60 degrees to each side of center. Due to sub
trim and programming limitations with today’s transmitters, your actual available control throw will be about 55 degrees
to each side. If you have a Piper Cub model, it is doubtful that you need, or want, this amount of control throw. I recommend
you reserve Direct Drive for aircraft that can use large throws. You can reduce the amount of throw down to about 30 degrees
with Direct Drive and still retain better resolution than a conventional setup.
In general, larger aircraft (33% up) can be retrofit easily with Direct Drive, even after construction. In
fact, it can be quicker to install than standard linkages. Quarter scale or smaller planes take more planning. This is because
the size of the servo proportionate to the size of the model increases. It can be done on any airplane, but the effort increases
inversely with the size of the aircraft.
The elevator and rudder servos will be mounted on the hinge lines in the tail of the plane. As a minimum you
will need two elevator servos and one rudder servo. If the aircraft design called for the servos to be mounted up front, you
will have to account for the change in center of gravity (CG) due to relocating the servos. On a large plane this is not normally
significant, as the relative weight of the servos, and the resulting CG shift, is small. On a 40 sized model, it may be severe.
If your plane called for rear mounted servos to begin with, then CG will not be significantly affected.