8/4/2019 Speed to fly between thermals
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What speed should I fly between thermals? Derek Ruddock Southern
Cross Gliding Club, Sydney November 2009
The material below is the opinion of the author and no official
endorsement by Southern Cross Gliding, Club or its
Training Panel and instructors, the Gliding Federation of
Australia or anyone else is claimed or implied.
What is the optimum speed to fly between thermals?Suppose you
have a typical day with a cloudbase of 4000 feet, and 2 knot
thermals spaced every 8k
(five miles) apart. In the distance there is a big cloud that
will give 6 knot thermal just over 32k
away.
What speed should you fly to the big cloud, and should you top
up in the 2 knot thermals on the
way?
This can be determined mathematically and plotted in Excel. The
sink rate of the gliders at any
particular speed can be derived from the polar plot.
Take four pilots at the cloudbase in identical DG300s.
Pilot Yellow is a recent solo pilot and is cautious of getting
low. He flies at the minimum sink speed
of 43 knots, and tops up in every thermal along the way.Pilot
Blue is more confident than Yellow, and believes that the breatest
distance can be covered by
flying at the best glide speed of 51 knots. He also knows that
by the look of the large cloud in the
distance there is likely to be good lift under it, and decides
not to take the 2 knot climbs.
Pilot Green is again more experienced. He estimates that he will
get a good 6 knot climb under the
large cloud, but as the cloud is a long way off, cautiously sets
his MacCready ring to 2 knots, which
tells him to fly at 65 knots.
Pilot Red has been to a couple of camps, has a 300 under his
belt, and knows it all. He correctly
estimates that the big cloud will give a six knot climb, sets
his MacCready ring to 2/3rds of this and
sets off 75 knots
What happens to the gliders?
After 25 minutes, Pilot Yellow is at 3274 feet, having taken
every climb to cloudbase, however he
is only 24km down track, and the other gliders have disappeared
into the distance. The big thermal
is likely to have died in the 6 minutes it would take to cover
the remaining 8km.
Pilot Blue has done better. He has ignored the 2 knot climbs,
and connects with the 6 knot thermal
at 1500 feet, has climbed almost to cloudbase, and is back on
cruise at a distance of 33km after 25
minutes.
Pilot Green, cruising at 65 knots connected with the 6 knot
thermal after 16 minutes climbed to
cloudbase and is already out of sight 9 kilometres ahead of
Pilot Blue, 42km from the start.
Pilot Red arrived in the vicinity of the big cloud after only 14
minutes, but at a height of just over
400 feet far to low to connect with the lift, and landed in the
paddock below Pilot Blue and PilotGreen.
So what is to be learned from this? Pilot Red has correctly
estimated the strength of the thermal in
the distance, and has reduced the MacCready setting to 2/3 of
that value in accordance to what he
has been taught, however he made no attempt to slow down and
extend his range when getting low.
Pilot Green has the correct idea. By reducing the MacCready
setting, and hence his speed, he has
reduced his sink rate, and as a consequence has extended his
range considerably, enabling him to
cover the greatest distance. Pilot Blue has done well, but is a
long way behind Pilot Green, with no
chance of catching up. Pilot Yellow, by taking every thermal,
has stayed high, but has covered
hardly any distance at all and if flying cross country would be
likely to be outlanding a long way
from home as the day died.
