flying machines-第11章
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which must not be overlooked。 No matter how
powerful the engine may be unless the propeller thrust
more than overcomes the wind pressure there can be
no progress forward。 Should the force of this propeller
thrust and that of the wind pressure be equal the result
is obvious。 The machine is at a stand…still so far
as forward progress is concerned and is deprived of the
essential advancing movement。
Speed not only furnishes sustentation for the airship；
but adds to the stability of the machine。 An aeroplane
which may be jerky and uncertain in its movements； so
far as equilibrium is concerned； when moving at a slow
gait； will readily maintain an even keel when the speed
is increased。
Designs for Propeller Blades。
It is the object of all men who design propellers to
obtain the maximum of thrust with the minimum expenditure
of engine energy。 With this purpose in view
many peculiar forms of propeller blades have been
evolved。 In theory it would seem that the best effects
could be secured with blades so shaped as to present a
thin （or cutting） edge when they come out of the wind；
and then at the climax of displacement afford a maximum
of surface so as to displace as much air as possible。
While this is the form most generally favored
there are others in successful operation。
There is also wide difference in opinion as to the
equipment of the propeller shaft with two or more
blades。 Some aviators use two and some four。 All
have more or less success。 As a mathematical proposition
it would seem that four blades should give more
propulsive force than two； but here again comes in one
of the puzzles of aviation； as this result is not always
obtained。
Difference in Propeller Efficiency。
That there is a great difference in propeller efficiency
is made readily apparent by the comparison of effects
produced in two leading makes of machinesthe Wright
and the Voisin。
In the former a weight of from 1；100 to 1；200 pounds
is sustained and advance progress made at the rate of
40 miles an hour and more； with half the engine speed
of a 25 horse…power motor。 This would be a sustaining
capacity of 48 pounds per horsepower。 But the actual
capacity of the Wright machine； as already stated； is 50
pounds per horsepower。
The Voisin machine； with aviator； weighs about 1；370
pounds； and is operated with a so…horsepower motor。
Allowing it the same speed as the Wright we find that；
with double the engine energy； the lifting capacity is
only 27 1/2 pounds per horsepower。 To what shall we
charge this remarkable difference？ The surface of the
planes is exactly the same in both machines so there
is no advantage in the matter of supporting area。
Comparison of Two Designs。
On the Wright machine two wooden propellers of
two blades each （each blade having a decided 〃twist〃）
are used。 As one 25 horsepower motor drives both propellers the
engine energy amounts to just one…half of
this for each； or 12 1/2 horsepower。 And this energy is
utilized at one…half the normal engine speed。
On the Voisin a radically different system is employed。
Here we have one metal two…bladed propeller with a
very slight 〃twist〃 to the blade surfaces。 The full energy
of a 50…horsepower motor is utilized。
Experts Fail to Agree。
Why should there be such a marked difference in
the results obtained？ Who knows？ Some experts
maintain that it is because there are two propellers on
the Wright machine and only one on the Voisin； and
consequently double the propulsive power is exerted。
But this is not a fair deduction； unless both propellers
are of the same size。 Propulsive power depends upon
the amount of air displaced； and the energy put into the
thrust which displaces the air。
Other experts argue that the difference in results may
be traced to the difference in blade design； especially
in the matter of 〃twist。〃
The fact is that propeller results depend largely upon
the nature of the aeroplanes on which they are used。
A propeller； for instance； which gives excellent results
on one type of aeroplane； will not work satisfactorily on
another。
There are some features； however； which may be safely
adopted in propeller selection。 These are： As extensive
a diameter as possible； blade area 10 to 15 per cent
of the area swept； pitch four…fifths of the diameter；
rotation slow。 The maximum of thrust effort will be thus
obtained。
CHAPTER X。
PROPER DIMENSIONS OF MACHINES。
In laying out plans for a flying machine the first thing
to decide upon is the size of the plane surfaces。 The
proportions of these must be based upon the load to be
carried。 This includes the total weight of the machine
and equipment； and also the operator。 This will be a
rather difficult problem to figure out exactly； but
practical approximate figures may be reached。
It is easy to get at the weight of the operator； motor
and propeller； but the matter of determining； before they
are constructed； what the planes； rudders； auxiliaries；
etc。； will weigh when completed is an intricate proposition。
The best way is to take the dimensions of some
successful machine and use them； making such alterations
in a minor way as you may desire。
Dimensions of Leading Machines。
In the following tables will be found the details as to
surface area； weight； power； etc。； of the nine principal
types of flying machines which are now prominently before
the public：
MONOPLANES。
Surface area Spread in Depth in
Make Passengers sq。 feet linear feet linear
feet
Santos…Dumont 。 。 1 110 16。0 26。0
Bleriot 。 。 。 。 。 1 150。6 24。6 22。0
R。 E。 P 。 。 。 。 。 1 215 34。1 28。9
Bleriot 。 。 。 。 。 2 236 32。9 23。0
Antoinette。 。 。 。 2 538 41。2 37。9
No。 of Weight Without
Propeller
Make Cylinders Horse Power Operator
Diameter
Santos…Dumont。 。 2 30 250 5。0
Bleriot。 。 。 。 。 3 25 680 6。9
R。 E。 P。 。 。 。 。 7 35 900 6。6
Bleriot。 。 。 。 。 7 50 1；240 8。1
Antoinette 。 。 。 8 50 1；040 7。2
BIPLANES。
Surface Area Spread in Depth
in
Make Passengers sq。 feet linear feet linear
feet
Curtiss 。 。 。 2 258 29。0
28。7
Wright。 。 。 。 2 538 41。0
30。7
Farman。 。 。 。 2 430 32。9
39。6
Voisin。 。 。 。 2 538 37。9
39。6
No。 of Weight Without
Propeller
Make Cylinders Horse Power Operator
Diameter
Curtiss 。 。 。 8 50 600 6。0
Wright。 。 。 。 4 25 1；100 8。1
Farman。 。 。 。 7 50 1；200 8。9
Voisin。 。 。 。 8 50 1；200 6。6
In giving the depth dimensions the length over all
from the extreme edge of the front auxiliary plane to
the extreme tip of the rear is stated。 Thus while the
dimensions of the main planes of the Wright machine
are 41 feet spread by 6 1/2 feet in depth； the depth over
all is 30。7。
Figuring Out the Details。
With this data as a guide it should be comparatively
easy to decide upon the dimensions of the machine required。
In arriving at the maximum lifting capacity the
weight of the operator must be added。 Assuming this
to average 170 pounds the method of procedure would be
as follows：
Add the weight of the operator to the weight of the
complete machine。 The new Wright machine complete
weighs 900 pounds。 This； plus 170； the weight of the
operator； gives a total of 1；070 pounds。 There are 538
square feet of supporting surface； or practically one
square foot of surface area to each two pounds of load。
There are some machines； notably the Bleriot； in which
the supporting power is much greater。 In this latter
instance we find a surface area of 150 1/2 square feet
carrying a load of 680 plus 170； or an aggregate of 850
pounds。 This is the equivalent of five pounds to the
square foot。 This ratio is phenomenally large； and
should not be taken as a guide by amateurs。
The Matter of Passengers。
These deductions are based on each machine carrying
one passenger； which is admittedly the limit at present
of the monoplanes like those operated for record…making
purposes by Santos…Dumont and Bleriot。 The biplanes；
however； have a two…passenger capacity； and this adds
materially to the proportion of their weight…sustaining
power as compared with the surface area。 In the following
statement all the machines are figured on the
one…passenger basis。 Curtiss and Wright have carried
two passengers on numerous occasions； and an extra 170
pounds should therefore be added to the total weight
carried； which would materially increase the capacity。
Even with the two…passenger load the limit is by no
means reached； but as experiments have gone no further
it is impossible to make more accurate figures。
Average Proportions of Load。
It will be interesting； before proceeding to lay out the
dimension details； to make a comparison of the proportion
of load effect with the supporting surfaces of various
well…known machines。 Here are the figures：
Santos…DumontA trifle under four pounds per square
foot。
BleriotFive pounds。
R。 E。 P。Five pounds。
AntoinetteAbout two and one…quarter pounds。
CurtissAbout two and one…half pounds。
WrightTwo and one…quarter pounds。
FarmanA trifle over three pounds。
VoisinA little under two and one…half pounds。
Importance of Engine Power。
While these figures are authentic； they are in a way
misleading； as the important factor of engine power
is not taken into consideration。 Let us recall the fact
that it is the engine power which keeps the machine in
motion； and that it is only while in motion that the machine
will remain suspended in the air。 Hence； to attribute the support
solely to the surface area is erroneous。
True； that once under headway the planes contribute
largely to the sustaining effect； and are absolutely essential
in aerial navigationthe motor could not rise without
themstill； when it comes to a question of weight…
sustaining power； we must also figure on the engine
capacity。
In the Wright machine； in which there is a lifting
capacity of approximately 2 1
