CHAPTER XIV
Biplane Driven by
Compressed-air Engine
The model aeroplane illustrated by Fig. 148 has been designed to suit the compressed-air plant fully illustrated and described in the preceding chapter. It is from the results obtained from the testing of the plant that the dimensions of a suitable model for it are determined; and while the design may suit the majority of the plants constructed from the illustrations shown in pp. 95 to 101, it is chiefly given to show the correct method of designing a “power-driven” machine, since the power unit (unlike the elastic motor) cannot be varied, and recourse to some established line of reasoning becomes essential.
The first thing to do, then, once the plant has been “tuned up,” is to ascertain the thrust obtainable from it. This is found by suspending the plant by the valve on a balance, with a container fully inflated, the weight registered being carefully noted. The container pressure should now be released, and the weight registered when the motor is running observed. By subtracting the former from the latter the thrust is obtained.
Thus, assuming the plant, at rest, to weigh 8 oz., and when running 12 oz., it is clear that the thrust is equal to 4 oz. Now, it is necessary to know the average thrust developed, since, as hitherto explained, the thrust is not constant, but gradually diminishes as the density of the air in the container approaches normal atmospheric conditions; that is, 14·6 lb. per square inch (known as an atmosphere). It is possible to obtain some very interesting data by plotting a graph of the thrust given off at various moments from the release of the pressure in the container. Meanwhile it can be taken as a good rule that the thrust registered after one-third of the effective run of the motor represents approximately the average thrust; and the figure given above (4 oz.) will serve for the purpose of illustration.
It is next necessary to know the weight of the model it will lift. It is well established that a plant will fly a machine weighing from four to six times the weight of the thrust it develops, although, of course, much depends on the efficiency of the model; the greater the complexity of frame members the lower the lift drag ratio, and consequently the lower the ratio between the thrust and the weight of the model. Compromising, and taking 5: 1 as the ratio, 20 oz. is obtained as the total weight of the plant and model.
The next point to be decided on is the loading, and as the model is to be a biplane a comparatively light loading can be used. In the case of the machine shown in side elevation by Fig. 146 and in front elevation by Fig. 147, 4 oz. has been taken as the loading per square foot. So that the total area of the wings will be
| 20 | ||
| — | = 5 sq. ft. | = 720 sq. in. |
| 4 |
A span of 54 in. for the top plane and 46 in. for the bottom one has been decided on, and by using a chord of 8¾ in. the total area of the wings vies very approximately with this figure, allowing a small margin for excess weight. The area of the tail, which is non-lifting, need not be taken into account. Although the “gap” is given as being equal to the chord, it could be made, if anything, ½ in. greater.
Now with regard to actual materials. Birch is to be used for the longitudinals, straight in the grain and of the cross-sections illustrated. The lower member is bent under steam to the curvature shown—of 6½-in. radius. Two vertical struts support the wings, and these should be cut from hickory. A short tie-strut secures the bottom longitudinal to the front inter-strut, the joint being made by means of side angle-plates bound into place. It will be found good practice to make a full-size drawing of the machine in side elevation, so that it can be used as a template to fit up the cross members—particularly with regard to the cutting of the angles.
The joint of the longeron to the cross member is shown separately at A (Fig. 146). The usual fish-plates are employed, so made that a small wiring plate is left protruding from the binding, to which the cross-sectional and longitudinal-sectional wires are made off.
Fig. 148.—Plan View of Biplane Driven by Compressed Air
The plant itself is slung into the framework by means of eight wires, each being made off to the wiring plates. Each should also be provided with a small ¾-in. wire strainer to enable the plant to be fixed quite rigidly—albeit permitting of its being removed for inspection or repairs. The wires from the engine itself are taken off from the four small eyes soldered to the stationary portion of the crank-shaft. Great care should be taken to ensure that the plane of rotation of the screw is at right angles to the main planes. A 1¼-in. dihedral is given to the bottom plane by means of the bracing wires passing between the inter-struts, and shown on the preceding page.
It has been thought advisable to attach a small rear wheel, to enable the model to rise off the ground with as little loss of power as possible. Such a wheel, with attachments, need weigh no more than ¼ oz., and is a great improvement over the cane skid usually employed.
Fig. 146.—Side Elevation of Biplane Driven by Compressed Air
In bracing the outriggers, or longerons, some care will be required to ensure their being quite true. It will be easier to finish each section off first, so that they are quite parallel at the joints.
The part plan view of the model (Fig. 148) will make the relative position of the various component parts quite plain. The two top tail outriggers pass through the fabric at the point where the spar is located, their front ends being pinned and cross bound to the wing spar, which is made of greater cross-section in the centre, so that its strength is not materially impaired through the piercing of it. Birch is to be used for the wing spars and ribs of the sections indicated.
The planes are ribbed at periods of 6 in. and given a camber of ¾ in., the greatest depth of which is 2¼ in. from the leading edge. It is far easier to impart the camber after the wing framework is made than to camber each rib separately. Each rib should be cut 1 in. longer than necessary, and pinned and glued to the spars, with ½ in. overlapping each of these latter. When the glue is quite set, the pins may be clinched over by supporting the wing on an iron weight and tapping them back flush to the spars.
The full-size section of the camber should be drawn upon a board, with which to check the accuracy of the first rib to be cambered (the end rib).
The ribs are cambered in a jet of steam, the convex or top sides being placed nearest to it. Having cambered the end rib carefully to agree with the drawing, the others may be matched to it. It will thus be easy to ensure that every rib is of the correct curvature, as any mistake in the steaming of the rib will distort the wing spar at the point of its attachment.
If, however, it is thought advisable to camber the ribs first, a wooden bending jig should be made, to enable several ribs to be bent at one operation. The ribs should be tied down to the jig with string, and thus held under the steam jet, being well dried in front of the fire before they are detached from the jig. All three spars pass underneath the ribs.
A very light fabric should be chosen, such as can be obtained from the model-aero accessory warehouses, or an unproofed Japanese silk can be used and varnished when on the wing. If this latter is used, it will be found advantageous to use a yellow hue, as this colour is least affected by the action of the varnish. But the covering of the wings must be left for the time being, for the reason that the sockets to which the inter-struts are made fast must first be attached. Further, the top plane must be covered after the tail outriggers have been assembled, as it is so much easier to make the joint between the wing spars and these latter before the fabric is attached.
To render it unnecessary to refer to the point further, it may be noted that the fabric is brought over the leading spar of each wing to pocket it out. It is much neater to sew the fabric along on the leading edge, as when glue is used an unsightly black smear shows through. The fabric should be stretched from end to end first, the fabric overlaps being glued on the bottom face of each end rib. Drawing-pins should be partially pressed into the ribs to secure the fabric until the glue is set.
At B in Fig. 148 is shown the method of securing the bottom plane to the inter-struts. Convenient notches are cut in the struts into which the plane is sprung. It will have been noticed from the side elevation (Fig. 146) that the width of the inter-struts increases towards the bottom or lower ends, and also that they incline slightly; this is to provide for the entry of the lower plane, since the top plane is attached outside the struts, while the bottom is placed inside them. At C is shown the method of attaching the inter-struts.
The tail is built up from split bamboo, ⅒ in. by ³/₃₂ in. in cross-section, and the rudders are framed up from No. 20 gauge piano wire. The ends of the rudder frames are forced through the longerons, and the ends bent back in alignment with them; they are then bound to the longerons with black three-cord carpet thread. The rudders are covered after being fixed to the outriggers. When it is necessary to adjust them, the piano wire will be found sufficiently ductile to admit of a warp being placed thereon.
Fig. 147.—Front Elevation of Compressed-air-driven Model
Eleven ribs connect the spars of the top plane and nine those of the lower, the camber of each being the same; that is, the same depth of camber is maintained throughout. Before the wings are covered, the angle-plates to which the inter-struts are fixed must be bound on; and these are cut from No. 30 gauge sheet tin. They should be cut less in width than the spar to which they are attached, in order that their sharp edges shall not cut through the binding. To prevent the plates from moving, they should be lightly sunk into the wing spar with two centre-punch dots, and a film of glue should also be spread over the face of the plate coming in contact with the spar.
The inter-struts are streamlined in cross-section (see Fig. 149); but they are to be left rectangular in section at their ends, to provide a flat surface for the plates to bed home on. The ends of the plates are turned back over the binding, which may be of the light machine variety.
The lower ends of the inter-struts are cut off to the same angle as the dihedral on the lower plane, to avoid distortion of the plane. Spruce or American whitewood may be used for them, the greatest cross section being ½ in. by ⅛ in. The greatest cross-section is situated at the middle of the strut, whence it tapers to ³/₁₆ in. by ⅛ in. Fig. 149 shows the attachment of the inner strut to the wing spar. In Fig. 150 are shown the brackets forming the guides for the axle, and also the supports for the rubber shock-absorbers. Piano wire is used for them. The width of the guide should be such that the umbrella-ribbing, which constitutes the inner portion of the axle, rides freely within it. The wheel axles are cut from No. 16 gauge piano wire, and they are soldered to the umbrella-ribbing, being sunk into the channel of the latter, bound with No. 30 gauge tinned iron wire, and then soldered.
Ordinary elastic, as used for a rubber-driven model, can be used as the shock-absorber, and it should be neatly and fairly loosely bound to the vertical guide, the axle of course being first seated therein. In order that the absorber brackets may maintain a vertical position, their ends are shaped to a form similar to the letter U. They are wire-bound to the skids and lightly soldered.
Fig. 149.—Details of Inter-struts.
Fig. 150.—Detail of Axle Guides.
Fig. 151.—Outrigger and Wing-spar Joint.
Fig. 152.—Tail Crossbar Joint.
Fig. 153.—Detail of Front Skid Angle-plates.
Fig. 154.—Tail Incidence-quadrant.
Fig. 155.—Details of Axle.
Fig. 156.—Rear Chassis Details.
Fig. 157.—Inter-strut and Longeron Joint.
The bottom plane only must be covered; it will be easier to cover the top plane when the machine is assembled, for it would be a difficult matter to secure the top outriggers to the spars were the fabric attached.
Having assembled the outriggers and completed the bracing of it, it will be possible to attach it to the wings.
Small elliptical holes are cut in the fabric of the lower wing, through which the central supports or stanchions pass, and the bottom plane is seated home in the notches alluded to in Fig. 148. Next, the top outrigger ends are fitted up, being cut off to correct length and halved on to the wing spars, as shown in Fig. 151. The vertical support is then glued, pinned, and cross-bound to the outrigger.
Great care will be necessary to ensure that the outriggers are quite central with the planes. A point to be made clear is that if in the fitting of the top outriggers one is cut even ¹/₃₂ in. short, the tail end of the machine may be ¾ in. out of centre. In order to check inaccuracy in this direction it would be advisable to mark the centre of the horizontal tail member, insert a drawing-pin, and take the measurement to the corner of the wing tip, on both sides of the model; the outriggers should be temporarily lashed to the wing spars, and gradually adjusted until they are located centrally with the planes. Perhaps it may be interesting to here mention that this is the method employed in locating the fuselage of full-size machines.
The bracing of the planes should now be undertaken. All lift wires should first be fixed, beginning from the wing tips. Just sufficient tension should be placed on each wire to ensure rigidity. A wooden straightedge should be used to reveal any distortion of the spar. The top plane must be given a slight dihedral, so that when the anti-lift wires are inserted it assumes a perfectly parallel position.
Fig. 158.—Biplane Driven by Compressed Air
The upper and lower longerons are spanned at the tail end with light spruce cross-bars, ⅛-in. by ¼-in. section, which are let into mortises cut in the longerons; and two vertical posts are halved on to these cross members (see Fig. 152), to provide the fulcrum about which the tail swings in the quadrant, to be referred to presently. They are spaced 4 in. apart, which is equivalent to the distance between two ribs; and on the outside of them a groove is cut in the centre of each to provide a seating for the two central tail ribs. These grooves must be cut V-shaped, the apex of the V facing the trailing edge of the post. The object of the groove is to form a guide for the tail when it is desired to alter the angle of it. A pin should be driven through the rib and into the groove to constitute the pivot on which the tail swings; and the ribs must be bound with fine thread on each side of the pin to prevent the rib from splitting. It will be found that it is better to bind the ribs before inserting the pin.
The central inter-struts are attached to the skids by angle-plates, and in Fig. 153 the form of these is given. It must be understood that there are two plates to each joint, one on each side of it, and for neatness and simplicity they can be cut from one piece of tin, both plates being thus formed in the one. No. 30 gauge tinplate is suitable. The plates are pinned, clinched, and bound into place, and constitute an exceedingly rigid piece of construction, which is needed in this portion of the machine, bearing as it does the impact of landing. Glue should be neatly brushed into all the joints. The tie-strut is to be streamlined as far as practicable without materially impairing the strength of it.
A very neat finish can be given to the binding if it is just brushed round with japan black, which shows up in pleasing contrast to the light brown varnish with which the framework is coated.
Fig. 154 gives the shape of the quadrant, which makes possible the variation to the angle on the tail. It is cut to a radius of 4 in., and is pinned into position. The pitch of the teeth is ⅛ in., and this facilitates a very fine adjustment. It should be so fixed that the tail springs tightly into notches, but not so tightly as to render adjustment difficult. Trial and error will be found the best method of locating its position.
Fig. 159.—Compressed-air-driven Monoplane
It was mentioned in the preceding chapter (see pp. 94 to 103) that the axle is composed of two portions, umbrella-ribbing and piano wire, and Fig. 155 shows the construction. It will be seen that the piano wire beds into the channel (which is fixed in a trailing position), wherein it is bound and soldered. The wheels are spaced apart by means of small brass-tube collars, soldered to the piano-wire axles in their respective positions. The axle itself, as mentioned earlier in the chapter, is attached between the shock-absorber brackets, being held there by means of suitable radius wires secured to any convenient part, the rubber binding forming the absorber. The radius wires are essential in order to maintain the lateral position of the axle relatively to the planes. Sufficient rubber binding is to be used to absorb the shocks the model is bound to receive, the exact quantity, of course, being impossible to define.
The rear wheel members are fixed to the longerons in the following manner. The ends are bent back parallel to align with the frame member. The apices of the V chassis members are soldered to the short axle carrying the back wheel, the axle being cut a length suitable to the hub of the wheel. No. 20 gauge wire is used for all portions of the rear chassis. Fig. 156 makes this clear.
Fig. 157 shows the joint of the trailing central inter-struts to the top longeron. It will be seen that the joint is a halved one, pinning and binding forming the security.
All woodwork may be polished by filling the grain with gold size, and finishing with a good varnish.
In flying the model the writer would point out that full pressure should not be given to the plant until adjustment has been completed, also the importance of tuning the machine by starting it from the ground, thus obviating many vexing smashes. Further, the rudder must be set to counteract torque; if the screw is of left-hand pitch, then torque will tend to bank the machine to the right, and the rudder must therefore be set to the left, and vice versa.
A sketch of the finished machine is given by Fig. 158, and a design for a monoplane driven by the same plant in Fig. 159.