Planning on buying a radio? Dual Rates and Exponential Explained
You are buying your first electric plane. Or perhaps you had a RTF and now plan to get an ARF and a new radio. You read all those feature lists and don't understand. 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 electric plane and the electrics, gliders and/or glow powered planes to follow.
DUAL RATES AND EXPONENTIAL EXPLAINED AND COMPARED
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.
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 a menu in the radio, you enter a percentage of the high rate to create a low rate, say 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 linier. 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 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 to but get big response when he needed it and there would be not 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. 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 my controls with about 35% exponential so that I have a softer response around the middle but 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.
While I have seen dual rates on a few "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.
Some 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 find adjustments and more easily 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 have been experimenting with exponential and find that I like it. I could see myself going totally to using exponential and doing away with dual rates all together, but that is not the case today. Right now I am having fun trying out different settings to see what works for me. I encourage you to do the same.
Clear skies and safe flying!
Last edited by aeajr; Feb 04, 2005 at 11:46 AM.
Based on your description, is it fair to say that exponential rates make it easier for a beginner? After reading through your description, it sounds like for most circumstances, either dual rates or exponential would help a beginner to lessen the tendancy to over control, but the exponential would allow one to get out of tight spaces quickly.
I ask because not only am I learing to fly, but I will be giving some young children ( 2 7 year olds ) some stick time. I wanted to make it easiest for them. Based on your article, dual rates would be the minimum thing that would make it easier for them, but a computer radio would provide that and the exponential response.
Yes, you have interperted the article properly.
Some computer radios will let you combine dual rates and expo as well.
On my Futaba 9C i have tripple rates where I can set rates or expo or both so I have three response levels set for each plane and they vary by the use of rates and expo.
For myself I usually set up top rates as 100% throw and 0 expo
Second level might be 100% throw and -35% expo - this is my normal flying position
Third level might be 70% rates and -30 to -50 expo - this is when I want very soft response. I use this when I am working a thermal and want very small adjustments around the middle of the stick, almost non-responseive till I get about 25 % out on the stick.
Just some thoughts. I am still experimenting with it, but I like Expo a lot!
Ed, I would like to hear your thoughts on high/low rates and servo travel-compared to expo. and servo travel....The more I have learned of computer radios (I am an old school Kraft and PCS series radios) I am starting to believe that using the expo. feature after learning the ins and outs is better for the servos...full travel so to speak...You have written an excellent article for those that are just learning and those that need to know and should start learning about the wonderful , if not amazing features , that are so useful ...I agree that is should be posted as a sticky, hear and in Radios....Thanks ......Tom TT
I find that the expo is all I need, except for a new plane which is completely unknown. After the airplane gets sorted out, I almost never use the dual rate switch. But having the expo set up makes for much more enjoyment of flying, and a plane can go from mild to wild in a moment. It makes one look like a better pilot, while sharpening skills at the same time.
SETTING SURFACE THROWS - EPA AND ATV
by Ed Anderson
aeajr on the forums
After you have built your model and installed your electronic
components, you will want to set your surface throws. If they are too
large, the model will be overly responsive and hard to handle. If they
are too small, you may have not adequate control of your model.
Regardless of what kind of radio you have, I feel it is always best to
do your first adjustment mechanically, at the servo and control horns.
The simple rule is that the further the control rod is from the hub of
the servo, the more movement you will get on the control surface. The
opposite applies to the control horn at the surface. The closer you
move the control rod to the control surface the more movement you will
get from the surface. By making a combination of changes at the servo
and the control horn, you can maximize or minimize your control throws.
Let's assume you have your control rod at the outermost hole on the
control horn of the servo, and the control horn hole closest to the surface.
will be your max throw position. If we stay with the rudder as the
example, measure the throw of the rudder when you move the control stick
all the way to one side. How does this compare to the recommended
throws in the instructions. If they recommend 1 inch left and right and
you have 1.5 inches, you want to reduce the throw or your model may be
In this case you can move the control rod at the servo and/or the
control horn to minimize the movement of the surface. Let's say you
move 2 holes up on the control horn and one hole in on the servo and you
hit the 1" mark, you are all set. However I have found that sometimes I
can not hit the desired throws. Either the movement is too large or too
small. Now what?
Staying with the mechanical approach you will either have to lengthen
the arm of the servo if you are trying to get more throw, or lengthen
the control horn if you are trying get less throw. While there may be
some formula for doing this, for the most part this is a trial and error
process and sometimes it doesn't really yield the desired results.
ATV and EPA
Today, all computer radios and many standard radios have End Point
Adjustment, EPA, or Adjustable Travel Volume, ATV features. Essentially
these are the same feature by different a name. They allow you to
control how far the servo arm moves when you give a full stick command.
This allows you to adjust how much surface movement you get by using a
dial, or by entering numbers into a menu. For convenience I am going to
call this feature EPA from here on, but you will understand that EPA and
ATV are essentially the same.
Mechanical First Please
First, I encourage you to make mechanical adjustments first, within the
limits of your standard servo and control horn. Use ATV and EPA after
you have done this. You will get the best service out of your servo if
you do the mechanical adjustments first.
Staying with the rudder as our example, you have gone to the innermost
hole on the servo arm, and the outer most hole of the control horn but
you still have to much throw. Using EPA, you go into the menu, or turn
a dial that controls how far the servo moves in response to a full throw
command. In other words, when I move the rudder stick all the way to
the left, how far do I want the servo to rotate in order to give me the
right amount of surface throw. On many radios this is expressed as a
percentage with 100 being full movement by the servo and 0 being no movement
of the servo.
When I was setting up my ZAGI slope wing, I was planning to use standard
servos and a three channel standard radio that did not have EPA as a
feature. No matter how I moved the control rods, I had way too much
movement on the elevons. This was going to make my plane very very
responsive; too responsive. I had to reduce the throws. I could have
moved to larger control horns on the elevons, but I felt this would create
opportunity for damage due to large horns that stuck out far from the
Fortunately I own a computer radio, so I changed the receiver to one
that was compatible with my ZAGI wing and my computer radio. I had
moved the control rods to minimize the control throws already. All I
had to do now was make some adjustments from the radio to get the
control throws I wanted. The operation took only a few minutes.
I have found that EPA/ATV is a very very valuable and useful feature.
Without it my models would be hard to adjust and I would have to go
through some difficult or inconvenient manipulations of the servo horns
or control horns. It is so much easier now with EPA/ATV on the radio.
When you go looking at radios, I encourage you to make sure it has this
What you need to know about receivers:
Clear skies and safe flying.
understanding dual and single conversion
here is a question i need an answer. i understand that i cannot fly a dual conversion rx with a single conversion tx right? well can i fly a single conversion rx with a dual conversion tx?
i have a bunch of single conversion rx's with a gws 4 channel tx and i would like to purchase a new computer radio. i am looking at a jr 6102 and need to know if it will fly my single conversion rx's
thanks in advance
As far as I am aware, it is only the receiver that is dual conversion or single conversion, the transmitter is just a transmitter. I think that the Tx crystal is a Tx crystal on the appropriate frequency, and dependant on whether the receiver is single or dual conversion, then the appropriate receiver crystal is installed.
That's my understanding of the situation, if I'm wrong would you please let me know.
so will any tx fly any rx considering it is the correct shift
i know my gws 4 channel will not communicate with a dual conversion rx. so will a radio such as a jr6102 communicate with a single conversion xtal. i don't think i can put a dual conversion xtal in a gws reciever.
anyway my bottom line on this is if i buy a jr 6102 fm radio will it work with my gws single conversion rx's??????????
question from a newbie
Trasmitters DO NOT come in single/dual conversion -- only receivers.
Dual conversion receivers need dual conversion crystals. Single conversion receivers need single conversion crystals.
Matching a transmitter to a reciever is simply a matter of shift (postitive or negative) and channel number.
Your GWS transmitter will communicate with a dual conversion receiver as long as they are of the same shift AND channel. Same applies to the JR radio you asked about.
You cannot put a dual conversion xtal in a GWS receiver unless it is a dual conversion receiver.
If you buy the JR radio it will work with your GWS single conversion receivers (again... assuming the shift and channel match).
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