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 super-adiabatic 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?

THERMAL STRUCTURE

Cut-off

Super adiabatic layer

Maturity

decayingThe three stages of a thermal life cycle

Thermalling

There are three stages in the life of a thermal

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

sink

core

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

Vertical air movements[simplified]

Fly here

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 m25 degr. bank angle

R = 85 m45 degr. bank angle

Comparison of circle diameters

45 degree angle of bank

Thermalling

Optimum angle of bank for strong and wide thermal

Shallow bank gives wide

radius of turn

Sink increases

rapidly above 50 degree of

bank

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

Positioning of a glider as a result of inaccurate flying

Half turn at 50kt and 40 degree angle of bank

Radius =80.5m

Half turn at 45kt and 45 degree angle of bankRadius=54.5m

• 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