Quest for Performance: The Evolution of Modern Aircraft
 
 
Part I: THE AGE OF PROPELLERS
 
 
Chapter 5: Design Refinement, 1939-45
 
 
Examples of World War II Aircraft
 
 
 
[120] Aircraft employed in World War II were usually designed to fill mission requirements in one or more of the following broad categories:
 
(1) Heavy bombers
(2) Attack and light bombers
(3) Fighters and interceptors
(4) Patrol and reconnaissance
(5) Transport and utility
(6) Training
 
Many aircraft specifically designed for use in one of these categories were later found to be useful in other categories with only minor modifications. There is no feasible way of describing all the outstanding World War II aircraft in such a short account as this one. A number of the books listed in the references at the end of this volume contain excellent detailed descriptions of the various aircraft used by the different warring powers during World War II. For those particularly interested in United States combat aircraft, reference 118 is highly recommended. Fighters and bombers of World War II are described in great detail in references 58 to 63. Combat aircraft of all the nations that saw operational service are described in reference 112.
 
A few examples of much-used United States bomber and fighter aircraft are illustrated and described below. These aircraft are representative of a vast array of very good aircraft produced by both Allied and Axis countries during World War II. The aircraft of no one country held a clear and continuing technical advantage over those of another country for very long. United States, British, and German aircraft were usually of about the same state of the art from a technological viewpoint. Detailed refinements discussed in the preceding paragraphs frequently spelled the difference between success and failure in combat operations. Essentially, all combat aircraft utilized in the World War II [121] period were, as previously described, designed to the same cantilever monoplane formula with retractable landing gear, variable-pitch propeller, and metal construction.
 
Bomber Aircraft
 
The bomber aircraft discussed here fall into the following categories: heavy, very heavy, and medium multiengine bombers, and single-engine Navy scout bombers. Discussed first are the heavy bombers.
 
Two outstanding heavy bombers that served with the U.S. Army Air Force were the Boeing B-17 and the Consolidated B-24. The two types are best remembered as the aircraft that carried out the United States strategic bombing offensive against Germany. The Boeing B-17 Flying Fortress, which first flew in prototype form in 1935, is described in chapter 4 and illustrated in figure 4.13. The B-24 Liberator, designed several years later than the B-17, was first flown as a prototype in December 1939, and the first production aircraft was delivered in 1941. The B-24, a four-engine, 56 000-pound bomber, had roughly the same gross weight and was designed for the same mission as the B-17 but differed radically in design concept and appearance from the Boeing aircraft. The B-24 bomber is illustrated in figure 5.7, and the characteristics of a B-24J are given in table III (appendix A). The most distinguishing features of the B-24 as contrasted with the B-17 were the high-aspect-ratio wing mounted atop the fuselage, the tricycle landing gear, and the two fins and rudders. The wing of the B-24 had a very high aspect ratio of 11.55 and employed the much-publicized Davis airfoil section that, according to the popular aviation literature of the day, was supposed to provide the aircraft with unusually efficient aerodynamic characteristics. Later, wind-tunnel tests showed that while the Davis airfoil had reasonably good aerodynamic characteristics it offered no marked superiority over contemporary airfoils of that time period. The high-wing position employed on the B-24 offered the distinct advantage of allowing the bomb bay, including bomb-bay doors, to be housed directly beneath the wing, thus permitting the bomb load to be located in the optimum position with respect to the aircraft center of gravity. The high wing, however, had the disadvantage of requiring the use of relatively long, heavy landing-gear struts. An examination of the data given in tables II and III shows that the zero-lift drag coefficient of the B-24 was 0.0406 (table III) as compared with 0.0302 (table II) for the B-17. Because of its high-aspect-ratio wing, however, the maximum lift-drag ratio of the B-24 was about the same...
 
 

ground view of a B-24
 
[122] Figure 5.7 - Consolidated B-24 heavy bomber. [NASA]

 
....as that of the B-17. The B-24 had a maximum speed of 290 miles per hour and, on a typical mission, could carry a 5000-pound bomb load for a distance (one way) of 1700 miles. The B-24 was equipped with four 14-cylinder Pratt & Whitney engines of 1200 horsepower each. These engines employed turbo superchargers, just as did the Wright Cyclone engines used on the B-17, and had a critical altitude of about 30 000 feet.
 
Both the B-17 and the B-24 were designed for high-altitude precision bombing in daylight without protection from fighter escort. In concept, the aircraft were to fly in close formation and protect themselves and each other with concentrated machine-gun fire. In accordance with this doctrine, the B-17G carried no less than 13 .50-caliber machine guns and the B-24J had 10 such guns. In spite of this formidable armament, however, combat experience showed an unacceptable loss rate from enemy air attack until fighter escort was provided for the bombers.
 
Another aspect of the United States strategic bombing offensive against Germany that deserves mention was the effect of the hostile high-altitude environment on the air crews. Neither the B-17 nor the B-24 was pressurized or heated. Temperatures in the range from -30 F to - 50 F were encountered at altitudes of 25 000 feet and above; and although crew members wore electrically heated flying suits, severe cases of frostbite were not uncommon. Later-generation bombers intended for high altitude operations were both pressurized and heated.
 
Somewhat over 18 000 B-24's were produced-more than any other American combat aircraft; furthermore, it was used as a bomber....
 
 

ground view of a B-29
 
[123] Figure 5 8 - Boeing B-29 very heavy bomber. [NASA]

 
...in every theater of operation. Among the B-24 types produced was cargo version known as the C-87 and a Navy patrol aircraft designated the PB4Y. The B-24 was a true workhorse and was used for many purposes other than its design role as a bomber.
 
Let us turn now to another and later class of bomber, the Boeing B-29. The B-29 was designated as a very heavy bomber by the U.S. Army Air Forces and, with a gross weight of 120 000 pounds (later to increase to more than 140 000 pounds), was the heaviest combat aircraft to be produced in quantity by any country during World War II. It grew from a requirement for an aircraft capable of carrying a significantly greater load for a longer range than was possible with either the B-17 or the B-24. The first test flight was made on September 21, 1942, and the first operational sortie was made on June 5, 1944 in a mission against Bangkok that originated in India. Truly, a phenomenally short development time for such an advanced aircraft. A major instrument in the defeat of Japan, the B-29 was used with great effectiveness in night raids against Japanese industry during the latter part of 1944 and in 1945. The aircraft also had the distinction, some may think a dubious one, of carrying the only atomic bombs ever dropped in war.
 
A B-29 is shown in figure 5.8, and characteristics of one version of the aircraft are given in table III. An examination of the photograph and accompanying data shows that the very-high-aspect-ratio wing (11.50) was mounted vertically in the midposition on the long, slim fuselage. In contrast with the earlier Boeing B-17, the B-29 had a tricycle landing gear with each leg having a two-wheel bogie and with the main gear retracting into the inboard engine nacelles. Each of the four [124] 18-cylinder Wright 3350 twin-row radial engines had two General Electric turbosuperchargers that gave the 2200-horsepower engines a critical altitude of about 30 000 feet. Air induction to the turbo superchargers was provided by ducts beneath the engine and gave the cowlings a distinctive oval shape. Engine power was transmitted to the air by means of four-blade controllable-pitch propellers. A notable design feature of the aircraft was the apparent lack of an identifiable windshield in front of the pilots' compartment. Actually, to reduce the drag associated with the usual type of windshield, the nose of the aircraft was transparent and provided visibility for both the pilots and the bombardier. Equipment innovations on the B-29 were pressurization and heating of the crew compartments and remotely controlled, power-operated gun turrets equipped with .50-caliber machine guns. Two of these turrets were on top and two were on the bottom of the fuselage. In addition, the tail gunner had two such machine guns as well as a 20-mm cannon.
 
In addition to its great weight, another indication of the size of the B-29 was provided by the wing span of 141.3 feet. By comparison, the wing span of a modern Boeing 727 jet transport is 108 feet. Other parameters of the B-29 included a maximum speed of 357 miles per hour at 25 000 feet and a zero-lift drag coefficient of 0.0241. This drag coefficient was substantially lower than the corresponding value of 0.0302 for the B-17G, and the maximum value of the lift-drag ratio of the B-29 was 16.8 as compared with 12.7 for the earlier bomber (tables II and III). Even with Fowler-type wing flaps, somewhat similar to the flap shown fourth from the top in figure 5.3, the stalling speed was 105 miles per hour. Such a stalling speed was considered quite high for so large an aircraft when the B-29 was introduced. For a ferry flight the aircraft had a range of about 5000 miles. A maximum payload of 20 000 pounds could be carried for 2800 miles; on a typical operational mission, 12 000 pounds of bombs could be carried for a distance (one way) of 3700 miles. The characteristics of the B-29 indicate that it represented a substantial advancement in design refinement as compared with earlier bombers.
 
The B-29 and its look-alike successor the B-50 continued in service with the United States Air Force for many years following the close of World War II. A number of these outstanding aircraft served their final years as tankers for air refueling of more modern, high-performance aircraft. The last of the B-50 tankers was retired in 1968.
 
In addition to strategic bombers, medium bombers and attack aircraft comprised another class of vehicle that usually had two engines [125] and were considerably lighter than the heavy, strategic type of aircraft. They were employed for short-range bombing missions and various types of ground support activities. The United States used a number of aircraft types in short-range bombing and ground support missions. Perhaps the best known of these aircraft were the North American B-25, known as the Mitchell, and the Martin B-26, known as the Marauder. The Martin B-26 is illustrated in figure 5.9, and some of the important characteristics of the aircraft are given in table III. The twin-engine B-26 follows the same high-wing monoplane formula as the Consolidated B-24 and had the same type of tricycle landing gear. Both the B-25 and the B-26 had the tricycle gear, and these aircraft, together with the B-24, set a precedent for landing gear design in future Air Force bomber aircraft. The B-26 was equipped with two of the new 18-cylinder Pratt & Whitney twin-row radial engines of 2000 horsepower each. Since the aircraft was intended to operate at medium to low altitudes, these engines were only mildly supercharged and developed 1490 horsepower each at 14 300 feet. The aircraft weighed 37 000 pounds fully loaded and had, for that day (1940) the exceedingly high wing loading of 56.2 pounds per square foot. By comparison, the B-17 had a wing loading of 38.7 pounds per square foot, and the Seversky P-35 fighter had a wing loading of 25.5 pounds per square foot. As a result of the high wing loading and relatively ineffective flaps, the stalling speed of the B-26 was a very high 122 miles per hour. The high stalling speed together with certain other characteristics made the B-26 a demanding airplane for the pilot, and many accidents occurred in training with this aircraft. As a result, the B-26 was....
 
 

ground view of a Martin b-26F
 
Figure 5.9 - Martin B-26F medium bomber. [Peter C. Boisseau]

 

[126] frequently referred to by such unflattering names as "widow maker" and "the flying prostitute" (i.e., no visible means Of Support). The zero-lift drag coefficient of the B-26 was 0.0314, which was considerably lower than the 0.0406 of the B-24 and about the same as the value of 0.0302 for the B-17. Other characteristics of the B-26 included a maximum speed of 274 miles per hour at 15 000 feet and the ability to carry a 4000-pound bomb load for a distance of 1100 miles. Armament consisted of 11 .50-caliber machine guns capable of being fired in various directions; several fixed, forward-firing guns were provided for ground attack use.

 
The Martin B-26 was ordered into production directly from the drawing board in September 1940, and a total of 5157 were built. The aircraft was used in both the European and Pacific theaters of operation but was little used in the peacetime Air Force following the cessation of hostilities in 1945. The North American B-25, counterpart of the B-26, was produced in greater numbers than the B-26 and is perhaps better known today because it was the aircraft used by James H. Doolittle in the famous Tokyo raid of April 1942. About 9800 models of the B-25 were constructed, and they served with the Air Force following World War II in a variety of training and support roles. In other countries, they remained as a primary bomber aircraft until comparatively recent years.
 
Multiengine bombers, such as those just discussed, usually dropped their bombs from a level flight attitude or, in the case of medium bombers in the ground attack mode, from a shallow dive. In contrast, an entirely different technique known as dive bombing was pioneered by the U.S. Navy during the decade preceding World War II. In this method of operation, the aircraft was put into a vertical or near-vertical dive at an altitude 15 000 to 20 000 feet and aimed directly at the target. Bomb release usually took place at about 3000 feet, after which the aircraft made a high-g dive recovery to a level flight attitude. Dive bombing was found to be especially suited for use against small, slow-moving targets such as tanks and ships and was employed with devastating effectiveness against Japanese naval forces during World War II.
 
Dive bombers were usually single-engine aircraft with a crew of two: a pilot and a rear-facing gunner situated behind the pilot. The most widely used U.S. Navy dive bomber during World War II was the Curtiss SB2C Helldiver series of which an SB2C-1 is illustrated in figure 5.10. The name "Helldiver" traced its origin to an earlier Curtiss dive bomber of biplane configuration that appeared in the 1930's. [127] With a wing span of nearly 50 feet and a normal gross weight of 14 730 pounds, the SB2C-1 was a large single-engine aircraft. Equipped with a 1750-horsepower twin-row radial engine, the aircraft had a maximum speed of 281 miles per hour at 12 400 feet; a stalling speed of 79 miles per hour facilitated operation of the SB2C-1 from the short deck of an aircraft carrier. Internal storage was provided in the fuselage for a 1000-pound bomb. Typically, the aircraft could carry this bomb load for a distance of 1100 miles. Armament varied with different models of the aircraft. In one arrangement, four .50-caliber machine guns were fitted in the wings and the observer had two .30-caliber guns.
 
Figure 5.10 shows the configuration of the SB2C-1 to have been entirely conventional for its time. A feature of the aircraft not evident in the photograph was the dive brakes used for limiting the speed of the aircraft while in its steep dive to the target. Trailing-edge split flaps that opened in a symmetrical configuration from the top and bottom surfaces of the wing were employed for this purpose; the symmetrical arrangement minimized the effect of flap deployment on longitudinal stability and trim. To reduce tail buffeting, the flaps were perforated with a large number of holes in the order of 3 inches in diameter (the exact size is not known). For landing, only the lower surface flaps were deflected. The need for dive brakes can be explained as follows: First, the normal acceleration, or g-load experienced by an aircraft during dive recovery, varies inversely as the radius of the pullout maneuver and directly as the square of the velocity; second, the accuracy with which the bomb can be dropped increases as the altitude of bomb release is reduced. Since 9 g's is about the maximum normal acceleration that a person can withstand and remain effective, the structural design...
 
 

ground view of a Curtiss SB2C-1
 
Figure 5. 10 - Curtiss SB2C-1 carrier-based scout bomber. (NASA]

 
[128]...of the Helldiver was based on this loading. Hence, the dive speed had to be limited to stay within design load limits and, at the same time, permit bomb release at the desired altitude. Most modern jet fighters, of course, employ some form of speed brake, but the use of such devices was not common practice on World War II aircraft except for aircraft designed for dive bombing.
 
First flight of the Helldiver took place in December 1940, and it first entered combat in November 1943. Including Canadian production, a total of over 7000 Helldivers were manufactured. The type was withdrawn from service in the U.S. Navy in 1949 after a long and useful career.
 
Fighter Aircraft
 
Each of the major Allied and Axis powers developed a series of effective fighter aircraft. The British Hawker Hurricane and Supermarine Spitfire will long be remembered, particularly as being responsible for the air victory in the critical Battle of Britain in 1940. The famous German Messerschmitt 109 was the principal antagonist of the Spitfire and Hurricane during the Battle of Britain, and together with the Focke-Wulf 190, formed the mainstay of the Luftwaffe fighter forces until the end of World War II. The Japanese Mitsubishi Zero probably is the best-remembered Japanese fighter in this country because of the role it played in the attack on Pearl Harbor in December 1941. The North American P-51 Mustang, the Republic P-47 Thunderbolt, and the Lockheed P-38 Lightning are the best known of the U.S. Army Air Force fighters employed in World War II; the Grumman F6F Hellcat and the Vought F4U Corsair are equally well remembered for the outstanding role they played as Navy fighters during the fierce conflicts in the Pacific area. A brief description of the North American P-51 and the Grumman F617 follows. These aircraft are considered typical of World War II land- and carrier-based fighter aircraft as employed by the United States armed forces. Because of its unusual configuration and interesting technical features, the Lockheed P-38 Lightning is also discussed.
 
The North American P-51 Mustang is considered by many to represent the highest level of technical refinement ever achieved in a propeller-driven fighter aircraft. The P-51 was originally designed to a British specification for use by the Royal Air Force (RAF) and was later adopted by the U.S. Army Air Forces. The aircraft was ordered by a British purchasing commission during the hectic days of April 1940, [129] with the understanding that the prototype was to be completed within 120 days. The prototype was completed on schedule; however, first flight was delayed until October 1940. The aircraft first saw combat service with the RAF in July 1942. At first, the aircraft was equipped with a 12-cylinder Allison in-line engine of about 1200 horsepower. With this engine, the aircraft was intended as a low-attitude fighter and ground-attack machine. Later, the North American airframe was mated with the British Rolls-Royce Merlin engine, and this combination resulted in one of the outstanding fighter aircraft of World War II The Merlin was a liquid-cooled engine that employed 12 cylinders arranged in a V-configuration and was equipped with a two-speed two-stage gear-driven supercharger. The engine developed 1490 horsepower at takeoff and was capable of producing 1505 horsepower under war emergency conditions at the critical altitude of 19 300 feet. The Merlin engine was produced under license in the United States by the Packard Motor Car Company.
 
The P-51 Mustang was produced in many variants, of which the most numerous and best known was the P-51D illustrated in figure 5.11. Specifications for the aircraft are given in table III. Figure 5.11 shows the aircraft was equipped with a low wing, which was a highly...
 
 

ground view of a P-51D
 
Figure 5.11 - North American P-51D fighter. [Peter C. Boisseau]

 

[130] ...favored wing position for fighter aircraft during World War II. The use of the in-line engine of low frontal area resulted in a fuselage of relatively low total wetted area and gave the aircraft a lean, streamlined appearance. The low frontal area of the in-line engine was one of the chief advantages cited for this type of power plant; the disadvantage was the vulnerability of the cooling system to enemy fire. The aft location of the cooling radiator and its associated inlet and internal flow system is of interest. The system was designed with the objective of obtaining a net thrust from the cooling air as a result of heat addition from the engine coolant. This feature no doubt contributed to the very low drag coefficient of the aircraft. The P-51 was also the first aircraft to utilize the NACA laminar-flow airfoil sections, discussed earlier. Although it is doubtful that any significant laminar flow was achieved on production versions of the Mustang, the low-drag airfoils did provide improved characteristics at high subsonic Mach numbers.

 
A typical value of maximum gross weight for the P-51D was 10 100 pounds, although this value varied to some extent depending upon the external armament and fuel load. The wing loading corresponding to the 10 100-pound gross weight was 43 pounds per square foot, and the power loading was 6.8 pounds per horsepower. A typical maximum speed was 437 miles per hour at 25 000 feet, and the stalling speed was 100 miles per hour. The zero-lift drag coefficient of 0.0163 was the lowest of any of the aircraft analyzed herein, and the corresponding value of the maximum lift-drag ratio was 14.6. The Mustang was therefore an extremely clean airplane. The aerodynamic cleanness of the aircraft was due, in large measure, to careful attention to detailed design and continued refinement of the aircraft during its production lifetime.
 
The Mustang was utilized in various types of fighter operations, including high-altitude air-to-air combat as well as ground-support and interdiction missions. It had a service ceiling of 40 900 feet and could climb to 20 000 feet in 7.3 minutes. Armament varied but usually consisted of six .50-caliber machine guns, three in each wing, and it could carry two 1000-pound bombs or six 5-inch rockets. Equipped with drop tanks, the P-51D had a range of 1650 miles at a speed of 358 miles per hour and an altitude of 25 000 feet. In contrast to the short range of contemporary British and German fighters, the range capability of the Mustang, as well as the P-47 and P-38, allowed it to be used with great effectiveness in escorting formations of B-17 and B-24 bombers on long-range missions. The P-51 was the only fighter to fly over three enemy capitals - Berlin, Rome, and Tokyo.
 
[131] A total of 14 490 aircraft of the P-51 series were constructed. The aircraft was used in all theaters of operation during World War II, was called into use by the U.S. Air Force again during the Korean War, and was used by a number of foreign air forces for many-years. Many P-51 aircraft are flying in the United States today as unlimited racing aircraft and even for executive transport use. A turboprop version of the Mustang has recently been proposed as a cheap, close-air-support aircraft for use by small, undeveloped countries in various parts of the world. An interesting history of the P-51 aircraft is given in reference 66.
 
Entirely different in configuration from the conventional single-engine fighter of World War II, the twin-engine Lockheed P-38 Lightning is depicted in early form in figure 5.12. In this unusual but highly practical arrangement, the pilot and armament were housed in the center pod, and the liquid-cooled engines together with cooling-air intakes, radiators, and turbosuperchargers were located in the twin booms that also supported the tall. The P-38 was the first fighter designed in the United States to be equipped with a tricycle landing gear: the nose gear retracted into the center pod; and the main gear, into the booms. It was also the first United States aircraft of any type to employ external surfaces composed of butt-joined metal skins with flush rivets. Other innovations employed in later versions of the aircraft included hydraulically boosted ailerons and provisions for use of partial....
 
 

aerial view of a YP-38
 
Figure 5.12 - Lockheed YP-38 twin-engine fighter. [Rudy Arnold via ukn]

 

[132] ...deflection (8°) of the trailing-edge Fowler flaps. Both of these modifications were intended to enhance maneuverability in combat. Powered controls and, to a lesser extent, maneuvering flaps are used on most modern jet fighters.

 
The P-38, intended as an interceptor with the mission of destroying enemy bombers at high altitude, was designed according to specifications issued in 1937 that called for speeds of 360 to 400 miles per hour (sources differ on the exact value) at 20 000 feet and the capability of reaching that altitude in 6 minutes. The specification also contained demanding requirements for range, endurance, and landing and takeoff field length. A single-engine aircraft could not meet the mission requirements with any engine available at that time. Hence, the P-38 employed two engines. First flight of the prototype XP-38 was in January 1939, and the aircraft was first deployed in Europe by the United States Army Air Force (USAAF) in the fall of 1942.
 
At a normal gross weight of 17 500 pounds and with a wing span of 52 feet, the P-38L, for which data are given in table III, was for its day a large fighter. All versions of the aircraft were equipped with Allison V-12 liquid-cooled engines; those on the P-38L developed 1470 horsepower each. Maximum speed was 414 miles per hour at 25 000 feet; stalling speed was 105 miles per hour. The P-38 could climb to 20 000 feet in 7 minutes and had a service ceiling of 44 000 feet. With internal fuel only, the aircraft had a range of 475 miles at 339 miles per hour, or 1175 miles at 195 miles per hour; with drop tanks, the range was 2260 miles.
 
Indeed, the P-38 was a high-performance aircraft. Even the prototype exceeded 400 miles per hour in 1939. Although its high speed was one of the great virtues of the P-38, this desirable characteristic was responsible for a serious problem encountered in the development of the aircraft. Little was known at that time about the problems associated with penetrating the Mach number regime characterized by large effects of compressibility (see discussion of fig. 5.6), and even less was known of means for alleviating such problems. A combination of the high speed reached in steep dives, together with a less than optimum high Mach number airfoil section, caused the P-38 to suffer severe compressibility problems. These problems manifested themselves in the form of buffeting, loss of control, difficulty in recovering from dives, and-in some cases-complete destruction of the aircraft. Many different modifications were tried before a successful solution to the problem was found. In the spring of 1942, NACA in conjunction with Lockheed devised a simple fix that came to be known as the dive-recovery [133] flap (not to be confused with the dive brake used on the SB2C. A short-span flap was located at the 30-percent-chord position behind the leading edge of the lower surface of the wing, just outboard of the booms. Deflection of these flaps in a high-speed dive increased the lift on the wings so that successful dive recovery was possible. Such flaps appeared on production aircraft beginning with the P-38J version. Among other aircraft employing this very effective device were the P47 Thunderbolt and the P-59 and P-80 jet fighters. (See chapter 11.)
 
Although never designed as a fighter for air-to-air combat with other fighter aircraft, the Lightning was widely used and highly effective in this role, particularly in the Pacific theater of operations. More Japanese aircraft were destroyed by the P-38 than by any other aircraft, and the two highest scoring American aces of World War II, Majors Richard I. Bong and Thomas B . McGuire, Jr., both flew the Lightning. It was used in all theaters in which the USAAF operated. As a fighter, several different combinations of armament were employed. Most aircraft had four .50-caliber machine guns and a 20-mm cannon located in the nose ahead of the pilot. Also, it could carry bombs weighing up to as much as 3200 pounds or 10 5-inch rockets. In addition to duties as a fighter, a photoreconnaissance version of the aircraft, designated F-5, saw extensive service. Many other types of military duty such as bombing and ground attack were performed by the P-38.
 
Nearly 10 000 P-38's, including all models, were produced. Several of these are still flying today in the hands of dedicated antique aircraft collectors, and they were used for many years after World War II in aerial survey work. German pilots in North Africa paid the P-38 a tribute of sorts when they dubbed it "Der gabelschwanz teufel" (the fork-tailed devil).
 
Navy fighter aircraft are intended primarily for operations from the short decks of aircraft carriers. Operation from an aircraft carrier poses certain constraints during the design of the aircraft. For example, the relatively short length of the flight deck (about 700 feet for the larger carriers employed during World War II) imposed restrictions on the stalling speed of the aircraft and thus required that Navy fighters have somewhat lower wing loadings than their counterparts in the USAAF. A tail hook must be provided to give rapid deceleration of the aircraft on touchdown, and this in turn required special strengthening of the rear portion of the fuselage. Furthermore, a carrier-based aircraft must be designed for higher landing sink rates than normally encountered in land-based aircraft; this higher sink rate requires a heavier landing gear and attachment structure. Since storage space both on the flight and [134] hanger decks is at a premium on an aircraft carrier, provision must also be made for folding the wings so that the required parking space is reduced. A number of aircraft companies specialized in the design and production of fighters for use on aircraft carriers. The Grumman Aircraft Engineering Company was one of the leading producers of Navy fighter aircraft during the 1930's (as it still is today), and the Navy entered World War II with the Grumman F4F Wildcat as its first-line fighter.
 
Early in 1941, Grumman began the design of a new fighter as a replacement for the Wildcat. Much combat experience had been obtained in the European conflict and was utilized in the design of the new aircraft. Following entry of the United States in World War II in December 1941, the Wildcat saw extensive service in combat against the Japanese. Although the Wildcat was a good aircraft, it was not really competitive with the Japanese Zero shipboard fighter. The lessons learned in action with the Zero were also incorporated in the design of the new Grumman fighter. The prototype of this aircraft, known as the F6F Hellcat, first flew in June 1942, and deliveries of combat aircraft were made to the Navy in early 1943. The first operational use of the Hellcat was in the attack on Marcus Island from the carrier USS Yorktown in August 1943. It is indeed remarkable that the aircraft could be developed from a prototype to combat status in little more than a year.
 
The Hellcat is illustrated in figure 5.13, and some of its characteristics are listed in table III. The aircraft was a rather bulky looking low-wing monoplane equipped with an 18-cylinder Pratt & Whitney twin-row radial engine of 2000 horsepower. The engine was equipped with a geared supercharger and gave 1970 horsepower at 16 900 feet. Although the USAAF deployed highly successful fighters with both air-cooled radial and liquid-cooled in-line engines, the U.S. Navy had employed air-cooled radial engines exclusively since the mid-1920's. Apparently, the Navy felt that the advantages of simplicity and reduced vulnerability to gunfire offered by the radial engine more than offset the disadvantages of increased frontal area. Although not evident in figure 5.13, the landing gear of the F6F retracted rearward and was enclosed within the wing root stubs. Outboard of the landing gear the wing could be rotated and folded aft so as to lie essentially flush along the sides of the fuselage to minimize the deck area required for the aircraft's storage.
 
The Grumman F6F was, for its day, a relatively large aircraft with a fully loaded weight of 12 441 pounds. The wing loading, however, was only...
 
 

ground view of a Grumman F6F-3
 
[135] Figure 5.13 - Grumman F6F-3 carrier-based fighter. [Peter C. Boisseau]

 
...37.3 pounds per square foot, which gave a relatively modest stalling speed of 84 miles per hour. The aircraft had a maximum speed of 375 miles per hour at 17 300 feet. In spite of its bulky appearance, the Hellcat was a clean aircraft having a zero-lift drag coefficient of only 0.0211. Range of the Hellcat was 1090 miles on internal fuel only, and with drop tanks it was 1590 miles. It had a service ceiling of 38 400 feet and an initial rate of climb of 3500 feet per minute. Its armament consisted of six .50-caliber machine guns, three in each wing, and two 1000-pound bombs or six 5-inch rockets.
 
The Grumman F6F Hellcat, of which 12 274 were produced, is considered by many to be the outstanding shipboard fighter of World War II. It was the standard carrier-based fighter employed by the U.S. Navy from mid-1943 until the end of World War II and accounted for the destruction of nearly 5000 enemy aircraft in air-to-air combat. The British Royal Navy took delivery of over 1100 Hellcats, which were used in operations from their carriers. The Hellcat was unusual, as compared with other combat aircraft employed in World War II, in that very few modifications were made to the aircraft during its service life. The F6F served for several years in the U.S. Navy following the close of the war.
 
Afterword
 
[136] The propeller-driven combat aircraft powered with reciprocating engines played a decisive role in World War II and reached a high level of perfection during that conflict. The revolutionary jet engine shaped the course of development of high-performance military aircraft in the post-World War II period. The propeller continued, of course, to be employed on various types of utility, transport, and patrol aircraft; but the development of the jet engine spelled the end for the high-performance propeller-driven fighter, bomber, and attack aircraft. The postwar development of propeller-driven aircraft has been primarily concerned with commercial and general aviation operations and is considered in the next chapter.
 

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