Aeroplane construction

The accurate erection and alignment or truing up of the aeroplane, is a cogent factor in ensuring that the best performance is obtained, and it is almost platitudinous to emphasize the fact that a machine incorrectly aligned gives inferior results in flight, entails greater attention on the part of the pilot, and may possibly seriously interfere with the general stability of the aeroplane. The degree of precision attained in the manufacture of the various components is reflected in the ease or otherwise with which the complete assembly is aligned; indeed, accuracy of erection is impossible without the close observance of limits and general trueness in the production of the different parts. For this reason the erection of the principal components is surveyed as a necessary preliminary to a consideration of their assembly in the complete structure.

Accurate Part Production.

In the production of the various struts, longerons and fittings of the fuselage, the wing spars, compression and interplane struts of the planes, the utmost accuracy must be observed. Although tolerances are permissible with regard to the overall dimensions of the struts, spars, longerons, etc., the lengths particularly of the fuselage struts should be absolutely correct to drawing. The bad effects of a strut, say 1 millimetre short, are not restricted to the particular component of which it forms a part, but are noticeable in one way or another in the complete structure. Similarly the ends of struts which are required to be square should be dead square, and those which are cut to a bevel should correspond with the correct angle. The result of the slightest discrepancy in this respect becomes speedily apparent when the defective struts are assembled, as the tension of the bracing wires will result in the strut becoming bowed or bent, this being due to the bedding down of the strut end in the socket or clip. It is also advisable to trim the ends in a machine after being sawn to something approaching the correct length, and the practice of sawing to dead length should not be permitted. The surface of a sawn strut end is formed of a number of more or less ragged fibres, which in position in the machine and under pressure of the bracing wires tend to gradually flatten down, this resulting in slack wires and loss of alignment. Absolute accuracy and uniformity of part production can only be obtained by the use of jigs, preferably of metal, and some form of jig should certainly be used for cutting the various struts to length. Referring again to the necessity of the strut ends being of the correct angle, it is surprising to note the effect of the smallest inaccuracy. The writer has frequently noticed fuselage struts considerably out of straight, the grain of the timber being sometimes advanced as the reason. However, the removal of the defective strut always resulted in its return to a straight condition. It should be realized that the effect of an initially bent strut is a reduction of strength, and as this may prove a source of danger, it is in itself sufficient reason for the rigid observance of length limits.

Drilling of Bolt Holes.

Of equal importance is the drilling of the various bolt holes for the attachment of the fittings. It is not always advisable to drill the holes in the spars and longerons before the fittings are applied, but in numerous instances this is possible, and where interchangeability is an important consideration it is imperative. The practice of setting out the positions of the various holes from a drawing and then drilling with a hand brace, is a procedure only justified when a small number of machines of a certain type are to be produced, and ought by now to be obsolete. Under such a system no two spars would be exactly the same, as owing to the influence of grain in the wood, the drill or bit always tends to “run” from the correct angle. Viewed from the aspect of quantity production such a practice is very deficient. It is only by the use of metal drilling jigs of suitable design that anything approaching absolute accuracy is possible. Such jigs should not only locate the hole, but should also form a guide for the drill. In the attachment of the fittings to a properly jig-drilled spar, it should not be necessary to again drill through, although this often occurs. Where this is done, there is a distinct possibility of the brace not being held true, which means that the hole becomes larger than necessary and not infrequently oval in shape. An additional bad point is the impossibility of detecting such a fault after the fitting is bolted on, and it may not be realized until a noticeably slack wire in the complete machine indicates the movement of the fitting. In the foregoing, absolute accuracy in the various fittings has been assumed, but unfortunately in practice almost the reverse is true. Variation generally occurs in built- or bent-up fittings, and is usually the result of jigs of either incorrect or bad design. Where the variation includes a hole out of position, the use of this fitting on a previously drilled wood part is only possible by the bad practice of drilling through with the results explained above. It will thus be realized that the uniformity and accuracy of component production is only attainable by the utmost precision in the manufacture of both wood and metal parts.

Locking of Bolts.

Throughout the complete machine it is necessary to lock the nuts of the bolts, to prevent their gradual loosening under the vibration of the engine, and different methods of accomplishing this are in use. Undoubtedly the best form of lock is by the use of a castellated nut and split pin. By this method one can readily ascertain whether or no a bolt is locked, while by the withdrawal of the split pin the bolt may be taken out. A disadvantage is that its use entails considerable drilling, so that a modification consists of fitting castellated nuts to all bolts liable to removal for minor adjustments; while elsewhere the threaded portion of the bolt is left a little longer than the nut, and then riveted over. Although this reduces labour, it is a somewhat destructive method; and it is also difficult to determine the adequacy of the riveting. Another method consists of filing the bolt end flush with the nut, and then centre punching three or four dots in the joint between nut and bolt.

This method is neat, the removal of a bolt is easily effected, and the fact that it has been used in the construction of some fast scouting biplanes is proof of its effectiveness.

Other systems include the use of two nuts, of a single nut soldered to the bolt end, and the various patent lock-washers, which in this country are not greatly in vogue. The practice of re-running down the threads of bolts to ensure ease in the application of the nut is not to be recommended—that is, indiscriminately done. Unless the die is properly adjusted there is a possibility of too much thread being taken off; the result, an extremely slack nut, being detrimental to general reliability. The durability of an aeroplane in service is dependent upon the good workmanship effected in the smallest and most insignificant detail. Moreover, it should be remembered that the absence of a split pin may eventually result in disaster.

Truing of Main Planes.

The planes or sections of a machine of the straight-wing type, as distinct from a machine possessing arrow-shaped or retreating wings, should, when erected on the fuselage, form a straight line from tip to tip. This feature is dependent upon (1) the trueness of the planes, and (2) the alignment of the attachments on the fuselage, the latter being considered under the fuselage heading. To ensure that the plane is quite square, it should be checked previous to covering by diagonal measurements on the wing spars, these being taken from accurate set positions such as are provided by the wing-root attachments and the interplane strut fittings. Should a difference in the diagonals exist, this can easily be rectified by a slight adjustment of the turnbuckles incorporated in the internal plane wiring. As the ribs of the plane are built up beforehand, and checked for correct contour by pattern, little variation should occur in the camber. A point where differences may occur is between the front spar and the leading edge, as the nose formers are generally inserted during the assembling of the plane. For the detection of faults in this direction the template illustrated by Fig. 34 in Chapter IV. is of great utility.

Fabric Covering of Planes.

The evenness and correct tautness of the fabric covering contributes largely to the trueness of the plane. Should the covering be stretched unevenly or too tightly, the application of the dope will cause distortion of the framework, which can only be obviated by re-covering. The bad effects of this is more noticeable with regard to the ailerons, elevators and rudder, which, being of very slender construction, are more liable to deformation. Twisted or warped control surfaces should never be used, as such surfaces not only offer increased resistance, but also interfere with the balance of the machine in flight.

Fuselage Erection.

As the fuselage constitutes the nucleus of the aeroplane, accuracy of alignment in this component is essential, and the degree of accuracy obtained in the complete erection depends largely on the correctness or otherwise of the fuselage. In different individual designs the methods employed for the construction of the body will be found to vary considerably. The process of erection adopted in many instances is to assemble the sides first, upon a table or bench upon which the correct disposition of the various parts have previously been set out. The wires are adjusted until the sides conform to the setting out, which are then packed up on a pair of trestles and the cross-struts attached. It now remains to align the body so that it is perfectly symmetrical in plan; and this is accomplished by marking the centre of each cross-strut, preferably before insertion in the fuselage, and then adjusting the plan-wires until a cord stretched from the stern-post to the nose covers each centre line. The cross or sectional bracing-wires are then tensioned until each diagonal coincides absolutely in length. This procedure answers very well for a small fuselage of simple construction, and of the wire-braced fabric-covered type; but where the forward portion is covered with ply-wood, and the top rails of the body are horizontal, viewed in side elevation, it is usual to true up on a bench. This consists of a wooden structure built up of strong sides, with legs at short intervals, the whole being well braced. The top surface, on which the body lays, is composed of boards placed wherever a plan-strut occurs. The bench should be rigidly fixed to a concrete floor, the top planed until it is level both longitudinally and transversely, and a centre line marked on each board, while these lines, checked with a fine steel wire stretched from end to end, should be in exact agreement with it. The fuselage, having been previously assembled, with the wires inserted and the plan struts accurately centred, is placed on it in an inverted position. All wires should be then slacked off, and the top, which is now underneath, should be wired until the centre on each strut coincides with the centres on the bench. The side wires are then tensioned until the stern post is vertical, or until various fixed points, such as wing-spar attachments, are in agreement with points marked on the bench and squared or lined up, and also until the longerons are touching every board. The sectional wires are then tightened and adjusted so that each diagonal is of the same length; and this will ensure the centre lines on the cross-struts connecting the bottom rails being plumb or vertical over the centre lines of the cross-struts connecting the top rails. Where the top rails of the fuselage are not parallel to the line of flight, but slope down towards the tail, it would be necessary, if the bench method is used, to construct it so that the boards conform to the slope. With the wire-braced fuselage minor adjustments to the wing-spar attachments, which predetermine the angle of incidence of the main planes, can be subsequently made. A type of fuselage which precludes this operation, and which demands extreme accuracy in construction, is that in which the bracing of the forward portion is effected by three-ply, all wiring in a vertical dimension being eliminated, this system being described in Chapter VIII. and illustrated by Fig. 66.

With this construction points such as the wing spar attachments are fixed, and cannot be altered after the fuselage is built, so that meticulous care must be taken in the setting of the short wing spars across the body, or the fittings to which the wing roots are anchored.

Where a joint occurs in the fuselage it is usual to build the tail separate from the front portion, and occasionally the two sections are trued up independently. This does not give such good results as when the two portions, although separately built, are joined together and trued up complete.

Checking of Fuselage.

To check the fuselage for alignment it should be placed on a pair of trestles, one underneath the forward undercarriage strut fixing and the other under a vertical strut a short distance from the stern post. The body should then be levelled up longitudinally by a straightedge placed on two short straightedges of exactly similar widths, one being placed at the front and the other towards the tail. It should then be packed up on the trestles until the top longerons are dead level across. At this point, if the body is in correct alignment, the engine-bearers would be level both longitudinally and transversely, the incidence of the main spar attachments should be correct and the stern post perfectly vertical in all directions. Other tests should include the placing of a straightedge at the nose, and another placed at the points where struts occur, should, when sighted across the top edges, be “out of wind,” that is in agreement. A point which should be carefully levelled is that portion of the fuselage towards the stern post to which is attached the fixed tail plane. Any inaccuracy here will result in the tail being twisted in relation to the main planes. Each fitting or attachment should also be equidistant from the stern post, and the effect of variation here will be evidenced by the tail plane being out of square with the centre line of the fuselage. Where the type of machine is such that the engine is supported on bearers of wood, it is usual to drill the holes for the accommodation of the holding-down bolts to jig before the bearer is built in the structure. In this case care should be taken to ensure that the corresponding bolt holes in each bearer are square with the centre line. Any deviation will result in the axis of the engine forming an angle with the centre line.

Alignment of Complete Machine.

In this connection it will be better to consider the alignment of a type of machine in common use: a tractor-biplane in which the upper plane is composed of two outer planes and a centre section, and the lower plane in two sections, each abutting against the side of the fuselage, this arrangement being shown in front elevation by Fig. 122. The first operation is the levelling of the fuselage transversely by placing the level across the engine bearers, and the attachment of the centre section, which is mounted upon four struts which have been previously cut to dead length and tested by jig. This, considered in front elevation, should be centrally placed over the body, and this is assured by adjustments in the wires A-A1. This can be checked by dropping a plumb-line from the centre plane spar ends and measuring the distance from the line to the side of the body, the distances on either side should, of course, coincide. The next point is to brace the outer sections to the correct dihedral. One method of accomplishing this, as shown by Fig. 122, is by the use of a dihedral board, this being prepared perfectly straight on one edge, the other being tapered to the desired angle. The wires are then adjusted until the straightedge is level. Another method is to use an ordinary straightedge placed along the top surface of the plane, the angle being measured with a protractor or clinometer, the latter instrument being most accurate. To check the dihedral a line can be stretched between points immediately above the top interplane struts on each side and then measuring to the centre section, but it would be difficult to detect differences in the angles of each wing. With regard to the undercarriage, the distances between lines dropped from the fuselage sides and the wheel centres should coincide.

Alignment of Machine in Side Elevation.

Considering the side elevation, Fig. 123, alignment here is concerned with the incidence of the main planes, the distance forward of the top plane from the lower plane or stagger, and the level of the engine bearers in relation to the top longerons of the fuselage.

The fuselage should be levelled longitudinally by placing the level on the engine bearers, assuming the engine is not in place. When the bearers are level, the top longeron should also be level, in any case the incidence of the plane should only be adjusted in relation to the engine bearers. To check the stagger, a plumb line should be dropped from the leading edge of the centre plane, and adjustments made with the incidence wires from the fuselage to the centre-plane struts, until the required distance forward from the leading edge of the lower plane is obtained. The incidence can be tested by a straightedge placed under the plane and a clinometer, as in Fig. 123, and another device sometimes used is shown by Fig. 124. This is made of dry wood, the lengths of the legs to the tops of the spars being obtained from a drawing of the wing section, and its incidence.

Plan Alignment of Machine.

In the plan view, Fig. 125, the distances AB and AC must be equal, the same applying to CD and BD. With modern machines external drift wiring is obsolete, so that discrepancies in these measurements must be rectified by alterations in the wiring of the fuselage, as it is inaccuracy at some point in the latter component to which the trouble may be ascribed. It is at this point that one realizes the need for precision in the construction of the fuselage. In Fig. 126 is shown a plan view in which the main plane is very obviously out of square with the centre line of the body, the amount is not likely to occur in actual practice, but it has been exaggerated in the drawing. The cause of this trouble can be traced to the short wing spars in the fuselage, to which the lower plane is attached, or in other cases to the fittings, to which the lower plane is anchored, being out of centre, possibly only an insignificant amount. The lengths of the fuselage wing spars are also possible causes of trouble. Assuming that the rear spar is the correct length, and the front spar is over the length, this would result, when the outer sections were attached, in the latter sloping backwards, which again emphasizes the need for accurate part production.

With regard to the tail plane, measurements taken from the extremities of the back spar to some fixed point forward on the fuselage, to the strut sockets on the planes, or to the rear wing spar anchorage, as in Fig. 125, should be equal.

The primary consideration with regard to the rudder and fin is that, viewed from the rear, they should be perpendicular, which can be verified by a plumb-line dropped from the top of the rudder-post. In plan view the fixed fin should correspond with the centre line of the fuselage although there are exceptions to this rule, notably where the fin is set over, to neutralize propeller torque, and in this case the measurements given in the general drawings must be adhered to.

Tension of Wires.

The correct tensioning of wires is a matter upon which some variation of opinion occurs. Although wires should not be left slack, conversely they should not be over-tensioned, as this results in the spars, wires, and struts, being initially stressed before any load due to flight is applied. In this connection the importance of even or uniform tension in the wires may be emphasized. The wires in one bay being of greater tension than those in an adjacent bay, is the frequent cause of bent or deformed struts. The more extended use of a tautness meter for the interplane wiring would result in greater uniformity and the more equal distribution of stresses.