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Quest for Performance: The Evolution of Modern Aircraft


Part I: THE AGE OF PROPELLERS


Chapter 2: Design Exploration, 1914-18


Heavy Bombers



[47] Most types of World War I aircraft, including fighters, were used at one time or another for tactical or ground-support bombing operations. The heavy bombers discussed in this section are what would be called strategic bombers in present-day terminology. They were used for bombing such targets as docks and harbor installations, rail yards, factories, and cities. The mission of these aircraft required them to have sufficient radius of action and payload capability to deliver a significant bomb load on a variety of targets and to carry enough defensive armament to offer a reasonable probability of mission success and safe return to base. Heavy bombers were used singly and in formations of several aircraft, on both day and night missions. Speed, maneuverability, and rate of climb were of secondary importance although a high ceiling was considered desirable.

The mission requirements for heavy bombers led to large, heavy (for that time period) multiengine aircraft just as they do today. Gross weights varied widely but usually fell in the range from 8000 to 16 000 pounds, and some of the German special-purpose R-planes weighed over 30 000 pounds (ref. 68). Two engines were used on most designs, although examples can be found of aircraft with three, four, and five engines. Most of the aircraft were multibay strut-and-wire-braced biplanes; however, several triplanes appeared, one of which is described [48] herein. At war's end, there were several German designs for highly advanced monoplane bombers that incorporated thick, cantilever wings (ref. 68). Construction of most heavy bombers consisted of a conventional wood framework covered with fabric.

The first of the large, heavy bombers was the Sikorsky Ilya Muromets, which first flew in February 1914. Caproni, Gotha, Friedrichshafen, A.E.G., Handley Page, and Vickers are a few other names that will be forever linked with the large, heavy bombers of the World War I era. Three heavy bombers, the German Gotha G.IV, the British Handley Page 0/400, and the Italian Caproni CA.42 are discussed here to give a glimpse of the size and characteristics of this class of aircraft.

Gotha G.IV

The name Gotha still evokes in the minds of some people the terrifying image of a group of large aircraft dropping bombs on the helpless citizens of a great metropolitan area. The Gotha gained this dubious distinction because of its use in the bombing raids on London in 1917 and 1918. Twenty-seven Gotha attacks were made in the course of about a year. Not a large application of strategic air power by World War II standards but enough to cause great consternation in an era when the English Channel was still mistakenly thought to ensure protection of the British Isles against a foreign invader. The Gotha raids, conducted first in daylight and later at night, actually caused little physical damage, but the psychological impact was such that badly needed British squardrons were recalled from the front to protect Britain against the German invader. Actually, several types of German aircraft participated in the bombing of London, but the name Gotha has for some reasons become synonymous with the bombing of helpless cities.

The Gotha model G.IV depicted in figure 2.21 was a triple-bay biplane equipped with two pusher-type engines mounted between the upper and lower wings, one on either side of the fuselage. The thin wings incorporated a small amount of sweepback to position the aerodynamic center in proper relation to the aircraft center of gravity. Horn balances were employed on the ailerons and rudder to reduce the control forces required to maneuver this very large aircraft. The landing gear had four main wheels; two were positioned below the bottom wing at the location of each engine nacelle.

The Gotha G.IV was manned by a crew of three: a single pilot and two gunners. The front gunner armed with a flexible machine gun was...




[49] Figure 2.21 - German Gotha G. IV twin-engine bomber; 1917. [ukn via Martin Copp]

... located in an open cockpit at the nose of the aircraft; this man also served as the bombardier. Behind the front gunner and just ahead of the upper wing was the pilot's cockpit. His flight controls consisted of the usual rudder bar and stick, but a "steering wheel," like that in an automobile, was mounted at the top of the stick and was used for deflecting the ailerons. The use of a full wheel, rather than a yoke as in modern aircraft, suggests that several revolutions of the wheel were required to move the ailerons through their full range of deflection. Aircraft response to control inputs must have been sluggish, and the piloting job must have seemed something like a wrestling match. The third crew member was another gunner located in an open cockpit behind the upper wing. His flexible machine gun could be utilized effectively in various quadrants above and to the sides of the aircraft and could also be fired downward and rearward through a sort of inclined tunnel that passed through the inside of the fuselage and opened on the bottom. The rear gunner could accordingly fire, through a limited angular range, at an aircraft attacking from below and to the rear. This feature proved to be a startling and unwelcome discovery to a number of unsuspecting Allied pilots.

The performance of the 8558-pound gross weight Gotha was not spectacular, as can be seen from the data in table I for the slightly [50] improved Gotha G.V. The maximum speed was only 87 miles per hour, which suggests a cruising speed at 75-percent power of about 78 miles per hour. This cruising speed, coupled with an estimated stalling speed of 56 miles per hour gave the pilot a very narrow speed corridor in which to fly and maneuver the aircraft. The maximum lift-drag ratio of 7.7 seems reasonably high for an aircraft festooned with so many struts, wires, wheels, and other protuberances. The usual load of the Gotha on a London raid consisted of six 110-pound bombs carried externally.

The reference sources indicate that more Gothas were lost in flying accidents than in combat with the enemy. Sluggish response to control inputs together with its narrow speed corridor may have contributed to the high accident rate. Many accidents occurred in landing. The fuselage was reportedly weak, probably because of the gun tunnel, and frequently broke in half on a hard landing.

All in all, the Gotha does not seem to have been the superb aircraft that its fearsome reputation would suggest. The reality, as with so many other aircraft, does not live up to the legend.

Handley Page 0/400

Like the Gotha G.IV, the Handley Page 0/400 illustrated in figure 2.22 was a multibay biplane equipped with two engines mounted between the wings and with a four-wheel main landing gear; two wheels were mounted below the lower wing at the location of each of the engine nacelles. The appearance of the British Handley Page bomber, however, was startlingly different from that of the German Gotha. The large gap between the wings, marked wing dihedral angle, and large span of the upper wing as compared with the lower are distinctive features in the appearance of the aircraft. Also in marked contrast to the pusher engine arrangement of the Gotha, the 0/400 employed a tractor configuration. Another distinctive feature, not evident in the photograph, is the tail assembly, which consisted of two horizontal surfaces arranged in a biplane configuration. A single fixed fin, centrally located between the two horizontal surfaces, and two all-moving rudders, also located between the horizontal surfaces but positioned near the tips, comprised the vertical tail surfaces. Horn-balanced ailerons and elevators were utilized to reduce control forces.

The wings folded rearward, just outboard of the engines, to a position parallel to the fuselage. This complication was dictated by a requirement that the aircraft fit into a standard-size Royal Air Force....




[51] Figure 2.22 - British Handley Page 0/400 twin-engine bomber; 1916-17. [USAF via Martin Copp]

....hangar. Apparently, the authorities responsible for aircraft procurement thought it more cost effective to complicate and perhaps compromise the aircraft than to build new hangars.
The crew of the Handley Page 0/400 usually consisted of four men. A gunner-bombardier, located in the nose of the aircraft, had two flexible machine guns. The two pilots were in an open cockpit behind the front gunner and just ahead of the upper wing; each pilot had a complete set of flight controls. The necessity for two pilots is suggested by the 9-hour flight maximum duration of the aircraft. The second gunner was located in a cockpit behind the upper wing and, as in the case of the front gunner, was provided with two flexible machine guns. In addition, a single flexible machine gun was mounted on the floor inside the fuselage and could be fired downward and rearward through a small trap door in the bottom of the fuselage. Apparently, the single rear gunner was expected to alternate between this gun and the two top-mounted guns, depending upon the position of the attacker. The frustration the single rear gunner must have felt in the event of a simultaneous attack from above and below can readily be imagined. [52] Surely, a second rear gunner must have been carried on missions in which aggressive attack by many enemy aircraft was anticipated.

The gross weight of the Handley Page was 14 425 pounds (table I), nearly 6000 pounds heavier than the Gotha, and the wing area was 1655 square feet as compared with 963 square feet for the German bomber. The maximum lift-drag ratio of the 0/400 was a very impressive 9.7, which was a full 26 percent higher than that of the Gotha. The Handley Page also had the higher top speed of the two aircraft. The 0/400 was large enough and had sufficient fuel capacity to deliver a 2000-pound bomb load on a target located 300 miles from home base and return safety. The bombs themselves were carried inside the fuselage in a vertical position ready for release. The Handley Page 0/400 seems to have been an outstanding aircraft for its time and, in most respects, superior to the Gotha except for its service ceiling of 8500 feet, which was less than half that attributed to the Gotha.

The size and certain other characteristics of the Handley Page 0/400 can be put in perspective by comparison with more modern aircraft. The wing loading and power loading of 8.7 and 20.5 are fairly close to the corresponding values of 6.9 and 18 for the famous Piper J-3 Cub (chapter 4), and the values of the maximum lift-drag ratio of the two aircraft are nearly the same. Thus, in a sense, the 0/400 can be likened to a 14 000-pound Cub, although the response to control inputs and the control forces required of the pilot must be considered as utterly different for the two aircraft. Cecil Lewis in reference 85 suggests the handling characteristics of the aircraft in the following quotation: "True, it was like a lorry in the air. When you decided to turn left, you pushed over the controls, went and had a cup of tea and came back to find the turn just starting.

Another interesting comparison of the Handley Page can be with the modern-day Boeing 727-200 jet airliner (chapter 13). The wing areas of the two aircraft are almost the same, but the 727 is nearly 15 times as heavy as the Handley Page, is about 7 times as fast, and has a value of the maximum lift-drag ratio more than twice that of the 0/400. All these changes occurred in a time span of a little less than 50 years.

The first Handley Page bomber was flown in 1915, and the 0/400 version appeared in 1916. About 800 Handley Page bombers of all types were built during the war. The model 0/400 continued in military service for several years after the war, and several were converted for use as civil transports. The 0/400 was scheduled for large-scale production in the United States for use by the American Expeditionary [53] Force in France. By the time hostilities ceased in November 1918, only 107 examples had been completed and all production contracts were soon terminated.

The principal legacy of the Gotha and Handley Page heavy bombers was the twin-engine, strut-and-wire-braced, open-cockpit biplane configuration that dominated bomber development for many years following the end of World War I. Various models of the Keystone bomber were employed by the U.S. Army Air Corps until the mid-1930's. These aircraft incorporated the same configuration concepts as the Gotha and Handley Page, with fewer struts and wires, more powerful engines, better structures, and marginally better performance.

Caproni CA.42

The name Caproni is an honored one in the annals of World War I aviation. The Italian firm bearing that name, along with Sikorsky in Russia, first flew heavy multiengine bombers in the year 1913, and Caproni bombers were used throughout World War I, not only by Italy but by England and France as well. Production of one version of a Caproni bomber was also planned in the United States but had not materialized at war's end.

All Caproni bombers had three engines. Two of these were mounted in a tractor arrangement, with one engine at the nose of each of two fuselagelike booms that connected the wings and tail assembly. The third engine was a pusher installed in the rear of a nacelle situated between the wings. Pilot and gunner-bombardier were in cockpits ahead of the pusher engine. The rear gunner(s) was located in several different positions in the various Caproni bomber. designs. Both biplane and triplane bombers were built by Caproni, with the number of biplanes produced far outnumbering the triplanes. About 200 Caproni bombers of all types were manufactured, of which about 30 were triplanes. In Italian service, these aircraft were extensively used for bombing targets in the Austro-Hungarian empire. Such raids originated in Italy and required round-trip flights across the Alps. Good high-altitude performance was accordingly an important design requirement.

Although production of Caproni biplanes far outnumbered the triplanes, the model CA.42 triplane bomber was selected for inclusion here because it represents an interesting application of the triplane formula to a very large aircraft. Some of the reasons for selecting a triplane configuration were given in the previous section describing the [54] Fokker Dr.-1 triplane fighter. For a very large airplane in which the physical dimensions are limited, perhaps by hanger size or tiedown area on the airfield, the triplane arrangement offers a higher effective aspect ratio for a given wing span and area than does a biplane. The triplane arrangement of the CA.42 probably derives from this argument since the aircraft had a very large wing area.

The Caprom CA.42 may be seen in figure 2.23 and offers a unique, if somewhat grotesque, appearance. The three wings were connected and braced by a veritable forest of struts and wires. A front view of the aircraft shows that the interplane struts were configured in a five-bay arrangement. The center nacelle containing the pusher engine, pilot, and forward gunner was attached to the undersurface of the center wing. The tips of the pusher propeller can be seen above and below the left fuselage-boom. A rear gunner was positioned in each fuselage-boom immediately behind the center wing. The boxlike pod on the lower wing housed the bombs. The main landing gear consisted of....



Figure 2.23 - Italian Caproni CA. 42 three-engine triplane bomber; 1917. [Stephen J. Hudek via Martin Copp]

[55] ....eight wheels in two clusters of four each, skids were located under each wing tip, and rather tall tail skids were at the rear. The large number of wheels was intended to distribute the weight of the aircraft on the ground and thus prevent the aircraft from becoming mired in the relatively soft turf airfields in use at that time. Three rudders were mounted on a single horizontal tall; a later version of the aircraft had a biplane horizontal-tail configuration. Ailerons were employed on all three wings.

A number of sources were consulted in assembling the data given in table I. Although the various sources were in essential agreement on the dimensions of the CA.42, discrepancies were found in the weight and performance data. Engines of different power were employed on a number of the production aircraft and may account for the confusion in the data. The specifications in table I were taken from reference 1 and are for the aircraft equipped with U.S.-built Liberty engines of 400 horsepower each. At 17 700 pounds gross weight, the CA.42 is the heaviest of the aircraft considered, and the maximum speed of 98 miles per hour is higher than that of either the Gotha or the Handley Page 0/400. The lower wing loading and more favorable ratio of power to weight, as compared with the other two bombers, probably gave it a good high-altitude capability for transalpine flying. One reference gives a flight duration of 7 hours and a maximum bomb load of 3200 pounds. Assuming a cruising speed of 89 miles per hour at 75-percent power, the CA.42 had an estimated range of about 600 miles, or the ability to deliver its bombs on a target 300 miles from home base and return safely.

The Caproni CA.42 seems to have had a very creditable performance when equipped with Liberty engines; with lower power engines the performance was not nearly so good. Perhaps the appearance of the Liberty engine relatively late in the war contributed to the small number of aircraft built. In one reference the aircraft was stated to be difficult to fly, but no specific details are given.

At least three CA.42 triplane bombers were sent to the United States for evaluation. One of these was to have been tested at Langley Field, Virginia, but was completely destroyed in a crash at Langley on its maiden flight in December 1917.

With this brief glimpse of the heavy bomber in World War I, attention is now focused on the two-seat army cooperation and light bomber types that constituted the workhorse aircraft of that era.