Question: My Texan II 90+ ARF uses two servos for the elevator and two servos for the rudder. Why do I have to use two? Can I use one high torque servo per control surface instead?

Answer: The Texan II, Harvard II and PC9 series of VMAR ARF models uses two servos for the elevator and two servos for the rudder. There are a couple of reasons for this and the logic behind the design is somewhat different for the elevator than the rudder.

 

In order to replicate the performance of this semi-scale airframe as well as the appearance we wanted to enable strong control and aerobatic capability. At the same time we do not like to force modelers into premium priced high torque servos. Standard servos are cheaper and almost all modelers have them on hand. Unfortunately, in order to drive the price down some servo makers have reduced the power of the motors to the point that torque on some standard servos is down near 30 oz-in or even less. For the big PC9 based airframe we needed torque of 55+ oz-in on the elevator and rudder and the only way to hit this mark with the weaker of the standard servos is to double them up.

 

In the case of the elevator, the two servo system is required to ensure sufficient torque using standard servos. A high torque single servo will work for the elevator in lieu of two standard servos if the single servo has torque of  at least 55+ oz-in. When trying this, test in flight before being totally confident that one servo works.

 

The dual servo rudder is a bit of overkill in some cases. Yes, a push-pull system like that used with two servos is inherently less likely to flutter or bog down under high loads when in knife edge flight. A high torque single servo will work for the rudder in lieu of two standard servos if the single servo has torque of  at least 55+ oz-in. When trying this, test in flight before being totally confident that one servo works. If you find flutter in the rudder in some conditions, go back to two servos. If you find knife edge flight is better one way than the other, you could be overloading the single servo or the control rod in the push mode. If possible use both rudder control rods with your single high torque servo to avoid the high load problems found with "push" only rod set ups. Test test test if you are departing from the two servo rudder.