Originally posted by squareloop
Wow that spreadsheet is interesting! How do you approximate what your average airspeed is though? That would be an important factor as to whether or not your servos are up to the task.
Aerobatic models are not typically not fast models. But the forces applied can be extreme, even at slow speeds. I'd suggest an air speed factor of 75mph.
[QUOTE]OK folks. Been thinking (oh no!!!) about this one last nite
Just as an example a surface needs 75 oz of torque minimum. Say I have a servo rated at 100 oz/inch. Setup: the rod is connected at 1" on both the servo arm and the control arm maintaining a 1:1 mechanical advantage. I also get a desired surface deflection of 50 degrees with the servo pegged at both endpoints. [/B]
No mechanical advantage here 1:1 is an equal advantage.
Now I move in the attachment points at 3/4" (.75) at both ends for an increased force value of 133 oz/inch (?). Assuming I moved them both exactly the same amount (1:1) the surface deflection should be the same (?).[/B]
133oz-in of Force is correct, this is realized at the end of the servo horn. No the surface deflection will be less. I would only move the servo horn pivot point to 3/4" this will increase your FORCE and provide a mechanical advantage. .75:1.
My question is: why would I use (or not use) one setup over the other? What am I sacrificing in exchange for the extra force in the second setup? [/B]
Short servo arms are desirable as are long control arms otherwise known as lever arms.
Short servo arms increase the Force realized but sacrifice the travel arc.
Short control arms will provide more travel movement but will require considerable more FORCE to do so.
Long servo arms provide more travel arc but reduce the FORCE delivered.
Long control arms provide a longer lever, thus requiring less FORCE to operate the surface but inherently reduce the travel arc.
Best setup is MAX ATV or End-point at TX, as short a control arm as will provide the desired surface deflection. Generally were limited to an absolute minimum with regard to control horns, this is measured from the hinge center line to the control horn pivot point. Always maintain at least a 1:1 ratio between both arms, preferably a mechanical advantage would be present (smaller servo arm than control arm).