ch2-4

Quest for Performance: The Evolution of Modern Aircraft


Part I: THE AGE OF PROPELLERS


Chapter 2: Design Exploration, 1914-18


Army Cooperation Aircraft



[56] The unglamorous two-seat aircraft, working in cooperation with army ground forces, formed the backbone of aerial activity in World War I and undoubtedly contributed more to military successes than any other class of aircraft. One of the primary functions of the much-heralded single-seat fighters was the protection of their own two-seaters and the destruction of those belonging to the enemy. Army cooperation aircraft performed a variety of diverse duties including photoreconnaissance, artillery spotting, observation of enemy troop movements, ground strafing, and daylight tactical bombing. Duties such as photoreconnaissance required steady and precise flying at a given altitude and along prescribed flight tracks if the photographs necessary for accurate mapmaking were to be obtained. All the while, the crews had to be constantly on the lookout for enemy air attack, and the steady flight path over enemy territory offered the antiaircraft gunners excellent opportunities for target practice. Certainly the men who flew these aircraft are among the unsung heroes of the First Great War.

The two-seater, as it evolved during the war, had the pilot in the front cockpit with one or two fixed, synchronized machine guns firing between the propeller blades; the observer was in the rear cockpit with one or two flexibly mounted machine guns in addition to the camera, wireless, or other special equipment. A steady platform was required for photoreconnaissance and bomb aiming, which meant that the two-seater had to be relatively stable; yet a certain amount of speed and maneuverability were required to avoid destruction by the enemy. Good high-altitude performance was another desirable characteristic. The correct mix of these sometimes conflicting requirements and the technical means for accomplishing that mix presented difficult design problems. In the early years of the war, two-seaters were often considered to be easy prey for fighter aircraft; but as designs improved, they gave an increasingly good account of themselves in combat operations.

The development of the two-seater presents little of technical interest beyond what has already been discussed in the preceding sections on fighters and bombers. A large number of two-seat types were developed during the war, and a number of configuration concepts, including monoplanes, biplanes, triplanes, and quadruplanes were investigated. As in the case of the fighters and bombers, however, the biplane emerged as the best compromise, consistent with the existing state of technology, between the various conflicting requirements. Three two-seat biplanes, the British B.E.2c, the German Junkers J-I, and the British DeHavilland DH-4 are described next.

[57] B.E.2c

It would be difficult to conceive of an aircraft so poorly adapted to the rigors of aerial combat as the long-lived series of British B.E.2 two-seaters designed by the government-controlled Royal Aircraft Factory. The prototype first flew in 1912, and a B.E.2a was the first British aircraft to land in France, on August 13, 1914, after the beginning of the war. The B.E.2c and other models of the B.E. series remained in production until July 1917. More than 3500 B.E.2-type aircraft were constructed, and, unbelievably, a single-seat fighter version, the B.E.12, was also produced. The British ace Albert Ball referred to this machine in the following succinct terms, ". . . a bloody awful aeroplane.

The B.E.2 was developed on the premise that inherent stability in an aircraft was a highly desirable characteristic that would contribute to flying ease and flight safety. Further, the military thinking in 1914 envisioned the use of the airplane in warfare solely as an instrument for supporting the army ground troops. Again, inherent stability seemed a desirable characteristic for such duties as reconnaissance and artillery spotting. Unfortunately, experience early in the war showed that a two-seater required speed, maneuverability, and a good rate of climb to survive. The B.E.2c had none of these characteristics, yet production of the aircraft continued; it was callously referred to as "cold meat" by German fighter pilots.

As shown in figure 2.24, the B.E.2c was a strut-and-wire-braced, double-bay biplane equipped with an in-line engine swinging a four-blade propeller. The 90-horsepower R.A.F. la engine itself was somewhat unusual in that it was air cooled. Ailerons were incorporated in both upper and lower wings, and the horizontal and vertical tail units had both fixed and movable surfaces. The large dihedral angle evident in the wings was dictated by the requirement for inherent lateral-directional stability. Unlike most two-seaters, the pilot sat in the rear cockpit and the gunner was in the front cockpit. Although the B.E.2c was equipped with a single machine gun, the field of fire between the wings and over the pilot's head and vertical tail limited the gunner's effectiveness. Because of the position of the lower wing relative to the gunner, the pilot had to operate the camera on photoreconnaissance missions in addition to flying the aircraft.

The data in table I show the 2142-pound gross weight of the B.E.2c to have had a disastrously low maximum speed of 72 miles per hour at 6500 feet. Not shown in the table is the climb-performance data that indicate that 45 minutes were required to climb to the low service ceiling of 10 000 feet. The zero-lift drag coefficient, drag area,...





[58] Figure 2.24 - British B.E.2c army cooperation aircraft; 1914. [ukn via Martin Copp]

... and maximum lift-drag ratio were comparable to many contemporary aircraft of the time; however, the ratios of power to weight and power to drag area were so low that only mediocre performance could be expected. In addition to its performance limitations, all reference sources indicate that the aircraft lacked maneuverability.
The shortcomings of the B.E.2c in armament, performance, and maneuverability resulted in a very poor front-line aircraft that was almost defenseless against determined enemy air attack. Untold numbers of British airmen perished in this monument to bureaucratic inertia and ignorance. The B.E.2c is presented here, not as an example of a good aircraft or, one having significant technical innovations, but as illustration of how an ineffective aircraft can be produced and fostered on the user long after it is obsolete. Similar examples can be found in the course of aeronautical history.

Junkers J-I

A bewildering variety of two-seat army cooperation aircraft were designed, developed, and operated by the Germans in World War 1. Albatros, AEG, Roland, DFW, Halberstadt, AGO, Aviatik, LVG, Junkers, and Rumpler are only a few of the companies that produced [59] army cooperation aircraft during the conflict. Some of these aircraft were designed for general-purpose reconnaissance duties, others for night bombing, and still others for the ground attack role in close cooperation with friendly ground troops. An interesting aircraft in this latter category, the Junkers J-I, is described here and is shown in figure 2.25.

The J-I biplane had a rather unusual appearance with thick, cantilever wings that were tapered in both planform and thickness ratio. Three-view drawings show that the aircraft was really a sesquiplane, with the bottom wing much smaller in span and chord than the upper wing. The small-chord lower wing, together with its position below the lower surface of the fuselage, afforded good downward visibility for the pilot in the front cockpit and the observer in the rear. The wings were connected to each other and to the fuselage by a rather complex cabane-strut arrangement. No interplane struts were used between the wings. 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 ...





Figure 2.25 - German Junkers J-I all-metal army cooperation aircraft; 1918. [Peter M. Bowers via AAHS]



[60] ....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. Power was provided by a six-cylinder, water-cooled, Benz Bz.IV engine of 200 horsepower. The aircraft was usually armed with two fixed, synchronized machine guns firing between the propeller blades and with a single flexible gun for use by the observer. Two downward-firing guns were sometimes installed for the observer, but the difficulty of aiming these guns from a low, fast-flying aircraft rendered them ineffective, and they were quickly removed. A radio link connecting the aircraft with friendly ground troops in the forward area was also generally provided.

The physical and performance data given in table I indicate that the J-I was a remarkable aircraft in many respects. The gross weight of 4748 pounds seems large for an aircraft of only 200 horsepower, and the useful load fraction of 0.19 is very low compared with the values of 0.30 to 0.35 shown in figure 2.20 for fighter aircraft. A low structural efficiency is accordingly suggested; however, the 0.20-inch steel shell of armor alone weighted 1036 pounds, according to reference 119, and no doubt contributed in large measure to the low apparent structural efficiency. The power loading of 23.9 pounds per horsepower is about the same as that of the B.E.2c and suggests a powered glider more than a fighting aircraft. The J-I, however, had a maximum speed of 96 miles per hour, could climb to 6560 feet in 30 minutes, and had an endurance of 2 hours, a very creditable performance for an aircraft of relatively low power. The good performance of the aircraft was due in large part to the low value of the zero-lift drag coefficient of 0.0335 and the high value of the maximum lift-drag ratio of 10.3. The J-I was among the most aerodynamically efficient of the World War I aircraft analyzed here.

The J-I proved in action to be a very effective weapon in ground-attack role for which it was designed. The prototype first flew in January 1917, but due to production difficulties the aircraft was not deployed in action until February 1918. Total production run was 227 aircraft. The Junkers J-1 incorporated many advanced engineering features and was a truly remarkable aircraft. It has not received proper recognition in the literature of World War I aviation.

DeHavilland DH-4

Although the DeHavilland DH-4 was an ordinary looking, strut-and-wire-braced, double-bay biplane, it occupies a unique niche in [61] aviation history as the only aircraft manufactured in the United States to serve in combat on the Western front in World War I. A total of 4846 DH-4 aircraft were built (under license) in the United States, and about 1600 of these reached France. They were all powered with the U.S.-designed and built 12-cylinder Liberty engine of 400 horsepower. The interesting story of the development of this outstanding engine is described in reference 45.

Two views of the DHA are shown in figures 2.26 and 2.27. The legend on the side of the aircraft pictured in figure 2.26 indicates that it was number 1000 off the United States production line and that it would leave (by ship) at 4:30 p.m., July 31, 1918. The DH-4 shown in figure 2.27 was the "pattern aircraft" sent from England to the United States in the summer of 1917 for use in developing production drawings for use by U.S. manufacturers¹. The photograph was made in the ....




Figure 2.26 - American-built (British-designed) DeHavilland DH-4 army cooperation aircraft; 1918. [Warren Bodie via AAHS]





[62] Figure 2.27 - National Air and Space Museum DH-4 on loan to the NASA Langley Research Center in 1967. Dr. Floyd L. Thompson, former Langley director, is shown with the aircraft. [NASA]


....fall of 1967 when this historic aircraft, on loan from the National Air and Space Museum, was exhibited at the Langley Research Center of the National Aeronautics and Space Administration on the occasion of its 50th anniversary. The gentleman in the photograph is Dr. Floyd L. Thompson, former director and longtime research leader at the Langley center.

The DH-4 was designed as a day bomber and general-purpose reconnaissance aircraft by Geoffery DeHavilland for the Aircraft Manufacturing Company (AIRCO). It first flew in August 1916, was deployed in March 1917, and subsequently served on all British fronts. DH-4's built in Britain were powered with a variety of engines, including the well-known Rolls-Royce Eagle powerplants of 250 horsepower. Only 1440 DH-4 aircraft were built in England.

[63] Figures 2.26 and 2.27 depict a very conventional-appearing biplane. Both photographs clearly show the maze of wires required to support the typically thin wings against flight and ground loads and to hold the wings in proper alignment. The aircraft had a conventional wood-frame structure covered with fabric, except for the forward part of the fuselage which was sheathed in plywood. An unusual feature of the aircraft was the large distance separating the pilot and the observer. The internal volume between the two cockpits was occupied by a large fuel tank. Communication between the pilot located under the top wing and the aft-placed observer was difficult, and the tank between the crew members was rumored to have a propensity for catching fire in an accident or when hit by enemy gunfire. As a consequence, the aircraft was sometimes unflatteringly referred to by crew members as the "Flaming Coffin."

The flight controls, which included ailerons on both upper and lower wings, were entirely conventional with the exception of the fixed portion of the horizontal tail, which could be adjusted in flight with a trim wheel located in the cockpit. The aircraft could accordingly be trimmed for zero longitudinal stick force as speed, weight, and altitude varied during the course of a flight. All modern aircraft have pitch trim capability, but this highly desirable feature was seldom found in World War I aircraft. Another unusual feature in the DH-4 was the tail skid that could be steered with the rudder bar; ground maneuverability was much enhanced by this feature. According to reference 28, the aircraft had light control forces and adequate stability and was easy to fly and land. Armament varied but usually consisted of two fixed, forwardfiring machine guns operated by the pilot and two flexible guns for use by the observer. On daylight bombing raids, 10 small bombs were mounted beneath the lower wing, 5 on either side of the fuselage; these bombs are visible in figure 2.27.

The data in table I for the DH-4 are for the Liberty-powered, American-built version of the aircraft. It was a relatively heavy machine with a gross weight of 4595 pounds, but the 400-horsepower engine gave it ratios of power to weight and power to drag area that were nearly the same as those for the Fokker D-VII fighter; the values of the maximum lift-drag ratio of the two aircraft were also nearly the same. Fighterlike performance might therefore be expected of the DH-4, and the maximum speed of 124 miles per hour certainly confirms this expectation. The rather high, for its day, stalling speed resulted from the 10.4-pound-per-square-foot wing loading in combination with the low [64] maximum lift coefficient of the thin wings. The aircraft had a service ceiling of 19 600 feet and could climb to 10 000 feet in 1.4 minutes.

Termination of hostilities in November 1918 resulted in cancellation of contracts in the United States for an additional 5160 aircraft. The end of the war, however, did not spell the end of the career of the DH-4 in the United States, as is seen in chapter 3.

¹ According to a recent publication, this aircraft has been identified by personnel of the National Air and Space Museum as the first DH-4 manufactured in the United States and is not, as was previously thought, a British-built pattern aircraft. See Boyne, Walter J.: The Aircraft Treasures of Silver Hill (New York: Rawson Associates).