It has been pointed out by experiment that certain forms of curved surfaces as applied to aeroplanes will lift more per horse power, per unit of square foot, while on the other hand it has been shown that a flat surface will lift more per horse power, but requires more area of surface to do it.
As a true pitch screw propeller is virtually a rotating aeroplane, a curved surface may be advantageously employed when the limit of size prevents using large plane surfaces for the blades.
Care should be exercised in keeping the chord of any curve to be used for the blades at the proper pitch angle, and in all cases propeller blades should be made rigid so as to preserve the true angle and not be distorted by centrifugal force or from any other cause, as flexibility will seriously affect their pitch speed and otherwise affect their efficiency.
How to Determine Angle.
To find the angle for the proper pitch at any point in the diameter of a propeller, determine the circumference by multiplying the diameter by 3.1416, which represent by drawing a line to scale in feet. At the end of this line draw another line to represent the desired pitch in feet. Then draw a line from the point representing the desired pitch in feet to the beginning of the circumference line. For example:
If the propeller to be laid out is 7 feet in diameter, and is to have a 7-foot pitch, the circumference will be 21.99 feet. Draw a diagram representing the circumference line and pitch in feet. If this diagram is wrapped around a cylinder the angle line will represent a true thread 7 feet in diameter and 7 feet long, and the angle of the thread will be 17 3/4 degrees.
Relation of Diameter to Circumference.
Since the areas of circles decrease as the diameter lessens, it follows that if a propeller is to travel at a uniform pitch speed, the volume of its blade displacement should decrease as its diameter becomes less, so as to occupy a corresponding relation to the circumferences of larger diameters, and at the same time the projected area of the blade must be parallel along its full length and should represent a true sector of a circle.
Let us suppose a 7-foot circle to be divided into 20 sectors, one of which represents a propeller blade. If the pitch is to be 7 feet, then the greatest depth of the angle would be 1/20 part of the pitch, or 4 2/10 inch. If the line representing the greatest depth of the angle is kept the same width as it approaches the hub, the pitch will be uniform. If the blade is set at an angle so its projected area is 1/20 part of the pitch, and if it is moved through 20 divisions for one revolution, it would have a travel of 7 feet.