The Aeroplane Speaks. Fifth Edition

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 angles are different, but their pitch (in this case altitude reached in a given time) is the same.