ORIGINAL: DDDrive
Resolution is the next issue. You are all absolutely correct about gaining resolution with a reduction linkage - in theory. If you use leverage to reduce your motion by 1/2, you should double your resolution. But, this logic neglects the deficiencies inherent to a linkage setup. Linkages have a small amount of play. If they had no play in the joints, they would bind - and any play is a loss of resolution. Add to this the deflection you get from the rubber servo mounting grommets, the deflection from servo arm twist, the deflection from the control horn...well, let's just say you are loosing much more resolution from your linkage than you realize. Increasing the amount of leverage just increases the forces imposed on the linkage, and that makes all of the deflections worse.
When you add it all up, you lose all of the theoretical resolution you thought you were getting from your linkage - and then some! Direct drive does not suffer from any of these resolution robbing problems.
Hope this helps...John
Respectfully, I disagree...
If you are talking about a 3D setup, ok. But if we want to do precision flight, a good linkage can actually REDUCE the free play. With the Direct Drive, your surface will have the free play of the servo gears. Ok... but with a good linkage, we can make the small gear play almost disappear: the result is that the surface is rock solid with almost zero play!
How can this be done? Properly choosing control horn & servo arm lengths, no mistery. For the interested people, I posted some math formulas on the pattern forum some time ago. The drawback of course is that you loose the amount of throw, but if we are interested in the best resolution, power and precision of centering a good mechanical setup is better by definition - it's physics.