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Bill Brown 302/03/2013 02:38:43
363 forum posts
234 photos

Hi guys & gals,

could any of you learned greater than i people please explain to me in leymans terms how exponential setting of your t.x. assis`ts with the control function?

I have read somewhere that within our field of use it "softens" the midrange controls, if this is the case i would assume that it would not be of advantage to the likes of the aerobatic or similar fliers who would demand an instant response on the controls, (possibly i could be wrong on this) researching the word gives an awful lot of mathematical jargon which does not enlighten me but makes me think it is another way of describing the theory of relativity.

Me , being of simple demands, would just like to know if it would enhance my flying experience and if so, in what way?.

Please don`t be shy, feel free to give me your angle on this.

I thank you in anticipation of your response.

Confused dummyindecision

Former Member02/03/2013 04:41:45

[This posting has been removed]

Dave 802/03/2013 05:42:46
31 forum posts

Hello Bill,

Your own description is accurate enough. Some people like instant response regardless of what model they fly be it something tame or wild. Some flyer's use it, some don't. It is a matter of personal preference. I personally like a bit of expo to tame my twitchy fingers although I did learn without expo. As Percy suggests, try it yourself. No harm will come to your plane and it will not do anything weird. Just gain a bit of height and play around with the values until you come up with something you like.

David Ashby - Moderator02/03/2013 07:02:25
10987 forum posts
1706 photos
614 articles

Bill, pop the word in the search box and you'll find some threads that should help

cymaz02/03/2013 07:26:09
9195 forum posts
1186 photos

This is quite good

GrahamC02/03/2013 07:46:32
1240 forum posts
196 photos

If you do try it on your favourite model remember that the softening around the centre effect is achieved with minus expo on Futaba and (I think Hitec) positive expo on Spekrum.

One way to observe the effect clearly is to set one of your flying surfaces to 100% and -100% expo and you will be able to see what it is doing as you move the control. Just don't forget to resort it to a sensible setting before you go flying!

Bob Bertram02/03/2013 09:19:16
295 forum posts
27 photos


As a newbie to all this and having had a Spektrum DX6i delivered yesterday, I found that article really good. Thanks for posting the link.


cymaz02/03/2013 09:42:53
9195 forum posts
1186 photos halo is slippingangel 2

Andy4802/03/2013 11:22:42
1495 forum posts
8 photos

If you've got a flight sim package which uses your own transmitter, that is the place to experiment with expo.

Bill Brown 302/03/2013 18:05:36
363 forum posts
234 photos

Many thanks all, especialy cymaz for the link, dead interesting and have saved it in my docs. Will definitely have to have a play.

Happy flying all! (may all your prangs be from low altitude) laugh


Frank Skilbeck02/03/2013 18:54:27
4681 forum posts
101 photos

Bill I think for conventional aeroibatic planes you are probably correct, but for the 3D type models which have very large (i.e. >45 Deg) movements for the prop handing manouvers the controls then become hypersensitive when flying normally, so expo helps here.

I have a couple of warbirds as well that like a lot of elevator after touch down to keep the tail down, but without expo they would be very sensitive in fast forard flight.

I usually start these days with 30% expo and then reduce it if I think the model is not responsive enough.

I must admit having learnt in the days of basic radios (with left and right servos) I'm all for all the electronic help I can get

Tom Sharp02/03/2013 22:48:01
387 forum posts

Definately a help for us shakey elderly pilots.

Biggles' Elder Brother - Moderator02/03/2013 23:51:40
15748 forum posts
1460 photos

I think the main benefit of expo lies in almost giving you the best of both worlds. At lot of the time you want very small control movements, if the response was linear (meaning equal stick movement gave you equal amounts of control movement throughout the range) then those small movements you need in most situations would be very tiny indeed at the stick level. For example, if you are flying a slow roll you want very small small corrections - and that's going to be difficult to do if we are talking about movements that amount to "breathing on the stick" almost! Ironically with a very aerobatic model this is often even more true as they can be very twitchy.

But sometimes you do want big control movements - a snap roll, a flick roll, a stall turn for example. They all call for big control movements to be put in very rapidly.

So we have a contradiction - sometimes we want small movements of the controls to match up to stick movements that are not so small, but at the same time we want "full scale deflection" to be quite large! Expo delivers exactly that. It "softens" the centre so that it is "desensitised" and allows managable degrees of stick movement for small, fine, control inputs. At the same time, as the stick movement ends the end of its travel, the "gearing goes up" and we get the big throws we want as well. One of the very few examples in life of having your cake and eating it!

Will it help you if you are learning? Probably yes. It will tend to make your flying smoother and help you "feel" the controls in "normal flight small movement conditions" rather better. As Dave says above - give it a go.


Bill Brown 303/03/2013 00:47:47
363 forum posts
234 photos

Thanks again all, the fog is clearing. yes

Ed Anderson05/03/2013 00:38:32
247 forum posts
10 photos


Dual rates and exponential allow you to change how responsive the plane is to your stick movements. If you have them set-up on a switch, you can make these changes while the plane is in flight. This might be useful as you move from take-off to normal flight. Perhaps an instructor has a trainer plane she would like share between new pilots and more experienced pilots. It would be convenient to be able to change the plane's behavior depending on the pilot without having to move the linkages.
Of the two, dual rates has been around longer and is simpler to understand.
Dual rates are based on changing how much a surface can move. Let's use
rudder set-up to illustrate this.
If your instructions say to set 1" of throw left and right, that would be
the recommended surface movement at full stick movement. When you move the
stick 1/4 of the way, you would get 1/4" of rudder movement. At 1/2 stick
you would get 1/2" of rudder movement. You get a direct, proportional and
linear relationship between stick movement and surface movement. At 100%
stick movement you get 100% of the maximum surface movement that you have
set. In this case 100% stick equals 1 inch.
With dual rates we can change to a second maximum at the flip of a switch.
Let's assume you have the standard throw set as the high rate. Then, using
the procedures outlined in your manual, you set low
rate at 50%. At this setting, when you move the stick all the way over you
will only get 1/2" of surface movement. However stick movement and surface
movement remain proportional. So at 1/2 stick movement we will get 1/2 of
the 1/2 inch maximum or 1/4 inch of surface movement. Your rudder movements
remain directly proportional but are now based on a smaller maximum.
We can say that control and response are both proportional and linear. That
is, all the way through the stick movement the rudder will move with us in a
linier fashion. If we move the stick 20% we gets 20% rudder. Move the stick
62% and the rudder will move 62% rudder movement. If we plotted a graph with
stick movement on one axis and rudder movement on the other, the graph would
have all points along a straight line at a 45 degree.
How does this effect the handling of the plane?
Continuing the example above, we have high rate, at full stick movement
equals 1" and low rate set at 1/2" maximum rudder movement.
On low rate, for each small movement of the stick, we get less movement of
the tail surface. So, on low rates the plane will be less responsive to the
same amount of stick movement. This may make it easier to fly as we can
make smaller adjustments when we move the stick. We have finer grain
On high, we get more movement of the rudder for each unit of movement of the
stick. We get a faster response from the plane for the same stick movement.
If you have ever worked with a precision tool or instrument, this is like
having course adjustment and fine adjustment.
As new flyers often have a tendency to over control the plane, it is not
uncommon to set-up the plane with smaller throws so that the pilot is less
likely to get in trouble by over controlling the plane. Later when she gains
confidence and the right feel for control, surface movements can be
increased to make the plane more responsive. Originally this had to be done
on the plane. Many RTF planes come set-up this way. They are set for mild
response for initial flights. Then the manual explains how to increase the
rates as the pilot gains experience. Some RTF planes now include a dual
rate style control on their radios.
With dual rates on the radio, this can be done at the radio rather than
working on the plane itself. This is much more convenient. Dual rates can
even allow the instructor to take control, flip to high rates and pull the
plane out of a tough situation that the student could not handle. Dual
rates can be very helpful during training.
Of course we can always have it the other way where the low setting is the
"standard" recommended by the instructions and a high setting might be our
aerobatic setting or our 3D setting where we want 1.5" of deflection at full
stick. This allows us to take the plane from mild to wild at the flip of a


Edited By Ed Anderson on 05/03/

Edited By Ed Anderson on 05/03/2013 00:43:54

Ed Anderson05/03/2013 00:40:47
247 forum posts
10 photos


Exponential changes the relationship between stick movement and surface
movement. When using exponential, stick movement and surface movement will
no longer be linear. What does that mean?
Exponential is going to allow us to shift some of the rudder response so
that we get a different amount in the early part of the stick movement as
compared to the later part. Let's stay with the rudder example above.
At 100% stick movement we would still get 100% surface movement, but at 50%
stick movement we might only get 25% rudder movement. This would be like
having low rates on the first half of the stick travel and high rates on the
second half of the stick travel. That would give us a "softer" response
around the center of the stick area, and a faster response toward the end.
How is this beneficial?
This gives us finer control when we are making those
typical small adjustments to the plane when we are cruising around, just
like low rates. However if we suddenly want a big surface movement to get
out of trouble, to respond to a gust of wind or to perform that big stunt,
still have the big surface movements we need without having to manually
switch to high rates. One of the criticisms of using a low rate for
is that it limits the pilot's ability to get out of trouble when you are on
low rate.
Let's look at that aerobatic or 3D pilot we mentioned above. He has BIG
surfaces and BIG throws set which makes the plane very responsive to small
inputs. If he were to set exponential rather than dual rates, then he could
have a very soft center to the stick. He could make fine adjustments when
needed, but get big response when he needed it and there would be no need
to flip a switch during the flight. Cool?
Let's try some examples that involve numbers. The numbers I am going to use
may not map directly to your transmitter as different manufacturers have
different interpretation of exponential and what the numbers mean, but the
overall impact on flying is the same. They just express it differently.
Let's say that under standard set-up conditions exponential will be
expressed as zero. This means we have the same linear response we have
always had. Now, if I put in -50% exponential, that might mean that for a
50% movement in the stick I only want to get 1/4 surface movement but
when I move the stick to 100% I want full 100% surface movement. An
input into the set-up menu of +50% might mean that for the first half of the
stick movement I want more of the total surface movement. This would
make the center area very responsive while leaving find grain control at
the ends of the stick movement. I am not sure where this would be used,
but that is how it would work.
It is important to note that exponential does not imply a sudden change in
rate. Rather it is a smooth change in rate. So the further we move the
stick, the faster we get more stick movement. If we were to plot the percent
stick movement to percent surface movement we would not get a straight
line as we normally get. We would get a curved line indicating that the
further we move the stick the less linear the relationship between the stick
and the surface.
This is one of those things you are just going to have to try to fully
understand. At first it seems it would make it difficult to predict how the
plane will behave depending on how much you move the stick. However in fact
most people tend to fly more by input/response rather than where the stick
is in its travel. You move the stick and watch the plane. After a while you
develop a good understanding of how the plane will respond to a given stick
movement, but you know that it will be influenced by wind, air speed, and
other factors.
I typically set up a switch with about 35% exponential so that I have a
softer response around the middle if I want it. That gives me gradually
faster response as I move toward the extremes of stick movement. On my
radio I have dual rates and exponential available and I can use them
together. I can also set them by surface.

Edited By Ed Anderson on 05/03/2013 00:41:32

Bill Brown 305/03/2013 21:58:24
363 forum posts
234 photos

Ed, Thanks very much for taking the time to explain in so much depth, your method of explanation is very easy to follow, now i think i could also explain exponential if i were asked.

Best regards


Steve Dorling01/06/2013 06:53:31
41 forum posts
1 articles

Small point - depending on how your mechanical links are set up - you're probably (almost undoubtedly...) flying with expo anyway (before you start with electronic tinkering)!

Ian Jones01/06/2013 10:30:17
3220 forum posts
1397 photos

Yes that's right Steve. Assuming the full range of the servo arm's movement is being used then the starting point is the equivalent of having a degree of positive (on Futaba) expo in. There is more movement around the neutral stick position. 1/2 the stick movement does not = 1/2 the surface movement. Half the stick movement does = 1/2 the servo arm movement. To get the stick movement and the control surface movement to proportionaly match each other some exponential adjustement is required.

I have explained this in detail in other threads but in short it is because the point at which the control rod is attached to the servo output travels in an arc. By the time the arc reaches the end of it's range it is moving the attachment point sideways as well as in the direction of the control rod so less of the servo movement is contributing to the control surface movement.

Adding or subtracting expononential changes the multiplier by which the position of the servo arm is calculated relative to the amount of stick input that is made.

So, a small amount of expo can be described as softening the stick movement around the nuetral position but this description is misleading if you want to understand what is actually happening which with a relatively small amount of expo adjustment is that the the control surface movement is being made to be more proportional to the amount of stick movement. Larger amounts of exponential adjustment take this effect to more extreme degrees eventually resulting in little movement around the neutral point and lots more at the end of the stick movement.

Probably the best way of getting to understand this is to use a spare memory slot on your Tx and dial in 100% expo on a control surface then slowly move the stick - you will soon see the effects of expo adjustment. On Spektrum TX's you can only do this on the memory the rx is bound to so if you do this test on a "live" memory make sure you have put it back where you started afterwards!

Edited By Ian Jones on 01/06/2013 10:33:26

Piper Cub30/06/2013 12:28:42
149 forum posts
Posted by Steve Dorling on 01/06/2013 06:53:31:

Small point - depending on how your mechanical links are set up - you're probably (almost undoubtedly...) flying with expo anyway (before you start with electronic tinkering)!

More than likely, only on aileron though.

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