The celebrated raid by 617 squadron on the Ruhr dams in 1943 is justifiably world famous. The mission and its principal characters Guy Penrose Gibson and Barnes Neville Wallis, commander and inventor respectively, has been well documented. The whole operation from War Cabinet approval through construction and testing of the bombs, modifications to the aircraft, training of the air crews and the raid itself was completed within ten weeks, a remarkable achievement. Keith decided to build a large model of Gibson’s Lancaster to coincide with the 50th anniversary of the raid.
Wingspan 15′ 7″
Controls: Ailerons, elevator, rudders, Inner and outer throttles, flaps, retracts, navigation and
altitude spotlights, bomb-spin and drop.
Keith started with the ASP (formally MAP) scale drawings 2781. This has good detail of the aircraft and considerable detail of the “Special B MkIII (Dambuster)”. The most obvious modifications from the basic MkI Lancaster are the omission of the mid-upper turret which was considered unnecessary, and saved weight, and the extensively modified bomb-bay. These drawings provided most of the essential information required to construct the aircraft.
Further research discovered that these drawings were incomplete and, in some areas, inconsistent with other reputable references. First, the bomb support trusses and position of the ‘VSG’ hydraulic rotation motor were incorrect. Another discrepancy was that in relation to the addition of a single Vickers ‘K’ gun in the rear fuselage underside. This was not fitted to the aircraft used in the raid.
The most difficult problem to resolve was the precise position and angle of the spotlights used to determine the height of the aircraft during the bombing run. Some references show them positioned so that they converge at 150ft. However, the raid was conducted at 60ft. The discrepancy is because the original ‘Upkeep’ bomb was designed to be used at 150ft. However, the disintegration of this bomb when hitting the water meant that the lower height had to be used. Few references have the information for this height. The front light was actually positioned in the camera position (to the rear and port side of the bomb-aimers compartment). The rear light was positioned on the bulkhead at the rear of the modified bomb bay.
In regard to the colour scheme used, ‘Dam Buster’ Lancasters were painted in standard camouflage upper surfaces and matt black lower surfaces. It was necessary to confirm whether or not the windows were painted (from 1943 onwards this was quite common as they were of little use in night operations and helped reduce glare from flares, searchlights etc. Research showed that they were unpainted.
The aircraft took almost five years to complete. It was started in 1988.This section details some of the main aspects of its construction.
The fuselage centre section is made from a cross braced 1/4″ ply box. Front and rear spar boxes were attached to the fuselage. Detachable ply wing spars were made and tested to 400 and 100 lbs for the front and rear spars respectively. These structures were doweled and glued to 1/4″ ply ribs running along the undercarriage bay.
The aerofoil is obechi covered foam covered in glass cloth. The wing was complicated to build due to the rigging angles; +5 degrees on the main plane and no dihedral on the inner panels, but 7.8 degrees on the other panels. The outer spars and braces have sweep-back and sweep-forward on the front and rear respectively.
The outer wing panels were supported by two aluminum dihedral braces. These were screwed and glued to a ply sandwich.
Additionally the outer panels were given 2 degrees washout for extra stability (this being the only non-scale part of the wing design.
The tail sections, were made form using ply formers and balsa sheet. The nose section was built-up using traditional methods. The main complication here being the cockpit which needed to have a scale like structure as it would be very visible through the 30″ glasshouse.
Keith did not possess a lathe and so the construction of the undercarriage was undertaken by a friend,Bunny Osborne.
The undercarriage oleo’s were made form mild steel tube, silver soldered. Springs rated at 140lb/inch were used allowing 1 1/4″ travel for a 3-4g landing load. They are attached to 1/2″ ply formers screwed and glued to the main spar-box and extended to form the inner engine mountings. The pneumatic jacks used started life as estate car tailgate struts.
The retracts have been completely reliable and the only difference in design from the full-size is that they are pneumatic using 150 psi air pressure rather than hydraulic. This pressure allows approximately six cycles.
The wheels are 10″ diameter.
The special ‘V’ shaped trusses to hold the bouncing bomb were formed from aluminum sheet. The drive spindles and clutch came from old cassette players. The belt drive is replicated by a turntable belt driven by a 12V Mabuchi electric motor also scavenged from an old cassette player. The ‘V’ trusses are held in compression to support the bomb by 4 tie-rods, the inner ends of which locate on a central ‘firing’ pin. When this pin is pulled, by a servo, compression springs attached to the tie-rods fling them outwards releasing the rotating bomb. This is as per scale. The servo also actuates a microswitch, cutting off the power to the spin-motor.
The spinners were drawn up from scale drawings. They were produced from a steel former.
The engine nacelles were made from beaten 0.5mm aluminum sheet. It took approximately 200 hours to produce the four. Each one is attached to the engine bay formers by 96 screws.
Keith also wanted the crew to be represented as accurate as possible. Pete of ‘Pete’s Pilots’ produced superb scale facsimiles of the crew and their what they were wearing for the raid.
The cockpit floor is actually the roof of the bomb-bay so the large space below was utilised to house the three battery packs, retract cylinders (3 butane torch canisters), retract valve and bomb spin motor. These items weighed approximately 11lbs.
The precise position of the spotlights were modelled using narrow-beam, 20W, Dichroic lamps. For bright sunlight 50W lamps were required. Some care was necessary to ensure adequate cooling though to prevent the danger of the airframe catching fire!
The 30″ glasshouse and bulbous turret structures were made from model railway tracks. The tracks silver soldered extremely well and provide a handy flange for the glazing. Litho plate strips were cut and stuck over the framework to enable the scale width and riveting of the frame to be modelled. These were attached with cyno.
The bomb-aimers nose blister was produced using a child’s beachball from which a former was made using dental plaster. The navigator’s blister at the rear of the canopy was vacuum formed with the aid of a Christmas tree glass borble.
13lb of nose weight was required to bring the centre of gravity to 15% mean cord.
Eleven channels were required to control the aircraft. Ailerons, elevator, rudders, inner and outer throttles, flaps, retracts, navigation and altitude spotlights, bomb-spin and drop. This required the use of two transmitters, three receivers and 24 servos.
The two primary receivers operate on a common frequency from the main transmitter and each controlled an aileron and elevator. The rationale was that if a receiver fails then there will be a elevator-aileron pair to enable some control of the aircraft. The third receiver, which has a separate transmitter and frequency, provides the outer throttle control plus all the ancillary functions. Each throttle channel has a failsafe.
The on-board receivers and servo power was supplied from a 6V, 4Ah Nicad pack and a 3V sealed lead-acid backup pack. Ancillary power to the spotlights, navigational lights and spin motor was provided by a 12V, 4Ah Nicad pack. The power to the servos was routed through a distribution board so prevent the high currents going thorough the receivers. The servos were placed close to the relevant surfaces keeping link lengths to less than 3″.
The aircraft was demonstrated at all the main model shows for several years. It was one of the largest and most complex models around at that time. It was very graceful in the sky and gave great pleasure to many people. Unfortunately, the aircraft no longer exists being destroyed in a tragic crash several years ago.
Keith and his Lancaster at Longhorsley.
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