aeroplanes-第8节
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of going into a 〃hole。〃
RESPONSIBILITY FOR ACCIDENTS。These so…called
〃holes〃 are responsible for many accidents。 The
outstretched wings； many of them over forty feet
from tip to tip； offer opportunities for a tilt at one
end or the other； which has sent so many machines
to destruction。
The high center of gravity in all machines makes
the weight useless to counterbalance the rising
end or to hold up the depressed wing。
All aviators agree that these unequal areas of
density extend over small spaces； and it is； therefore；
obvious that a machine which is of such a
structure that it moves through the air broadside
on； will be more liable to meet these inequalities
than one which is narrow and does not take in such
a wide path。
Why； therefore； persist in making a form which；
by its very nature； invites danger？ Because birds
fly that way！
THE TURNING MOVEMENT。This structural arrangement
accentuates the difficulty when the machine
turns。 The air pressure against the wing
surface is dependent on the speed。 The broad
outstretched surfaces compel the wing at the outer
side of the circle to travel faster than the inner
one。 As a result； the outer end of the aeroplane
is elevated。
CENTRIFUGAL ACTION。At the same time the
running gear； and the frame which carries it and
supports the machine while at rest； being below
the planes； a centrifugal force is exerted； when
turning a circle； which tends to swing the wheels
and frame outwardly； and thereby still further
elevating the outer end of the plane。
THE WARPING PLANES。The only remedy to
meet this condition is expressed in the mechanism
which wraps or twists the outer ends of the planes；
as constructed in the Wright machine； or the
ailerons； or small wings at the rear margins of the
planes； as illustrated by the Farman machine。
The object of this arrangement is to decrease the
angle of incidence at the rising end； and increase
the angle at the depressed end； and thus； by manually…
operated means keep the machine on an even
keel。
CHAPTER IV
FORE AND AFT CONTROL
THERE is no phase of the art of flying more important
than the fore and aft control of an airship。
Lateral stability is secondary to this feature； for
reasons which will appear as we develop the
subject。
THE BIRD TYPE OF FORE AND AFT CONTROL。
Every aeroplane follows the type set by nature
in the particular that the body is caused to oscillate
on a vertical fore and aft plane while in
flight。 The bird has one important advantage；
however； in structure。 Its wing has a flexure at
the joint； so that its body can so oscillate independently
of the angle of the wings。
The aeroplane has the wing firmly fixed to the
body； hence the only way in which it is possible
to effect a change in the angle of the wing is by
changing the angle of the body。 To be consistent
the aeroplane should be so constructed that the
angle of the supporting surfaces should be movable；
and not controllable by the body。
The bird； in initiating flight from a perch； darts
downwardly； and changes the angle of the body to
correspond with the direction of the flying start。
When it alights the body is thrown so that its
breast banks against the air； but in ordinary flight
its wings only are used to change the angle of
flight。
ANGLE AND DIRECTION OF FLIGHT。In order to
become familiar with terms which will be frequently
used throughout the book； care should be
taken to distinguish between the terms angle and
direction of flight。 The former has reference to
the up and down movement of an aeroplane；
whereas the latter is used to designate a turning
movement to the right or to the left。
WHY SHOULD THE ANGLE OF THE BODY CHANGE？
The first question that presents itself is； why
should the angle of the aeroplane body change？
Why should it be made to dart up and down and
produce a sinuous motion？ Why should its nose
tilt toward the earth； when it is descending； and
raise the forward part of the structure while ascending？
The ready answer on the part of the bird…form
advocate is； that nature has so designed a flying
structure。 The argument is not consistent； because
in this respect； as in every other； it is not
made to conform to the structure which they seek
to copy。
CHANGING ANGLE OF BODY NOT SAFE。Furthermore；
there is not a single argument which can be
advanced in behalf of that method of building；
which proves it to be correct。 Contrariwise； an
analysis of the flying movement will show that it is
the one feature which has militated against safety；
and that machines will never be safe so long as
the angle of the body must be depended upon to
control the angle of flying。
_Fig。 11a Monoplane in Flight。_
In Fig。 11a three positions of a monoplane are
shown； each in horizontal flight。 Let us say that
the first figure A is going at 40 miles per hour；
the second； B； at 50； and the third； C； at 60 miles。
The body in A is nearly horizontal； the angle of
the plane D being such that； with the tail E also
horizontal； an even flight is maintained。
When the speed increases to 50 miles an hour；
the angle of incidence in the plane D must be
decreased； so that the rear end of the frame must
be raised； which is done by giving the tail an angle
of incidence； otherwise； as the upper side of the
tail should meet the air it would drive the rear
end of the frame down； and thus defeat the attempt
to elevate that part。
_Fig。 12。 Angles of Flight。_
As the speed increases ten miles more； the tail
is swung down still further and the rear end of
the frame is now actually above the plane of flight。
In order； now； to change the angle of flight； without
altering the speed of the machine； the tail is
used to effect the control。
Examine the first diagram in Fig。 12。 This
shows the tail E still further depressed； and the
air striking its lower side； causes an upward movement
of the frame at that end； which so much decreases
the angle of incidence that the aeroplane
darts downwardly。
In order to ascend； the tail； as shown in the second
diagram； is elevated so as to depress the rear
end； and now the sustaining surface shoots upwardly。
Suppose that in either of the positions 1 or 2；
thus described； the aviator should lose control of
the mechanism； or it should become deranged or
〃stick；〃 conditions which have existed in the history
of the art； what is there to prevent an accident？
In the first case； if there is room； the machine
will loop the loop； and in the second case the machine
will move upwardly until it is vertical； and
then； in all probability； as its propelling power is
not sufficient to hold it in that position； like a
helicopter； and having absolutely no wing supporting
surface when in that position； it will dart
down tail foremost。
A NON…CHANGING BODY。We may contrast the
foregoing instances of flight with a machine having
the sustaining planes hinged to the body in
such a manner as to make the disposition of its
angles synchronous with the tail。 In other words；
see how a machine acts that has the angle of flight
controllable by both planes；that is； the sustaining
planes； as well as the tail。
_Fig。 13。 Planes on Non…changing Body。_
In Fig。 13 let the body of the aeroplane be horizontal；
and the sustaining planes B disposed at
the same angle； which we will assume to be 15
degrees； this being the imaginary angle for illustrative
purposes； with the power of the machine
to drive it along horizontally； as shown in position
1。
In position 2 the angles of both planes are now
at 10 degrees； and the speed 60 miles an hour；
which still drives the machine forward horizontally。
In position 3 the angle is still less； being now
only 5 degrees but the speed is increased to 80
miles per hour； but in each instance the body of
the machine is horizontal。
Now it is obvious that in order to ascend； in
either case； the changing of the planes to a greater
angle would raise the machine； but at the same
time keep the body on an even keel。
_Fig。 14。 Descent with Non…changing Body。_
DESCENDING POSITIONS BY POWER CONTROL。In
Fig。 14 the planes are the same angles in the three
positions respectively； as in Fig。 13； but now the
power has been reduced； and the speeds are 30；
25； and 20 miles per hour； in positions A； B and C。
Suppose that in either position the power should
cease； and the control broken； so that it would be
impossible to move the planes。 When the machine
begins to lose its momentum it will descend on a
curve shown； for instance； in Fig。 15； where position
1 of Fig。 14 is taken as the speed and angles
of the plane when the power ceased。
_Fig。 15。 Utilizing Momentum。_
CUTTING OFF THE POWER。This curve； A； may
reach that
