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Old 12-03-2012, 08:31 PM   #11
AEAJR
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DUAL RATES AND EXPONENTIAL EXPLAINED AND COMPARED
by Ed Anderson
aeajr on the forums

Perhaps you are buying your first plane. Or perhaps you are thinking about
upgrading your radio. You read all those feature lists and don't understand
what they do for you. This may help you understand two of the features that
are listed so you can get the radio that will really help you enjoy this new
plane and future planes, whether electric, glow, gas or glider.

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.

Changing how the plane responds might be useful if we move from normal
flight to highly aerobatic flight. The large throws for aerobatics might
make the plane "twitchy" or hard to control during normal cruising around.
Dual rates
and exponential, when tied to a switch, or some other trigger can be changed
while the plane is flying. They are used for similar reasons but accomplish
the task in different ways.


DUAL RATES

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
control.

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
switch. However having it set to high might make the plane uncomfortable for
"normal" flying so we switch to low.

OK? You with me so far? If not, go back and read through it again as the
next section is based on your understanding of dual rates. Imagine how your
plane will behave on high and low rates. When you are comfortable then you
can go on to the next section.


EXPONENTIAL

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,
we
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
"flyability,
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.

Computer Radios

While I have seen dual rates on a some "standard" radios I have never seen
exponential. So for this discussion, we are going to assume that exponential
is a feature of computer radios. If you don't have a computer radio, this
might be a reason to move up to one.

Whether you ever use dual rates or exponential is, of course, is up to you.
However I would encourage you to give them a try if you have them. They are
just tools and like all tools, it takes a little while to get the feel of
how to use them. So, if the first time you try dual rates you don't see an
overwhelming benefit, don't walk away. Try different settings.

Most radios will allow you to set different rates to each surface. So, for
example, my ZAGI flying wing slope glider has dual rates set up on the
elevator. Tailless planes like this tend to be very sensitive to pitch, so
under normal circumstances I find I like to have the elevator controls set
on a low rate. However when I want to "crank it up" and get aerobatic that
low rate does not give me the action I want, so I flip the switch and get
the kind of pitch control I want for stunts.

On my 3M sailplane I find I like to have high rates set up for launch where
the plane can get in trouble very quickly and I might need a fast response,
but then switch to lower rates for normal flying. I even have a third rate
set for working thermals which is lower so I can make very fine adjustments
to get the most lift I can out of each thermal.

On my electric planes, I tend to have a mild and wild set-up for cruising
and for stunting.

I constantly experiment. Overall I find I like exponential better
than dual rates. However as I have become more experienced I find, for my
gliders and parkflyers, that I am using them less and less. I now fly with
no expo or DR most of the time. But I still have an Expo setting on a
switch so I can go to it if I wish. When I am teaching someone I find that
they often do better if I use some Expo to "soften the center".

Have fun trying out different settings to see what works for you.

Clear skies and safe flying!

How to select your first radio
http://www.wattflyer.com/forums/showthread.php?t=68741

Long Island Silent Flyers
www.lisf.org
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