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165
The Aeroplane Speaks. Fifth Edition
·
H. Barber
70
The
best
cambers
for
varying
velocities,
angles
of
incidence,
and
thickness
of
surface,
are
found
by
means
of
wind-tunnel
research.
The
practical
application
of
all
this
is
in
taking
the
greatest
care
to
prevent
the
surface
from
becoming
distorted
and
thus
spoiling
the
camber
and
consequently
the
lift-drift
ratio.
4.
Aspect
Ratio.--This
is
the
proportion
of
span
to
chord.
Thus,
if
the
span
is,
for
instance,
50
feet
and
the
chord
5
feet,
the
surface
would
be
said
to
have
an
aspect
ratio
of
10
to
1.
For
a
given
velocity
and
a
given
area
of
surface,
the
higher
the
aspect
ratio,
the
greater
the
reaction.
It
is
obvious,
I
think,
that
the
greater
the
span,
the
greater
the
mass
of
undisturbed
air
engaged,
and,
as
already
explained,
the
reaction
is
partly
the
result
of
the
mass
of
air
engaged.
I
say
"undisturbed"
advisedly,
for
otherwise
it
might
be
argued
that,
whatever
the
shape
of
the
surface,
the
same
mass
of
air
would
be
engaged.
The
word
"undisturbed"
makes
all
the
difference,
for
it
must
be
remembered
that
the
rear
part
of
the
underside
of
the
surface
engages
air
most
of
which
has
been
deflected
downwards
by
the
surface
in
front
of
it.
That
being
so,
the
rear
part
of
the
surface
has
not
the
same
opportunity
of
forcing;
the
air
downwards
(since
it
is
already
flowing
downwards)
and
securing
there
from
an
upwards,
reaction
as
has
the
surface
in
front
of
it.
It
is
therefore
of
less
value
for
its
area
than
the
front
part
of
the
surface,
since
it
does
less
work
and
secures
less
reaction--i.e.,
lift.
Again,
the
rarefied
area
over
the
top
of
the
surface
is
most
rare
towards
the
front
of
it,
as,
owing
to
eddies,
the
rear
of
such
area
tends
to
become
denser.
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