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165
The Aeroplane Speaks. Fifth Edition
·
H. Barber
82
Now,
should
some
gust
or
eddy
tend
to
make
the
surface
decrease
the
angle,
i.e.,
dive,
then
the
C.P.
moves
forward
and
pushes
the
front
of
the
surface
up.
Should
the
surface
tend
to
assume
too
large
an
angle,
then
the
reverse
happens--the
C.P.
moves
back
and
pushes
the
rear
of
the
surface
up.
Flat
surfaces
are,
then,
theoretically
stable
longitudinally.
They
are
not,
however,
used,
on
account
of
their
poor
lift-drift
ratio.
As
already
explained,
cambered
surfaces
are
used,
and
these
are
longitudinally
unstable
at
those
angles
of
incidence
producing
a
reasonable
lift-drift
ratio,
i.e.,
at
angles
below
about
12°.
A
is
a
cambered
surface,
attitude
approximately
vertical,
moving
through
the
air
in
the
direction
M.
Obviously
the
C.P.
coincides
with
the
transverse
centre
line
of
the
surface.
With
decreasing
angles,
down
to
angles
of
about
30°,
the
C.P.
moves
forward
as
in
the
case
of
flat
surfaces
(see
B);
but
angles
above
30°
do
not
interest
us,
since
they
produce
a
very
low
ratio
of
lift
to
drift.
Below
angles
of
about
30°
(see
C)
the
dipping
front
part
of
the
surface
assumes
a
negative
angle
of
incidence
resulting
in
the
downward
air
pressure
D,
and
the
more
the
angle
of
incidence
is
decreased,
the
greater
such
negative
angle
and
its
resultant
pressure
D.
Since
the
C.P.
is
the
resultant
of
all
the
air
forces,
its
position
is
naturally
affected
by
D,
which
causes
it
to
move
backwards.
Now,
should
some
gust
or
eddy
tend
to
make
the
surface
decrease
its
angle
of
incidence,
i.e.,
dive,
then
the
C.P.
moves
backwards,
and,
pushing
up
the
rear
of
the
surface,
causes
it
to
dive
the
more.
Should
the
surface
tend
to
assume
too
large
an
angle,
then
the
reverse
happens;
the
pressure
D
decreases,
with
the
result
that
C.P.
moves
forward
and
pushes
up
the
front
of
the
surface,
thus
increasing
the
angle
still
further,
the
final
result
being
a
"tail-slide."
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