Servos (7/13)

We’ve covered a lot of information that comes together here in the Servos screen.  This would be a great time to review the ‘Important Info on Servo Travel’ that we developed early in the class, especially the concepts of weights, default and extended limits, effect of trim and subtrim on endpoints, and the equation for calculation of servo travel.  Here’s a short cut if you want to revisit that information, and you can use your browser Back button to return here. 

Let’s do a safety check before proceeding.  If your Rx is installed in a model that uses electric flight power, please remove the prop before we go any further.  I know you’ve probably done this many times without getting hurt;  I just don’t want this class to be an injury for anyone.  Please remove the prop for the rest of our Basic Airplane set up class.

Here’s what the top of our Servo screen looks like before we configure the servo behaviors:

Servos screen rev 3

OpenTX provides a line for each of the possible 32 radio channels, and the inputs we configured in Inputs are shown on the channels we assigned in Mixer.  Each line/channel controls at least one servo or other device such as an ESC from that position on the receiver.  Each line contains 7 fields that determine how that servo/device will respond to the channel.

The vexing part of the Servos screen is that almost everything affects almost everything else.  Typically you desire one outcome (“I want to stop my plane from rolling right”).  You take an appropriate action (add left aileron trim).  You get the desired result (airplane stops rolling), but you also get an undesired not-so-obvious effect (“I can’t roll left as well but the right roll is still good.”)

OpenTX gives you more flexibility to shape behavior than most any closed-source radio that you could buy.  The result is that you are given choices and shown interactions that most closed source radios decide for you and then obscure the interactions.  In this Servos section of the class, we help you “do the math” if you want to fine tune your airplane to your preferences, and then we suggest simplifying set-up approaches if you want to say, “Heck with the math, I just want to get my plane flying”.  The bottom line is that the more your airplane is built straight and square, is properly balanced, and the controls are installed well, the less you will need trim and sub-trim to get it to fly well.  Many of the complexities of the Servos screen are to help correct for deficiencies in the airplane.

Let’s begin with the small stuff —  trim and subtrim.  Trim and sub-trim have identical outcomes to the servo, but subtrim is set on the Servos screen with the plane on the bench, while trim is adjusted from the Tx trim buttons with the airplane in flight.  Sub-Trim is used to achieve the best mechanical geometry of the servo/arm/linkage/horn physical set up to get control surfaces at their balanced neutral condition when the Tx sticks and Trims are centered.  Trims are in-flight adjustments to achieve “hands-off” straight/level flight of the aircraft.

Please check to insure that your Tx trimmers are all centered before proceeding, and navigate to the first field to the right of the line name on the Servos screen.  The label at the top of the screen changes to inform you that you’re in the sub-trim field.  Increasing the sub-trim value moves that control surface toward the “increasing signal” side of the servo center while decreasing the value moves it to the other side.  Note that an increasing signal may be a clockwise or counterclockwise rotation of the servo depending upon other decisions, including servo selection.  Adjust the sub-trim value to center your control surfaces.  If your sub-trim is more than about +/- 5%, take a look at your mechanical set up to see if you can improve the control surface centering without as much sub-trim.

If the subtrim behavior is left in a linear mode (more about this below), then the subtrim acts just like holding the stick that amount to one side of center.  But since the stick still begins from its physical center, if you continue to move the stick to its limit on that side, then the signal to the servo max’s out before the stick does.  If you move the stick to the opposite side, the stick stops before the servo signal reaches its maximum.  To the trimmed side, you still have the full 100% (512 uS) of signal available but it arrives earlier.  To the side away from the trim, you have less than 100% signal available to the servo by the amount of the trim given to the other side.  So addition of subtrim in a linear manner causes you to lose a little something on both sides of the stick.

The final field to the far right of each line is called sub-trim mode and shows a default ‘∆’ symbol.  This mode selection “ramps down” the stick-to-servo sensitivity and extends servo travel as necessary to cause both the stick and servo to max out together at the limiting values on both sides of the adjusted starting point.  This default works well for simple set ups, i.e., multiple servos driven by a Y-cable or no mixes of other channels with this control channel.  The variable stick-to-servo sensitivity can cause unexpected effects where mixes and curves are employed in the channel, so the ‘=’ alternative choice is available to retain the linear relationship at the expense of the tradeoffs described above for trim and sub-trim.

You may think “Gee that’s dumb, just make the new stick-center point 5% higher”.  That is actually available to you at the “PPM center” field further to the right in the channel line that shows a default value of “100%” or “1500us”, i.e., the servo center point ready to be edited.  That choice is sometimes used in lieu of sub-trim to achieve good servo geometry.  More on this below.

The relationships are summarized in the following graphic prepared by the OpenTX developers:

Types of Servo Sub-Trim
Types of Servo Sub-Trim

 Each option has it’s advantages and disadvantages, so it becomes a user preference rather than a right/wrong decision.  Notice that the two sub-trim curves (blue & red) show adjustment on one side of center while PPM Center Shift (green) shows adjustment to the other side.  If these choices aren’t enough, custom curves are available from the servos screen to tailor servo response to your individual needs — an advanced feature available in the field on the screen to the left of the PPM Center Shift.  

My own personal preference is to use the sub-trim mode with the default ‘∆’ relationship because if works nicely and feels OK.  If you prefer the ‘=’ linear sub-trim response, then adjusting the servo geometry with PPM center shift may also be worth considering as an option.  Just be certain you limit servo travel to non-damaging values with any of these choices.  Play with the available combinations and decide for yourself what you prefer.

Worth repeating:  If you have a good build and hardware installation, the complexities of trim and sub-trim fade away.  If your set up needs a lot of sub-trim and/or trim for whatever reason, OpenTX provides unlimited flexibility to customize the servo response.  

A final feature related to trim and sub-trim is located at the very bottom of the Servos screen and is pretty cool.  A LP ENT on the “Trims to Subtrims” line will add all flight Trims to their sub-trim values and reset Trims to zero. This can be handy at the field following a trim-and-balance flight, but presents the same risks described above from adjusting sub-trims, especially if the Trims were large.  This feature fills the need for those who prefer to fly with the Trims zero’d out.  

Basic Airplane pg 7 Servos Screen 2

Basic Airplane pg 7 Servos Screen 2

That innocent looking arrow pointing to the right is the field that reverses normal servo travel direction.  Navigate to this field and the ENT button toggles the arrow from normal (arrow right) to reverse (arrow left).  One source of possible confusion is that servo travel also can be reversed in the Inputs and Mixer screens by entering negative weights.  We strongly recommend reversing servo direction in the Servos field because doing so with weights in Inputs or Mixer can confuse the relationships that are defined in those screens as well as cause trims to act opposite to its related control stick.

It’s time to wrap up the Servos screen by setting our hi-rate control deflections.  This is done according to the equation:

net wt % in Inputs
(up to 100%)
net wt % in Mixer
(net wts greater than 100% scaled back to 100%)
% of Servo Travel Limit (up to 100%, or up to 150% if extended limits)
= Percent of +/- 100% (512uS) travel to Servo


Recall that the default servo travel limits stop at 100% unless the Extended Limits box was checked in the Model Set Up screen (2/13) which will allow up to 150% .  Otherwise any number less than the default 100% (or 512uS) may be entered in the “% of Servo Travel Limit” field.  There are two fields in the Servo screen, one for the positive-going side of center and one for the negative-going side.  If you move the stick for the control that you’re editing, the dashed line between these two fields becomes an arrow pointing to the field for that side of the stick.  Adjust that servo limit field to get the max control throw you need in Hi Rates for a given control surface.  Don’t over-travel the servo or the control surface to avoid damaging your equipment.  Having adjusted the Hi Rate deflections, the reduced rate servo deflections are already determined by the settings you entered in Inputs.

Tip:  Remember that one-sided stick option in Inputs?  That would let you treat each side of a stick as a different input with different rates, expo, and servo trim, sub-trim, and travel limits if you really need it.  Otherwise, reduced rates will have the same effect on both sides of a given input stick.  Yep, different rates available on either side of a stick — another advanced feature if you’re really into fine tuning.

Recall that the Channel Monitor is always available to show the effects of servo travel limits, extended limits, trim, and sub-trims as you work through the lines in of the Servos screen.  LP MENU is a short cut and EXIT returns.

Let’s review what we’ve learned and tie these ideas together with the command chain:  What Goes Where.


Servos (7/13) — 2 Comments

  1. Could you please explain how the above (really useful graph) for subtrim/ppm relates to mixing. I have a wing (elevons only) where the maximum throw (using the mixing screen) is 65% on the ailerons and 20% on the elevators. it is imperative elevator throws are identical. I am assuming using ppm or subtrim = are equivalent while values are higher than the maximum weight set in the mixing screen? Thanks for your thoughts & the brilliant resource

Leave a Reply