The two ten-inch propellers with which the model is fitted have a theoretical pitch of twelve and one-half inches. The propellers are carved from blanks one-half inch thick, the blades of the completed propellers having a maximum width of one inch at a radius of three inches. The propeller shafts are made from No. 16 piano wire and have small washers for bearings. Each propeller is driven by three strands of ¹⁄₄″ strip elastic. The rubber is given 1700 to 1750 turns and revolves the propellers at 1150–1200 r.p.m., when the model is in flight.
The model usually runs over the surface of the water for a distance of from two to three feet before it rises, after which it climbs at a very steep angle to the necessary altitude. The model seems, when in flight, to be slightly overpowered but this is misleading. The rubbers usually unwind in from 85 to 90 seconds. On four out of six flights this model has made a duration of between 98 and 100 seconds which is rather unusual for a model of this type.
THE RUDY FUNK DURATION MODEL
Of the many different types of duration models that have made their appearance during the year of 1915 perhaps the model described herewith, constructed and flown by Mr. Rudolph Funk, of the Aëro Science Club, was one of the most successful. Unlike most models the propellers of this model are bent and not cut. This model made its appearance during the latter part of 1915, on several occasions having flown for over 100 seconds duration. [Diagram 15].
While retaining the important characteristics of his standard model, slight changes have been made. Instead of the usual wire for the construction of the frame of the wings, bamboo is used in its place for lightness and strength. The wing frames are single surfaced, China silk being used for covering. The “dope” which is used to render the silk airtight is made by dissolving celluloid in banana oil. This in turn is applied to the silk with a soft brush.
The camber of the main wing is ³⁄₄″ at the center, with a slight reduction towards the negative tips; it also has a dihedral angle of 2 degrees. The main beam, which is secured to the under side of the frame for rigidness, is of spruce 1″ by ⁵⁄₆₄″, tapering to ³⁄₄″ × ⁵⁄₆₄″. The ribs for the main wing and small wing or “elevator” are cut from solid pieces of bamboo ³⁄₁₆″ thick by ¹⁄₄″ wide. These pieces of bamboo are first bent to the proper camber and are then cut into strips each ¹⁄₁₆″ wide. The ribs are next tapered to a V at the bottom, toward the trailing edge, as shown in [diagram 15], and also toward the entering edge. To accommodate the entering and trailing edges of the frame, each rib is slit slightly at both ends. Both edges of the frame are then inserted in the slots at the ends of the ribs and bound around with silk thread.
Diagram 15
The frame is composed of two sticks of silver spruce 38″ in length, ⁵⁄₁₆″ × ³⁄₁₆″, tapering to ¹⁄₄″ × ⁵⁄₃₂″, held apart by a streamline bamboo cross brace in the center. An additional brace of bamboo is securely fastened across the frame toward the front. The propeller brace consists of a streamline-cut piece of bamboo 12¹⁄₂″ in length by ³⁄₈″ in width at the center, tapering to ¹⁄₄″ toward the ends. The propeller brace is inserted in slots cut in the rear ends of the frame members, then bound and glued.
The propellers are bent from birch veneer, the bending being done over an alcohol flame as illustrated in [diagram 15]. But first of all the blades are cut to shape, sandpapered and finished before they are bent. As shown in the drawing a slot is filed in the hub of each blade to enable the propeller shaft to pass through when both have been glued together. The blades are then glued and bound together, first by placing a piece of wire in the slots to insure their being centered and also to prevent their being filled with glue. After this has been done each propeller is given three coats of the same dope as is used on the wings.