Latest Posts

When you set up a new model on your radio using default settings, it is likely that the maximum movement of the servo is very close to 45 degrees in each direction.  This is so for several reasons but the key facts are as follows:

• Most servos “out of the box” are made to  rotate to a maximum of around 60 degrees in each direction or 120 degrees overall.
• The radio defaults the Travel or End Point Adjustment to a value of 100 (brands vary as to the maximum allowed, typically either 125 or 150.
• The radio also defaults the Dual or Triple rate settings to 100 with the maximum typically being 125.

For purposes of this example I’ll use 125 as the maximum for the “Rates” and 150 as the maximum for the travel as that is what my Spektrum radio uses and that is what I have tested with.  Though the 100 value is not really 100 “percent” of anything (other than 100% of the default) most folks refer to it that way.  I feel like this creates an issue as many have trouble understanding how these functions work because they are thinking in terms of percentages and they really are not so I will just specify the value from this point on and avoid the confusing terminology.

Since the radio is set to a Travel maximum value of 100 out of a possible 150 (in each direction) the maximum rotation of the servo output shaft is 100/150 or 2/3rds of the 60 degrees max in each direction.  I.E. the servo rotates 45 degrees in each direction in this default configuration.  If we were to up the travel value to 150, we would then get the full 60 degree designed maximum rotation of the servo.  Simple enough.  This Travel setting is not commonly something assigned to a switch with the intention of changing it to allow for certain maneuvers or flying in certain conditions.  It is simply set and forgotten or never even looked at.

On the other hand a Rate switch is a commonly used setup.  Most folks will use the Rate switch to allow for lesser or greater travel as needed.  To understand how this works let’s return to the default setting for Travel (100) and adjust the rate settings only.  Let’s assume we have a dual rate switch assigned.  In position 1 it is set for a rate of 100 and at setting 2 it is set for our maximum setting of 125.  In position 1 we are at our standard default setting and the servo arm moves 45 degrees in each direction as we move the appropriate stick through the complete available motion.  When we switch to setting 2, we get a bit more travel.  As you might guess we get about 7.5 degrees more travel in each direction.  Not quite to our maximum, but half way between our default 45 and our maximum 60 degrees.

So now it gets interesting because there is some interaction between the two.  After doing some testing, here is what I found.  If you leave the Rate setting at 125 and set the Travel to 125, we can now reach our maximum of 60 degrees of motion.  Further increasing the Travel to 150 will not change (apparently) anything.  Maximum throw is still at the same 60 degrees.  But beware!  There is something a bit less obvious going on.  The motion is smooth and continuous with both set to 125 or with the Travel set to 150 and the Rate set to 100, but when you set both to maximum a “dead spot” is created at the high end of stick motion!!  The last 25% or so of stick motion results in no movement at the servo.  What is apparently happening is that the radio has interpreted the combination of settings such that the maximum output is reached before we run out of stick motion.  I can’t imagine why this would ever be a desirable outcome.  I would suggest that you either choose to limit your maximum Travel to 125 and use the Rate setting to reach the maximum travel, or limit your Rate setting to 100 and use the full Travel setting range as desired.

During my recent posting about making servo linkage adjustments on my Slick, I found I had set the standard rate on the elevator to 33.  This accomplished what I needed to do at the time, limiting my throw so that the Slick wouldn’t snap without full or nearly full application of elevator stick input but I wanted to get more out of my servo so I shortened the arm and managed to get the setting up to 66.

(you can read about that here: WH Slick Linkage Changes)

That was all to the good, but should I try to get more?  How much of my precision am I giving up?  If I did want to get more throw in the future, how much more could I get without reversing these changes?

First of all I’ll look at precision.  Here’s how the math works out.  If my servo is capable of 2048 steps and I only get all of those steps when I have maximum throw (60 degrees in each direction) then my setting of 33 in my Rates combined with my Travel setting defaulted to 100 was really limiting the precision that both the servo and radio are capable of!  I was limiting my commands to a maximum of 2/3rds (45 degrees versus 60) of the original steps because of my Travel setting and then limiting it to use only 1/3rd of that possible throw.  If my math is any good, I was using maybe 22% or 450 of the available 2048 steps.  With my new configuration I still have the 100 Travel setting but I’m now using 2/3rds of those available steps which doubles the available steps to about 900.  Hopefully this allows for more precision and less “slop” in the system.  I am covering the same distance with twice the precision and that should result in more precise control and more exact centering.  Even more of these changes (shorter servo arms and/or longer control horns) may be in the future but I’d like to do a bit of test flying before making more changes.  For now I think this will be more than adequate.  I hadn’t really noticed any elevator slop or lack of precision during past flights, but with many of these adjustments it can often be a case of not realizing what you were missing!

Finally, let’s look at travel.  I know that my new setup gets me about 27 degrees of rotation at the servo and a little over 10 degrees or 3/4″ of motion at the elevator itself.  This is slightly less than half of the available 60 degrees of rotation so I should be able to slightly more than double the existing throw if I should ever decide to do so.  While 20 degrees or 1.5″ of travel isn’t what most 3D guys would consider huge, it’s far more than I will likely need for flying IMAC style precision aerobatics.

Based on these observations I certainly can continue down this path a bit farther but that decision will be based largely on actual flight testing.  At this point that means waiting on some favorable weather.