Rounding a turnpoint in sink

I simulated this last year. The theoretical answer
between a and b in still air is to pull up and make
a tighter turn at thermalling speed. Making the turn
at cruise speed loses more altitude due to the higher
induced drag from turning at higher speed, which is
worse than the losses associated with a 1G pullup and
a lower energy turn. I recall the calculated difference
is about 100', so it's not a huge deal. I don't think
the answer changes with airmass movement.

With respect to option d, generally it's a good idea
to avoid voluntarily flying in sink. It's only worth
it if you think the savings in climb time will exceed
the additional 4 miles in higher sink (going from the
near side of the 1-mile cylinder to the far side and
back again).

For example, if you are getting 500 fpm in sink, at
90 knots over 4 miles it will cost you about 1200 feet.
If you anticipate a climb at 8 knots instead of 5 knots
then you'd need to climb about 3200 feet to make up
the time loss from having to climb the additional 1200
feet (breakeven math is: 3200ft/800fpm = 4 min. and
2000ft/500fpm = 4 min.)

This assumes that you don't need extra altitude to
avoid getting low on your egress from the sink area
and back to the last thermal you took. It also doesn't
take into consideration different McCready speeds that
might narrow the gap a bit. Also you need to consider
some probabilities: The probability that the sink will
continue and the probability that the thermal on the
far side will be that much better.

9B

At 14:30 21 July 2004, Mark Navarre wrote:

From: CV e
What is the most effective technique for making the
turn ?

a. Tight turn at the current speed (90 kts) and carry
on according to McCready.
b. Slowing down to thermalling speed, turning tight
and
accelerating back to 90 kts. This would end up
being
a kind of chandelle-like maneuver since you'd be
pulling up sharply, turning while still in the
climb,
and diving in quick succession.
c. A half loop with a roll on top. OK, I suspect this
might be theoretically best, but let's suppose
you
rule this out due to a load of loose objects in
the
cockpit: maps, pen, camera, sandwiches, drinking
water etc. and you are not really into advanced
aerobatics anyway.
d. Carrying on for half a mile (or a mile ?) past the
turnpoint where a cloud is building and you expect
to find lift to turn in.
e. Anything else.

Any manuvering as in b or c will cost more than a plain
efficient turn, option
a would be fine if there is a crosswind to the course,
turn into the wind for
the shortest ground track around the turn. D is the
best option for two
reasons: you know you have to fly back out through
sink, so tanking up is good,
also if the wind is generally along the outbound course,
you will be drifting
toward the next turn while climbing. This is based
on the old logic of
arriving low at downwind turns and high at upwind turns.
Another tactical
point: flying into the turnpoint on the outbound leg
when the wind is a
headwind going in to the turn may mark lift to be used
after rounding the turn.
E, anything else. Why were you flying so long in sink?
Altering course to
left or right may have put you in better air.





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Mark Navarre
2/5 black ace
LoCal, USA
remove brain to reply
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