August 2007 33
Tuning a twin-engine
airplane: twice the
trouble or twice the fun?
Above: The engine is
inverted and the fuel-tank
position replicates the
tank location of the
intended twin.
Left: Side-mounting an
engine in a test rig
replicates the twin
installation for testing
and breaking in.
TwTwini n TTuunni nigng
BY ERIC
HENDERSON
If programmed correctly, twin engines could make this airplane handle on the ground
and fly as if they were one power plant.
THERE IS NOTHING like the sound of
two engines beating out a synchronous sound
as you hold your model’s line in a
scintillating low pass down the center of the
runway. On the other hand, it does not sound
right or handle very well if one of a twin
aircraft’s engines is not set up to run as well
as the other.
A twin-engine airplane is an attractive
“next step” once you have become proficient
at flying an RC airplane. Twins are much
more readily available now than in the past
because of an increasing selection of twinengine
ARFs. There are several basic
versions you could call twin trainers and a
range of more scalelike models that could
tempt you to “go twin.”
When everything is working right, a twin
is not that much different to fly from a
single-engine model except for the
sumptuous sound the former produces. Time
spent working on a good initial setup greatly
improves your chances of success.
Twins can be difficult to handle on the
ground because of the outboard position of
the engines, which are well away from the
airplane’s centerline. During takeoff a
centered fin and rudder does not have much
steering effect until the airplane gains speed.
Up to that moment you are basically relying
on your engines to pull the model in a
straight line.
One engine pulling more than the other
makes a twin hard to steer on the ground or
guide in the air. It can also cause the model to
do a nasty tuck-under of one wing in a
banked turn if your aircraft is going a bit
slow.
Getting both engines to come up equally
from idle is more important than having them
A twin-engine Mosquito heads out for
another successful mission with both
engines in perfect harmony.
08sig2.QXD 6/22/07 10:59 AM Page 33
run the same at full throttle. The following series of actions will
greatly improve your chances when operating a twin.
Some are tried and proven techniques you can greatly enhance by
taking advantage of the functionality of today’s computer radios.
Many of the methods apply regardless of the transmitter’s features. A
twin is a wonderful opportunity to take full advantage of a high-end
computer radio’s servo-mix programming options.
The most important thing to do is to first get your engines to be
reliable and transition smoothly from low to high throttle. This is the
primary “secret” of successfully operating a twin-engine model.
Famous football coach Vince Lombardi said that success is what
happens when preparation meets opportunity. With twin power plants
it is all in the setup and engine preparation.
To get your engines to operate equally
you must run them. If they are new you must
break them in until they can be run at full
throttle without overheating. Then they must
idle reliably and transition well throughout
the throttle range.
If you have an old trusty trainer or a used
sport airplane around your “hangar,” you can
use it to break in your engines one at a time
and simulate the final installation. A model
with easy access to the idle adjustment, such
as one with no cowl, would be ideal.
If possible use a similar installation to the
twin you will fly to simulate the intended
operating environment. For instance, many
Scale models have the engine inverted,
which puts the tank high relative to the
carburetor.
If you can install the engines in a test
airplane, with the same engine attitude and
34 MODEL AVIATION
Photos by the author
Above: The first
stage in matching
the second engine’s
rpm to that of the
first engine.
Left: The primary
throttle is mixed
with a second slave
channel.
Top left: Set full throttle angle the same on both throttle servos. Top right: Make sure
the opening is not stalling the servo arm’s full throw. Above left: Setting the low-end
arm position should be the same for both engines. Above right: This starting point will
change when the engine is run.
tank relativity, you will be way ahead of
the game. You can preadjust the engines to
run as if they were already in the intended
twin.
You can compare maximum rpm and
finely tune the low end. Then test the
transition to high throttle. Use a tachometer
and not just your ears! And please take all
rpm readings from behind the propeller.
With engines broken in and preset for
reliable operation, install them in the twin
with the throttles hooked up as equally as
possible. You need to set the servo throws as
mechanically close as possible.
The goal is to set the servo-arm and
throttle-arm angles to get the best throttle-toservo
geometric relationship. Then obtain the
same relationship for both engines.
Start by setting one engine with the
throttle servo plugged into the receiver’s
throttle channel. In this example I am using a
nylon clevis on the throttle arm and an EZ
connector on the servo arm. Set the throttleservo
endpoints (ATV, or travel adjustment)
to 100% each way and then perform the
following steps.
1) Move the throttle stick to the halfway
position with the throttle-trim lever in the
halfway position.
2) Position the servo arm 90° to the
pushrod. (The pushrod is not always parallel
to the servo.)
3) Move the throttle stick to the fully
open position.
4) Move the pushrod to fully open the
08sig2.QXD 6/22/07 11:01 AM Page 34
August 2007 35
Use a dial-type function knob on your
radio to operate the low-end trim
settings of the slave channel/engine.
Keep advancing the throttle to each point mix and then adjust
the throw until the rpm are the same.
Secure the airplane to the ground. Take all rpm readings from
behind the propellers.
Applying right rudder increases the left engine’s low-end rpm to
assist the left turn.
Applying left rudder increases the right engine’s low-end rpm to
assist the left turn.
Taxiing a twin-engine aircraft is much easier if the engines help you.
Use a small amount of channel mixture settings so the knob behaves like it is a trim
lever.
Twin Steps
1) Make sure the engines run as reliably
as possible before installation.
2) Ensure that the throttles are as
mechanically equal as possible.
3) Set the carburetor needles to the
individual needs of the engine.
4) Use two tachometers simultaneously
to make synchronization easier.
5) Optimize engine setup by channelmixing
the throttle servos.
6) Employ throttle-curve mixing to
allow the engines to be matched perfectly.
7) Improve ground handling by mixing
throttle with rudder control. MA
—Eric Henderson
08sig2.QXD 6/22/07 11:02 AM Page 35
throttle and then temporarily tighten the EZ
connector.
5) Bring the throttle stick back to the low
position and increase or decrease the endpoint
percentages to establish the carburetor’s
“idle” position. (This position should have
been visually established during the engine
test phase. It does not have to be correct and
will most likely change slightly when you run
the engine. For now it will be good enough
for the radio/engine setup.)
6) You will now have two percentage
readings on your throttle channel: one at
100% for high throttle and one for the lowthrottle
position. Let’s say that it had to be
reduced to 70% to get a satisfactory throttle
opening for an idle. That is a total throw of
170%.
7) Divide the total by two, which will give
an equal percentage in each direction of 85%.
8) Loosen the EZ connector and dial 85%
for the high- and low-throttle percentages.
9) Move the throttle stick to the fully open
position.
10) Move the pushrod to fully open the
throttle and then tighten the EZ connector.
You have set the throttle action in the best
part of the servo rotational movement, which
will give the high and low throttle travels
some symmetry.
To set the second throttle servo, plug it
into the throttle slot in the receiver and
replicate steps one through 10. This will give
you two throttles that have been set equally.
At this stage you could use a Y lead to drive
36 MODEL AVIATION
both servos from the one throttle channel.
Radio Programming: It is not unusual to
hear that you don’t need a fancy radio to fly a
twin; just use a Y lead and set the needles
correctly. This may be true in some situations,
but a computer radio takes you into a whole
new world where you can set the individual
engines and mix the channels to obtain an
equal run-up all the way through the throttle
range.
Computer radios let you go way beyond
the Y-lead approach. This is because you can
use two separate channels for the throttles.
You can still operate both engines from one
throttle stick, and at this point you are equal to
the “Y-lead approach.”
However, once you use the two-servo
channel-mix option, the servos can be
individually adjusted for high-end and idle
settings. Each servo, hence each engine, can
also have its own dedicated throttle trim lever
or knob.
More exciting is going up another level
and matching both engines’ rpm all the way
through the throttle range. The “Y-lead
approach” won’t let you go there!
For this example I used a JR10SXII, but
any radio with a “point-mixing” graph
between a master and a slave servo can do the
job. The second throttle servo is usually
plugged into an auxiliary channel that is
mixed with the one plugged into the receiver
throttle.
If you are right-handed, plug the left motor
servo into the throttle channel. This will be the
first engine you start for a flying session. It
will be running when you start the second
engine.
With the left engine running, you need to
shift your body over to start the right engine.
Moving to the right engine will keep your
hand movements and precious body parts
away from the left engine’s already spinning
propeller.
If you use the starter in a left-handed
fashion, plug the right-hand servo into the
throttle channel and start the right-hand
engine first. The left- and right-hand
directions apply when looking from behind
the airplane.
During the channel-mixing process select
a series of points on the mix graph displayed
on the radio. In this example I mixed the
throttle channel with the AUX 3 channel. For
now select a straight line on the graph and set
the slave servo so it moves equal to the master
throttle servo. You will further tune the point
mixes later at the field.
The mix program asks if you want the
throttle trim lever to be active. You answer
“yes” because you want to be able to bring
both idles up and down at the same time.
Similarly, if you employ a throttle-kill option,
you can mix “kill” to the throttle servo and the
computer radio takes care of the rest.
At this stage I can add a control that will
the knob either way had a small effect on the
servo movement, as with a throttle trim lever.
You do not want the auxiliary throttle
trim to be active all the way through the
throttle-stick movement. You can prevent
and control this by using the throttle-stick
position option as a switch. Once the throttle
stick is advanced beyond approximately 15%
open, the trim mix can be turned off.
Running the Engines: Secure the airplane
with a reliable restraint. Start the left engine
and adjust for optimum running performance.
Turn that engine off and start the second
engine. It should be close in regards to
throttle range. You can adjust the low-end
rpm with the AUX 4 trim knob.
If this is not enough, you might have to
adjust the low-throttle endpoint. Do this by
going to the mix settings between the left
engine and the right engine rather than using
the endpoint option. The point-mix
relationship is where you want to make all
the tuning adjustments.
The next thing to do is find two similar
tachometers. A good safety practice is to tape
each one to a small stake and position one
behind each engine’s propeller blades. Start
the engines, warm them up, and then set the
idle rpm to be equal with the radio trims.
Anything between 2,000 and 2,500 rpm will
be good for this exercise.
This next step may become your favorite
thing to do. Go to the throttle-servo’s mixadjustment
graph on your computer-radio
screen. Advance the throttle to the first point
on the graph. Increase or decrease the mix
percentage for the right engine until it holds
the same rpm as the left engine.
Let’s say the left engine is showing 3,250
rpm and the right engine is showing 3,100
rpm. Bump up the mix until the right engine
is also reading 3,250 rpm.
Move the throttle to the next point on the
graph and match the rpm again. Do this for
all the points on the graph, including the
maximum rpm. The JR 10SXII and 10X
systems will let you create seven points on
the mix graph and turn that into an
exponential curve between each point.
Now you have two synchronized engines
that will pull your twin airplane in a straight
line at all throttle positions. However, it will
not help you taxi your model on the runway.
At this point you may hear that familiar
voice saying “You don’t need a fancy radio
to taxi a twin.” This may be true in certain
situations, but there is nothing like being able
to swing a twin’s tail around with a throttleassisted
rudder.
The inboard wheel can be made to almost
stay put as the airplane spins around. It is as
if there were a pilot onboard doing the job. It
makes turning on grass much easier for a taildragger.
This is especially advantageous for a
warbird twin, such as a Mosquito, that loves
to tip forward at any opportunity.
If you have come this far with your
computer-radio mixing, you might as well
have throttle-assisted turning—if for no other
reason than you can!
You should have no trouble getting one
engine to increase in rpm by applying rudder.
The way to set this up is to get “left” rudder
application to make an rpm-increase request
to the “right” engine and vice versa.
How much throttle mix is a matter of
experimentation? An increase to 3,500 rpm
has proven to be manageable during taxi
turns.
Also, using the throttle as a switch lets
you disengage this option during flight and
takeoff. Remember that nothing happens
unless you apply the rudder in the first place.
If you have done all the preceding, you now
have a tuned-up twin-engine model. You will
have improved your airplane’s flying
capabilities, engine operation, and general
handling characteristics.
You can run each engine individually.
You can move both engines’ idle at the same
time. Adjust and balance the right engine
with an auxiliary trim knob. You can taxi
your model like a full-scale twin and
maneuver it in tight spaces. Most of all, you
have improved your aircraft’s chances of
success on takeoff and landing.
Now all you have to do is accept a lowlevel
mission over Holland to release
freedom fighters from the clutches of their
evil captors. Your tuned-up Mosquito is
warming its Merlins for the dawn raid and
awaiting its pilot! MA
Eric Henderson
[email protected]