Thermals.
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All diagrams are based on classic
ideas. The weather for the day
will modify the weather as it
pleases.
It is your job to predict how the
classic structures will be
modified on a given day.
On each slide we should look as a
glider pilot.
“What are we going to do.”
Teasing
zone
Generally
in Blue
thermals.
Core
A classic thermal
DOWN
UP
Cross section of classic thermal
The strong lift is in the core.
So stay there.
The effect of wind shear, moves the upper
part of the thermal down wind.
So be prepared to shift the thermal that way at
the shear height.
The effect of sun shadows.
Stay on the “Sunny side of the street”
As the sun rises the thermal source becomes blocked so the
thermal bends to receive heat energy.
Mid day the thermal source is cut off, less predictable thermals.
Wind effect on thermal.
A thermal
bubble rises at
an angle
proportional to
the wind and
thermal
strength.
The result is that the
into wind part of the
thermal is the strongest.
Wind Direction
8 Kt
4 Kt
2 Kt
Whisp
Developing
Developed Cu with
dark concave base
Decay and
sink.
Development of Cumulus Clouds
Thunderstorms
When the thermal dumps water lift can be found
around the edge of the thermal. Be careful.
Very Rapid Storm development with Alto Cu.
Morning
Afternoon
The cloud grows rapidly being
further heat
all the
Ingiven
the morning
therefrom
is Alto
Cu at
available
heatnot
from
the ground,
higher
levels,
contactable
by
The
cloud
reaches
freezing
level
Rain
will
to
come
As
heating
develops
Cu
will
form
Eventually
the Cu
rise
to
meet
the
gliders,
butstart
indicating
unstable
air
As
the
thermal
energy
isfrom
given
toproducing heavy rain and sink, lifting
dust
and
creating
local
squalls,
infor
addition to lift around the edge of the
corners
of
the
Strong
at
a Alto
lower
level
than
the
Alto
Cu,
Alto
Cu
adding
the
thermal
in
the
upper
levels.
Poor
lift energy
atlift
the
Cu
thecloud.
Alto
Cu
builds
sink.
may
rapidly
move
glider
pilots
with
the
possibility
of
giving
stronger
lift
for
the
to
theThe
instability
of
the
upper
layer
lower
levels
till
later
in
the glider
rapidly
incloud
height
developing
into
a being steered by the upper level
winds.
Time
tointo
be on
ground and well tied down.
being
sucked
thethe
cloud.
pilot.
of
Alto
Cu. lift
morning.
Cb.
Strong
for
glider
pilots.
Once the thermals get established
after say approximately 9:30 the
day begins to organize itself.
How thermals effect one another
Why good thermal sources do not always work.
As thermals go higher so they go
further apart.
If you leave the top of a thermal you
will pass through about 3 thermals
before you have to land, on any given
day.
After about 2:30 the heating to the
ground reduces but the thermal
height still goes higher.
As there is less energy to go into
producing thermals, so they are not
produced so often they therefor
become further apart.
The same theory goes for
unsuitable ground, i.e. after
previous wetting from rain.
The thermal bubble
Rock
Crop
Bush
The Hot Layer.
Understanding this is critical.
Street
Development
A
C ro p
R ock
B ush
The Bush acts as a reservoir for hot air, increasing in heat and volume to the leeward
side.
The rock acts as a trigger for the release of the thermal.
A
C ro p
R ock
B ush
Thermals continue to stream off Bush.
The underside of the thermal bubble is a low pressure and sucks the residue of hot air
from the crop.
As the thermal bubbles drift down wind they continue sucking from the crop but not
with so much strength but certainly enough to help guide you to the source.
The upper level of the thermal is still strong and sucking.
The lower levels are narrower in all cross sections.
A
B
C
C ro p
D
R ock
B ush
See the Movie
• Thermal Movie
Random scatter of Cu on a non wind day, NOT TO SCALE
. 200 M max in diameter.
Thermals stretched by wind into streets. About 15-30M
wide. To the same scale as previous slide.
It can be seen that if you travel cross wind on a windy day
you will not have to travel very far before finding lift.
More realistic streets with Key Holes.
Join the thermal by heading into wind
Leave the thermal cross wind to find the next thermal.
BINGO!
Why Large Rocks don’t give lift
The novices track around a task.
Novice
Expert
A quicker way around the task.
Andrew Repton’s path around a task.
Thermalling tips
• Naturally we need to be able to fly
At 45Deg and a constant speed
• This is for two critical reasons
• To be able to maintain a constant position in
space
• To be able to stay in the centre of a thermal
when you have cored it.
Approaching the thermal
When we feel the pre thermal turbulence SLOW
DOWN ALWAYS.
With a thermal diameter of 150m it will take a glider
travelling at 120 kts 2.5 seconds to pass through
85 kts 3.5 seconds
60 kts 5 seconds
GIVE YOURSELF TIME TO THINK AND
SAMPLE.
Which Way to turn
Pre Thermal
Turbulence
Rising Air
Sinking Air
Still Air
Three possible ways to approach a
thermal.
Glider
Enters
Sink
Pilot
Pre
Thermal
gets
into
Acceleration
Detects
Approaches
Pilotofand
Buffet
core
upwards
forces
stop,
Thermal
Mentally
SLOWS
thermal
into
acceleration
pilot
using
switches
60-65kts
first
turn. on.
and
full
lifting
control
of
right
movements
wing.
turns to
right.
Sink
1kt
2kt
3kt
Lift
Glider
Pilot
Pilot
Detects
Detects
Enters
Pre
Thermal
Sink
Approaches
acceleration
Little
Pilot lift
Buffet
andbut
Thermal
and
lifting
continues
of to
right
Mentally
SLOWS
flying
wing
so
straight.
turns
switches
60-65kts
on.
to the right.
Acceleration
forces stop,
pilot using
Pilot
gets into
full control
core
of
movements
thermal
turns to in
first
right.turn.
Sink
1kt
2kt
3kt
Lift
Glider
Enters
Sink
Pre
Acceleration
Thermal
Pilot
Detects
Approaches
Pilot stop,
Buffet
forces
maintains
upwardsanda
Thermal
Mentally
SLOWS
pilot
using
to
very
tight
acceleration
switches
on.
60-65kts
full
control
turn
for
180
but no wing
movements
Deg.
lift.
turns .
Pilot opens
out turn to 45
Deg and has
thermal core
in one turn.
Sink
1kt
2kt
3kt
Lift
Turn the wrong way Option 1
Turn the wrong way Option 2
Why
VERY
Staying in
the maintaining
same location
in accu
anglespace.
of bank and speed is impo
Now We Move the
Our Centre.
We now make a
new centre
We keep in the
same location.
Move Straight or Reduce Bank.
Aggressive roll out of turn
and fly straight.
Or Roll out gradually?
Result
If a slow roll out is used you
must predict the change in lift
in advance.
When to leave
• When the thermal strength drops to the
same strength as the start of the next
thermal you will get to.
Look for lift ahead
• In your climb you should be looking for the
next thermalS
• And assessing wind drift.
How long do we have to make
decisions?
• 2.5 seconds to get between the edge of the
thermal and the middle
• Not much time
• You need it to be instinctive
• Practice.
Practice Technique
• See how many 1000ft climbs you can do in
2 hours
• A 2000ft climb counts as one climb!
• You can go back to the same thermal but
you must leave it
• Compete with friend.
Articles on
• www.jamescooper.com.au