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_Fig。 22。 Stream lines along a plane。_
STREAM LINES ALONG A PLANE。In Fig。 22； A
is a flat plane； which we will assume is 10 feet
from the front to the rear margin。 For convenience
seven stream lines of air are shown；
which contact with this inclined surface。 The first
line 1； after the contact at the forward end； is
driven downwardly along the surface； so that it
forms what we might term a moving film。
The second air stream 2； strikes the first stream；
followed successively by the other streams； 3； 4；
and so on； each succeeding stream being compelled
to ride over； or along on the preceding mass of
cushioned air； the last lines； near the lower end；
being； therefore； at such angles； and contacting
with such a rapidly…moving column； that it produces
but little lift in comparison with the 1st；
2d and 3d stream lines。 These stream lines are
taken by imagining that the air approaches and
contacts with the plane only along the lines indicated
in the sketch； although they also in practice
are active against every part of the plane。
THE CENTER OF PRESSURE。In such a plane the
center of pressure is near its upper end； probably
near the line 3； so that the greater portion of the
lift is exerted by that part of the plane above
line 3。
AIR LINES ON THE UPPER SIDE OF THE PLANE。
Now； another factor must be considered； namely；
the effect produced on the upper side of the plane；
over which a rarefied area is formed at certain
points； and； in practice； this also produces； or
should be utilized to effect a lift。
RAREFIED AREA。What is called a rarefied area；
has reference to a state or condition of the atmosphere
which has less than the normal pressure or
quantity of air。 Thus； the pressure at sea level；
is about 14 3/4 per square inch
As we ascend the pressure grows less； and the
air is thus rarer； or； there is less of it。 This is a
condition which is normally found in the atmosphere。
Several things tend to make a rarefied
condition。 One is altitude； to which we have just
referred。
Then heat will expand air； making it less dense；
or lighter； so that it will move upwardly； to be
replaced by a colder body of air。 In aeronautics
neither of these conditions is of any importance
in considering the lifting power of aeroplane surfaces。
RAREFACTION PRODUCED BY MOTION。The third
rarefied condition is produced by motion； and generally
the area is very limited when brought about
by this means。 If； for instance； a plane is held
horizontally and allowed to fall toward the earth；
it will be retarded by two forces； namely； compression
and rarefaction； the former acting on the
under side of the plane； and the latter on the upper
side。
Of the two rarefaction is the most effectual；
and produces a greater effect than compression。
This may be proven by compressing air in a long
pipe； and noting the difference in gauge pressure
between the ends； and then using a suction pump
on the same pipe。
When a plane is forced through the air at any
angle； a rarefied area is formed on the side which
is opposite the one having the positive angle of
incidence。
If the plane can be so formed as to make a large
and effective area it will add greatly to the value
of the sustaining surface。
Unfortunately； the long fiat plane does not lend
any aid in this particular； as the stream line flows
down along the top； as shown in Fig。 23； without
being of any service。
_Fig。 23。 Air lines on the upper side of a Plane。_
THE CONCAVED PLANE。These considerations
led to the adoption of the concaved plane formation；
and for purposes of comparison the diagram；
Fig。 24； shows the plane B of the same length and
angle as the straight planes。
In examining the successive stream lines it will
be found that while the 1st； 2d and 3d lines have
a little less angle of impact than the corresponding
lines in the straight plane； the last lines； 5； 6
and 7； have much greater angles； so that only line
4 strikes the plane at the same angle。
Such a plane structure would； therefore； have
its center of pressure somewhere between the
lines 3 and 4； and the lift being thus； practically；
uniform over the surface； would be more effective。
THE CENTER OF PRESSURE。This is a term used
to indicate the place on the plane where the air
acts with the greatest force。 It has reference to
a point between the front and rear margins only
of the plane。
_Fig。 24。 Air lines below a concaved Plane。_
UTILIZING THE RAREFIED AREA。This structure；
however； has another important advantage； as it
utilizes the rarefied area which is produced； and
which may be understood by reference to Fig。 25。
The plane B； with its upward curve； and at the
same angle as the straight plane； has its lower
end so curved； with relation to the forward movement；
that the air； in rushing past the upper end；
cannot follow the curve rapidly enough to maintain
the same density along C； hence this exerts
an upward pull； due to the rarefied area； which
serves as a lifting force； as well as the compressed
mass beneath the plane。
CHANGING CENTER OF PRESSURE。The center of
pressure is not constant。 It changes with the
angle of the plane； but the range is considerably
less on a concave surface than on a flat plane。
_Fig。 25。 Air lines above a convex Plane。_
In a plane disposed at a small angle； A； as in
Fig。 26； the center of pressure is nearer the forward
end of the plane than with a greater positive
angle of incidence； as in Fig。 27； and when
the plane is in a normal flying angle； it is at the
center； or at a point midway between the margins。
PLANE MONSTROSITIES。Growing out of the idea
that the wing in nature must be faithfully copied；
it is believed by many that a plane with a
pronounced thickness at its forward margin is one
of the secrets of bird flight。
Accordingly certain inventors have designed
types of wings which are shown in Figs。 28 and
29。
_Fig。 28 Changing centers of Pressures。_
_Fig 29。 Bird…wing structures。_
Both of these types have pronounced bulges；
designed to 〃split〃 the air； forgetting； apparently；
that in other parts of the machine every effort is
made to prevent head resistance。
THE BIRD WING STRUCTURE。The advocates of
such construction maintain that the forward edge
of the plane must forcibly drive the air column
apart； because the bird wing is so made； and that
while it may not appear exactly logical； still there
is something about it which seems to do the work；
and for that reason it is largely adopted。
WHY THE BIRD'S WING HAS A PRONOUNCED
BULGE。Let us examine this claim。 The bone
which supports the entire wing surface； called the
（pectoral）； has a heavy duty to perform。 It is so
constructed that it must withstand an extraordinary
torsional strain； being located at the forward
portion of the wing