When do you use exponential?

[Erfolg]

As with the majority, I use exponential, at 50% on aileron and elevator.

With me it is not about the model needing it, it is me.

[Martin Harris - Moderator]

Posted by Ian Jones on 27/12/2012 19:43:25:

I wish Spektrum hadn't confused the issue by reversing the direction of + & - expo already established by existing market leaders like Futaba & Hitec.

I was alerted to a clubmate having problems with a model - when I got it down for him we found he'd set 40% expo - positive - on his Futaba equipped model after experimenting with upping the control throws. It did make it a little interesting to fly...

[Biggles' Elder Brother - Moderator]

I'll bet it did!

BEB

[Chris Bott - Moderator]

I wish Spektrum hadn't confused the issue by reversing the direction of + & - expo already established by existing market leaders like Futaba & Hitec.

I think you'll find JR were doing things differently to Futaba long before Spektrum arrived on the scene.

Whether Futaba or JR were first I haven't the foggiest.

[Alan Randall]

I always dial in some expo, particularly on ailerons. How much depends on the size of the ailerons, the throws, and the speed of the model. My DX8 has 3 rates so I set varying amounts to experiment.

I have seen one or two fliers, who brag that they fly by the seat of their pants, roll models into the runway or the field by getting into an aileron see-saw by over correcting on large throws and/or large ailerons.

[MikeS]

Like most it depends on the aircraft. Being that I fly scale i like to add 30% for my P40 (50" wing) size and 20% for my Pawnee (80" wing).

Mike

[Jungmeister]

I wonder if the interest in expo is greater among Mode 2 flyers. I fly Mode 1 and it is easy to avoid interaction between Roll and Pitch simply because the sticks are separated. Expo can help beginers to fly smoother.

Having said that, there is a place for some expo on some of my models.

[Danny Fenton]

I use expo, sometimes as high as 80% and not just on fast models but to make super smooth flying a reality for me. I usually set about half that initailly for the first few flights, then increase until I am happy. I learned to fly on equipment that did not have expo nor servo reversing. New computer equipment is great and so powerful, to ignore their capablities just seems daft. It did take a while to get used to hi expo values though

Mick Reeves also uses up to 80% and servo slow...... and he knows a thing or two

Cheers

Danny

[Turbycat]

It all depends on the model for me. Sometimes I'll use dual rates as well. I like to tame down my aerobatic stuff with quite a bit of expo, 45% is generally what I use. Mainly only the ailerons and elevators but sometimes on the rudder (if it is a large effective one). If I'm maidening a model I'll always dial a bit in just to take edge off of the centre stick movement. My little delta I built had 50% in for a start but I've dialled a bit out down to 30% now I've got the hang of it. I don't think I have any on my large Tiger Moth. Just a bit of differential and a little aileron/ rudder mix to switch in if I'm feeling lazy and can't be bothered to balance the turns manually

Andy

[IanN]

I'm with the majority. I always start with some on ail/ele

Edited By IanN on 28/12/2012 09:15:43

[Steve Hargreaves - Moderator]

Nearly always.....its like having dual rates without flicking the switch....

I prefer my models to be quite touchy & set them up with lots of movement & then tame them down again with expo.....

[Nev Haycox]

I'm still very much a beginner to all this, but, right or wrong I usually dial in 25% on all but the rudder to start off with.

Nev.

[Erfolg]

This is a question for BEB.

The graph shown on the screen, although called exponential on Futaba, does not appear to be of the form e^x. As at x= 0 y does not equal 1. If to that form it is not the simple equation.

What has bothered me for some time, my understanding all values of x would be +VE.

So why has Futaba chosen negative values.

Just curious, really.

Edited By Erfolg on 28/12/2012 13:39:35

[Biggles' Elder Brother - Moderator]

Hi Erflog, you are a naughty man! You got me curious now!

Doing some "back of the envelop" type calculations the form of the function is something like:

Output=(EXP(-C x P x Input /100) - 1) / (EXP(-C x P / 100) -1)

'Input' is the stick movement, equals 0 at the centre point and 1 at full deflection.

'P' is the percentage expo set - uses the Futaba model - ie negative for less movement around nuetral

'C' - is a scaling factor - (fiddle factor!) a value of about 7.5 seems to produce similar curves to what you get from my 8FG Tx.

The "-1" is in here to make it go through the zero-zero point.

'Output' is the resulting servo arm measurement.

I worked this out myself - then did an internet search which threw this up from a guy at the Technical University of Delft in Holland who seems to have arrived at a very similar conclusion! But his fiddle factor - sorry scaling factor! - is 2.5. That doesn't seem to work well for me. And he doesn't scale the percent figure in quite the same way. But the general form of the equation is the same.

BEB

Edited By Biggles' Elder Brother - Moderator on 28/12/2012 16:11:42

[Geoff S]

As mode 2 flyer I always put some expo on aileron and elevator to give me some dead band in the middle and to minimise my hand cross-coupling the two controls. Due to a high spinal injury my right side doesn't work too well, particularly my right hand, so the expo and a using a tray helps a huge amount. I didn't start RC until after my injury I can't imagine why I started using mode 2 but it's too late to change now ... and I like being able to scratch my nose and still fly

Geoff

[Bob Cotsford]

I wonder how many sets actually produce a true exponential relationship? I bet most are more of a smoothed dog-leg where only the rate change point is adjustable.

[Erfolg]

Hmmm, BEB, not the simple e^x function that schoolboys were so familiar with.

Your function is the type that used to scare the hell out of me as a schoolboy , until, I had spent the whole night sketching out the graph, then comparing the results with the rest of the class, well some that would leave me alone. If I had been born later I would have just plugged it into the Microsoft graphing programme, using a spread sheet. How kids have it easy today, Ggrrrrrrrrrrrrrr!!!!!!!!!!!!!!!!!

I guess the rather unusual Futaba function is related to the functionality of the IC/chip set?

[Biggles' Elder Brother - Moderator]

I would guess that the function is probably implimented as a look up table in firmware. The "few set points" is probably not correct because my Tx certainly produces a nice smooth-like exponential function on the graphics screen - and if it can do it there why not do it for the actual output? - but I don't actually know how its done and interestingly Futaba seem to view it as a bit of a secrete - I can't find anywhere were they just say "this is the function we use" - I can't see why not.

BEB

[David Ashby - Moderator]

197 votes and I have to say I'm surprised that close to half always add expo.

[Martin Harris - Moderator]

Why - did you think the readership were too thick to use it?

Edited By Martin Harris on 28/12/2012 20:26:36

[David Ashby - Moderator]

Well yes......no

It's just that not all models need it.

[Steve Hargreaves - Moderator]

I don't think its a model thing David...I think its more a pilot thing.....depends on how you like your models set up.....I tend to view Expo as being like dual rates without flicking switches.....nice & easy around the middle of the stick but full wang when you need it.....

[Shaunie]

I don't think so much it's whether all models need it. I think it's more of a user comfort/preference thing.

I didn't like expo when I first used it, but I stuck with it and now hate flying without it (the only thing I've got on linear at the mo is a Thunder Tiger OBL 40 trainer that I use as a camera plane, and I nearly stacked that in the summer when I suddenly ran out of elevator turning onto finals! Expo and more travel would have been a boon then.).

I typically use about 30% on low rates and 50% on high. I like the fact that it "de-twitches" the aircraft about neutral but gives enough travel to be fun, or get you out of trouble, with the sticks in the corners. Having got used to the aircraft on low rates switching to high then gives a similar feel about the stick centres but permits more lunacy.

S.

[Biggles' Elder Brother - Moderator]

Posted by Martin Harris on 27/12/2012 19:08:30:

A wise man (can't remember who) once wrote that the point of adding negative expo (in Futaba parlance) was to nullify the positive expo introduced by the rotary output from the servos. I believe he made a good point.

This point has been rattling around in head since Martin posted it. Then Erf's post asking about the nature of the exponential function used got me really thinking about it all - I'm decorating so I have a lot of time to think at the moment!

At first glance Martin's comment makes perfect sense. Imagine the servo's neutral postion being with the arm at 90 degrees to the body, then initial rotation will indeed cause much more straight line movement than the last few degrees would as the arm comes in-line with the servo body. So the translation, or straight line motion, of the control rod or snake is indeed affected by a non-linearity (a sine function actually) that is in the opposite sense to the non-linearity we use when we add expo.

BUT,....the control rod is of course connected to the control horn at the other end and this converts the straight line motion back into rotary motion as it deflects the control surface by rotating it about its hinge point. This second conversion perfectly restores the linearity. If the servo arm and the control arm are the same length then the angle the control surface rotates will be exactly the same as the angle of rotation of the servo - a perfect 45 degree linear relationship. If they are different lengths its still linear - but there will be a gain or attentuation in the amount of rotation.

So, that appears to debunk the idea of an intrinsic non-linearity that needs to be "corrected for" by expo.

BUT,... is the aerodynamic effect linear? That is, does aileron or elevator effect vary linearly with servo rotation? Afterall that's of primary interest to us. Well the force generated by a deflected control surface is proportional to the out-of-plane projected area of the surface. That in turn is related to the sine of the control surface's angle of rotation. This "vertical projection" is shown in the figure below.

That vertical projection will be equal to the width of the aileron multiplied by the sine of the angle of rotation. And sine is a non-linear function the non-linearity of which is indeed in the opposite direction to the "correction" we insert when adding expo.

BUT,... assuming the rotation of the control surface is relatively small - say less than 30 degrees - then the non-lineraity of the sine function is very slight indeed - far less than the effect of the amount of expo we typically put in.

So, to conclude all this rambling.

1. I don't think that actual control movement is rendered non-linear by the rotation to translation effect of the servo and its control rod - because the effect is reversed at the other end.
2. The aerodynamic effect - what we are really interested in - is indeed non linear with respect to control surface deflection - but not, I think, by enough to justify expo as a means of correction at the levels we typically use it.

I've always said that having too much time to think is a dangerous thing - amazing what you can come up with when painting a window frame!

BEB

[Doug Ireland]

In my opinion it's probably the best development in proportional control.