The past five years have witnessed amazing progress in the technical development of naval aviation. This progress has touched all relative fields and there is no indication that the end is in sight. Many very interesting transitions have taken place during this period, the most notable of which are the passing from water-cooled to air-cooled engines and the perfection of the latter in practically all power ranges; the elimination of wooden structures, and the advent of metal; the use of the metal propeller as a replacement for the wooden propeller; the perfection of the catapult for launching planes from ships; the remarkable improvement in performance of naval airplanes; and the decided increase in size of naval airships. These are the high lights of the development during this five-year period.
One of the most interesting transitions which has taken place recently in naval aviation has been the perfection of the aircooled engine in all power ranges from a modest beginning of one satisfactory aircooled power plant developing approximately 200 hp. There are now in production and in service proved units covering the complete range from 150 hp. to 600 hp. Although not losing touch with water- cooled power-plant development the Navy in the interest of lighter and smaller planes for compact storage aboard ship has been forced to the adoption of the aircooled engine as standard. It has vigorously sponsored the development of this type and the soundness of the Navy’s policy in this regard is proved by the almost universal adoption of air-cooled engines for commercial planes, and the increasing application to military types in other organizations. The part which the Navy has played in the development of the air-cooled engine has not been the part of a passive purchaser, but has been an active one. Naval officers and engineers have been directly responsible by their study and suggestions for many of the improvements which have taken place.
Today power under sea-level conditions is not alone sufficient for naval aircraft. The battle altitude has increased 100 per cent in the past five years because of the increase in aircraft ceilings. Power and speed at high altitudes are therefore required nowadays for combat, and the fighting pilot obviously wants the maximum performance at these high altitudes. To accomplish this, superchargers, variable pitched propellers, manifold pressure regulators, etc., have been developed, each of which is an engineering project of considerable magnitude and complexity in itself.
The recent adoption of the dive-bomber type of naval aircraft has likewise added additional power-plant requirements which have been successfully met. The engine and installation for such planes must be capable of operating satisfactorily in a vertical attitude and to withstand the increased r.p.m. from acceleration in a dive. Diving operations result in a terrific beating of the engine. It is only within recent years that engines have been built which can withstand such treatment. The average speed in a dive is about 2,500 to 2,800 r.p.m.
The passing of the wooden propeller with its inherent defects of susceptibility to climatic conditions, erosion by the elements, and its non-variable blade setting has also taken place during the past five years. The metal propeller with its adjustable blade feature has come into use as standard equipment of naval air units. The present standard detachable-blade aluminum-alloy type of propeller has a wide range of usefulness as its pitch may be adjusted on the ground and set to give best performance. As a result, a given propeller design is often suitable for use on several types of airplanes.
The Navy is continually experimenting with new propeller developments. Hollow- steel propellers have recently been flight tested and found to compare favorably with the aluminum-alloy propeller in both weight and performance. These propellers when whirl tested, demonstrated superior strength in comparison with present service types. An experimental order of magnesium propellers has also been procured and, if the magnesium propeller proves suitable and sufficiently corrosion resistant for service use, a considerable saving in weight will be effected. Similar experimental projects for the testing and development of controllable pitch propellers have been undertaken.
Another outstanding feature of the recent progress in naval aviation has been the replacement of wood by metal in the construction of aircraft. It has been realized for years that wood for structural parts for aircraft is in a measure unreliable. The unknown qualities which affect its strength are always present. Wooden members used in seaplane construction gain weight by absorbing moisture, and suitable woods have been more and more difficult to obtain. Hence there has been a definite movement toward metal construction of naval aircraft. The trend was first to steel, then to a combination of steel and duralumin and the present practice is to use light aluminum alloys entirely. These insure more dependable structures, give greater strength, and are lighter.
This substitution of metal members for the corresponding wood members, has involved an improved method of design and construction of such far-reaching effects that the modern aircraft structure bears very little resemblance to its wooden predecessor of five years ago. Fuselages have evolved from stick and wire to welded and built-up tubular, to so-called monocoque. Hulls have changed from the wood planked truss construction adapted from fuselages to the monocoque type. Wings have developed from the solid routed beam and wood rib assembly, to the built-up wood beam, to the metal beam and rib, to the stressed skin wing. Likewise, control surfaces have changed from wood to metal, and landing gear from bunge cord absorbers and tail skids to oleo absorbers, tail wheels, and low-pressure tires.
These developments which have caused such a great change in the airplane structure have made necessary a radical change in design calculations and methods. New basic design data had to be accumulated and, in order not to handicap this structural evolution, the need for this data had to be foreseen. It has been the policy of the Bureau of Aeronautics to encourage and finance basic structural research by naval establishments such as the Naval Aircraft Factory, and by other government agencies, such as the Bureau of Standards and the National Advisory Committee for Aeronautics. Private contractors have also been encouraged along this line by the placing of contracts for experimental airplanes and structural parts.
Airplane structures must have, first, the requisite strength, and second, the minimum weight. In order to satisfy these requirements, the strength while sufficient, must not be excessive. The amount of strength required is a function of the loads to be withstood. The Bureau of Aeronautics has therefore expended a great deal of thought and effort recently in determining more exactly the loads which naval airplanes are required to withstand both during the extremely severe maneuvers which are demanded by naval tactics and in landing on the decks of carriers in a restricted area. This research conducted by the bureau has made possible the development and application of more precise methods of structural analysis with a view to providing the increased strength to withstand the severity of present-day operating requirements of naval aircraft without a corresponding increase in their weight.
The research work in the Navy’s wind tunnel and model basin at the Washington Navy Yard has yielded valuable returns by way of improvement in the water characteristics of both hulls and floats, improved wing sections, and reduction of parasite drag, and has generally enlarged the fund of scientific knowledge in aerodynamics. Recent tests have covered such subjects as the effect of basic form and of modifications thereof on the drag of struts, the effect of fairings and location on nacelle interference, the drag of several series of typical airplane bodies and floats, and the effectiveness of various types of radiators and radiator cowlings. Tests have also been made on a number of airfoil models fitted with various kinds of slots and flaps. Another investigation covered the effects of various general types of center section cut-outs on airfoil models. Studies of a somewhat similar nature cover the effects of various types of fairings on wing tips and control surfaces. An investigation now under way is concerned with the proper proportions between the elevator area and the stabilizer area for typical designs. A new 6-foot 4-inch diameter open- throat modern wind tunnel has just been completed to replace the present 4 by 4- foot tunnel which is too small for the work required. With the greater capacity of the new wind tunnel, it will be possible to complete tests much faster than at present.
The Navy increased its aeronautical research facilities a few years ago by the establishment of an aeronautical engine laboratory at the Philadelphia Navy Yard and a flight test section at. the Anacostia Air Station. These research units permit the Navy to conduct tests on all kinds of planes, engines, and equipment, and gain first-hand information as to their characteristics. The rapid progress in the art of flying within the past few years has placed a tremendous load on all the Navy’s research activities, as new ideas are presented for testing and investigation in much greater number than heretofore.
An important tendency in connection with the improvement in performance of naval aircraft is the return from the multipurpose to the single-purpose plane. The only advantage of the former is to reduce somewhat the total number of airplanes and spares required. This, however, is more than outweighed by the reduction in performance which is unavoidable in multi-purpose designs, and by the poor disposition of crew and material, even under best compromise conditions. The present tendency is to place the crew and equipment in the best arrangement for a single purpose and to design the airplane around this arrangement. The designer attempts to secure the smallest and lightest airplane possible to carry the given load and fulfill a given mission.
Certain types of naval aircraft used by the fleet are subject to restrictions on maximum span, which prevent the immediate adoption of the high aspect ratios now used extensively on commercial monoplanes. This handicap is being met by better wing arrangements, better wing sections, and reduction of drag. In those types where the restrictions are not absolute, as with fighter and observation planes, and flying boats, there is a pronounced tendency to go to the monoplane arrangement.
The past year has broadened the field considerably in amphibian development. Several new naval types completely house the wheels in the retracted position and at the same time have more reliable operating mechanism. This was achieved at a great weight reduction over those now in service use. These latest retractable units hold forth promising possibilities for use in types other than amphibian.
Parallel to the development of single-purpose airplanes, the growth of naval radio has been along similar lines. There are now in use, or being developed, distinct equipment for the various types of airplanes, namely, patrol, scouting, observation, etc. Radio sets have been greatly increased in power without appreciable increase in weight. The engine-driven generator is supplanting the wind-driven unit. Ignition shielding has been adopted to secure the best radio performance. To aid naval airplanes, which under certain conditions fly several hundred miles or more over the ocean under conditions of poor visibility, in the return to their ship, radio-compasses or homing devices have been developed by the Navy. These homing devices installed on naval airplanes guide the lost or confused pilot so accurately that unless he stops beforehand, he inevitably runs down the antenna from which the homing signals are being transmitted. The instances that these homing devices have been of service have many times repaid the Navy’s efforts in developing them.
The steady development of instruments for naval aircraft has been largely along lines of standardization, reduction in size and weight, and methods of test. Improvements consist in smaller and more reliable equipment and better installations, particularly as regards more convenient grouping of the instruments and the indirect lighting of panels. Notable improvements have also been made in equipment for blind flying and navigation. Interest in gyroscopic instruments and their effect on the simplification of instrument flying has been revived by the introduction of a new type of artificial horizon and direction indicator. At the present time, the Bureau of Aeronautics with the cooperation of the Sperry Gyroscope Company, has under development a gyrocompass for use in aircraft. The development of a satisfactory gyrocompass will eliminate the present difficulties encountered with magnetic compasses due to large magnetic masses, such as engines, near the compass.
The details of armament equipment are for the most part confidential, but steady progress has been made during the past five years in perfecting armament material such as bombs, torpedoes, machine guns, sights, bomb racks, and the many accessories which are necessary to insure the operation of such major items. Provision for the carrying of armament material has been especially emphasized recently. During the World War, and for a number of years afterward, airplanes were designed and built primarily as mere flying machines and military equipment for either offense or defense was generally hung on as an afterthought. Today a naval airplane when compared with a commercial airplane of similar size shows wide variations. One will find that machine guns are carefully lodged in the wings or fuselage of present-day naval airplanes, and not simply secured in place after the plane has been completed. Bomb racks in many instances are similarly treated. Protection from the wind blast for the rear gunner is much more adequately supplied than formerly. Instead of a bomber standing in the wind blast in the bow of an airplane and trying to operate his sight while bracing himself against an 80-mile an hour breeze, as was the case five years ago, he is comfortably seated or kneeling behind a non-shatter- able plate glass window and can operate his bomb rack as easily as he rings a doorbell instead of pulling a toggle with all his strength.
Of the distinctly naval equipment developed by the Bureau of Aeronautics in the recent past, none is more important than the emergency flotation gear. Primarily, its purpose is the protection of personnel; secondarily, the preservation of material. Since its adoption, many lives have been saved, and in addition the value of airplanes and equipment salvaged has been a good percentage of the total cost of the flotation gear. In naval maneuvers, landplane fighters, scouts, bombers, and torpedo planes are sent from their aircraft carriers on missions which take them far over the water. In case of a forced landing, by means of the emergency flotation gear, the airplane is kept afloat in a horizontal position thereby furnishing a reasonably stable floating platform, usually the upper wing. This serves as a refuge for the crew and as a large, easily identified object to aid the searching parties.
The gear itself consists of a carbon dioxide cylinder secured in the fuselage, containing the proper charge. Fastened to its head is a special type of valve which, when the pilot pulls the release, allows the liquid to flow through copper tubing to a small piston valve adjacent to each bag. The latter valve first releases the bag from its compartment, and then directs the flow for inflation. In the smaller types of airplanes, two bags are used, usually housed in a flush, covered compartment in the upper wing. In the large torpedo and bombing planes, a three-bag installation is employed, two being housed in containers inside of the fuselage cowling forward and the third inside of the rear of the fuselage. It may be interesting to know that the entire emergency flotation gear for a 4,000- pound airplane weighs only 60 pounds, and that from the pull of the release to full inflation of the bag requires only 40 seconds.
The Bureau of Aeronautics now has under development a valve for this gear weighing about two pounds which will operate automatically and thus will save the lives of personnel in crashes at sea which are of sufficient violence to stun the crew, but not completely to demolish the airplane. This valve, in addition, incorporates the temperature release, the indicating device, and the mechanical operation of the present valve. It is actuated by the pressure due to a hydrostatic head of about eighteen inches of water acting upon a diaphragm. When thoroughly tested and approved, this valve will replace the manual ones now in service.
The operation of catapults for the launching of planes from ships has been changed during the past five years from one requiring the most careful supervision of highly trained personnel, to an everyday routine procedure aboard all the battleships and cruisers of the fleet. There has been a steady increase in capacity, reliability, and simplicity of catapult operations and further development of this aviation apparatus is going ahead at a rapid pace.
On the carriers, effort has been concentrated on speeding up operations, including take-offs, landings, and faster handling of planes both on deck and below. Brakes and tail wheels are now standard equipment for all carrier planes. The tail wheels are unique in that they are full swiveling, and also lockable in the fore-and-aft position. Both of these factors are necessary in different phases of carrier activities. Torpedo planes attached to carriers now have folding wings, and this feature may be extended to other types. Moreover, continual changes and improvements are being made in the carrier’s arresting gear, all for the same purpose.
The recent high rate of progress in the technical development of naval aircraft is well evidenced by the remarkable improvement in their performance. From low- powered aircraft with limited load factors and performance, a steady progression has been made to the powerful, heavily armed fighting units that are at present launched from catapults from the decks of carriers. As an example, five years ago, the fighting plane made a high speed of 122 miles per hour, and had a ceiling of 13,500 feet; today, one of the fighting class has a high speed of 175 miles per hour, and has a service ceiling of over 27,000 feet. Similar progress has been made in all the various types which go to make up our naval air forces.
In its lighter-than-air work, the Navy has been a pioneer. The facilities which now exist for the construction of rigid airships in this country and the engineering knowledge on this subject which is now available, have been due entirely to the Navy’s realization of the possibilities of lighter-than-air craft and its activities in this field. Five years ago, the Navy had the only rigid airship then in operation throughout the world, the U.S.S. Los Angeles. This airship is still in active operation, and a recent inspection indicated that its material condition will permit from two to four years’ more service. Another Navy airship, the U.S.S. Akron, is under construction and almost completed. It will be placed in commission within the next few months, and will, when completed, be the largest in the world, with a volume of 6,500,000 cubic feet. A sister-ship of the U.S.S. Akron, the ZRS-5, is under contract at the present time. These two new airships will have a high speed of 73 knots, and a cruising radius of 183 hours at 50 knots. They will be used for long distance scouting and each will carry five or more high performance fighting planes to add to their protection.
Important lighter-than-air development and training work has been carried on during the past five years utilizing the Los Angeles and two nonrigid airships. Extensive full flight research has shed light on airship problems that were hitherto somewhat obscure. Water recovery apparatus, an important adjunct to helium-filled airships, has been improved. Materials entering into the construction of airships have been developed and perfected. Handling methods have been tried out and the present mobile mooring mast evolved which permits a large reduction of the man power formerly required to handle airships. Promising steps toward the development of a satisfactory oil-burning engine for airships have been taken, and there is every indication that a suitable engine of this type will be available in the near future.
One of the greatest objections to dirigibles was the danger caused by the use of hydrogen gas. When the Navy first desired to use helium as a substitute, this material was so rare and the methods of production so little known, that it could be had only in small quantities, and at prohibitive expense. After extensive studies, the Navy built and operated at Fort Worth, Texas, a plant where large scale production at reasonable cost became an accomplished fact. Today this production is being handled by the Bureau of Mines, and helium is also being produced by private manufacturers.
In conclusion, it may be said that this technical development of naval aviation has been invaluable not only to the Navy, but also to the aeronautical industry as well. The studies, research work, and experimentation which the Navy has made for its own use, have in almost all cases been held non-confidential and have been published for the benefit of the industry. Naval aviation is in a healthy state of development at the present time and it is ready to do its part in any emergency.