What does x% expo mean?

[Gabriel]

Hello,

Can anyone explain what does X% expo mean? What is the impact on the servo travel?

Let's say I move the stick half way (50%) towards right? If the control were linear, then I would expect the servo to move 50% of full travel. What's going to be the servo movement if I apply 30%, 50%, 80% expo? Full travel stick is going to be full travel servo for both linear and expo, I guess, but what about the mid points?

In other words, what is the equation (math welcome) that defines the impact of how much expo one applies? Is it something standardized? Is it proprietary to each manufacturer?

I would like to emphasize that the question is not related to how to set a transmitter, rather what is behind that setting?

Thank you,
Gabriel

[Tom Hastie]

I think it's pretty proprietary. I quick google search didn't yield anything. I'm gonna have to resort to describing what I've seen.

My Futaba 9C shows a graphical "curve" of the servo travel. The curve always starts at the origin (0,0 or 0 stick deflection=0 servo deflection) and always ends at the top right corner (call it 1,1 or full stick deflection= full servo deflection). These two points are constant and don't change with the amount of expo you dial in.

With zero expo dialed in, the curve diagram shows a striaght line connecting 0,0 to 1,1.
As you dial in negative expo, the curve becomes parabola shaped. Still passing through 0,0 and 1,1 but the slope of the curve at 0,0 decreases and the slope of the curve at 1,1 increases. The radio maintains a smooth transition between the two points.
Positive expo does the reverse. Increases the slope at 0,0 and decreases the slope at 1,1.

When I have my radio handy, I can take some pictures of the screen to show you the shape of the curve. It should be fairly easy to make a curve fitting equation that will take a % and turn that into an amount of curvature. However, I think you'll have trouble figuring out the exact curve futaba (or anyone else for that matter) is using without some reverse engineering. (you might be able to take some measurements of the stick position vs. receiver pulse width to get an idea of the curve they're using)

Tom

[Gabriel]

Thank you, Tom.

True. Google doesn't seem to be of too much help. Went that path before. Also tried to ask on rcgroups but people don't seem to get the idea that I don't need to know what to dial on my Tx. Anyways...

We are on the same page. The family of curves have to start at 0,0 and end at 1,1 (that's one quadrant view), whether they are linear or expo. I have come up with an equation that does that already, but there is one parameter that needs to be tweaked in order to get more linear or more expo shape. What I want to know is a reasonable range for that parameter, i.e. what the experienced guys have come up with. I don't want to reinvent the wheel.

Now, thank you for the offer, but please don't bother taking pictures of the screen. That might be way off, its sole purpose being just to look nice on the screen. If you have some spare time (no rush) and we can get together somewhere, somehow, I can prepare a little device and measure that myself. The only thing I would like to ask you is to power on the transmiter, dial in different amounts of expo and move the control in steps, so I can record the channel variation versus the stick movement. The transmitter has to be PPM, though. And if you agree to help me with that, I would like to know in advance how many channels are there (the fewer the better) in order to measure them.

Thanks a bunch,
Gabriel

[Tom Hastie]

Ah yes… managed to find that thread on RCGroups now :D… now I also realized you did some posting in that highly entertaining, yet not very informative thread on why the current draw of a motor increase when you put a large prop on the motor :p heh heh…

Your only solution is probably to do some measurements if you want the “exact” effect of 80% expo. Not many people will know the exact expo equation as it’s a very subjective thing and even on the same plane, the amount of expo someone else wants to fly with will be different from the amount that I choose to fly with. Personally when I set up a new airplane I follow the control throws given the in manual, but I’ve never followed the expo settings. I just set some mid range expo and tweak it from there. On your VTO, I’d say just set up your Rx to it moves the servo 20% of it’s throw in the first 50% of stick movement, and experiment from there. The exact setting you end up with will depend on your preference of how you like the airplane to feel. Case in point, there was a Funtana (no names mentioned :p) that someone asked me to fly that was flyable, but felt REALLY squirrelly… after landing it, I checked the expo setting… it was set to POSITIVE expo. so The control surfaces moved more around the neutral. The airplane was still quite flyable, just not smoothly :p… I guess I’m trying to say that there’s nothing magical about the “80%” setting you see in the setup from the VTO (I read your other posts on the subject on RCGroups)…

That being said, I don’t mind letting you record what my transmitter expo looks like. I still usually fly at the west end field in the winter, on the weekends, so if you wanted, you could record my radio and its expo effect sometime there. Could hookup over email to co-ordinate what time I’d go flying on any particular weekend.

See ya,
Tom

[Gabriel]

Thanks Tom,

"it moves the servo 20% of it’s throw in the first 50% of stick movement"
This sounds like a good point to start experimenting with.

I'll catch up with you by e-mail. What RF channel are you using? I forgot.

Gabriel


EDIT: Tom, thank you for the offer, but in the mean time someone posted a set of measurements which allowed me to figure out the curves.

[Skybolt]

Hi Gabriel,

Regarding your question about expo on radio.

Personally I use expo setting a lots on my planes.
The expo setting will increase (+) or decrease (-) the sensitivity of your control surface in the middle position for elevator, aileron and rudder. For throttle it will affect the idle position.

Example:
For aileron and elevator I use negative expo to make the center of the control less twitchy but you keep the same deflection at full stick.
Positive expo is used on the rudder for an airplane with wide fuselage because they have more turbulence and have sluggish rudder response in the central area.
Negative expo will give you a more linear rpm response with throttle stick movement.

The amount of percentage is a trial and error until you find what you want. No two planes are the same.

Hope this will help you.