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 feet 1 3/5 inches.
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.
Pages:
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144