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165
The Aeroplane Speaks. Fifth Edition
·
H. Barber
24
"I
have
tried
to
make
that
as
clear
as
possible,"
he
said.
"It
may
appear
a
bit
complicated
at
first,
but
if
you
will
take
the
trouble
to
look
at
it
for
a
minute
you
will
find
it
quite
simple.
A
is
the
normal
and
proper
direction
of
motion
of
the
Aeroplane,
but,
owing
to
a
gust
of
air,
it
takes
up
the
new
nose-down
position.
Owing
to
Momentum,
however,
it
does
not
fly
straight
along
in
that
direction,
but
moves
more
or
less
in
the
direction
B,
which
is
the
resultant
of
the
two
forces,
Momentum
and
Thrust.
And
so
you
will
note
that
the
Angle
of
Incidence,
which
is
the
inclination
of
the
Surfaces
to
the
Direction
of
Motion,
has
decreased,
and
of
course
the
Lift
decreases
with
it.
You
will
also
see,
and
this
is
the
point,
that
the
Tail
Surface
has
lost
a
higher
proportion
of
its
Angle,
and
consequently
its
Lift,
than
has
the
Main
Surface.
Then,
such
being
the
case,
the
Tail
must
fall
and
the
Aeroplane
assume
its
normal
position
again,
though
probably
at
a
slightly
lower
altitude."
"I'm
afraid
I'm
very
stupid,"
said
Efficiency,
"but
please
tell
me
why
you
lay
stress
upon
the
words
'in
effect.'"
"Ah!
I
was
wondering
if
you
would
spot
that,"
he
replied.
"And
there
is
a
very
good
reason
for
it.
You
see,
in
some
Aeroplanes
the
Tail
Surface
may
be
actually
set
at
the
same
Angle
on
the
machine
as
the
Main
Surface,
but
owing
to
the
air
being
deflected
downwards
by
the
front
Main
Surface
it
meets
the
Tail
Surface
at
a
lesser
angle,
and
indeed
in
some
cases
at
no
angle
at
all.
The
Tail
is
then
for
its
surface
getting
less
Lift
than
the
Main
Surface,
although
set
at
the
same
angle
on
the
machine.
It
may
then
be
said
to
have
in
effect
a
less
Angle
of
Incidence.
I'll
just
show
you
on
the
Blackboard."
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