Radio Control Aerobatics - 2009/05

104 MODEL AVIATION
This transference from the goal of precise
flying to precise building is a fairly
normal piece of human behavior.
I spent four years of my professional
career as a quality assurance manager. Did
the job come home with me? It did not
take long before it spilled over into my
car-maintenance schedules. Soon I was
upgrading the home heating system and
improving the windows and roof
insulation.
If you compete in RC Pattern, it
probably won’t be long before natural
progression takes over and precision
becomes a big part of your airplane
construction. I doubt if it would be hard to
accept the next jump either.
If you are into precision, there is only a
short hop to buying more expensive, ergo
higher-quality components. You will tend
to buy the “best.”
What the best is can be debated
forever, but a Pattern pilot will tend to use
the latest hardware, lightest materials,
tightest connections, most precise servos,
etc. wherever
possible. It could
be a better hinge
or a connector that
has less slop in
the pushrod-tohorn
connection.
Exotic pushrod
materials are
common, as are
the almost
[[email protected]]
Radio Control Aerobatics Eric Henderson
Also included in this column:
• Aerodynamics isn’t in the
rules
• Airplanes that help you do
better
• Flying tips
The meaning and effect of precision
Above: Pattern models have size and
weight rules, but you can design them
any way you want. As long as Richard
Lewis’s midfin design is shorter than 2
meters, he is good to go.
Right: Chad Northeast has a fuselage that
has more area than the wing. As the
model rolls, it keeps flying level with little
corrective input.
THE PRIMARY INTENT of calling an
event “precision aerobatics” is to illustrate
that there is a requirement for accuracy in
the maneuvers that are performed. You can
find this detail in the AMA Competition
Regulations, which is available to AMA
members upon request.
A secondary meaning of the word
“precision” overflowed into the design and
selection of materials used in RC
Aerobatics (Pattern) model construction.
standard carbon-fiber wing tubes.
Endless attempts are made to “add
lightness,” and all are aimed at keeping
the model’s all-up weight below the
weight maximum. A ball-race clevis (not
to be confused with ball-link clevises) has
great accuracy, because it exhibits no
detectable slop between the horn and the
clevis.
Pattern pilots go to extraordinary
lengths to set precise engine alignment
and wing and stabilizer incidence. For
precise flying, these settings are critical
and can take many days or even weeks of
test-flying to get right.
Designers are trying to create the
neutral airplane. In brief, this model stays
at the same angle and attitude until
instructed otherwise. It will not roll level,
as a trainer will, if left alone.
A neutral airplane is almost impossible
to achieve with propeller-driven models.
(FAI Pattern does not allow jets.) You
have to deal with the P-factor, or reaction
to the propeller’s rotation against the air.
Some enterprising individuals came up
with counter-rotating propellers, but so far
the side thrust, downthrust solution is the
only one holding its own.
It may surprise you to know that
mechanical/physical rules have little
effect on the design of Pattern airplanes.
Some may argue that the change to
Turnaround schedules in roughly 1985
had a significant effect on the competition
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May 2009 105
Above: Shown is 1/4 degree. This stabilizer incidence
setting gives level upright flight at all throttle
settings when the elevator is at neutral.
Left: The relationship of the wings and stabilizers to
the fuselage and engine are critical to making
Pattern performances easier.
This ball-race clevis costs roughly $10 and has no play or slop in
it. There will be one at each end of a pushrod/wire. Fanaticism is
precision.
The elevator servo is buried inside the fuselage and covered by
the stabilizer—not just to keep airflow clean, but to have the
lightest, shortest connection.
models that were subsequently designed.
That is probably true, but the
construction rules did not motivate or
prevent the changes. There is still nothing
in the sporting code to hold back your
imagination when looking to design a
Pattern aircraft.
I must accept that Pattern may give you
the impression of having many rules, but I
hastily insert that most of them are related
to competition issues—not aerodynamic
design. It is a misconception that Pattern
design constraints can stop you from
making your model meet any aerodynamic
shape that your heart desires.
Some influences are competitive, such
as symmetrical wings. These do not create
lift at a zero angle of attack.
Pattern airplanes do maneuvers that
need a wing to fly at all attitudes and
angles, especially inverted. To fly the right
way up, we do so with the wing at a
slightly positive angle of attack.
This works great until you go inverted.
Then, to stay level, the angle of attack is
adjusted back to positive with the elevator.
The less elevator or rudder needed, the
better.
When not required to provide lift, the
symmetrical wing cuts through the air at a
zero angle of attack. This would be the
case in knife edge or going straight up or
down.
The best Pattern airplanes I have flown
were neutral in all aspects of flight, except
for a little pull to the canopy in a long
power-off down-line. This was easy to fix
with slight down-elevator trim mix at low
throttle using a computer radio.
Have you ever wondered why your
model drops its nose in a roll, sometimes
radically? Try a really slow roll (with a bit
of altitude, please), and you will see a
pronounced drop in height at a certain
moment as the airplane rolls over.
What Pattern aircraft designers did first
was figure out what was happening. Most
pilots can perform a roll with a sport
airplane, but what kind of roll? Anyone
can bang the aileron stick over and hold
his or her breath.
A precision Aerobatics slow roll has
some requirements that the pilot must
meet. It needs to last a minimum of three
“long” seconds. (A second can feel like a
long time when there is the potential of
your pride and joy plummeting to earth
because you deliberately made it go
inverted.)
For grins, add the constraint that the
model must roll around its central axis
(often called an axial roll) in a straight
line, with no weaving up or down along its
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106 MODEL AVIATION
track. Then add that it must be flown
parallel to the runway at roughly 150 yards
out, no matter what the wind is doing. The
roll rate must also be constant; a quick flip
out at the end would not score well.
Great pilots can get most models to roll
close to the needs of the scoring rules.
However, it would not be easy with an
airplane such as, say, a Piper Cub. It has a
quasi flat-bottom wing section and
responds with a rolling action to rudder
input. That means the roll rate will
accelerate and decelerate when you add
the rudder correction to keep the nose up.
A superhuman pilot could anticipate all
of this interaction and adjust the controls
all the way through the roll. The rest of us
normal humans would need some help
from the airplane. That’s precisely what
Pattern model designers decided to do:
make aircraft that help you do better
maneuvers!
Initially, the design thinking focused on
the side area of the fuselage, so fuselages
got taller. This was because this extra area
helped in the part of the roll where the
model was mainly flying on its side. The
airplanes still dropped their noses when
rolling onto their sides, but the corrective
rudder inputs got much smaller.
There have been many designs in which
extra side area was created using a central
vertical fin, as shown at the Pattern Nats a
couple years ago by Richard Lewis. He
could have put another fin on the bottom
and created an X-plane if he could have
come up with a way to land that idea!
There is nothing in the rules to stop his
design, other than the total span could not
be more than 2 meters.
As designers got more feedback, they
realized that as an airplane rolls, there is a
moment between the wing providing the
lift and the fuselage taking over where the
lift disappears. A square edge on a flatbottomed
fuselage causes a sudden change
in lift. What they did was increase the
girth of the fuselage and made it oval in
section; they made the corners go away.
In today’s designs, just as the rolling
wing stops, providing the necessary lift,
the fuselage smoothly takes over. The
current fashion of oval large-side-area
fuselages handles the transition to the
knife-edge position. All of this takes some
pressure off of the pilot and helps produce
more precise maneuver execution.
The new designs still need corrective
input from the tail feathers, but if you do a
slow roll, with no other inputs, the overall
height loss is small compared to that of
their predecessors. These designs help the
top pilots do better maneuvers and make it
easier for others to advance their flying
skills more quickly.
In the past, I have compared precision
Aerobatics to Formula 1 cars. I have had to
rethink this position when it comes to ease
of driving/flying. In Formula 1, the cars
may go much faster, but I doubt if they
have been made easier to drive than a
sports car.
If you try a Pattern model for general
sport-flying, you will get an airplane that
is smooth and does what you tell it to do.
It may be a bit hard to slow down,
resulting in longer landings, but it will be
precise!
I have a couple of flying tips/suggestions
that you can try at your field. First, why
enter a loop at full speed? All you really
do is skid into the loop and quickly burn
off speed.
See what happens instead of hurtling
into a loop versus entering at threequarters
throttle and adding full power as
your airplane goes up. You might be
pleasantly surprised by how good the loop
works out.
Second, when executing a Stall Turn,
try not cutting the power and waiting to
apply the rudder. While your model is
going up the vertical line, drop back to
half throttle and then feed in the rudder
just as the airplane stops. As it comes
over, go to low throttle.
It will rotate on a dime and look
professional. You can dictate which way
the stall goes and not be at the wind’s
mercy. MA
Dress Code
We were flying at a small local event at an
airstrip in Debot, Alberta. Shawn Scott was
flying his Logo 500 in an area that had a ditch
right behind him. Seeing the opportunity for a
different shot angle, I went into the ditch and
was able to capture the photo.
What else can I say? “Every bird catches a
worm sometime.”
What I like about this photo is the mood it
portrays. Shawn’s clothes, the helicopter’s
position—just like the shirt reads. Looking at the
latest MA and its gallery of photos, it seems that
there are a lot of other contributors with an
exceptional photographic eye. It’s a very
welcome sight.
The image was taken using a Nikon D200
camera with a Nikkor 70-200mm F2.8 lens. It
has minor Photoshop editing, adjusting the
sharpness and levels. MA
—Dwayne Doll
Grande Prairie
Alberta, Canada
E-mail your high-resolution “Viewfinder”
photo and a short note telling the airplane or
helicopter story to [email protected].
Viewfinder
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