On Sep 23, 6:26*pm, Martin Gregorie
wrote:
On Wed, 23 Sep 2009 15:00:04 +0000, Derek Copeland wrote:
I think this rather depends on the degree of instability in the air. On
relatively stable days the usual thermal sources often don't seem to
work. You need a large area to remain undisturbed for some time until
enough hot air is available to give a decent thermal when triggered.
Often things like towns and motorways don't work because there is too
much trigger activity going on and only tiny weak thermals will form
that go to no great height.

'Flapping' (as its called in FF jargon) only works in calm conditions
with weak lift. To be successful it requires an area of 'stuck-down' warm
air that can be broken loose by some vigorous milling about.

A few years ago at a WC we and the Russians, at adjacent poles, each had
one man still to fly in the last 5 mins of a round. We could feel a
thermal building and the thermal detectors showed the air temp was
building but it wasn't likely to go before the end of the round, so we
decided to try to break it loose and started flapping. The Russians saw
what we were doing and joined in. The thermal got broken loose and both
models climbed away in it just before the hooter at the end of the round.

Flapping is common at major Euro and World level events where the fliers
have retrieval teams available to flap, but that was the only time I've
seen or helped to get a thermal going before the model was launched. Its
more usual to flap under a model that's been launched before the bubble
has broken away and is coming down.

--
martin@ * | Martin Gregorie
gregorie. | Essex, UK
org * * * |

Shat is a thermal detector?

Bill Snead