From Wood to Metal
If airplanes were to become useful instruments of military service, their individual capacity had to increase. The airplane of World War I carried too little payload and fuel, its performance marginal. For aircraft designers, that meant the useful load - crew, fuel, bombs, cameras, etc. - had to increase significantly, and the speed, altitude ceiling, and range had to become much greater. To achieve this goal, designers had to transition from the wood-and-fabric biplane to the all-metal, streamline monoplane. It would require creating the technology of aircraft materials and structures.
For most of recorded history wood has been the primary structural material used for large structures subjected to dynamic loads. In recent times wood has been largely replaced by steel, aluminum, and fiberglass composites. This transition has been due mostly to problems associated with moisture control and joining efficiency, rather than to a lack of attractive material properties of the wood itself.
With the arrival of manned flight lightweight structural capability became paramount for the success of aviation. At this point the true limitations of past wood technologies were addressed. For the first 30 years of the development of the airplane, wood was the primary structural material. Pressures to develop safe, reliable, lightweight structures fueled research and development efforts that, for the first time, began to scientifically characterize wood properties. Aircraft engineers quickly realized that even the best mechanically fastened wood joints could transfer only a little over 30 percent of downstream wood material capability. Thus, the full material capability of wood had rarely been utilized in any of the dynamic wood structures of the past.
The primary structural material of World War One aircraft was wood, which was well suited to the small shops and skilled craftsman of the period. However, the properties of wood are anisotropic, and strength characteristics can vary depending on supply source. A potential alternative was metal, but it would not be a simple matter of substituting metal for wood. Early attempts to use aluminum and steel did not immediately satisfy the goal of weight reduction. The low stress levels in the airframes of this period required only a fraction of the inherent load bearing capacity of metal. Designing with metal as though it were wood often resulted in metal parts being fabricated to minimum gauge dimensions, yet this size was still larger and heavier than necessary for component strength. Further applications of metals to full-scale airframes likewise did not demonstrate promising results, with empty weights increasing rather than decreasing.
Most historians will agree that the Fokker D.VII was the best fighter of the Great War. It was neither the fastest nor the most maneuverable, nor did it possess the best rate of climb or have the most firepower. The DVII's greatest strength was that it had no weaknesses. The fuselage was metal-frame constructed, angular but strong. The canvas sides of the fuselage were stretched over the internal metal frame.
There had been an all-metal plane as early as World War I, but it was an exception. Created in 1915, the Junkers J-1 was the first cantilevered wing all metal airplane. Developed for low-level, front-line observation and attack, the Junkers J 1 was the first all-metal aircraft to go into series production anywhere in the world. Like all Junkers aircraft, the J-1 incorporated an all-metal structure. The wing was composed of 0.08-inch corrugated aluminum alloy skin riveted to an internal framework of aluminum alloy tubing. The engine and crew were encased in an armored shell formed from 0.2 inch sheet steel. The aft portion of the fuselage consisted of a metal alloy frame covered with fabric in early models but with sheet metal in later versions. Although heavy, cumbersome, and slow to take off from rough ground, the J-1 was immensely strong and well suited for low-level observation/attack. Its weight, combined with the relatively heavy metal construction, resulted in a slow aircraft, but provided effective protection against ground-fire.
Most airplanes of the WWI period and the 1920s had been primarily of wood and fabric construction, although many later ones had tubular steel fuselage frameworks. It initially appeared impractical to make a very small airplane of metal, since an all-metal airplane of minute size seemed certain to be heavier than if it had been built of wood. Among the very large airplanes, hoever, the advantage was distinctly the other way, and the aerial giant of the future seemed only likely to be realized by the fullest possible use of steel and aluminum.
If metals were to become a primary material, new techniques for light-weight airframe construction would be necessary. Successful aircraft design results from finding the best balance between the strength of the airframe and its weight. Decreasing weight improves performance, but may risk inadequate structural strength. Higher flight performance requires stronger structure, as the airloads increase with the square of the velocity (doubling the speed from 100mph to 200mph increases the nominal airloads by four), resulting in a tendency for increasing weight. It is a vicious cycle, one that easily diverges to an overweight, poor performing aircraft.
Acquiring the knowledge for constructing all-metal airplanes would be a long, arduous process, with gains coming in small increments. It was a high-risk endeavor, with uncertain reward for commercial firms, but well suited for long-term government sponsorship. Eventually it would take a full decade to achieve success.
The Gallaudet DB-1 of 1921 and the Engineering Division CO-1 of 1922 were unsuccessful attempts to construct an all-metal military aircraft. They were seriously overweight, compromising their useful load and flight performance.
The Stout ST-1, the first all-metal airplane designed for the Navy, was successfully test flown by Eddie Stinson on 25 April 1922. Although the ST-1 twin-engine torpedo plane possessed inadequate longitudinal stability, its completion marked a step forward in the development of all-metal aircraft. Henry Ford's Ford Motor Company's involvement in aeronautics extended from 1925 to 1936. Interested in doing for aviation what he had already succeeded in doing for the automobile industry, Henry Ford bought the Stout Metal Airplane Company and its idea for an all-metal airplane; he hoped this would be the foundation for his new aviation enterprise.
The NM-1, an all-metal airplane, was first flown at the Naval Aircraft Factory on 13 December 1924. This aircraft was designed and built for the purpose of developing metal construction for naval airplanes and was intended for Marine Corps expeditionary use.
By the end of the 1920s, biplanes were becoming obsolete and manufacturers turned to building all-metal monoplanes. Boeing Aircraft led this technological revolution with welded steel tubing for fuselage structure. This soon became standard in the industry until it was replaced by monocoque sheet metal structures in the mid-1930s.
Seattle lumber magnate William E. Boeing started manufacturing aircraft in 1916. By the 1920s, his company produced passenger and mail airplanes as well as military fighters and bombers. Boeing's first all-metal monoplane was the Monomail, designed to carry cargo and mail, and the single unsuccessful XP-9 monoplane fighter. Boeing's enterprise was one of the first manufacturing concerns to benefit from the innovations brought forth by the airplane design revolution when the company moved toward specialization in multi-engine aircraft. The Boeing Company pioneered the all-metal "modern" airplane, the Model 247.
On March 31, 1931, Knute Rockne, the famous football coach, was killed when a wooden Fokker trimotor crashed. It had suffered a structural failure partly because of its wood construction. Consequently, the Civil Aeronautics Authority grounded the plane and insisted on so many modifications that the Fokker was taken out of service, leaving the company to return to solely European production. The industry realized that it had to come up with a safer plane-an all-metal plane.
By the early 1930s, aircraft design and construction technology throughout the world had advanced to the point where it was possible to mass-produce all-metal airplanes. Metals quickly gained favor as a safer material for most larger and faster aircraft. Metals not only possessed more consistent properties but could be fabricated with a high degree of reliability by a semiskilled work force. In comparison, woodworking required a high degree of skill that took a long apprenticeship to acquire.
The US Army Air Corps' first all-metal monoplane bomber was the Boeing B-9. Produced during 1932-1933, the B-9 was outclassed by its contemporary all-metal Martin B-10 and only seven were purchased. The Air Corps' first all-metal fighter was the Consolidated P-25 of 1933. Although only two were procured, the P-25 design was modified into the P-30, later redesignated the PB-2, of which 54 were purchased in 1935. The first all-metal fighter ordered in quantity was the Boeing P-26; 139 were purchased from 1932-1936.
The all-metal airplane had many opponents. Because it was metal, it was naturally heavier than previous wooden airplanes and thus consumed more fuel. Because of its weight, the plane was also harder to control if it suddenly pitched into a spin. Some even thought an all-metal airplane couldn't glide to safety if the engines cut out. But the all-metal aircraft was stronger and could carry fuel in larger tanks, the stiffer metal allowed for better control of the ailerons and stabilizer, and the aerodynamics of a wing surface were the same, regardless of its composition.
Because of the limitations of early adhesives, bonded wood joint technology did not become fully viable until the mid-1930's, when more advanced adhesives became available. This late development, combined with a lack of uniform, consistent wood physical properties that could be relied upon in a quality control effort, limited the use of wood in the then rapidly developing aircraft industry.
Some efforts to keep aviation-oriented wood technology alive persisted in both the United States and Great Britain. With the coming of World War II and the ensuing shortages of all metal materials, the substitution of wood in aircraft and other highly sophisticated structures became crucial to the war effort. For the first time a serious effort was begun to perform the necessary testing so that an engineering data base could be established for wood materials.
The De Havilland Aircraft Company of Great Britain developed a unique stressed-skin monocoque shell design that was the culmination of 23 years of experience in wooden aircraft. The chief structural feature of this design was a wood composite sandwich of birch veneers over a unidirectional balsa core. The design for De Havilland's Mosquito bomber using this advanced structural concept was conceived in 1939. This extremely successful airplane was in full-scale production in 1941 and saw much service in World War II. This two-man-crew wooden bomber, one of the most advanced aircraft of its day, had a level flight speed of over 400 mph and was capable of carrying a 3000-1b bomb load. Operating at fighter speed without armament, it had a 1500-mile range.
Major pioneering efforts in wood technology ended at the close of World War II. One reason was that aluminum alloy technology evolved quickly in response to the needs of modern aircraft. This was compounded by wood's past image, traditions, limitations, and folklore. However, the main reason wood lost favor was related to maintenance. Lack of a viable moisture protection system for a completed structure was at the heart of the problem. All wooden structures need some reasonable moisture stability to prevent internal stressing and fungus attack.
The old wood technology of ships had evolved to the point where it could successfully deal with large changes in wood moisture content, but the rot problem was never solved. Although the development of all-bonded joints solved the major structural limitation of wood construction, moisturerelated problems persisted. By 1945, moisture problems were perceived by the aircraft engineering community as a fundamental unresolved dilemma that severely limited wood as a viable engineering material for high-performance dynamic structures. Another major drawback was the lack of adequate quality controls that could be implemented in large-scale manufacturing efforts with mass production.
Once the all-metal airplane body went into full production, there was no longer any reason to shelter these aircraft in hangars, except during maintenance, repair, and outfitting operations. The major construction challenge then was to provide enough hangar space to handle the enormous servicing capacity required to keep an airborne fighting force in the sky.
|Join the GlobalSecurity.org mailing list|