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165
The Aeroplane Speaks. Fifth Edition
·
H. Barber
65
The
surface
acts
upon
the
air
in
the
following
manner:
As
the
bottom
of
the
surface
meets
the
air,
it
compresses
it
and
accelerates
it
downwards.
As
a
result
of
this
definite
action
there
is,
of
course,
an
equal
and
opposite
reaction
upwards.
The
top
surface,
in
moving
forward,
tends
to
leave
the
air
behind
it,
thus
creating
a
semi-vacuum
or
rarefied
area
over
the
top
of
the
surface.
Consequently
the
pressure
of
air
on
the
top
of
the
surface
is
decreased,
thus
assisting
the
reaction
below
to
lift
the
surface
upwards.
The
reaction
increases
approximately
as
the
square
of
the
velocity.
It
is
the
result
of
(1)
the
mass
of
air
engaged,
and
(2)
the
velocity
and
consequent
force
with
which
the
surface
engages
the
air.
If
the
reaction
was
produced
by
only
one
of
those
factors
it
would
increase
in
direct
proportion
to
the
velocity,
but,
since
it
is
the
product
of
both
factors,
it
increases
as
V^2.
Approximately
three-fifths
of
the
reaction
is
due
to
the
decrease
of
density
(and
consequent
decrease
of
downward
pressure)
on
the
top
of
the
surface;
and
only
some
two-fifths
is
due
to
the
upward
reaction
secured
by
the
action
of
the
bottom
surface
upon
the
air.
A
practical
point
in
respect
of
this
is
that,
in
the
event
of
the
fabric
covering
the
surface
getting
into
bad
condition,
it
is
more
likely
to
strip
off
the
top
than
off
the
bottom.
The
direction
of
the
reaction
is,
at
efficient
angles
of
incidence,
approximately
at
right-angles
to
the
neutral
lift
line
of
the
surface,
as
illustrated
above;
and
it
is,
in
considering
flight,
convenient
to
divide
it
into
two
component
parts
or
values,
thus:
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