Jet Lift Or Wing Lift?

The primary motivation behind these developments is the widespread realisation that conventional airfields are becoming increasingly vulnerable to attack. Interest in V/STOL combat aircraft first took serious form in the 1950s, with the spread of tactical nuclear weapons to all major air forces, although engine technology was then hardly up to the job. Even today’s Rolls-Royce Pegasus turbofan with a thrust/ weight ratio of 7:1 still necessitates a short take-off (rather than allowing VTO) at normal gross weight in the various members of the Harrier family that it powers.

Although undoubtedly a worthwhile technological achievement, the British Aerospace Harrier did not gain widespread acceptance, since by 1965 NATO thinking had switched to a graduated response, i.e., a few days of war with conventional explosives, before slogging it out with various levels of nuclear weaponry. The matter of making air forces independent of runways therefore lost its urgency.

One major factor that brought V/STOL back in from the cold was the advent of dedicated airfield attack weapons in the 1970s. Reports of a runway “dibber” bomb being used in the Six Days War of 1967 were false (Arab airfields were in fact destroyed by standard free-fall 225kg bombs delivered in steep dive attacks along the runway centre lines but such advances were not long delayed.

France’s Matra Duranda (named after a mythological sword that could pierce rock) employs a braking parachute to give a steep impact from low-level delivery, and a boost rocket to punch the warhead down through the concrete, where it explodes to maximum effect. Weighing only 195kg, Durandal would typically be dropped in a stick of 10, each bomb penetrating up to 40cm of concrete and destroying a runway area of 150-200sq metres. The minimum launch altitude is 60 metres, and the maximum launch speed is 550 knots. The Spanish EXPAL BRFA has a similar method of operation, but weighs 330kg and penetrates 60cm of concrete, destroying an area of 180sq metres.

The British airfield attack weapon JP233 will be even more effective, in fact, it is so impressive that the US has bought in on the program. Developed by Hunting Engineering, it is intended to be deployed primarily on RAF Tornados and USAF F-111s. In this case, the aim is to deny the enemy the use of any linear system (runway, road, railway, etc) for an extended period. Released in a low-level, high-speed pass, JP233 scatters one type of submunition to disrupt the surface area on the target, and a second type to discourage any attempt at repairs (presumably with a mixture of time delay and influence fuses).

Looking further into the future, America’s cruise missile will be able to navigate to an airfield at a low level, line up with the runway by means of an optical contrast system, and scatter munitions along it. Similar developments are clearly open to the Soviets, who led the world in small anti-ship missiles, and have the capability to produce a whole range of anti-airfield weapons. Indeed, from the Warsaw Pact viewpoint this category of armament is even more attractive than it is to NATO, since Western combat aircraft (with one notable exception in the Harrier) lack the capability to operate from grass. Traditionally, Soviet fighters have had soft tyres and rocket attachment points, making them less dependent on runways, at least for take-off.

At present the main WarPac threats to NATO air bases take the form of massed strikes by aircraft such as the MiG-27 Flogger and Su-19 Fencer and hits by surface-surface missiles with massive conventional explosive or chemical warheads. Despite well-publicised NATO exercises with aircrew and groundcrew in full protective NBC kit, it is obvious that chemical weapons would seriously degrade the effectiveness of operations from conventional bases.

While airfield attack considerations are forcing the trend toward improved take-off and landing performance in land-based aircraft (either to operate from bombed runways or to disperse from airfields), in the maritime context there is a corresponding desire to get away from the traditional concentration of air power in a small number of ships. This is the US Navy’s thinking behind the plan announced in 1977 to develop a family of fixed-wing V/STOL aircraft to replace the Service’s conventional aircraft during the 1990s. Below the super-power level, many navies see conventional fixed-wing aircraft as requiring a ship that is far· too expensive in construction and operation to contemplate, whereas an aircraft designed from the outset for very low take-off and landing speeds would enable them to stay in the naval air power game with relatively inexpensive ships.

Current Programs

Given this strong motivation to develop V/STOL or STOL military aircraft, the question is whether the jet lift or the wing lift category is likely to gain prominence in the 1900s. As a basis for this discussion, it may be worthwhile to summarise where existing programs in these categories stand at the present time.

Dealing firstly with V/STOL, the aircraft of greatest interest to the RAN is the McDonnell Douglas AV-8B Advanced Harrier. This is based on the British Aerospace Harrier AV-BA, but has an enlarged yet lightweight wing of supercritical section and carbon fibre construction, increased fuel volume, improved intakes, ground cushion devices under the fuselage, modified nozzles to optimise lift in STO, and two extra weapon pylons. The AV-BB is expected roughly to double the warlord or radius of the AV-BA from a given ground roll.

However, it is worth noting that the AV-BB is a dedicated close support aircraft for the US Marine Corps. It does not have the air defence capability that the RAN would demand, although this could doubtless be added at a price. Initial operational capability (IOC) is scheduled for 1985, which would suit the Melbourne replacement well enough.

Air defence is, however, the primary role of the BAe Sea Harrier, deliveries of which began to the Royal Navy last year. The first of four RN squadrons (No 800) will embark on HMS Invincible, the first of the Service’s three new anti-submarine cruisers, later this year. The Sea Harrier retains the wing of the land-based Harrier, but has a raised cockpit, multi-mode radar in the nose, a revised nav-attack system, and new weapons to suit its air defence, reconnaissance, and strike roles.

For land-based operations, BAe is proposing the so-called Harrier Mk 5 with an enlarged “tin” wing, extra pylons, reduced outrigger track to suit highway take-offs, and additional fuel space. At time of writing, the RAF is deciding whether to support this development, or to seek government approval to buy AV-8Bs from McDonnell. This manufacturer has been promoting the AV-8B in the UK on the basis that foreign support would enhance the prospects of US government funding continuing, although the RAF order would add only 60-100 aircraft to the USMC figure of 250-350.

The British manufacturers are in a difficult position, since both BAe and Rolls-Royce stand to gain more financially in the short-term by supporting the AV-BB, although this policy would clearly damage export prospects for the Sea Harrier and would pass the V/STOL lead to America. Both BAe and RR would feel happier if assured of UK government support for a British supersonic V/STOL technology demonstrator.

The RAF is in an equally difficult position, in that the design of the Harrier Mk 5 emphasises manoeuvrability (hence the large leading edge root extensions), whereas the AV-8B wing was designed primarily for high lift in STO. In the RAF context the Mk 5 was originally on safe ground in being designed as a simply retrofit Job (which is impossible with the AV-8B wing), but this concept now appears to have lost favour. Further complicating the matter, the Mk 5 would require more engine development because of the weight of the “tin” wing, but it would be less costly to manufacture.

In addition, although the AV-8B appears to have reached its design aim in regard to STO, reports of its in-flight performance (particularly its maximum speed and manoeuvrability) have been less than ecstatic. The second of the two YAV-8Bs was therefore considerably modified, but crashed after the engine failed to relight following a high altitude surge on November 15, just before the RAF was due to evaluate it. This assessment was accordingly delayed to February or March, when the other YAV-8B was due to have the modifications.

The AV-8B was excluded from the US Navy plan to develop a family of V/STOL aircraft for the 1990s. In the early days of this program, priority was given to V/STOL-A, a large general-purpose subsonic aircraft to perform ASW, cargo, AEW, marine assault, SAR, tanker and other missions. Initial estimates put IOC at 1988. However, in 1978 it was decided to reduce V/STOL-A to a technology study, and switch the emphasis to V/STOL-B, a supersonic fighter for air defence and strike duties. At present it appears that some of the V/STOL-A missions will be carried out by advanced forms of helicopter (e.g. the Bell tilt-rotor, and the X-Wing), while others will go to a relatively small aircraft, designated V/STOL-0. Little information is available on this project, other than that Grumman and Vought are involved.

The V/STOL-B supersonic fighter was scheduled for IOC in the mid-1990s, although this could well be delayed. Various powerplant concepts are under study, including RollsRoyce’s augmented vectored-thrust engine, as demonstrated in the 1960s in the form of the Pegasus and the BS100 (the latter then slated for the ill-fated Hawker P1154).

Although the simplest approach to supersonic V/STOL, in a flat-riser the single vectored-thrust engine is oversized for cruise, with consequent penalty in fuel consumption. One popular alternative is to optimise the propulsion engine(s) for in-flight demands, turn the thrust downward at low speeds, and add sufficient lift engines to give V/STOL capability. The result is known as the “lift plus lift/cruise” concept.

Reportedly promoted by General Electric, RALS (Remote Augmentation Lift System) would take bypass air from propulsion turbofans in the rear fuselage, and duct it forward to combustion chambers to produce jet lift, balancing the thrust of the deflected core flow. This is also known as the “bleed and burn” system.

Jet augmenters have often shown thrust boosts of around 70 per cent in small-scale laboratory tests, but typically produce a 20 per cent thrust loss when tried full-scale. Nevertheless, the US Navy hopes to fly an “augmented wing” (some people call it “demented wing”) technology demonstrator based on the Rockwell XFV-12A.

Conceptually the simplest option in powerplant terms, the VATOL (Vertical Attitude Take-Off and Landing) aircraft would operate from a tilting platform at the stern of the ship. Pilot orientation problems would be minimised by hingeing the forward fuselage, but the VATOL aircraft would still suffer the traditional tail-sitter’s problem of lacking an STO overload capability.

During the 1980s these V/STOL-B options will presumably be reduced in number, to provide the US Navy with a basis for practical development. The program may also win support from the US Air Force, which is coming to recognise the potential benefits of a V/STOL strike fighter for European operations, and from the Royal Navy, which will need a supersonic replacement for the Sea Harrier around the end of the century.

Turning to STOL fighters, the simplest is the Saab Viggen, currently the most advanced European combat aircraft in service. Using a close-coupled canard optimised for lift at low speeds, plus a mobile landing aid to minimise touchdown scatter, automatic speed control on the approach, a beefed-up landing gear to eliminate the need for a round-out, and thrust reversal activated by main gear touchdown, the Viggen can be operated from dispersed sites on short straight stretches of highway.

Panavia claims shorter take-off and landing distances for Tornado, although this aircraft is far more complex, having a variable-sweep wing and sophisticated leading-and trailing edge flaps. However, in take-off demonstrations, the Tornado appears to lack the acceleration of the Swedish aircraft. On the other hand, Viggen’s short landing technique (which is more akin to naval practice) seems to be so hard on the aircraft that it is rarely practised.

Although this discussion is primarily concerned with combat aircraft, it should be noted that STOL military transports have also been developed. The McDonnell Douglas YC-15 has its four engines blowing directly on its trailing edge flaps at low speeds, while the Boeing YC-14 and Antonov An-72 Coaler each have two overwing engines for upper surface blowing. However, the market as yet appears to be showing little interest in such aircraft, presumably because of their high cost and limited payload-radius performance.

Personal View

The forthcoming struggle between V/STOL and STOL is thus a highly complex matter, depending on technological developments somewhat nebulous operational requirements, and budgetary constraints. In attempting to predict the outcome, the writer can only aim to identify the critical factors and give a personal opinion of which concepts will succeed.

Dealing firstly with transports, in land-based terms, it is difficult to identify a requirement involving true vertical operation that cannot adequately be met by advanced forms of helicopter, hence STOL aircraft appear likely to prevail over performance V/STOL transports. Four-engined aircraft such as the YC-15 are likely to be favoured from considerations of engine failure and accessibility, structure weight, and interior noise. However, in regard to naval transports, there may well be a genuine need to combine high transit speeds with vertical landings, hence the V/STOL aircraft will come into its own. Turning to combat aircraft, the struggle between jet lift and wing lift depends firstly on the ground roll available. There is a break-even point in the region of 500 metres, beyond which STOL is the natural solution, and below which jet lift is practically essential. However, in considering operations from conventional bases, studies of runway bombing produce residual ground rolls around this figure, rather than pointing clearly to either concept.

Make no mistake: a modern well-defended airbase with wide twin runways and good taxiways and perimeter tracks (and full provisions for rapid runway repairs) would still need heavy sustained bombing to reduce the available ground run to a few hundred metres, but there is no doubt that (for example) the Warsaw Pact could achieve this at selected airfields in West Germany for a limited period.

However, in dispersed operations Harriers have shown significant advantages in vertical landings, which permit approaches from any direction, and have potential for much reduced weather minima. Vertical landings are also favoured for naval operations, since they eliminate the need to clear the deck and turn the ship into wind.

In the writer’s view, STOL aircraft will prevail for airfield operations, since they involve less development. For example, the Northrop F-18L with thrust reversers added would be quite a useful short field aircraft. However, an increasing number of air forces will demand the capability to disperse some combat aircraft away from conventional bases, which in turn will require V/STOL capability. Likewise the majority of navies with fixed-wing aviation elements will switch to V/STOL to reduce the cost of carriers and achieve more flexible operations.

However, while V/STOL combat aircraft improve to reach high supersonic speeds and to minimise the penalties of jet lift, STOL aircraft may change to acquire some of the attributes of V/STOL fighters. For example, both the Tornado and Viggen will probably be developed to use thrust reversal in a dogfight situation, something currently only possible with the Harrier.

Likewise, it is just conceivable that an aircraft such as the F-18 (with moderate wing loading, high thrust/weight ratio, and leading edge extensions) could take-off unassisted from a short deck run, using the Sea Harrier’s ski-jump. Landings would still require an arrester, but the ship would be smaller than normal, and the catapult would be eliminated.

With technological advances in prospect for both the V/STOL and STOL concepts, the future cannot be predicted with any certainty. All that is clear is that both jet lift and wing lift will have their parts to play in the development of combat aircraft over the next two decades.