The early part of the 1950s saw Westland pursuing a design for a large heavy-lift helicopter for both civil and military applications. The new model was to be gas turbine powered. April 1951 saw the Yeovil firm responding to a British European Airways (BEA) requirement for a passenger helicopter. The Westland submission was for a 30-seat helicopter powered by a single Armstrong Siddeley Double Mamba engine. A year later saw a proposal for an even larger design, a fifty-seat machine with three Napier Eland gas turbines or four Rolls-Royce Dart engines. Projects in 1953 included a giant 400 seat Westland W.90 troop transport. None of these three transport helicopters were developed past the preliminary design phase.
In January 1954, Westland actively considered the licence production of the Sikorsky S-56, albeit with the original Pratt & Whitney radials replaced by Rolls Royce, Tyne or Mamba or even Napier Elands. The change-of engine would see the power plant side by side above the roof of the main cabin. The chairman of Westland's, Sir Eric Mensforth, later wrote to the Ministry of Supply (MoS) asking for funding for the S-56 development. July 1954 saw the company provide the MoS with a specific proposal that padded out their original submission. Further negotiations saw the company estimate that they could produce twelve S-56s by June 1958. Of the twelve quoted, five would have turboshaft engines. The lobbying by Westland carried little weight with the Government department and the project proposals failed to gain Governmental backing.
On 29th June 1955 negotiations were completed for Westland to extend their inter-company licensing agreement with Sikorsky to cover the S-56s main rotor, gearbox unit, tail rotor component as well as the transmission and control system. The rotor had five blades and featured a 72-foot diameter, the tail rotor was four bladed. Despite the loss of official backing, Westland took the decision to proceed with a private venture design for a heavy lift transport helicopter.
At the time, Westland had a prominent and heavy commitment to the Westland Wessex programme for the British military forces. This meant very few staff, engineering resources of financial allocations could be allocated to the new project. From the outset, two versions were identified; a short range civil transport and a flying crane. The civil transport would see the helicopter carrying around 40 passengers on trips of about 100 miles. The flying crane would be capable of carrying under-slung loads and outsize cargo. It would have the ability to lift a 15,000 lb load. It was designed to be able to carry up to fifty-one armed troops or four Jeeps.
One of the features of the Westland development was the location of the engines on the cabin roof. This enabled the maximum exploitation of the internal volume, whether for cargo or passengers. The design also provided a new rotor drive system. The prototype was fitted with a pair of 2,920 shp Napier Eland E.229A turboshafts, which were a relatively light engine at 1,500 lb. This was the principal reason the cabin roof option could be exploited. The company became one of the pioneer manufacturers to take advantage of turboshaft engines good power to weight ratio in this way.
Westland's previous experience with the Whirlwind and Wessex models had shown the essential requirement for extensive testing for all of the dynamic components. In order to achieve this, a ground test rig was set up. March 1956 saw a flying test rig constructed to integrate the control and engine systems. The thinking behind the flying rig was to save time and money by developing areas such as the flight deck instruments and controls while evaluating the flying characteristics. The prototype, which carried the factory designation WG.6, was designed as the simplest welded steel tube space frame possible. It located the cockpit, engines, transmission, undercarriage and rotor systems in their correct positions for the civil passenger variant. Initially there was no specific fuselage layout.
The fuselage framework section tapered towards the rear and was basically a rectangular shape. The space frame was made up of steel tube modules that were jig-welded. These modules formed a two-part structure with four longerons and twelve rectangular frames. The frames had longitudinal and transverse diagonal compression struts. The forward section made up around 50% of the overall length and consisted of five bays of equal width and height. St the front of the tubular frame was the glazed fish head monocoque twin-seat cockpit area. The flight test observers station and instrumentation was located in a boxed off area behind the pilots.
The aft part of the fuselage was made up of six bays of tapering rectangular section to which the tail rotor pylon was attached. Economy was one of the major factors in the design of the Westminster an existing Westland or Sikorsky components were utilised where possible. An example of this was the two 250 gallon fuel tanks, which were carried in the forward fuselage, were standard Westland Whirlwind overload units with an additional section incorporated. The undercarriage, attached to this part of the fuselage was originally intended to be the same as that fitted to the Wessex. However, design office calculations showed a risk of ground resonance and therefore a simpler design using one Dowty oleo-pneumatic shock absorber strut was used instead. The main wheels were from a Bristol B.170 Freighter cargo aircraft but with a reduced weight, achieved by removing 50% of the brakes. The Sikorsky S-56 provided the tail wheel unit.
On the prototype the rotor system was five bladed main rotors and a four bladed anti-torque tail rotor. The main rotor had offset flapping hinges and hydraulically damped drag hinges. The Westminster was fitted with anti-coning and droop restrainers.
The landing gear was fixed with two wheels and a tailwheel. The helicopter was fitted with Dowty air and oil shock struts. The main wheels were manufactured by Dunlop, whereas the tailwheel was made by Goodyear.
The power plant installed in the Westminster was basically a constant-speed system in which variations in the load caused by pitch changes of the rotor provided a power demand ob the engines. Load changes caused a change in the revs per minute figures, which was picked up by the sensor in the governor on the engines, which subsequently adjusted the fuel input to maintain a constant speed.
Electric de-icing of both rotors was planned for production examples. The Westminster, in its cargo guise, was to have special freight loading doors and hoist facilities. An external slinging point was to be provided for those loads too bulky to be carried internally.
After the construction of the second example some development was put into proposals for a short-range version and a variant with detachable pods for carrying cargo or passengers.