Thermalling
The importance of efficient thermalling
How thermals are formed
Finding thermals
Thermal selection
Centering
Staying centred
Leaving
Mike Codling May 2008
Efficient Thermalling – why ?
 X-country speed = 0 while thermalling
 3hr flight, difference between 4 knot av. and
4.5 kn av. is approx 32 secs/ thermal or
10mins 40 seconds for flight.
 Better speed, further difference, best use of
day.
How thermals are formed
 Parcel of air warmer than the surrounding air
 Source
 Trigger
What things come to mind that effect thermal generation?
•Sunlight
•Wind
•Slope
•Soil Type
•Shadow lines
•Moisture level
•Inversions
•Vegetation
•Time of day
•High level cloud
Thermal Sources
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Dark ploughed paddocks
Red soil paddocks
Vegetation – evening
Gullies in the sun
Quarries, mines
Fires
Power stations, industry
Rock faces
Thermal Triggers
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Tree lines
Buildings
Ridges
Turkey nest dams
Creek/ river lines
Tractors
Stages of a thermal
THERMAL STRUCTURE
The three stages of a thermal life cycle
decaying
Maturity
Cut-off
Super adiabatic layer
Finding Thermals – blue days
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Know the wind direction
Look for sources
Look for triggers
Account for wind, slope, sun angle
Dust Devils
“In order to find a thermal, I think like one”
Finding Thermals – CU days
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Look for CUs ;-)
Note cycle timing
Account for wind, sun angle
Check for dark, flat bottoms or concave
Check for swirls, wisps
Avoid “hanging” remnants
Avoid clouds lower than others nearby
If in doubt fly to centre whilst high
If low treat as blue day
Thermal Selection
 Look for correct timing of cycle
 Look for energy lines, streets and future
options
 Check out the other gliders for bank angle
 Modify selection strategy for time of day
Plan on for every 1000ft of convection, 1 knot of climb rate
Thermal Entering
Feel the rising wing and decide whether to turn,
when to turn, and then which way to turn.
•Reduce speed in rough air, not too slow
•Keep momentum through sink
•Pull up in lift
•Avoid strong pull-ups, push overs
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Never fly through the same patch of bad air twice.
Always shift towards the stronger part of the thermal
Air is smooth in the core
Feel will always precede the vario
Turning
 Maintain attitude A.R.E.
 Adjust once per turn and
wait to note effect
 Good angle of bank (45deg
in Aust)
R = 149 m
25 degr. bank angle
R = 85 m
45 degr. bank angle
Use wires, straws, or
instrument screws to
determine 45 degree
bank
Thermal Centering – simple method
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
Worst Heading Centering Method
When vario is weakest (A) roll out (B), wait for 2 seconds as vario rises,
roll back in (C)
Good for low hours as low workload
Leaving
 Use McCready rule for leaving
– current climb = expected next climb
 Decide exit strategy well below cloudbase
 Build speed during last turn or …
 Turn across the centre ( not when sharing a
thermal)
 Exit upwind or downwind to avoid sink
 Have cruise speed before sink
Thermalling Safety
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Assume that all other aircraft have not seen you.
Always be aware of what is happening around you.
Always keep other sailplanes in sight and ensure that you can be seen.
A sailplane that remains stationary in the canopy and is growing larger,
is going to hit you unless you take avoiding action.
The first sailplane in the thermal has the right of way.
Join the thermal to turn in the same direction.
When joining ensure that other sailplanes do not have to manoeuvre to
avoid you.
If there is more than one sailplane thermalling in different directions
turn in the same direction as the one at your height.
Do not pull up or dive sharply in the thermal.
Do not pull up in front of another sailplane.
Be Predictable
Enjoy !