ORIGINAL: Ben Lanterman
Giant scale folks make an effort to make the rudder pull-pull with 4 servos ganged up for rudder. Then for control "goodness" they go to an aft mounted servo or two servos on each stabilizer half and a short pushrod to the elevator.
Why the difference? Certainly a tight pull-pull system is just as tight as a direct servo connection. Or the reverse, if direct is so good why not use it on the rudder?
Another question. Why use a pull-pull when a big diameter carbon fiber tube push rod would be rigid and let you put the servos up in the equipment bay?
Granted dual elevator servos per elevator half gives some reduncy in failure analysis.
Well I can point to a specific time in the pattern/F3A world, mid->late 80's (gee has it been that long?) when weight was a big factor. It was quicly realized that 2 lengths of thin cable or kevlar cord was lighter than a CF pushrod & essentially did the same job on teh rudder control, so out it went. Yes it was also done on elevators, but to a lesser extent. Pattern types want discreet control over both elevator halves, which meant getting the geometry right & keeping it that way over hundred of vibrating flights. It also meant 4 cables vs 1 pushrod & ideally 2 servos to slave each elevator half without it becoming a rats nest in there. When the 4 bangers came along it basically meant gobs of power for all, so the weight thing could be relaxed a bit. With bigger fuses & smaller more powerful servo's comes the opportunity to sink the servo very close to the deflection surface which is probably the best from a slop free standpoint. But in the bigger scale stuff when you have to gang servos together for power, the current setup might be a function of where servos can best be accomodated. So to answer your question, probably some practical reasons & probably just old habits! <g>