Thermalling
Part 1
A simplified guide on how to fly a
“thermal”
GFA Coaching
Thermalling
• A thermal may form if a parcel of air is
warmer than that which it surrounds.
• One of the basic skills in gliding is to
recognise a thermal as you approach it.
Thermalling
Thermals form near the
ground in the superadiabatic layer and
need a nudge to
release
In this example note
the super-adiabatic
layer behind the tractor
and the release of the
thermal.
Tractors trigger
thermals can you think
of any other causes
that may help trigger a
thermal?
Thermalling
THERMAL STRUCTURE
The three stages of a thermal life cycle
decaying
There are three
stages in the
life of a thermal
Maturity
Cut-off
Super adiabatic layer
Thermalling
Thermalling
• The thermal rises and broadens with
altitude.
• The pilot needs to be sensitive to the rising
air and feel the acceleration of the
sailplane as it approaches the thermal.
Thermalling
core
sink
Thermalling
Feel the rising wing and decide whether to turn, when to turn, and
then which way to turn.
Thermalling
A-B turning into lift.
B-C steep angle of bank due to weakening lift.
C-D shallow angle of bank due to strengthening lift.
D maintain normal 45 degree angle of bank while in steady lift
Thermalling
• Rule 1 Never fly through the same patch
of bad air twice.
• Rule 2 Always shift towards the stronger
part of the thermal
Thermalling
Worst heading centering method
Thermalling
Fly
here
Vertical air movements[simplified]
Thermalling
• It has been found that in Australia a bank
angle of 45 degrees is close to the
optimum.
• This gives a higher sink rate for a smaller
radius of turn but with the result of a higher
climb rate
Thermalling
R = 149 m
25 degr. bank angle
R = 85 m
45 degr. bank angle
45 degree
angle of bank
Comparison of circle diameters
Thermalling
Sink
increases
rapidly above
50 degree of
bank
Shallow bank
gives wide
radius of turn
Optimum angle of bank for strong and wide thermal
Thermalling
Angle of bank, when established in the
thermal to optimise climb use approximately
45 degree of bank.
Use wires, straws, or instrument screws to determine 45
degree bank
Thermalling
A- represents the minimum speed at which the glider
will still fly any slower and the aircraft stalls
B- is the rate of minimum sink, the optimum if the aim
is to sink as slowly as possible
C- is the speed for best glide ratio
Thermalling
Half turn at 50kt
and 40 degree
angle of bank
Radius =80.5m
Half turn at 45kt
and 45 degree
angle of bank
Radius=54.5m
Positioning of a glider as a result of inaccurate
flying
•
To fly accurately fly attitude
1. Nose attitude for steady speed.
2. Bank angle for rate and radius of turn
Thermalling
Use of ” top rudder” for optimum thermalling performance