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Strictly speaking, both versions are correct depending
on your perspective... :-)
If
you're measuring the velocity relative to the pilot, i.e. the ground speed
of the
model,
then version 1 is correct. However if you're measuring the air-speed
of the
model
then version 2 is correct.
In
reality the model doesn't care about the ground unless it hits it,
while it has
to fly through the air all the
time and hence it's behaviour is totally
determined
by
it's airspeed. Therefore version 2 is a
far better description of how DS works:
the
aircraft is actually loosing air-speed in both the top and
bottom turns, and
gaining air-speed as it transits the boundary zone
in each direction.
What
causes the confusion is that most fliers tend to judge the speed
of their
models
relative to their fixed position, and from this perspective the
model
seems to really accelerate around the top turn.
In fact this is simply another
instance of the infamous 'down-wind turn' myth -
although the model's speed
relative to the ground-based pilot has increased
(because it's now flying with
the
wind rather than into it), the model's actual airspeed hasn't
changed.
It's
easy to see that it's not the top turn adding the energy by cutting it
out
of the
circuit altogether - start on the front side, drop into the back side,
turn
and
exit straight back out to the front side. If the site is DS'able and you
get
it
right the model will end up out the front higher and/or faster than it
started.
You
probably already do this to find the best spot when
feeling out a new DS
site
or testing the conditions.
Ciao -
Chris
******
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