{
    "system": "GoGuides Verified Text",
    "api_version": "verified-text-v1",
    "status": "ok",
    "response_type": "verified_text_record",
    "source_key": "britannica_1926",
    "source_title": "Encyclopaedia Britannica (1926)",
    "license_code": "public_domain",
    "attribution": null,
    "license_url": null,
    "chunk_id": "1926:aeronautics:09ad2d6adb7e",
    "title": "AERONAUTICS",
    "section": null,
    "hash_alg": "sha256",
    "hash_sha256": "1d2a02dd3e3a6d71dc8474a4313a09fd8979b73b439a83d5105119d210da216b",
    "normalizer": {
        "name": "ggnorm",
        "version": "1.0"
    },
    "verified_text": "from the early experiments of s. p. langley and the wright bros. until the beginning of the world war the progress in aeronautics was not marked. through the efforts of lord haldane, which resulted in the establishment of the advisory committee for aeronautics in 1909, research had made a start in england at the national physical laboratory and at farnborough, while in germany the pottingen school had been founded and the importance of a asluay of fundamental principles was being recognised. aircraft, as we know them, were a creation of the war, and military requirements dominated design and practice; the im- portance of civil aviation was barely recognised; the history of aeronautics, as the reader of sir w. raleigh’s war in the air realises, formed no small part of the history of the war. the five years (1920-5) have shown a rapid advance in many directions; the theory of the aerodynamic action between a flying-machine and the medium in which it flies has been put on a new basis which has already, as the subsequent pages will show, borne good fruit, which promises to be still more fruitful in the future. aircraft are now being designed for the definite purposes for which they are required; that to some extent was true in the earlier period, but the purposes were military, not civil, and the knowledge at the designer’s disposal was far less than he now possesses, thanks to investigation and research. now, as then, to combine strength and lightness is the problem which confronts the designer; metallurgists have realised this and are giving him new materials to work with and in this way they are raising new problems in design. the internal combustion engine, without which aviation could not exist, is being studied from every point of view with the ob- ject of increasing efficiency, of discovering methods to make it as effective at 20,000 ft. as at sea-level, of lessening the number of failures in the air and rendering it as immune from accident as a locomotive or marine turbine. - methods of navigation are being devised to render air trans- port safe and certain; wireless has been enlisted in the effort; the neon beacon and the leader cable illustrate the manner in which aviation, in its desire to put safety first, calls other sciences to its aid. the meteorologist is asked for frequent weather forecasts and is told that there is an urgent need for information as to the nature of gusts in the wind, their intensity, and the rapidity with which they change both in direction and amount. in all direc- tions aeronautics rests on investigation and research. aerodynamics since a sound theory is the basis of good practice, we com- mence the detailed study of our subject with aerodynamics, dealing with the laws which regulate the flow of air and the mo- tion of bodies through the air. here the advances have been marked; not that the practice of flight has lagged behind, as will be clear when we come to treat of the design and structure of air- craft, their navigation and their use as a means of transport ever growing in importance. but in england and indeed throughout the world, theory and practice have gone hand in hand, pilots have appreciated the work done in laboratories and, by the aid of wind tunnels, have been ready to test the conclusions of the theo- rist, and have dared unhesitatingly to carry out the manoeuvres required for the verification of his theories. acrodynamics is a branch of the larger subject of hydrody- namics, which deals with the motion of fluids in general, but as its name indicates has been concerned with the properties of the all-pervading fluid, water, or rather with those of a simplified water, identical in some of its main properties with water, but still differing from it sufficiently to allow the mathematician to solve his otherwise intractable equations. the main difference is the assumed absence of any action of a frictional nature either between the particles of the fluid them- selves or between those particles and the surface of any solid which may be moving through the fluid; the fluid is said to have no viscosity. now one of the results of this assumption is that if a solid, of which the surface is free from sharp edges or entering angles, be aeronautics moved uniformly through such a fluid it will experience no re- sistance, unless the velocity exceeds a certain limit depending on the shape of the solid; the fluid moves smoothly round the sur- faces of the solid and the lines of flow, the streamlines, unite where they meet and pass off together; the resultant pressures balance and there is no force on the solid. but if the velocity be too great, or if the body have a sharp edge, theory shows that the pressure at some point of the surface becomes negative; now a fluid cannot support a tension or negative pressure, the motion therefore breaks down. streamlines leave the body at different points enclosing a mass of dead water which is dragged along by the solid; the surface pressures no longer balance and the solid experiences resistance. a streamline shape, e.g., an airship hull or a fish’s body, is a shape in which this failure to maintain streamline motion and to avoid the dead water does not occur at the velocities at which the body is required to move and which therefore would experience no resistance. viscosity, etc., of air —but real air and water differ from the hydrodynamical fluid in that they have viscosity; air, moreover, can become pressed into a smaller volume, though this property does not greatly modify the motions with which we are con- cerned; the viscosity, however, is all important and the enquiry as to its effect on the conclusions just drawn is vital. it is here that aerodynamics has made some of its most marked advances during the period since the world war. in a narrow band close to the surface of the body which we are considering (e.g., an aeroplane wing) moving at right angles to its edge, the motion is modified greatly by viscosity; the layer of air adjacent to the surface sticks to it and is drawn for- ward with it, and the motion thus caused is communicated to the adjacent layers and superposes on the streamline motion a cir- culation round the wing opposite to the direction of motion above the wing, and with it below. the velocity above the wing is thus increased, and the pressure reduced relative to the streamline flow, while below the wing the reverse is the case; thus the circulation which we may regard as a vortex or whirl round the wing causes an upward forced lift on the wing. friction factor.—again, the friction between the layers of air adjacent to the surface shows itself as a drag on the wing, while eddies are set up in the boundary of the dead water, due in part to the friction between it and the rapidly moving streams by which it is surrounded, and these add to the drag. if we con- sider the case of a wing the span of which is very long (infinite in a mathematical sense) in comparison with its chord, the mo- tion will all take place in planes at right angles to the front edge, and will be the same in any such plane; the loading on the wing will also be uniform. in this case expressions have been found for the lift and drag. we owe to lanchester the original conception of this vortex motion round the length of the wing, and to prandtl and his school the original development of the mathematics, while the problem has been carried further by other workers, notably glauert at the r.a.e. and monk in america. the fact that the wing is finite in length modifies the conditions near the ends; in consequence the simple expressions for the lift and drag need corrections and these have been evolved by making an assumption as to the manner in which the loading falls off as the wing tips are approached. experiments at the national physical laboratory and elsewhere have shown a good agree- ment with the theory which thus constitutes one of the most notable advances of the past few years. but many more are described in some detail in the section on aerodynamics and the rest of the articles; among these we may pick out as vital the growth of our knowledge in the means of controlling an aeroplane in flight, especially when at or above the stalling incidence, or when performing difficult manoeuvres, such as spinning. spin factors-——tvhe spin is a method developed during the war for causing an aeroplane to lose height rapidly; its nose is pointed downward and as it descends it spins about an axis ap- proximately vertical. at the same time a method of bringing the machine out of the spin was devised. but in some cases the method failed. why? this question has led to a whole series of aeronautics investigations, in part theoretical or on models in the wind tunnels, in part, full-scale, involving daring flights by skilled pilots, aimed in both cases at the measurement of the forces on an aeroplane when spinning and the discovery of the laws which govern that motion; as a result we know now far more about the dynamics of a spin. or again accidents occur most frequently as a result of forced landings. as the speed drops there comes a point at which the controls cease to act; the machine is said to be stalled. how does its condition differ from that of a machine in ordinary flight, what are the peculiarities of the motion of the air about the machine which lead to this disastrous result? it is a problem on which many keen minds have been exercised in the near past, both from the theoretical and practical side, and, though a com- plete solution has not been reached and there is much to learn, work resulting in a larger rudder and a combination of wing-slots and ailerons has shown that experimental machines can be con- trolled with safety at speeds considerably below the stall. the langley field wind tunnel—in concluding this de- scription of the more purely theoretical advances reference must be made to the high pressure wind tunnel at langley dield, vir- ginia, u.s.a. the meaning of the term reynolds number is explained in the article aerodynamics and the necessity, if model experiments are to be compared with those of full scale, that they should be concluded either at the same reynolds num- ber or under conditions where the effect due to a change in the number is not marked. this result the high pressure tunnel has enabled us to reach and the detailed conclusions of the work carried out by the able staff employed at langley vield were eagerly awaited in 1926. design and structure and so we pass on to matters concerning design and structure. here we notice at once a marked difference between air and land transport. the railway wagon or the motor-lorry rests on the ground; their weights are carried thus. ships and aircraft are supported by the medium through which they travel and in the case of aircraft heavier than air the motor must provide not only the means of propulsion but also support. this it does as a con- sequence of the speed at which the aeroplane travels, but it fol- lows that light-weight construction is essential. strength too is needed and the problem of the designer is to combine strength and lightness in a form which will experience small resistance to rapid motion. in this his problem differs from that of the bridge- ' builder or the engineer; they, it is true, do not wish to waste material but at the same time a reduction of the weight of the structure they are designing by a few pounds is rarely important; to the aero engineer it is essential. thus a host of new problems arise. five years ago an aeroplane almost without exception was built of wood and wire and canvas; while wood is still the rule, all-metal aircraft exist and promise to replace the wood. metal construction renders desirable new designs and methods of con- struction and the demand for a light but strong metal calls for difficult and accurate research into metallurgical problems (see electrometallurgy; metallurgy). lift and drag.—again, the lift and drag depend on the speed of the aeroplane, varying as the square of the speed; so that if with a given wing we increase the spced to obtain a greater lift we are met also with a greater drag. but the proportion the lift bears to the drag depends on the shape of the wing; thus we are confronted by the problem of selecting a form of wing which has a high ratio of lift to drag and is free from such other disad- vantages as a large displacement of the centre of pressure (the point through which the resultant lift acts), the variations in the angle which the wing makes with the direction of motion. safety first—once more safety is of the first importance and thus the aircraft must be controllable, completely under the command of the pilot, but it must also be stable within limits, z.€., 4 must, ii for any reason it is caused to deviate from its desired course, tend to return to that course and not require the unison between the eye and hand of an ever-waichful pilot in order to prevent disaster. : 33 in early days the attainment of many of these desiderata was a matter of chance; now the designer sets before himself a definite aim and usually reaches it in his completed machine. special purposes and design——and that aim includes the special purpose for which the machine is to be used; in years past a machine which could fly could be utilised for many diverse pur- poses, ¢.g., bomb-throwing or passenger transport; now specifica- tions are drawn giving in minute detail all that is required. in this connection an extract from the first report of imperial airways—the company working the traffic from england to the continent—which has been published is of great interest to all concerned. after referring to the difficulties and to the high maintenance costs involved in the use of machines not specially designed, the report continues:— there is a prospect that the modern machines now coming along will be simpler in construction, require less expenditure of man-hours on maintenance, spend more of their time in the air earning, and ike of more advanced design will not become so quickly out of ate. slirscrew and engine-—we come now to the method of pro- pulsion, the airscrew and the engine which drives it. the air- screw is used exclusively for this purpose and probably will continue to be so employed; it is of interest to notice the differ- ence in this respect between a bird and an aeroplane; the wings of a bird both support and propel it; an aeroplane has wings to carry its weight, but an airscrew in rotatory motion is needed to give it speed. nature has not fashioned an organ which can be maintained in continuous rotation about an axis. the atrscrew and tts problems —the airscrew of to-day is made of metal more often than in the past, but its shape and its efficiency, which is very high, have not been greatly modified. much more, however, is now known of the theory of an airscrew and the mechanism of the air motions by which the rotational energy driving the screw is transformed into the translational energy which propels the aircraft. it is now fully realised that the blade of a propeller is an aerofoil or aeroplane wing in rapid circular motion about one end. an aeroplane wing is ordinarily driven forward in a straight line in a direction at right angles to its length, and lift and drag forces result; the one acting at right angles to the wing and lifting it, thrusting it upward in a direc- tion perpendicular to that of its motion, the other dragging it back and requiring the action of an external force to maintain the motion. 7 if now the wing be spun about an axis perpendicular both to its chord and its span and passing through one tip, similar results follow; each section is moving forward, though with a speed which varies as its distance from the axis; each section ex- periences a lift and a drag, varying of course from section to sec- tion, but the lifts combine to form a force thrusting the wing parallel to the axis, while the resultant of the drags is a torque or twisting force about the axis which requires to be balanced by an impressed torque if the motion is to continue. thus the torque about the axis, produced by the engine, be- comes a thrust parallel to the axis driving the wing in that direction. an airscrew has two or four such blades placed sym- metrically round the axis and shaped suitably to produce, for a given speed of revolution, the maximum thrust. the great advance of the last few years has consisted in the fuller appreciation of this similarity and the realisation of the fact that the methods of calculation outlined above as applicable to the wings of an acroplane, the circulation theory of the hft, apply also to the action of the airscrew. vortices are formed around the blades of the screw and it is to these vortices that the thrust is due. engine problems.—but power must be supplied to the air- screw if the machine is to fly, and it was the advent of the internal combustion engine that made flying possible. and here again weight is the essence of the problem; weight includes not only that of the engine itself but that of the fuel it burns and of the oil which makes its motions possible. a comparison of the data as to engines given in any of the earlier text books with those 34 printed in the present article shows what the advance has been, an advance due to many causes. metallurgical research has given strong metals capable of standing the high temperatures requi- site, while at the same time it has provided us with lighter metals of strength sufficient for the purpose with improved thermal conductivity and other advantages. thermodynamic reasoning points out the need for a high compression ratio in the cylinder and experiments of great delicacy are guiding us to understand more clearly the nature of the combustion which takes place there, the meaning of detonation and some of the means of avoiding it. the valve mechanism has its own difficulties and some of these may perhaps be overcome by the use of a sleeve valve type of construction; in all directions there has been ad- vance, and some engines are far superior for their purpose to those left us by the world war. navigation problems —and now, having described the ma- chine, its structure and its methods of propulsion, we come to the consideration of its navigation through the air, the difficulty of knowing the vertical when flying out of sight of land, the mysterious behaviour of the compass under certain conditions, first unravelled at farnborough by keith lucas, the new com- passes much improved by recent work, the problem of straight flying in a fog, turn indicators, gyro controls and the like, wire- less direction-finding and the methods devised for night landing at an aerodrome. in all these directions advances of no small amount have been made, and while there is much to jearn the progress has been great. commercial aviation —commercial aviation is almost a new idea since the war and embraces a wide field of enquiry. air transport began under serious difficulties, but from the day when, in rorg, the air transport and aviation co. dispatched its first acroplane to fly on a regular commercial service to the pres- ent time the progress on the whole has been one of marked ad- vance—advance accompanied by setbacks due to ignorance of the problems to be solved and want of experience as to the methods of meeting the difficulties (see flying). airships (q.v.) are described in a separate article drawing at- tention to two important assets distinguishing airships from aeroplanes. the one is the advantage derived from increase of size, the other the ability to remain aloft without use of power. a clear account of the principles which must guide airship de- velopment follows and then we have details of construction of a modern sized ship with particulars of its possible performance and carrying capacity. the advantages which follow from mast- mooring are fully pointed out. the article on seaplanes (q.2.) deals with flying boats and seaplanes. the problems calling for solution when seacraft are in the air do not differ essentially from those which have to be solved for land-craft. rising from and alighting upon water, however, involve fresh considerations. the boats and floats are so constructed that when, alter starting from rest on the water, a certain speed is attained—the tramp speed it is called—the craft rises in and planes or skims over the water. up to this point the resistance has increased with increasing speed; as the ‘“‘tramp speed” is passed there is a marked drop in resistance, for the same expenditure of power the craft begins to move more rapidly, attaining finally its flying speed when it rises in the air. the determination of the “tramp speed,’ and the proper adjustment of airscrew and engine to attain a full efficiency at this speed, constitute a difficult problem for the designer. again the impact caused by landing on the water sets up serious stresses in the hull calling for most careful design to combine the necessary strength and lightness. (ee dg) aeroplane.—the main problem that confronts the designer of any means of transport is to find a working compromise between the various demands of those who maintain and use the service. there is no conflict between the interests of the operator and those of the user. their common requirements are safety, speed, comfort and adherence to a published time-table. these all influence the cost of the service, but the measure of each which will enable a service to pay its way can be found only after more experience than is yet available in air transport. the way aeroplane in which the need for safety reacts on the design of aeroplanes! is referred to below, and for information on what has been achieved the article flyine should be consulted. adherence to a time-table involves, from the point of view of design, a general robustness of details and attention to the facility with which minor replacements can be made. comfort requires a certain minimum space and weight of equipment for each passenger, and an improvement in general means simply a fall in the paying load per aeroplane and hence a rise in fares. the main advantages which air transit has to offer as a com- pensation for a reduced measure of safety, comfort and relia- bility, are speed and freedom from interruption. the air is the only medium of transport which connects, without interruption, all places on the globe, and it may be anticipated that the con- sequent possibility of transit free from “ changes’ may prove to be nearly as valuable as the speed which can be maintained. stalling speed —yor reasons which are explained in the article aerodynamics a normal aeroplane, such as those illustrating the article fiy1inc, cannot be maintained in flight below a certain minimum stalling speed, which, for a modern commercial aero- plane, with fixed wings,? varies between 50 and 65 m. per hour. on the other hand, it is possible to give it an engine powerful enough to drive it at between go and 120 m.p.h. while leav- ing a margin of some 25% of its total weight for passengers or goods. ‘the impossibility of steady flight dcfow the stalling speed con- stitutes the chief danger of air travel. many attempts have been made to avoid this source of danger by devising an acroplane which would be capable, as a whole, of remaining stationary in the air. such machines have generally employed either lapping or rotating wings, the so-called ornithopters and helicopters. the former have shared the legendary fate of icarus. of the latter there is at the moment only one example which offers a prospect of success. this, the de la cierva autogiro, is, how- ever, fundamentally similar to an aeroplane with fixed wings, in that the rotation of its wings depends on a forward movement of the whole machine through the air, which in turn is attained by means of an ordinary ‘engine and propeller, or airscrew, with horizontal axis. hence the machine cannot “ hover,” but its minimum speed is much lower than that of any existing aero- plane. on the other hand, it seems to be inherently less economi- cal of power than an ordinary aeroplane. it appears therefore that its application will be rather to circumstances where the very low minimum speed which at the moment of touching the ground may be as low as 10 m.p.h. and consequent ability to land safely in emergency in confined spaces are of special value, than to normal commercial aviation. minimum speed of flight-—there is no fundamental obstacle 1o reducing the minimum speed of a normal aeroplane to some 25 m.p.h., instead of the 50 to 65 m.p.h. quoted above as typical of modern practice. in fact the earliest flying machine had mini- mum speeds of this order. but the general proportions of such a machine would be uneconomical from every other point of view. in particular almost the whole of the available weight would be absorbed by the, structure and power plant, leaving no margin for paying load. the relation between the total weight (w ib.), the area’ of the wing (s sq. ft.) and the acrodynamic coefficient (kg) which deter- mines their lift at any given speed (v ft./sec.) (see aerodynamics) is w =krpsv?, p being the density of the air. this may be put in the form* v= ra4/e miles per hour a > this article deals only with aircraft which are “ heavier than air. 2 ‘the movable flaps or ailerons at the tips of the wings of such an aeroplane are control surfaces and are referred to below. 3 the area of the wings of an aeroplane is the sum of the areas of each separate plane (if there are more than one). the area of each plane is the area of its projection on a plane (in the strict sense of the word) through its front and rear edges, ‘for standard ground level density. aeroplane where w=w/7s and is called the wing loading, in pounds per square foot. fig. 1 shows the possible combinations of w and ky, which will give any selected speed. fig. 2 shows the cross section by a vertical plane parallel to the direction of flight of some modern wing sections. for these the maximum value of ky, (which in virtue of the above relation corresponds to the minimum flying speed) is between 0-55 and 0-65, represented by the shaded band in fig. t. modern com- mercial aircraft using such wings have loadings between 9 and 14 ib. per square foot. areduction in the stalling speed can be made only by reducing the wing loading or by using a shape of wing section which speed 25mp.h. imps. 65 mph wt maximum k \\s for wings - of fig 2 modern commercial , , aeroplanes i i u lift coefficient i f 1s o 10 wing loading « (ibs.] sq ft) fic. 1.—diagram showing the possible combinations of wing loading and lift coefficient which will give any selected speed. will give a higher maximum value ef k;,. the former course leads to a reduction in the paving load. much attention has therefore been devoted to the latter method. the most effective of the high lift wing sections which have been devised as yet is the slotted wingof f. handley page (fig. 3). with this the maximum lift coefficient can be increased some 50° or more. the action of the device is, broadly speaking, to cause the flow characteristic of stalling (see aerodynamics) to occur at a larger angle of incidence! the tncrease in lift ts dependent on the relative position of the main and auxiliary wings, 7.e., on the size and shape of the slot between them, which can be varied in flight. with the slot closed the characteristics are nearly the same as those of a normal wing. in this respect form, a, (fig. 3) is superior to form, b, but in practice there are, with both, constructional difficulties in the way of ensuring a smooth and unbroken surface when the slot is closed. another method which has been widely used, is to vary the curvature, or camber, of the wing (fig. 4). in general, the greater the curvature of a wing section the higher is the maximum lit, but the lower the efficiency at high speeds. this device attempts to secure both high lift and high efficiency by enabling the pilot to alter the curvature to suit the conditions of flight. the in- crease of lift attainable in this way is much less than can be —— se a fic. 2.-cross sections of modern \"‘ wing sections.” achieved with a slot, the maximum being about 10%. an au- tomatic form of this device has also been used, the trailing flap being held down by elastic cord and therefore rising as the air pressure increases, j.e., as the speed increases. by combining beth these methods very high maximum lifts have been ob- tained, and it is probable that such a combination will be used on civil aircraft in the near future. the chief obstacles to the use of these variable wings in com- mercial aircraft are, first, the increase in weight involved, not 1 angle of the chord to the direction of flight. the chord of the wing sections in fig. 2 is indicated by a broken line. 35 only in the devices themselves, but also in the larger control surfaces which they require; second, the lack of a clear demand on the part of those who travel by air for a reduction in stalling speed. the increase in weight is not as great as the increase in lift (at the same speed), but the whole aeroplane is appreciably more costly, and in the absence of adequate credit for the bene- fits derived it is natural that designers should hesitate to adopt such devices. power required for flight when an aeroplane of conventional design and normal pro- portions is flying horizontally? the power of the engine is dis- posed of in the following ways:— (a) about 25° is lost owing to the inefficiency of the airscrew. (6) from 30% to 45% is used in overcoming the resistance of the air pig. 3.— wandley-page ‘ slotted wing.” to the passage of the hody, including the engine itsclf, the under- carriage and the exposed part of the wing structure, etc. (¢) from 45° to 30% is similarly absorbed by the resistance of the wings. the choice between the alternative figures in (6) and (¢) depends on ihe speed of flight, the former corresponding to low, and the latter to high speeds. the resistance of the parts inclucled in (a) varies practically as the square of the speed of flight, and the cor- responding power thercfore as the cube. the resistance of the wings (¢) may be further subdivided as follows :— (c)t. from a third to three-quarters is due to the equivalent of friction of the air on the surface of the wings. (see the profile drag.) this varies as the square of the speed, and the power as the cube. if the area of the wing is changed, the profile drag changes in pro- portion, each wing section has a typical coefficient of profile drag, and, apart from maximum lift, this is in fact nearly all that distin- guishes one section from another. (¢)2. from two-thirds to a quarter is due directly to the action of the wings as generators of a lifting force, the induced drag of prandt} (see aerodynamics). this varies inversely as the square of the speed, and the power therefore inversely as the speed. it does not lope on the shape of the wing section, or on the area of the fic. 4.—method of varying curvature of wing. wings, but (for a given lifting force, 7.e., weight) only on their overall width or span, being inversely proportional to the square of the span. this induced resistance is the essential feature in which trans- port by aircraft heavier-than-air differs from all other methods, for in no other form is it necessary to expend energy in order to sustain the vehicle. about 8% of the fuel used by a normal aero- plane is used in this way (fig. 5). this division of the resistance of the wings into two parts, one of which increases with the speed of flight, while the other de- creases, the former dependent (broadly speaking) only on the section and shape of the wings, the latter independent of these factors and governed by the extent to which the span of the wings allows the aeroplane to “ grip’ the air (see airscrew), was originally suggested by f. w. lanchester. it lay dormant for some years, largely because there was not forthcoming any adequate theory by means of which it would be given quantita- tive expression in terms of the dimension of the wings. this expression has been rendered possible by the work of l. prandtl of gettingen and his collaborators, with great advantage to the technique of aeroplane design. its chief merit is that it enables a designer to examine the economics of wing proportions without continual resort to model experiments. it has been abundantly verified by comparison with experiments, both in great britain 2 in air of a given density. 26 and elsewhere, and its applications have been extended by work- ers in great britain, in particular h. glauert. the experiments made have not enabled its fundamental ideas to be verified directly, but have shown that it adequately accounts for very varied observations. many engineering theories stand on a similar footing. in normal flying conditions, as opposed to the special con- ditions characteristic of arising from and alighting on the ground, the speed of an acroplane may vary from 1-2 to 2-0 times its stalling speed. the upper nine of these two figures can be raised by the provision of more power, that given corresponding to a machine having about 60 b.h.p. for each 1,000 lb. of total weight, which is a normal figure for a commercial aeroplane. at its top peed the engine is working at its full power, which js not an economical condition from the point of view, either of the life of the engine or of fuel consumed per ton-mile. the most 60 moh. 100 m.p.h. airscrew loss 25 (induced 7% } (undercarriage 9) (struts & wires 9) 100 brake horse power fic. 5.—diagram showing how the power required for flight depends on the speed. economical speed, taking into consideration all the various fac- tors, is about 1-5 times the minimum speed. the diagrams in fig. s show the way in which the power required for flight depends on the speed, and incorporate the figures given above relating to the distribution of the power among the various parts. the maximum b.h.y. of the engine has been taken to be roo and the minimum speed 50 m. per hour. these diagrams illustrate the rapid rise of power required with the speed of flight. for such an aeroplane the most economical speed would be about 75 m.p.h., requiring about 50 brake horse- power. ii the engine throttle is fully opened at the lower of the two speeds considered above, the aeroplane will climb. the angie of climb for modern commercial acroplanes is from 1 in toto 1ini3s. for high-powered war machines it may rise to 1 in 6, or more. when the engine is cut off, the aeroplane will lose height, its path being inclined downwards at a corresponding angle, and the necessary power will be supplied by gravity. the angle of descent for a modern aeroplane is from 1 in 6 to1in 8 thusa passenger in an aeroplane will not in general be subjected to greater changes of inclination of the direction of motion than one who travels by road. improvement of performance the above proportions apply without material change to most modern commercial acroplanes. the main directions in which improvements in the efficiency of the machines are being sought are as follows: airscrew efficiency.—a loss of between a quarter and a third of the power of the engine in the transmission gear would hardly be tolerated in road or rail transport. transport by water, however, is burdened by a loss identical in character and of similar amount. it arises from the fact that the propulsive force is obtained by driving a fluid backwards. (sce airscrew.) aeroplane resistance of the body.—the majority of aeroplanes have in the nose of the body either an air-cooled engine, or a water- cooled engine with a radiator. with this arrangement the resist- ance of the body cannot be separated from that which is incurred in cooling the engine. this latter is cquivaient toa reduction in the power of the engine of from 15° to 25° at the cruising speed of a normal commercial seronlane: at higher speeds the loss is greater. it is natural to endeavour to recover some of this by placing as much as possible of the load carried (fuel, crew, passengers) behind the radiating surfaces. but a large part of the loss remains, because such parts as these could, at least in principle, be placed in a stream-line body of very low resistance. such bodies would, however, be heavy, and the conventional tractor aeroplane is by no means a bad compromise, especially when an air-cooled engine is used. when a very high speed is required, as for example in racing acroplanes, the cooling resistance can be completely eliminated by using a water-cooled engine in combination with a radiator whose cooling surface is part of the smooth surface of the wings. such a radiator is heavy and is exposed to damage, reasons which prevented the development of the scheme during the war. it has, however, been used in the united states, by the curtiss co., and the success of their acroplanes in the recent speed con- tests is largely duc to the use of the equivalent of about one-third of the wing surface of a normal acroplane as a radiator. but the consensus of the opinion of designers in great britain appears to be that it is as yet unsuitable for employment in com- mercial aeroplanes. resistance of the undercarriage—the undercarriage of an aeroplane has a high resistance, mainly due to the wheels. if the wheels could be withdrawn into the body in flight an appre- ciable gain would result. mechanical difficulties are involved owing to their large bulk, but the problem is worthy of attention. resistance of the bracing of the wings.—there has been a tend- ency, more noticeable in american, dutch, french and ger- man than in british designs, to eliminate external wing bracing entirely. there is little doubt but that this results in a heavier structure and a relatively less economical aeroplane than one in which a less extreme construction 1s adopted—an example of a general principle of all engineering design that extremes are seliom economical. the matter is dealt with below. but it may be remarked that there has been apparent for some time a tend- ency to reduce the amount-of exposed bracing, of all kinds, without entirely eliminating it. distribution of weight vig. 6 illustrates the relative proportions of the total weight taken up by the various sections into which it is convenient to divide the component parts. the figures apply to an average commercial aeroplane of moderate size. the distribution of weight varies little from one machine to another, even over a considerable range of size, provided the minimum speed, the cruising speed and the duration of flight are kept fixed. structure —the “ structure ”’ includes a great deal which is not part of the essential structure of the aeroplane. a change in the specified strength affects about 60 to 70% only of the nominal structure weight. iwetghts—the weight of the power plant depends mainly on the desired maximum speed of the aeroplanc, and on the weight of the engine and its equipment per brake horsepower. ‘there is generally a notable reduction in this item if an air-cooled engine is used instead of a water-cooled engine. the weight of fuel and tanks is dependent on the speed and duration of thght specifed, and on the standard of fuel economy reached by the engine. the latter factor varies little for modern engines, both air- and water- cooled engines now giving one b.h.p. hour for about }!% lb. of petrol. the crew and equipment tend naturally to absorb a progressively smaller proportion of the total weight as the size of the acroplane increases; this is an important advantage possessed by the large aeroplane. paying load.—the paying load, being the balance after the aeroplane above items have been accounted for, is from 20 to 30°% of the gross loaded weight of the acroplane. compared with other means of transport this is a high proportion. for example, a modern motor-car of medium power, weighing some 2,500 lb. empty, will carry four passengers and a driver, equivalent to a paying load of some 650 lb. or 20% of the whole. both a railway train and a ship carry a much smaller percentage of paying load. that all - structure 35/7 eb ey ay oak nt | undercarriage 5%, iiscellaneods 3 /o engine, airscrew, power plant i9 ye hadiator & vater, lontro/s. crew & equipment 67 teens sle passengers paying load 267% luggage goods fic, 6.—distribution of weight in average commercial aeroplane. these means of transport are able to offer lower fares than air- craft is due partly to the high running costs of an aeroplane per passenger mile, but mainly to the fact that the majority of travellers are not yet convinced that air transport is safe, and therefore do not use it. the essential need for economy of weight in aircraft is due, not to the fact that only one-quarter of its weight is available for paying load, but to the high cost of carrying this load by air, as compared with other ways. a saving in the weight of some part of the aeroplane of 1°% of the whole allows a reduction of the cost of transport by 4%. external form of aeroplanes there is a striking lack of variation in the appearance as scen in plan of all modern aeroplanes. the main wings have a large span (fig.7) in relation to their chord, the average ratio of span to chord being about seven. this isa direct consequence of the fact that the resistance due to the generation of lift varies inversely as the square of the span. in some aeroplanes the chord diminishes towards the tip of each wing (fig. 7b) and occa- sionally the wings, although of uniform chord, are “ swept back ” (fig. 7c). neither of these modifications affects the aero- dynamic efficiency appreciably. ailerons.—near the tip of each wing is an aileron or wing flap hinged about axes 1:1 (fig. 7a) and interconnected so that an upward movement of one is accompanied by a downward movement of the other. they are opcrated by a sideways move- ment of the pilot’s control column, and are the means by which the wings are kept level in straight ilight, or banked for a turn, as may be required. tail plane —at a distance of some three chords behind the leading edge of the wings is the tail plane, in area about one- ninth that of the wings. in steady flight its function js to bal- ance or trim the aeroplane, 7.e., to ensure that the resultant of all the air forces on the aeroplane passes through the centre of gravity. (see aerodynamics.) it is generally in two parts. the front part, some 60% of the whole, is normally fixed, though means are provided by which the pilot can alter its angle of inci- dence by rotating it about an axis such as 2-2 (fig. 7a), thus enabling the aeroplane to be trimmed for flying at various specds. this operation is analogous to trimming the sails of a ship so that it carries no weather or lec helm. in a modern acro- plane a movable tail plane is generally considered indispensable. the flevater.—the rear part of the tail, or elevator, is hinged on an axis such as 3-3 (fig. 7a), and connected to the control column in such a way that a forward movement of the pilot’s 37 hand depresses the trailing edge of the elevator and causes the machine to dive, and vice versa. the elevator is the primary control organ of an acroplane, since by it the pilot alters the angle of incidence of the wings and hence controls the speed. fig. 8 shows these and other control organs in more detail, and the forms which they take in more modern machines will be seen in the plate with the article on flying. the appearance of aeroplanes in front and side elevation is more varied than that in plan, owing mainly to the use of one or more planes. as many as four planes have been used on com- paratively modern aeroplanes, but the great majority of existing machines have either one or two planes. monoplanes and biplanes—an analysis of the aeroplanes described in jane’s all the world's aircraft for 1925 shows that, fic. 7.—comparison of plans of modern aeroplanes. out of a total of nearly 400 types, 32° are monoplanes and 68% biplanes, with two triplanes and no quadruplanes. a similar analysis for 1922 gives, for rather more than half the number of aeroplanes, 22% monoplanes, 74% biplanes, with eight tri- planes and one quadruplane. these figures show that the biplane is still the preponderating type, but that the proportion of mono- planes is increasing. the increase during the three years covered is divided between biplanes and monoplanes in the ratio of 9 to 7 approximately. this trend is due partly to the development of wing sections of great depth in relation to their chord, and partly to a desire to eliminate external wing bracing. it is much more conspicuous in foreign than in british design, as will be scen from fig. 9, in which separate analyses are shown diagram- matically for aeroplanes produced in great britain, france, germany, the united states and other countries. it should be noted that the figures given above refer to types and include 38 both commercial and war aeroplanes and seaplanes. some of these naturally exist only in small numbers, and some may be confined even to one experimental machine, but there is evi- dence that all have actually been made and flown. there is no room for doubt that had the above analyses dealt with numbers of aeroplanes, instead of types, the preponderance of the biplane . would have been much greater. the monoplane-—confining attention to monoplanes and biplanes, the chief variations in front elevation are shown in fig. ro. the parasol monoplane (a) is used mainly where it 1s desired to have some of the advantages of external bracing. the monoplane with body above the wing (b) has rather less resistance and is less liable to damage by wind when on the ground, an important advantage. the former type is the more numerous, and the ratio about 3 to 1, but the latter is favoured by the german firm of junkers which has supplied a large pro- portion of the machines used on german air lines. the biplane —the biplane with wings of equal span (c) is the most common form of aeroplane, but wings of unequal span bracing wire fic, 8.—tractor biplane of typical composite (wood and steel) construction. aeroplane ~° |the speed of flight, it is desirable that the relative position of wings and body shall be such that in horizontal flight at cruising speed the body may be nearly horizontal. but the attitude of the body in relation to the wings has no effect whatever on their angle of incidence relative to the direction of flight. failure to realise this has been responsible for many fruitless inventions of wings whose angular position on the body can be varied. the vertical stabilising and controlling surfaces, a fixed fin anda movable rudder, connected to a rudder bar on which the pilot’s feet rest, are situated near the tail plane. they perform prac- tically the same function as the rudder of a boat and can be seen in more detail in fig. 8. position of airscrews and engines—in the above sketches the airscrews indicate the position of the engines. in modern aeroplanes the pusher position of the airscrew is seldom used except in single-engined flying boats, the vast majority of modern aeroplanes using tractor airscrews. when two or more engines are used, it is customary to distribute them along the wings, as the transmission of their power by means of shafts and frevator ly x 2 = ~s! skis bs. : akg : (part of the covering of the wings, body and control surfaces is removed to show the internal construction.) (cl) (sometimes called sesquiplane, 7.c., 1% plane) have many advantages. the proportion between types (c) and (d) is about 3 tor. fig. 10 (e) shows a large biplane with two or more engines. in all these the right and left hand wings are not in line, but form a very flat upward vee. this dihedral angle (which is gen- erally given as the angle between either wing and the horizontal, and varies from o° to 5°) is introduced in order to improve the lateral stability of the machine. the corresponding side elevations of these aeroplanes are givenin fig. rr. ina biplane the rclative position of the planes as seen in this view varices as shown in (c), (d) and (e). in (c) and (d) the planes are staggered, z.c., the line joining the leading edge of the upper and lower planes is not perpendicular to their chords. the angle of stagger is generally between 0° and 30° and a variation over this range has little if any effect on the efficiency of the wings. in (d) the chord of the lower wing is smaller than that of the upper wing, a common variant, especially when the spans are also unequal (fig. 10). in (c) the wings have equal chords and no stagger. the tail plane is here shown as a biplane, in which form its weight can be made jow ior a con- siderable area. set of the wings.—the angle at which the chord of the wings is set in relation to the centre line of the body is gencrally about 5 degrees. though often termed the angle of incidence, it has no connection with the accepted sense of that word. its magnitucle is the result of a compromise between a number of conflicting requirements, a not unimportant one being the comfort of the passengers. the true angle of incidence being determined by gearing from a central engine-room to airscrews in suitable posi- tions involves a prohibitive weight, which is generally not less than 13 ib. per h.p. transmitted, or about half the weight of the engine itself. as seen in front elevation the arrangements shown in fig. 12 have been used. considerations of rudder control in the event of the failure of one engine make it desirable to place the engines as near to one another as the airscrews permit. overlapping airscrews have been used, but have generally given rise to troublesome vibra- tions. it is also desirable that the axes of the airscrews should be, as far as possible, on the same horizontal level as the centre of gravity, in order to avoid giving rise to couples tending to make the aeroplane rotate in a vertical plane when the throttles are altered. stability and control the general principles of stability and control are dealt with in the article aerodynamics and the stabilising and controlling surfaces have been referred to above. the necessary disposition and dimensions of these surfaces are decided in practice chieily by direct comparison with previous designs. since, however, acro- planes are not merely scale copies of one type, some rational means of using the accumulated experience of previous work is essential, and it is this that research into the fundamentals of stability and control is able to supply. needless to say, even these methods break down when the aeroplane in question is not broadly similar to those from which the data are derived, and then nothing short of a thorough investigation, assisted by model aeroplane experiments, will suffice. very often, even with the best that can be done in this way, uncertaintics remain, and the designer is forced, as in other branches of engineering design, to rely upon his ability to draw conclusions from insuflicient premises. stability. stability, the property of returning to a steady state of motion when deliberately or accidentally disturbed, im- plies as a necessary preliminary the existence of equilibrium of trim in that state. an acroplane is trimmed for a stated steady condition of flight by an adjustment of its tail plane, the eleva- tors being left free. the degree of stability generally considered desirable for a commercial acroplane can be attained by arrang- ing the size of tail plane and the fore and aft position of the centre of gravity within certain limits. the further back the centre of 1922 1925 6 @ 60.27.26 227 12 3378 13 97 32h n n monoplanes titte. aon n nf : n . biplanes ; 1 f . nnseussn ef nn 94 91 4073 74 78h 88 67 22 87 53 = x 6s > gg 8 sw sy c« se ec ese ee. eel s ns > ~ s 222,35 = sexx § = ssig8 & seiss8 ¥ 5 ae os ss \"93 8 & a) x= ; © = kn froportion of re 2 worlds aeroplanes q ] | | produced in 1 l] | each country 28 24 14 18 16h 32 27 15 12 14fz fic. 9.—diagram illustrating world production of acroplanes. (n.b. the analysis refers to types of aeroplanes, not numbers of machines.) gravity, the larger is the necessary tail area. the fore and aft position of the centre of gravity in relation to the wing is defined by its distance from a plane passing through the jeading edge of the wing perpendicular to the chord for a monoplane; for a biplane an equivalent monoplane wing is substituted. in normal aeroplanes this distance is usually about one-third the chord, though it has varied between one-quarter and one-half. the height of the centre of gravity of an aeroplane in relation to the wings has a secondary effect on its stability. in particular a low position of the centre of gravity is not necessary for stability. with the first position stability will be secured providing the product of the ratios area of tail plane! ——_—_______——. and area of wings distance of tail plane from centre of gravity chord of wings is greater than about 0-35. the latter ratio is usually about 3, and the former one-eighth. the fore and aft control required can generally be obtained area of elevator if the ratio is about two-fifths. area of fixed tail plane lateral stability depends mainly on the provision of an ade- quate fin and dihedral angle. while desirable for the comfort of the pilot, lateral stability is of minor importance compared with lateral control. aileron control.—ailerons of an area between ye and 1g of that of the wings are usually provided, and give ade- quate control at all normal flying speeds. at low speeds, how- ever (more accurately, at angles of incidence of the wings in the neighbourhood of stalling), not only are normal ailerons in- effective, but they may actually be a source of danger, their use aggravating an inherent tendency of the stalled wings to force the aeroplane into a spiral dive or spin. this has undoubtedly been a prolific cause of accidents, of a type which generally involves the deaths of all on board. as the result of research carried out in great britain under the auspices of the aeronautical research 1 including that of the elevator. 39 committee means of curing this vice are now available. the most successful device so far developed involves the principle of the handley page slotted wing (see fig. 3). | rudder control.man effective rudder is as essential for safety at low speeds as eflective ailerons. the efficiency of any given rudder depends on its being arranged so that the body does not shield it, but it is generally found that when the product of the ratios area of rudder and fin distance of rudder from centre of gravity area of wings semi-span of wings is above 1/20, a satisfactory control is obtained. the latter ratio is usually about 1, so that the former should not be less than one-twentieth. the area of the rudder is generally about 23 that of the rudder and fin combined. and origin of aerodynamic data of design the designer bases his calculations on data derived from four main sources: (1) theoretical investigations, such as the lanchester- prandtl theory of the action of wings. (2) experiments on e fic. 10.—variation in front clevation of monoplanes and biplanes. models in wind tunnels. (3) experiments on full scale aero- planes in flight. (4) analysis of the measured performance of acroplanes. references to the first of these divisions will be found in the article on aerodynamics, where also the principles underlying experiments on models are dealt with. the wind tunnel.—the wind tunnel has played a great part in the development of aeronautics. many manufacturing firms possess their own tunnels, used mainly for measurements of the resistance of acroplane bodies, etc., with a view to improving performance. national wind tunnels exist in many countries, gencrally reserved in ptinciple for research on problems of wide fic. 11.—side elevations of acroplanes shown in fig. ro. interest and application. in great britain such tunnels exist at the national physical laboratory and the royal aircraft establishment, while, as part of the equipment of university institutions where the scientific problems of aeronautics are inves- tigated, wind tunnels were constructed as at east london college in 1910. the main advantage of model experiments is the ease with which the experimenter can control the conditions. ‘they enable suggestions for improvements to be developed to the stage at which a trial on the full scale is possible. : 40 most of the leading aircraft-producing countries carry out full scale experiments. in great britain they are mace at the royal aircraft establishment under the control of the director of scientific research at the air ministry. owing to the expense involved, private firms can do little work of this kind, but there is active co-operation between them and the air ministry, with great benefit to aeronautics. the results of such work will be found in the annual technical reports of the aeronautical research committee. the measurements of speed and climb which are made during the acceptance tests of all aeroplanes are a valuable source of ©) clo __ alla —_o0g—— fic. 12.—arrangements of two or more engines. information for the designer. their analysis enables an overall comparison between various types to be made on a rational basis, but necessarily lacks something in precision. tor each type differs from the others in many features, so that, for example, an observed difference in speed cannot safely be ascribed to any one peculiarity. but with suitable precautions reliable informa- tion can be obtained in a convenient form. it is found that the factors which have the greatest influence on the speed and climb of an aeroplane are the horsepower per unit of total weight and the total weight carried per unit of wing surface. for further details of the method, and its limitations, references given should be consulted. safety of air travel the risks to which those who travel by air are exposed are probably the chief influence which hinders the development of air transport. these risks are associated with the following causes: (1) physical failure of the pilot, (2) fire when in flight, (3) collision when in flight, (4) breakage of the structure, (5) forced landing. dangers of the first fout types are not peculiar to aircraft and the precautions they involve are common to trans- port by road, rail and sea. the measures adopted in air trans- port have practically climinated them. it is mainly the dangers which may arise when an aeroplane is forced to land that loom large in the minds of those who might otherwise travel by air. while it cannot be said that these fears are unwarranted, yet in so far as the design of aircraft is involved the position was rapidly improving in 1926. forced landings —the need for a forced landing arises pri- marily from failure of the power plant, generally due to a defect devcloping in some minor part and not to a breakage in the engine itself. deprived of its power an aeroplane must eventually descend. in practice, from the height at which most civil fly- ing takes place (about 3,000 ft.), about three minutes are avail- able in which the pilot may choose a landing place. ii the failure occurs before the aeroplane has reached its working height, the time, and the corresponding area of ground in which this landing placeisto befoundarerecucedin proportion. theseconsiderations unfortunately result in the pilot having to execute somewhat sharp turns in order to place the aeroplane correctly, and it is during these that the control ability of the machine is most severely tested. the ultimate danger lies in the fact that the speed of horizontal travel at the moment of landing cannot possibly be less than the stalling speed. in practice with a typical commer- cial aeroplane it will be between 65 and 75 m.p.h. the pilot, therefore, endeavours to make the most of what space is avail- able. this consideration, combined with the psychological effect aeroplane of the apparent increase in the aeroplane’s speed as the ground is approached,! lead him to carry out’ the manoeuvres at a low speed, where the inherent characteristics of the aeroplane are least favourable and the controls least powerful (see above). the dangers associated with a forced landing may be reduced in three ways: (1) the probability of complcte failure of the power plant may be reduced by subdivision. machines with three engines (see above) are now coming into favour, capable of maintaining their height with any two engines and of greatly prolonging the time of descent with one engine only working. (2) the controls may be so arranged that they give the pilot complete command over the attitude of the aero- plane right down to the stalling speed (see above). (3) the stall- ing speed may be reduced (see above), so that the space required for landing, and the consequent need for sharp turns, etc., are also reduced. reference has been made to what is now possible in the arrangement of controls. but even with controls as normally arranged, there is little room for doubt but that a limitation of the stalling speed of civil aircraft to some 45 m.p.h. would greatly reduce the principal danger of flying. a reduction in stalling speed reduces the space required for landing and there- fore increases the proportion of the accessible ground which is suitable. it reduces the danger in the event of over-running the selected landing ground, and it reduces the psychological effect on the pilot of the approach of the ground, in the opinion of some by no means the least valuable advantage. such a limitation would probably involve for a time an increase in fares, since the aeroplanes would be more costly. this might temporarily retard the development of flying, but in the end the gain in safety could react only to its advantage. structural design: the general problem the aeroplane shown in fig. 8 was designed about 1914, but in its general aerodynamic and structural design it is not funda- mentally different from the majority of modern aeroplanes. regarded as a problem in structural design, a normal aeroplane wing, such as that shown in fig. 8, is a double cantilever, 7.c., two similar cantilevers joined at their roots, carrying a load distributed approximately uniformly over its span. the inten- sity of the loading begins to diminish appreciably at about one chord from the tip, but is still 2 of its value at the centre, at a distance of 1¢ chord from the tip. the wing itself is essen- tially a lamina, the wing sections most commonly used (sce fig. 2) having a greatest depth of from !/\\4 to ? chord. the resultant loading is nearly perpendicular to the chord, but its line of action (as seen in sicle elevation) intersects the chord at a point (the so-called ‘‘centre of pressure ’’) whose distance from the leading edge of the wing varies from 4 to § chord in normal flight, but may travel much further to the rear, even beyond the trailing edge—in special conditions. as a whole, therefore, the wing structure must be capable of taking torsion. from general principles it would appear that the lightest way of bracing such a lamina would be to stiffen it internally by longitudinal and transverse beams, spars and ribs, and to sup- port these by external bracing. the arrangement of two planes, one above the other, connected by struts and wires to form a braced tubular girder, suggested itself to the earliest experi- menters in flight. such a solution is seen in fig. 8 and is typical of the majority of modern aeroplanes. alternatively, by tapering the wing in plan (fig. 7b) and thus both reducing the intensity of loading outwards from the centre, though not, it appears, in proportion to the reduction of the chord, and increasing the available depth at the root, the whole of the necessary structure could be contained inside the covering. this is the principle of the internally braced wing characteristic of many foreign designers. it has the advantage of avoiding the resistance of the exposed bracing which is characteristic of the first solution. comparison of monoplane and biplane-—many variants of these two main schemes have been used, but in general there is a clear division between them. the first undoubtedly produces a 1 toa passenger in an acroplane 1,000 or 2,000 ft. up, there is gen- erally no sensation of speed relative to the ground. aeroplane lighter structure and a more compact acroplane. for, with the same total weight and wing area, the biplane has a smaller span and chord and requires a shorter body and smaller tail. with the aid of the prandtl theory it is possible to arrive at comparative overall dimensions for a normal biplane and a monoplane of otherwise similar characteristics. such a com- parison is shown in fig. 13. the span of the monoplane, (full lines) is about 10% larger than that of the biplane (broken lines), and its chord 65% larger. the length of the body and the tail area required are each increased about 28%. each type has other advantages and disadvantages, but the mere existence of many examples of each indicates that technical opinion is divided on f i gaa al aa tae es oe sees, eaeh es —— { i ! wd licesienioestindcetnnditantinntia deeaitn tentententientioedinttedicatien fic, 13.—comparison of plans of normal monoplane and biplane. their merits, and to a great extent experience in the design and manufacture of one or the other is a dominating factor in the choice between them. refinement in details —the details of the structural design depend largely on the materials used (see below). but the main problems are not essentially different from those of the design of ordinary engineering structures, except in one particular. the importance of saving weight in every item of an acroplane makes it economically possible to carry refinement in design to a point not usually attempted in any other structure. for the same reason elaborate calculations are made during the design, and the effect of every minor strengthening is carefully consid- ered. all aeroplane structures are highly redundant and initial stresses are imposed on the members by tightening the bracing wires, in order to increase the stiffness of the structure. the design of modern aeroplane structures takes account of these features as far as it is possible to do so, in contrast to the general tendency of modern bridge design, for example, which tends to avoid them as sources of uncertainty (sce bripcrs). where saving of weight is a prime consideration such a course is not possible. strength of acroplane siructures.—since an aeroplane must be able to manoeuvre rapidly, many systems of air pressure, differ- ing both in distribution and in intensity, must be taken into account. how rapidly it should manoeuvre depends on its class, e.g., a small war machine must be able to execute safely any manoeuvre of which an aeroplane is capable, whereas less exact- ing conditions suffice for a large commercial machine. spins, rolls, loops have all to be considered and experiments have been made in order to ascertain the distribution of the air pressure in such manoeuvres. the fundamental manoeuvre is, however, that of rapid recovery from a steep dive at a high speed, and here the combined effect of centrifugal force and gravity pro- duces an equivalent gravity (see aerodynamics) several times the normal. it can be shown that, given the necessary control power and indifference to his own safety, the pilot of an aero- plane could impose in this way forces up to 1o or 12 times those which the structure has to bear in the normal flight. as much as seven times normal gravity has been recorded in experi- ments under war conditions. ai this number, the ratio between the effective and the normal value of gravity, is termed the load factor for the whole machine under the stated conditions. a normal commercial acroplane need never experience a load factor of as much as 2 in flight. in practice it is designed to bear a factor of from 4 to 6, dependent on its size and class. (see r and m 673, the report of the load factors sub-committee of the aeronautical research committee.) this is the basis of the specified strength of mod- ern commercial aeroplanes. the ratio between the maximum load factor considered in the design, for any stated form of dis- tribution of air pressure, and the factor in any actual condition of flight appropriate to that distribution, is the nearest approach to the factor of safety commonly used in engineering. for a commercial aeroplane the factor of safety, as thus defined, seldom falls below 3. in normal level flight it is of the order of 5. for a war aeroplane it may fall to 13, so that the most highly stressed member is loaded to two-thirds of its breaking load, or even less, but only in extremely rapid manoeuvres. a commercial aeroplane has a true factor of safety, a margin of strength not called upon under the worst conditions which it normally experiences. the load factors corresponding to irregu- larities in the air are very small. so long as it is in the air, an aeroplane is exposed to much smaller risk of the failure of its structure by stress of weather than is a ship at sea. any com- mercial aeroplane which conforms to the official criterion of strength, and has thereby secured an air-worthiness certificate without which it may not carry passengers, is capable of per- forming many manoeuvres such as looping and spinning with complete safety. materials used in construction composite structures—the materials which have been most commonly used hitherto for the main structural members of aeroplanes are wood and steel, with a fabric covering for the wings, the wood used is chiefly spruce of the highest grade, and in many aeroplanes the whole of the main structure, with the exception of ties and joints, is of this material. ties and the joints or fittings by which they are attached to the other mem- bers are generally of steel. steel tubes are used in some parts, either, as in the undercarriage, on account of their robustness, or, as in the control mechanism, because of their efficiency in transmitting torsion. the general arrangement of the members of a conventional acroplane structure of this composite character will be seen in fig. 8. the cross section of the wing spars has the i section commonly used in general structural enginecring, though the thickness of the web and ilanges is much larger in proportion to the overall dimensions of the section than is usual in steel i girders. the wings of the aeroplane shown in fig. 8 are in biplane form with external bracing. composite construction has also been applied to wings with no external bracing, generally, but not invariably, monoplanes. the dutch firm of fokker is the chief european exponent of such methods. their wing struc- tures are built mainly of wood, the spars being box girders. the covering of the wings is of three-ply wood, which also enters largely into the ribs and spars. in aeroplane construction the primary requirement is light- ness, and a modern composite aeroplane structure is on the whole nearly as light as one of any other type of construction so far developed, for the same strength. at present it is also very much cheaper. nevertheless, in so far as it embodies wood it suffers from the following disadvantages:— (1) wood of asuitable grade cannot be freely obtained. (2) when exposed to the atmosphere, particularly to large changes in tem- perature and humidity, wood deteriorates more rapidly than steel or other metals. (3) wood is not a reliable material. its external appearance is often misleading as an inclication of its internal con- dition. (4) in aeroplane structures glue and woodscrews cannot be entirely avoided. both are sources of weakness and uncertainty. (5) when a wooden structure is involved in an accident, many of its members break completely, and splinter and the structure dis- integrates, whereas metal members often merely bend, and an all- metal structure generally preserves much of its original shape. the a2 passengers in a wooden aeroplane are thus exposed to greater danger in case of an accident. (6) wood has no inherent resistance to fire and cannot be rendered fire resisting. these disadvantages of wood have been the chief incentive to the development of all-metal construction for aeroplanes. all-metal structures—the development of all-metal struc- tures by british designers has been mainly confined to replacing each of the wooden members of the composite structure described above by a metal member, using either steel or duralumin. in such a structure the load to be borne by each member is of a fairly simple character and can be estimated fairly closely. hence a comparison of the merits of a composite and an all- metal structure on these lines can be reduced to a comparison of typical members. on the other hand european and american designers have used all-metal construction chiefly for monoplanes with no external bracing. the most prominent example is the german firm of junkers.! a wing of this kind cannot readily be considered in detail. in fact it is definitely regarded by its designer as a struc- tural whole. its structure is highly redundant in character and the corrugated metal covering combines the function of the ribs and fabric found in most other aeroplanes, andin addition contrib- utes largely to the torsional stiffness of the whole wing, a func- tion which in a biplane is performed mainly by the wire bracing. the relative advantages and disadvantages of these two dis- tinct types of aeroplanes are concerned more with their general characteristics than with the materials of their structures. relative cost of wood and metal —the world’s present demand for aeroplanes is uneconomical in character. the total volume of the demand is small and it is spread over a large number of types. this reacts to the disadvantage of all-metal aeroplanes in particular. wood is an economical material when small quantities of many types of articles have to be manufactured. the same raw material is required, and it can readily be shaped to efficient sections by the same simple tools and equipment. with metal, the raw material must be prepared by special plant into the necessary forms (tubes, strip, wire, etc.) and these proc- esses are economical only when large quantities of one jorm are required. but given the demand for a standard article, metal is cheaper than wood. many of the metal details of acroplanes have been standard- ised since early in the war, and nearly all the raw materials are now the subject of specifications issued by the british engi- necring standards association. but there is nothing as yet to correspond with the standard sizes of steel beams (i, angle, 1see h. junkers, journal of royal aeronautical society (sept. 1923). aeroplane channel, etc.) which are essential to economy in general struc- tural engineering, though design is now approaching the stage when such a step will be possible. against the high first cost, however, must be offset the longer life and greater reliability of metal, and it cannot be doubted that wood as a material for the construction of aircraft is rapidly becoming obsolete. physical properties of materials —the disadvantages of wood as a material for the structure of an aeroplane, outlined above, give an idea of what should be the characteristics of a satisfac- tory material. there is the usual difficulty, common to most problems of engineering design, of assessing the relative impor- tance of each of these characteristics. in fact, their relative posi- tions in the scale change with time and circumstances. but, owing to the low weight of the composite structures characteristic of modern aeroplanes, the technical problem is simplified into that of finding means of making, at the worst equally light, and preferably lighter structures from materials that do not share with wood the disadvantages mentioned. in table i. are given the strength and specific gravity of various materials which have been, or are likely to be, used for the main structural mem- bers of aircraft. they fall into three classes: wood, steel and light non-ferrous metals. the comparatively short life of wood has been quoted as one of its main disadvantages. both steel and aluminium alloys need protection against corrosion (prob- ably the most promising metal from this point of view being stainless steel). although it would be misleading to suggest that all the problems of protection of metal structures have been solved, the experience gained suggests that effective protection can be given against all the ordinary causes of corrosion. it is customary to regard the single figure obtained by divid- ing the strength by the specific gravity as a figure of merit on which the material can be judged. such figures are not given in table i. as they are not, in fact, a reliable criterion for all types of structural members. struts and beams of thin metal—the types of membcrs responsible for the larger part of the weight of the structure of an acroplane are two, struts and beams, the latter very often having to serve also as struts. such members have essentially a variation of stress over their cross section, so that when the most heavily stressed point reaches the limiting stress of the material (this may be taken to mark the failure of the member as a whole), the majority of the material is under a lower stress. hence the density of the material divided by its limiting stress is not in general proportional to the weight of the member. in general, for a given area of cross section and a given limit- ing stress, the more the material can be spread out, e¢.g., in the fic. 14.—examples of metal construction of spars and ribs for aeroplane wings. method of attaching rib by means of spring stecl clips rivetted to struts, pressing into corrugations in the spar. in stecl, showing wide rib and attachment of wing to aeroplane. (c (a) section of corrugated steel spar, showing (b) inner end of wing ) corrugated steel spar, using transverse tubes to stiffen webs, and duralumin strips to stiffen flanges. (d) internal construction of wing in duralumin. spar built up from sheet and angles. ribs made from tube and channels. (ce) simplified wing spar in duralumin. (n.b. the greatest depth of each of the spars shown is about 5 inches.) aerotherapeutics parle. 3 ae ee effective : ultimate strength material ae iftc tensile in compres- tavuy strength sa (tons/sq. in.) (tons/sq. in.) wood spruce o-45 about 3 2 steel mild (soft). : 7:8 26 15 plain carbon stee bar for ties (cold- rolled) as 7:8 70 plain carbon stee strip (cold-rolled and blued) . 7°8 60 50 34% nickel steel stri (hardened and tempered) 78 90 go light metals aluminium (cold- rolled) 2-6 10 8 duralumin 2-85 of 17 | form of a circle, the greater will be the strength of the strut or beam, but the thinner will the material become. it appears, however, that a limit is reached for steel when the thickness is about 1/50 the radius beyond which an increase in the radius causes a decrease in strength. the member will then fail owing to local buckling of the material, at a load which corresponds to a maximum stress in the material, as calculated in the con- ventional way, which is below the limiting stress. the design of metal structures for aircraft hinges round the discovery of shapes of cross section which will enable the limiting stress of the material to be reached, and will be convenient for manufac- ture. the members must also be robust enough to ensure that ordinary handling does not damage them, a serious problem with thin sheets or tubes. limiting stress —the limiting stress in question is probably what is termed the yicld point in compression, the stress at which plastic strain begins, for materials such as mild steel. for high tensile steel and light alloys, in practice the aluminium alloy known as duralumin its the only one used, there is no defi- nite yield point and the limiting stress is accordingly somewhat uncertain. shapes for members (sec fig. 14) have been devised! in which the greatest stress at failure is some 60 to 70% of the ultimate tensile strength of the materials (see table [., column headed “ effective strength in compression’’) and there is reason to suppose that this is not far from the best that can be achieved. some improvement may be anticipated from an extension of these results to still higher grades of stecl, or from the develop- ment of a light alloy stronger than duralumin, but at the present time attention is concentrated mainly on the improvement of manufacturing methods. the ruling principle in the shapes of cross section which have achieved these results with steel is corrugation of the thin sheet material used. the theory of the collapse of corrugated material awaits development, design being at present largely empirical. but the general nature of the strengthening effect of corrugation is well understood. a flat sheet of thin metal buckles readily uncer compressive stresses. corrugation, in any direction which does not make too great an angle with the direction of the stress, stiffens it against such buckling. moreover, it greatly reduces the influence of the inevitable local irregularities in actual mate- rials, and gives robustness to what would otherwise be a very easily damaged member. it has been found essential in struts and beams of thin metal, to corrugate fongitudinully, transverse cor- rugations have proved ineffective. steel strip of thickness as low as 1/100 in. has been used in aeroplane spars. the material is either celd-rolled medium carbon steel, whose essential properties have been improved by a process known as blucing (heating to some 350 c.), but is otherwise not heat treated, or a nickel chromium alloy steel, hardened and tempered. hitherto the strip has been formed 43 into the final shape cold and without any subsequent treatment. this restricts the sections which can be produced and the materials which can be used owing to the need for ductility in order to avoid cracks. progress has been made in the direc- tion of forming the shapes while the material is in an annealed state and heat treating subsequently. this method will probably supersede ‘‘ hard ” drawing and rolling. for spars of the size shown in fig. 14, it has been found that the low specific gravity of duralumin makes it possible to use material of such a thickness that corrugation is not always neces- sarv. the resulting shapes (fig. 14) resemble those used in bridge construction, though the lattice girder seen in fig. 14d is found in practice to be heavier than the simpler shape in e. it is probable that, for the size of spar required in the largest present-day aeroplanes of the type under discussion, it 18 more economical of weight to use a spar composed of a few parts (such as a, c and e) than to build it up from many pieces. on the other hand for larger beams, such as are used in airships, a lattice construction is lighter. probably the lightest all-metal construction for a wing structure using spars and ribs, etc., combines both steel and duralumin, the former for the main members (spars and struts) and the latter for the subsidiary ribs and edges. with the methods outlined above it 1s now pos- sible to make an all-metal aeroplane certainly as light as, and sometimes lighter than, the corresponding composite machine. | (w.s. f.) aerotherapeutics.—(see 1.270).—in addition to the general use of open air and sunshine, there are several particular ways of using air for therapeutic purposes. thus oxygen en- riched air is used for those made ill through breathing irritant gases such as were used in warlare. these gases evoke oedema of the lungs and danger of suffocation through want of oxygen. so, too, in cases of pneumonia, shock and failing circulation, oxy- gen inhalation is useful. it is administered either by a mask or in a special chamber in which the patient is put. in the latter case great precaution must be taken against the danger of fire. oxygen containing 3° of carbon dioxide provokes deep breath- ing, and is useful in cases when the lungs require expanding, as after operations for empyema, when hypostatic congestion of the lungs threatens, and in cases of poisoning by carbon monoxide, as in ordinary coal-gas poisoning, etc. this mixture is particularly useful for breathing during induction of and after anaesthesia with ether, the deep respiration inducing on the one hand anaesthesia quickly, and washing the ether out of the body on the other hand at the end of the anaesthetic period (sce anaesthetics). compressed air.—the breathing of compressed air in a special chamber has been used but this probably has no other action than that of increasing the concentration of oxygen in the air breathed. the breathing of nitrous oxide and oxygen in a compressed air chamber at half an atmosphere extra pressure gives deep and safe anaesthesia; an operating theatre constructed as such a chamber would be valuable. air contamination —the contamination of the air with cer- tain dusts is important in those affected with hay fever and asthma, e.g., the pollen of plants, spores of aspergillus mould, dandruff of horses, cats, dogs, feathers, to one or other of which dusts certain people have a sensitivity similar to that in anaphy- laxis. it has been found possible to relieve such patients by send- ing them to the high alps, to sea or by having them sleep in a special dust-proof chamber. some may be relieved by vaccina- tion with the specific exciting substance (sce therapeutics). air may be cooled so as to resemble alpine air, but such a method is difficult and of little utility in the sick-room compared with the advantages of natural breathing of open air. warming and drying air for respiration is of no advantage, but steaming the air is useful in cases of bronchitis. in a mercurial or sulphur bath, the patient, enveloped in a sheet, sits on a chair beneath which a lamp is placed both to volatilise the drug and produce a.steamy atmosphere. the vapour is absorbed by the skin. this treatment is used for syphilis and also for scabies and other affections of the skin. 44 rarefied air.—rarefied air is used locally for cupping and thus producing local congestion, the blood being drawn into the part to which the cupping apparatus is applied at the expense of other organs. a similar result is got by local application of heat, poultices, etc. in operations where the chest cavity is opened airis blown into the lungs through a tube introduced through the jarynx in order to keep the lungs distended. ozonised air.—ozonised air has been used in treatment of phthisis and wounds. ozone is an irritant to the lungs and can be used safely only in very weak concentrations, ¢.g., in a con- centration just perceptible to the smell. it takes away the power to smell bad odours, but otherwise has no valuable prop- erties which have been proven. its chief use, then, is for deodorising offensive smells. ae otel sb)",
    "source_url": "https://archive.org/details/encyclopaedia-britannica-encyclopaedia-britannica.-3-encyclopaedia-britannica-inc.-1926",
    "observed_at": "2026-05-17 11:59:27",
    "integrity": {
        "hash_check": "match",
        "hash_scope": "full_normalized_text",
        "computed_sha256": "1d2a02dd3e3a6d71dc8474a4313a09fd8979b73b439a83d5105119d210da216b"
    },
    "machine_use": {
        "read": true,
        "cite": true,
        "decision": "verified_public_domain_text"
    },
    "goguides_data_license": "https://www.goguides.com/data-license",
    "goguides_data_license_version": "1.0",
    "documentation": {
        "white_paper_url": "https://www.goguides.com/white-paper.php",
        "pdf_url": "https://www.goguides.com/whitepapers/goguides-ai-source-clearance-white-paper.pdf"
    }
}