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the air thirty miles an hour； or sixty miles； with
the expenditure of the same power。
An automobile； if propelled at sixty； instead of
thirty miles an hour； would require an additional
power in doing so； but an airship acts differently；
within certain limitations。
When it is first set in motion its effective pull
may not be equal to four pounds for each horse
power； due to the slow speed of the propeller； and
also owing to the great angle of incidence which
resists the forward movement of the ship。
INCREASE OF SPEED ADDS TO RESISTANCE。Finally；
as speed increases； the angle of the planes
decrease； resistance is less； and up to a certain
point the pull of the propeller increases； but beyond
that the vacuum behind the blades becomes
so great as to bring down the pull； and there is
thus a balance；a sort of mutual governing motion
which； together； determine the ultimate speed
of the aeroplane。
HOW POWER DECREASES WITH SPEED。If now；
with the same propeller； the speed should be
doubled； the ship would go no faster； because the
bite of the propeller on the air would be ineffective；
hence it will be seen that it is not the amount
of power in itself； that determines the speed； but
the shape of the propeller； which must be so made
that it will be most effective at the speed required
for the ship。
While that is true when speed is the matter of
greatest importance； it is not the case where it is
desired to effect a launching。 In that case the
propeller must be made so that its greatest pull
will be at a slow speed。 This means a wider
blade； and a greater pitch； and a comparatively
greater pull at a slow speed。
No such consideration need be given to an automobile。
The constant accretion of power adds
to its speed。 In flying machines the aviator must
always consider some companion factor which
must be consulted。
HOW TO CALCULATE THE POWER APPLIED。In a
previous chapter reference was made to a plane
at an angle of forty…five degrees； to which two
scales were attached； one to get its horizontal pull；
or drift； and the other its vertical pull； or lift。
PULLING AGAINST AN ANGLE。Let us take the
same example in our aeroplane。 Assuming that
it weighs 900 pounds； and that the angle of the
planes is forty…five degrees。 If we suppose that
the air beneath the plane is a solid； and frictionless；
and a pair of scales should draw it up the incline；
the pull in doing so would be one…half of its
weight； or 450 pounds。
It must be obvious； therefore； that its force； in
moving downwardly； along the surface A； Fig。 60；
would be 450 pounds。
The incline thus shown has thereon a weight B；
mounted on wheels a； and the forwardly…projecting
cord represents the power； or propeller pull；
which must； therefore； exert a force of 450 pounds
to keep it in a stationary position against the surface
A。
In such a case the thrust along the diagonal
line E would be 900 pounds； being the composition
of the two forces pulling along the lines D； F。
THE HORIZONTAL AND VERTICAL PULL。Now it
must be obvious； that if the incline takes half of
the weight while it is being drawn forwardly； in
the line of D； if we had a propeller drawing along
that line； which has a pull of 450 pounds； it would
maintain the plane in flight； or； at any rate hold
it in space； assuming that the air should be moving
past the plane。
_Fig。 60。 Horizontal and Vertical pull。_
The table of lift and drift gives a fairly accurate
method of determining this factor； and we refer to
the chapter on that subject which will show the
manner of making the calculations。
THE POWER MOUNTING。More time and labor
has been wasted； in airship experiments； in poor
motor mounting； than in any other direction。
This is especially true where two propellers are
used； or where the construction is such that the
propeller is mounted some distance from the motor。
SECURING THE PROPELLER TO THE SHAFT。But
even where the propeller is mounted on the engine
shaft； too little care is exercised to fix it securely。
The vibratory character of the mounting
makes this a matter of first importance。 If there
is a solid base a poorly fixed propeller will hold
much longer； but it is the extreme vibration that
causes the propeller fastening to give way。
VIBRATIONS。If experimenters realized that an
insecure； shaking； or weaving bed would cause a
loss of from ten to fifteen per cent。 in the pull of
the propeller； more care and attention would be
given to this part of the structure。
WEAKNESSES IN MOUNTING。The general weaknesses
to which attention should be directed are；
first； the insecure attachment of the propeller to
the shaft； second； the liability of the base to
weave； or permit of a torsional movement； third；
improper bracing of the base to the main body of
the aeroplane。
If the power is transferred from the cylinder
to the engine shaft where it could deliver its output
without the use of a propeller； it would not
be so important to consider the matter of vibration；
but the propeller； if permitted to vibrate；
or dance about； absorbs a vast amount of energy；
while at the same time cutting down its effective
pull。
Aside from this it is dangerous to permit the
slightest displacement while the engine is running。
Any looseness is sure to grow worse； instead
of better； and many accidents have been
registered by bolts which have come loose from
excessive vibration。 It is well； therefore； to have
each individual nut secured； or properly locked；
which is a matter easily done； and when so secured
there is but little trouble in going over the machine
to notice just how much more the nut must
be taken up to again make it secure。
THE GASOLINE TANK。What horrid details have
been told of the pilots who have been burned to
death with the escaping gasoline after an accident；
before help arrived。 There is no excuse for
such dangers。 Most of such accidents were due
to the old practice of making the tanks of exceedingly
light or thin material； so that the least
undue jar would tear a hole at the fastening
points； and thus permit the gasoline to escape。
A thick copper tank is by far the safest； as this
metal will not readily rupture by the wrench which
is likely in landing。
WHERE TO LOCATE THE TANK。There has been
considerable discussion as to the proper place to
locate the tank。 Those who advocate its placement
overhead argue that in case of an accident
the aeroplane is likely to overturn； and the tank
will； therefore； be below the pilot。 Those who
believe it should be placed below； claim that in
case of overturning it is safer to have the tank
afire above than below。
DANGER TO THE PILOT。The great danger to the
pilot； in all cases of accidents； lies in the
overturning of the machine。 Many have had accidents
where the machine landed right side up； even