按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
level in the airon an 〃even keel；〃 as a sailor would
say。 This simile is particularly appropriate as all aviators
are in reality sailors； and much more daring ones
than those who course the seas。 The latter are in craft
which are kept afloat by the buoyancy of the water；
whether in motion or otherwise and； so long as normal
conditions prevail； will not sink。 Aviators sail the air
in craft in which constant motion must be maintained in
order to ensure flotation。
The man who has ridden a bicycle or motorcycle
around curves at anything like high speed； will have a
very good idea as to the principle of maintaining equilibrium
in an airship。 He knows that in rounding curves
rapidly there is a marked tendency to change the direction
of the motion which will result in an upset unless
he overcomes it by an inclination of his body in an opposite
direction。 This is why we see racers lean well
over when taking the curves。 It simply must be done
to preserve the equilibrium and avoid a spill。
How It Works In the Air。
If the equilibrium of an airship is disturbed to an
extent which completely overcomes the center of gravity
it falls according to the location of the displacement。
If this displacement； for instance； is at either end the
apparatus falls endways； if it is to the front or rear； the
fall is in the corresponding direction。
Owing to uncertain air currentsthe air is continually
shifting and eddying； especially within a hundred feet or
so of the earththe equilibrium of an airship is almost
constantly being disturbed to some extent。 Even if this
disturbance is not serious enough to bring on a fall it
interferes with the progress of the machine； and should
be overcome at once。 This is one of the things connected
with aerial navigation which calls for prompt；
intelligent action。
Frequently； when the displacement is very slight； it
may be overcome； and the craft immediately righted by
a mere shifting of the operator's body。 Take； for illustration；
a case in which the extreme right end of the
machine becomes lowered a trifle from the normal level。
It is possible to bring it back into proper position by
leaning over to the left far enough to shift the weight
to the counter…balancing point。 The same holds good as
to minor front or rear displacements。
When Planes Must Be Used。
There are other displacements； however； and these are
the most frequent； which can be only overcome by manipulation of
the stabilizing planes。 The method of procedure
depends upon the form of machine in use。 The
Wright machine； as previously explained； is equipped
with plane ends which are so contrived as to admit of
their being warped （position changed） by means of the
lever control。 These flexible tip planes move simultaneously；
but in opposite directions。 As those on one end
rise； those on the other end fall below the level of the
main plane。 By this means air is displaced at one point；
and an increased amount secured in another。
This may seem like a complicated system； but its
workings are simple when once understood。 It is by
the manipulation or warping of these flexible tips that
transverse stability is maintained； and any tendency to
displacement endways is overcome。 Longitudinal stability
is governed by means of the front rudder。
Stabilizing planes of some form are a feature； and a
necessary feature； on all flying machines； but the methods
of application and manipulation vary according to the
individual ideas of the inventors。 They all tend； however；
toward the same endthe keeping of the machine
perfectly level when being navigated in the air。
When to Make a Flight。
A beginner should never attempt to make a flight
when a strong wind is blowing。 The fiercer the wind；
the more likely it is to be gusty and uncertain； and the
more difficult it will be to control the machine。 Even
the most experienced and daring of aviators find there
is a limit to wind speed against which they dare not
compete。 This is not because they lack courage； but
have the sense to realize that it would be silly and useless。
The novice will find a comparatively still day； or one
when the wind is blowing at not to exceed 15 miles an
hour； the best for his experiments。 The machine will be
more easily controlled； the trip will be safer； and also
cheaper as the consumption of fuel increases with the
speed of the wind against which the aeroplane is forced。
CHAPTER XIII。
PECULIARITIES OF AIRSHIP POWER。
As a general proposition it takes much more power to
propel an airship a given number of miles in a certain
time than it does an automobile carrying a far heavier
load。 Automobiles with a gross load of 4；000 pounds；
and equipped with engines of 30 horsepower； have travelled
considerable distances at the rate of 50 miles an
hour。 This is an equivalent of about 134 pounds per
horsepower。 For an average modern flying machine；
with a total load； machine and passengers； of 1；200
pounds； and equipped with a 50…horsepower engine； 50
miles an hour is the maximum。 Here we have the equivalent
of exactly 24 pounds per horsepower。 Why this
great difference？
No less an authority than Mr。 Octave Chanute answers
the question in a plain； easily understood manner。 He
says：
〃In the case of an automobile the ground furnishes a
stable support； in the case of a flying machine the engine
must furnish the support and also velocity by which the
apparatus is sustained in the air。〃
Pressure of the Wind。
Air pressure is a big factor in the matter of aeroplane
horsepower。 Allowing that a dead calm exists； a body
moving in the atmosphere creates more or less resistance。
The faster it moves； the greater is this resistance。
Moving at the rate of 60 miles an hour the resistance；
or wind pressure； is approximately 50 pounds to the
square foot of surface presented。 If the moving object
is advancing at a right angle to the wind the following
table will give the horsepower effect of the resistance
per square foot of surface at various speeds。
Horse Power
Miles per Hour per sq。 foot
10 0。013
15 0 044
20 0。105
25 0。205
30 0。354
40 0。84
50 1。64
60 2。83
80 6。72
100 13。12
While the pressure per square foot at 60 miles an hour；
is only 1。64 horsepower； at 100 miles； less than double
the speed； it has increased to 13。12 horsepower； or exactly
eight times as much。 In other words the pressure
of the wind increases with the square of the velocity。
Wind at 10 miles an hour has four times more pressure
than wind at 5 miles an hour。
How to Determine Upon Power。
This element of air resistance must be taken into consideration
in determining the engine horsepower required。
When the machine is under headway sufficient
to raise it from the ground （about 20 miles an hour）；
each square foot of surface resistance； will require nearly
nine…tenths of a horsepower to overcome the wind pressure；
and propel the machine through the air。 As
shown in the table the ratio of power required increases
rapidly as the speed increases until at 60 miles an hour
approximately 3 horsepower is needed。
In a machine like the Curtiss the area of wind…exposed
surface is about 15 square feet。 On the basis of this
resistance moving the machine at 40 miles an hour would
require 12 horsepower。 This computation covers only
the machine's power to overcome resistance。 It does
not cover the power exerted in propelling the machine
forward after the air pressure is overcome。 To meet
this important requirement Mr。 Curtiss finds it necessary
to use a 50…horsepower engine。 Of this power； as
has been already stated； 12 horsepower is consumed
in meeting the wind pressure； leaving 38 horsepower
for the purpose of making progress。
The flying machine must move faster than the air to
which it is opposed。 Unless it does this there can be no
direct progress。 If the two forces are equal there is no
straight…ahead advancement。 Take； for sake of illustration；
a case in which an aeroplane； which has developed a
speed of 30 miles an hour； meets a wind velocity of
equal force moving in an opposite direction。 What is
the result？ There can be no advance because it is a
contest between two evenly matched forces。 The aeroplane
stands still。 The only way to get out of the difficulty
is for the operator to wait for more favorable conditions；
or bring his machine to the ground in the usual
manner by manipulation of the control system。
Take another case。 An aeroplane； capable of making
50 miles an hour in a calm； is met by a head wind of 25
miles an hour。 How much progress does the aeroplane
make？ Obviously it is 25 miles an hour over the ground。
Put the