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History of Aviation - Chapter 2

TYPES OF MILITARY AIRPLANES AND THEIR USES

Modern Airplanes Combining Best Features of Previous Experiments.-The modern airplane, of which the Curtiss training machine used at the U. S. Aviation Schools is typical, is a combination of the best features referred to above. It is of the biplane type for, as shown by Chanute, rigid trussing is thus possible, an advantage sufficient to offset the slight loss of efficiency which exists in the biplane. The landing gear consists of two wheels provided with shock absorbers; the body is of the general stream-line type, enclosed from front to back, containing comfortable seats for the passengers and enclosing the motor and tanks away from the wind. The motor is at the front where, in an accident, it will not be on top of the pilot. The warping effect is obtained by hinging flaps at the wing tips, the same effect being obtained while at the same time leaving the whole wing structure rigid and strong rather than flexible and weak, as was the case in the early warping type of machines.

Military Airplanes of Today.-In the modern airplane, therefore, we see that matters of efficiency, to which the Wrights gave great attention, have sacrificed in favor of convenience, particularly 4vor of power and speed. This is the effect of demands for airplanes where power, speed, ability to climb fast are vital requirements. ie from or to destroy an enemy, high speed ability to climb fast are, of course, prerequisites. r, from the standpoint of safety in man-it is desirable to have a reserve of power and Therefore, the design of military machines has tended in a given direction up to the present. New considerations have arisen on this account, such as for instance the question of landing. Fast machines in general make high-speed landings, and are for that reason dangerous. The original Wright machines were built to land at such a slow speed that ordinary skids were sufficient to take the shocks. Nowadays the high-powered airplane is likely to come to grief in landing more than at any other time. The question of stability in flight has of recent years been treated mathematically and experimentally, using of course the fundamental system of "three axes control's first applied by the Wrights. It has been found that by properly proportioning the tail surfaces and properly arranging the Wings and center of gravity, any desired degree of stability may be obtained, such that a machine may be made almost self-flying or, if preferred, may be made very sensitive. All of the above features of design have had consideration in the latest types of military airplanes. Observe the high speed of the latest speed scouts, where power is concentrated exclusively on speed and climbing ability and landing speed is dangerously high. We see the advent of the triplane scout, which is an attempt to secure slow landing speed combined with high flying speed. We see machines with the motor and propeller in the rear, or with two motors, one to each side of the body out in the wings, the object being to avoid interference of the propeller with the range of gun fire. In short, we see the effect of many military considerations on the design of the airplane. It will be interesting at this point to survey what are these military uses of the airplane.

Aerial Fighting.-Fighting in the air is the most spectacular use to which military airplanes have been put. The first requirements in a fighting airplane are speed and climbing ability and these must be obtained at all, costs, because speed and climb are weapons of defense and offense second only in value to the gun itself. The concentration of motive power for speed and climb requires that as little weight as possible be used; and therefore the fastest fighters are designed to carry only one person and are very light and of course very small. It is usual to have one gun fixed to the body and firing through the propeller in the case of a tractor, and a second adjustable aim gun pointing upwards over the top wing. This gives the pilot a chance to fire a round at the enemy while "sitting on his tail" or following from behind; and then when diving below the enemy the second gun is available for Scouting overhead. These very high-speed fighters cult to land, due to their speed, and are only for the highest-trained pilots. ecting Artillery Fire.-The friendly airplane out over the enemy's positions, soars above target, sends back signals by wireless to the ~r battery regarding the effect of fire; practically dictating the success of artillery operations. Reconnaissance.-The friendly airplanes go out, usually in squads for the sake of protection, and observe by means of photographs or vision size of enemy troops, batteries, trenches, lines of communication, etc.; report the situation to headquarters as a source of daily photographic record of the operations of the enemy, to such an extent that any change of the enemy's position can be analyzed. Of course the value of reconnaissance is lessened when the enemy disguises his gun emplacements, etc. In reconnaissance machines it is important to have two persons, one to steer and the other to scan the countryside. The reconnaissance machine is therefore a two-place type which may or may not have armament. It need not be so fast, especially when convoyed by fighting speed scouts. The two-place machines are frequently used for fighting, in which case the pilot will have a gun fixed to the body and shooting through the propeller, and the passenger, especially in German machines, will also have a gun mounted in the turret so that it may be shot in a variety of directions by the passenger.

Bomb Dropping.-This maneuver requires squadron flights to be of great value. The fundamental characteristic of a bombing airplane is its ability to carry great weight. Such machines are of comparatively large size and not particularly fast. Weight carrying is of course incompatible with speed and climbing ability and- therefore the bombing machine must be a compromise if it is to have any reasonable speed. It may be said that airplanes compare very unfavorably with dirigible balloons for bomb raids because the latter are able to carry several tons of bombs as against the airplane's quarter of a ton.

Locating Submarines.-For coast patrol or submarine spotting, the airplane is an important factor, for from an airplane it is possible to see for a considerable depth into the water, and to locate hostile submarines.

Training Student Aviators.-The training machine on which prospective aviators secure their flying instruction may be considered as a type in which great speed and power is not essential, but in which reliability and ease of control is desirable. The typical military training airplane in this country is a single-motor tractor of moderate horsepower (about 100) having of course the seats in tandem and furnished with dual control so that operation may be from either pilot's or passenger's seat. The dual-control system of training which prevails in this country differs from the French method of starting the pupil out alone to try his wings; it enables the pilot to keep a constant eye upon the pupil's con-'and to correct them, instantly as they are in error before any damage is A possible improvement in the dual-controlling machine will be the substitution of side by seats for tandem seats. At present, communication is difficult due to the great noise of the motor; but with the adoption of side by side seats such as is used in naval training schools, the pilot and pupil will be able to communicate to better advantage.

Types of Airplanes.-To suit the foregoing purposes flying machines exist in seven distinct different shapes at the present time, namely: monoplanes, biplanes, triplanes single-motor tractors, single-motor Pushers, double-motor machines and marine airplanes. The last four types may be either mono-Planes, biplanes or triplanes. In order to under- stand the adoption of one or the other type for military use, it is well to run over the characteristic of the seven types mentioned.

Monoplanes.-The simplest form of airplane is the -monoplane which is fashioned after the manner of a bird (see Fig. 34). There are two things to say in favor of the monoplane: first, that the passengers have an unobstructed view forward and range of gunfire upward because there is no wing above them; second, the aerodynamic efficiency of the monoplane is superior to any other type. But when the bird design is applied to a man-carrying apparatus, it becomes impracticable to construct spars to take the place of the bird's wing bones; and therefore to give the wings proper strength it becomes necessary to truss them with numerous tension wires stretching from the running gear out to various portions of the wings. There are also wires running from a vertical mast above the body to a point on the top part of the wing; these wires, while they give the wing no added strength during a flight, are necessary in order that the shock of landing shall not break the wings off sharp at the shoulder. It is characteristic of mono-plane construction that from a point below the body and also from a point above the body a number of heavy wires run outward to various points on the wings; and it may be said that the strength to be secured from this construction is not all that could be desired.

Biplanes.-The biplane is an improvement over the monoplane from the latter standpoint; in the there are two parallel surfaces separated by sticks or struts, thus forming parallelograms are susceptible of being trussed by means of -wire diagonals in a manner familiar and well understood in case of bridges. It is possible to up biplane wings of great rigidity and strength system, much more easily than in case of monoplanes. However, the biplane type is from the t of efficiency inferior to the monoplane. This is due to the fact that the vacuum above the bottom wing which is so necessary for high duty is somewhat interfered with by the upper wing; thus while in a biplane the upper wing operates about as efficiently as it would operate in a monoplane, yet the lower wing has its efficiency materially reduced and the resulting overall efficiency of a biplane compared area for area with the monoplane is about 85 per cent, as great. However, recent developments of the airplane have more or less put efficiency in the background and as a result today the biplane is more popular than the monoplane. In addition to the greater strength of biplane wings their span may be less than the monoplane for the same supporting area. This makes them less unwieldy. Moreover, for certain reasons a biplane machine of high speed may be landed at a lower speed than equivalent monoplane5

Triplanes~What is true of the biplane is more true in almost every item of the triplane, that is, it 15 comparatively strong, compact, and of low landing speed, but of reduced efficiency.

Single-Motor Tractors.-The single-motor tractor received its name simply because the propeller is in front and draws the machine forward; but this location of the propeller necessitates a distinct type of airplane, wherein the power plant is located at the very nose of the machine. The tractor type has the pilot and passenger located in or to the rear of the wings in order that their weight may balance the weight of the motor. This means that the view and range of fire of the passengers is obstructed in a forward direction by the wings, and in machines such as the U. S. training machine, the passenger, who is practically in the center of the wings, can not look directly upward nor directly downward. Moreover, as concerns gun fire, the propeller of atractor obstructs the range straight ahead. In the tractor the tail is supported at the rear and on the same body which contains the motor and passengers; this body constitutes a stream-line housing for the machinery, seats, etc., and therefore has low wind resistance. The tractor is a very shipshape design, compact and simple and is at present the prevailing type on the European war front. However, it has disadvantages which are only overcome in other types. One of these disadvantages is of course the obstruction to range of gun fire. The present practice in fighting airplanes is simply to shoot the gun straight tfirough the circle of rotation of the propeller on the assumption that most of the bullets will get through and that those which hit the shank of the propeller blade will be deflected by proper armoring. An attempt is made to insure that all the shots will get through by connecting the gun mechanism mechanically to the motor shaft in such a way that bullets will be discharged only at the instant when their path is unobstructed by a propeller blade. This practice is possible of course only in guns which are fixed immovably to the airplane.

Single-motor Pusher Airplanes.-The pusher type has popularity because the propeller and motorrotate to the rear of the passenger, who takes his place in the very front of the body and has an open range of vision and gun fire downward, upward and sideways. Another point in favor of the pusher is that the oil and fumes of the motor do not blow into his face as in the case of the tractor. The disadvantage of the pusher is that the motor, being located behind the pilot, will be on top of him in the case of a fall. Another disadvantage is that the body can not be given its shipshape stream-line form because to do so will interfere with the rotation of the propeller. Therefore, the body is abruptly terminated just to the rear of the wings and it is just long enough to hold the passenger and the motor, the propeller sticking out behind. The tail surfaces are then attached to the airplane by means of long outriggers springing from the wing beams at points sufficiently far from the propeller axis so as not to interfere with the propeller.

Double-motor Machines.-In order to combine the advantages of the tractor and pusher types and eliminate their disadvantages, the double-motor machines have been developed. In these there is no machinery whatever in the body either in front or back, and the passengers may take seats at the extreme front as is desirable. The body then tapers off to the rear in stream-line form and supports the tail surfaces. The power plants are in duplicate and one is located to each side of the body out on the wings. It is customary to enclose each of these two motors in a casing so that the whole power plant presents a more or less stream-line shape to the wind, the propellers projecting from the front or rear of these stream-line shapes. It may be said that in the double-motor airplane it makes very little difference whether the propeller is in front or behind so that while a "twin-motor" machine may be more accurately specified as a "twin-motor pusher" or a "twin-motor tractor," it is usually sufficient indication of a machine's characteristics to call it a twin-motor machine.

By adopting this twin-motor form we bring in new disadvantages. One of these is due to. the fact that the heavy motors are now located some distance from the center of gravity of the machine. This requires stronger supporting members between the motor and the body. It also makes the lateral control comparatively logy for now the heavy masses are far from the center of gravity, resisting the pilot's efforts to use the lateral control. The second disadvantage in the twin-motor type results from possible

stoppage of either motor. In this case, of course, the propelling force is some distance off center and is also reduced to one-half its value requiring energetic exercise of the control wheel to maintain equilibrium. It is reported, however, that twin machines can continue to fly and even climb with only one motor running. In this country the twin-motor type has not developed as was hoped at first, and on the European firing lines it is not so numerous as the single-motor tractor type.

Marine Airplanes.-The possibility of mechanical ffight having onee been established and wheels having been applied to the airplane so that it could start ~ from and land on the ground, the logical next step was to substitute some form of boat for the wheels so that ffights could be made over the water.

Experiments were made in France by M. Fabre in this direction and in this country by G. H. Curtiss. :1 The latter, in his flight down the Hudson from Albany to New York, equipped his airplane with a light float to provide against forced landing in the river. Pursuing this general idea he made some experiments under the auspices of Alexander Graham Bell's Aerial Experiment Association, in which a canoe was substituted for the wheels, and in which an attempt was made to start from the surface of the water. Success did not come at first and this plan gave no satisfaction. Curtiss next turned his attention to the hydroplane type of boat and made a series of experiments at San Diego. The hydroplane appeared to be much better adapted to his purpose than the canoe had been, and he was able to obtain success.

The Hydro-airplane (or "Seaplane").-From analogy to the airplane one might at first imagine that a suitable h~ydroplane would have a wide span and fore and aft length; but such proportion would give a very poor stability on the water, and would require auxiliary hydroplanes in the same way that an airplane requires auxiliary guiding surfaces. So Curtiss, with his customary eye for simplicity and convenience, adopted a type of hydroplane which bad the general proportions of an ordinary boat, i.e., was long and narrow, thus obviating the necessity of auxiliary hydroplanes at the tai.l of the machine. To prevent the machine's tipping over sidewise, "wing pontoons" were attached at the lower wing tips to prevent capsizing.

The Flying Boat.-In the early bydro-airplane, which was thus developed, the motor and pilot were above in the usual position in the wings, while the hydroplane itself was a considerable distance below the wings. Thus there was a good deal of bead resistance. Curtiss set about reducing this bead resistance as far as possible and tried to incorporate the pilot's seat with the hydroplane pontoon. The outcome of his endeavor was that he developed a boat with a tapering stern. The pilot, gasoline tanks, etc., are located inside of the hull; the tapering; stern provides a backbone to which the tail surfaces can be readily attached; the wings fixed to the sides of the hull in a manner analogous to the wing ~ fastenings of the modern military airplane; and the ~ motor alone remains exposed to the wind. This is the flying boat; its action on the water is analogous to the action of the hydroplane for the bottom of this boat bull is made in hydroplane form; indeed, in the latest types of flying boat, the hydroplane area is increased by extending it to right and left of the boat hull. The flying boat is an ingenious combination, wherein the characteristics of the hydroplane are combined with the seaworthiness of the ordinary boat, and at the same time wind resistance is reduced to a minimum.

The hydro-airplane remains in use, however, being preferable to the flying boat for certain purposes, and often is termed seaplane.

Future of the Airplane.-In order to.be commercially successful and have a commercial future after the war, the following weak points in airplane design must be rectified.

1. Motor.-Airplane motors are imperfect and unreliable at present and there must be considerable progress before this type of motor which is very light and delicate can be considered as reliable or can be made in large enough quantities to cut down the cost. 2. Landing.-The necessity of landing at considerable speed, say 40 to 50 miles per hour, requires a wide flat space, such as is not easy to find, and if the present type of airplane is to become commercially numerous, a large number of landing fields must be developed all over the country. 3. Danger.-The airplane is by no means so dangerous as the public has been led to think from the exploits of the daredevil circus performers of the past 10 years; with careful manipulation it will make trips day after day without any damage. However, it is not a foolproof machine and there remains an element of danger on this account, which it is hoped will one day be eliminated. 4. Future Uses of the Airplane.-Future uses of the airplane are many after the war is over. The postal service of several governments are considering this means of mail delivery; the sports use as in the past will continue to flourish; express carrying may be expected in inaccessible countries where railroads and roads do not give access and where high-speed delivery by countless airplanes would aid materially in the development of newly opened countries. For airplane transportation will require no expensive right-of-way, rubber-tire renewals, etc. Minor uses of airplanes are on such duties as forest-fire patrol, working at life-saving stations, etc.

American Airplane Industries.-The magnitude of the airplane industry in this country is great, although not so great as in Europe. Leading business men have invested in this industry with the firm belief that it will become a profitable one, irrespective of war. We see a number of leading bankers and also automobile manufacturers in various parts of the country putting their money into this new industry. Now that a great demand has sprung up on our side of the water for airplanes, we will expect to see this industry increase more rapidly still. The only result can be, from all the interest and importance attached to aviation, that after the war is over, _ large commercial uses will develop which will offer employment to those who go into the work at this time for military reasons. No one can predict _ exactly what turn the situation will take, but there is every indication that aviation has graduated from the primary class of experimental work and is to be considered now as an industry along with the automobile business, motor-boat business, etc.