Armstrong Whitworth AW.681

Armstrong Whitworth Aircraft

Industry	Aerospace, Defense
Fate	Project cancelled
Headquarters	Coventry , United Kingdom
Products	Military aircraft
Parent	Hawker Siddeley Aviation

The Armstrong Whitworth AW.681, alternatively referred to as the Whitworth Gloster 681 or Hawker Siddeley HS.681, was a conceptual British military transport aircraft from the early 1960s designed for long-range operations with STOL capabilities. Developed by Armstrong Whitworth Aircraft, the design aimed to incorporate both Short Takeoff and Landing (STOL) and Vertical Takeoff and Landing (VTOL) functionalities.

This aircraft was intended to meet the criteria outlined in Operational Requirement 351, a specification heavily shaped by the NATO NBMR-4 standard. Armstrong Whitworth and the British Aircraft Corporation both submitted proposals, while engine manufacturers Bristol-Siddeley and Rolls-Royce Ltd competed to supply powerplants. After the requirement was revised to include VTOL capabilities, both designs underwent significant modification. Ultimately, Armstrong Whitworth’s AW.681 design was selected as the preferred candidate.

On 5 March 1962, the UK Government granted Armstrong Whitworth approval to advance with a detailed design study and prototype development, targeting an initial flight around 1966. Nonetheless, a governmental shift led to the cancellation of the HS.681 programme in February 1965. Despite Armstrong Whitworth’s attempts to reduce costs, culminating in the HS.802 variant proposal, the project was not reinstated. Following the termination of the AW.681 initiative, the decision was made to acquire the American Lockheed C-130 Hercules as an immediate solution to meet the Royal Air Force’s transport needs.

Background

By the year 1960, several significant British military transport aircraft projects, including the Short Belfast and the Vickers V-1000, had been either discontinued or significantly scaled back. Author Derek Wood attributes these setbacks primarily to insufficient backing from their main client, the Royal Air Force (RAF). One remaining viable prospect was Operational Requirement 351, which called for a capable medium-range cargo aircraft intended to succeed the piston-engine Blackburn Beverley and Handley Page Hastings models in RAF service. The RAF had defined the performance parameters for this requirement, specifying that the aircraft must demonstrate Short Takeoff and Landing (STOL) capabilities while carrying a minimum payload of 35,000 pounds (15,900 kg).

Various manufacturers submitted proposals in response. The British Aircraft Corporation (BAC) presented two concepts: the BAC.222, which was a domestic adaptation of the American Lockheed Hercules, and the BAC.208, an original design equipped with deflected-thrust engines and multiple lift fans. Wood remarked that the BAC.222 held strong potential for sustained production and represented an excellent fulfillment of the requirement; he further noted that both Lockheed and BAC actively campaigned for its adoption.

Armstrong Whitworth, then part of Hawker Siddeley Aviation, proposed the AW.681 design. The company opted to power this model with the Rolls-Royce RB.142 Medway, a turbofan engine under development at the time; according to Wood, Armstrong Whitworth considered this engine a solid foundation for a broader program targeting both civil and military applications.

Selection

A considerable delay occurred between the submission of proposals and the British government's decision on the requirement. Wood observed that strong advocacy within Britain's aerospace sector for a domestically developed aircraft may have discouraged the government from pursuing a joint venture with American firms, despite investigations into such possibilities.

A revised version of the requirement introduced a need for four deflected-thrust engines supplemented by multiple lift engines mounted beneath the wings to enable effective Vertical Takeoff and Landing (VTOL) performance. Armstrong Whitworth updated their AW.681 design accordingly; these revisions included vectored thrust nozzles and boundary layer control (BLC), among other enhancements. There was also significant interest in incorporating the Rolls-Royce Pegasus engine. BAC also modified their designs to comply with this updated specification; however, Wood indicates that by this stage Armstrong Whitworth had become the favored contender.

After thorough evaluation of all submissions, Armstrong Whitworth’s AW.681 was chosen to advance into development. On 5 March 1962, then Aviation Minister Julian Amery announced government approval to proceed with the HS.681 project, including plans for detailed studies and a prototype expected to conduct its initial flight by 1966. At that time, an order for approximately fifty units was anticipated. Additionally, arrangements were made to subcontract about twenty percent of production work to Short Brothers located in Belfast, Northern Ireland.

Termination

In 1964, after the establishment of a new government led by the Labour Party, a reassessment of major military projects was initiated. By February 1965, the government declared the cancellation of the HS.681 programme, opting instead to acquire the American Lockheed Hercules.

Armstrong Whitworth made a final attempt to revive the project by proposing a non-STOL variant named the HS.802. This version aimed to reduce expenses by utilizing the same wings and engines as those found on the HS.801 Nimrod maritime patrol aircraft. However, this redesign sacrificed the aircraft’s STOL capabilities in favor of cost savings.

Despite these efforts, the HS.802 failed to attract any orders, rendering further development impractical. Consequently, Armstrong Whitworth abandoned the project and closed its Coventry plant, resulting in approximately 5,000 job losses. Criticism of the programme's cancellation pointed to overly ambitious RAF requirements that pushed aircraft capabilities beyond feasible limits and a failure to finalize those specifications.

Design

The AW.681 featured a monoplane design with a circular fuselage cross-section, swept shoulder-mounted wings, and a high-set T-tail. Its rear fuselage was raised to incorporate large clamshell loading doors alongside a ramp, supplemented by multiple side-mounted fore-and-aft cabin doors for additional access. Configured for RAF use, it was designed to carry up to 60 paratroopers. The retractable main landing gear was housed within prominent bulges along the lower fuselage sides.

Propulsion was planned to come from four Rolls-Royce RB.142 Medway turbofan engines equipped with vectored thrust nozzles attached to pylons beneath the wings. After a competitive selection process between Rolls-Royce and Bristol-Siddeley, Armstrong Whitworth chose the RB.174-11 Medway engine variant, expected to deliver up to 13,800 pounds of thrust, with more powerful versions potentially reaching about 20,000 pounds. To enhance STOL and VTOL performance, thrust deflectors were intended to be installed around each engine.

The design incorporated boundary layer control (BLC) technology employing blown flaps on the wing leading edges; blown air would also operate the ailerons, flaps, and slats. The integration of BLC with thrust vectoring was expected to provide significant STOL capabilities powered solely by the Medway engines. Additional performance improvements were considered through water injection and afterburner (reheat) systems.

One variant envisioned VTOL capability but required substantial modifications. One approach involved supplementing the four Medway engines with eighteen 6,000 pounds-force (27,000 N) RB.162-64 lift engines. Alternatively, replacing the Medways with four Bristol Siddeley Pegasus ducted turbofans—the same engines used on the Harrier jump jet—was proposed. The Pegasus 5 or 6 models were projected to deliver roughly 18,000 pounds-force (80,000 N) thrust each. Both configurations were suggested as viable options for achieving practical VTOL operation.

See also

Bibliography

• Wood, Derek. Project Cancelled. Macdonald and Jane's Publishers, 1975. ISBN 0-356-08109-5.

External links

• Flight magazine, 1964 - HS.681