serves to protect the machine and aviator from shock or injury in
touching the ground, but also aids in getting under headway. All the
leading makes, with the exception of the Wright, are furnished with a
frame carrying from two to five pneumatic rubber-tired bicycle wheels.
In the Curtiss and Voisin machines one wheel is placed in front and two
in the rear. In the Bleriot and other prominent machines the reverse is
the rule--two 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:

First--Approximate weight of the machin