Consolidated B-24 Liberator
The Consolidated B-24 Liberator is an American heavy bomber, designed by Consolidated Aircraft of San Diego, California. It was known within the company as the Model 32, and some initial production aircraft were laid down as export models designated as various LB-30s, in the Land Bomber design category.
At its inception, the B-24 was a modern design featuring a highly efficient shoulder-mounted, high aspect ratio Davis wing. The wing gave the Liberator a high cruise speed, long range and the ability to carry a heavy bomb load. In comparison with its contemporaries, the B-24 was relatively difficult to fly and had poor low-speed performance; it also had a lower ceiling and was less robust than the Boeing B-17 Flying Fortress. While aircrews tended to prefer the B-17, General Staff favored the B-24 and procured it in huge numbers for a wide variety of roles. At approximately 18,500 units – including 8,685 manufactured by Ford Motor Company – it holds records as the world's most produced bomber, heavy bomber, multi-engine aircraft, and American military aircraft in history.
The B-24 was used extensively in World War II where it served in every branch of the American armed forces, as well as several Allied air forces and navies. It saw use in every theater of operations. Along with the B-17, the B-24 was the mainstay of the US strategic bombing campaign in the Western European theater. Due to its range, it proved useful in bombing operations in the Pacific, including the bombing of Japan. Long-range anti-submarine Liberators played an instrumental role in closing the Mid-Atlantic gap in the Battle of the Atlantic. The C-87 transport derivative served as a longer range, higher capacity counterpart to the Douglas C-47 Skytrain.
By the end of World War II, the technological breakthroughs of the Boeing B-29 Superfortress and other modern types had surpassed the bombers that served from the start of the war. The B-24 was rapidly phased out of U.S. service, although the PB4Y-2 Privateer maritime patrol derivative carried on in service with the U.S. Navy in the Korean War.
Design and development
Initial specifications
The Liberator originated from a United States Army Air Corps request in 1938 for Consolidated to produce the B-17 under license. After company executives including President Reuben Fleet visited the Boeing factory in Seattle, Washington, Consolidated decided to submit a more modern design of its own.The new Model 32 combined designer David R. Davis's wing, a high-efficiency airfoil design created by unorthodox means, with the twin tail design from the Consolidated Model 31 flying boat, together on a new fuselage. This new fuselage was intentionally designed around twin bomb bays, each one being the same size and capacity of the B-17 bomb bays.
In January 1939, the USAAC, under Specification C-212, formally invited Consolidated to submit a design study for a bomber with longer range, higher speed and greater ceiling than the B-17. The specification was written such that the Model 32 would automatically be the winning design. The program was run under the umbrella group, "Project A", an Air Corps requirement for an intercontinental bomber that had been conceived in the mid-1930s. Although the B-24 did not meet Project A goals, it was a step in that direction. Project A led to the development of the Boeing B-29 and Consolidated's own B-32 and B-36.
Design
The B-24 had a shoulder-mounted high aspect ratio Davis wing which proved highly efficient in all phases of flight, allowing a relatively high airspeed and long range. Compared to the B-17, it had a larger wingspan but a lower wing area. This gave the B-24 a 35-percent higher wing loading. The relatively thick wing allowed the loading of more fuel while delivering increased lift and speed, but pilots complained that it tended to become less responsive when fully loaded at altitude and in bad weather. The Davis wing was also more susceptible to ice formation than contemporary designs, causing distortions of the aerofoil section and resulting in the loss of lift, with unpleasant experiences drawing such comments as, "The Davis wing won't hold enough ice to chill your drink". The wing was also more susceptible to damage than the B-17's wing, making the aircraft less able to absorb battle damage.The wing carried four supercharged Pratt & Whitney R-1830-35 Twin Wasp engines mounted in cowlings borrowed from the PBY Catalina that turned 3-bladed variable-pitch propellers.
The tailplane featured two large oval vertical stabilizers mounted at the ends of a rectangular horizontal stabilizer. As early as 1942, it was recognized that the Liberator's handling and stability could be improved by the use of a single vertical fin. The single fin was tested by Ford on a single B-24ST variant and an experimental XB-24K: it was found to improve handling. However, all Liberators were produced with twin oval fins, with the exception of eight preproduction B-24N aircraft. The B-24N was intended as a major production variant featuring a single tail. Over 5000 orders for this version were placed in 1945, but they were cancelled due to the end of the war. The single fin did appear in production on the PB4Y Privateer derivative.
The B-24's spacious, slab-sided fuselage was built around two central bomb bays that could accommodate up to of ordnance in each compartment. The forward and aft bomb bay compartments were further split longitudinally with a centerline ventral catwalk just wide, which also functioned as the fuselage's structural keel beam.
An unusual four-panel set of all-metal, tambour-panel "roller-type" bomb bay doors, which operated very much like the movable enclosure of a rolltop desk, retracted into the fuselage. These types of doors created a minimum of aerodynamic drag to keep speed high over the target area; they also allowed the bomb bays to be opened while on the ground since the low ground clearance prevented the use of normal bomb bay doors. The occasional need during a mission for crewmen to move from fore to aft within the B-24's fuselage over the narrow catwalk was a drawback shared with other bomber designs.
The Liberator carried a crew of up to ten. The pilot and co-pilot sat alongside each other in a well-glazed cockpit. The navigator and bombardier – who could also double as a nose or wiggly ear gunners – sat in the nose, fronted on the pre-B-24H models with a well-framed "greenhouse" nose with some two dozen glazed panels and with two flexible ball-mounts built into it for forward defensive firepower using.30 caliber Browning M1919 machine guns. The radio/radar operator sat behind the pilots, facing sideways and sometimes doubled as a waist gunner. The flight engineer sat adjacent to the radio operator behind the pilots; he operated the upper gun turret, located just behind the cockpit and in front of the wing.
Up to four crew members could be located in the waist, operating waist guns, a retractable lower ball turret gun, and a tail gun turret matching the nose turret. The waist gun hatches were provided with doors. The ball turret was required to be retractable for ground clearance when preparing to land as well as for greater aerodynamic efficiency. The tail gunner's powered twin-gun turret was located at the end of the tail, behind the tailplane.
The B-24 featured a tricycle undercarriage, the first American bomber to do so, with the main gear extending out of the wing on long, single-oleo strut legs. It used differential braking and differential thrust for ground steering, which made taxiing difficult.
Armament
The defensive armament of the B-24 varied from transport variants, which were usually unarmed, to bombers armed with up to ten.50 caliber M2 Browning machine guns located in turrets and waist gun positions.Early model Liberators were fitted with a top-mounted turret, a tail turret and single machine guns located in the waist and in the glazed nose. The B-24D initially featured upper, belly and tail turrets, plus swiveling single guns in the waist and on either side of the nose. The belly turret was a periscopically sighted Bendix model. The turret proved unsatisfactory and was soon replaced by a tunnel gun, which was itself omitted. Later D models were fitted with the retractable Sperry ball turret.
The B-24H saw the replacement of the glazed 'green house' nose with a nose turret, which reduced the B-24s vulnerability to head-on attacks. The bombsight was located below the turret.
Long-range naval patrol versions often carried a light defensive armament. Being on long-distance patrols, they generally flew outside the range of enemy fighters. Also, the necessity of range increased the importance of weight and aerodynamic efficiency. Thus naval patrol often omitted top, belly and nose turrets. Some were fitted with a belly pack containing fixed, forward-facing cannon.
Prototypes and service evaluation
The U.S. Army Air Corps awarded a contract for the prototype XB-24 in March 1939, with the requirement that one example should be ready before the end of the year. Consolidated finished the prototype and had it ready for its first flight two days before the end of 1939. The design was simple in concept but, nevertheless, advanced for its time.Consolidated incorporated innovative features such as a tricycle landing gear and Davis wing.
Compared to the B-17, the proposed Model 32 had a shorter fuselage and 25% less wing area, but had a greater wingspan and a substantially larger carrying capacity, as well as a distinctive twin tail. Whereas the B-17 used 9-cylinder Wright R-1820 Cyclone engines, the Consolidated design used twin-row, 14-cylinder Pratt & Whitney R-1830 "Twin Wasp" radials of. The maximum takeoff weight was one of the highest of the period.
The new design would be the first American heavy bomber in production to use tricycle landing gear – the North American B-25 Mitchell medium bomber's predecessor, the NA-40 introduced this feature in January 1939 – with the Consolidated Model 32 having long, thin wings with the efficient "Davis" high aspect ratio design promising to provide maximum fuel efficiency. Wind tunnel testing and experimental programs using an existing Consolidated Model 31 provided extensive data on the flight characteristics of the Davis airfoil.
Early orders, placed before the XB-24 had flown, included 36 for the USAAC, 120 for the French Air Force and 164 for the Royal Air Force. The name "Liberator" was originally given to it by the RAF, and subsequently adopted by the USAAF as the official name for the Model 24. When France fell in 1940, their aircraft were re-directed to the RAF. One outcome of the British and French purchasing commissions was a backlog of orders amounting to $680m, of which $400m was foreign orders, US official statistics indicating tooling, plant and expansion advanced the previously anticipated volume of US aircraft production by up to a year. A consequence of the British orders went beyond requests for specific modifications: as the RAF accepted some designs while rejecting others, American production was – to some extent – re-directed along specific lines that accorded with British doctrine, the B-24's capacious bomb bay and ability to carry 8,000 lb ordnance a case in point.
After initial testing, the XB-24 was found to be deficient in several areas. One major failure of the prototype was that it failed to meet the top speed requirements specified in the contract. As built, the XB-24 top speed was only 273 mph instead of the specified 311 mph. As a result, the mechanically supercharged Pratt & Whitney R-1830-33s were replaced with the turbo-supercharged R-1830s. Additionally, the tail span was widened by and the pitot-static probes were relocated from the wings to the fuselage. The XB-24 was then re-designated XB-24B—these changes became standard on all B-24s built starting with the B-24C model.
In April 1939, the USAAC initially ordered seven YB-24 under CAC contract # 12464. The US policy at the time, despite neutrality, was that American requirements could be deferred while its Allies could immediately put US production into the war effort. The added advantage was the American types could be assessed in the European war zone earlier. Thus the first six YB-24 were released for direct purchase under CAC contract # F-677 on 9 November 1940. These aircraft were redesignated LB-30A. The seventh aircraft was used by Consolidated and the USAAC to test armor installations as well as self-sealing fuel tanks. Initially, these aircraft were to be given USAAC serials 39–681 to 39-687. Due to deferments of the US requirements, the US purchase was twice postponed, and the serial numbers were changed to 40–696 to 40-702. When the RAF purchased the first six YB-24 aircraft, the serial numbers were reassigned to an early batch of B-24D funded by the deferment.