Model Aeroplanes And Their Engines

To find the center of the block two lines should be drawn from the opposite corners, their point of meeting being approximately in the center—near enough for all practical purposes to insure greater accuracy. Similar lines should be drawn from the corners on the Bottom side of the block of wood. A hole ³⁄₃₂ of an inch in diameter should be bored through the center thus obtained, through which the propeller shaft will be inserted when the propeller is finished. The two sections of the propeller blades drawn in diagrammatical form on the Top of the block, should be marked respectively Blade 1 and Blade 2, as shown in diagram 1. The block is then ready for the commencement of the actual cutting. In cutting out the propeller, Blade 1 should be held in the left hand and the knife in the other, with the blade of the knife on the straight edge of Blade 1. The cutting should be carried out very carefully with attention constantly paid to Fig. 2, and should be stopped when the line shown in Fig. 2 has been reached. The semi-blade should then be sandpapered until a small curve is obtained by which the propeller will be enabled to grip the air.

To cut Blade 2, Blade 1 should be held in the left hand and Blade 2 cut until the line shown in Fig. 3 is reached, after which the sandpapering process is carried out in the same manner as in the case of Blade 1. During all of the foregoing operations it must be clearly borne in mind that the Top of the blank propeller must always face upward, and the cutting should always be done on the Straight lines. Should the straight edge be cut on one edge of the blank propeller and the curved edge on the other, it would result in the blades of the finished propeller having a tendency to push in opposite directions and in consequence no propulsion of the model would be possible.

Attention should next be turned to the back of the propeller blank on which the manner of cutting is exactly like that suggested for the top side, with the exception that instead of cutting along the Straight lines, the cutting is done along the Curved lines. In this part of the work great care is to be exercised for by the time the necessary cutting has been done on the back of the propeller the entire structure is very fragile and one excessive stroke of the knife may result in destroying the entire propeller blade. By constantly holding the wood to the light it is possible to determine with a reasonable degree of accuracy the evenness of thickness. To complete the Bottom side of the propeller the blade should be sandpapered as was the top.

The method of cutting the second propeller is exactly that used in cutting the first propeller, only that the diagram shown in Fig. 4 should be used. This will result in two propellers being made that will revolve in opposite directions in order to produce even and balanced propulsion. If both propellers revolved in the same direction the effect would be to overturn the model.

In diagram 1 the propellers are shown with the straight edge as the entering or cutting edge of the blade. Some of the model builders prefer the curved edge as the cutting edge (diagram 2). It is significant that Mr. Frank Schober, a well known model constructor, tested both designs on his compressed air driven model, and while both propellers were the same in weight, diameter and pitch, the one having the straight edge as the cutting edge was found one-third more efficient.

When the propellers have been given a light coat of shellac they should be laid aside until the assembling of the complete model.

By following the foregoing instructions a simple and effective set of propellers will be produced. But in order to vary the experimental practice of the constructor various other diagrams, Nos. 3 and 4, illustrating suitable designs, are provided and can be made by applying the above general theory and using the diagrams herewith.

One of the most important considerations in the construction of a model is the making of the wings. To obtain the greatest efficiency the wings must be carefully designed, with due attention to whether the model is being constructed for speed, duration or climbing ability. Attention should be given to streamline construction; that is, the parts of the wing should be so assembled that the completed wing would offer the least possible resistance to the air, if the best results are to be obtained.

For the main wing three strips of spruce, each 30″ in length, two of them being ³⁄₁₆″ × ¹⁄₄″ and the third ³⁄₁₆″ × ¹⁄₁₆″ are required. To make them thoroughly streamline all edges should be carefully rounded off and all surfaces should be smooth. A strip of bamboo at least 20″ long, ¹⁄₂″ wide, ¹⁄₈″ thick, should be cut into pieces, each piece to be 5 in. long. To secure the necessary curve, ¹⁄₂″ depth, the pieces of bamboo should be held in steam and slowly bent in a manner closely resembling the skids of an ordinary bobsled. When the curvature has been obtained, care should be exercised in cutting each piece into four longitudinal strips, from which twelve should be selected to be used as ribs, each to be ¹⁄₈″ wide. The bending of the bamboo preliminary to making the ribs is done in order to secure uniformity of curvature.

When this has been done the ribs are ready for fastening to the sticks—entering and trailing edges—and each must be attached an equal distance apart. In order that the ribs may be evenly spaced it is necessary to put a mark every 3″ on the larger stick or entering edge of the wing, and also on the flat stick or trailing edge. The main beam which is of the same dimensions as the entering edge is afterwards fastened across the center of the wing, and does not necessarily need to be thus marked, as it is fastened to the ribs after the ribs have been attached to the entering and trailing edges of the wing frame. By holding the ribs one at a time so that the curved edge rests upon the entering edge where the mark indicates, as shown in diagram 5, they should be fastened thereon by means of thread and glue. The rear end of the rib must be fastened to the trailing edge where the mark indicates, also by thread and glue.

After all ribs have been thus securely fastened to both edges of the frame the third stick, or main beam, should be attached to the frame on the underside, the fastening being made at the highest point of the curve of each rib. This main beam prevents the wing covering from drawing in the end ribs and adds very materially to the strength of the entire wing structure. To cover the wings fiber paper may be used and is a suitable material, but the best results, from a standpoint of flying efficiency and long service, are obtained by the use of China silk.

The frame of the forward wing or elevator is made in the same manner as is the main wing, but it is only 12″ in span by 4″ in chord, and is constructed without the use of a main beam. This wing has only five ribs which are made in the same manner as those for the rear wing, and each is placed a distance of 3″ apart.

A piece of silk measuring 2″ longer and 2″ wider than each of the wing frames should be used in covering the wings, and this can be held in position by the use of pins prior to the actual sewing. The extra inch of silk on all sides of the frame is placed around the under side of the frame—in order that it can be made thoroughly taut when the silk has been sewn close to the edges of the frame. After the silk has been sewn close to the edges the pins may be removed and the surplus silk that hangs from the under side of the frame may be cut off. To make this silk airproof it should be coated with a thin coat of shellac or varnish and the wings should be thoroughly dry before being used. This coating, in addition to airproofing, will assist in making the covering perfectly taut, and also in making the wing ready for service when the entire model is ready to be assembled.

As all other parts of the model are attached to the frame in addition to its having to stand the strain of the tightly wound rubber strands which serve as the motive power for the model, it must be made strong. It is therefore necessary to exercise care and judgment in making certain that the different units that make up the frame are rightly proportioned and are of the proper material. Just as in the large sized aëroplanes there are many types of bodies, so there are many different types of frames in use in model construction, but the standard, and for all practical purposes the best frame, resembles the letter A in shape, hence the name A type. The lightness of the frame depends entirely on the materials used and the manner in which it is constructed.

Some model flyers use but a single stick for the frame, but generally the A type frame is preferred for the reason that it is more durable, the wings can be more securely attached to it, and that it is possible of developing very much better results.

To construct such an A type frame 2 main sticks to serve as frame side members are necessary and are made from spruce. Each member should be 36″ in length, ³⁄₈″ in depth by ¹⁄₄″ in width. By rounding the edges and smoothing the various surfaces with sandpaper streamline effect will be secured and will add to the efficiency of the machine as well as to its appearance. When the side members are placed in A formation the extremity of the sticks at which they meet should be so tapered in the inner sides that when they meet and are permanently fastened the result will be a continuance of the general streamline effect. The permanent fastening of the frame side members at the point of the A may be accomplished by using either strong fish glue or better, a good waterproof glue and then have the jointure reinforced by securing a piece of ³⁄₃₂″ steel wire 3″ in length and placing the center of it at the point of the A, afterwards bending the wire along either outer edge of the frame side members, putting as much pressure on the wire as the strength of the structure will permit; after this the reinforced jointure should have thread wound around it to insure even greater strength. About ¹⁄₂″ of the wire on each side of the point should be left clear and afterwards turned into a loop as shown in diagram 6, for the purpose of attaching the hooks that hold the rubber strands. To hold the side members apart at the rear end and for a propeller brace, a piece of bamboo 10″ long, ¹⁄₈″ thick by ¹⁄₂″ in width is required and this should be fastened to the extreme rear ends of the frame side members, allowing the propeller brace to protrude on either side 1¹⁄₂″ as illustrated. To put the propeller brace in position a slot ¹⁄₂″ deep by ¹⁄₈″ wide should be cut into the rear ends of the frame side members for the reception of the propeller brace. After the brace has been placed in position the outer edge should come flush with the rear ends of the side members. To hold the brace in place thread and glue should be used in the same manner as described for the point of the frame side members. Between the point of the frame and the propeller brace two bamboo pieces, one 9″ long and another 2¹⁄₃″ long, should be used as braces for the general strengthening of the structure. The longest piece should be secured across the top of the frame about 9″ from the rear and the shorter piece about 9″ from the point.

When these two braces are in position the next matter that calls for the attention of the constructor is the matter of getting into position at the two outer extremities of the propeller brace bearings for the propellers. For this purpose two pieces of ³⁄₃₂nd inch brass tubing, each ³⁄₄th of an inch long, should be used, and should be fastened to the underside of the propeller brace, at each extremity of that brace, by the use of thread and glue. Sometimes greater efficiency is secured by putting these pieces of bronze tubing about ¹⁄₄″ from the end. Some model constructors make a very neat jointure here by soldering the piece of tubing to a strip of thin brass, which is bent over the end of the propeller brace and bound and glued thereon. In fastening the bronze tubing to the propeller brace it should be so adjusted that it will run parallel to the side members of the frame and will therefore offer the least possible resistance to the shaft of the propeller when the rubber strands have been attached.

When the frame has been completed a coat of shellac should be applied to the entire structure to render it damp-proof.

The proper assembling of the parts of the model is as essential to good results as is the designing and making. Parts, although properly made, if improperly placed in relation to each other will very often lead to trouble. Therefore very great care must be exercised in the assembling process.

When all the parts have been prepared and are ready to be assembled the first thing that should be done is to mount the propellers in position. This must be done very carefully on account of the fact that the propeller shafts are easily bent and if bent the result is considerable trouble, for such a bend in the propeller shaft will cause the propeller to revolve irregularly with a consequent loss of thrust. Before inserting the propeller shafts in the tubing 4 washers each ¹⁄₄″ in diameter should be cut from hard metal, and a hole large enough for the propeller shaft to pass through should be bored in the center of each washer. The metal washers should be passed over the straight ends of the shafts which extend from the rear of the tubing, after they have been inserted in the tubing, and in this manner the cutting into the hubs of the propellers which would follow is avoided. The propellers are now to be mounted and this is accomplished by allowing the ends of the shafts, which extend out from the rear of the tubing, to pass through the hole in the hub of each propeller. In mounting the propellers it is absolutely necessary to have the straight edge of the propellers to face the point or front end of the model. The propeller shown in Fig. 4 of diagram 1, should be mounted on the left side of the frame to revolve to the left, while the propeller shown in Fig. 1 should be mounted on the right side of the frame to revolve to the right. When the propellers have thus been mounted the one-half inch of shafting which extends out from the hubs of the propellers should be bent over to grip the propeller hub and thereby prevent the shaft from slipping during the unwinding of the rubber strands. For the reception of the rubber strands to provide motive power a hook must be formed in each shaft and this can be done by holding securely that portion of the shaft which extends toward the point of the model, while the end is being formed into a hook as illustrated in diagram 7.

Eighty-four feet of ¹⁄₈th″ flat rubber is necessary to propel the model. This should be strung on each side from the hooks (see diagram) at the front part of the model to the propeller shafts at the rear of the model. In this way 14 strands of rubber will be evenly strung on each side of the frame. To facilitate the winding of the rubbers two double hooks made of ³⁄₃₂″ steel wire to resemble the letter S, as shown in diagram 7, should be made. One end of this S hook should be caught on the frame hook, while the other end is attached to the strands of rubber, and to prevent the possible cutting of the strands a piece of rubber tubing is used to cover over all wire hooks that come in contact with the rubber strands providing propelling power.

The wings are mounted on the top side of the frame members by means of rubber bands and in placing them upon the frame it should be noted that the entering edge of each wing must face the point or front of the model. The wings must be so adjusted on the frame that they result in perfect side balance which means that there is an even amount of surface on either side of the model. To secure a longitudinal balance it will be found that the entering edge of the main wing should be placed approximately 8″ from the propeller brace or rear of the model, and the entering edge of the small wing or elevator approximately 6″ from the point. But it is only by test flying that a true balance of the entire model can be obtained. To give the necessary power of elevation (or lifting ability) to make the model rise, a small block of wood about 1″ long by ¹⁄₄″ square must be placed between the entering edge of the small wing and the frame of the model.

After the wings have been thus adjusted and a short test flight made to perfect the flying and elevating ability of the model, and this test flight has been satisfactory, the model is ready for launching under its full motive power.

In the preliminary trials of a model close attention must be paid to the few structural adjustments that will be found to be necessary and which if not properly and quickly remedied will result in the prevention of good flights or even in possible wrecking of the model. Careful designing and construction are necessary but it is equally as important that the model should be properly handled when it is complete and ready for flying.

The approximate idea of the balance of a model can be secured by launching it gently into the air. If the model dives down point first it indicates that the main wing should be moved a little toward the front. If it rises abruptly the main wing should be moved slightly toward the rear. In this way by moving the wing forward or rearward until the model glides away gracefully and lands flat upon the ground, proper adjustment of the balance can be effected. If when launching from the hand the model should curve to the left the main wing should be moved slightly to the left of the frame members. And if the curve is to the right the main wing should be moved in that direction. This process can be continued until the model flies in the course desired.

The winding of the rubber strands to get the necessary propelling power is an important detail. The model should be firmly held by some one at the rear with the thumb on either side member, pressing down on the jointure and with the four fingers of each hand gripping the under side of the frame members, and in this way holding the model steady and until the rubber strands have been sufficiently wound. With the hands in this position the propellers, of course, cannot and should not revolve. The hooks attached to the rubber strands at the point or front of the model should be detached from the side members and affixed to the hooks of the winder. A winder may be made from an ordinary egg beater as is shown in diagram 8. When the hooks attached to the rubber strands at the point of the model have been affixed to the winder the rubbers should be stretched four times their ordinary length (good rubber being capable of being stretched seven times its length) and the winding commenced, the person winding slowly moving in towards the model as the strands are wound. If the ratio of the winder is 5 to 1, that is if the rubber is twisted five times to every revolution of the main wheel of the winder, 100 turns of the winder will be sufficient for the first trial. This propelling power can be increased as the trials proceed. When the winding has been accomplished the rubber hooks should be detached from the winder hooks and attached to the hooks at the front of the side members as shown in the diagram.

In preparation for launching, the model should be held above the head, one hand holding it at the center of the frame, the other in the center of the propeller brace in such a way as to prevent the propellers from revolving. When the model is cast into the air if it is properly adjusted it will fly straight ahead.

A precaution which is sometimes worthy of attention before the launching of the model under its full power is to test out the propellers to find out whether or not they are properly mounted and whether they revolve evenly and easily. To do this the rubber strands may be given a few turns, enough to revolve the propellers for a brief period, while the machine is held stationary. If the shafts have been properly inserted in the hubs of the propellers and have not been bent during the winding of the rubbers, the propellers will revolve evenly and readily. If the propellers revolve unsteadily it indicates that there is a bend in the propeller shafts or the propellers have not been properly balanced. If the trouble is a bend in the shaft, it must be removed before the model is launched on actual flight. If the propeller does not revolve freely the application of some lubrication (such as vaseline) to the shaft will eliminate this trouble. With these adjustments made satisfactorily, the model can be launched with the anticipation of good flying.

The preceding instructions and discussions have dealt with different parts of a simple model to be used as a hand-launched type of model. The experience which will come as the result of flying this type of model for a period will undoubtedly tend toward a desire on the part of the constructor to make his model more nearly represent a large sized aëroplane and will make him want to have his model rise from the ground under its own power. Such a model is known as an R. O. G. type, that is, rises off the ground.

To meet this desire all that it is necessary to do is to make a chassis, or carriage, which can be secured to the frame of the model, and with extra power added, will result in a practical R. O. G. model. In constructing such a chassis or carriage it is necessary to bear in mind that it must be made sufficiently strong to withstand the shock and stress which it will be called upon to stand when the model descends to the ground.

For the main struts of the chassis two pieces of bamboo each 9″ in length are needed and these should be bent over 1″ on one end as shown in the diagram, that they may be fastened to the under side of the frame members, one on either side, at a point on that member 12″ from the front. Two similar pieces of bamboo, each piece about 7″ in length, are required to act as braces between the frame members and the main chassis struts. Each end of each of the braces should be bent over in the same direction and in the same manner as that described for the main strut so that the fastening to the main frame member and the main chassis strut may be accomplished. Steam may be used in bending the ends of the pieces of bamboo. To make the landing chassis sufficiently stable to withstand landing shocks a piece of bamboo 9″ should be fastened from either side of the main chassis struts at the point where the chassis brace on either side meets with main strut. The ends of this cross brace should be bent in similar fashion to the other braces to enable its being fastened easily and securely.

Two small wheels constitute the running gear for the front part of the chassis, for which two pieces of ¹⁄₁₆″ steel wire each 2¹⁄₄″ long are required. These small wires are fastened to the bottom ends of the main struts, and to accomplish this the wire should be bent in the center at right angles; one leg of the angle is attached to the bottom end of the main strut as shown in the diagram. Disks for wheels may be cut from a bottle cork which should be ³⁄₄″ in diameter by approximately ¹⁄₄″ in thickness. The edges should be rounded off to prevent chipping. Before mounting the wheels on the axles which have been provided by the wires attached to the bottom of the main struts, a piece of bronze tubing ³⁄₃₂″ inside diameter and ³⁄₁₆″ long should be inserted in the center of each disk. To secure the least possible resistance on the revolutions of the wheels, there should be placed on the wire axles pieces of bronze tubing similar in diameter and ¹⁄₈″ in length on either side of the wheel (see illustration). When the wheel is thus placed in position with the pieces of bronze tubing on either side about ¹⁄₄″ of the axle wire will extend from the outward end of the outside piece of tubing. This should be bent over the tubing to prevent its falling off and at the same time hold the wheel securely in position.

For the rear skid a piece of bamboo 6″ long is used, one end of which is curved as in a hockey stick so that it will glide smoothly over the ground. The other end of the rear skid should be bent over about ¹⁄₂″ so that it can be securely fastened to the propeller braces, as illustrated in the diagram. Two 7″ pieces of bamboo are required to act as braces for the rear skid. Both ends of each brace strut are bent over ¹⁄₂″ in the same direction, one end of each strut is securely fastened to a side member 3″ from the rear and the other end of each strut is fastened to the rear skid, at their point of meeting as shown in diagram 9, the method of attaching being the same as in the case of the forward portion of the chassis. All joining should be accomplished by first gluing the braces and then binding with thread. When completed, the rear skid should glide along the ground in bobsled fashion, thus preventing the propellers from hitting the ground.

In making such a chassis or carriage the endeavor should be made to use, as near as possible, the same weight of material on either side of the model so as little interference as possible will be made with the general balance of the model in flight.

Having satisfactorily developed the hand launched model and the model rising off the ground under its own propulsion the constructor will next turn his mind to the question of having his model rise under its own power from the surface of the water in the fashion of passenger-carrying hydros and flying boats. This will be accomplished by the use of pontoons attached to a specially designed chassis.

Three pontoons are necessary and these should be made as light as possible. Each pontoon should be made 6″ long, 1″ deep toward the forward part, by ³⁄₄″ at the rear and 2″ wide. The side members of each pontoon are made from pieces of thin white pine wood ¹⁄₃₂nd of an inch thick, slightly curved up at the front and sloped down toward the rear. Small niches should be made on the top and bottom sides of the pontoons into which the cross braces are inserted and glued. Further reference to diagram 10 will show that at the extreme forward end of the sides a cut is made large enough to receive a flat piece of spruce ¹⁄₁₆″ wide. Another cut of the same dimensions is made at the extreme rear end. Still further cuts are made on the top and bottom sides of the pontoons, the forward cuts measuring 1¹⁄₂″ from the front and the rear cuts 1¹⁄₂″ from the rear, to join the sides of the pontoons as illustrated in diagram 10. Six pieces of ¹⁄₁₆″ flat spruce are required for the rear pontoon, the ends of which are held in position by glue. For the forward pontoon only 4 braces are required in so far as the ends of the two main brace spars of the forward part of chassis are inserted in the cuts on the top sides of the pontoon. These brace spars measure 10 inches in length and are made from bamboo ¹⁄₈th inch in diameter, which necessitates enlargement of the cuts on the top sides of the forward pontoons so that the extreme ends of the spars can be inserted in the cuts in the place of the braces. To complete the rear pontoon and prepare it for covering, three strips of ¹⁄₈″ bamboo are required for struts. Two of these strips should measure 9″ in length and should be attached to the front of the pontoon on the inner side as shown in diagram 10. Thread and glue should be used in attaching the ends of the strips to the pontoon. To enable fastening to the frame the upper ends of the bamboo strips should be bent over about ¹⁄₂″. The third strip should measure 8″ in length and is attached to the upper and lower braces toward the front of the pontoon as shown in the diagram. It is necessary that this strip be secured in the approximate center of the pontoon to insure a good balance. For the purpose of securing the upper end of the third strut to the center of the propeller brace a piece of wire 1¹⁄₂″ long should be secured to the upper end of the strut and looped as shown in diagram 10. The three pontoons should now be covered with fiber paper and it is necessary to exercise care to avoid punctures. For the purpose of coating the fiber paper to render it waterproof, a satisfactory solution can be made by mixing banana oil with celluloid until it has attained the desired thickness, after which it should be applied to the covering of the pontoons with a soft brush.

For the main strut of the forward portion of the chassis two pieces of ¹⁄₈″ bamboo, each 11″ in length, are required and these should be bent over 1″ on one end as shown in the diagram, that they may be fastened to the under side of the frame members, one on either side at a point on that member 11″ from the front. Two similar pieces of bamboo, each piece 8″ in length, are required to act as braces between the frame members and the main chassis struts. Each end of the braces should be bent over in the same direction and in the same manner as that described for the main struts so that the fastening to the main frame member and the main chassis struts may be accomplished. Steam or an alcohol lamp may be used in bending the ends of the pieces of bamboo. To make the chassis sufficiently stable a piece of bamboo 7¹⁄₂″ should be fastened from either side of the main chassis struts at the point where the chassis brace on either side meets with the main strut. The ends of this cross brace should be bent in similar fashion to the other braces to enable its being fastened easily and permanently.

For the accommodation of the pontoons two strips of flat steel wire, each 4″ in length, should be attached to the ends of the main struts, about one inch from the bottom, the farthest ends should be bent to grip the second spar which joins the pontoons. Note diagram 10.

To further strengthen the chassis a strip of flat steel wire sufficiently long enough should be bent so that ¹⁄₂″ of the central portion can be securely fastened to the center of the cross brace as shown in diagram 10. The two outer ends should be bent down and are fastened to the wires which are attached to the bottom ends of the struts. This method of attaching the forward pontoons enables the constructor to adjust them to any desired angle and also detach them when not in use.

A model hydroaëroplane is one of the most interesting types of models and if properly taken care of will afford the constructor many pleasant moments.

Although the method of determining the balance of an R. O. G. or a model hydroaëroplane is exactly the same as that of a hand launched model, the manner of launching is somewhat different. Instead of holding the model one hand in the center of the frame and the other at the rear as in the case of the hand launched model, in launching an R. O. G. or hydro, the model should be rested upon the ground or water, as the case may be, with both hands holding tightly to the propellers. Then when about to let the model go release both propellers instantly. If the model has sufficient power and it has been properly adjusted it will glide over the surface of the ground or water for a short distance, then rise into the air. Should the model fail to rise into the air additional strands of rubber should be added, after which it should be rewound and a second attempt made.

Should the model fail to respond after the addition of extra rubber, the indications are that something requires further adjustment. Perhaps the pontoons need further elevation if the model is a hydro, or if it be an R. O. G. model the forward wing may require an increase of elevation. In any event the model should be carefully examined and adjustments made where necessary, after which the model should be tested for balance and elevation. If satisfied with the behavior of the model after test flights have been made, another attempt should be made to launch the model from the ground or water.

On no account try to fly the model in the house, or see, supposing the model is of the R. O. G. type, if it will rise from the dining room floor. This advice may seem unnecessary, but it is not so, for there has been quite a number of instances in which the above has been done, nearly always with disastrous results, not always to the model, more often to something of much greater value. The smashing of windows has often resulted from such attempts, but generally speaking pictures are the worst sufferers. It is equally unwise to attempt to fly the model in a garden in which there are numerous obstructions, such as trees and so forth. A wrecked model is very often the result of such experimenting. The safest way to determine the flying ability of any model is to take it out in an open field where its flight is less apt to be interrupted.

The frame is braced further by an upwardly arched strip of bamboo, as shown in diagram 11, this strip being 2¹⁄₂″ in height. At the top of this brace are two bronze strips of No. 32 gauge brass, one above the other, one on top of the brace and the other below.

Adjacent the ends of these strips of metal are perforations through which pass bracing wires, one of which wires runs to the front of the frame where a hook is mounted for its reception, and the other two wires extend to the rear of the frame where they are secured to the propeller brace. The propeller brace consists of a strip of streamlined spruce 11³⁄₄″ in length, the propellers being at an angle, thus clearance is allowed ¹⁄₄″ wide at the center, tapering to ³⁄₁₆″ at the ends. The ends of the propeller brace extend out one inch from the side members of the frame, to allow room for the rubber strands to be used as motive power. In order to avoid slotting the ends of the side members of the frame so that the propeller brace can be secured therein, thin strips of bamboo are secured above and below the end of each side member, by binding with silk thread and gluing, the space between these bamboo strips being utilized for the brace which is securely bound and glued therein. The propeller bearings consist of strips of very thin bronze (No. 32 gauge), about ³⁄₁₆″ in width, bent over ⁵⁄₈″ strips of German silver tubing, the tubing being soldered to the bronze strips and the propeller brace, which fits between the upper and lower portions of the bronze strips, is securely bound and glued thereto.

The propellers are cut from solid blocks of pine, and are 12″ in diameter. The blade, at its widest portion, measures 1³⁄₈″. The blades are cut very thin, and in order to save weight, they are not shellacked or painted.

The propeller shafts are of piano wire (No. 20 size) to fit the tubing used in the bearings, pass through the propellers and are bent over on the outer side to prevent turning. A few small bronze washers are interposed between the propellers and the outer ends of the tubing to minimize friction when the propellers are revolving. Twelve strands of rubber are used for each propeller, the rubber being ¹⁄₈″ flat.

The wings are both double surfaced, and are of the swept back type. The span of the main wing is 28¹⁄₂″, with a chord of 6¹⁄₂″. The elevator has a span of 15″ with a chord of 4³⁄₄″. The main wing has eleven double ribs, these ribs being built up on mean beams of spruce ¹⁄₁₆″ × ³⁄₁₆″, the front beam being placed 1¹⁄₄″ from the entering edge, and the second beam being 2″ back from the front beam. The entering and trailing edges are formed from a single strip of thin split bamboo, all the joints being made by binding with thin silk and gluing.

The elevator is constructed in like manner, except that it only has seven ribs, and the measurements are as above set forth. Both planes are covered with goldbeater’s skin, sometimes known as “Zephyr” skin, which is first glued in place and then steamed, which tightens the same on the plane, and given a coat of preparation used for this purpose.

(SINGLE TRACTOR MONOPLANE, 116 seconds DURATION RISING FROM WATER)

The Hittle World record model hydroaëroplane, designed and constructed by Mr. Lindsay Hittle of the Illinois Model Aëro Club, is perhaps one of the most interesting types of models yet produced. The establishing of this record illustrates the value of careful designing and construction and offers to the beginner an example which might be followed if good results are sought. In having broken the world’s model hydroaëroplane record with a tractor type model Mr. Hittle accomplished a feat of twofold importance. First, in having advanced the possibilities of the tractor model, and, second, in illustrating the value of scientific construction. The previous record for this type of model has been but 29 seconds, just one-fourth of the duration made by Mr. Hittle’s model.

Mr. Hittle’s model shows many new and original features not hitherto combined on any one model. Note diagram 12. The model is of extremely light weight, weighing complete but 1.75 ounces. The floats and their attachments have been so designed as to offer the least possible wind resistance. In fact every possible method was utilized in order to cut down weight and resistance on every part of the model. As a result of this doing away with resistance an excellent gliding ratio of 8³⁄₄ to 1 has been obtained.

For the motor base of the model a single stick of white pine ⁵⁄₆″ deep and 45″ in length is used. On the front end the bearing for the propeller is bound with silk thread and a waterproof glue of the constructor’s own composition being used to hold it secure. For the bearing a small light weight forging somewhat in the shape of the letter “L” is used, this being made streamline. At the rear end of the engine base is attached a piano wire hook for the rubber. The stabilizer consisting of a segment of a circle measuring 12″ × 8″ is attached to the under side of the engine base. The rudder measuring 3¹⁄₂″ × 3¹⁄₂″ is attached to the stabilizer at the rear of the engine base.

The wing is built up of two beams of white pine with ribs and tips of bamboo and has an area of 215 square inches.

The wing which has a total span of 43″ and a chord of 5¹⁄₈″ is built up of two beams of white pine with ribs and tips of bamboo and has a total area of 215 square inches. The wing is given a small dihedral and the wing tips are slightly upturned at the rear.

The trailing edge is longer than the entering edge the ribs being placed somewhat oblique in order to secure an even spacing. The wing is attached to the frame by two small bamboo clips which hold it rigidly and permit easy adjustment and is set at an angle of about 4 degrees with the line of thrust.

Both the floats which take practically the whole weight of the machine are situated directly under the wing just far enough behind the center of gravity to prevent the model from tipping backward. These floats are attached to the engine base by means of streamlined bamboo struts. Bamboo is also used in the construction of the float frames. A single float of triangular sections is situated just behind the propeller. The entire weight of the floats and their attachments is but .23 ounces.

The propeller which consists of four blades is built up of two propellers joined together at the hubs and securely glued, the completed propeller having a diameter of 10″ with a theoretical pitch of 14″. The blades are fairly narrow, tapering almost to a point at the tips. The propeller is driven by five strands of ³⁄₁₆th″ strip rubber at about 760 r.p.m. when the model is in flight. At the time when the model made its record flight of 116 seconds the rubber was given 1500 turns which is not the maximum number of turns. At other times the model has flown satisfactorily with less turns of the rubber. While in the air the model flies very slow and stable notwithstanding its light weight and large surface. On three occasions the model has made durations of approximately 90 seconds which rather dispenses the possibility of its being termed a freak.

One of the most notable results of the National Model Aëroplane Competition of 1915 was the establishing of a new world’s record for flying boats. Considering that the model flying boat is a difficult type of model to construct and fly, the establishing of this new world record of 43 seconds is remarkable. Credit for this performance is due Mr. Robert La Tour of the Pacific Northwest Model Aëro Club, who designed, constructed and flew the model flying boat which is herewith described and illustrated. Diagram 13.

The frame is made of laminated spruce 40″ in length, made of two strips glued together. They are ³⁄₈″ × ¹⁄₈″ at the center tapering to ³⁄₁₆″ × ¹⁄₈″ at the ends. The cross braces are of split bamboo and are fastened to the frame side members by bringing them to a wedge at the ends and then inserting them into slots in the sides of the frame side members and are finally drilled and bound to the latter. The rear brace is of streamlined spruce ¹⁄₄″ × ¹⁄₈″; this butts against the frame side members and is bound to them. The propeller accommodations are made of brass.

The propellers are 10″ in diameter with a 19″ pitch. These are carved from a block of Alaska cedar 1¹⁄₄″ wide by ³⁄₄″ thick. Of course the propellers may also be made from white pine. To turn the propellers 15 strands of ¹⁄₈″ flat rubber are used.

Bamboo about ¹⁄₁₆″ square is used to obtain the outline of the wings. The main wing has a span of 33″ with a chord of 5¹⁄₂″. Split bamboo is used for the making of the 9 ribs. The wing spar or brace is of spruce ³⁄₁₆″ × ¹⁄₈″ and is fastened below the ribs as illustrated in diagram 13. The elevator is constructed in like manner but has a span of only 17″ × 4³⁄₄″ and has only 5 ribs. A block ³⁄₄″ high is used for elevation. Both wings have a camber of ¹⁄₂″ and are covered on the upper side with silk doped with a special varnish and a few coats of white shellac.

The boat is 20″ long, 3″ in width and shaped as shown. The slip is ¹⁄₂″ deep and is located 7″ from the bow. The rear end is brought down steeply to avoid the drag of the water on this point when the boat is leaving the surface of the water. Spruce ³⁄₆₄ths of an inch thick is used for the making of the sides, but the cross bracing is of slightly heavier material, there being six braces used throughout. The rear brace is much heavier in order to withstand the pull of the covering and to receive the ends of the wire connections. The outriggers or balancing pontoons are constructed of the same material as that of the boat and are held together by a spruce beam 18″ long, ¹⁄₂″ wide by ³⁄₁₆″ thick, streamlined. This beam is fastened to the boat by means of three brads to permit changing if necessary. The lower edges of the outriggers should clear the water about ¹⁄₈″ before the steps on the boat leave the water. The boat and outriggers are covered with silk, shrunk with a special solution and then coated several times with white shellac. It is a good plan to shellac the interior walls of the boat and pontoons before covering to prevent them from losing their form by becoming soft from the influence of water in the case of a puncture.

The boat is connected to the frame at its front by two steel wires, their ends being inserted into the cross members of the boat, and then brought up along the sides, crossed and then bound to the frame. A similar pair of connecting wires are used to connect the rear end of the boat to the rear end of the frame. A U-shaped wire is bound to the outrigger beam and frame. A single diagonal strip of bamboo is also fastened to the outrigger beam with a brad, its upper end being bound to the cross bracing of the frame, making a very solid connection.

Under ideal weather conditions this model will fly on 12 strands of rubber with the possibility of a better duration than has been made. But, however, with 15 strands the model will rise at every attempt. More rubber, however, causes the bow of the boat to nose under and to accommodate this increase of power the boat should be lengthened.

(TWIN PROPELLER HYDROAËROPLANE, 100.6 SECONDS RISING FROM WATER)

During the National Model Aëroplane Competition of 1915 held under the auspices of the Aëro Club of America, a number of new world records were established, one of which was for twin propeller hydroaëroplanes. The credit for this record is due Mr. Ellis C. Cook of the Illinois Model Aëro Club, who succeeded in getting his model hydroaëroplane—which by the way is a rather difficult type of model to operate—to rise from the water and remain in the air for a duration of 100.6 seconds. This model is of the common A frame design with the floats or pontoons arranged in the familiar fashion, two forward and one aft. The model is fairly light, weighing, when complete, 3.33 ounces, ¹⁄₂ ounce of which is made up in rubber strands for motive power. Diagram 14.

The frame is made of two sticks of white pine for side members, each member measuring 38¹⁄₄″ in length, ⁵⁄₁₆″ in depth, by ¹⁄₈″ in width. These are cut to taper toward the ends where they are only ¹⁄₈″ in width by ³⁄₁₆″ in depth in the front and rear respectively. Three “X” strips of streamlined bamboo measuring ³⁄₁₆″ in width by ³⁄₆₄ths of an inch in depth, are used for bracing the frame between the front and rear and are arranged as shown in diagram 14. The propeller bearings are of small streamlined forgings of light weight, and are bound to the rear end of each side member first by gluing, then binding around with thread. The front hook is made of No. 16 piano wire and is bound to the frame as shown in diagram 14. The chassis which holds the floats or pontoons is made of ³⁄₃₂″ bamboo bent to shape and bound to the frame members. By the use of rubber strands the floats are attached to the chassis; the forward ones being attached so that angle may be adjusted.

The main wing has a span of 36″ and a chord of 5″ and is constructed of two white pine beams each 39″ long, with bamboo wing tips. The ribs, seven in number, are also made of bamboo and are spaced along the edges of the wing at a distance of 4¹⁄₂″ apart. The “elevator” or front wing has a span of 14″ and a chord of 3¹⁄₄″, the framework of which is made entirely of bamboo. The entering edge of this wing is given a slightly greater dihedral so that the angle of incidence at the tips is greater than at the center. By this method the added incidence in the front wing is obtained. By the use of rubber bands both wings are attached to the frame.

The two forward floats are spaced eight inches apart and are of the stepped type, the step being 3¹⁄₂″ from the front and has a depth of ¹⁄₈″. These two floats are separated by two bamboo strips as shown in the diagram, which are tied to the rounded portion of the under carriage by small rubber bands. By the sliding of these strips back and forth the necessary angle of the floats may be obtained to suit conditions. The floats are built up with two thin pieces of white pine for sides, separated by small pieces of wood about one-half the size of a match in cross section. Chiffon veiling which is used for the covering of the wings, is also used for the covering of the floats, after which it is covered with a special preparation to render both the wings and the floats air and water-tight.

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.

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.

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.

The propeller bearings are turned out of ¹⁄₃₂″ bronze tubing, the length of each bearing being ¹⁄₂″. Steel washers are slipped over the propeller shaft, between the bearing and propeller to insure smooth running. The propeller shafts are made from steel hatpins which are heated at both ends, one end of which is bent into a loop to receive the rubber strands, the other end being bent around the hub of the propeller to prevent the shaft from slipping during the unwinding of the rubbers. Two strips of brass, each ¹⁄₄″ × 2″, are bent around the one-half inch bearing and soldered. The brass strips are then glued and bound onto the ends of the propeller brace as shown in diagram 15.

(TWIN PUSHER BIPLANE 143 SEC. DURATION RISING FROM THE GROUND)

Since the beginning of model flying very little attention has been paid to the model biplane. Practically all records are held by model aëroplanes of the monoplane type. With this fact in view, the record established by Mr. Wheeler with his Twin Pusher Biplane is extraordinary, in so far as it surpasses many of the monoplane records. This model is a very slow flyer, and has excellent gliding ability. At the time when this model flew and broke the world’s record, the greater portion of the flight consisted of a beautiful glide of 86 seconds’ duration, after the power gave out, making it possible for the model to remain in the air for a duration of 143 seconds.

The frame consists of two I-beams, each 48″ in length, running parallel, and spaced by cross pieces, each piece 11¹⁄₂″ long. The bearing blocks used made it possible for the propellers to clear by one-half inch. Two 12″ expanding pitch racing propellers are used and these are mounted on ball bearing shafts. The main upper plane has a span of 34″ with a chord of 5″, the lower plane being 26″ by 5″. The elevator consists of two planes, each measuring 14″ by 5″. Cork wheels are used, each being one inch in diameter. For motive power one-eighth inch flat rubber is used, this being coated with glycerine to prevent sticking.

A hole ¹⁄₈″ in diameter should be drilled in the center of each head so as to enable the connecting of the intake pipes. By the use of soft wire solder the heads should be soldered into the ends of the cylinders as shown in Figure 1-d.

The pistons should now be made; for this purpose two additional male ferrules are required. These should be made to operate freely within the cylinders by twisting them in a rag which has been saturated with oil and upon which has been shaken fine powdered emery. When they have been made to operate freely they should be cut down one-half inch from the closed end as shown in Figure 5-a. For the connecting rods, 2 pieces of brass tubing, each ¹⁄₈″ in diameter by 1¹⁄₄″ long, are required, and, as illustrated in Figure 6, should be flattened out at either end and through each end a hole ³⁄₃₂″ in diameter should be drilled. For the connecting of the piston rods to the pistons, studs are required, and these should be cut from a piece of brass rod ¹⁄₄″ in diameter by ¹⁄₂″ in length. As two studs are necessary, one for each piston, this piece should be cut in half, after which each piece should be filed in at one end deep enough to receive the end of the connecting rod. Before soldering the studs to the heads of the pistons, however, the connecting rods should be joined to the studs by the use of a steel pin which is passed through the stud and connecting rod, after which the ends of the pin are flattened, to keep it in position as shown in Figure 5-a.

For the outside valve mechanism and also to serve in the capacity as a bearing for the crankshaft, a piece of brass tubing ¹⁄₄″ in diameter by 1¹⁄₂″ long is required. Into this should be drilled three holes, each ¹⁄₈″ in diameter, and each ¹⁄₂″ apart as shown in Figure 4. Next, for the valve shaft and also propeller accommodation, secure a piece of ³⁄₁₆″ drill rod 2″ long. On the left hand side of the valve shaft, as shown in Figure 3, a cut ¹⁄₃₂″ deep by ¹⁄₂″ in length is made 1″ from the end. Another cut of the same dimensions is made on the right side only; this cut is made at a distance of ³⁄₈″ from the stud end.

As shown in Figure 1-f, the crank throw consists of a flat piece of steel, ³⁄₃₂″ thick, ³⁄₈″ in length by ¹⁄₄″ in width. At each end of the crank throw a hole ³⁄₁₆″ in diameter should be drilled, the holes to be one-half inch apart. Into one hole a piece of steel drill rod ³⁄₃₂″ in diameter by ¹⁄₄″ long is soldered, to which the connecting rods are mounted, as shown in Figure 1-f. Into the other hole the stud end of the crank throw is soldered.

Before making the tank it is most desirable to assemble the parts of the engine, and this may be done by first fitting the pistons into the cylinders as shown in Figure 1-b, after which the cylinders should be lapped one over the other and soldered as shown in Figure 1-a. When this has been done a hole one-fourth of an inch in diameter should be drilled half way between the ends of the cylinders, and into this hole should be soldered one end of the valve casing shown in Figure 4. For the inlet pipes as shown in Figure 1-c secure two pieces of ¹⁄₈″ brass tubing and after heating until soft, bend both to a shape similar to that shown in Figure 1-c. When this has been done solder one end to the end of the cylinder and the other in the second hole of the valve shaft casing. The valve shaft should now be inserted in the valve shaft casing and the connecting rods sprung onto the crank throw as shown in Figure 1-d. To loosen up the parts of the engine which have just been assembled it should be filled with oil and by tightly holding the crankshaft in the jaws of a drill the engine can be worked for a few minutes.

The tank is made from a sheet of brass or copper foil 15″ long by ¹⁄₁₀₀₀″ thick. This is made in the form of a cylinder, the edges of which are soldered together as shown in Figure 2. Sometimes this seam is riveted every one-half inch to increase its strength, but in most cases solder is all that is required to hold the edges together. For the caps, or ends, the tops of two small oil cans are used, each can measuring 2¹⁄₂″ in diameter. To complete the caps two discs of metal should be soldered over the ends of the cans where formerly the spouts were inserted, the bottoms of the cans having been removed. The bottom edges of the cans should be soldered to the ends of the tank as shown in Figure 2. Into one end of the completed tank a hole large enough to receive an ordinary bicycle air valve should be drilled. Figure 2. Another hole is drilled into the other end of the tank, into which is soldered a small gas cock to act as a valve. Figure 2. This should be filed down where necessary, to eliminate unnecessary weight. To connect the tank with the engine, a piece of ¹⁄₈″ brass tubing 3″ long is required, the ends of which are soldered into the holes in the valve shaft casing nearest the cylinders, as shown in Figure 1-ee. As shown in Figure 1-ee, a hole ¹⁄₈″ in diameter is drilled in one side of this piece, but not through, in the end nearest the tank. Another piece of brass tubing ¹⁄₈″ in diameter is required to connect the tank with the engine, one end of which is soldered to the cock in the tank, the other in the hole in the pipe which leads from the engine to the tank, illustrated in Figure 1-ee, thus completing the engine.

In conclusion it is suggested that the builder exercise careful judgment in both the making and assembling of the different parts of the engine in order to avoid unnecessary trouble and secure satisfactory results. After having constructed an engine as has just been described, the constructor may find it to his desire to construct a different type of engine for experimental purposes. The constructor therefore may find the descriptions of satisfactory compressed air engines in the following paragraphs of suggestive value.

In the accompanying illustration is shown a model aëroplane of monoplane type driven by a three-cylinder rotary engine which was constructed by Edward Willard Dart of South Norwalk, Connecticut.

The engine was constructed after several months of patient labor. Careful judgment was exercised in the drafting of the plane and likewise in the assembling of the engine for it is absolutely essential that all parts be properly fitted as to enable the engine to run smoothly. In designing the wings every detail was taken into consideration to insure good flying.

The main wing has a spread of 58″ and 7″ in chord. The elevator measures 23″ in spread and 6″ in chord. In the construction of both wings bamboo ribs are used, the frames being covered over with China silk and coated with celluloid solution. The main wing is made in two sections to facilitate quick adjustment to the fuselage.

One of the latest developments in the field of model flying is the McMahon compressed air driven monoplane. This model was built to be used as either a tractor or pusher, but in view of its ability to balance more easily as a pusher most of the experiments have been carried out on this machine as a pusher. The machine in itself is simple and inexpensive to construct, the chief portion of the expense being involved in the making of the engine. By using the machine as a pusher a great deal of protection is afforded both the propeller and engine, and this protection helps to avoid damaging the propeller or engine, which would mean an additional expenditure for repairs, thus minimizing the cost of flying the model.

The frame has been made to accommodate both the tank and engine, and this is done by using two 30″ strips of spruce, each ¹⁄₄″ wide by ³⁄₈″ deep, laid side by side, a distance of three inches apart, up to within 10″ of the front, as shown in the accompanying photograph. No braces are used on the frame, as the tank, when securely fastened between the frame, acts in that capacity.

The wings are made in two sections, each section measuring 24″ in span by 8″ in chord, consisting of two main spars, ³⁄₁₆″ in diameter, one for the entering edge and one for the trailing edge. To these edges, at a distance of three inches apart, are attached bamboo ribs, 18 in all, each measuring 8″ in length by ¹⁄₈″ in width by ¹⁄₁₆″ thick. The wings are round at the tips, and have a camber of approximately one-half inch, but they are not set at an angle of incidence. Light China silk is used for covering and after being glued over the top of the wing frame is given two coats of dope to shrink and fill the pores of the fabric. A good “dope” for the purpose can be made from celluloid dissolved in banana oil. The wing sections are attached to the frame and braced by light wire. The forward wing or “elevator” is made in the same manner as the main wing, but should measure only 18″ × 3″. Instead of being made in two sections as the main wing, the forward wing is made in one piece.

The chassis is made by forming two V struts from strong steel wire sufficiently large enough so that when they are attached to the frame of the model the forward part will be 9″ above the ground. One V strut is securely fastened to either side of the frame, at a distance of 8″ from the front. A 7″ axle is fastened to the ends of these struts. On the axle are mounted two light wheels, each about 2″ in diameter. The chassis is braced by light piano wire.

The rear skid is made in the same manner as the forward skid, only that the ends of the struts are brought together and a wheel 1 inch in diameter is mounted at the bottom ends by means of a short axle. The struts are not more than 7¹⁄₂″ long, thus allowing a slight angle to the machine when it is resting upon the ground.

The machine complete does not weigh over 7 ounces. The power plant used in connection with this model is of the two cylinder opposed engine type, with tank such as has just been described in the foregoing chapter.

The tank is mounted in the frame by drilling a ¹⁄₁₆″ hole through either end of the tank, through which a drill rod of this diameter can be inserted. About ³⁄₄ths of the drill rod should extend out on each side of the tank, to permit the fastening of the tank to the frame side members. This method of mounting the tank serves two purposes to a satisfactory degree. First, it permits secure fastening; second, as the rods are passed through the side and cap of the tank they help materially in preventing the caps from being blown off in the event of excessive pressure.

Two of the most enthusiastic advocates of the compressed air engine for use in model aëroplanes are Messrs. Frank Schober and Rudolph Funk, both members of the Aëro Science Club. For a number of months both these gentlemen have experimented with compressed air engines of various designs, until they finally produced what is perhaps one of the most satisfactory rotary engines now in use, from a standpoint of simplicity and results.

As can be seen from the accompanying illustration, this little engine is remarkably simple in appearance. The engine complete, with equipment, weighs at the most but 14 ounces. The cylinders, three in all, are stamped from brass shells for strength and lightness. The pistons are made from ebony fiber. The cylinders have a bore of ⁵⁄₈″, with a piston stroke of ¹⁄₂″. The crank case is built up from a small piece of brass tubing and is drilled out for lightness. The crankshaft is hollow, and is supported at the rear by a special bearing which acts as a rotary valve, admitting the intake through the crankshaft and permitting the exhaust to escape through a specially constructed bearing.

The tank is constructed of 30 gauge sheet bronze, wire wound, and fitted at the ends with spun brass caps. The actual weight of the engine alone is 2¹⁄₂ ounces, the tank and fittings weighing 11¹⁄₂ ounces, making the total weight of the complete power plant 14 ounces.

Another interesting type of compressed air engine that has been developed in America is the Schober four cylinder opposed engine. While this engine is different in appearance from most compressed air engines, it has been made to work satisfactorily and is consistent with the same high class construction that is displayed in most all of Mr. Schober’s engines. The accompanying diagram 18 illustrates the method of operation of the four cylinder engine.

The crank case is constructed from four pieces of 24 gauge spring brass, substantially connected in the form of a rectangle, the top and bottom being left open. The front and rear walls have flanges which engage the inside of the side walls and are secured thereto by four small screws on each side, thereby making it an easy matter to take the crank case apart.

The four cylinders are made from drawn brass shells and have a bore of ¹⁄₂″ and stroke of ¹⁄₂″. The pistons are made of solid red fiber. The two-throw crankshaft is built up of steel with brass webs. The bearings are of steel. The valves, being overhead, are driven by a gear mounted at the end of the crankshaft, the gear driving the valve shaft by means of a gear on that shaft, with which the crankshaft gear meshes. The valve arrangement, as shown in diagram 18, consists of four recesses cut into the valve shaft, two of which allow the air to pass from the inlet pipes, which lead into the valve chamber at the center of same, to two of the cylinders at once, while the other two recesses allow the exhaust to pass from openings in the sides of the valve chamber.

The cylinders are secured to the side plates of the crank case so that when those side plates are removed, the cylinders are removed with them. The pipes are detachable at their centers; small pipes running to the heads of the cylinders extending into the larger pipes which run to the valve chamber. This arrangement is shown in the end view of the engine. A 17″ propeller is used in connection with this engine.

As the accompanying diagrams 19 and 20 and photograph show, the engine is of the four-cycle, horizontal opposed type, having two cast-iron cylinders of 1¹⁄₄″ bore and 1³⁄₈″ stroke. Each cylinder is cast in one piece, and as the engine is air cooled, they are cast with radiating fins. One h.p. is developed at 1500 r.p.m. The total weight of the engine, gasoline tank and propeller is 7¹⁄₂ lbs. In preparing the design of this engine, the designs of similar full-sized aëro engines were followed as far as possible. The pistons are similar to those used on large aëro engines and are fitted with two rings; the crankshaft is turned out of two inch special bar steel, and is carried in two phosphor-bronze bearings. There is no special feature about the connecting rods, these being of the standard type, but very strong and light. To enable the two cylinders to be exactly opposite one another, the connecting-rods are offset in the pistons and are connected to the latter by gudgeonpins. The aluminum crank case is extremely simple, being cylindrical and vertically divided. The inlet valves are automatic, the exhaust valves being mechanically operated; the camshaft is driven from the main shaft by two-to-one gearing.

To assist the exhaust, and also the cooling, small holes are drilled round the cylinder in such a position that when the piston is at the inner end of its stroke, these holes are uncovered, thus permitting the hot exhaust to escape, and so relieve the amount passing through the exhaust valves. The commutator is also driven off the camshaft, as shown in the drawing. No distributor is fitted to the commutator, as small ones are somewhat troublesome and very light coils are obtainable at a reasonable price.

The gasoline tank is made of copper in streamline form, and is usually fitted to the back of the crankcase, thus reducing the head resistance, but if desired it can be fitted in any other position. The action of the carburetor can be easily seen from the drawings; it is of the surface type and much simpler, lighter and quite as efficient as the spray type. Specially light and simple spark plugs are used, that give very little trouble. The propeller used in connection with this engine is somewhat out of the ordinary, having been specially designed for this engine, and patented. The propeller is made entirely of aluminum and has a variable pitch, this being easily obtainable, as the blades are graduated so that any desired pitch, within certain limits, may be given at once. The results of a series of tests on a 30 inch propeller are shown on the accompanying chart, and from it the thrust as certain speeds with a certain pitch can be obtained. Taking the engine running at 1540 r.p.m. with a pitch of 15″, the thrust comes out at 9¹⁄₂ lbs., or more than the weight of the engine and accessories.

Although numerous model constructors in America are experimenting with model gasoline engines, the Midget Gasoline Engine, the product of the Aëro Engine Company, Boston, Massachusetts, is perhaps the most satisfactory up to the present time. An engine of this type was used by Mr. P. C. McCutchen of Philadelphia, Pennsylvania, in his 8 foot Voisin Type Biplane Model, for which he claims a number of satisfactory flights.

The engine is made from the best iron, steel, aluminum and bronze and the complete weight including a special carburetor, spark plug and spark coil is 2¹⁄₂ lbs. From the top of the cylinder head to the bottom of the crank case the engine measures 7″. It is possible to obtain from this engine various speeds from 400 to 2700 r.p.m., at which speed it develops ¹⁄₂ h.p. The propeller used in connection with this engine measures 18″ in diameter and has a 13″ pitch.

The Midget ¹⁄₂ H. P. gasoline engine

It might be of interest to know that one of the parties responsible for the development of this engine is Mr. H. W. Aitken, a former model maker and who is now connected with one of the largest aëro engine manufacturing companies in America.

The principal parts used in Mr. Harris’s steam power plant was an engine of the H. H. Groves type, twin cylinder, ⁷⁄₈″ bore with a piston stroke of ¹⁄₂″. The boiler was made from 12″ of ³⁄₁₆″ × 20″ G. steel tubing, weighing 10.5 ounces. The blow lamp consisted of a steel tube, ⁵⁄₃₂″ × 22″ G. wound round a carbide carrier for a nozzle. The tank was made of brass ⁵⁄₁₀₀₀″ thick. The pump, ⁷⁄₃₂″ bore, stroke variable to ¹⁄₂″, fitted with two non-return valves (mushroom type) and was geared down from the engine 4.5 to 1.

The Langley steam driven model, of which so much has been said, and which on one occasion flew a distance of one-half mile in 90 seconds, had a total weight of 30 lbs., the engine and generating plant constituting one-quarter of this weight. The weight of the complete plant worked out to 7 lbs. per h.p. The engine developed from 1 to 1¹⁄₂ h.p. A flash type boiler was used, with a steam pressure of from 150 to 200 lbs., the coils having been made of copper. A modified naphtha blow-torch, such as is used by plumbers, was used to eject a blast or flame about 2000 Fahrenheit through the center of this coil. A pump was used for circulation purposes. With the best mechanical assistance that could be obtained at that date, it took Professor Langley one year to construct the model.

About ten months after Langley’s results, some experiments were carried out by the French at Carquenez, near Toulon. The model used for the experiments weighed in total 70 lbs., the engine developing more than 1 h.p. As in the Langley case, twin propellers were used, but instead of being mounted side by side, they were mounted one in front and the other behind. The result of these experiments compared very poorly with Langley’s. A flight of only 462 feet was made, with a duration of a few seconds. The maximum velocity is stated to have been 40 m.p.h. The span of this model was a little more than 6 meters, or about 19 feet, with a surface of more than 8 square meters, or about 80 square feet.