ne 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 passengers--we 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 machines--538 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 you
are following uses 500, it is simply a matter of making your planes
one-fifth larger all around.

The Cost of Production.

Cost of pro