Andy Blackburn wrote:
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
I would have thought the more significant factor is that
the time needed for a 180 deg change of heading is much
longer at a higher speed and you'd be travelling around
a longer distance as well, losing valuable seconds.
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.
It is not immediately, or intuitively clear to me whether
it changes or not. A key issue would be to minimise the time
spent in the sink area.
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).
Hmmm. If I fly one mile out and one mile back I make
a total of two miles. I donīt follow the bit about
the mysterious "cylinder" that makes it 4, but it
sounds like a good idea to avoid it
)
Anyways, what I meant to imply in the question, and
which I was perhaps not sufficiently clear about,
was not that you go out to that thermal to stop
and climb in it (you might want to of course if
it was strong enough and depending on wind as well,
but that was not part of the question) but merely
in order to perform the pullup and 180 deg turn
and subsequent acceleration in rising air rather
than in sink.
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.)
Well, I think it is not only about making up for the
height loss. There is also the time spent going past
the turnpoint, and returning to it. All that time will
be "wasted" in the sense that your progress on the
task is zero during that time.
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
Naturally. This was just a theoretical question about
TP-rounding technique as such. All sorts of other
factors and circumstances will of course be part of
the decision making in a real situation.
As somebody pointed out, a prevailing wind would of
course complicate the issue further as well.
Thanks for the answer.
CV