The model aeroplane requires more work than ordinary kite construction. It also requires more patience and greater accuracy, because each part of the little aircraft must be made just so, assembled just so, and "tuned-up" just so, to produce a model which will give a good account of itself.
Of course your first model will probably not be perfect. But if you do your work correctly and carefully it will fly, and the experience you have acquired will make it possible to turn out a more nearly perfect second model.
Many types of model aeroplanes have been devised, but those of the simplest form of construction have made the best showing. The majority of record-breaking models have been of one type—a triangular framework, equipped with two planes, and a pair of propellers operated by a pair of rubber-strand motors.
A most successful model of this type is shown in Fig. 34, and described and illustrated on the following pages. This model has a distance record of 1620 feet made at the Aero Club of Illinois' aviation field at Cicero, Chicago, where it flew 16 feet beyond the fence of the 160 acre field.
The model weighs but 5½ ounces, has 9-inch propellers of 27 inch pitch, and is in every essential a speed machine.
Fig. 34.—Launching a Model Aeroplane.
The first part of the model to make is the triangular Fuselage, or motor base.
This consists of two side sticks, splines, or spars (A, Fig. 35) of straight-grained white pine cut to the dimensions marked upon the drawing, with their bow ends beveled off for a distance of 1¼ inches, glued together, and bound with thread. The stern ends have a spread of 8 inches, and are braced at that distance by the separator B (Fig. 35).
This separator is fastened flatwise between sticks A, and its edges are reduced as shown in the small section drawing of Fig. 37 so they will offer less resistance to the air.
This piece is fastened between sticks A with brads. Separators C, D, and E are of the sizes marked in Fig. 35, and of the proper length to fit between side sticks A at the places indicated on the drawing. They are cut oval-shaped, as shown in the small section drawing in Fig. 37.
Figs. 35 and 36.—Working-drawings of Model Aeroplane Designed and Built by Harry Wells.
This Model has a record of 1620 feet made at the Aero Club of Illinois' Aviation Field at Cicero, Chicago.
Before fastening the separators in position,
The Thrust Bearings for the propellers, and the end plates for connecting the wire stays, must be prepared. Figure 38 shows a dimensioned detail of the thrust bearings, and Fig. 37 shows how they are bound to the ends of sticks A with thread. These are cut out of brass, bent into the shape shown, and have a hole pierced through the folded tip for the propeller-shaft to run through, another through one end for the brad to pass through that pins stick A to B, and another through the other end to fasten the end of the wire stays to. The small detail in Fig. 37 shows the end plates for the wire stays. These are made no longer than is necessary for the connecting holes for the wire-stay ends. Pierce a hole through the center of each plate for the brad to pass through which fastens sticks A to the ends of the separators. The plates are bound to sticks A with thread.
Fig. 37.—Detail of Fuselage and Motor of the Wells Model.
Fig. 38.—Detail of Thrust Bearing, Propeller-shaft, and Connections.
Fig. 39.—Detail of Bow Hook and how Rubber Motor is Connected to it.
The Bow Hooks support the bow ends of the rubber motor, and are made upon the ends of a piece of heavy piano-wire bent V-shaped to fit over the ends of sticks A (Fig. 39). Bind the wire to the sticks with thread, coating the thread with glue to make it hold fast (Fig. 37).
The Main Plane has a framework built as shown in Fig. 40, with the front or entering-edge, and the rear or following-edge, made of sticks of white pine or other light-weight wood, and the ribs and tips on the ends made of No. 16 gauge aluminum wire. The ends of the frame sticks are cut away on their outer edge, to receive the ends of the wire forming the tips, and the ends of these wires, and the laps of the wire ribs, are bound in position with thread, and the thread then coated with glue to hold it in position.
The Elevator, or front plane, has a framework made as shown in Fig. 41. Its entering-edge is a stick, and its following-edge, ribs, and end tips, are made of No. 16 gauge aluminum wire. You will notice by Fig. 41 that the center ribs cross the following-edge of the frame and are bent up in the form of a flat loop. This loop rests against the under side of the fuselage, and gives the elevator its proper angle for stability (Fig. 36). The tips are bent up to add stability.
The frames of the main plane and elevator are covered with china-silk, which may either be sewed or glued in place, and this is given a thin coat of shellac to make it air-tight and taut. The covering must be put on smoothly to reduce to a minimum what is known as skin resistance—the resistance that the plane makes to the air while passing through it.
The main plane and elevator are held to the fuselage by means of rubber-bands slipped beneath them and over the fuselage, and unlike the planes of the majority of models, are fastened to the under side of the fuselage. Figure 36 shows the approximate position of the elevator. That of the main plane will vary under different air conditions, sometimes being placed over the separator C, and at other times closer to separator B than is shown in Fig. 35. Therefore, you must adjust your plane and elevator—this operation is known as tuning—to suit the condition of the atmosphere, until you find the positions where they will give the machine the greatest stability. A great factor in the successful flight of a model aeroplane lies in properly tuning the planes, both laterally and longitudinally, and of course the planes must balance at their centers, in order to make the machine balance properly.
Framework.
Fig. 40.—Detail of the Main Plane Framework of the Wells Model.
Fig. 41.—Detail of the Elevator Framework.
Fig. 42.—Detail of Fin.
The Fin directly over the center of the elevator (Figs. 34 and 36) is provided for stability, and may be used as a rudder by turning it slightly to one side or the other. It is made of No. 34 gauge sheet aluminum, cut to the form shown in Fig. 42. Its vertical edge is bent around a piece of heavy wire, as shown in the plan detail of Fig. 42, and the lower end of the wire is fastened upright between the bow ends of sticks A.
Fig. 43.—The Wells Model Propeller.
The Propellers are the most difficult part of the model aeroplane to make. They must be very accurately cut, and must be of identical size and pitch. The pitch of a propeller is, theoretically, the distance forward that it advances in one complete revolution.
Figure 43 shows one of the propellers of Harry Wells' machine, which is 9 inches in length and has a 27-inch pitch. Figure 44 shows
How to Prepare the Propellers. The pair must be opposites, that is, one must be of right-hand pitch and the other of left-hand pitch, or, in other words, the upper end of the right-hand pitch propeller turns to the right, and that of the left-hand pitch propeller turns to the left, when viewing them from the rear.
Fig. 44.—How to Prepare a 9-inch Propeller.
Step A consists in properly planing up a straight-grained block of white pine 1½ inches thick, 2 inches wide, and 9 inches long, with its sides and ends straight and true, for
The Propeller Blank. Draw a line around the four faces of this block at the exact center of the length. Then on faces C and D, lay off a distance of ½ inch on the center-line, measuring from the edge of face B, for the thickness of the propeller-hub, and draw diagonal lines from the upper and lower left-hand corners of faces C and D to the end of the hub center-line (Step B). Then cut away the portions outside of these lines, as shown in Step C. Lay out the hub upon faces A and B of the block, with a ½-inch diameter, and bore a small hole through the center to receive the propeller-shaft (Step C). Draw diagonals from the corners to the center-line of the hub (Step D); then cut away the wood outside of these lines (Step E).
The next step (F) consists in laying out the form of the propeller blade upon all four sides and ends of the block, and Step G is the final one of cutting out the propeller, scooping out its blades concave on one side, and carving them convex on the opposite side. A very sharp knife must be used for cutting; and the work must be done slowly and carefully, because the least slip is likely to ruin the propeller. The entering-edge of each blade is the almost straight edge, and should be cut very thin. The ends of the blades should also be cut thin, while the hub should be cut away as much as can safely be done without weakening the propeller.
When you have completed cutting the propellers, place them at their centers across the edge of a knife-blade, and if they do not balance perfectly, locate the trouble and correct it. Finish the work with fine emery-paper, and then shellac it. Some boys glue silk over the ends of their propeller blades, for a distance of ½ inch or so, to reinforce them and make them less likely to split.
The Propeller-shafts are made of heavy piano-wire, bent into a hook at one end (Fig. 38) to receive the rubber strands of the motor, and cut of the right length to extend through the hole in the bearing, through a glass bead, through the propeller, and then to bend over the side of the hub (Figs. 37 and 38). By bending over the end of the shaft against the hub, it is held securely in place.
The Motors consist of twelve strands of 1/8-inch flat rubber, each, and as these are 1 yard in length, exactly 24 yards of rubber are required. The rubber is not connected direct to the hooks on the bow and propeller-shafts, as the wire would quickly cut through the strands. Instead, small rings are bent out of wire, with pieces of small rubber-tubing slipped over the wire, and the ends of the rubber strands are looped through these rings and bound in place with thread (Fig. 39). The wire rings are then slipped on and off the hooks quickly. As light and heat cause rubber to deteriorate, you must remove the motors from the machine after use, pack away in a covered box, and keep in a cool place, in order to get the longest life possible out of the rubber.
It has been found that rubber motors can be wound much farther by lubricating them with glycerine. It is only necessary to put a few drops of the glycerine upon a clean cloth, and rub it over the outside strands; then wind the motors, and it will work over the surface of the inner strands until all parts are covered.
Fig. 45.—A Home-made Motor Winder.
Fig. 46.—The Kind of Egg-beater to Use.
Fig. 47.—How the Motors are Connected to Winder for Winding.
Of course the rubber motors must be twisted an equal number of turns, in order to make the propellers work the same, and this is usually done with an ingenious winder made from an egg-beater, which winds both motors simultaneously.
The Home-made Motor-winder shown in Fig. 45 is made from a Dover egg-beater (Fig. 46). To convert the egg-beater into a winder, it is necessary to cut off the loop ends and the center pivot wires on which the loops turn. Then bend the cut-off ends of the loops into hooks, and punch them to fit over the pivot wire ends, as before (Fig. 45). The ends of the pivot wires must be riveted to keep the hooks in position.
Figure 47 shows
How the Egg-beater Winds the Motors. While an assistant supports the model by the propeller end, you remove the motor rings from the hooks on the bow of the fuselage, and slip them on to the hooks of the egg-beater. Then you turn the crank of the winder, counting the turns as you do so, and when you have wound the motors as far as you wish, slip off the motor rings, and slip them back on to the bow hooks of the model aeroplane. Motors of models like that shown in this chapter are wound one-thousand turns or more for each flight.
Wind the Motors Slowly, especially after the first row of knots begin, as it puts the rubber to the least amount of strain by doing this. Quick winding not only strains the rubber but makes the knots form in bunches, and uneven winding, of course, produces an uneven unwinding.
The propellers must be held after the motors have been wound, to keep them in check. Figure 34 shows
The Position to Take for Launching a Model from the hand. The machine should not be thrown forward, as the movement would cause too great a disturbance of the air, resulting in the machine losing its stability, and probably upsetting. The best method is to give the model a slight push that will start it off at a speed a trifle under that produced by its propellers.
HOME-MADE TOYS FOR GIRLS AND BOYS
BOOKS BY A. NEELY HALL
LOTHROP, LEE & SHEPARD CO., BOSTON
Published, August, 1915
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