Gary Osoba <<wosoba>>
writes:

Here is an excellent paper that John Cochrane has written about
the value of course deviations. I did a similar analysis some years back, and
presented parts of it in lectures at SSA Conventions and ESA gatherings during
the late 90's and early 2000's. These featured themes and phrases such as "When
Slower is Faster" and "The Shortest 'Distance' Between Two Points is
Convoluted." Not implying curved space/time and speaking metaphorically about
"Distance," of course.

Whereas MacCready theory as normally applied matches cruising speeds for
straight glides in response to variable air masses, the approaches John’s paper
explores introduce what is often a more useful variable, that of the varied
course line.

The other grand regime for piloting strategy in a discontinuous atmosphere is
dynamic maneuvering - this is the area I have been working in. This places the
focus on when, how quickly, how frequently and in what manner to make changes
from one cruising speed or another and one heading to another, optimized to
harvest energy or minimize its losses. This strategy, for example, could allow
for counterintuitive results when applied to Graph 1 of the paper with Pilot B
arriving at his destination much more quickly than Pilot A when dynamic
maneuvering is properly applied. This assumes that both pilots had a *reason* to
make deviations where they did, according to the paper's impetus.

John has a writing style that is clear, concise, and cogent. He focuses on the
key relationships in decision-making. The quantitative conclusions will shift
due to lower performance in hang gliders, but the trigonometric analysis of
polar graphs with an appropriate hang glider polar will obviously hold true.
This is an excellent paper on the subject.

http://ozreport.com/docs/deviations_I.pdf

http://soaringcafe.com/2011/10/deviations-part-i/

John H. Cochrane writes:

I’ve been torn in my efforts to fly faster lately. For a few
years, I’ve been working hard to learn to follow “energy lines,” and especially
to stay out of the rivers of sink that seem to ruin too many race days. I’ve
been trying to emulate my buddy Herb Kilian at the Chicago Glider Club, who is
regularly wipes us all out of the sky by floating along in lift while I bore a
hole in the sky. But then came Ephrata, and a long set of conversations with
Bill Elliot. He’s been trying to go as straight as possible to minimize course
deviations, and obviously that’s working great for him. Maybe I’m wandering too
far around the sky? Who is right? Now that flying season has been replaced by
goof-off-at-the-office season, it’s time to think it through. Here are some
principles I’ve come up with:

1. Degrees matter, not miles. Many pilots measure their deviations by how many
miles they have gone off course. This is a mistake. The only thing that matters
is how many degrees you go off course.

2. Small deviations are cheap. Once past 45 degrees though, the costs start to
spiral up dramatically. Going more than 45 degrees off course is really costly.

Small deviations are nearly costless. The lines each start at the thermal the
pilot will find straight ahead, but they start moving horizontally to the right.
It’s worth making small — 10-20 degree course deviations for even very slightly
better thermals.

Make smaller deviations in stronger weather. If you can find a 5 knot thermal
straight ahead, you need to know there is a 10 knot thermal to make a 30 degree
course correction worthwhile! Bill is right at Ephrata, where thermals were
strong but isolated.

Make big course deviations in weaker weather. Herb is right in Chicago. Here it
is typical to find two knots straight ahead, but 3-4 knots if you go chasing
clouds. That means up to 30-40 degree course deviations are ok.

Make really big course deviations in very weak weather. If you’re going to end
up climbing at 1 knot, it’s worth really going anywhere to avoid that fate. Even
50 degrees off course for a 2 knot thermal is justifiable.

This is part of the general point: Avoid weak lift. It’s much better to climb at
3 knots always than to have half your thermals at 1 knot, and half your thermals
at 5 knots. Minutes per foot matter, not feet per minute.

Don’t go past the wall. Each line explodes vertically off the chart at some
point. If you fly 4 sufficiently far off course, you go slower than flying
straight and taking a 2 knot thermal, even if you can find an infinitely strong
thermal off course.