arf Mystique Manuel D'utilisation page 44

What to do when you find a thermal
Probably the first thing you need to be absolutely sure
of is if you are in lift. Often a sailplane may find what
we call a stick thermal. It's a tongue-in-cheek term
meaning you may have been carrying some additional
speed and the model will climb by pulling elevator.
One of the best signals you will see when the model
is truly in lift is it will slightly speed up and the nose
of the aircraft will be down slightly. The model will
feel more agile and responsive. Once you have found
your lift and you're sure it is lift, start circling in a
moderate circle, about a 50–75' radius. The next
thing you need to do to determine is how big the
thermal is. Once circling, you may notice that your
model may drop on one side of the thermal and be
more buoyant on the other. The perimeters of most
thermals are clearly marked by downward flowing air.
If you have seen an atomic bomb cloud, then this is
a good visual for you to understand what a thermal
can look like. The center has fast, rising air and the
outside has downward, rolling air, often known as the
edge of the thermal, or the thermal wall.
In the middle of the day when thermals are at their
strongest, the thermal wall can be very distinct and
violent, yet in the morning and late evening much
softer. Keeping this in mind, the main objective is to
make sure you are completely inside the thermal.
This is called centering or coring the thermal. You will
need to constantly make adjustments to stay in the
center of the thermal. Keep checking you are getting
an even climb all the way around each circle flown, as
you may not be completely centered in the thermal.
Often, especially if it is a windy day, thermals will drift
with the wind. Most will travel directly downwind. One
thing to remember is your model will also drift with
the wind, especially when circling. Thus, once you
have established the core of the thermal, your model
will naturally drift with the thermal, much the same
as a free flight model will. One mistake people often
make is that they don't allow their model to drift with
the thermal, which causes them to fall out of the front
or side of the thermal as it drifts downwind. If this
happens, then you need to look again and re-acquire
the thermal.
44
In-Flight Adjustments for
Performance and Conditions
• Pitch Attitude
• Minimum Sink Speed
• Maximum Lift/Drag (L/D) Speed
• Best Penetration Speed
Once the fundamentals of launch, trim and control of
the model are learned, it's time to consider getting
the most out of its ability to perform. To do that,
you must learn how to trim your model for maximum
performance, whatever the current conditions are
at the time. The key to trimming for maximum
performance is to become knowledgeable about, or
aware of, three key speeds: minimum sink, maximum
lift/drag (L/D) and best penetration.
These three speeds are what we call airspeeds,
not ground speeds (the aircraft's speed across the
ground). Thus, the airspeed of the plane is relative to
the air mass surrounding it.
Pitch Attitude
To determine the airspeed, you will have to watch
carefully for its pitch attitude. Pitch attitude can
best be described as the amount (degree) the nose
of the aircraft is above or below a line relative to the
horizon. The angle of attack term is used to describe
the angle between the chord (width) of the wing and
the direction the wing moves through the air.
Nose
Longitudinal
Axis
Pitch
Attitude
Line Relative to Horizon
Center of
Gravity
Horizon
Increasing Angle of Attack
Relative Wind Direction
Minimum Sink Speed
In our discussion of thermals, we know sink is the
cooler air moving downward to replace the warm air
that is rising. Minimum sink speed is the speed at
which a glider loses altitude most slowly. As the term
then implies, minimum sink speed gives the glider the
maximum amount of time aloft from a given altitude.
This is the speed to fly at when you are circling in
thermals, or whenever you need the maximum lift the
glider can produce. The pitch attitude will appear to
be more nose-up.
To determine what this speed is for your model, fly it
at a slow speed, slowing down until it just stalls. Then
trim it to fly at a speed just above where it begins
to stall. Observe the pitch attitude at this speed.
You will need to practice flying at this speed without
stalling so you can come back to it whenever you want
to, especially when you are in a thermal or trying to
maintain maximum lift.
Maximum Lift/Drag (L/D) Speed
This is the speed at which you can fly the maximum
distance for a given altitude. It's used when you move
from one thermal to another, or when you need to
cover the maximum distance over ground. This will be
a moderately faster airspeed than the minimum sink
speed. You will have to experiment by starting from
the minimum sink speed and adding small amounts
of down trim to increase speed slightly. It will take
practice until you are familiar with the attitude at this
speed. Remember, you will be flying slightly faster at
a lower pitch attitude as compared to minimum sink
speed.
Best Penetration Speed
This is the speed at which the model will travel
forward against the wind or a thermal, as far and
as quickly as possible. This speed will vary with the
conditions, such as windy situations or very strong
thermals. You will want to use this speed to escape
from very strong lift (or sink). This speed has a more
pronounced nose-down appearance, which will vary
with the conditions encountered. It will also not be a
consistent attitude, but vary with the strength and
direction of the lift/sink or wind.
Once you have learned to launch and control your
model in a consistent manner, you will want to start
practicing these three speeds. Remember, these are
trim speeds, so you will be using your trim lever to
obtain them. For maximum performance, remember
to use trim sparingly and don't depend on the stick,
as you will only impart small movements that result in
drag and battery drain.
Practice smooth control inputs and use the trim lever.
This is why you trimmed the model in the test flights
and then set the mechanical linkages to reflect the
trim your trim levers back to neutral. Now you know
why we performed that procedure, to allow you to use
the trim lever for in-flight trim and have better control
over flight performance.
There are other things that can be done to bring the
performance level of your model to its absolute best.
However, they should not be attempted until you have
become proficient in the launch, control and trim of
your model.
The more you learn how to trim your model for
optimum performance, the more fun you can have
chasing thermals!
Slope and Alpine Lift
Another form of soaring is slope lift. This lift is
caused by wind rushing over a hill, cliff or any solid
land mass that has more than 30 degrees of slope.
As the air hits the hill or slope, it will be redirected
in an upward motion, thus creating lift. The best
example of this are hang gliders that are soaring
on the cliff faces. They maintain flight by soaring on
the updrafts created by the sea breezes hitting the
cliffs, which is known as slope lift. This sort of soaring
is a lot of fun, and with your model you can always
motor back to a safe landing if the lift falls away. The
important thing to understand with slope type lift
is the wind must be almost directly blowing up the
face of the hill or slope. Any more than a 20-degree
variation may cause more turbulence than actual lift.
Alpine soaring has been popular in Europe and is
also becoming popular in the U.S. Basically, it is
the extreme end of thermal soaring. As thermals
develop deep on the valley floor, they rise up the
mountainside, reaching their climax at the top of the
mountain. This is often marked by a strong breeze
blowing at the top of the mountain, which is in fact a
fully-developed thermal. One of the benefits of your
model is what is called a power-assisted sailplane.
Even though it does have an electric motor that will
allow quite a steep climb, the primary purpose of the
motor is launching and returning to a safe landing
point.