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165
The Aeroplane Speaks. Fifth Edition
·
H. Barber
80
DIRECTIONAL
STABILITY
is
its
stability
about
its
vertical
axis,
and
without
which
it
would
have
no
tendency
to
keep
its
course.
For
such
directional
stability
to
exist
there
must
be,
in
effect,[16]
more
"keel-surface"
behind
the
vertical
axis
than
there
is
in
front
of
it.
By
keel-surface
I
mean
everything
to
be
seen
when
looking
at
an
aeroplane
from
the
side
of
it--the
sides
of
the
body,
undercarriage,
struts,
wires,
etc.
The
same
thing
applies
to
a
weathercock.
You
know
what
would
happen
if
there
was
insufficient
keel-surface
behind
the
vertical
axis
upon
which
it
is
pivoted.
It
would
turn
off
its
proper
course,
which
is
opposite
to
the
direction
of
the
wind.
It
is
very
much
the
same
in
the
case
of
an
aeroplane.
The
above
illustration
represents
an
aeroplane
(directionally
stable)
flying
along
the
course
B.
A
gust
striking
it
as
indicated
acts
upon
the
greater
proportion
of
keel-surface
behind
the
turning
axis
and
throws
it
into
the
new
course.
It
does
not,
however,
travel
along
the
new
course,
owing
to
its
momentum
in
the
direction
B.
It
travels,
as
long
as
such
momentum
lasts,
in
a
direction
which
is
the
resultant
of
the
two
forces
Thrust
and
Momentum.
But
the
centre
line
of
the
aeroplane
is
pointing
in
the
direction
of
the
new
course.
Therefore
its
attitude,
relative
to
the
direction
of
motion,
is
more
or
less
sideways,
and
it
consequently
receives
an
air
pressure
in
the
direction
C.
Such
pressure,
acting
upon
the
keel-surface,
presses
the
tail
back
towards
its
first
position
in
which
the
aeroplane
is
upon
its
course
B.
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