The Armstrong Whitworth AW.681, also known as the Whitworth Gloster 681 or Hawker Siddeley HS.681, was a projected
British long-range
STOL
A short takeoff and landing (STOL) aircraft is a conventional fixed-wing aircraft that has short runway requirements for takeoff and landing. Many STOL-designed aircraft also feature various arrangements for use on airstrips with harsh conditio ...
military transport aircraft design of the early 1960s. Developed by manufacturer
Armstrong Whitworth Aircraft, it was intended to be capable of achieving both
Short Takeoff and Landing
A short takeoff and landing (STOL) aircraft is a conventional fixed-wing aircraft that has short runway requirements for takeoff and landing. Many STOL-designed aircraft also feature various arrangements for use on airstrips with harsh condition ...
(STOL) and
Vertical Takeoff and Landing
A vertical take-off and landing (VTOL) aircraft is one that can take off and land vertically without relying on a runway. This classification can include a variety of types of aircraft including helicopters as well as thrust-vectoring fixed-win ...
(VTOL) performance.
The AW.681 was designed to satisfy the performance specification of
Operational Requirement 351, which had been heavily influenced by the
NATO
The North Atlantic Treaty Organization (NATO, ; french: Organisation du traité de l'Atlantique nord, ), also called the North Atlantic Alliance, is an intergovernmental military alliance between 30 member states – 28 European and two No ...
specification
NBMR-4. Both Armstrong Whitworth and the
British Aircraft Corporation
The British Aircraft Corporation (BAC) was a British aircraft manufacturer formed from the government-pressured merger of English Electric Aviation Ltd., Vickers-Armstrongs (Aircraft), the Bristol Aeroplane Company and Hunting Aircraft in 1 ...
vigorously competed to be awarded the contract, as well as engine manufacturing
Bristol-Siddeley and
Rolls-Royce Ltd to power it. Both submissions had to be revised heavily following an update to the Requirement that added VTOL performance. Armstrong Whitworth's AW.681 submission emerged as the favoured option for selection.
On 5 March 1962, Armstrong Whitworth received UK Government authorisation to proceed with a detailed project study and the construction of a
prototype
A prototype is an early sample, model, or release of a product built to test a concept or process. It is a term used in a variety of contexts, including semantics, design, electronics, and Software prototyping, software programming. A prototyp ...
, which was projected to fly around 1966. However, following a change in government, during February 1965, the cancellation of the HS.681 programme was announced. Despite efforts to cheapen the design by Armstrong Whitworth, culminating in the HS.802 proposal, there was no reversal of this decision. Following the AW.681's termination, it was decided to purchase
American
American(s) may refer to:
* American, something of, from, or related to the United States of America, commonly known as the "United States" or "America"
** Americans, citizens and nationals of the United States of America
** American ancestry, pe ...
-built
Lockheed Hercules as an off-the-shelf means of fulfilling the RAF's transport requirements instead.
Development
Background
By 1960, several major British programmes in the field of military transport aircraft had been terminated or considerably curtailed, such as the
Short Belfast
The Short Belfast (or Shorts Belfast)Mondey 1981, p. 228. is a heavy lift turboprop freighter that was built by British manufacturer Short Brothers at Belfast. Only 10 aircraft were constructed, all of which entered service with the Royal Air ...
and the
Vickers V-1000; according to author Derek Woods, these outcomes had been largely due to a lack of meaningful support from their principal customer, the
Royal Air Force
The Royal Air Force (RAF) is the United Kingdom's air and space force. It was formed towards the end of the First World War on 1 April 1918, becoming the first independent air force in the world, by regrouping the Royal Flying Corps (RFC) and ...
(RAF).
One promising opportunity remaining was
Operational Requirement 351, which called for a capable medium-range freighter to replace both the
Beverley
Beverley is a market town, market and minster (church), minster town and a civil parishes in England, civil parish in the East Riding of Yorkshire, England, of which it is the county town. The town centre is located south-east of York's centre ...
and
Hastings
Hastings () is a large seaside town and borough in East Sussex on the south coast of England,
east to the county town of Lewes and south east of London. The town gives its name to the Battle of Hastings, which took place to the north-west ...
then in service with the RAF. Officials within the service had heavily shaped the specified performance criteria of this requirement; specifically, the aircraft was to possess
Short Takeoff and Landing
A short takeoff and landing (STOL) aircraft is a conventional fixed-wing aircraft that has short runway requirements for takeoff and landing. Many STOL-designed aircraft also feature various arrangements for use on airstrips with harsh condition ...
(STOL) capabilities, which it was to perform while carrying a minimum payload of 35,000lbs.
[Wood 1975, p. 225.]
In response, multiple proposals were received from various manufacturers. The
British Aircraft Corporation
The British Aircraft Corporation (BAC) was a British aircraft manufacturer formed from the government-pressured merger of English Electric Aviation Ltd., Vickers-Armstrongs (Aircraft), the Bristol Aeroplane Company and Hunting Aircraft in 1 ...
(BAC) decided to offer two designs; the BAC.222, which was a localised development of the
American
American(s) may refer to:
* American, something of, from, or related to the United States of America, commonly known as the "United States" or "America"
** Americans, citizens and nationals of the United States of America
** American ancestry, pe ...
Lockheed Hercules, and the indigenously-developed BAC.208, which would have been furnished with deflected-thrust engines and multiple lift fans.
[Wood 1975, pp. 225-226.] According to Woods, the BAC.222 proposal possessed very strong long-term production prospects and represented an excellent solution to the requirement; he also noted that both Lockheed and BAC aggressively lobbied for its construction.
Armstrong Whitworth, which was by then a constituent of
Hawker Siddeley Aviation
Hawker Siddeley was a group of British manufacturing companies engaged in aircraft production. Hawker Siddeley combined the legacies of several British aircraft manufacturers, emerging through a series of mergers and acquisitions as one of onl ...
, submitted their own design, designated AW.681. The company had chosen to adopt the
Rolls-Royce RB.142 Medway, an in-development
turbofan
The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft engine, aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and "fan": the ''turbo'' portion refers to a gas turbine engine which ac ...
engine, to power the type; according to Wood, Armstrong Whitworth viewed the engine as a suitable basis for a broader programme covering both civil and military markets.
[Wood 1975, pp. 226-228.]
Selection
There was a lengthy interval between the submissions and the British government making any decisions regarding the requirement.
Wood observed that there was considerable pressure within the British aircraft industry for an indigenously developed design to be favoured, which had perhaps dissuaded the government from opting for a collaborative programme with the Americans despite explorations of such arrangements.
[Wood 1975, p. 226.]
A second issue of the requirement called for the presence of four deflected thrust engines, which were to be suitably augmented by several lift engines housed underneath the wings to allow the aircraft to achieve a viable
Vertical Takeoff and Landing
A vertical take-off and landing (VTOL) aircraft is one that can take off and land vertically without relying on a runway. This classification can include a variety of types of aircraft including helicopters as well as thrust-vectoring fixed-win ...
(VTOL) capability. In response, Armstrong Whitworth issued multiple revisions of their AW.681 proposal to accommodate this new need; these revised submissions incorporated both
vectored thrust
Thrust vectoring, also known as thrust vector control (TVC), is the ability of an aircraft, rocket, or other vehicle to manipulate the direction of the thrust from its engine(s) or motor(s) to control the attitude or angular velocity of the ve ...
nozzles and
boundary layer control (BLC) amongst other features; there was also considerable interest in adopting the
Rolls-Royce Pegasus engine.
[Wood 1975, p. 228.] BAC also revised their designs in line with the requirement change, although Wood notes that Armstrong Whitworth had already emerged as the front runner for selection around this time.
Following a competitive evaluation of the submissions, the AW.681 proposal was selected for further development. On 5 March 1962,
Julian Amery, the
Aviation Minister, announced that the UK Government had authorised the go-ahead for the HS.681 and that, in addition to a detailed project study being conducted, a
prototype
A prototype is an early sample, model, or release of a product built to test a concept or process. It is a term used in a variety of contexts, including semantics, design, electronics, and Software prototyping, software programming. A prototyp ...
of the aircraft was projected to perform its
maiden flight
The maiden flight, also known as first flight, of an aircraft is the first occasion on which it leaves the ground under its own power. The same term is also used for the first launch of rockets.
The maiden flight of a new aircraft type is alwa ...
sometime during 1966.
Around this time, it was anticipated that a total of 50 aircraft would be ordered. Furthermore, arrangements were also made for around 20 percent of the production work to be
sub-contracted to
Short Brothers
Short Brothers plc, usually referred to as Shorts or Short, is an aerospace company based in Belfast, Northern Ireland. Shorts was founded in 1908 in London, and was the first company in the world to make production aeroplanes. It was particu ...
in
Belfast
Belfast ( , ; from ga, Béal Feirste , meaning 'mouth of the sand-bank ford') is the capital and largest city of Northern Ireland, standing on the banks of the River Lagan on the east coast. It is the 12th-largest city in the United Kingdo ...
,
Northern Ireland
Northern Ireland ( ga, Tuaisceart Éireann ; sco, label= Ulster-Scots, Norlin Airlann) is a part of the United Kingdom, situated in the north-east of the island of Ireland, that is variously described as a country, province or region. Nort ...
.
Termination
During 1964, following the formation of a new government dominated by the
Labour Party, a review of all ongoing large military programmes was announced. During February 1965, the cancellation of the HS.681 programme was announced by the government; in its place, it would procure the
American
American(s) may refer to:
* American, something of, from, or related to the United States of America, commonly known as the "United States" or "America"
** Americans, citizens and nationals of the United States of America
** American ancestry, pe ...
-built
Lockheed Hercules to meet the RAF's needs instead.
In response to the order's cancellation, Armstrong Whitworth pursued a last-minute effort, promoting the development of a non-STOL version of the HS.681, designated the HS.802. To achieve lower costs, this proposal would have adopted both the same wings and engines as had been used on the
HS.801 Nimrod maritime patrol aircraft
A maritime patrol aircraft (MPA), also known as a patrol aircraft, maritime reconnaissance aircraft, or by the older American term patrol bomber, is a fixed-wing aircraft designed to operate for long durations over water in maritime patrol roles ...
.
[Wood 1975, p. 227.] Wood notes that the redesign, while likely being cheaper, came at the sacrifice of the type's STOL capabilities.
Despite lobbying, the HS.802 failed to secure any commitments, making further development untenable; accordingly, Armstrong Whitworth shelved the project entirely. As a consequence of the cancellation, the Armstrong Whitworth factory in
Coventry
Coventry ( or ) is a City status in the United Kingdom, city in the West Midlands (county), West Midlands, England. It is on the River Sherbourne. Coventry has been a large settlement for centuries, although it was not founded and given its ...
was closed, resulting in the loss of 5,000 jobs. Wood critiqued the programme and its cancellation, attributing some responsibility to overly-ambitious requirements of the RAF that had pushed the limit of aircraft capabilities, as well as the failure to settle those requirements.
[Wood 1975, p. 231.]
Design
In its basic configuration, the AW.681 was a
monoplane
A monoplane is a fixed-wing aircraft configuration with a single mainplane, in contrast to a biplane or other types of multiplanes, which have multiple planes.
A monoplane has inherently the highest efficiency and lowest drag of any wing confi ...
with a circular cross-section fuselage, featuring a
swept shoulder-mounted wing and a high
T-tail
A T-tail is an empennage configuration in which the tailplane is mounted to the top of the fin. The arrangement looks like the capital letter T, hence the name. The T-tail differs from the standard configuration in which the tailplane is ...
.
The rear
fuselage
The fuselage (; from the French ''fuselé'' "spindle-shaped") is an aircraft's main body section. It holds crew, passengers, or cargo. In single-engine aircraft, it will usually contain an engine as well, although in some amphibious aircraft t ...
was upswept, accommodating both sizable
clamshell loading doors and a ramp; further egress was to have been provided by several side-mounted fore-and-aft cabin doors.
In the configuration proposed for the RAF, the AW.681 would have been able to accommodate a maximum load of 60
paratroop
A paratrooper is a military parachutist—someone trained to parachute into a military operation, and usually functioning as part of an airborne force. Military parachutists (troops) and parachutes were first used on a large scale during World ...
s. The retractable main
undercarriage
Undercarriage is the part of a moving vehicle that is underneath the main body of the vehicle. The term originally applied to this part of a horse-drawn carriage, and usage has since broadened to include:
*The landing gear of an aircraft.
*The ch ...
was accommodated within large bulges on the lower sides of the fuselage.
[Wood 1975, pp. 228-229.]
The AW.681 was to have been powered by an arrangement of four
Rolls-Royce RB.142 Medway turbofan
The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft engine, aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and "fan": the ''turbo'' portion refers to a gas turbine engine which ac ...
engines; these would have been combined with a series of
vectored thrust
Thrust vectoring, also known as thrust vector control (TVC), is the ability of an aircraft, rocket, or other vehicle to manipulate the direction of the thrust from its engine(s) or motor(s) to control the attitude or angular velocity of the ve ...
nozzles mounted upon pylons underneath the wings. The selection of the powerplant was subject to a considerable fight between engine manufacturers Rolls-Royce and
Bristol-Siddeley. Ultimately, Armstrong Whitworth opted for the RB.174-11 model of the Medway, which was projected to generate a maximum thrust of 13,800lb, although more powerful models would have attained around 20,000lb according to Rolls-Royce.
[Wood 1975, pp. 227-228.] For improved STOL/VTOL performance, deflectors were to be installed around the engines.
[Wood 1975, p. 229.]
As designed, the AW.681 was set to feature
boundary layer control (BLC), which would have used
blown flaps mounted upon the
leading edge
The leading edge of an airfoil surface such as a wing is its foremost edge and is therefore the part which first meets the oncoming air.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', page 305. Aviation Supplies & Academics, ...
s of the wing; the
aileron
An aileron (French for "little wing" or "fin") is a hinged flight control surface usually forming part of the trailing edge of each wing of a fixed-wing aircraft. Ailerons are used in pairs to control the aircraft in roll (or movement around ...
s,
flaps, and
slat
Slat, slats, or SLAT may refer to:
* Slat (aircraft), aerodynamic surfaces on the leading edge of the wings of fixed-wing aircraft
* a Lath, a narrow strip of straight-grained wood used under roof shingles or tiles
* Vertical or horizontal pieces ...
s would all have been operated using blown air.
The combination of BLC and thrust vectoring would have provided the AW.681 with a high degree of STOL performance, which could be achieved exclusively using its Medway engines. To achieve further performance gains, the use of both
water injection and
reheat
An afterburner (or reheat in British English) is an additional combustion component used on some jet engines, mostly those on military supersonic aircraft. Its purpose is to increase thrust, usually for supersonic flight, takeoff, and combat ...
were proposed options.
One proposed variant of the AW.681 would have been capable of VTOL performance, albeit requiring extensive alterations in order to achieve this. In one configuration, it would have been powered by an additional eighteen
RB.162-64 lift engines in addition to its four Medway engines.
Alternatively, the Medway engines could have been replaced by four
Bristol Siddeley Pegasus
The Rolls-Royce Pegasus, formerly the Bristol Siddeley Pegasus, is a British turbofan engine originally designed by Bristol Siddeley. It was manufactured by Rolls-Royce plc. The engine is not only able to power a jet aircraft forward, but also ...
ducted-flow turbofan engines, the same power plant as used on the
Harrier jump jet. The specific version of the Pegasus projected for use was 5 or 6, which would have been rated at around .
“Pegasus engine variants.”
harrier.org.uk, Retrieved: 20 September 2019. Both approaches were proposed as being capable of obtaining a realistic VTOL capability.
Specifications (proposed STOL)
See also
References
Citations
Bibliography
*
* Wood, Derek. ''Project Cancelled''. Macdonald and Jane's Publishers, 1975. .
External links
{{DEFAULTSORT:Armstrong Whitworth Aw.681
1960s British military transport aircraft
AW.681
VTOL aircraft
Cancelled military aircraft projects of the United Kingdom
Quadjets
High-wing aircraft
T-tail aircraft