Inter-thermal Speed To Fly

On Jan 5, 8:56*am, T8 wrote:
On Jan 5, 8:51*am, "Ken Kochanski (KK)"
wrote:



Morning,

I'm reviewing some STF topics to correct some fallacies that seem to
have crept into my understanding of STF theory and application ... I
though I knew this stuff ... or maybe I forgot ... I am a senior now,
you know ... :-))

So, if I have MC set at 3 ... and I am crusing XC to the next three
knotter ... should I slow in lift and speed up in sink ... *or will I
have a faster average speed if I just hold speed steady ... i.e. at
the velocity appropriate for MC = 3 in still air. I see pilots doing
both ...

Also, do all flight computers compute inter-thermal STF with the
formula that does not include a wind component - as identified in
Reichmann's texts, for example.

Anybody have an excel program that will plot polars ... including the
tangent to the shifted origin you get when *when you change airmass
sink ... or tail/head winds ..

Gracias, Happy new Year ...
KK

Heretic alert... :-)

My suggestion: understand the theoretical points that Reichmann makes,
then throw that damned book away. *But excel spreadsheets are fun to
play with on the laptop in front of the wood stove in January, so
don't let me dissuade you there.

My semi-obvious observations, shared with many others:

1. *Achieved XC speed vs cruise speed for all of these speed to fly
models goes through a very broad optimum.
2. *The models all ignore transient losses -- your glider is optimized
for 1.0 gee flight
3. *Slower than "optimum" cruise speed enhances range, gives better
chance of finding really good thermal for next climb, often results in
higher XC speed.
4. *The vario only tells you about where you've been.

Better approach -- my $0.02 -- choose your speed based on what you
anticipate encountering in the next 60 seconds. *Fly smooth (IIRC, you
already do). *I think of it as STF theory with the sharp edges
polished off. *So what if you are "wrong" a lot of the time. *See how
you do next to the guy that is chasing needles. *And it's more
enjoyable flying this way, too. *Basically, I'm providing
justification here for the way a lot of us already fly, consciously or
not.

Arrrrgh. *January sucks.

But happy new year just the same.

-Evan Ludeman / T8

You should read some of John Cochrane's analyses on the subject,
especially "Just a little faster, please"

Classic McCready theory is just fine for optimizing speed provided
that the next thermal strength is at least as good as the number you
have dialed in on your ring/computer and that you actually find a
thermal. However, it doesn't make any allowance for the chances of
finding a thermal.

The probability of finding a thermal depends on how far you can fly
and the closer you are to the ground, the smaller this distance is.
Many pilots use a more aggressive McCready setting when high and dial
it back as they get closer to the ground to increase range.

On the question of speed variations on encountering lift and sink
between thermals, you will find all sorts of different practices. I
once flew in the back seat with a good cross-country pilot who
subscribed to the very aggressive "push in sink and zoom in lift" camp
and I was puking in minutes! Not many fly like that any more, as it's
generally agreed that the aerodynamic losses of accelerating and
decelerating outweigh any small gains. I fly at a more-or-less
constant speed, but speed up gently in long runs of sink and slow down
gently in lift - pretty much what Tuno describes.

if the wind speed is constant throughout the airmass you are flying
in, it has little bearing on your overall speed, but will affect your
final glide.

Mike