Why not on helis?

Kurt my response was in no way aimed at anyone. I just find it hard to believe that we muddle the waters with over complicated answers.
After reading some of this I wouldn't touch expo.

When the OP asked "is there a formulae for calculating Expo" I just asssumed he was asking if there was a formula the transmitter uses for expo, which it obviously does, part of its programming. If the OP was asking if there is a formulae to work how much to set expo too...its up to your personal preference. The way I set mine I dont need dual rates, even though I have maximum throws on my Pitts (45 Deg deflection), if I need to make small adjustments to a control surface in flight I can do it comfortably using what I have set for expo, and still have huge deflections in manoeuvers. I fly a wide variety of planes with different amounts, and to me I just adjust the settings until it feels natural for the plane I am flying. I also use a different expo setting for up and down elevator.

Exactly, try then fly, the only real formula if you wish to call it that is what you yourself dial in that makes it comfortable for your thumbs and a particular airframe. It's that simple, for the most part the rest of this is just allot of BS you don't need to think about. Just try it, fly it and then you will know.

Bob

As I assume the OP's question to be, yes, and its coded in the transmitter, and it may or may not be consistent from brand to brand or even different transmitters within a brand. Thats why you're getting the answers you are, tinker and try.

IMHO it's more a matter of personal preference than anything a model requires. I have none at all on some of mine.

You won't know how much you need until you fly the model. And you may even change your mind several times as you get used to that particular model and how it is balanced and the throws are set. Guys flew VERY well for many years before it was even an adjustable feature.

What I found was that anything over 40% - even on hot 3D models - just makes it a jerky on-off-on sensation to the stick and becomes counterproductive. Instead of finesse you have to bang the sticks around. I don't believe now I have anything over 35% (that is on my little 15" EDF Sabre that is tres squirrely to elevator) and normally I use 25% to 15%. Just enough to lessen the center for landing or smooth, straight flight. I normally fly high rates throughout the flight.

Had a bad one when switching between transmitters and thought I was switching to low rates for a fast pass and instead hit CROW at speed with my Hot Stik. Did an instant 90° down and I pulled out just in time to shear off the landing gear (which tore off the horizontal stab) and rip up the teeth of all four aileron servos. Thought I had saved it until I walked over and saw it was a fatal pancake. Learned a lesson on being consistent with how I assign switches on my various transmitters.

Jetmech05...totally agree with you! But I love the math.

Kurt

Expo will make it smoother around the centre. The more expo, the smoother it will be for more stick travel. If it is jerky, then you either need more expo, you need to be smoother with your inputs or most likely a combination of both. With a low percentage of expo, it will be jerky if you are moving the sticks quickly past the point where the expo is set for. If you are banging the sticks around, then your expo/throw settings may need adjusting to suit your flying style.

It does make it smoother in the center. But the flatter the center the more rapidly the surfaces have to move in the outer reaches to get to full deflection. If you have a surface that can move 40° up and down, but the first 50% of stick travel only moved the surface 10°, then the last 50% of stick travel has to pick up the remaining 30°. For me that gives a jerky response for some maueuvers.

As I said - personal preference.

Yes, everything is personal preference. What works for some, doesn't work for others.

More expo will give a smoother action for longer, but as you say it does have to pick up the rest sometime and with higher expo it does come on pretty quick. But when you get that much deflection, it is most likely going to be a bit jerky anyway especially with quick direction changes.

That's why dual/tripple rates or flight modes are good. Less travel can be set for maneuvers that don't require full deflection and you won't need a high amount of expo, which will stop the jerky movements when going to full travel on the reduced throw rate.