effect than the surface in front.]

[Footnote 17: The reason the C.P. of an inclined surface is forward of
the centre of the surface is because the front of the surface does most
of the work, as explained on p. 62.]

CHAPTER III

RIGGING

In order to rig an aeroplane intelligently, and to maintain it in an
efficient and safe condition, it is necessary to possess a knowledge of
the stresses it is called upon to endure, and the strains likely to
appear.

STRESS is the load or burden a body is called upon to bear. It is
usually expressed by the result found by dividing the load by the number
of superficial square inches contained in the cross-sectional area of
the body.

[Illustration: Cross Sectional area]

Thus, if, for instance, the object illustrated above contains 4 square
inches of cross-sectional area, and the total load it is called upon to
endure is 10 tons, the stress would be expressed as 2-1/2 tons.

STRAIN is the deformation produced by stress.

THE FACTOR OF SAFETY is usually expressed by the result found by
dividing the stress at which it is known the body will collapse by the
maximum stress it will be called upon to endure. For instance, if a
control wire be called upon to endure a maximum stress of 2 cwts., and
the known stress at which it will collapse is 10 cwts., the factor of
safety is then 5.

COMPRESSION.--The simple stress of compression tends to produce a
crushing strain. Example: the interplane and fuselage struts.

TENSION.--The simple stress of tension tends to produce the strain of
elongation. Example: all the wires.

BENDING.--The compound stress of bending is a combination of compression
and tension.

[Illustration]

The above sketch illustrates a straight piece of wood of which the top,
centre, and bottom lines are of equal length. We will now imagine it
bent to form a circle, thus:

[Illustration]

The centre line is still the same length as before being bent; but the
top line, being farther from the centre of the circle, is now longer
than the centre line. That can be due only to the strain of elongation
produced by the stress of tension. The wood between the centre line and
the top line is then in tension; and the farther from the centre, the
greater the strain, and consequently the greater the tension.

The bottom line, being nearest to the centre of the circle, is now
shorter than the centre line. That can be due only to the strain of
crushing produced b