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165
The Aeroplane Speaks. Fifth Edition
·
H. Barber
83
It
is
therefore
necessary
to
find
a
means
of
stabilizing
the
naturally
unstable
cambered
surface.
This
is
usually
secured
by
means
of
a
stabilizing
surface
fixed
some
distance
in
the
rear
of
the
main
surface,
and
it
is
a
necessary
condition
that
the
neutral
lift
lines
of
the
two
surfaces,
when
projected
to
meet
each
other,
make
a
dihedral
angle.
In
other
words,
the
rear
stabilizing
surface
must
have
a
lesser
angle
of
incidence
than
the
main
surface--certainly
not
more
than
one-third
of
that
of
the
main
surface.
This
is
known
as
the
longitudinal
dihedral.
I
may
add
that
the
tail-plane
is
sometimes
mounted
upon
the
aeroplane
at
the
same
angle
as
the
main
surface,
but,
in
such
cases,
it
attacks
air
which
has
received
a
downward
deflection
from
the
main
surface,
thus:
The
angle
at
which
the
tail
surface
attacks
the
air
(the
angle
of
incidence)
is
therefore
less
than
the
angle
of
incidence
of
the
main
surface.
I
will
now,
by
means
of
the
following
illustration,
try
to
explain
how
the
longitudinal
dihedral
secures
stability:
First,
imagine
the
aeroplane
travelling
in
the
direction
of
motion,
which
coincides
with
the
direction
of
thrust
T.
The
weight
is,
of
course,
balanced
about
a
C.P.,
the
resultant
of
the
C.P.
of
the
main
surface
and
the
C.P.
of
the
stabilizing
surface.
For
the
sake
of
illustration,
the
stabilizing
surface
has
been
given
an
angle
of
incidence,
and
therefore
has
a
lift
and
C.P.
In
practice
the
stabilizer
is
often
set
at
no
angle
of
incidence.
In
such
case
the
proposition
remains
the
same,
but
it
is,
perhaps,
a
little
easier
to
illustrate
it
as
above.
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