Vought F4U Corsair

Vought F4U Corsair

Vought F4U Corsair was one of the most successful and renowned fighter aircraft of World War II. Developed since 1938, it didn’t make it to serial production until mid-1942. Initially disqualified by the US Navy for carrier service, it was handed over to land-based US Marine Corps fighter units, where it soon proved its worth in combat.

It was fast, robust and heavily armed, which also made it an excellent weapon against ground targets. When it was finally qualified for carrier operations, before long it became (along with F6F Hellcat) the primary aircraft of US Navy fighter and fighter-bomber squadrons.

The Concept

In early 1935 BuAer (Bureau of Aeronautics) of the US Navy issued a request for proposals for a new carrier-based fighter aircraft, a future successor of biplane fighters Boeing F4B and Grumman FF, operated by the Navy at that time, as well as Grumman F2F and F3F, which were shortly to enter service. The Navy accepted XF2A-1, the project submitted by Brewster company. The prototype, powered by Wright R-1820 Cyclone engine, was first flown in December 1937. BuAer also ordered – as an alternative, in case Brewster’s project failed to meet expectations – Grumman’s new fighter powered by Pratt & Whitney R-1830 Twin Wasp engine, which soon evolved from biplane XF4F-1 to monoplane XF4F-2. The prototype XF4F-2 was first flown in September 1937, even earlier than its competitor, but problems with its powerplant slowed down its development. Hence, F2A-1 Buffalo entered service as the first US monoplane carrier-based fighter. Eventually however, it was the Grumman fighter, the improved F4F-3 model later known as Wildcat, which proved a much more capable aircraft and in early forties became the standard US Navy carrier fighter.
Meanwhile, on 1st February 1938, a few months after first flights of XF4F-2 and XF2A-1, BuAer offered a tender for yet another carrier fighter. The new aircraft was expected to reach at least 350 mph (563 kph) maximum speed at 20,000 feet (6096 meters), have stall speed no higher than 70 mph (113 kph) and range of 1000 miles (1609 km). It was to be armed with four machine guns. As was the case three years earlier, BuAer didn’t insist on any particular powerplant or design concept, besides the obvious requirement that the aircraft was capable of operating from carrier decks. In fact, for the Navy the priority was the aircraft’s high performance, especially maximum speed. BuAer was so focused on this factor that it became a standing joke among Vought designers, who claimed that the Navy gave them only three requirements: firstly – speed, secondly – speed, thirdly – more speed.

corsair 1

In response to this latest tender, in April 1938 Chance Vought Aircraft Division of United Aircraft Corporation, based at East Hartford, Connecticut, submitted two projects of a classic, single-engined, single-seat fighter: V-166A (known to BuAer as Vought A), powered by a proven R-1830 Twin Wasp engine, and V-166B (Vought B) powered by a new XR-2800 Double Wasp engine constructed by Pratt & Whitney, still under development at that time. The latter powerplant was a huge, two-row, 18-cylinder radial engine with a displacement of 2800 cubic inches (45.9 l). The prototype XR-2800-2 (B-series) was equipped with two-stage, two-speed supercharger with intercooler. Its maximum rated power on takeoff was 1,850 hp at 2,600 rpm, and it could develop 1,500 hp of continuous power at 2,400 rpm (at 17,500 ft). In comparison, the XR-1830-76 engine powering XF4F-3 prototype produced only 1,200 hp on takeoff and 1,000 hp at 19,000 ft.
The designers from Pratt & Whitney expected soon to increase the takeoff power to 2000 hp. The R-2800 was the first American 18-cylinder engine and the first producing such enormous power for its time. The decision to mount it in a new aircraft design was a risky one, for it was a completely new and unproven powerplant; on the other hand, there was a good chance that the Navy would get its desired high-performance fighter.1
BuAer, having analyzed all submitted proposals, selected the one referred to as the Model V-166B and powered by XR-2800-2 engine. On 11th (or 30th, according to other sources) June 1938 Vought was awarded a contract No. 61544 authorizing the company to build a prototype of the new fighter, which was given military designation XF4U-1 and serial number (BuAer Number, BuNo) 1443. The same month BuAer signed a similar contract with Grumman for building a prototype of XF5F-1, a twin-engined fighter, and in November also with Bell for building XFL-1 prototype powered by Allison V-1710 inline engine. Of these three very novel fighter aircraft designs, only Vought XF4U-1 made it to serial production.

The Design

The team of designers working on technical details of XF4U-1 included Frank C. Albright (as project engineer, replaced in January 1941 by John Russell “Russ” Clark); Paul S. Baker and William C. Schoolfield (as aerodynamics engineers); James Shoemaker and Donald J. Jordan (propulsion engineers). The overall supervision of the project was given to Rex Buran Beisel, the company’s chief engineer. Beisel also had a decisive voice in choosing particular design solutions.

The aircraft design was conform to the requirement of achieving the greatest possible maximum speed. The key to success was obviously the R-2800 engine and careful aerodynamic refinement of the airframe. Double Wasp was 52.8 in (1,342 mm) in diameter – a little less than R-1820 (54.25 in; 1,378 mm) but significantly more than R-1830 (48.03 in; 1,220 mm). However, it was almost twice as heavy and much longer, which meant that it had to be coupled to a fittingly big and sturdy (thus heavy) airframe. In fact, XF4U-1 was the largest and heaviest of all American single-engined fighters built to date. Its empty weight was about 7500 lbs (3,400 kg) – about 2700 lbs (1,200 kg) more than XF4F-3 and over 3300 lbs (1,500 kg) more than XF2A-1.
In order to harness the engine’s tremendous power, a large-diameter propeller was necessary. Hamilton Standard company (which, like Pratt & Whitney, was a division of United Aircraft Corp.) came up with a purpose-designed, three-bladed constant-speed propeller of 13 foot 4 inch (4.06 meter) in diameter – the largest of all hitherto used in single-engined aircraft. Now the question was, how to achieve sufficient propeller clearance. Longer landing gear struts were not an option, because it would be very difficult (if not impossible) to fit them inside wings, and reinforced struts would increase the aircraft’s weight. The designers came up with a very innovative solution – the inverted gull wing, with wings bent down from the root at the angle of 23 degrees, then canted gently upwards (the dihedral at the upward bend was 8.5 degrees).2 The main landing gear was attached at the lowest point of the ‘crank’ in the wing, so the struts could be comparatively short and light.
Such wing design had another very welcome feature. The wings were attached to the fuselage at a nearly perfect right angle, which was an optimal solution as far as aerodynamics went, as it helped minimize interference drag – there was no need for mounting wing-root fairings (overlaying intersections between wings and fuselage). Wing roots, which were the thickest part of the wing, housed oil coolers, with air inlets at the wing leading edges and vent doors at the bottom surface of the wings. The same inlets supplied air for both stages of the supercharger (depending on the engine operating condition, one or both stages could be employed) and intercooler.3 The air outlet of the intercooler was located under the fuselage. All these features allowed the designers to retain a clean airframe and further reduce drag. The only disadvantage of such wing layout was a complex structure of the main spar (connecting both halves of the wing center section to each other and to the fuselage), which also had to have the shape of a flattened W letter when looked from the front.
The wings were of single-spar construction, with an auxiliary rear spar in the wing center section. They consisted of rectangular-shaped center section and two tapering outer panels with rounded tips. The outer panels could be folded up over the canopy for maintenance and storage at carrier decks. The aircraft’s height with folded wings was 16 ft 4 in (4.98 m), and its width 17 ft 0.61 in (5.19 m). In order to accommodate oil coolers and main landing gear wells in the wing center section, the designers chose a fairly thick NACA 23000 series airfoil, which thickness ratio changed smoothly from 18% at the roots to 15% at the junction with outer panels, and 9% at the wingtips. Main landing gear retracted rearward, with the wheels rotating through a 90 degree to lay flat in a fully enclosed wheel well. Wheel dimensions were 32×8 in (813×203 mm), and the wheel track was 12 ft 1 in (3.68 m). Interestingly, the main landing gear, especially the flat strut covers, could act as aerodynamic brakes reducing the aircraft’s diving speed. A pilot could select to lower the main landing gear only, without lowering the tail wheel and arresting hook. However, in practice this option was rarely used.
Wings were equipped with ailerons and flaps, the latter subdivided into three separate sections. The three-section arrangement was necessary because of the bent shape of the wing and the folding of the outer wing panels. Two flap sections were carried by the wing center section, and one by the outer wing panel. The flaps were of a new type, known as slotted deflector flaps, designed and patented by Roger W. Griswold, and used for the first time in OS2U Kingfisher observation-scout floatplane.
The flaps were fitted with deflector plates attached to the flaps’ leading edges. When the flaps were being lowered, the plates shielded from above the slot between the flaps and wings, deflecting the airflow (hence their name). In this way greater angles could be obtained without stalling the flap and having the air break away from the upper flap surface and thereby cause the flap to lose its lift. The new flaps were much more effective than the standard slotted flaps, although their construction was practically the same.
The flaps were all metal except for outer sections, which were fabric-covered. Ailerons had wood frames and plywood skinning; the port aileron was fitted with a trim tab. Outer wing panels had metal construction and fabric skinning aft of the main spar, except for an area inboard under the gun bay, which was metal covered.

corsair 2

The size of the fuselage was determined by the engine diameter. Immediately aft of the engine the fuselage gradually tapered towards the tail, changing the cross-section from round to oval. Technologically, the fuselage was divided into front section, mid section and aft section. The front section housed a spacious cockpit with a fixed windshield and a backwards sliding canopy streamlined into the fuselage’s outline. Although the cockpit was located near the wings’ trailing edges, it offered a fairly good field of view ahead and downward on either side of the nose because of the bend in the wings. The cockpit had no floor, only small footrests directly in front of the rudder pedals. The bottom of the fuselage featured a small teardrop shaped window allowing the pilot to see directly beneath the aircraft.
The front part of the fuselage was attached to the wing center section at the main spar. The engine bearer, made of welded steel tubes, was attached to the front part of the fuselage. The engine had a closely fitting cowling, with 18 cowl flaps completely encircling the fuselage aft of the engine. The engine and its accessory compartment was covered with detachable panels, which offered easy access to the engine, supercharger and other installations.
Mid section accommodated radio set and other equipment. The rear section of the fuselage housed the tail wheel bay covered with doors, and the rearmost tip was used for mounting a retractable arrester hook. Tail planes were tapered, with rounded tips. Small fin, to which a broad-chord rudder was attached, was located ahead of the horizontal stabilizers. Rudder and elevators were all fitted with trim tabs, and elevators additionally with balance tabs.
While constructing the fuselage, the designers made extensive use of spot welding – an innovative method developed together with engineers of Naval Aircraft Factory in Philadelphia and used for the first time also in OS2U floatplane. Instead of a dense ‘skeleton’ of longerons and formers, with small panels of skinning riveted to it, a light framework was used, consisting of only several main bulkheads and four main longerons (two lower and two upper ones). Large duralumin sheets (the largest of these sheets measured 48×102 in; 122×259 cm) had stiffeners spot-welded to the inboard sides. Each of these sheets is preshaped by stretching over forms, and some of them incorporate compound curvatures. The preassembled sheets were then flush-riveted to the framework. Such skinning was much more rigid and smoother, and it didn’t crease during riveting. Furthermore, this method produced a lighter and more durable fuselage than in case of the classic, fully riveted semi-monocoque construction.

bl corsair mapka1

The aircraft’s armament comprised two fuselage mounted Browning M2 .30 inch (7.62 mm) machine guns firing through propeller arc, with muzzles in the upper engine cowl ring, and two Browning M2 .50 inch (12.7 mm) machine guns, one in each outer wing panel. The ammunition boxes for the fuselage mounted guns had a capacity of 750 rounds, while the box for each wing mounted gun had a capacity of 300 rounds. Additionally, there were two small bomb bays with five cells each for small 5.20 lb (2.36 kg) fragmentation bombs, 40 in all, inside outer wing panels. The idea was to use these bomblets for breaking enemy bomber formations. The upper sides of the outer wing panels had built-in compartments for inflatable air bags, designed to help the aircraft remain afloat in the event of a ditching at sea, and give the pilot time to safely get out of the cockpit and retrieve a life-raft stowed in a recess behind the pilot’s headrest.
The fuel was contained in four wing tanks (two in wing center section and two in outer panels), 273 US gallons (1,033 l) in all. To keep the Corsair aerodynamically as clean as possible, the designers made no provision for external auxiliary drop tanks. Hydraulic installation was used for a variety of tasks: lowering and retracting landing gear, flaps and arrester hook, opening and closing landing gear bay doors, folding and unfolding outer wing panels, operating cowl flaps, flow splitters inside air inlets, vent doors and wheel brakes, as well as recharging guns.

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