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.
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
a few notes:
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
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.
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.
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.
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
out several copies of the protractor
and size them for use
each servo individually to an extension long enough for access
the servos in the airframe, exactly how they will be installed
your radio, programmer, y-cable and choice of control linkages
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.
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
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.
1 - Drilling for symmetry
2 - Placing the servo horn / clevis hinge point on the surface
control horns need to be installed in a manner that the contour
of the control surface
wont cause the horn to change angles in the hole drilled in
the hard point or leave gaps in the retainers.
To achieve that you must use either horns with self-aligning
retainers or machine / sand down the rigid retainers to match
the contour of the surface. See figure 3 below for a clarification
of the servo horn components. In figure 4, notice that the horn
retainers are matching the surface contour as required. This
is not hard to do, a grinder or sanding block makes quick work
of this task.
3 - servo horn components
4 - horn retainers need to conform to contour of surface
- make sure there is no gap on either retainer
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).
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).
: 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.
5 - installing protractor on servo
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.
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 - pivot arm center point aligned with hinge line
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. Dont 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
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.
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.
7 - trimming the DuBro horn for perfect alignment of pivot
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.
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.
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 - determining and marking end points with the protractor
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.
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.
RX elevator channel to the programmer
up RX, TX, and programmer
ALL trims on the transmitter (don't forget mixes and sub trim)
programmer into TX signal pulse test (step 7 in programmer manual)
pulse value and record it
programmer, power off TX, RX
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
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
the servo arm from the servo, leaving all linkages attached
up the servo using the programmer
a reset (do NOT have linkage connected.. very important)
power and put the programmer in manual mode
the programmer knob to the exact microseconds reading you took
in step 1
servo now is at the TX center
the servo arm, getting as close to 90 degrees as possible on
that it is likely the arm will not align exactly with 90 degrees,
this is OK
the programmer into end point, neutral and fail safe mode (EPAneuFS)
the knob, rotate until you see --><-- and servo acquires
center has been acquired, use knob to rotate servo to 90 degrees
M to lock the new center point
now will be setting the left hand and right hand end points:
programmer will now be in Left Hand end point set mode
the knob to move the servo to its LH high mark on the protractor
sure the center of the servo arm is perfectly aligned with the
mark you made
UP/L to store the LH end point
programmer will now be in Right Hand end point set mode
the know to move the servo to is RH high mark on the protractor
sure the center of the servo arm is perfectly aligned with the
mark you made
DN/L to store the RH end point
IMPORTANT - press EXIT to save your new settings
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.
If everything was set up correctly both sides will travel and
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.
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
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 (dont 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.
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.
left to do now but secure everything and go FLY!