Some questions about servos

[ACITceva]

I've been doing a lot of research and I've figured out a lot of the technical aspects regarding servos - coreless vs brushless, plastic gears vs metal gears, analog vs digital, voltages, speed, torque etc.

I'm having trouble with some of the more abstract parts though. It feels like everybody else understands this and I've not really had any luck finding documentation:

What do the sizes of servos represent? Standard, micro, sub micro, mini etc. I see them used all the time but the definition of each seems "fuzzy". If you have a kit that says it requires "sub micro" servos can you just choose any servo listed as sub micro and it should physically fit? Problem is I see people using micros on kits that call for sub micros and they fit perfectly. I think I've even seen the same servo listed with different sources as sub micro in one place and micro in another. So do you really have to just look at the specific dimensions of the servos and figure it out for yourself? Or just ask other people and see what THEY use?

Regarding servo speed... obviously lower is faster given how this spec is always shown and you want the fastest possible servo for your tail. But how do you know what's fast enough? Is there some rule of thumb for example that says "this is fast enough for cyclic for this size heli with this type of flying" and "this is fast enough for rudder for this size heli with this type of flying".

[ArchmageAU]

Measurements are approximate: (length, width, depth of main body of servo)
nano - 0.74" x 0.30" x 0.44" - one screw hole top/bottom
sub-micro - 0.89" x 0.45" x 0.65" - one screw hole top/bottom
micro - 1.17" x 0.47" x 0.78" - one screw hole top/bottom
mini - 1.27" x 0.66" x 0.93" - two screw holes top/bottom
standard - 1.57" x 0.78" x 1.05" - two screw holes top/bottom

Dependant on the frame you are mounting you may be able to shave some interior diameter of the mount point and put in a larger servo. Generally the size is a guide and one size fits most within a size bracket.

Not only do you want speed for tail servos, but also durability (they do A LOT of work changing position many times a second, more than cyclics). The higher cost for speed and durability is not usually required for cyclic servos (which have 3 to move a swash plate, but the static load on these servos can be greater than the tail servo).

There is no hard and fast rule as to how fast is fast enough. The more experienced you are with flying, the more you will "feel" how different servos change the handling of a model.

Hope this helps.

[rhodesengr]

just to add a little.....
It is not a bad idea to take a good hard look at what others are using. Almost every model heli has it own sub-forum and within each model's sub-forum there are always numerous threads about servo choices. There is never a consensus opinion about any one servo but it will help you narrow down to some tried and true choices.

Especially on smaller models, not every servo within a size class will be an easy fit. For example, some helis require the servo ball to be facing the servo and not all servos can be set up that way.

I'll use no name servos in a plane but in a heli I always go with the name brands.

[ACITceva]

Thank you both for the information... it does indeed clear up some of the missing pieces for me!