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165
The Aeroplane Speaks. Fifth Edition
·
H. Barber
73
THE
MARGIN
OF
POWER
is
the
power
available
above
that
necessary
to
maintain
horizontal
flight.
THE
MARGIN
OF
LIFT
is
the
height
an
aeroplane
can
gain
in
a
given
time
and
starting
from
a
given
altitude.
As
an
example,
thus:
1,000
feet
the
first
minute,
and
starting
from
an
altitude
of
500
feet
above
sea-level.
The
margin
of
lift
decreases
with
altitude,
owing
to
the
decrease
in
the
density
of
the
air,
which
adversely
affects
the
engine.
Provided
the
engine
maintained
its
impulse
with
altitude,
then,
if
we
ignore
the
problem
of
the
propeller,
which
I
will
go
into
later
on,
the
margin
of
lift
would
not
disappear.
Moreover,
greater
velocity
for
a
given
power
would
be
secured
at
a
greater
altitude,
owing
to
the
decreased
density
of
air
to
be
overcome.
After
reading
that
you
may
like
to
light
your
pipe
and
indulge
in
dreams
of
the
wonderful
possibilities
which
may
become
realities
if
some
brilliant
genius
shows
us
some
day
how
to
secure
a
constant
power
with
increasing
altitude.
I
am
afraid,
however,
that
will
always
remain
impossible;
but
it
is
probable
that
some
very
interesting
steps
may
be
taken
in
that
direction.
THE
MINIMUM
ANGLE
OF
INCIDENCE
is
the
smallest
angle
at
which,
for
a
given
power,
surface
(including
detrimental
surface),
and
weight,
horizontal
flight
can
be
maintained.
THE
MAXIMUM
ANGLE
OF
INCIDENCE
is
the
greatest
angle
at
which,
for
a
given
power,
surface
(including
detrimental
surface),
and
weight,
horizontal
flight
can
be
maintained.
THE
OPTIMUM
ANGLE
OF
INCIDENCE
is
the
angle
at
which
the
lift-drift
ratio
is
highest.
In
modern
aeroplanes
it
is
that
angle
of
incidence
possessed
by
the
surface
when
the
axis
of
the
propeller
is
horizontal.
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