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！
than 500 feet。 Over a level area it is certain that
it is much less； but in some sections of the country；
where the hill ranges extend for many miles；
at altitudes of three and four hundred feet； the
upper atmosphere may be affected for a thousand
feet above。
Prof。 Lowe； in making a flight with a balloon；
from Cincinnati to North Carolina； which lasted
a day and all of one night； found that during the
early morning the balloon； for some reason； began
to ascend； and climbed nearly five thousand
feet in a few hours； and as unaccountably
began to descend several hours before he landed。
Before it began to ascend； he was on the western
side of the great mountain range which extends
south from Pennsylvania and terminates in
Georgia。 He was actually climbing the mountain
in a drift of air which was moving eastwardly；
and at no time was he within four thousand feet
of the earth during that period； which shows that
air movements are of such a character as to exert
their influence vertically to great heights。
For cross country flying the safest altitude is
1000 feet； a distance which gives ample opportunity
to volplane； if necessary； and it is a height
which enables the pilot to make observations of the
surface so as to be able to judge of its character。
But explanations and statements； and the experiences
of pilots might be detailed in pages； and
still it would be ineffectual to teach the art of flying。
The only sure course is to do the work on
an actual machine。
Many of the experiences are valuable to the
learner； some are merely in the nature of cautions；
and it is advisable for the beginner to learn what
the experiences of others have been； although they
may never be called upon to duplicate them。
All agree that at great elevations the flying
conditions are entirely different from those met
with near the surface of the ground； and the history
of accidents show that in every case where
a mishap was had at high altitude it came about
through defect in the machine； and not from gusts
or bad air condition。
On the other hand； the uptilting of machines；
the accidents due to the so…called 〃Holes in the
air；〃 which have dotted the historic pages with
accidents； were brought about at low altitudes。
At from two to five thousand feet the air may be
moving at speeds of from twenty to forty miles
an hour；great masses of winds； like the trade
stream； which are uniform over vast areas。 To
the aviator flying in such a field； with the earth
hidden from him； there would be no wind to indicate
that he was moving in any particular direction。
He would fly in that medium； in any direction；
without the slightest sense that he was in a gale。
It would not affect the control of the machine；
because the air； though moving as a mass； would
be the same as flying in still air。 It is only when
he sees fixed objects that he is conscious of the
movement of the wind。
CHAPTER XIII
THE PROPELLER
BY far the most difficult problem connected
with aviation is the propeller。 It is the one great
vital element in the science and art pertaining to
this subject which has not advanced in the slightest
degree since the first machine was launched。
The engine has come in for a far greater share
of expert experimental work； and has advanced
most rapidly during the past ten years。 But；
strange to say； the propeller is； essentially； the
same with the exception of a few small changes。
PROPELLER CHANGES。The changes which have
been made pertaining to the form of structure；
principally； and in the use of new materials。 The
kind of wood most suitable has been discovered；
but the lines are the same； and nothing has been
done to fill the requirement which grows out of
the difference in speed when a machine is in the
act of launching and when it is in full flight。
PROPELLER SHAPE。It cannot be possible that
the present shape of the propeller will be its ultimate
form。 It is inconceivable that the propeller
is so inefficient that only one sixty…fifth of the
power of the engine is available。 The improvement
in propeller efficiency is a direction which
calls for experimental work on the part of inventors
everywhere。
The making of a propeller； although it appears
a difficult task； is not as complicated as would appear；
and with the object in view of making the
subject readily understood； an explanation will be
given of the terms 〃Diameter；〃 and 〃Pitch；〃 as
used in the art。
The Diameter has reference to the length of
the propeller； from end to end。 In calculating
propeller pull； the diameter is that which indicates
the speed of travel； and for this reason is
a necessary element。
Thus； for instance； a propeller three feet in
diameter； rotating 500 times a minute； has a tip
speed of 1500 feet； whereas a six foot propeller；
rotating at the same speed； moves 3000 feet at the
tips。
PITCH。This is the term which is most confusing；
and is that which causes the most frequent
trouble in the mind of the novice。 The term will
be made clear by carefully examining the accompanying
illustration and the following description：
In Fig。 76 is shown a side view of a propeller
A； mounted on a shaft B； which is free to move
longitudinally。 Suppose we turn the shaft so the
tip will move along on the line indicated by the
arrow C。
Now the pitch of the blade at D is such that it
will be exactly in line with the spirally…formed
course E； for one complete turn。 As the propeller
shaft has now advanced； along the line E； and
stopped after one turn； at F； the measure between
the points F and G represents the pitch of the propeller。
Another way to express it would be to
call the angle of the blade a five； or six； or a seven
foot pitch； as the pitches are measured in feet。
_Fig。 76。 Describing the Pitch Line。_
In the illustration thus given the propeller shaft；
having advanced six feet； we have what is called
a six foot pitch。
Now； to lay out such a pitch is an easy matter。
Assume； as in Fig。 77； that A represents the end
of the blank from which the propeller is to be cut；
and that the diameter of this blank； or its length
from end to end is seven feet。 The problem now
is to cut the blades at such an angle that we shall
have a six foot pitch。
_Fig。 77。 Laying out the Pitch。_
LAYING OUT THE PITCH。First； we must get the
circumference of the propeller； that is； the distance
the tip of the propeller will travel in making
one complete turn。 This is done by multiplying
7 by 3。1416。 This equals 21。99； or； practically； 22
feet。
A line B is drawn； extending out horizontally
along one side of the blank A； this line being made
on a scale； to represent 22 feet。 Secondly； at the
end of this line drawn a perpendicular line C； 6
feet long。 A perpendicular line is always one
which is at right