Читать книгу Building and Flying an Aeroplane - Charles B. Hayward - Страница 5

The student may enter upon the business of building to any extent that his inclination or his financial resources or his desire to experiment may lead him. The simplest stage, of course, is that of model building and there is a great deal to be learned from the construction and flying of experimental models. This has become quite a popular pastime in the public schools and some very creditable examples of work have been turned out. The apparent limitations of these rubber-band driven models need not discourage the student, as some of the school-boy builders have succeeded in constructing models capable of flying a quarter mile in still air and their action in the air is wonderfully like the full-sized machines.

Models with Rubber-Band Motor. The limitations of the available power at command must be borne in mind, as the rubber-band motor is at best but a poor power plant. It is accordingly not good practice to have the spread of the main planes exceed 24 inches, though larger successful models have been built. In attempting to reproduce any of the well-known models, difficulty is often experienced in accommodating the rubber-band motor to them, as even where the necessary space is available, its weight throws the balance out entirely, and the result is a model that will not fly. This has led to the production of many original creations, but these, while excellent flyers, would not serve as models for larger machines, as of necessity they have been designed around their power plants. The rubber bands for this purpose may be purchased of any aeronautic supply house. The most practical method of mounting the motor is to attach it to the rear end of the fuselage, usually a single stick, which is accordingly made extra long for that purpose. At the other end it is attached to a bent wire fastened to the propeller in order to revolve the latter. An easy way to wand up the motor is to employ an ordinary egg beater, modified as described below, or a hand drill, inserting a small wire yoke in the jaws in place of the usual drill, or bit. This yoke is placed so as to engage the propeller blades, and the latter is then turned in the opposite direction, storing energy in the rubber band by twisting its strands tightly.

Fig. 1. Details of Main Frame of Rubber-Band Driven Aeroplane Model

For those students who do not care to undertake an original design at the outset, or who would prefer to have the experience gained by building from a plan that has already been tried, before attempting to originate, the following description of a successful model is given. This model can not only be made for less than the models sold at three to five dollars, but is a much more efficient flyer, having frequently flown 700 feet.

Main Frame. The main frame of the model monoplane consists of two strips A of spruce, each 28 inches long, and measuring in cross section 1/4 by 3/8 of an inch. As shown in Fig. 1, the two strips are tied together at the front with strong thread and are then glued, the glue being spread over and between the windings of the thread, Figs. 1 and 2. The rear ends of these strips are spread apart 4 1/4 inches to form a stout triangular frame, and are tied together by cross bars of bamboo B and C which are secured to the main strips A by strong thread and glue.

Fig. 2. Details of Forward Skids of Aeroplane Model

Propellers. The propellers D are two in number and are carried by the two long strips A. Each propeller is 5 inches in diameter, and is whittled out of a single block of white pine. The propellers have a pitch of about 10 inches. After the whittling is done they are sandpapered and coated with varnish. The thickness of the wood at the hub E₂, Fig. 3, of the propeller should be about 5/8 inch. At the rear ends of the strips A, bearing blocks E₁ are secured. These bearing blocks are simply small pieces of wood projecting about 5/8 inch laterally from the strips A. They are drilled to receive a small metal tube T₂ (steel, brass, or copper), through which tube the propeller shaft T₁ passes.

Fig. 3. Details of Propeller and Rudder of Aeroplane Model

The propeller shaft itself consists of a piece of steel wire passing through the propeller hub and bent over the wood, so that it can not turn independently of the propeller. Any other expedient for causing the propeller to turn with the shaft may obviously be employed. Small metal washers T₃, at least three in number, are slipped over the propeller shaft so as to lie between the propeller and the bearing block.

That portion of the propeller shaft which projects forwardly through the bearing block E₁ is bent to form a hook T₄. To the hook T₁ rubber strips T₂ by which the propellers are driven, are secured. The rubber strips are nearly as long as the main strips A. At their forward ends they are secured to a fastening consisting of a double hook G H, the hook G lying in a horizontal plane, the hook H in a vertical plane. The hook holds the rubber strips, as shown in Figs. 1 and 4, while the hook H engages a hook T. This hook is easily made by passing a strip of steel wire through the meeting ends of the main strips A, the portions projecting from each side of the strips being bent into the hooks I.

Skids. Three skids are provided, on which the model slides, one at the forward end, and two near the rear end. All are made of bamboo. As shown in Fig. 2 the front skid may be of any length that seems desirable. A 6-inch piece of bamboo will probably answer most requirements. This piece N is bent in opposite directions at the ends to form arms Z and U, The arm Z is secured to the forward ends of the two strips A, constituting the main frame, by means of thread and glue. The strips and skid are not held together by the same thread, but the skid is attached to the two strips after they have been wound. Hence, there are two sets of windings of thread, one for the two strips A themselves, and another for the skid and the strips. Strong thread and glue should be used, as before. In order to stiffen the skid, two bamboo struts W will be found necessary. These are bent over at the ends to form arms V₁, Fig. 2. Each of the arms is secured to the under side of a strip A by strong thread and glue. The arms X are superimposed and tied to the bamboo skid V with strong thread and glue.

Fig. 4. Details of Rear Skids on Aeroplane Model

The two rear skids, of which one is shown in Fig. 5, consist each of two 5-inch strips of bamboo S, likewise bent at either end in opposite directions to form arms S₂ and S₃, The arms S₂ are fastened to the strips A by strong thread and glue. To stiffen the skids a strut C₁ is provided for each skid. Each strut consists of a 3-inch strip of bamboo bent over so as to form arms C₂. Strong thread and glue are employed to fasten each strut in position on the strip and the skid. In the crotch of the triangular space B₁, a tie bar J, Figs. 4 and 5, is secured by means of thread and glue. This tie bar connects the two skids, as shown in Figs. 1 and 4, and serves to stiffen them. The triangular space B₁ is covered with paper, preferably bamboo paper. If bamboo paper is not available, parchment or stiff light paper of some kind may be used. It does not need to be waterproof. Thus triangular fins are formed which act as stabilizing surfaces.

Fig. 5. Enlarged Details of One Rear Skid, Aeroplane Model

Main Planes. The main planes are two in number, but are different in size. Contrary to the practice followed in large man-carrying monoplanes, the front supporting surface is comparatively small in area and the rear supporting surface comparatively large. These supporting surfaces L and P are shown in detail in Figs. 6 and 7. It has been found that a surface of considerable area is required at the rear of the machine to support it, hence, the discrepancy in size. Although the two supporting surfaces differ in size, they are made in exactly the same manner, each consisting of a thin longitudinal piece of spruce R, to which cross pieces of bamboo Q are attached. In the smaller plane, Fig. 7, all the cross pieces are of the same size. In the larger plane, Fig. 6, the outer strips S are somewhat shorter than the others. Their length is 2 1/2 inches, whereas the length of the strips Q is 3 1/2 inches. In order to allow for the more gradual tapering of the plane, around the outer ends of the longitudinal strips R and the ribs Q a strip of bamboo is tied. The frame, composed of the longitudinal strip and cross strips, is then covered with bamboo paper, parchment paper, or any other style light paper, which is glued in place.

Fig. 6. Details of Main Plane of Aeroplane Model

Fig. 7. Details of Smaller Plane of Aeroplane Model

The forward or smaller plane has a spread of 8 1/2 inches and a depth of 3 1/4 inches. The main plane has a spread of 20 inches and a depth of 3 1/2 inches at the widest portion. The author has made experiments which lead him to believe that the tapering form given to the outer edge of the plane improves both the stability and endurance of the machine.

The planes are slightly arched, although it will be found that flat planes will also give good results. The rear edge of the main plane should be placed 4 1/4 inches distant from the forward edge of the propeller block E₁.

The front plane must have a slight angle of incidence, just how much depends upon the weight of the machine, the manner in which it is made, and various other factors. This angle of incidence is obtained by resting the front portion of the plane on two small blocks N, Figs. 1 and 2, which are fastened to the top of the main strip A by strong thread and glue.

Fig. 8. Device for Winding up Rubber-Band Motors

The height of the blocks N should be about 1/4 inch, although this will necessarily vary with the machine. The blocks should be placed approximately 4 inches from the forward end of the machine. The front end of the forward plane should be elevated about 1/4 inch above the rear end, which rests directly on the main strips.

Both the front and rear planes L and P are removably lashed to the frame by means of ordinary rubber bands, which may be obtained at any stationery store. These rubber bands are lettered M in Fig. 1.

Winding the Rubber Strips. The rubber strips can be most conveniently wound up by means of an egg beater, slightly changed for the purpose. Fig. 8. The beater and the frame in which it is carried are entirely removed, leaving only the main rod E, which is cut off at the lower end so that the total length is not more than 2 or 3 inches. The two brass strips D on either side of the rod, which are attached to the pinion Q meshing with the large driving wheel H, are likewise retained. A washer F is soldered to the rod near its upper end, so as to limit the motion of the small pinion and the brass strips D attached to the pinion. Next a wire B is bent in the form of a loop, through which loop the central rod passes. The ends of the wire are soldered to the side strips D. Lastly, a piece of wire C is bent and soldered to the lower ends of the side strips. In order to wind up a rubber strip, the strip is detached from the forward end of the model, and the hook A slipped over the wire C. The opposite end of the rubber band is held in any convenient manner. Naturally the two strips must be wound in opposite directions, so that the two propellers will turn in opposite directions. By stretching the rubber while it is being wound, more revolutions can be obtained. It is not safe to have the propeller revolve more than 700 times. The ratio of the gears of the egg-beater winder can be figured out so that the requisite number of twists can be given to the rubber bands for that particular number of revolutions.

Model with Gasoline Motor. The next and somewhat more ambitious stage is the building of a power-driven model, which has been made possible by the manufacture of miniature gasoline motors and propellers for this purpose. Motors of this kind, weighing but a few pounds and capable of developing 1/4 horse-power or more, may be had complete with an 18-inch aluminum propeller and accessories for about $45. As is the case with the rubber-band driven model, the monoplane is the simplest type to construct, and the dimensions and details of an aeroplane of this type are given here. It will be found that a liberal-sized machine is required to support even such a small motor. The planes, Fig. 9, have a spread of 7 feet 8 inches from tip to tip, each wing measuring 3 1/2 feet by a chord of 15 inches. They are supported on a front and rear wing spar of spruce, 1/2 by 3/8 inch in section, while the ribs in both the main plane and the rear stabilizing plane measure 1/8 by 1/2 inch in cross section. There are eight of these spruce ribs in the main plane, and they are separately heated and curved over a Bunsen burner, or over a gas stove, which is the same tiling. They are then nailed to the wing spars 6 inches apart. The main spars of the fuselage are 7 feet long and they are made of 1/2 by 3/8 inch spruce, the struts being placed 1 1/2 feet apart, measuring from the rear, with several intermediate struts to brace the engine bed. Instead of using strut sockets for the fuselage, which would increase the cost of construction unnecessarily, a simple combination of a three-way wire fastener and a wire nail may be resorted to. The shape of these fasteners is shown at A in Fig. 9. They may be cut out of old cracker boxes or tin cans (sheet iron) with a pair of shears, the holes in the ends being made either with a small drill or by driving a wire nail through the metal placed on a board, and filing the burrs off smooth. A central hole must also be made for the 1 1/2 inch wire nail which is driven through the main spar and the fastener then slipped over it. As indicated, this nail also serves to hold the strut. A drop of solder will serve to attach the fastener to the nail. The front of the fuselage is 9 inches square, tapering down to 6 inches at the rear. The height of the camber of the main planes is 1 1/2 inches and the angle of incidence is 7 degrees, measured with relation to the fuselage. The non-lifting tail plane at the rear which is to give the machine longitudinal stability, measures 4 feet in span by 14 inches in depth.

Fig. 9. Details of Power-Driven Aeroplane Model

The running gear or front landing frame is made of 1/2 inch square spruce, all joints being made with 1/16 by 1 inch bolts. Aluminum sleeves, procurable at an aeronautic supply house, are employed for the attachment of the rubber springs and the radius rods running down to the wheels, which may also be purchased ready to install. Old bicycle wheels will serve the purpose admirably. Light steel tubes 1/2 inch in diameter are used to run these aluminum sleeves on. Two other steel tubes are joined to the lower corner of the frame by flattening them at the ends and drilling with a small hole for a nail. These are run diagonally up to the fuselage and serve as buffers to take the shocks of landing. For bracing the wings, two similar tubes are fastened to form a pyramid on top of the main plane just back of the engine. From these, guys are run to the wings as shown. The engine bed is made of 1/2 by 3/4-inch white pine, and to make it solid it is carried as far back as the rear edge of the main plane. The batteries and coil are directly attached to this plane, care being taken in their placing to preserve the balance of the machine. The rudder measures 14 inches square and is made of 3/8-inch square spruce, reinforced with tin at the joints, as it is necessary to make the frame perfectly rigid. Both sides are covered with fabric. In this case a 1-horse-power motor furnishes the necessary energy and it is fitted with an 18-inch aluminum propeller which it is capable of turning at 2,400 r.p.m. The carbureter and gas tank are made integral, and the gasoline and oil are both placed in this tank in the proportion of about four parts to one, in order to save the weight of an extra tank for oil.

Flights of half a mile are possible with this model in calm weather, but a great deal of measuring and testing of the fuel is necessary in order to regulate the flight, and "grass-cutting" should be practiced by the builder in order to properly regulate the machine. Trials have shown that the flat non-lifting tail on the fuselage gives excellent longitudinal stability, the machine rising nicely and making its descent very easy angle, so that it is seldom damaged by violent collisions in landing.