Cross-Section
Showing Halving of Block
Fig. 168.—Four-bladed Screws
Waterproofing Silk for Model Aeroplanes.—A waterproofing solution can be made of pure coach varnish reduced in consistency with turpentine in the proportion 2: 1. It is, however, more important to make the fabric airtight than waterproof. This solution accomplishes both. Rubber lubricant is made of 1 part of graphite, 6 parts of pure soft soap, 1 part of glycerine, 4 parts of water, and 1 part of salicylic acid, boiled together and allowed to cool.
Making Four-bladed Air-screws.—[Fig. 168] on the preceding page shows how truly helical and also four-bladed screws are carved. Four-bladed screws are not so efficient for models as two-bladed ones. The drawing also shows the method of marking out and also of halving the blocks together at the centre, so that the four blades are at right angles to one another. A view of an ordinary twin-blade screw of similar design is appended to give some idea of the finished shape of the blades. The pitch should not exceed the circumferential measurement of the disc swept by the propeller; that is to say, the pitch angle, or the angle made by the propeller tip with the axis, should not exceed 45 degrees. The angles along the blade are determined in the manner illustrated by [Fig. 168]. A line is laid off to any convenient scale equal to the circumference of the propeller disc = π × diameter. The tip angle (or pitch angle) may now be produced and the triangle completed by erecting a line at right angles to the first, or the pitch may be erected perpendicularly to the circumferential line to the spiral scale and the pitch angle line drawn in. The circumferential line may now be divided into a number of equal parts and the points connected up. In the illustration three points have been taken, which will be enough for a small propeller. Cardboard templates should be cut to these angles and the blades checked at the points corresponding. The balance of the screw must be attended to and is a most cogent factor in such a small design. Marine-screws are exceedingly inefficient when working in air, and their use for driving model aeroplanes, etc., is to be strongly deprecated. The formula for propeller pitch is: p = πd tan A, where p = pitch, π = 3·14, d = diameter of propeller, tan A = tangent of pitch angle.
Fig. 169.—Securing Wooden Planes
Fixing Planes.—[Fig. 169] shows a neat and effective method of securing the planes.