🚀 go-pugleaf

During the holidays, I read Dave Thornburg's book on soaring, so I thought I'd
pass on my impressions.

I really liked the first part of the book, which concentrates mostly on thermal
soaring. There are lots of ideas that make you think, and the humourous writing
style is very entertaining. You really share the author's enthusiasm and wonder
as he talks about "the perfect thermal", and gains insights from his friend the
old buzzard. I especially liked the fact that a lot of time was spent on
studying the air from the ground - "feeling" what it is doing, in the chapters
"The River of Air" and "Stalking the Invisible Thermal".  Dave's free-flight
background helped here, and is something that pure RC'ers can learn from.  The
contest tips were also interesting to me, since (never having flown a contest)
I had not thought of things like using a 180 degree landing pattern.

After a chapter on flyaways, Dave spends two chapters on slope soaring which
are good for the rank beginner, but offer little for anybody with some slope
time under their belts. At one point he falls into the classic trap of the
"downwind turn" when he warns people about downwind turns on the slope. His
advice is good, but his explanation is off. He says "The second you turn
downwind, the nose drops...why is a downwind turn on the slope any different
from a downwind turn when you're thermal flying? Answer: it isn't. Any time you
turn downwind, you model has to accelerate (relative to the earth) in order to
catch up with the river of air and regain a comfortable air speed." What he
SHOULD have said is that models don't feel the wind, only changes in the wind
such as gusts and gradients. On a slope, there is a remarkable gradient set up
by the venturi effect. The wind speed increases over the lip of the hill. If
you fly toward it by initiating a turn into the hill, you are taking chances
since the increasing wind speed will feel like a tailwind to the plane and it
will temporarily lose airspeed, making the turn sluggish.

In Part Two: DESIGN, Dave gets more into the nitty gritty of sailplane design.
In fact, he gets in over his head. After really enjoying the first part of the
book the second part brought me "back to earth", so to speak. He starts off by
saying that model aircraft aerodynamics texts (such as Martin Simons' book) are
good references, but that they are just that - references to be used like a
dictionary, only when you get stuck. He should have stayed strictly with this
idea and kept to rule-of-thumb design, but he unfortunately gets into
explanations that are better left to the aforementioned texts. This is
especially true when he talks about stabilizer design, pitch stability, and
center of gravity location - he totally mixes up wing center of pressure, wing
center of lift, and airplane neutral point, combining these into a single
beastie he most often refers to as "center of lift envelope". They are three
totally separate and important ideas, but to Dave they are the same.

There are some good things in part two of the book, but at least for me, the
author had already lost his credibility in aerodynamic matters.  It was no
surprise to me then that I again read the words "...all those little molecules
that go over the top have to travel farther than the ones that go underneath.
This means that they have to move faster than the molecules below in order to
get to the same meeting-point (the trailing edge) at the same time. All this
extra speed lowers the air pressure on the wing's top surface, sucking the
whole wing upwards. Behold: lift!"

I hope those who read the second part of the book come away with the INTENTIONS
of the author, and not his exact words...

Cheers!
Max
--
Max Feil  mkfeil@bnr.ca | Disclaimer:
Bell-Northern Research  |   What do I know? I'm just a Nerd on the Big Ranch.
P.O Box 3511 Station C, |
Ottawa, Ontario, Canada.| "Enrich The Soil, Not EveryBody's Goal" Peter Gabriel