Without perfect symmetry an airplane will snap out of high elevator maneuvers, roll in loops and generally not handle well during precision or pattern flight. For that reason it is imperative that split elevator surfaces move equally both in speed and travel.

You now have that fancy Hitec programmer and a bunch of Hitec Digital servos but you are stuck trying to figure out how to program them to get this perfect elevator symmetry?

Well here is what the manual won't tell you - perfect results every time!

First, a few notes:

Note #1

This setup procedure is geared  toward giant scale planes that have a symmetrical tail servo setup. What this means is that the servos are at the same position fore/aft, and up/down within the fuselage or set within the stabilizer  (i.e. the servo   positions mirror one another and the linkages are the same length and in the same basic angular position side to side).  

Note #2

This tutorial is geared exclusively toward the use of the Hitec Programmer with Hitec digital servos, however it would be easily adapted for JR/Matchbox users as well any others using the SmartFly equalizer, etc.  The same principles apply, but obviously instead of the programmer you use the procedure to set end points and center points using the appropriate device manual.

Note #3

It is my firm belief that one cannot achieve perfect symmetry by using the trim, sub trim or mixes on the transmitter.   It is for that reason I will only use a Y-Harness to connect my elevator servos on 2-servo (i.e. split elevator) setup and not use two separate receiver channels. This setup provides for simplicity and can even save weight on longer cable runs.

Note #4

This is not a beginners guide! It is assumed that you have an idea where the volume (throw) needs to be for your airframe and that you have selected the appropriate servo arms, linkage, and horns to allow for your target.  It also assumes you have some basic understanding of the programmer, end points, and center point.

Since I fly IMAC and 3D I set up all my planes to have a maximum of 45*  surface throws on all surfaces and reduce them based on the flight mode I am in.   The examples below are based on 45* volume target but the procedure is very easily adapted to lesser volumes if required by changing the Adjustable Travel Volume (ATV) numbers at the radio. .

• Print out several copies of the protractor and size them for use
• Connect each servo individually to an extension long enough for access
• Mount the servos in the airframe, exactly how they will be installed
• Have your radio, programmer, y-cable and choice of control linkages ready

Geometry 101

It is said that garbage in = garbage out.  This could not be more true in the setup of your linkage.   Without a properly set up linkage, no matter what you do on the radio end  you will not have matched travel and your aircraft is going to be more difficult to trim.   Getting equal throws from your elevator halves is 90% mechanical 10% electrical.  If this procedure is performed correctly you will not need to touch your radio trim, sub trim or any mixes.

Geometry Step One:

The elevator hardpoints need to be drilled identically – same location, same angle, same distance from the hinge line.   To do this  build a jig out of foam or clamp to support the elevator halves as shown so that the drill bit is absolutely perpendicular to the surface neutral center line. The neutral center line or zero deflection line as it also is called, is an imaginary reference that runs fore and aft of the surface chord (see figure 1, horizontal line). It is the point where on the aircraft, the surface is absolutely neutral.

Use a drill press or other means of drilling an accurate hole.   By drilling the surfaces together you will have the holes in the same spot in relation to the hinge line on both halves.  This will make things much easier later when fabricating the pushrods such that they will both be the same length (see figure 1).   Also note where the pivot arm centerline is when placing the holes in the hard points. Some horns like the DuBro, allow the pivot arm holes to be drilled so placement during this step is not critical.   On fixed pivot arm horns like the Nelson, it is imperative that one get the exact distance to the hinge line dead on.  (see figure 2). Once the holes are drilled, fit the servo horn to verify everything is correct and adjust as necessary.

Geometry Step Three:

In order to get the symmetry perfect, the linkages must be identical in length, alignment and travel.  This takes a few tricks but is easily accomplished with a paper protractor and a good eye. To correctly set up the linkage the servo must be installed in the airplane and a protractor mounted to the airframe in such a manner as to allow the servo arm center to be in line with the center of the protractor. Ideally, the center point of the protractor will be the center line of the servo arm pivot point (see figure 5).

 Simply cut out the protractor and align its center where the servo arm pivot point is and use tape to hold it in place.  Do this on both servos.   The orientation of the servo is unimportant, the only thing that needs to happen here is that the protractor 90* mark needs to be perpendicular to the control direction (nose-tail in this example)  Once the protractors are installed, the servo arm is moved manually to  the 90 degree mark  (see figure 5). 

Note : it is unimportant at this time whether the servo is centered so do not use the radio to perform this step - move the servo arm manually and don't worry about electrical center.

Figure 5 - installing protractor on servo

Geometry Step Four:

The example above demonstrates a fuselage mounted tail servo setup but it can easily be adapted for stabilizer mounted servos as well.  If your servo is in a bay or hatch (Comp-Arf, H9 Extra, Extreme Flight YAK 54, etc) you will need to get creative in how you mount the protractor, but it can be done.  Once the 90 degree position is achieved with all servos,  tape or clamp the elevators into dead center position (i.e. neutral).  On elevators with counterbalances this is easy.  On conventional elevators, you will need to use scrap balsa or some other means to sandwich the assembly and secure the surfaces to neutral or have an assistant hold them. It is best to use clamps however as any movement will create errors.

 Once the surfaces are in neutral,  mount  the control clevis arm.   It is imperative that the hole in the clevis hinge arm is perfectly centered in the hinge centerline (see figure 6). 

This example shows the ideal conditions for linkage setup – a perfect 90 degree geometry at neutral.  Most likely, this is not achievable due to the design of the airframe, servo placement, horn size, etc.    Don’t worry too much if your setup is not “square” as this one is.     As long as the centerlines are on and the linkages the same length, that is the important part.   It is important to get as close to square as your setup will allow. The DuBro horns are the best suited for this application as the pivot arms are much longer than needed. This allows fitting and trimming to get an exact alignment with the hinge center line.  Fit and trim one side, then make the other one identical.  The benefit of this type of horn and being able to drill your own pivot point is that it negates the need to be dead on in drilling the location of the horn hole in the surface. By drilling the hole by eyeball where you want it, then fine tuning the final setup on the plane by drilling the hole in the arm gets a perfect alignment every time.

The arm is mounted on the horn, and then the hole is drilled at center while looking at the hinge line from the end of the surface.  The excess material on the arm is cut off and discarded (see figure 7).   This method is very consistent plane-to plane. 

Geometry Step Five:

The final step in the mechanical setup is to install the control rods.  Install the ball link on the servo arm and the clevis/pin at the horn.   Measure the rod length so that the threaded portion of the rod goes into the ball link or clevis approximately 5/16” (It is easy to fabricate custom carbon fiber rods at this point, so that one can get the size exactly where needed). Hangar 9 Pro Links or R/C car turnbuckles also work well for this.

Thread the rod into each part so that the servo remains locked  at the 90 degree position on the protractor and the surfaces remain locked in neutral.  Ideally, the rod has penetrated each boss (on the clevis and on the ball link) equally.   Again, reference figure 6.   With both sides mechanically identical now, remove the tape as we are ready to align them electrically.   Move the surfaces to their limits and make sure they move the same side to side and up / down and that there is no binding.   If you have everything right (including the hinging) this should not be an issue. 

With everything mechanically perfect now,   move one elevator half 45 degrees up and 45 degrees down from neutral (or whatever your maximum end points are going to be, not all planes need 45 degrees)  Using a thin pencil – mark the point at which the exact center of the control arm aligns with the protractor degree marking (note - 45 degrees deflection will usually result in more than 45 degrees read at the protractor due to mechanical advantage).   These are the end points.    Transpose the marks you just made on the other protractor being very careful to get them exact.      You now have the end points and the center point for each servo. See figure 8.

Figure 8 - determining and marking end points with the protractor

Servo Programming

Now that the mechanical part is done, we can do the easy part –  programming the servos to match the mechanical alignment we just did.   This is written such that you have some basic knowledge of using the servo programmer and that one servo has already been reversed by the programmer for use with the y-harness. This is very easy to do if the geometry is correct, in fact, it will only take five minutes if you did everything right above.

Programming Step One:

Before we do any programming we will want to determine where our transmitter dead center is. We need to do this as it will effect the accuracy of the servo programming. JR transmitters center at 1500 microseconds. Futaba is 1560 microseconds. It is not uncommon for these values to be off slightly due to calibration or aging of the transmitter.

• Connect RX elevator channel to the programmer
• Power up RX, TX, and programmer
• Zero ALL trims on the transmitter (don't forget mixes and sub trim)
• Put programmer into TX signal pulse test (step 7 in programmer manual)
• Read pulse value and record it
• Disconnect programmer, power off TX, RX

Programming Step Two:

Now we will need to set each servo's center point and end points. Follow this work flow to the letter and you will be pleasantly surprised how quick and accurately you will get it done.

The programmer will only perform this operation on one servo at a time, so have both servo leads ready to be connected to the programmer individually. The procedure will be covered once, but you will need to perform it two times, once for each servo.

• Remove the servo arm from the servo, leaving all linkages attached
• Power up the servo using the programmer
• Perform a reset (do NOT have linkage connected.. very important)
• Cycle power and put the programmer in manual mode
• Rotate the programmer knob to the exact microseconds reading you took in step 1
• The servo now is at the TX center
• Install the servo arm, getting as close to 90 degrees as possible on the protractor
• Note that it is likely the arm will not align exactly with 90 degrees, this is OK
• Put the programmer into end point, neutral and fail safe mode (EPAneuFS)
• Select Center mode
• Using the knob, rotate until you see --><-- and servo acquires center
• Once center has been acquired, use knob to rotate servo to 90 degrees on protractor
• Push M to lock the new center point

You now will be setting the left hand and right hand end points:

• The programmer will now be in Left Hand end point set mode
• Use the knob to move the servo to its LH high mark on the protractor
• Make sure the center of the servo arm is perfectly aligned with the mark you made
• Press UP/L to store the LH end point
• The programmer will now be in Right Hand end point set mode
• Use the know to move the servo to is RH high mark on the protractor
• Make sure the center of the servo arm is perfectly aligned with the mark you made
• Press DN/L to store the RH end point
• VERY IMPORTANT - press EXIT to save your new settings

Connect the second servo to the programmer.   Repeat the procedure exactly for the other side noting the end points transcribed on the protractor in the mechanical setup section above.  

Programming Step Three:

Testing! If everything was set up correctly both sides will travel and end equally.  

Disconnect the programmer and install the y-harness and both servos to the elevator channel on the rx.    DO NOT TURN ON RX!     Turn on your radio and set the up and down elevator ATV points at 150% and ensure that your trim and sub trims are set to ZERO and that no mixes are engaged.    

Now turn on your radio.     Move the elevator up and down and note from behind if the surface movement is symmetrical –  if it is you hit a home run – your linkages are perfect and in alignment and they will travel to the limits you have established.   

If there is some variation in volume left to right with respect to the end points it is not a big deal..  Reconnect the programmer to the second servo (don’t touch the first one) and  carefully experiment with the end points on the second servo until they match the first.  This is a very easy task and can be usually done on the first try with an assistant holding the opposite surface at the end points already established  by “lining them up” by eye behind the aircraft as you move the programmer knob. 

Once the surfaces move in unison, program the high, middle and low rates using ONLY the dual rate or flight mode settings and you are done.    Do not use sub trim or stick trim, in fact these should be at zero prior to first flight after setup.

Nothing left to do now but secure everything and go FLY!