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reverse is the ruletwo wheels in front and one in the
rear。 Farman makes use of five wheels； one in the；
extreme rear； and four； arranged in pairs； a little to the
front of the center of the main lower plane。
In place of wheels the Wright machine is equipped
with a skid…like device consisting of two long beams
attached to the lower plane by stanchions and curving
up far in front； so as to act as supports to the horizontal
rudder。
Why Wood Is Favored。
A frequently asked question is： 〃Why is not aluminum；
or some similar metal； substituted for wood。〃
Wood； particularly spruce； is preferred because； weight
considered； it is much stronger than aluminum； and this
is the lightest of all metals。 In this connection the following
table will be of interest：
Compressive
Weight Tensile Strength Strength
per cubic foot per sq。 inch per sq。 inch
Material in lbs。 in lbs。 in lbs。
Spruce 。 。 。 。 25 8；000 5；000
Aluminum 162 16；000 。。。。。。
Brass （sheet） 510 23；000 12；000
Steel （tool） 490 100；000 40；000
Copper （sheet） 548 30；000 40；000
As extreme lightness； combined with strength；
especially tensile strength； is the great essential in flying…
machine construction； it can be readily seen that the
use of metal； even aluminum； for the framework； is
prohibited by its weight。 While aluminum has double the
strength of spruce wood it is vastly heavier； and thus
the advantage it has in strength is overbalanced many
times by its weight。 The specific gravity of aluminum
is 2。50； that of spruce is only 0。403。
Things to Be Considered。
In laying out plans for a flying machine there are five
important points which should be settled upon before
the actual work of construction is started。 These are：
FirstApproximate weight of the machine when finished
and equipped。
SecondArea of the supporting surface required。
ThirdAmount of power that will be necessary to
secure the desired speed and lifting capacity。
FourthExact dimensions of the main framework
and of the auxiliary parts。
FifthSize； speed and character of the propeller。
In deciding upon these it will be well to take into
consideration the experience of expert aviators regarding
these features as given elsewhere。 （See Chapter X。）
Estimating the Weights Involved。
In fixing upon the probable approximate weight in
advance of construction much； of course； must be assumed。
This means that it will be a matter of advance
estimating。 If a two…passenger machine is to be built
we will start by assuming the maximum combined
weight of the two people to be 350 pounds。 Most of
the professional aviators are lighter than this。 Taking
the medium between the weights of the Curtiss and
Wright machines we have a net average of 850 pounds
for the framework； motor； propeller； etc。 This； with
the two passengers； amounts to 1；190 pounds。 As the
machines quoted are in successful operation it will be
reasonable to assume that this will be a safe basis to
operate on。
What the Novice Must Avoid。
This does not mean； however； that it will be safe to
follow these weights exactly in construction； but that
they will serve merely as a basis to start from。 Because
an expert can turn out a machine； thoroughly equipped；
of 850 pounds weight； it does not follow that a novice
can do the same thing。 The expert's work is the result
of years of experience； and he has learned how to construct
frames and motor plants of the utmost lightness
and strength。
It will be safer for the novice to assume that he can
not duplicate the work of such men as Wright and Curtiss
without adding materially to the gross weight of
the framework and equipment minus passengers。
How to Distribute the Weight。
Let us take 1；030 pounds as the net weight of the machine
as against the same average in the Wright and
Curtiss machines。 Now comes the question of distributing
this weight between the framework； motor； and
other equipment。 As a general proposition the framework
should weigh about twice as much as the complete
power plant （this is for amateur work）。
The word 〃framework〃 indicates not only the wooden
frames of the main planes； auxiliary planes； rudders；
etc。； but the cloth coverings as welleverything in fact
except the engine and propeller。
On the basis named the framework would weigh 686
pounds； and the power plant 344。 These figures are
liberal； and the results desired may be obtained well
within them as the novice will learn as he makes progress
in the work。
Figuring on Surface Area。
It was Prof。 Langley who first brought into prominence
in connection with flying machine construction the
mathematical principle that the larger the object the
smaller may be the relative area of support。 As explained
in Chapter XIII； there are mechanical limits as
to size which it is not practical to exceed； but the main
principle remains in effect。
Take two aeroplanes of marked difference in area of
surface。 The larger will； as a rule； sustain a greater
weight in relative proportion to its area than the smaller
one； and do the work with less relative horsepower。 As
a general thing well…constructed machines will average
a supporting capacity of one pound for every one…half
square foot of surface area。 Accepting this as a working
rule we find that to sustain a weight of 1；200 pounds
machine and two passengerswe should have 600
square feet of surface。
Distributing the Surface Area。
The largest surfaces now in use are those of the
Wright； Voisin and Antoinette machines538 square
feet in each。 The actual sustaining power of these machines；
so far as known； has never been tested to the
limit； it is probable that the maximum is considerably
in excess of what they have been called upon to show。
In actual practice the average is a little over one pound
for each one…half square foot of surface area。
Allowing that 600 square feet of surface will be used；
the next question is how to distribute it to the best
advantage。 This is another important matter in which
individual preference must rule。 We have seen how
the professionals disagree on this point； some using
auxiliary planes of large size； and others depending upon
smaller auxiliaries with an increase in number so as to
secure on a different plan virtually the same amount of
surface。
In deciding upon this feature the best thing to do is
to follow the plans of some successful aviator； increasing
the area of the auxiliaries in proportion to the increase
in the area of the main planes。 Thus； if you use 600
square feet of surface where the man whose plans y