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Jan 21, 2009, 05:39 PM
Flies Like Rock
Thread OP

To expo or not to expo....that is the question


I would be curious about who thinks some negative expo on these sensitive slope planks (weasel, bee,moth, etc) is a good idea, particularly for those who are "skill challenged"? (that would be me )

the guys that I fly thermal with have encouraged me to get rid of any expo on my thermal ships and just learn to fly them.

Tony W
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Jan 21, 2009, 06:04 PM
Walk like a camel
gizzo's Avatar
I think maybe you mean reduced rates? I don't use expo at all, but I imagine negative expo would mean lots of throw initially, trailing off to much less the further you push the stick? That would just be weird. Rates are great and I use reduced rates on wings and planks, normal (100%) on R/E gliders, dual rates on A/E slopers.
Jan 21, 2009, 06:34 PM
Crikey never leave beer behind
steve wenban's Avatar
To me the use of expo on any slopie is about as useful as an elevator in a single story
building. Rates on the other hand are the perfect answer to the planks meger suface control throw requirements . Seem to me that Expo is a sort of Power flyers thing .
On the other hand if you mean negative differencial well yes that works on some planks I know the Moth works well with it.
Jan 21, 2009, 06:42 PM
Phil.Taylor's Avatar
negative expo?
on a sweet flying Weasel?
(someone tell me what I'm missing out on here - got the Weasel, aint got no expo?)
with 80% expo (+ve) on a Kulbutin All-Moving-Tail on high rates = 80 degrees +/- movement
Jan 21, 2009, 06:43 PM
DS Junkie
Screamin' Eagle's Avatar
I think he means softening the throws around center. Negative expo does that on Futaba and HiTec radios, may be different on other systems.

I use expo on some plane and not others. For frontside, I'd rather have expo than dual rates (if either is necessary) so I don't have to be flipping switches. For DS I use reduced rates for the backside.
Jan 21, 2009, 06:49 PM
Phil.Taylor's Avatar
Geesh - so not only do Futaba servos go the wrong way, but Futaba expo is opposite ways around too?
Explanation time - expo (+or- whatever Tx) can actually be useful, to tame wild throws & make a plane flyable with small stick movement, whilst still leaving the wild throws for mad aerobatics with full stick movements. Even my Voltij serious aerobat only uses 30% expo on ailerons & AMT - it would be fine without, you just have to be very gentle on the sticks...
Your thermal guys may be right - no expo, just learn to fly the plane. If its too twitchy, turn down the rates (especially on a plank) - my Weasel uses 100% rates for aileron, 25% for elevator - and no expo

who learnt to fly RC long ago in the dark ages before expo was invented
Last edited by Phil.Taylor; Jan 21, 2009 at 06:58 PM.
Jan 21, 2009, 06:55 PM
Registered User
SchiessCo's Avatar
On flying wings and F3F style planes I use a little expo on the elevator only. On PSS stuff it's just straight control input. I do have a couple 3D planes that need 40-50% just to be flyable - big control surfaces with lots of throw.

That said, I've gone through periods where I used lots of expo, and others were I've preferred none at all. Give it a try and see if it works for you.

Jan 21, 2009, 07:03 PM
find me @samotage
samotage's Avatar
I generally run some center softening on my high rate settings, whuch on pointy planes has some pretty high rates. This makes for controllable flight in this mode.

Low rates typically I run linear.

Jan 21, 2009, 07:14 PM
Flies Like Rock
Thread OP

Great feedback.

Yes..I was referring to softening the amount of control surface movement around the center of the stick. In other words, your throws around center do not be be so microscopic. On both my Optic-6 (hitec) and Royal Evo (Multiplex) transmitters, negative Expo does just that. Not sure if some transmitters use a different naming conventions for positive vs. negative expo.

I also have dual rates available and will certainly use that.

Steve- You mentioned aileron differential on the Moth. I have that capability. How and why do you use that? With my Mioth being a few hours from completion, I am interested in what you have to say.

Tony W
Jan 21, 2009, 07:37 PM
Phil.Taylor's Avatar
Sorry - yep, on a JR Tx its +ve, not negative expo - same softening-about-centre effect.

Re. negative differential on a plank (that's more down aileron than up) - yes, it can be helpful, cos planks use reflexed airfoils & less reflex (down aileron) can give less drag - the opposite of what happens to the wing on a tailed plane. I dont have a Moth, but have had other planks. For the Moth - get it trimmed out CofG & elevator throws first (use rates to get the "elevator" throw small - very small as you get the CofG back) Then you may want to try tweaking negative diff - but start off without.

Jan 21, 2009, 07:41 PM
Registered User
Daemon's Avatar
My preference is for no expo. I only use it when I've done
all the CG tuning, and quintuple checked for slop in my servos
and linkages and such, and still can't get it to not be pitchy
around center at high speed. Then I put a little bit of negative expo
on the elevator to calm it down around center. I had to do this
on my Halfpipes. On other wings and planks, never needed it.

I've also used expo on my Thorn because with its super heavy hardwood
wings, it takes more throw to get the wing rotating when
moving at low speeds, but then gets very twitchy at high
speeds as the aerodynamic forces take over on the full span control
surfaces. I tried rates, but was just constantly switching from
high to low and back again, so finally gave up and added some expo
on ailerons and it seems happier.

Some people use expo on the aileron channel of their PSS sleds
to give them smooth control around center, and crazy roll rates.
I cure that with my left stick mix (rudder -> aileron at 100%)
and then fly low rates on the right stick most of the time.
When I push both sticks over, the roll turns into a blur, but
my high speed runs are still super smooth and linear, without
the "slightly dead" feel of expo.

Given a choice between rates and expo, I'll choose rates.

Jan 21, 2009, 08:02 PM
If it flies - it's good
Fred_L's Avatar
aileron differential on the Moth
I have both the Moth and Weasel. In my experience all these 2ch plank style aircraft don't roll axially due the reflex built into the airfoil. This results in the up going elevon being much more effective than the down going elevon, and up going = pitch up, so the rolls are quite barrelly ie. non axial.
A little negative differential which causes the down going elevon to move more than the up going one can make the rolls axial. Experimentation is required to find the right value. I have -15% on my Moth.

On the issue of exponential, I don't use it as I find I get caught out by the non linearity of it all. I can be soft on the sticks as required.
I find that dual rates are the way to tame sensitive elevators on planks. I use full rates on elevator only for spinning when I need that maximum elevator deflection.
Jan 21, 2009, 08:13 PM
Crikey never leave beer behind
steve wenban's Avatar
Steve- You mentioned aileron differential on the Moth. I have that capability. How and why do you use that? With my Mioth being a few hours from completion, I am interested in what you have to say.

Tony W[/QUOTE]
Tony My Mate Fred L flys the same slope an its was his use of negative differencial I was tlaking about I remember the day of expriment that lead him to the correct balance of movement and yep he get a pretty good axial roll out of the moth.
Jan 21, 2009, 08:28 PM
Flagstaff, AZ
dawsonh's Avatar
I use rates or flight phases quite a lot... for my acro gliders. I don't change my elevator or rudder throw that much, but like varying amounts of aileron... one set up for one revo per second... one faster... one a bit slower.

I don't use much expo... a little bit to soften the centers. The only exception is the "Mad" amounts of elevator on my Kulbutin... about 70 each way. It is un-flyable with out a lot of expo. Although I rarely fly it with that rate.

On my electric 3-D's I use a moderate amount of expo.

I guess it boils down to... if you have really big control surface throws (or may a very sensitive plank) you need expo to tone down the centers.

Using too much expo is usually the kiss of death for beginners.... since it tends to exaggerate their tendency to over-control.
Jan 21, 2009, 08:34 PM
Texas Ranger
Neil Walker's Avatar
My understanding of the use of expo is that it is for use in conjunction with dual rates. You have high rate, and low rate. You have expo on your high rates so that around center stick it has the same feel as low rates, so there isn't too big a change when you switch between the two. It follows from this that if you find your low rates need expo then it's probably better just to reduce them.

There's also a pretty good post about expo and throws by Tom Copp of from the F3X forum: - kind of specific to F3F slope racing but still pretty valid IMO.

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