m Notes on Rigging — FOR - ^^ == Air Mech anics WASHINGTON Press of Gibson Bros., Inc. The person charging this material is re- sponsible for its return to the library from which it was withdrawn on or before the Latest Date stamped below. Theft, mutilation, and underlining of books are reasons for disciplinary action and may result in dismissal from the University. UNIVERSITY OF ILLINOIS LIBRARY AT URBANA-CHAMPAIGN SEP 2 2 ml ^2 3. NOTES ON RIGGING FOR AIRPLANE MECHANICS. IMPORTANCE OF GOOD RIGGING. It is impossible to exaggerate the importance of CARE and ACCURACY in rigging. The pilot's life, the speed and climb of the airplane, its control and general efficiency in fhght, and its duration as a useful machine all depend upon the rigger. Consider that while the engine may fail the pilot may still glide safely to earth BUT, if the airplane fails, then all is lost. The responsibility of the rigger is, therefore, very great, and he should strive to become a sound and reliable expert on all matters relatmg to his art— for an art it is, and one bound to becOm^ncjpa^ingly impor- tant as time passes. FLIGHT. First of all he must have a sound idea of flight and stability. Flight is secured by driving through the air a surface or surfaces inclined to the direction of motion. Such inclination is called the ANGLE OF INCIDENCE. (See Fig. 1.) Side of <5t/r-foce /Vt P/r-ec^tor? of Moi-ior? A- /in^lt of IncK^ertce r-iG. 1 ■ - Lift. In this way the surfaces, that is, the lifting planes, secure a lift from the air, and, when the speed through the air is sufficient, the lift will become greater than 1 2 the weight of the airplane, which must then rise. Bear in mind that the hft is ahvaj^s trying to collapse the planes upwards. Drift. The resistance of the air to the passage of the air- plane is known as drift, and this is overcome by the propeller thrust, which thrusts the airplane through the air and so overcomes the drift. Bear in mind that the drift is always trying to collapse the plane back- wards. (See Fig. 2.)' Thus you will see by this diagram that there are f • forces to consider. The LIFT which is opposed to the WEIGHT, and the THRUST which is opposed to the DRIFT. The lift is useful — the drift is the reverse of useful. The proportion of lift to drift is known as the LIFT-DRIFT RATIO. This is of paramount impor- tance for upon it depends the efficiency of the airplane. In rigging an airplane the greatest care must be taken to PRESERVE THE LIFT-DRIFT RATIO. Always keep that in mind. 3 Angle of Incidence. The angle of incidence is the incHnation of the hfting surfaces. If the angle of incidence is increased over the angle specified in your rigging instructions then both the lift and the drift are increased also — and the drift is increased in greater proportion than the lift. If, however, the angle of incidence is decreased, then the lift and the drift are decreased and the lift decreases in greater proportion than does the drift. You see then that in each case the efficiency is lessened, because the proportion of lift to drift is not so good as would other- wise be the case. Balance. The whole weight of the airplane is balanced upon, or slightly forward of, the center of the lift. If the weight is too far forward then the machine is nose heavy. If the weight is too far behind the centre of the lift then the airplane is tail heavy. GET A GOOD UNDERSTANDING OF THE ABOVE BEFORE GOING ANY TURTHER. Stability. By the stability of the airplane is meant the tendency of the airplane to remain upon an even keel, and to keep its course; that is to say, not to fly one wing down, tail down, or nose down, or to try and turn off its course. Directional Stability. By directional stability is meant the natural ten- dency of the airplane to remain upon its course. If this did not exist it would be continually trying to turn to the right or to the left, and the pilot would not be able to control it. For the airplane to have directional stability it is necessary for it to have, in effect, more keel surface behind its turning axis than there is in front of it. By keel surface is meant everything you can see when you look at the airplane from the side of it — the sides of the body, landing gear, wires, struts, etc. Directional stability is sometimes known as "weather- 4 cock" stabilit3^ You know what would happen if, in the case of the weathercock, there was too much keel surface in front of its turning axis, which is the point ^ upon which it is pivoted. It would turn the wrong way. That is just how it is with an airplane. Directional stabiUty will be badly affected if there is more drift, i. e., resistance, on one side of the airplane than there is on the other side. This may be caused as follows.: 1. The angle of incidence of the mam planes or the tail plane may be wrong. If the angle of incidence on one side of the machine is not what it; should be, there will be a difference in the drift betw een the two sides of the airplane, with the result that it will turn off its course. 2. If the ahgnment of the fuselage fin in front of the rudder, is not absolutely correct, that is to say, if it is turned a little to the left or to the right, instead of being in line with the center of the machine and dead on in the direction of flight, it will act as an enormous rudder and cause the machine to turn off its course. 3. If the dihedral angle is wrong, that may have a bad effect. It may result in the propeller not thrusting from the center of the drift, in which case it will pull the macliine a little sidew^ays, and out of its course. 4. If the struts and stream line wires are not adjusted to be dead on in the hne of flight, then they will pro- duce additional drift on then side of the airplane, with the result that it will turn off its course. 5. There is still one other reason why the airplane may be directionally bad, and that is DISTORTED SURFACES. You must understand that the planes are "cambered," that is, curved to go through the air with the least possible drift. If, perhaps, owing to the leading edge, spars or trailing edge getting bent, the curvature is changed, it will result in changing the amount of drift on one side of the airplane, which will then have a tendency to turn off its course. Lateral StabOity. By lateral stability is meant the sideways balance of the machine. The only possible thing that can make 5 the machine fly one wing down is that there is more lift on one side than on the other. That may be due to the following reasons: 1. The angle of incidence may be wrong. If the angle of incidence is too great, then it will produce more lift than on the other side of the machine, and if the angle of incidence is too small, then it will produce less lift than on the other side, the result being that in either case the machine will try to fly one wing down. 6 2, Distorted surfaces. — If the planes are distorted, then their camber or curvature is changed and the lift will not be the same on both sides of the airplane, and that, of course, will cause it to fly one wing down. (See Fig. 3.) Longitudinal Stability. By longitudinal stabiUty is meant the fore and aft balance. If that is not perfectly right then the machine will try to fly nose down or tail down. This may be due to the following reasons: 1. The stagger may be wrong. The top plane may have drifted back a little and this will probably be due to some of the wires having elongated their loops or having pulled the fittings into the wood. If the top plane is not staggered forward to the correct degree, , then that means that the whole of its lift is moved backwards and it will then have a tendency to lift up the tail of the machine too much. In such a case the machine would be said to be "nose heavy." A Hnch error in the stagger will make a very considerable difference to the longitudinal stabiUty. 2. If the angle of incidence of the main planes is not right, that will have a bad effect. If the angle is too great it will produce an excess of lift, and that will lift up the nose of the machine and result in its trying to fly tail down. If the angle is too small, it will produce a decreased Uft and the machine will try to fly nose down. 3. When the machine is longitudinally out of balance the usual thing for the rigger is to rush to the tail plane, thinking that its adjustment relative to the fuselage must be wrong. This is, indeed, sometimes the case, but it is the least hkely reason. It is much more likely to be one of the first two reasons given or the one given below : The fuselage may have been ivarped upwards or down- wards, thus giving the tail plane an incorrect angle of incidence. If the tail plane has too much angle of incidence it will make it lift too much and the machine will be ''nose heavy." If the tail plane has too little angle of incidence then it will not lift enough, and the machine will be "tail heavy." 7 4. If the above three points are all correct, then there is a possibility of the tail plane itself having assumed a wrong angle of incidence, in which case it must be cor- rected. In such event, if the machine is nose heavy, the tail plane should be given a small angle of incidence. If the machine is tail heavy then the tail plane must be given a larger angle of incidence, BUT be careful not to give the tail plane too great an angle of incidence, because the longitudinal stabiUty of the airplane entirely depends on the tail plane being set at a much smaller angle of incidence than the main plane, and if you cut the difference down too much the machine will become uncontrollable longitudinally. Some- times the tail plane is set on the machine at the same angle of incidence as the main plane, but it actually engages the air at a lesser angle owing to the air being deflected downwards by the main planes, thus (see Fig. 4): Maiff Planes fl^ackir?^ flir Fig. 4- Propeller Torque. Owing to propeller torque the airplane has a ten- dency to turn over sideways in the opposite direction 8 to which the propeller revolves. In some machines this tendency is rather marked, and it is offset by increasing the angle of incidence on the side tending to fall^ and in some cases by also decreasing the angle of incidence of the side tending to rise. In this way more lift is secured on one side of the machine than on the other, and so the tendency to overtm-n is corrected. It is common practice to offset propeller torque by merelv increasing the angle of incidence on one side. It is far better, however, to give half such increase on one side, at the same time making a similar decrease on the other side. In this way the angle is nearer the normal angle and the efficiency of the airplane is not so much disturbed. Wash In. Wash-In. — When the angle of incidence is increased, that is called "Wash-IN." Wash-Out. — When the angle of incidence is decreased, then that is called "Wash-OUT." Sometimes a wash-out is given to both sides of the main plane. This decreases the di-ift towards the wing tips, and consequently decreases the effect of gusts upon them. It also renders the ailerons more effective (see Figs. 5 and 6). ^/////////////////////7m Stresses and Strains. In order to rig a machine intelligently it is necessary to have a correct idea of the work every wire and every part of the airplane is doing. 9 The work the part is doing is known as STRESS. If, owing to undue stress, the material becomes distorted, then such distortion is known as STRAIN. Fi6. 6. Compression. The simple stress of compression produces a crushing strain. As an example, the interplane and fuselage struts. Tension. The simple stress of tension results in the strain of elongation. As an example, all the wires. Bending. The compound stress of bending is composed of both tension and compression. Now we will suppose we are going to bend a piece of wood. Before being bent it will have the following appearance (see Fig. 7) : You see that the top line, the bottom line, and the center line are all of the same length. Now we will bend it right round into a circle, thus See Fig. 8). . 10 The center line is still the same length as it was before being bent, but you will note that the top line, being on the outside of the circle, must now be longer than the center line. That can only be due to the strain of elongation. That is produced by the stress of tension, so you see that the wood between the center Une and the line on the outside of the circle is in tension, and the greatest tension is on the outside of the circle, because there the elongation is greatest. You will notice that the line on the inside of the circle, which before being bent was the same length as the center line, must now be shorter because it is nearest to the center of the circle. That can only be due to the strain of crushing. That can only be produced by a state of compression, so you see that the wood between the center line and the inside line is in compression, and the greatest compression is nearest to the inside of the circle, because there the crushing effect, i. e., the strain, is greatest. 11 By this you will see that the wood near the center line is doing the least work, and that is why it is possible to hollow out the center of spars and struts ^without unduly weakening them. In this way 25 to 33 per cent of the weight of wood in an airplane is saved. Shear. Shear stress is such that when the material breaks under it, one part slides over the other. As an example, the locking pins. Some of the bolts are in a state of shear stress also because, in some cases, there are lugs underneath the bolt heads from which wires are taken. Owing to the tendency of the wire the lug is exerting a sideways pull on the bolt and trying to break it in such a way as to make one part of it slide over the other. Torsion. Stress of torsion. — This is a twisting stress composed of compression, tension, and shear stress. As an example, the propeller shaft and crank shaft of the engine. NATURE OF WOOD UNDER STRESS. Wood, for its weight, takes the stress of compression the best of all. For instance, a walking stick of about half pound in weight, will, if kept perfectly straight, probably stand up to a compression stress of a ton or more before crushing, whereas if the sam^e stick is put under a bending load it will probably collapse to a stress of not more than about 50 pounds. That is a very great difference and since weight is of greatest importance in an airplane, the wood must, as far as possible, be kept in a state of direct compression. This it will do safely as long as the following conditions are carefully observed : Conditions to be Observed. 1. All the spars and struts must be perfectly straight (see Fig. 9). This diagram shows a section through an interplane strut. If it is to be prevented from bending, then the 12 stress of compression must be equally disposed around the center of strength. If it is not straight, then there will be more compression on one side of the center of strength than on the other side. That is a step towards getting compression on one side and tension on the other side, in which case it will be forced to take a bending stress for which it is not designed. Even if it does not break it will, in effect, become shorter, and thus throw out of adjustment all the wires attached to the top and bottom of it, with the result that the flight efficiency of the airplane will be lessened, I besides an][undue and dangerous jstress^j^being^Ithrown upon other wires (see Fig. 10). 2. Struts and sparsTmust be symmetrical. That is meant that the cross sectional dimensions must be correct, as otherwise there will be bulging places on the outside, with the result that the stress will not be evenly disposed around the center of strength, and a bending stress will be produced. 3. Struts, spars, etc., must he-undamaged. Remem- ber that, from what has been said about bending stresses, the outside fibers of the wood are doing by far the most work. If these get bruised or scored, then the strut or spar suffers in strength much more than one might think at first sight, and if it ever gets a tendency to bend it is likely to go at that point. 13 4. The wood must have a good clear grain with no cross-grain, knots or shakes. Such blemishes mean that the wood is in some places weaker than in other places, and if it has a tendency to bend, then it will go at those weak points. 5. The struts, spars, etc., must be properly bedded into their sockets or fittings. To begin with, they must be a good ijushing or gentle tapping fit. They must never be driven with a heavy hammer. Then, again, they must bed well down, all over their cross-sectional area; otherwise the stress of compression will be taken on one part of the cross-sectional area with the result that it will not be evenly disposed around the center of strength, and that will produce a bending stress. Fic 10. The bottom of the strut or spar should be covered with some sort of paint, bedded into the socket or fitting, and then withdrawn to see if the paint has stuck all over the bottom of the fitting. 6. The atmosphere is sometimes much damper than at other times, and this causes the wood to expand and contract appreciably. This would not matter but for the fact that it does not expand and contract uniformly, but becomes unsymmetrical, i. e., distorted. This should be minimised by well varnishing the wood to keep the moisture out of it. 14 Function of Interplane Struts. — These struts have to keep the planes apart, but this is only part of their work. They must keep the planes apart so that the latter are in their correct attitude. That is only so when the spars of the bottom plane are parallel with those of the top plane. The chord of the top plane must also be'parallel to the chord of the bottom plane. If that is not so then one plane will not have the same angle of incidence as the other one. You may think that all you have to do is to cut all your struts the same length, but that is not the case. Sometimes, as illustrated in Fig. 11, the rear spar is not so thick as the main spar, and it is then necessary to make up for that lack of thickness by making the rear struts correspondingly longer. If that is not done, then the top and bottom chords will not be parallel, and the top and bottom planes will have different angles of incidence. Also the sockets or fittings or even the spars upon which they are placed sometimes vary in thickness, and this must be offset by altering the length of the struts. The proper way to proceed in order to make sure that everything is right is to measure the distance between the top and bottom spars by the side of each strut end; if that distance, or "gap" as it is called, is not as specified in your rigging diagram, make it correct by changing the length of the strut. When measuring the gap between the top and bottom spars always he careful to measure from the center of the spar, as it may be set at an angle, and the rear of the spar may be considerably lower than its front (see Fig. 11). Boring Holes in Wood. — It is a strict rule tha-t no spar may be used which has an unnecessary hole in it. Before boring a hole its position must be passed on by whoever is in charge of the shop. The hole should be of a size that the bolt can be pushed in, or, at any rate, not more than gently tapped. Bolts must not be hammered in, as it may split the spar. On the other hand, a bolt must not be slack in the hole, as in such a case it may work sideways and spht the spar, not to speak of throwing out of adjustment the wires leading from the lug or socket under the bolt head. 15 Washers. — Under the bolt head, and also under the nut, a washer must be placed; a very large washer compared with any other form of engineering. This is to disperse the stress over a large area of the wood, otherwise the washer may be pulled into the wood and weaken it, besides possibly throwing out of adjustment the wires attached to the bolt or fitting. Locking. — Now as regards locking the bolts. If spht pins are used, be sure to see that they are used in such a way that the nut cannot possibly unscrew. If it is locked by burring over the bolt, do not use a heavy Fig. 11 hammer and try to spread the whole head of the bolt. That might damage the woodwork inside the plane. Use a small, hght hammer, and gently tap around the edge of the bolt until it is burred over. Turnhuckles. — A turnbuckle is composed of a cen- tral barrel into each end of which is screwed an eye-bolt. Wires are taken from the ends of the eyebolts, and so, by turning the barrel, the wires can be adjusted to their proper tension. Eyebolts must be a good fit in the barrel; that is to say: not too slack and not very tight. There is a rule that they must be screwed into the barrel for a distance equal to not less than^twice their 16 diameter, but it is better to screw them in a good deal more than that. Now about turning the barrel to get the right adjustment. The barrel looks solid, but really it is hollowed out and is much more frail than it appears. For that reason it must NOT be turned by seizing it with pliers, as that may distort it and spoil the bore. The proper method is to pass a piece of wire through the hole in the center and use that as a lever. When you have got the correct adjustment, you niust lock the turnbuckle to prevent it from unscrewing. It is quite possible to lock it in such a way as to allow it to unscrew a quarter or half a turn, and that will throw the wires out of the very fine adjustment neces- sary. The proper way is to use the locking wire in such a way as to oppose the tendency of the turn- buckle to unscrew, thus (see Fig. 12) : Ti;gpiBiycK/.t- Internal Turnhuckles. — Turnbuckles on the internal wires inside the planes must be well greased and served round with adhesive tape. WIRES. The following points must be carefully observed where wire is concerned: 1. Quality. — It must not be too hard or too soft. An easy practical way of learning to know the quality of wire is as follows: Take three pieces of wire all of the same gauge and each about a foot in length. One piece should be too soft, another piece should be too hard, and the third piece of the right quality. Fix them in a vise about an inch apart and in a vertical position, and with the light from a window^ shining upon them. Burnish them if necessary, and you will see a bar of light reflected from each wire. 17 Now bend the wires over as far as possible. Where the soft wire is concerned it will squash out at the bend, and you will see this because the band of light will have broadened out there. In the case of the wire which was too hard the band of light will broaden out very little at the turn, but if you look carefully you will see some little cracks or roughness on the surface. In the case of the wire of the right quality, the band of light may have broadened out a very little at the turn, but there will be NO cracks or roughness on it at all. By making this experiment two or three times you will soon learn to know good wire from bad, and also learn to know the strength of hand necessary to bend the right quality. 2. It Must Not he Damaged. — That is to say, it must be unkinked, rustless and unscored. 3. As Regards Keeping Wire in Good Condition. — Where the outside wires are concerned they should be kept well greased or oiled, especially where bent over at the ends. In the case of internal bracing wires which cannot be reached for the purpose of re-greasing them, you will prevent them from rusting by painting them with lacquer. You must be very careful to see that the wire is perfectly clean and dry before painting with lacquer. A greasy finger mark is sufficient to keep the lacquer from sticking to the wire. In such a case there will be a little space between the lacquer and the wire. Air can enter there and cause the wire to rust under the lacquer. Tension of Wires. The tension to which you adjust the wires is of the greatest importance. All the wires on an airplane should be of the SAME TENSION, otherwise the air- plane will quickly become distorted and fly badly. As a rule the wires are tensioned too much. The tension should be SUFFICIENT TO KEEP THE FRAME- WORK RIGID. Anything more than that changes the factor of safety, throws various parts of the framework into undue compression, pulls the fittings into the wood, and will, in the end, distort the whole framework of the airplane. 18 Only experience will tell you what tension to employ and assist you in making all the wires of the same ten- sion. Learn the construction of the various types of airplanes, the work the various parts do, and cultivate a touch for tensioning the wires by constantly handling them. Wires with No Opposition Wires. In some cases you will find wires having no opposition wires as the overhang in the Curtiss machines. In such cases be extremely careful not to tighten such wires beyond taking up the slack. They must be a little slack, as otherwise they wiU distort the top spar down- ward. That will change the camber (curvature) of the plane and result in changing both the lift and drift at that part of the plane. Such a condition will cause the machine to lose its directional stability and also to fly one wing down. Wire Loops. , Wires often bent over at the end in the form of a loop. These loops, even when made perfectly, have a ten- dency to elongate, thus spoiling the adjustment of the wires. Great care should be taken to minimise this as much as possible. The rules to be observed are as follows: 1. The size of the loop should be as small as possible within reason. By that is meant that it should not be so small as to create the possibility of the wire breaking. 2. The shape of the loop must be symmetrical. 3. The loop should have good shoulders in order to prevent the ferrule from slipping up. At the same time the shoulders should have no angular points. 4. When the loop is finished it should be undamaged, and it should not be, as is often the case, badly scored (see Fig. 13). Strained Wire Cables. No splice should be served with twine until it has been inspected and passed by whoever is in charge of the shop. Should a strand become broken then the cable must be replaced by another one. Control cables 19 have a way of wearing out and fraying wherever they pass round pulleys. Every time an airplane comes down from flight the mechanic should carefully examine the cables wherever they pass round the pulleys, and, if he finds a strand broken, he should report that fact at once. The ailerons balance cable on the top of the top plane is often forgotten, since it is necessary to fetch a step ladder to examine it. DON'T OVERLOOK THIS Wif^f Loops Wron^ Kif?d oi Wire Loop f'XQ^ \ O 3^ Pefor/z>a-fiotp o-f Wron^ . ' '~ Corree/- F^orr/r of Wire Loo/^ ADJUSTMENTS. ^ Control Surfaces.— The greatest care must be exer- cised in properly rigging the aileron, rudder and ele- 20 vator, for the pilot entirely depends upon them in managing the airplane (see Fig. 14). The ailerons and elevators should be rigged so that WHEN THE MACHINE IS IN FLIGHT they are in a fair true hne with the surface in front and to which they are hinged. If the surface to which they are hinged is NOT A LIFTING SURFACE, then rig the controlling sui'face to be in a fair true line with the surface in front. If the controlling surface is hinged a little below what it would be if it was in a fair true line with the surface in front. This is because in such a case it is set at an angle of incidence. This angle will, when the machine is flying, produce lift and cause it to lift a little above the point at which it has been rigged on the ground. It is able to hft owing to a certain amount of slack in the control wu-e holding it — and you can't adjust the control wire to have no slack, because that would cause it to bind against the pulleys and make the operation of it too hard for the pilot. It is, therefore, necessary to rig it a little below what it would be if it was rigged in a fair true line with the surface in front. Remember that this only applies when it is hinged to a lifting surface. The greater the angle of incidence of the lifting surface in front, then the more the controlling surface will have to be rigged down. As a general rule you will be safe in rigging it down so that the trailing edge of the controlling surface is ^ to I of an inch below where it would be if it was in a fair true hne with the surface front — or half an inch down for every 18 inches of chord of the controlling surface. 21 When adjusting the controlling surfaces the pilot's control levers must be in a neutral position. It is not sufficient to lash them in that position. They should be blocked into position. Remember that controlling surfaces must never be adjusted with a view to altering the stability of the machine. Nothing can be accomplished in that way. The only result will be that the control of the airplane will be spoiled. CONTROL CABLES. The adjustment of the control cables is quite an art, and upon it will depend to a large degree the quick and easy control of the airplane by the pilot. The method is as follows: After having rigged the controlling surfaces reniove the blocking which has kept the control levers rigid. Then sitting in the pilot's seat, move the control levers smartly. Tension up the control cables so that when the levers are smartly moved there is no perceptible snatch or lag. Be careful not to tension up the cables more than necessary to take out the snatch. If you tension them too much the cables will bind round the pulleys and result in hard work for the pilot and also in throwing dangerous stresses upon the controlUng surfaces, which are sometimes of rather flimsy construc- tion. It will also cause the cables to fray round the pulleys quicker than would otherwise be the case. Now, after having tensioned the cable sufficiently to take out the snatch or lag, place the levers in their neutral position and move them backwards and for- wards not more than eighth of an inch either side of the neutral position. If the adjustment is correct you should be able to see the controlling surfaces move. If they do not move then the control cables are too slack. FLYING POSITION. Before rigging the machine it is necessary to place . it in what is known as its "jflying position." In the case of an airplane fitted with a stationary engine this is best secured by blocking up the machine 22 so that the engine foundations are perfectly horizontal both LONGITUDINALLY and LATERALLY. This is done by placing a straight edge and a spirit level on the engine foundations, and you must be very careful indeed that the bubble is exactly in the center of the level. The sUghtest error will be much magnified towards the wing tips and tail. Great care should be taken to block the machine up RIGIDLY. In case it gets accidentally distm'bed during the rigging of the machine, you should constantly verify the flying posi- tion by running the straight edge and the spirit level over the engine foundations. CAREFULLY TEST THE STRAIGHT EDGE FOR TRUTH BEFORE USING IT, for, being usually made of wood, it will not remain true long. Place it lightly in a vise, and in such a position that a spirit level on top shows the bubble exactly in the center. Now slowly move the level along the straight edge. The bubble should remain exactly in the center. If it does not, then the straight edge is not true, and must be corrected. NEVER OMIT DOING THIS. Angle of Incidence. One method of finding the angle of incidence is as follows (see Fig. 15). The corner of the straight edge must be placed under- neath and against the CENTER OF THE REAR SPAR and held in a horizontal position parallel to the ribs. This is secured by using a spirit level. The set measurements for the angle of incidence will then be from the top of the straight edge to the CENTER of the bottom surface of the main spar, or it may be from the top of the straight edge to the lowest part of the leading edge. Be careful to take the adjustment from the center of the spar and to see that the bubble is exactly in the center of the level. Remember that all this will be useless if the machine has not been accu- rately placed in its "flying position." This method of finding the angle of incidence must be used under every part of the plane where struts occur. The method should not be used midway between the struts because, in such a place, the spars 23 may have taken a slight permanent set up or down — not sufficiently bad to make any material difference to the fljing of the machine, but quite bad enough to throw out the angle of incidence adjustment. If the angle of incidence is not right, correct it as follows: If it is too gi'eat then the rear spar must be warped up until it is right, and this is done bv slacking off ALL THE WIRES going to the top of the struts and then tightening ALL THE WIRES going to the bot- F-zg. 15. tom of |the [struts. If the angle is less [than'Jit should be then slack off ALL THE WIRES going to the bottom of the struts and tighten ALL THE WIRES going to the top of the struts until you have the correct adjustment. Do not attempt to secure the adjustment by merely adjusting the incidence ■s^dres. This is a veiy bad practice indeed and while, owing to the airplane being of such ffimsy construction, it may be possible to change the angle of incidence by 24 adjusting merely the incidence wires, the result of such practice is to throw other wires into undue tension, and that will cause the framework to become distorted. Dihedral Angle. One method of securing the dihedral angle, which is the upward inclination of the wings towards their tips, is as follows; this method will, at the same time, give you the angle of incidence (see Fig. 16) : , A- One Mef&ad »/ Hotif/^a Sfr/ nj B±_finefh er • The strings, drawn very tight, must be taken over both main and rear spars of the top plane. They must run between points on the spars just inside the outer struts. The set measurement is then from the strings down to four points on the main and rear spars of the the center section plane. These points should be just inside the four center section struts; that is to say, as far as possible away from the center of the center section. DO NOT ATTEMPT to take the set measurement near the center of the center section. The string should be as tight as possible, and, if it can be done, the best way to arrange it is as shown in the diagram, i. e., by weighting the string down to the spars by means of weights. This will give a tight and MOTIONLESS string. Do not use the method shown in the left side of the diagram, if the weight on the end 25 of the string causes the wing tip to distort. However careful the above adjustment is made there is almost sure to be some slight error, and it is necessary to take certain check measurements, as follows: Each bay must be diagonally measured, and such diagonal measurements must be the same on each side of the airplane. As a rule these diagonal measure- ments are taken from the bottom sockets of one strut to the top socket of another strut, but this is all wrong because of possible inaccuracies due to faulty manu- facture. The points between which the diagonal measurements are taken must be at fixed distances from the butt of the spars, such fixed distances being exactly the same on each side of the machine, thus (see Fig. 17) : " J P ^^^^ 3/ 7 Piaaonal 1 Mos^ f-qual Q- ^ - - Pis-lanct from fl fa B<0Mosf j-t^ual Pisfartce V io Boff. — 1 • £ c - ■• »e+h F-rcn/ 4 K^tar Bays hfloff Check tJ . Diagonal 1 must = 4. Diagonal 2 must = 3. Distance from A to Butt must = distance from D to Butt. The same applies to B and C and to E and F. Both Front and Rear Bays must be checked. It would be better still to use the center line of the fuselage instead of the butts of the spars, but for the fact that such method is a troublesome one. Dihedral Board. Another method of securing the dihedral angle (and also the angle of incidence) is by means of the dihedral 26 board. The dihedral board is a Ught handy thing to use but leads to many errors, and should not be used unless necessary. The reasons are as follows: The dihedral board is probably not true. If you MUST use it, then be very careful to test it for truth beforehand. Another reason against its use is that you have to use it on the spars BETWEEN THE STRUTS, and that is just where the spars may have a Uttle permanent set up or down which will, of course, throw out the accm-acy of the adjustment. Then again, there may be inequaUties of surface on the spar due to faulty manufacture. The method of using it is as follows (see Fig. 18) . F-16. 16. If the dihedral board is used then the bays must be carefully diagonally measured as explained above. Whatever the method is used to secure the angle of incidence, be sure that, after the job is done, THE SPARS ARE PERFECTLY STRAIGHT. STAGGER. The stagger is the distance the top plane is in advance of the bottom plane when the machine is in flying posi- tion. The set measurement is obtained as follows (see Fig. 19). A, set measurement when taken along Prolongation of Chord. B, set measurement when taken along Horizontal Line. Plumb lines must be dropped over the leading edges wherever struts occur, and also near the fuselage. The set measurement is taken from the front of the lower 27 leading edge to the plumb line. Remember that it makes a difference whether you measure along a hori- zontal line (which can be found by using a straight- edge and a spirit level) or along a projection of the chord. The correct line along which to measure is laid down in the rigging diagram accompanying each machine. If you make a mistake and measm-e along the wrong line this may make a difference of a quarter of an inch or more to the stagger, with the certain result that the airplane will be nose heavy or tail heavy. A' Se^ Measuremer-/ W/^e^ Taker? fffof7^ ProfoA>^a-f/ov ef Chofe^ When the adjustment of the angles of incidence, dihedral angle, and the stagger have been secured RUN OVER ALL OF THEM AGAIN AS, IN MAKING YOUR LAST ADJUSTMENT, YOU MAY POSSIBLY HAVE THROWN OUT YOUR FIRST ONE. OVER ALL ADJUSTMENTS. Now you should take the following over-all measure- ments (see Fig. 20). 28 The straight lines "A" and "C" must be equal. The point "B" is the center of the propeller thrust, or, in the case of a "pusher" machine, the center of the nacelle. The points "D" and "E" are marked on the main spar; and must in each be the same distance from the butt of the spar. Do not attempt to make 3 "D" and "E" merely sockets on the outer struts as they may not have been placed quite accurately by the manufacturers. The lines "A" and "C" must be taken from both top and bottom spars— two measure- ments on each side of the airplane. ^ Now measure the distance between I* and U and "H" and "G." These two measurements must 29 be equal. "G" is the center of the fuselage or rudder post. "F" and "H" are points marked on both top and bottom rear spars, and in each case must be the same fixed distance from the butt of the spar. Remem- ber that "F" and "H" must not be taken to be the sockets of the outer rear struts. Two measurements on each side of the akplane. If these over-all measure- ments are not correct then it is probably due to some of your drift or anti-drift wires being too tight or too slack. It may possibly be due to the fuselage being out of truth, but, of course, you should have made quite sure that the fuselage was true before rigging the rest of the machine. Again, it may be due to the internal bracing wires not being accurately adjusted but, of course, that should have been done before covering the planes with fabric. PROPELLERS. The last thing to go on to the machine is usually the propeller, by which time there is usually a rush to get the machine out. You must, however, be very careful to see that this is fitted on true and straight. This is easily verified by bringing the tip of one blade round to graze some fixed object such as a trestle. Mark the place where the tip of the blade touches it. Now bring the tip of the other blade round and it should be within an eighth of an inch of the mark. If it is not so, then it is probably due to some of the propeller bolts being pulled up too tight. It may be due to the propeller itself not being true. FUSELAGE. The methods of trueing fuselages are laid down in the rigging diagrams accompanying each machine. After having adjusted the fuselage according to the specified directions, then arrange it on trestles in such a way as to make about three-quarters of the fuselage towards the tail stick out unsupported. In this way you wiU get as near as possible to flying conditions and, when it is in this position, you should run over the adjustments again. If this is not done the fuselage 30 may be out of truth but perhaps appear all right when supported by trestles at both ends as, in such case, its weight may keep it true as long as it is resting upon the trestles. TAIL PLANE. The exact angle of incidence of the tail plane is given in the rigging diagram. Be careful to see, however, that the spars are horizontal. If they are tapered spars then see that their center lines are horizontal. After the tail plane has been rigged, support the machine so that the tail is unsupported as explained above. Then verify the adjustment and make sure that the tail plane spars are horizontal when the machine is in flying position. RUDDER, AILERONS, ELEVATOR. These controlling surfaces must not be distorted in any way. If they are held true by bracing wires then such wires must be carefully adjusted. If they are distorted and there are no bracing wires with which to true them up, then the matter should be reported, as it may be necessary to replace some of the internal framework. LANDING GEAR. The landing gear must be very carefully aligned as laid down in the rigging diagram accompanying each machine. 1. Be very careful to see that the landing gear struts bed well down into their sockets. If this is not done then after having a few rough landings, they will bed down farther and thi'ow the landing gear out of align- ment, with the result that the machine will not taxi straight. 2. When rigging the landing gear the airplane must be blocked up in its flying position and sufficiently high so that wheels are off the ground. When in this position the axle must be horizontal. 3. Be very careful to see that the shock absorbers are of equal tension, and that the same length of elastic and the same number of turns are used in the case of each absorber. 31 HANDLING OF AIRPLANES. An extraordinary amount of damage is done by the mishandling of airplanes and in blocking them up from the ground in the wrong way. The golden rule to observe is: Produce No Bending Stresses. 1. Remember that nearly all the wood of the airplane is designed to take the stress of direct COMPRESSION and it cannot be safely bent. In blocking up an air- plane from the ground the blocking must be used in such a way as to come underneath the interplane struts and the fuselage struts. Padding should always be placed on the points upon which the airplane rests. 2. When puUing the machine along the ground always, if possible, puU from the landing gear. If necessary to pull from elsewhere then do so by grasping the interplane struts as low down as possible. 3. As regards handling parts of airplanes. Never lay anything covered with fabric on a concrete floor, as any slight movement will cause the fabric to scrape over the concrete with resultant damage. 4. Struts, spars, etc., should never be left about the floor, as in such position they are likely to become dam- aged, and it has akeady been explained how necessary it is to protect the outside fiber of the wood. Remem- ber also that wood easily becomes distorted. This particularly appUes to the interplane struts. The best method is to stand them up in as nearly vertical posi- tion as possible. NOTES FOR KEEPING THE AIRPLANE IN GOOD CONDITION. Cleanliness. — The fabric must be kept clean and free from oil, as that will rot it. To rake out dirt or oily patches try acetone. If that will not do the job try gasohne, but use it sparingly or otherwise it will take off an unnecessary amount of dope. If that wiU. not remove it, then hot water and castile soap will do so. Use water sparingly, as otherwise it may get inside the planes and rust the internal bracing wires, or cause some of the wooden framework to swell. 32 The wheels of the landing gear have a way of throwing up a great deal of mud onto the lower plane. This should be taken off at once. Do not allow it to dry and do not try to scrape it off when dry. If dry then it must be moistened first as otherwise the fabric will be spoiled. Controlling Wires. — After every flight pass your hand over the control wires and carefully examine them near pulleys. If only one strand is broken the wire must be changed. Don't forget the aileron balance wire on the top plane. Once a day try the tension of the control wires by smartly moving the control levers about as explained elsewhere. . Wires. — See that all wires are kept well greased or oiled, and that they are all in the same tension. When examining your wires be sure to have the machine on level ground as otherwise it may get twisted, throwing some wires into undue tension and slackening others. The best way, if you have time, is to block the machine up into its flying position. If you see a slack wire do not jump to the conclusion that it must be tensioned, perhaps its opposition wire is too tight, in which case slacken it and possibly you will find that will tighten the slack wire. Carefully examine all wu'es and their connections near the propeller, and be sure that they are snaked round with safety wire, so that the latter may keep them out of the way of the propeller if they come adrift. Distortion. — Carefully examine all sm-faces, including the controlling surfaces, to see whether any distortion has occurred. If the distortion can be corrected by the adjustment of wires well and good, but if not then report the matter. Adjustment. — Verify the angle of incidence, the dihedral angle, the stagger, and the over-all measure- ments as often as possible. Landing Gear. — Constantly examine the alignment and fittings of the landing gear, and the condition of axle, tires, shock absorbers, wheels and the tail skid. Control Surfaces. — As often as possible verify the rigging position of the ailerons and elevator. 33 Locking Arrangements. — Constantly inspect the lock- ing arrangements of all tiu-nbuckles, bolts, etc. Outside Position. — The airplane, when outside its shed, must always stand FACING THE WIND. If this is not so, then the wind may catch the controlling surfaces and move them sharply enough to darnage them. If the airplane must be moved dm^ing windy weather then the control levers should be lashed fast. Sighting. — ^^Tienever you have the opportunity, practice sighting one strut against another to see that they are parallel. Standing in front of the machine, which, in such a case, should be on level ground, sight the center section plane against the tail plane and see that the latter is in line. Sight the leading edge against the main spars, the rear spars and the traihng edges, taking into consideration the "wash-in" or the "wash- out." You will be able to see the shadow of the spars through the fabric. By practising this sort of thing you wiU, after a time, become quite expert, and will be able to diagnose by eye faults in efficiency, stabiUty, and control. PROPELLERS. The sole object of a propeller is to produce THRUST. The thrust overcomes the drift of the airplane, and draws ;or pushes the airplane through the air. The thrust must,^be equal to the drift of the airplane at flying speed. If it is not equal to the drift then the airplane cannot secure its proper speed. The thrust will be badly affected if any of the following conditions are not as they should be (see Fig. 21) : Pitch-Angle. — The propeller screws through the air and its blades are therefore set at an angle. This angle is known as the "pitch-angle" and it must be correct to HALF A DEGREE. It is, of course, smaUer towards the tips of the blades just as in the case of the pitch-angle of a screw. Pitch. — The^pitch is the distance the propeller will screw the air in one revolution, supposing the air to be^soHd. As a matter of fact the air is not soHd, and gives back to the thrust of the propeller blade so that the propeller does not travel its full pitch. Such 34 "give-back" is known as "slip." For instance, the pitch of the propeller may be perhaps 10 feet, and the propeller may have a slip of 2 feet. The propeller would then be said to have 20% slip. To test the pitch-angle the propeller is mounted on a shaft, the latter being mounted upon and at right angles to a beam. The fac of the beam must be per- fectly straight and true. Now select a spot some distance (say about 2 feet) from the center of the propeller and, by means of a protractor, find the angle the chord of the blade makes with the beam. Then lay out the angle out on paper thus (see Fig. 22) : X ' Jln^/e of hc>def7ce as /rr f^,g. dO. ^ ^ ^ ' • Pi'am. (Se/ec^ed per Ca /tu/a//o/y) X 3 It/f, C ' Len^fb of Line C is fhe F,^ch of Pro^e//fr X = Angle of incidence as in first diagram. XX = Diameter (selected for calculation) X 3.1417. C : Length of line C is the pitch of propeller. The line marked CHORD represents the chord of the propeller. The line marked CIRCUMFERENCE 35 represents the face of the beam. The angle the two lines make is the angle you have found by means of the protractor. We will suppose, for the sake of example, that the point at which you have taken the angle if 2 feet, from the center of the propeller. Find the cu'cumference at the point by doubling the 2 feet (which is the radius) and then multipljdng the result by 3.1417, thus: (2 feet X2 feet) X3. 1417 -12.5668 feet, i. e., the cir- cumference at that part of the propeller. Bring it down in scale, and mark it off from the point "A" and along the circumference line. -Now draw the line marked pitch from "B" (the end of the circumference measurement of 12.5668 feet) and at right angles to the circumference line. Weigh/ Boli Hol