The aluminum 2" PDRS202 and 4" PDRS204 servo arms are ideal for models ranging from 92"-140" wingspan. Both 3D and IMAC pilots use the same arms - but - with an important difference. The 3D-pilot uses the outer hole position to maximize control surface throw while the IMAC-pilot uses the innermost hole to maximize precision.
The point being, control link location is more a question of how you fly the model. For example, if you are going to take your giant scale model down on the deck - like it's an $80 foamy - to perform maneuvers like a Wall or Terminator, then you need huge control throw and for this, you use the outermost mounting hole. Alternatively, if you're an IMAC-type pilot and perform smooth maneuvers like loops, point-rolls, stall turns and spins, plus the occasional Lomcovák, then you're going to use the innermost mounting position for maximum precisio.
Experienced pilot (building and flying since the early 1970s). However, without question, I'm in the latter category - a club pilot. My point? I just purchased an 88" wingspan Extra 260 for a 50cc twin I have laying around. Servos arms? Believe it or not, while I could reach into the parts bin for a set of PDRS202 and PDRS204 aluminum arms, I'm using our heavy duty polymer servo arms, instead.
Specifically the PDRS101 for ailerons and elevators, and a PDRS102 for the rudder (equipped with pull-pull cables). Aren't these servo arms much too short? Won't this result in insufficient throw? Maybe yes, if the intent is to impress the peanut gallery with how far the control surfaces can move. But for a guy like me who isn't performing a waterfall maneuver and whose torque rolls happen at 100', the super long aluminum arms are a waste.
Frankly, for the maneuver class I actually fly, the heavy duty polymer arms will be perfect because I'll get plenty elevator throw for a snap roll or Lomcovák with the 1" arm - without the model being so sensitive I have to dial in either huge amounts of expo, or resort to flying with a switch (by engaging dual rates). Plus there's the attendant loss of precision that comes of using long servo arms! Ditto the ailerons will be more than sensitive enough with 1" arms to roll faster than I want. All while being 'perfect' for slow rolls, point rolls, 3-roll maneuvers once again without losing precision. This last is the trade off that comes with using the super long servo arms. I mean look at the size of the control surfaces on modern acrobatic models! After all, how far do you think they have to deflect to horse the model around? When the surfaces are huge, not far, believe me!
Now, please don't take what I am saying as impugning your skills as a pilot, or otherwise 'dissing' you but instead, I am asking you to reflect on how you 'really' intend to fly the model. If you are an aggressive 3D-pilot, then without question the PDRS201 and PDRS202 aluminum servo arms are what you want even with a small model like my 50cc Extra. But if you're a club pilot, save your money and use the inexpensive HD polymer arms, instead.
My point? I want you to be a happy camper and simple physics are at play in this decision. Specifically, the longer the lever, the less power and precision you're going to get. For example, on a 1" arm our DS360DLHV (which is rated at 360oz-in) will, on a 2" arm, only output half as much torque, or 180oz-in. And note, this is true for any servo, of any brand, it's just physics!
Also, don't forget, with a longer servo arm, both the backlash within the gear train as well as the free play within the linkages will be magnified. For example, you may believe the linkages are really tight but I can assure you they have several thousandths of free play (or they would bind). Same holds for gears that simply won't work without backlash. Note, while I am referring to the 1-3/4" arm the exact same hols true for the double arm version, the 3-3/4" arm typically used for pull-pull setups on rudder.
Anyway, look at the other end of the servo linkage route - like the tip of the rudder which is easily 8" from the hinge line. Now the little bit of free play within the linkage and backlash in the gear train is magnified, which means a terrible loss of precision around neutral (where the control surfaces spend 90% of their time). Do you take the meaning?
When it comes to money, while the aluminum arms are roughly $9-10 a pop, you can get a 5-pack of the PDRS101 heavy duty polymer arms for $4 (plus another $2 for the heavy duty PDRS102 double arm). Since these are the arms I'm using for my own 50cc model I want to bring them to your attention on the off chance you're a club pilot like me instead of some 3D god. I only mention this in an attempt to ensure you're really happy with what you build because a model that impresses the know-nothings in the pits with incredible control surface swings is a miserable model to live with the other 99% of the time.
Moreover, make no mistake, I'm happy enough taking your money for either type of servo arm, but if you're going to be my customer 10 years from now it won't just be because you're satisfied with my servos, but because you understand you can seek my advice and know I speak truth irrespective of how large the invoice is for what you get form us. Believe me, I know I can make a bigger sale with the aluminum arms, but this doesn't change the facts that for a smaller model, e.g. 30cc and 50cc class, the polymer arms are usually a far better choice because a) they'll deliver plenty of throw while b) affording a safety factor (by breaking in a crash rather than destroy the servo).