A few bits i've learned about Exponential:
1. Good reminder about the difference between Futaba and SPektrum/JR radios. + and - expo may result in an opposite effect on different brands.
2. With any brand...if you have Expo set to 0%...then you will still have exponential movement of your controls...unless you are using linear servos. The natural geometry of radial actuation with a standard servo causes the control to be most sensitive at the center of servo travel...and least sensitive at the ends of the servo travel.
Expo was added to radios to counteract the natural exponential movement of a radial servos....there is no simple mechnical way out of it. If you have a shorter servo arm and a longer control surface arm...it will make the bad exponential motion worse...and it will take more opposite exponential programing to counteract the effect. As tdford was saying....as you use a larger amount of your servo travel arm, you get better resolution....but you also end up with more "bad" expo. Many people use this becuse they want to amplify the servos mechnical advantage....by using a shorter servo arm...and longer contro surface arm....so you have to use all of your servo travel.
3. When someone says "I don't like expo...i have mine at 0" the control of the surface will be exponential....in the bad way. You have to add some exponential to achieve linear (as a function of % of control input to % of control output) in other words....everybody should be using some expo. That said....people lived without it forever...and some just dont care....and some aircraft have so much stability that it does not make so much of a difference.
4. You may have 20% expo....but you still may not be linear. Depending on the motion ratio between the servo arm and the control surface arm, 20% expo might not even get you to linear. Between 2 planes ...unless you are talking about an ARF where you have confirmed you have all your linkages in the exact same holes on the servo and controls...you cant compare expo. The expo variable on your radio is just a relative number. the expo "feel" you like plane will be different for most planes....because it is based on the servo arm setup (motion ratio), percentage of total servo rotation used,...and of course the sensitivity/stability of the individual plane.
5. the only way to know what expo you really have is to put a degree meter on your control surface and map the change in degrees of deflection to the percentage of -/+ control input.
6. most people don't care about any of this...and just play with it until it feels right....but they should at least understand the principals. At the same time most people agree that they expect to have linear control response....and possibly some helpful expo to calm down a twitchy airplane. In general it always helps to add expo (or - expo for Futaba) but if you map out your actual linearity...which is easy....you may find that you need more expo than you think...just to get to linear.
