On May 29, 9:22*am, John Cochrane
wrote:
Thoughts?

9B

Here's my 2 cents.

If you're racing, not maximizing glide over the ground, and if you're
far from a turnpoint -- meaning you will certainly have to thermal
before you get to the turnpoint -- then as everybody notes, the wind
speed is irrelevant.

That assumes that thermals drift with the wind, and are as easy to
core going upwind as downwind. Thermals actually drift a bit slower
than the wind, and are anchored to ground sources. That means that
going upwind is harder; you're effectively in a lower-performing
glider, so in fact you have to fly more cautiously. *I seem to have an
easier time centering when going downwind as well; that may be because
I hit the obvious core first rather than be seduced by the driblets
off downwind of the core. I also seem to stay in contact with streets
better going downwind. (In general, better performing gliders use
slightly higher Mc settings, because they are less likely to get in
trouble)

But back to theory which ignores all this stuff. The calculations in
"upwind/downwind" assume you're *near the turnpoint. Here you're
making the decision "do I climb at x before the turnpoint or do I
wait, round the turnpoint and climb at y?" *It's only valid if the
latter is an option before hitting the ground!

In any decent wind, it's surprising how much difference there is
between x and y. On the other hand, the graph quantifies common sense:
if you are in an 8 knot thermal and all the other thermals are 3
knots, take it even if it's upwind! *The rule of thumb about turning
upwind low isn't always right.

I bug the clearnav team to put these numbers in about once a week.
When you're above glideslope to the next turnpoint, it could show the
equivalent Mc "after the turn" to your current Mc. So far no luck, but
they may correctly perceive that there are about 3 of us who
understand and care about this number.

Many people make the mistake of thinking wind affects final glide. It
does not (except for the above meteorological considerations). There
does come a point, gliding in to the wind, that lowering your
MacCready setting actually results in a worse glide. You'll see that
-- you get low, turn down the Mc, and all of a sudden you're even
lower! ouch!

If that isn't enough, you need a thermal, and the thermal has to be
stronger than this minimum Mc setting. If you're going downwind, a
slightly negative Mc setting will result in a better glide. *I also
encourage my favorite insturment makers to not allow the Mc setting to
go below the value that gives the best glide over the ground, and
allow it to go slightly negative downwind. Again, I think they rightly
perceive this as unnecessary nerdiness.

In both cases, there really is no valid reason at all for cruising at
a lower Mc setting than the weakest (smooth, bottom to top average,
including all centering etc) thermal you'd take. Equivalently, if
you're cruising at Mc 2 and the weather gods grant you a smooth,
guaranteed 3 knot thermal, you're better off taking it and then
cruising at Mc 3 for a while. This is very hard to swallow, but it's
true.

John Cochrane

Okay I modelled this out just to see what the speed to fly
implications are - that is, what's the optimal STF going into an
upwind/downwind turnpoint depending on whether you thermal before or
after the turn and what kind of penalty do you pay flying off the
optimal speed.

First, it confirms John's theory on how picky you should be about
thermals and the consequences of going into a turnpoint low versus
high depending on the wind. If the there is strong wind it matters A
LOT how you do this. Just by way of a real world example - a couple of
years ago I stretched a glide into an upwind turnpoint on a contest
day when the winds were 30 knots on the nose, then caught a strong
thermal on the following downwind leg. The math shows that this cut my
total task time by more than 12 minutes! If you assume 20 miles of
glide into the turn, the time savings vary from 3 minutes for 15 knots
of wind up to 12 minutes for the aforementioned 30 knots of wind.

The other surprising thing is how much the optimal speed to fly varies
depending on whether you are planning to take one more thermal before
the turn or wait until after - the speed differences can be more than
20 knots, though the penalties for flying a bit too slow are only a
couple of mph.

Note to BB: when you did the theory for optimal MAT strategies did you
include the high/low turnpoint strategy for the up/downwind versus
crosswind courses?. If you assume even a few thousand feet of high/low
strategy I think the up/downwind approach may dominate the crosswind
one. I think it may also pay to up the number of legs to maximize the
number of times you can play the high/low turnpoint game per mile of
task. Try programming THAT into a ClearNav. :-)

9B