o that the widest part will
be at or near the dotted line 3, say two-thirds of
the distance from the center of the blade.

This is the practice with most of the manufacturers
at the present time, and some of them
claim that this form produces the best results.

FOR HIGHER SPEEDS.--Fig. 83 shows a propeller
cut from a blank, 4" x 6" in cross section, not
laminated.

_Fig. 83. Cut from a 4" x 6" Single Blank._

It should be borne in mind that for high speeds
the blades must be narrow. A propeller seven
feet in diameter with a six foot pitch, turning
950 revolutions per minute, will produce a pull of
350 pounds, if properly made.

Such a propeller can be readily handled by a
forty horse power motor, such as are specially
constructed for flying machine purposes.

INCREASING PROPELLER EFFICIENCY.--Some experiments
have been made lately, which, it is
claimed, largely increase the efficiency of propellers.
The improvement is directed to the outline
shape of the blade.

The typical propeller, such as we have illustrated,
is one with the wide part of the blade at
the extremity. The new type, as suggested, reverses
this, and makes the wide part of the blade
near the hub, so that it gradually tapers down to
a narrow tip.

Such a form of construction is shown in Fig.
84. This outline has some advantages from one
standpoint, namely, that it utilizes that part of
the blade near the hub, to produce a pull, and
does not relegate all the duty to the extreme ends
or tips.

_Fig. 84. A Suggested Form._

To understand this more fully, let us take a
propeller six feet in diameter, and measure the
pull or thrust at the tips, and also at a point half
way between the tip and the hub.

In such a propeller, if the blade is the same
width and pitch at the two points named, the pull
at the tips will be four times greater than at the
intermediate point.



CHAPTER XIV

EXPERIMENTAL GLIDERS AND MODEL AEROPLANES


AN amusing and very instructive pastime is
afforded by constructing and flying gliding machines,
and operating model aeroplanes, th