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165
The Aeroplane Speaks. Fifth Edition
·
H. Barber
23
"That's
easy,"
said
Grandfather
Gravity.
"I'm
so
heavy
that
if
I
am
too
low
down
I
act
like
a
pendulum
and
cause
the
Aeroplane
to
roll
about
sideways,
and
if
I
am
too
high
I'm
like
a
stick
balanced
on
your
finger,
and
then
if
I'm
disturbed,
over
I
go
and
the
Aeroplane
with
me;
and,
in
addition
to
that,
there
are
the
tricks
I
play
with
the
Aeroplane
when
it's
banked
up,[6]
i.e.,
tilted
sideways
for
a
turn,
and
Centrifugal
Force
sets
me
going
the
way
I'm
not
wanted
to
go.
No;
I
get
on
best
with
Lateral
Stability
when
my
Centre
is
right
on
the
centre
of
drift,
or,
at
any
rate,
not
much
below
it."
And
with
that
he
settled
back
into
the
Lecturer's
Chair
and
went
sound
asleep
again,
for
he
was
so
very,
very
old,
in
fact
the
father
of
all
the
Principles.
And
the
Blackboard
had
been
busy,
and
now
showed
them
a
picture
of
the
Aeroplane
as
far
as
they
knew
it,
and
you
will
see
that
there
is
a
slight
Dihedral
Angle,
and
also,
fixed
to
the
tail,
a
vertical
Keel
Surface
or
fin,
as
is
very
often
the
case
in
order
to
ensure
the
greater
effect
of
such
surface
being
behind
the
vertical
turning
axis.
But
Efficiency,
growing
rather
critical
with
her
newly
gained
knowledge,
cried
out:
"But
where's
the
horizontal
Tail
Surface?
It
doesn't
look
right
like
that!"
"This
is
when
I
have
the
pleasure
of
meeting
you,
my
dear,"
said
Longitudinal
Stability.
"Here's
the
Tail
Surface,"
he
said,
"and
in
order
to
help
me
it
must
be
set
in
effect
at
a
much
less
Angle
of
Incidence
than
the
Main
Surface.
To
explain
we
must
trouble
the
Blackboard
again,"
and
this
was
his
effort:
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