Hi Everyone,

I have a servo arm use question. I have a Futaba 6exa-pcm TX and the servo's are the S3004's. I have noticed in the hardware kit, there are #4 6-armed star type arms and 1 Large 4 arm star arm. On the servo's themselves, they have the round servo disc. My question is, what type of servo arm would you all suggest to start with. This is my first build and soon to be flight with instructor. I have a LT-40 trainer I'm building, and I am at the point of getting ready to mount the servos in the servo tray.

Thanks All

CC

depends on the type of hookup you have on your pushrods. sometimes the round ones will bind on some types of hookups so I usually use one of the star type arms and even then sometimes I have to cut off some of the extra arms if they interfere

Use the star.If you don`t like the extra arms you can cut them off. Look at your fuselage plan, it shows the servo tray with the servos and arms.

DON'T cut anything yet! I, personally, don't like the round arms. They are too small and won't transfer the servo's full torque. When everything is installed and batteries charged, center the trims and turn on both the TX and RX. The servos will center. Take an arm and fit it on the output shaft. If the arm you want to use is not exactly 90 degrees to the servo case, rotate it to another arm. One will fit at, or very close to, 90 degrees. NOW, cut off the others. Or, if you have room, leave them on for another installation. The servo arm length should be the same, or a little shorter, than the control horn length. This is the most sound mechanical setup for transferring maximun torque/movement to the control surface. A short servo arm/long control horn gives the most torque at the loss of range of movement. A long servo arm/short control horn gives more movement at the loss of torque. Servo torque is rated at ounces/1", so both the servo arm and control horn should be about that length.

Dr.1

Thanks all!! Appreciate your quick responses.

I just installed the same system, Futaba 6EXA TX with S3004 servos, in a Sky Raider Mach II. I used a star servo control arm for the throttle and it does not seem to provide the same amount of stroke as the one sided Hytec servo arm I had previously used on a Sky Raider Mach I (the Hytec servo arm does not fit the Futaba servo gear). I have not installed the engine yet so I'll check for adequate throttle travel tonight. If necessary, to gain additional throttle travel can I set the starting point (idle) for the servo arm at say 235 degrees instead of 270 degrees (i.e. half throttle would be achieved at 270 degrees, 0 degrees being orientated towards the engine)?
Thanks,

it does not seem to provide the same amount of stroke as the one sided Hytec servo arm

Read my post above. If the Hitec servo arm is shorter than the Futaba arm, the throw at the engine throttle arm will be less.

Dr.1

On the throttle servo you would want mid-throttle on the carb to have a servo arm position approx perpendicular to the servo body. Just like neutral elevator/aileron/rudder is mid-travel and also perpendicular to the servo body.