Quest for Performance: The Evolution of Modern Aircraft
 
 
Part I: THE AGE OF PROPELLERS
 
 
Chapter 8: Boats in the Sky
 
 
Early Military Flying Boats, 1914-20
 
 
 
[172] The years of World War I provided the stimulus and proving ground for the development of the flying boat into a useful and versatile class of aircraft. It was employed primarily for reconnaissance, patrol, and bombing operations. Each of the warring nations developed and operated military flying boats. Three pace-setting Curtiss patrol boats designed in the United States, or derived from Curtiss designs, are discussed in the next section, after which the first aircraft to cross the Atlantic Ocean is described.
 
Three Curtiss Patrol Boats
 
The world's first flying boat was designed, built, and flown by the American aviation pioneer Glenn H. Curtiss in 1912. The small single-engine biplane that was the centerpiece of this historic event was the progenitor of an entire family of single and multiengine flying boats that served with United States and British forces during World War I and made the name Curtiss almost synonymous with flying boat during that world conflict. Typical of the Curtiss flying boats developed during this period were the twin-engine H-16 and the HS-2L, a scaled-down single-engine version of the H-16. The two aircraft are shown in figures 8.4 and 8.5. After the American-developed Liberty engine (ref. 45) became available, both boats were equipped with this 400-horsepower power plant. A multibay, strut-and-wire-braced biplane configuration with the engine(s) mounted between the wings was employed for both [173] aircraft. Whereas landplanes employing the biplane configuration usually had the lower wing attached near the bottom of the fuselage, the H-16 and the HS-2L had the entire wing assembly mounted atop the hull. The horizontal tail was mounted high on the vertical fin located at the rear of the fuselage extension above the hull. A single step together with a shallow vee bottom and wide beam characterized the hull itself. The lateral stabilizing floats can be clearly seen beneath the tips of the lower wing in figures 8.4 and 8.5. The finlike surfaces on top of the upper wing near the tips assisted in providing the necessary structural support for the large overhang of the wingtips. Barely visible in the photograph are the bracing wires that extended both inboard and outboard from the top of this surface to the wing. Wires also extended downward and inboard from near the upper wingtips to the juncture of the lower wing and the outboard interplane struts. A somewhat similar type of bracing arrangement was employed on the Handley Page 0/400 bomber shown in figure 2.22 and the Curtiss JN-4H Jenny shown in figure 3.1. The fabric covering was intended to reduce the drag of the struts projecting above the wing.
 
 
ground view of a Curtiss H-16
 
Figure 8.4 - Curtiss H-16 twin-engine flying boat; World War I era. [USN via
Martin Copp]
 
 
hangar view of a Curtiss HS-2L
 
[174] Figure 8.5 - Curtiss HS-2L single-engine flying boat; World War I era. [mfr via Martin Copp]
 
The wings and tail surfaces of the H-16 and HS-2L were of conventional (for that time period) wood-frame construction covered with fabric. The hull construction consisted of a laminated wood-veneer covering attached to a wood frame. An undesirable characteristic of wooden-hull flying boats was a tendency to absorb a certain amount of water over a period of time. To some degree then, the aircraft empty weight was a variable and known only within approximate limits. Accordingly, conservatism had to be exercised in estimating allowable fuel and payload weights to avoid a risk of operating in an overweight condition.
 
The statement is frequently made that no United States-designed aircraft served in France in World War I (see chapter 2); this certainly applies to fighters, bombers, and army cooperation aircraft used in operations over the Western front. American naval forces, however, operated both the H-16 and the HS-2L over the coastal waters of France, with the first operational flight of the HS-2L occurring on June 13, 1918. After the cessation of hostilities, both of these long-lived aircraft continued in operation with the United States Navy until the late 1920's. Some of them, declared surplus by the Navy, were sold to civil [175] operators who employed them in short-lived airline operations. Some were also used as rum-runners during the prohibition era in the 1920's.
 
The British Royal Navy operated several types of Curtiss flying boats long before the United States entered the war but were not entirely satisfied with their hydrodynamic performance. New aircraft utilizing Curtiss-designed wings and tail surfaces, but incorporating an improved two-step hull that was more suited to the rough waters of the North Sea and gave a reduced takeoff distance, were accordingly developed. The work was carried out at the Felixstowe Naval Air Station, and the resulting flying boats were designated F-1 to F-5 depending on the Curtiss boat from which they were derived. These aircraft were highly successful, so much so that the United States Navy had over 200 of the F-5 version, derived from the H-16, built under contract in this country where they were given the designation F-5L. Those built here were equipped with the Liberty engine, which accounts for the "L" in the designation. Many of these aircraft were constructed by the Curtiss company and, since they had Curtiss - designed wings and tail, were usually referred to as Curtiss F-51, flying boats although that designation is not entirely accurate. The F-5L arrived on the scene too late to see action with United States forces during World War I, but in the postwar years it was a mainstay with Navy patrol squadrons until finally withdrawn from service in 1928.
 
The inflight photograph of an F-5L flying boat (fig. 8.6) clearly shows the configuration of this classic, triple-bay biplane aircraft. The two pilots sitting side by side, as well as the two men in the front gunners' cockpit, are clearly visible. The cockpit for the rear gunner, located behind the wings, is partially obscured in the photograph. A crew of three or four normally manned the aircraft. Armament consisted of several flexibly mounted machine guns together with a bomb load of up to four 230-pound bombs.
 
The wings of the F-5L are obviously similar to those of the H-16 shown in figure 8.4. In early versions of the F-5L the tail assembly was also the same as that of the H-16; but as shown in figure 8.6, an enlarged tail equipped with horn-balanced elevators and rudder was fitted to the F-5L. The modified tail was incorporated in all F-5L boats in the early 1920's. Noteworthy in figure 8.6 are the exposed cylinders and crankcases of the engines and the large automobile-type radiators positioned above the propellers. The flared sides of the broad-beamed hull are called sponsons (not to be confused with the winglike lateral stabilizing surfaces also called sponsons, which are discussed later).
 
 
aerial view of a Curtiss F-5L
 
[176] Figure 8.6 - Curtiss F-5L twin-engine flying boat; World War I era. [USN via Martin Copp]
 
The physical and performance characteristics of the H-16, HS-2L, and the F-5L, flying boats are given in table IV. That they were relatively large, heavy, and low-performance aircraft is indicated by the data presented. To put these characteristics in the proper context with respect to multiengine landplanes of that same time period, the following comparative data are given for the Curtiss F-5L flying boat and the Handley Page 0/400 twin-engine bomber (for which data are given in table I):
 
 

Aircraft

Wg

b

S

Vmax

CD,0

(L/D)max

Handley Page 0/400

14 425

100.0

1 655

94

0.0427

9.7

Curtiss F-5L

13 600

103.8

1 397

90

0.0694

8.2

 
The data show the two aircraft are roughly the same size and weight, but the landplane is about 5 percent faster than the flying boat. In comparing the aerodynamic characteristics of the two aircraft, the zerolift drag coefficient is about 60 percent higher and the maximum [177] lift drag ratio is about 18 percent lower for the Curtiss F-5L flying boat than for the Handley Page 0/400 landplane. The large, low length-beam ratio, two-step hull of the flying boat no doubt contributed to its degraded aerodynamic performance, a price exacted for the operational versatility of the flying boat.
 
The configuration arrangement of the three Curtiss aircraft just described set a style in patrol-boat design that continued in the United States for many years.
 
The NC Boats, First Across the Atlantic
 
The NC boats are discussed here not so much for their advanced design features but rather because one of their number, the NC-4, was the first aircraft to fly across the Atlantic Ocean and thus secured for the type a permanent place in the annals of aviation history. The origins of the NC flying boat can be traced to a request issued by the British in 1917 for a long-range patrol aircraft for antisubmarine operations over the open ocean. In addition, it was desired that the aircraft be capable of flying from the United States to the British Isles, thus shortening delivery time and saving much-needed cargo space on available surface shipping. An aircraft designed to meet these specifications emerged from a collaboration between engineers of the United States Navy and the Curtiss company -hence the NC designation. The first of these flying boats, NC-1, was flown in the late fall of 1918, and the other three, NC-2, NC-3, and NC-4, were completed in the spring of 1919. These aircraft were built by the Curtiss company under the supervision of Glenn H. Curtiss himself, subsequently, six more NC boats were constructed by the Naval Aircraft Factory located in Philadelphia, Pennsylvania. Since World War I ended in November 1918, none of the NC boats were completed and tested in time to fill the wartime role for which they had been designed.
 
The United States Navy, however, organized a mission to fly across the Atlantic Ocean, a feat not yet accomplished, with the use of NC-1, NC-3, and NC-4. Bound for Plymouth, England, the three aircraft left Trepassy Bay, Newfoundland, on May 16, 1919, with intended refueling stops in the Azores and Portugal. The mission was under the overall command of Commander John H. Towers, who also served as commander of NC-3. The three aircraft soon became separated after leaving Newfoundland; and because of uncertainties as to their position and deteriorating weather conditions, NC-1 and NC-3 landed in the [178] ocean, short of the Azores, with the hope locating the position with radio equipment and subsequently continuing the flight. Although the crew was rescued, NC-1 capsized and sank because of heavy seas; NC-3, unable to take off again, taxied 200 miles to the Azores but was too badly damaged to continue the flight. Arriving at Plymouth, England, on May 31, 1919, NC-4 successfully completed the mission and thus became the first aircraft to fly across the Atlantic Ocean. The six-man crew for the flight was commanded by Lt. Albert C. Reed, who also served as navigator, and included two pilots, a radio operator, and two flight engineers. The chief pilot for the flight was Lt. (jg.) Walter Hinton. An interesting account of this historic flight is contained in reference 80.
 
The Navy-Curtiss NC-4 flying boat is shown in figure 8.7. The configuration of the aircraft featured a typical Curtiss three-bay, strutand-wire-braced biplane wing arrangement mounted on top of a short, wide-beam, single-step hull designed by Navy engineers. The tail assembly was supported by wire-braced outriggers extending back from the top wing and the stern of the hull. This arrangement was chosen to minimize hull weight, as compared with designs like the F-5L, to place the tail high above the waves in rough seas, and to provide the rear gunner with a wide field of fire. The horizontal tail consisted of a biplane arrangement with three rudders mounted between the horizontal stabilizing surfaces. Fins were fixed ahead of the outboard rudders. Power was supplied by four of the ubiquitous, American-designed, 400-horsepower Liberty engines, located in three nacelles. One engine was carried in each of the outboard nacelles, and two engines in a tractor-...
 
 
Curtiss NC-4 afloat
 
Figure 8.7 - Four-engine Navy-Curtiss NC-4 was first across the Atlantic Ocean; 1919. [ukn via Martin Copp]
 

[179] ....-pusher arrangement were mounted in the center one. The NC-1 was initially flown with only the three tractor engines but was found to be so underpowered that the fourth pusher engine was installed.

 
As described in connection with the H-16, the wings and tail surfaces of the NC boats were of wood-frame construction covered with fabric. The wooden frame of the short, broad-beam hull was covered on the sides and bottom with two layers of planking that were glued together with a sheet of canvas in between and had a three-ply wood veneer turtle deck. The outriggers supporting the tail were of wooden box beam construction.
 
A glance at the physical characteristics of the NC-4 given in table IV shows that it was indeed a large aircraft. At a gross weight of 27 386 pounds, it was about twice as heavy as the F-5L and had a wing span of 126 feet as compared with 103.8 feet for the F-5L. The wing area of 2380 square feet was 70 percent greater than that of the F-5L and was only about 18 percent less than that of the modern Boeing 707 jet transport (chapter 13). The performance data show a maximum speed of only 85 miles per hour and estimated values of the cruising and stalling speeds of 77 and 67 miles per hour, respectively. Accordingly, the aircraft had to be carefully flown and maneuvered within the narrow speed range available to it. At 0.0899, the zero-lift drag coefficient of the NC-4 was the highest of any of the aircraft for which data are given in table IV, and the value of 7.0 was the lowest of any of the maximum lift-drag ratios shown. The maximum range of the aircraft is given in reference 109 as 1470 miles; with such a low value of maximum lift-drag ratio, this range could only be possible with a large aircraft having a relatively low empty weight as compared with gross weight. In spite of its shortcomings in aerodynamic efficiency, the NC-4 fulfilled these weight requirements and was able to make the Atlantic crossing for which it was designed.
 
Shortly after completion of its historic flight in 1919, the NC-4 was presented to the Smithsonian Institution, which completely restored the aircraft for the 50th anniversary of the famous flight in 1969. Today, the NC-4 may be seen at the United States Naval Air Museum located at the Naval Station in Pensacola, Florida.
 
 
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