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165
The Aeroplane Speaks. Fifth Edition
·
H. Barber
68
Above
is
illustrated
the
flow
of
air
round
two
objects
moving
in
the
direction
of
the
arrow
M.
In
the
case
of
A,
you
will
note
that
the
rarefied
area
DD
is
of
very
considerable
extent;
whereas
in
the
case
of
B,
the
air
flows
round
it
in
such
a
way
as
to
meet
very
closely
to
the
rear
of
the
object,
thus
decreasing
DD.
The
greater
the
rarefied
area
DD,
then,
the
less
the
density,
and,
consequently,
the
less
the
pressure
of
air
upon
the
rear
of
the
object.
The
less
such
pressure,
then,
the
better
is
head-resistance
D
able
to
get
its
work
in,
and
the
more
thrust
will
be
required
to
overcome
it.
The
"fineness"
of
the
stream-line
shape,
i.e.,
the
proportion
of
length
to
width,
is
determined
by
the
velocity--the
greater
the
velocity,
the
greater
the
fineness.
The
best
degree
of
fineness
for
any
given
velocity
is
found
by
means
of
wind-tunnel
research.
The
practical
application
of
all
this
is,
from
a
rigging
point
of
view,
the
importance
of
adjusting
all
stream-line
parts
to
be
dead-on
in
the
line
of
flight,
but
more
of
that
later
on.
2.
Angle
of
Incidence.--The
most
efficient
angle
of
incidence
varies
with
the
thrust
at
the
disposal
of
the
designer,
the
weight
to
be
carried,
and
the
climb-velocity
ratio
desired.
The
best
angles
of
incidence
for
these
varying
factors
are
found
by
means
of
wind-tunnel
research
and
practical
trial
and
error.
Generally
speaking,
the
greater
the
velocity
the
smaller
should
be
the
angle
of
incidence,
in
order
to
preserve
a
clean,
stream-line
shape
of
rarefied
area
and
freedom
from
eddies.
Should
the
angle
be
too
great
for
the
velocity,
then
the
rarefied
area
over
the
top
of
the
surface
becomes
of
irregular
shape
with
attendant
turbulent
eddies.
Such
eddies
possess
no
lift
value,
and
since
it
has
taken
power
to
produce
them,
they
represent
drift
and
adversely
affect
the
lift-drift
ratio.
Also,
too
great
an
angle
for
the
velocity
will
result
in
the
underside
of
the
surface
tending
to
compress
the
air
against
which
it
is
driven
rather
than
accelerate
it
downwards,
and
that
will
tend
to
produce
drift
rather
than
the
upwards
reaction,
or
lift.
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