Here we go the reading part of this build.
Lancaster
300px-Avro_Lancaster_B_I_PA474 by
Gary Wilcox, on Flickr
Royal Air Force Avro Lancaster B I PA474 of the Battle of Britain Memorial Flight.
Role Heavy bomber
National origin United Kingdom
Manufacturer Avro
Designer Roy Chadwick
First flight 9 January 1941
Introduction February 1942
Retired 1963 (Canada)
Status Retired
Primary users Royal Air Force
Royal Canadian Air Force
Royal Australian Air Force
Number built 7,377
Unit cost £45–50,000[
Developed from Avro Manchester
Variants Avro Lancastrian
Developed into Avro York
Avro Lincoln
The Avro Lancaster is a British four-engined Second World War heavy bomber. It was designed and manufactured by Avro as a contemporary of the Handley Page Halifax, both bombers having been developed to the same specification, as well as the Short Stirling, all three aircraft being four-engined heavy bombers adopted by the Royal Air Force (RAF) during the same wartime era.
The Lancaster has its origins in the twin-engine Avro Manchester which had been developed during the late 1930s in response to the Air Ministry Specification P.13/36 for a capable medium bomber for "world-wide use". Originally developed as an evolution of the Manchester (which had proved troublesome in service and was retired in 1942), the Lancaster was designed by Roy Chadwick and powered by four Rolls-Royce Merlins and in one version, Bristol Hercules engines. It first saw service with RAF Bomber Commandin 1942 and as the strategic bombing offensive over Europe gathered momentum, it was the main aircraft for the night-time bombing campaigns that followed. As increasing numbers of the type were produced, it became the principal heavy bomber used by the RAF, the RCAF and squadrons from other Commonwealth and European countries serving within the RAF, overshadowing contemporaries such as the Halifax and Stirling.
A long, unobstructed bomb bay meant that the Lancaster could take the largest bombs used by the RAF, including the 4,000 lb (1,800 kg), 8,000 lb (3,600 kg) and 12,000 lb (5,400 kg) blockbusters, loads often supplemented with smaller bombs or incendiaries. The "Lanc", as it was affectionately known, became one of the more famous and most successful of the Second World War night bombers, "delivering 608,612 long tons of bombs in 156,000 sorties". The versatility of the Lancaster was such that it was chosen to equip 617 Squadron and was modified to carry the Upkeep "Bouncing bomb" designed by Barnes Wallis for Operation Chastise, the attack on German Ruhr valley dams. Although the Lancaster was primarily a night bomber, it excelled in many other roles, including daylight precision bombing, for which some Lancasters were adapted to carry the 12,000 lb (5,400 kg) Tallboy and then the 22,000 lb (10,000 kg) Grand Slam earthquake bombs (also designed by Wallis).[5] This was the largest payload of any bomber in the war.
In 1943, a Lancaster was converted to become an engine test bed for the Metropolitan-Vickers F.2 turbojet. Lancasters were later used to test other engines, including the Armstrong Siddeley Mamba and Rolls-Royce Dart turboprops and the Avro Canada Orenda and STAL Dovern turbojets. Postwar, the Lancaster was supplanted as the main strategic bomber of the RAF by the Avro Lincoln, a larger version of the Lancaster. The Lancaster took on the role of long range anti-submarine patrol aircraft (later supplanted by the Avro Shackleton) and air-sea rescue. It was also used for photo-reconnaissance and aerial mapping, as a flying tanker for aerial refuelling and as the Avro Lancastrian, a long-range, high-speed, transatlantic passenger and postal delivery airliner. In March 1946, a Lancastrian of BSAA flew the first scheduled flight from the new London Heathrow Airport. In 1963, the last remaining Lancasters were retired by the RCAF.
Origins
In the 1930s, the Royal Air Force (RAF) was primarily interested in twin-engine bombers. These designs put limited demands on engine production and maintenance, both of which were already stretched with the introduction of so many new types into service. Power limitations were so serious that the British invested heavily in the development of huge engines in the 2,000 horsepower (1,500 kW) class in order to improve performance. During the late 1930s, none of these were ready for production. Both the United States and the Soviet Union were pursuing the development of bombers powered by arrangements of four smaller engines, the results of these projects proved to possess favourable characteristics such as excellent range and fair lifting capacity. Accordingly, in 1936, the RAF also decided to investigate the feasibility of the four-engined bomber.
The origins of the Lancaster stem from a twin-engined bomber design that had been submitted in response to Specification P.13/36, which had been formulated and released by the British Air Ministry during the mid 1930s. This specification had sought a new generation of twin-engined medium bombers suitable for "worldwide use". Further requirements of the specification included the use of a mid-mounted cantilever monoplane wing, all-metal construction; the adoption of the in-development Rolls-Royce Vulture engine was also encouraged". Various candidates were submitted for the specification by such manufacturers as Fairey, Boulton Paul, Handley Page and Shorts; all submissions were designed around two-engine configurations, using the Rolls-Royce Vulture, Napier Sabre, Fairey P.24 or Bristol Hercules engines. The majority of these engines were under development at this point; while four-engined bomber designs were considered for specification B.12/36 for a heavy bomber, wings which mounted two pairs of engines were still in the experimental stage and required testing at the Royal Aircraft Establishment(RAE), the resulting increase in overall weight of adopting a stronger wing also necessitated further strengthening of the overall aircraft structure.
In response, British aviation company Avro decided to submit their own design, designated the Avro 679, to meet Specification P.13/36. In February 1937, following consideration of the designs by the Air Ministry, Avro's design submission was selected along with Handley Page's bid being chosen as "second string". Accordingly, during April 1937, a pair of prototypes of both designs were ordered. The resulting aircraft, named the Manchester, entered RAF service in November 1940. Although considered to be a capable aircraft in most areas, the Manchester proved to be underpowered and troubled by the unreliability of the Vulture engine. As a result, only 200 Manchesters were constructed and the type was quickly withdrawn from service in 1942.
Flight testing
As early as mid-1940, Avro's chief design engineer, Roy Chadwick, had been working on an improved Manchester design. This redesign was powered by four of the more reliable but less powerful Rolls-Royce Merlin engines, specifically adopted the form of the Merlin "Power Plant" installations which had been developed by Rolls-Royce for the earlier Beaufighter II, installed on a larger wing. Initially, the improved aircraft was designated as the Type 683 Manchester III, it was subsequently renamed as the Lancaster. The prototype aircraft BT308 was assembled by the Avro experimental flight department at Ringway Airport, Manchester; the prototype was constructed from a production Manchester airframe, which was combined with a new centre section designed to accommodate the additional engines. On 9 January 1941, test pilot H. A. "Sam" Brown performed the prototype's maiden flight at RAF Ringway, Cheshire.
220px-Avro_Lancaster_bombers_nearing_completion_at_the_A_V_Roe_&_Co_Ltd_factory_at_Woodford_in_Cheshire,_1943 by
Gary Wilcox, on Flickr
Flight testing of the new aircraft quickly proved it to be a substantial improvement on its predecessor, aviation author Jim Winchester referred to the Lancaster as being "one of the few warplanes in history to be 'right' from the start." The first prototype was initially outfitted with a three-finned tail layout, a result of the design having been adapted from the Manchester I; this was quickly revised on the second prototype, DG595, and subsequent production Lancasters to the familiar larger elliptical twin-finned tail unit that had also been adopted for the later-built Manchesters, discarding the stubby central third tail fin. The adoption of the enlarged twin fins not only increased stability but also provided for a greater field of fire from the dorsal gun turret position. The second prototype was also outfitted with more powerful Merlin XX engine.
Some of the later orders for Manchesters were converted in favour of the Lancaster; both bombers shared various similarities and featured identical design features, such as the same distinctive greenhouse cockpit, turret nose and twin tail. The designs were so similar that an entire batch of partially constructed Manchesters could be completed as Lancaster B I aircraft instead. Based upon its performance, a decision was taken early on to reequip twin-engine bomber squadrons with the Lancaster as quickly as possible. In October 1941, the first production Lancaster, L7527, powered by Merlin XX engines, conducted its first flight.
220px-Avro_Lancaster_aircraft_under_construction_at_the_A_V_Roe_&_Co_Ltd_factory_at_Woodford_in_Cheshire,_1943._TR1384 by
Gary Wilcox, on Flickr
Avro received an initial contract for 1,070 Lancasters. The majority of Lancasters that were manufactured during the war years were constructed by Avro at their factory at Chadderton near Oldham, Greater Manchester and were test flown from Woodford Aerodrome in Cheshire. As it was quickly recognised that Avro's capacity was exceeded by the wartime demand for the type, it was decided to form the Lancaster Aircraft Group, which comprised a number of companies that undertook the type's manufacture, either performing primary assembly themselves or producing various subsections and components for the other participating manufacturers.
In addition to Avro, further Lancasters were constructed by Metropolitan-Vickers (1,080, also tested at Woodford) and Armstrong Whitworth. It was also produced at the Austin Motor Companyworks in Longbridge, Birmingham, later in the Second World War and postwar by Vickers-Armstrongs at Chester as well as at the Vickers Armstrong factory, Castle Bromwich, Birmingham. Belfast-based aircraft firm Short Brothers had also received an order for 200 Lancaster B Is, but this was cancelled prior to any aircraft have been completed. Only 300 of the Lancaster B II, which was outfitted with Bristol Hercules engines, were constructed; this had been produced as a stopgap modification as a result of a shortage of Merlin engines due to fighter production have had a higher priority placed upon it at that time.
The Lancaster was also produced overseas. During early 1942, it was decided that the bomber should be produced in Canada, where it was manufactured by Victory Aircraft in Malton, Ontario. Of later variants, only the Canadian-built Lancaster B X was produced in significant numbers. A total of 430 of this type were built, earlier examples differing little from their British-built predecessors, except for using Packard-built Merlin engines and American-style instruments and electrics.[ In August 1942, a British-built Lancaster B I, R5727, was dispatched to Canada as a pattern aircraft, becoming the first of the type to conduct a transatlantic crossing. The first Lancaster produced in Canada was named the "Ruhr Express". The first batch of Canadian Lancasters delivered to England suffered from faulty ailerons; this error was subsequently traced to the use of unskilled labourers.
Further development
The Lancaster B I was never fully superseded in production by a successor model, remaining in production until February 1946. According to aviation authors Brian Goulding and M. Garbett, the Lancaster B I altered little during its production life, partially as a result of the sound basic structure and design; of the visible changes, the fuselage side-windows were deleted, the Perspex chin of the bomb-aimer was enlarged, and a larger astrodome was provisioned. Various additional bumps and blisters were also added, which would typically house radar equipment and radio navigational aids. Some Lancaster B I bombers were outfitted with bulged bomb bay doors in order to accommodate increased armament payloads.
Early production Lancaster B Is were outfitted with a ventral gun turret position. In response to feedback on the lack of application for the ventral turret, the ventral turret was often eliminated during the course of each aircraft's career. While some groups chose to discard the position entirely, various trials and experiments were performed at RAF Duxford, Cambridgeshire and by individual squadrons. A total of 50 Austin-built Lancaster B Is were constructed to a non-standard configuration, having a Frazer Nash turret installed directly above the bomb bay; this modification was largely unpopular due to its obstruction of the internal walkway, hindering crew movements. Various other turret configurations were adopted by individual squadrons, which included the removal of various combinations of turrets.
The Lancaster B III was powered by Packard Merlin engines, which had been built overseas in the United States, but was otherwise identical to contemporary B Is. In total, 3,030 B IIIs were constructed, almost all of them at Avro's Newton Heath factory. The Lancaster B I and B III were manufactured concurrently and minor modifications were made to both marks as further batches were ordered. The B I and B III designated was effective interchangeable simply by exchanging the engines used, which was occasionally done in practice. Examples of modifications made include the relocation of the pitot head from the nose to the side of the cockpit and the change from de Havilland "needle blade" propellers to Hamilton Standard or Nash Kelvinator made "paddle blade" propellers.
Design
Overview
220px-Lancaster_B_MkI_44_Sqn_RAF_in_flight_1942 by
Gary Wilcox, on Flickr
The Avro Lancaster was a British four-engined strategic bomber that was used as the RAF's principal heavy bomber during the latter half of the Second World War. The typical aircraft was powered by an arrangement of four wing-mounted Rolls-Royce Merlin piston engines, each of which drove a set of 13 ft diameter de Havilland Hydromatic three-bladed airscrews. While not optimal, the Lancaster was capable of flying the return journey home on only two operational engines, along with very limited distances on a single running engine. Goulding and Garbett have claimed that experienced Lancaster pilots were often able to out-manoeuver Luftwaffe fighters. It possessed largely favourable flying characteristics, having been described by aviation authors Brian Goulding and M. Garbett as being: "a near-perfect flying machine, fast for its size and very smooth...such a delightfully easy aeroplane to fly...there are instances of Lancasters having been looped and barrel-rolled, both intentionally and otherwise".
The Lancaster benefited from a structure that possessed considerable strength and durability, which had been intentionally designed to maximise structural strength-per-weight; this resulted in the Lancaster being capable of withstanding some levels of damage resulting from attacks by hostile interceptor aircraft and ground-based anti-aircraft batteries. However, during the first year of the type's career, some instances of structural failures were encountered on Lancaster B Is and a number of aircraft were lost in accidents as a result of the design limitations having been greatly exceeded. Compared with other contemporary aircraft, the Lancaster was not an easy aircraft to escape from; in a Halifax, 25 per cent of downed aircrew bailed out successfully, and in American bombers (albeit in daylight raids) it was as high as a 50 per cent success rate while only 15 per cent of the Lancaster crew were able to bail out.
The Lancaster uses a mid-wing cantilever monoplane configuration. The wing is constructed from five separate main sections while the fuselage is likewise composed of five sections. Aside from a few elements, such as the fabric-covered ailerons, the Lancaster's oval-shaped fuselage had an all-metal covering. All of the wing and fuselage sections were manufactured separately, during which they were outfitted with all of the required equipment in advance of final assembly being performed, as a measure intended to accelerate the rate of production. The Lancaster was equipped with a retractable main undercarriage and fixed tailwheel; the hydraulically-actuated main landing gear raised rearwards into recesses within the inner engine nacelles. The distinctive tail unit of the aircraft was outfitted with a large twin elliptical fins and rudder arrangement.
Crew accommodation
203px-G_for_George_pilot_1944_AWM_UK2055 by
Gary Wilcox, on Flickr
161px-Lancaster_flight_engineer_WWII_IWM_CH_12289 by
Gary Wilcox, on Flickr
The standard crew for a Lancaster consisted of seven men, stationed in various positions in the fuselage. Starting at the nose, the bomb aimer had two positions to man. His primary location was lying prone on the floor of the nose of the aircraft, with access to the bombsight controls facing forward, with the bombsight computer on his left and bomb release selectors on the right. He also used his view out of the large transparent perspex nose cupola to assist the navigator with map reading. To man the Frazer Nash FN5 nose turret, he stood up placing himself in position behind the triggers of the twin .303 in (7.7 mm) guns. Ammunition for the turret was 1,000 rounds per gun (rpg). The bomb aimer's position contained the nose emergency hatch in the floor; at 22 by 26.5 inches (560 by 670 mm) (two inches narrower than the Halifax escape hatch) it was difficult to exit through while wearing a parachute. Operational research experts, including British scientist Freeman Dyson, amongst others, attempted unsuccessfully to have the escape hatch enlarged.
On the roof of the bomb bay the pilot and flight engineer sat side by side under the expansive canopy, with the pilot sitting on the left on a raised portion of the floor (almost all British bombers, and most German bombers, had only a single pilot seat as opposed to American practice of carrying two pilots, or at least having controls for two pilots installed). The flight engineer sat on a collapsible seat (known as a "second dicky seat") to the pilot's right, with the fuel selectors and gauges on a panel behind him and to his right. The pilot and other crew members could use the panel above the cockpit as an auxiliary emergency exit while the mid-upper gunner was expected to use the rear entrance door to leave the aircraft. The tail gunner escaped by rotating his turret to the rear, opening the door in the back of the turret, passing into the fuselage, and clipping on a parachute that was hung on the side wall. He could then exit through the rear entrance door.
151px-Lancaster_bomb_aimer_1943_CH_8789 by
Gary Wilcox, on Flickr
213px-G_for_George_wireless_operator_and_navigator_AWM_UK2052 by
Gary Wilcox, on Flickr
Behind the pilot and flight engineer, and behind a curtain fitted to allow him to use light to work, sat the navigator. His position faced to port with a chart table in front of him. An instrument panel showing the airspeed, altitude, and other information required for navigation was mounted on the side of the fuselage above the chart table. The wireless operator's radios were mounted on the left-hand end of the chart table, facing the rear of the aircraft. Behind these and facing forwards the wireless operator sat on a seat at the front of the main spar. On his left was a window, and above him was the astrodome, used for visual signalling and by the navigator for celestial navigation.
Behind the wireless operator were the two spars for the wing, which created a major obstacle for crew members moving down the fuselage even on the ground. On reaching the end of the bomb bay the floor dropped down to the bottom of the fuselage, and the mid-upper gunner's turret was reached. His position allowed a 360° view over the top of the aircraft, with two Browning .303 Mark IIs to protect the aircraft from above and to the side. The mid-upper gunner sat on a rectangle of canvas that was slung beneath the turret and would stay in position throughout the flight. Ammunition for the turret was 1,000 rounds per gun.
To the rear of the turret was the side crew door, on the starboard side of the fuselage. This was the main entrance to the aircraft, and also could be used as an emergency exit. The Elsan chemical toilet, a type of aircraft lavatory, was located near the spars for the tailplane. At the extreme tail-end of the fuselage, the rear gunner sat in his exposed position in the tail turret, which was entered through a small hatch in the rear of the fuselage. Depending on the size of the rear gunner, the area was so cramped that the gunner would often hang his parachute on a hook inside the fuselage, near the turret doors. Neither the mid-upper nor the rear gunner's position was heated, and the gunners had to wear electrically heated suits to prevent hypothermia and frostbite.
Armament
Defensive armament
Lancaster_Mk_I_gun_positions by
Gary Wilcox, on Flickr
220px-A_gun_turret_on_a_restored_WW2_Lancaster_bomber_-b by
Gary Wilcox, on Flickr
The Avro Lancaster was initially equipped with four Nash & Thomson Frazer Nash hydraulically operated turrets mounted in the nose, tail, mid-upper and underside. The original tail turret was equipped with four Browning .303 Mark II machine guns and all other turrets with two such machine guns.
Late on in the war, as a result of statistical analysis, Freeman Dyson, put forward a case for the removal of the majority of the Lancaster's defensive armament. He argued that this would reduce the overall loss rate as it would have the benefit of increasing the Lancaster's cruise speed by up to 50 mph (80 km/h) (assuming the bomb load was not increased at the same time), and thus make the bomber harder to shoot down. He also considered that the modification would be justified regardless of the envisioned decreased loss rate as, by requiring fewer crew to serve as defensive air gunners, that would be a lower number of human losses incurred with each aircraft lost.
Nose turret
Only the FN-5A nose turret which was similar to the FN-5 used on the preceding Avro Manchester, the Vickers Wellington and the Short Stirling remained unchanged during the life of the design, except in instances where it was removed entirely.
Ventral turret
The ventral (underside) FN-64 turret quickly proved to be dead weight, being both difficult to sight because it relied on a periscope which limited the gunner's view to a 20 degree arc, and too slow to keep a target within its sights. Aside from early B Is and the prototype B IIs, the FN-64 was almost never used. When the Luftwaffe began using Schräge Musik to make attacks from below in the winter of 1943/1944, modifications were made, including downward observation blisters mounted behind the bomb aimer's blister and official and unofficial mounts for .50 in (12.7 mm) machine guns or even 20 mm cannon, firing through the ventral holes of the removed FN-64. The fitting of these guns was hampered as the same ventral position was used for mounting the H2S blister, which limited installations to those aircraft fitted with bulged bomb bays which interfered with the H2S.
Mid-upper turret
220px-A_No._57_Squadron_Lancaster_mid-upper_gunner_in_his_turret,_February_1943._CH8795 by
Gary Wilcox, on Flickr
The mid-upper (dorsal or top) turret was an FN-50 on early examples and the very similar FN-150 with improved sights and controls on later examples. On all but the earliest examples this turret was surrounded by a coaming which provided a track for a cam operated interruptor device which prevented the gunner from shooting the tail of his own aircraft. The Mk. VII and late Mk. X Lancasters used the heavier, electrically-controlled Martin 250 CE 23A turret equipped with two .50 inch machine guns which was mounted further forward to preserve the aircraft's longitudinal balance, and because it had an internal mechanism to prevent firing on the aircraft itself, it did not require a coaming. Other experimental turrets were tried out, including the FN-79 and the Boulton-Paul Type H barbette system.
Tail turret
170px-Lancaster_tail_gunner_IWM_CH_12776 by
Gary Wilcox, on Flickr
The tail turret was the most important defensive position and carried the heaviest armament. Despite this, the turrets used, starting with the FN-20, were never entirely satisfactory and numerous designs were tried. The FN-20 was replaced by the very similar FN-120 which used an improved gyroscopic gun sight(GGS). Many rear gunners insisted on having the centre section of perspex removed from the turret to improve visibility. The transparencies were difficult to see through at night, particularly when trying to keep watch for enemy night fighters that appeared without notice astern and below the aircraft when getting into position to open fire. This removal of perspex from the turret was called the "Gransden Lodge" modification. Ammunition for the tail turret was 2,500 rounds-per-gun. Due to the weight, the ammunition was stored in tanks situated near the mid-upper turret's position and fed rearward in runways down the back of the fuselage to the turret.
Gunners using both the FN-20 and 120 removed perspex and armour from the turret to improve visibility, but trials by the RAF showed that a Mosquito night fighter was still able to get within a very short distance of the tail gunner without being spotted, confirming what the Luftwaffe had already realised. The Rose turret attempted to improve on the FN turrets by being completely open to the rear (improving visibility and allowing easier emergency egress) and by being fitted with two .50 inch machine guns and was installed in a small number of Lancasters but never became common.
Ultimately radar, rather than improved visibility, made the turret more effective. The FN-121 was the Automatic Gun Laying Turret (AGLT), an FN-120 fitted with Village Inn gun-laying radar. Aircraft fitted with Village Inn were used as bait, flying behind the main formations to confront the night fighters that followed the formations and shot down stragglers. This significantly reduced operational losses; and gun-laying radar was added to the last versions of the turret. Before the end of the war Lancasters built in the UK standardised on the FN-82 fitted with two .50 inch machine guns and fitted with gun-laying radar as production allowed, which was also used on early models of the Avro Lincoln. The disadvantage of all radar and radio transmitting systems is that attacking forces can locate aircraft by picking up transmissions.
Bombs
141px-Lancaster_bomb_bay_Jan_1944_IWM_CH_18554 by
Gary Wilcox, on Flickr
223px-Lancaster_area_bombing_load_IWM_CH_18371 by
Gary Wilcox, on Flickr
An important feature of the Lancaster was its unobstructed 33 ft (10 m) long bomb bay. At first, the heaviest bomb carried was the 4,000 lb (1,800 kg) high capacity HC "Cookie". Bulged doors were added to 30 per cent of B Is to allow the aircraft to carry 8,000 lb (3,600 kg) and later 12,000 lb (5,400 kg) "Cookies". The Lancaster also carried a variety of smaller weapons, including the Small Bomb Container (SBC) which held 236 4 lb (1.8 kg) or 24 30 lb (14 kg) incendiary and explosive incendiary bomblets; 500 lb (230 kg) and 1,000 lb (450 kg) General Purpose High Explosive (GP/HE) bombs (these came in a variety of designs); 1,850 lb (840 kg) parachute deployed magnetic or acoustic mines, or 2,000 lb (910 kg) armour-piercing (AP) bombs; 250 lb (110 kg) Semi-Armour-Piercing (SAP) bombs, used up to 1942 against submarines; post 1942: 250 lb (110 kg) or 500 lb (230 kg) anti-submarine depth charges.
In 1943, 617 Squadron was created to carry out Operation Chastise, the raid against the Ruhr dams. This unit was equipped with B.III (Specials), officially designated the "Type 464 (Provisioning)", modified to carry the 9,250 lb (4,200 kg) "Upkeep" bouncing bomb. The bomb bay doors were removed and the ends of the bomb bay were covered with fairings. "Upkeep" was suspended on laterally pivoted, vee-shaped struts which sprang apart beamwise when the bomb-release button was pressed. A drive belt and pulley to rotate the bomb at 500 rpm was mounted on the starboard strut and driven by a hydraulic motor housed in the forward fairing. The mid-upper turret was removed and a more bulbous bomb aimer's blister was fitted; this, as "Mod. 780", later becoming standard on all Lancasters, while the bombsight was replaced by a simple aiming device. Two Aldis lights were fitted in the rear bomb bay fairing; the optimum height for dropping "Upkeep" was 60 ft and, when shone on the relatively smooth waters of the dam's reservoirs, the light beams converged into a single spot when the Lancaster was flying at the correct height.
Towards the end of the war, attacking special and hardened targets, other variants of B I Specials were modified to carry the 21 ft (6.4 m) long 12,000 lb (5,400 kg) "Tallboy" or 25.5 ft (7.8 m) long 22,000 lb (10,000 kg) "Grand Slam""earthquake" bombs. Aircraft intended to carry the "Grand Slam" required extensive modifications. These included the removal of the dorsal turret and of two guns from the rear turret, removal of the cockpit armour plating (the pilot's seatback), and installation of Rolls-Royce Merlin Mk 24 engines for better take-off performance. The bomb bay doors were removed and the rear end of the bomb bay cut away to clear the tail of the bomb. Later the nose turret was also removed to further improve performance. A strengthened undercarriage and stronger mainwheels, later used by the Avro Lincoln, were fitted.
Specific bomb loads were standardised and given code names by Bomber Command:
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Scott
