so necessitate a very high undercarriage to keep the
propeller off the ground, and such undercarriage would not only produce
excessive drift, but would also tend to make the aeroplane stand on
its nose when alighting. The latter difficulty cannot be overcome by
mounting the propeller higher, as the centre of its thrust must be
approximately coincident with the centre of aeroplane drift.

MAINTENANCE OF EFFICIENCY.

The following conditions must be observed:

1. PITCH ANGLE.--The angle, at any given point on the propeller, at
which the blade is set is known as the pitch angle, and it must be
correct to half a degree if reasonable efficiency is to be maintained.

This angle secures the "pitch," which is the distance the propeller
advances during one revolution, supposing the air to be solid. The air,
as a matter of fact, gives back to the thrust of the blades just as
the pebbles slip back as one ascends a shingle beach. Such "give-back"
is known as _Slip_. If a propeller has a pitch of, say, 10 feet, but
actually advances, say, only 8 feet owing to slip, then it will be said
to possess 20 per cent. slip.

Thus, the pitch must equal the flying speed of the aeroplane plus
the slip of the propeller. For example, let us find the pitch of
a propeller, given the following conditions:

Flying speed ... 70 miles per hour.
Propeller revolutions ... 1,200 per minute.
Slip ... 15 per cent.

First find the distance in feet the aeroplane will travel forward in one
minute. That is--

369,600 feet (70 miles)
----------------------- = 6,160 feet per minute.
60 " (minutes)

Now divide the feet per minute by the propeller revolutions per minute,
add 15 per cent. for the slip, and the result will be the propeller
pitch:

6,160
----- + 15 per cent. = 5.903 feet.
1,200

In order to secure a constant pitch from root to tip of blade, the pitch
angle decreases towards the tip. This is necessary, since the end of the
blade travels faster than its root, and yet must advance forward at the
same speed as the rest of the propeller. For example, two men ascending
a hill. One prefers to walk fast and the other slowly, but they wish to
arrive at the top of the hill simultaneously. Then the fast walker must
travel a farther distance than the slow one, and his angle of path
(pitch angle) must then be smaller than the angle of path taken by the
slow walker. Their pitch angle