Help calculating Speed To Fly for headwind and tailwind

On May 29, 6:41*pm, Dave Nadler wrote:
On May 29, 12:22*pm, 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

A common thread in this discussion is the need to know the
ACTUAL average climb over an entire thermal. Otherwise
you're typically plugging too high a number into McCready
theory, which doesn't work.

For you SN10 pilots, this is why the instruments I've
designed prominently display TAv - Thermal Average...
Use it ! Don't use the 20 second averager peak...

For John, perhaps time to switch back ;-)
And if you don't, you can still use the SN10 in my
plane to plan your task on the ramp ;-)

Hope this helps,
Best Regards, Dave "YO electric"

And SeeYou has the VarA navbox (20 sec average) and the more useful
VarT (average since circling) navbox and on the statistics page shows
last four thermal averages, as well as rough graphical guide of the
entry and exit heights of those last four thermals (sometimes handy to
remind you what the climb was) and the average vario for all four last
thermals.

The stats pages also show global flight average vario, on task
average vario, and last 60 minute average vario. Sorry but I think
that beats the SN10 vario stats. But the stats pages don't show how
long pilots spend starting at all this stuff trying to work out what
is what :-)

I fly with a VarT navbox.

Darryl