RC Scale Aerobatics - 2012/10

In my last column, I explained how a
pilot could change the response of an
airplane to suit his or her particular
needs. This month, I will continue to
discuss how to obtain a proper setup for
your aircraft.
In addition to the importance of
having an airplane react in a preferred
manner, it is equally important to
examine the various radio programming
and the linkage installation of the throttle
servo.
Many will agree that the radio
installation and functionality, as well
as the engine, are the heart and soul of
your model. I have helped many pilots
program their airplanes only to find that
some did not take the time to properly
program settings that pertain to the
throttle servo.
I will discuss which servo should be
used for throttle control, the various
programming functions I adjust for
proper functionality of the servo (which
include sub trims, adjustable travel
volume, etc.), the need and use of two
idle positions in a given flight, and review
proper linkage installations.
The 35% Hangar 9 Extra 300 is a great choice for Aerobatics competition and a good all-around
airplane. This month, we will take a look at throttle linkage setup and various procedures
required to ensure a reliable idle.
The author prefers to use a 4-40 ball link on the
pushrod end that secures the carburetor, as well
as the servo arm.
You shouldn’t have to worry about an
inconsistent idle at any point during your
flight. After all, a dead-stick approach can
be the difference between first and last
place in a competition! Without further
delay, let’s begin!
A Potential Problem
Typically, any maneuver that requires
the throttle stick to be at idle can be
problematic if the throttle servo and
Because the Extra 300
needed a long pushrod, the
author opted to insert an allthreaded
4-40 pushrod from
Du-Bro into a carbon tube.
J-B Weld was applied to the
ends to secure the pushrod.
linkage are not properly set up. In the
2012 Basic schedule, maneuver six is
a stall turn, which is also known as a
Hammerhead; maneuver 10 is a one-anda-
half-turn positive spin. The engine could
potentially quit while performing either of
these maneuvers.
A pilot could have an idle that is set too
low. Once the airplane is rotating, either at
the top of the stall turn or during the spin,
the engine may stop. The servo linkage
may bind and result in an unpredictable
idle position, which can also cause the
engine to stop functioning. I have often
seen it happen to others!
Please use this column as a guide for
the throttle servo and the other linkages
that are located on your airframe. Many
of these principles are applicable to all
servos.
Throttle Servo Selection
When someone asks me what servos
I recommend for a competition-level
aircraft, which can be 50cc and larger,
I urge him or her to invest in a highperformance
servo for all applications.
The airplanes we fly are often expensive.
Having a servo fail, even on the throttle,
can be fatal for an aircraft.
Many manufacturers produce high-
voltage servos. These servos take the
voltage of a two-cell LiPo battery, which
puts out approximately 8.4 volts on
a fresh charge. If this is your choice,
make sure that either the throttle
servo is high voltage, or that you use a
voltage regulator for the throttle servo.
Running a servo at a voltage higher than
recommended by the manufacturer
can harm the servo and/or shorten its
lifespan.
Make sure that your servo centers
properly and always comes back to the
same end points. If it doesn’t, make sure
that the servo arm is properly secured
on the servo output spline. If your servo
features a metal output spline, use
threadlocker for the servo screw that
holds the servo arm in place.
Regardless of whether or not your
servo features a metal or plastic output
spline, perform routine maintenance to
ensure that the servo arm is properly
secured. The threads on the output spine
of a plastic-geared servo can strip, causing
the servo screw to loosen and back out
during flight. Make sure that all servo
screws are properly secured before taking
to the skies.
As with anything mechanical in
nature, a servo can
fail at any time for
any reason. Maybe
there was a cold
solder joint within
the servo that vibrated loose in flight or
maybe a servo gear stripped internally.
If either of the two scenarios happened,
the result could be catastrophic for your
airplane. The engine may stop working at
an rpm that is too high to land.
If you are new to the world of Giant
Scale Aerobatics gas-powered models,
a neat gadget to have is an ignition
cutoff. Smart-Fly offers one that I have
seen in many airplanes, and it adds
approximately 0.3 ounces to your
aircraft. This unit allows you to kill
the engine any time while in flight—
especially if you have a throttle servo
fail—so you can safely land the aircraft.
Let’s Look at Linkages
Installing a servo and linkages is simple
and is outlined in most airplane assembly
manuals. It is important to ensure there is
no friction within the movement of the
throttle linkage, and that it is properly
supported.
Last year, I built a Hangar 9 Sukhoi
SE and the Extra 300. Both require
a lengthy throttle pushrod. With the
Sukhoi, a throttle pushrod is inserted
within a plastic tube and wood supports
properly align the pushrod from the
throttle servo to the throttle arm on
the carburetor. This pushrod was long
because the throttle servo was mounted
next to the fuel tank. With differing
amounts of throttle, the pushrod can’t
bend or flex because of these supports.
On the Hangar 9 Extra 300, the
throttle servo is mounted on the motor
box a few inches from the firewall. For
pushrods, my first choice is to use a
titanium turnbuckle whenever possible.
However, the longest turnbuckle I found
was 5 inches long, and I needed much
more overall length.
I used a 4-40 threaded pushrod from
Du-Bro and inserted it in a carbon sleeve.
I used J-B Weld to secure the pushrod
within the carbon tube and used two
ball links to secure the pushrod to
the throttle servo and the arm on the
carburetor. No flex, guaranteed!
Study the motion of travel of the
throttle servo arm and how it aligns with
the corresponding arm on the carburetor.
If you notice that the servo does not
return to the same idle position, study
the linkage setup to ensure there is no
friction. Also, ensure that the servo is not
trying to “drive” the pushrod to an angle
that the carburetor doesn’t allow.
Proper Programming 101
When you begin the programming
process on any new airplane, it is always
good to perform critical steps before
making any mechanical adjustments.
Make sure all sub trim values are at
zero. Then, make sure the adjustable
travel volume, which is also known as
end point adjustment, is either at the
default value of 100 or at the maximum
values suggested by your transmitter’s
manufacturer.
Make sure that no throttle curve exists
and that the throttle trim is at the center
position. After all of this is done, you
are ready to begin the initial required
programming.
If you are using a JR or Spektrum
transmitter, bind the aircraft again after
you complete the programming process.
I will now reveal why.
Let’s say that you are setting up
your airplane and find that you need
to reverse the direction of the throttle
servo. If you use the reverse function to
reverse the servo direction, what was
once recognized as low throttle during
the initial bind process will now become
full throttle! If there are any issues with
the transmitter signal in a given flight and
the aircraft goes into failsafe, it will be at
maximum power.
If the airplane is on the ground and
you accidently turn the transmitter off,
it will switch to maximum power. Make
sure you re-bind your aircraft once
all servos are traveling in the proper
direction.
Two Different Positions
I covered some various programming
techniques on which I’ll expand.
Although I do not use it, I have seen
others utilize two different idle positions
in a given flight. Some people use a
landing idle and a flight idle, which are
assigned to a two-position switch on the
transmitter.
With the flip of a switch, you can
have your aircraft idle down for landing.
However, you won’t have to worry about
the engine stopping in a maneuver such
as a spin or stall turn because you can use
a higher idle—which is called your flight
idle—after you’re in the air.
If you go this route, don’t use excessive
flight idle. Use enough rpm so that the
engine has a low idle, but one that slows
down enough to stall the airplane for a
proper spin entry. You don’t want to give
away easy points when competing!
This can be done using a
programmable mix on your transmitter.
Assign throttle as both the master and
slave channel for the mix, resulting
in what is called a throttle-to-throttle
mix. Assign this mix to a switch of your
choice.
Enter a value or values into the mix
so that two different idle positions exist.
The process is straightforward.
Final Thoughts
I urge you to always take your time
and properly set up your aircraft. Many
variables can cause one failure.
If your engine stops in flight, don’t
panic and remember to keep the
airspeed up. If you feel that you will be
unable to make the runway, observe your
surroundings and try to find a smooth area
to land your airplane. When landing, keep
your wings level and stretch the approach.
Balance your airspeed and apply a slight
flair as the airplane is about to land, in an
effort to minimize damage.
With time and practice, these survival
instincts will become second nature. In the
meantime, follow the principles outlined
to achieve a reliable mechanical setup.
Until next time, fly hard!