le is too small, it will produce a decreased
lift, and the aeroplane may have a tendency to fly "nose-down."

3. _The fuselage_ may have become warped upward or downward, thus giving
the tail-plane an incorrect angle of incidence. If it has too much
angle, it will lift too much, and the aeroplane will be "nose-heavy." If
it has too little angle, then it will not lift enough, and the aeroplane
will be "tail-heavy."

4. (The least likely reason.) _The tail-plane_ may be mounted upon
the fuselage at a wrong angle of incidence, in which case it must
be corrected. If nose-heavy, it should be given a smaller angle of
incidence. If tail-heavy, it should be given a larger angle; but
care should be taken not to give it too great an angle, because the
longitudinal stability entirely depends upon the tail-plane being set
at a much smaller angle of incidence than is the main surface, and
if that difference is decreased too much, the aeroplane will become
uncontrollable longitudinally. Sometimes the tail-plane is mounted on
the aeroplane at the same angle as the main surface, but it actually
engages the air at a lesser angle, owing to the air being deflected
downwards by the main surface. There is then, in effect, a longitudinal
dihedral as explained and illustrated in Chapter I.

CLIMBS BADLY.--Such a condition is, apart from engine or propeller
trouble, probably due to (1) distorted surfaces, or (2) too small an
angle of incidence.

FLIGHT SPEED POOR.--Such a condition is, apart from engine or propeller
trouble, probably due to (1) distorted surfaces, (2) too great an angle
of incidence, or (3) dirt or mud, and consequently excessive
skin-friction.

INEFFICIENT CONTROL is probably due to (1) wrong setting of control
surfaces, (2) distortion of control surfaces, or (3) control cables
being badly tensioned.

WILL NOT "TAXI" STRAIGHT.--If the aeroplane is uncontrollable on the
ground, it is probably due to (1) alignment of undercarriage being
wrong, or (2) unequal tension of shock absorbers.

CHAPTER IV

THE PROPELLER, OR "AIR-SCREW"

The sole object of the propeller is to translate the power of the engine
into thrust.

The propeller screws through the air, and its blades, being set at an
angle inclined to the direction of motion, secure a reaction, as in the
case of the aeroplane's lifting surface.

This reaction may be conveniently divided into two component parts or
values, namely, Thrust and Drift (see illu