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RADIO CONTROL
BY BOB YOUNG
Radio-controlled gliders
While most people think of radio-controlled
aircraft as powered models there is a whole
branch of radio control devoted to gliders.
These can be large, fast and very acrobatic
models which are great to watch and even
better to fly.
In this series of articles we will look
at the subtle art of glider flying, including slope and soaring models. As well,
we will look at some of the technology
used in this very demanding aspect of
R/C modelling. We will begin with the
two metre class of soaring glider and
continue up through slope soaring to
the exotic international F3B competition class.
The two metre class glider is an
officially recognised class of model
which is sanctioned by the Model
Aeronautical Association of Australia
(MAAA). The rules are deliberately
aimed at producing a simple model
which is ideal for introducing mod
ellers to the rigorous art of competition glider flying. As a result, clubs
run regular competitions for this class)
often with a yearly point score) and
they are quite popular.
The simple model places few demands on the radio equipment and a
This close-up view of one of the home-made winches clearly shows the high
standard of the workmanship involved. Long metal stakes are used to anchor
the winch to the ground, while a car starter motor drives the pulley. Power
comes from a heavy-duty 12V car battery.
70 Silicon Chip
basic two-channel system will suffice.
These simple models usually take
little time to build, unlike their ultra-complex cousins, the F3B models.
These place extraordinary demands
on the radio and require a high level
of manual skill and the innovative
application of materials technology to
produce a competitive model.
The two metre and F3B models
are primarily intended to be used on
flat fields, using hand-tow, bungee or
winch-launching and therefore must
be designed to withstand the quite
consider
a ble launch stress. Thus
these types of models are essentially
thermal or soaring models, unlike
the slope soaring models which are
designed to ride the wave lift from
hills. The two types of models have
completely different design parameters and we will look at slope soarers
in due course.
There are various methods of scoring for soaring competitions but one
simple and popular method is the
MAAA thermal soaring point system. Under these rules, each pilot is
allowed eleven minutes working time
with a maximum eight-minute flight
in this period. Thus, there is sufficient
time for a re-launch if required.
There is no limit to the number
of re-launches. Landing points are
scored by measuring the distance from
the model’s nose to a marked spot after
the model has come to rest. One point
is given for each second of flight time
up to the maximum eight minutes. If
the flight exceeds eight minutes then
the clock starts to run backwards with
one point being deducted for each
second over eight minutes.
If the flight exceeds eleven minutes
then there are no landing points allowed. If the model lands more than
one hundred metres away from the
marked spot, then the flight does not
score at all. It’s not meant to be easy
and contests are won on margins of
one or two points. Any reader wishing
to know more about the fine points of
the rules should refer to the MAAA
rules book.
Reading the sky
Everyone has heard about thermals
but what are they? Thermals are bubbles of hot air which originate on the
ground due to local differences in
heat absorption which may be due
to ground texture, colour or composition.
Thus a bitumen car park located in a
field covered in moist green grass will
generate a local hot spot. The air over
the bitumen will gradually warm up
and a hot bubble of air will form over
the bitumen. This will increase in size
until the bubble eventually becomes
large enough to break away through
the colder air above it. The bubble
will start to rise, increasing in size as
the air pressure drops with altitude.
As the bubble leaves the ground, it
often sucks up grass seeds and small
insects along with it. Birds quickly
find these bubbles and feed off the
debris, thus signalling to the alert pilot
that here is a thermal to be used when
you are ready. Ultimately, the bubble
reaches an altitude (and therefore a
temperature) at which the originally
warm air can no longer hold its moisture and the familiar puffy little cumulus-nimbus cloud begins to form.
Thus when you look across a clear
blue sky and see those little cotton
puffs scattered around, you are looking at the end of a great little thermal
in each cotton puff.
Flying soaring models requires
a very high level of under
standing
on the part of the pilot in regards to
the formation of thermals, weather
patterns, local conditions and all of
the very subtle information available
on any one field at any given time.
It is no accident that the same small
group of pilots dominate soaring competitions. They are usually the very
experienced pilots who have learned
their craft well and can read the subtle
signs available to all on the field but
observed only by the few.
For example, birds soaring in a
thermal are a dead giveaway. Small
birds zooming around feeding on the
debris give another clue. Wind-puffs,
changes in wind direction and lots of
equally subtle pointers are there like
Taking the strain – Peter Abel about to launch an F3B model built by Phil Bird.
Note that the winch line has been stretched over his shoulder in order to take
the strain off the model.
signposts in the sky for the experienced glider pilot, all pointing the way
to victory at competition level. It takes
years to absorb and you have enough
learning to deal with just this aspect
of glider flying without having to cope
with a complex model as well. Thus
the two-metre class competition uses
simple models which are relatively
easy to fly.
Launch methods
The original and simplest launch
method is hand-tow. It requires one or
more runners to run across the field,
leaping ditches, fences and fallen trees
while keeping an eye on the model
at all times to ensure the tow speed
is not too high. If the tow speed is
too high, the model will try to move
sideways, resulting in the familiar
kite-type looping circle, often striking
the ground in the process if the model
cannot flick off the line beforehand.
The strain of excessive launch speed
can also tear the wings off the model.
An additional hazard is stalling of the
tailplane which will result in a frantic
small looping circle which can wrap
the towline around the model with
no hope of it releasing. This can be a
sad spectacle, as you might imagine.
Obviously hand tow is only for the
young at heart or the very fit. It can
result in excellent launches as the
tow team can move around the field,
keeping the model on the line until a
thermal comes through, at which time
the model is released. The drawback
with hand-tow is that it is difficult to
fly the model and tow at the same time,
which means that another person or
persons are required.
This can lead to inconsistency in
the launch if the same people are not
available each contest. It also requires
May 1998 71
Bob Young (foreground) watches as his own-design glider is launched for round
5 of the Heathcote Cup. According to Bob, it was his “last chance” to redeem
himself but as it turned out, this flight was no better than his previous efforts.
a very skilled tow team to launch a
large model and into a thermal to boot
and skilled people of this order are
not easy to find.
A more popular method is the bungee launch in which a fixed length of
nylon line is attached to a length of
bungee rubber. This is drawn back
to a suitable tension and the model
is re
leased. The rubber snaps back
to its original length and the model
climbs away to launch height. This
method is simple and consistent and
one man can operate the system quite
comfortably.
However, the most popular method
for contest work is the winch launch.
In this method a turn-around pulley is
placed at one end of the field and the
towline looped though the pulley and
returned to the winch. Fig.1 shows the
basic concept.
The line is fitted with a small parachute and a tow ring. The tow-ring is
attached to the top of the parachute so
that, during the launch, the tension on
the line keeps the parachute closed.
When the model is released the chute
opens, slowing the decent of the line
and allowing the winch to wind in the
line before it reaches the ground. The
rules call for the turn-around pulley to
be placed 200 metres from the winch,
with a maximum of 400 metres of line
on the drum.
Winch launches are very spectacular, especially for the larger models,
and the glider climbs away vertically
at a speed that takes your breath away
for the first few launches. Driving
these powerful winches takes a deal of
skill as it is very easy to rip the wings
off the model if you launch too hard.
However, used correctly, winches
result in one-man launches of great
consistency and excellent height.
A typical winch system consists of
an automotive starter motor with solenoid and foot switch, a winch drum
with ratchet and a 12V car battery.
The foot-switch applies voltage to the
solenoid which in turn switches the
starter motor.
The motor drives the drum directly
and the ratchet allows the line to be
recovered after the launch. The footswitch is used by the pilot to power
Fig.1: winch rules call for the turnaround pulley to be placed 200 metres
from the winch with a maximum of
400 metres of line on the drum.
72 Silicon Chip
the winch during launch and takes a
little getting used to. Depending on
the wind speed, the foot-switch may
have to be pulsed on and off to adjust
the launch speed.
There are very strict rules governing
the design of the winches for it is here
that real advantages can be obtained
by a lavish application of technology
and money. In the late 1980s, those
techno-junkies, the Germans, developed a series of winches for their
F3B team which were microprocessor
controlled.
The winches were mounted on
hydraulic rams and the microprocessor sensed the ram pressure which
reflected the towline tension. It then
adjusted the winch speed to compensate for the wind speed. The cost of
these engineering marvels? – $10,000
each and the German team had four
of them!
The rules now call for a winch
motor of no less than 15mΩ armature
resistance powered by a 12V battery
of a designated size and capacity. No
electronics are permitted in the winch
switch
ing, only electromechanical
switches. Most winches use a simple
foot-switch to actuate one or more
solenoids. Parallel solenoids are
sometimes used for extra reliability.
Once the glider has been launched it
is incumbent upon the pilot or winch
operator to wind in his line as quickly
as possible to prevent any crosswind
component blowing the line across
the other winch lines on the field. In a
contest there may be many winch lines
laid out in parallel and there is nothing worse than attempting a re-launch
with the clock ticking away, only to
find that someone has laid their line
right across the top of your line.
Once the model has left the line,
the real work begins. In still air with
no lift of any kind, or worse still
sinking air, it is possible to be back
on the ground inside three minutes,
even from an excellent launch. This
means a re-launch if the pilot considers that he can improve his position
by doing so.
In good conditions a glider can stay
up for hours but this is of no benefit
in a contest. The rules call for the
model to be back on the ground in
eight minutes or the clock starts to
run backwards. So every trick in the
book is used to milk eight minutes
from each flight and it is a measure of
the pilot’s skill to place the model on
Another of the winches is shown here and once again the high standard of the
workmanship is evident. The wheels and the handle at the top make it easy to
move the unit from one place to another.
The launch winches come in all shapes – it all depends on the ingenuity of the
builder and the materials to hand. This was the line-up for the Heathcote Cup,
held back in March. There are very strict rules governing the design of winches.
the ground as close to eight minutes
as possible and with the nose resting
on the spot landing marker.
If all has gone to plan and the
pilot wins the round he is given the
maximum number of points (1000)
and the battle begins anew. There are
usually four to six rounds flown in a
contest, depending on the number of
competitors and the time available.
Contest glider flying is good fun
and calls for a high level of team effort
to ensure a successful day. It can be
very physical, chasing towlines and
retrieving gliders, and it’s all done
against a ticking clock and in a spirit
of good-natured competi
t iveness.
For those who enjoy the intelligent
application of technology mixed with
outdoors exercise and a real and subtle
appreciation of nature, I can strongly
recommend it.
SC
May 1998 73
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