Short take-off and vertical landing; a close look at an historical technical breakthrough.

Short Take-off and Vertical Landing

A Close Look at an Historic Technical Breakthrough

Vertical take off was not really a new idea. At the beginning of the 1940s, the Germans developed the Bachem 339 Natter, a rocket-engined fighter which took off from a tower just like the Space Shuttle with the help of four solid propellant boosters and was recovered by parachute. Not until the emergence of the jet engine was the concept revived.

From the end of the 1950s onwards, there was a blossoming of several exotic designs whose main merit was to clarify the problem and to "darwinize" potential solutions. Almost every manufacturer had a go, with the exception of Bell, who tackled the problem the other way round and launched research into the tilt-rotor. Thirty years later, Bell's V-22 Osprey is a remarkable example of rewarding tenacity.

The various designs which were tested at the time can be divided into five main families: * The "tail-sitters", which rested on the ground on their tail, taking-off vertically: in the USA the Ryan X-13 (1956) and in France the SNECMA "Atar Volant" (1959). * The "flow-switchers", with twin 2D thrust diverters and aerodynamic "gearing": e.g. the Lockheed XV-4A (1963), Ryan XV-5A (1964) and Rockwell XFV-12A (1978). * Aircraft using pure dynamic lift. Several small, fixed jet engines located in the wing and fuselage provided vertical thrust while the cruise engine installation was conventional: e.g. the Shorts SC-1 (1960), Dassault Balzac (1963), Dassault Mirage IIIV (1966) and Lockheed XV-4B (1966). * Hybrids (lift engines plus a cruise engine supplying vectored thrust), e.g. the EWR VJ-101C (1963), Dornier DO31 (1967), VAC 191B (1972) and the Soviet Yak-36 Freehand (1976). * Pure vectored thrust: e.g. the Bell X-14 (1958), the British P.1127 (1960) and the Soviet Yak-38 Forger (1967).

The prototypes mentioned above are only a few of the numerous designs which were flight-tested. Most of them, at least in the West, were fitted with British engines.

Of all the configurations tested during the past three decades, only two survived and were developed to the production stage: the Soviet Yak-38 Forger and the British P.1127, which gave birth to the Kestrel and in turn the Harrier family.

It should be noted and kept in mind for further reference that, with the exception of the fifth category (pure vectored thrust), all prototypes in the first four categories could only take off vertically and were therefore deprived of the short rolling take-off option which was later to prove such an advantage due to the extra payload it permitted. The British very early on foresaw advantages of the rolling take-off and this is why they developed a simple, reliable and above all very fast-acting (100[degrees]/s) vectoring system.

Where do We Stand Now, Thirty Years On?

Today, with the exception of a few British Harrier units, the NATO Tactical Air Forces still depend on their 2 400 metres-long, paved runways. The number of such suitable airstrips available in Western Europe is well below a hundred. The potential enemy knows their exact position and they are not likely to move overnight. It should not be forgotten that during the past three decades, anti-runway munitions have made tremendous strides. Not only are they now extremely accurate, using submunition-dispensers carried in stand-off guided missiles, the pilots no longer have to overfly the target. What is more, these munitions are now interspersed with a mix of anti-personnel mines intended to slow down or prevent the repair work.

To pretend that airbases can be defended 100% against air attacks at a reasonable cost is another typical example of refusing to face unpalatable facts. Who can say how many runways will still be usable at dusk on D-day? The most pessimistic optimists maintain that even on a cratered runway an undamaged section 500 or 600 metres long can always be found to allow take-offs. Granted. However, the real problem is landing. A modern conventional fighter can take off on a very short strip thanks to its 1 g acceleration at full throttle. But on landing its deceleration is about 0,25 g on a dry surface and never exceeds 0,5 g even when making full use of thrust reversers (only fitted on Tornados and Viggens). While on take-off the pilot can use every foot of the available lenght by releasing brakes at the very edge of the paved surface, the precision of the touchdown point on landing is far from being so accurate, never being less than within about a hundred metres even for a very experienced pilot. By and large, the minimum length required for landing is at least twice that required for take-off.

To close this chapter, it is an odd fact that the only West European countries whose tactical air forces can reasonably hope to survive an all-out war are two neutral powers, Sweden and Switzerland. With a population of 8.3 million people, Sweden maintains 500 modern combat aircraft (France has barely 200 more) sheltered in tunnels and operated from a score of narrow strips scattered in the forests or from highway sections. The 300 Swiss fighters are safely protected in caves dug in the mountains, together with all their logistics. Runways are generally sited in deep, narrow valleys, surrounded by 3,000-metre mountains which provide the cheapest and most efficient protection against air attacks.

At Sea

Even more than on land, gigantism at sea has now become a chronic affliction that leads inexorably to a budgetary and operational dead-end. The well-proven deck-landing system aboard ships has not changed in more than half-a-century, consisting of arresting wires on the deck and a tail hook on the aircraft.

Launching and recovering aircraft at sea still require a strong relative wind over deck, exactly fore-and-aft. The carrier - and her escorts - must therefore constantly change course and speed according to the tactical situation, even if a single aircraft has to be launched or recovered. All this takes time and planning and wastes ship's fuel.

A Carrier Vessel Battle Group (CVBG) at sea is far more vulnerable than it was three decades ago. The threat of new weapons like long-range anti-ship missiles and nuclear attack submarines is such that defending the so-called sanctuary in which the carrier is supposed to move around freely and safely takes up an ever-increasing proportion of its shipborne aviation, in fact considerably more than 50%. Paradoxically on its "attack" carriers, the US Navy spends the bulk of its budget on defensive assets.

The net result of these aberrations is that the "cost-effective" size of an aircraft carrier able to operate modern conventional fighters is now close to 100 000 tonnes at a unit price of roughly $ 4 billion, which is an awful lot of eggs in one basket. At that rate, even the USA can hardly keep up. Former Secretary of the Navy John Lehman's "Maritime Strategy" called for maintaining 15 super-carriers in the fleet, but the actual projected number is already dwindling to 12.

The British Answer

Despite official scepticism and the abandonment of VTOL development by most manufacturers, the British, alone, did not give up. Pragmatic, inventive, stubbornly sticking to their own views and with supreme disregard for outside opinion they casually went their own way. They grasped very early on that thrust vectoring was the right approach.

Incidentally, the basic configuration of the four vectoring nozzles was brought to England by a French engineer, Michael Wibault, who in 1956 approached Bristol Aero Engine (now part of Rolls-Royce) with his Gyroptere design, the ancestor of what was to become the Harrier.

The experimental Hawker P.1127 built on the Gyroptere design first flew in 1960 equipped with a Rolls-Royce Pegasus jet engine. Then came the Kestrel and eventually its production derivative (more than 90%) the Harrier, which entered service in the Royal Air Force in 1969. The Royal Navy joined in much later and rather hesitatingly with the Sea Harrier (a "navalized" Harrier GR3) which only entered fleet service in 1979. But how could the Navy have guessed at the time that the ski-jump take-off technique would so enhance the payload performance of the aircraft? Incidentally, turning the Harrier into Sea Harrier was achieved at a "cost" of only 45 kg in empty weight, as compared with several hundred kilograms for the navalization of a conventional fighter.

Interestingly, the costs of the initial development stage (the first three years on the P.1127 and the first four years on the Pegasus engine) were entirely borne by the manufacturers without any order, grant or subsidy from the government. This was very fortunate for had the government funded the project, it would certainly have imposed its own solution, which at the time was the hybrid concept (separate lift plus lift and cruise engines), and there would today be no Harrier. The programme suffered from the usual inter-services rivalry. At the time, the RAF considered it essential for a fighter to have a speed of Mach 2 and therefore showed very little interest (except that it saw in the Harrier the spectre of the re-emergence of the Fleet Air Arm). When the Navy finally came round to accept the STO/VL concept, it had to take care to conceal its true intentions. HMS "Invincible", the lead ship of a batch of three new aircraft-carriers, was designated "through-deck cruiser" and at the time of her commissioning, the Navy selected a name which no previous carrier had ever borne.

From 1971 onwards, the US Marine Corps ordered a total of 110 Harriers, including eleven two-seaters under the American designation AV-8A to be operated from LPHs and LPDs. For years, the Marines had been trying to achieve the operational self-sufficiency which only the STO/VL could provide. They had nasty memories of the abrupt departure of the Navy carriers at Guadalcanal in 1942, leaving them in the lurch with no air support. Their initial experiences with the AV-8A proved very satisfying and they are now in the process of acquiring a total of more than 300 AV-8Bs, the American version of the Harrier II developed jointly with the UK and now in production in the USA by McDonnel Douglas. A first batch of 72 aircraft was authorized at the rate of 24 yearly in FY 1989, 90, and 91.

The Lessons of the Falklands Campaign

The Harrier is far from being an unproven newcomer. In service for almost twenty years, its various versions had logged more than half-a-million hours of flying time by the end of 1986. During the Falklands conflict in 1982, the 28 Sea Harriers operating from HMS <<Hermes>> and <<Invincible>> shot down 23 Argentinian aircraft, while on the British side not one was lost or even hit in air combat. Ten Harrier GR3s from the RAF, whose pilots had no previous deck training, were also engaged. Four Harriers/Sea Harriers were lost in accidents and five were shot down by the Argentinian ground-to-air defense. The operational attrition rate was never greater than 0.5% per sortie. Aircraft serviceability never fell below an astonishing 85% throughout the campaign.

Appalling weather conditions, which would almost certainly have precluded operating conventional aircraft from a large carrier, very seldom kept the Harriers/Sea Harriers idle on deck. Some were recovered in almost zero visibility (less than the ship's deck-width) or in extreme sea-state conditions with the flight deck moving vertically through as much as 10 metres.

In people's minds, the Harrier is now the symbol of the Falklands campaign, much as the helicopter gunship is of the Vietnam war.

The Harrier: a Few Facts

The non-reheated Rolls-Royce Pegasus turbofan of the Harrier/Sea Harrier/AV-8B has of course been upgraded since that of the original Kestrel. Its thrust is now in excess of 10 tonnes, almost double what it was 25 years ago, but its general architecture remains unchanged. The four nozzles, arranged in a rectangle, two either side under the wing and two further aft, can be rotated through 100 [degrees] from fully aft to about 10 [degrees] forward of the vertical. The control mechanism is simple (a "bicycle-type chain" driven by pneumatic actuators) and fast (100 [degrees]/s). The front nozzles exhaust more than 60% of the air flow (at 360 m/s and 110 [degrees] C), the aft nozzles 40% (at 550 m/s and 650 [degrees] C).

In dynamic flight, the aircraft is controlled in pitch/roll/yaw and trim through the Reaction Control System (RCS) consisting of small, variable-area shut-off valves located at the wing tips and at both ends of the fuselage, fed from HP compressor bleed at about 10 kg/[cm.sup.2] and 400 [degrees]. These valves are controlled by stick and rudder the same way as ailerons, rudder and elevators in aerodynamic flight. They start to operate automatically when the main nozzles are vectored down to 20 [degrees] regardless of airspeed, the pilot having to take no specific action and being in fact unaware of what type of control (aerodynamic or dynamic) he is operating when he moves his stick or the rudder pedals. On the ground, the front wheel of the tandemtype main undercarriage can be steered with the rudder pedals. In the cockpit, the thrust vectoring lever is the only additional control that distinguishes the Harrier from a conventional fighter. Located within the throttle box, the lever has an adjustable stop for short take-off. This allows the pilot to preselect the vectoring angle at the selected lift-off speed or lift-off point, according to the landing run available and other usual parameters (load, wind, temperature, elevation, etc.). The take-off run is then initiated with the nozzles fully aft. When the pilot reaches the selected lift-off speed (or the end of the ramp in a ski-jump takeoff), he slams the nozzle lever to its preselected stop and is airborne, about two-thirds on engine power and one third on wings.

Specific Operating Procedures and Limitations

The Harrier is not a helicopter. In the hover, it is less sensitive to gusts and wind direction. It is less manoeuvrable than a helicopter, particularly in the vertical axis. The aircraft is a bit "sluggish" and hence slower to recover from over-control. Touch-down is not as accurate as in a helicopter but typically within about one metre of the intended point.

On take-off, when applying full throttle, the pilots should be careful not to "drift" on the tyres since the engine takes several seconds to reach its maximum thrust (one aircraft just skidded overboard during the Falklands campaign).

For various reasons, notably due to the design of the tandem-type under-carriage, an aircraft at a weight significantly greater than maximum hover weight cannot be recovered at airspeeds below 70 to 80 knots, thus precluding a carrier landing at this weight without an arresting system. In any case, the gear is not designed for high vertical impact velocities.

The Harrier is not a good glider. Its lift-to-drag ratio is of the order of 3:1. Ejection is the only emergency procedure. Contrary to a widely spread legend, wooden decks (as in the Spanish "Dedalo") do not catch fire due to the hot exhaust jet. On a steel deck, one can walk barefoot from where a Harrier just took off. The Pegasus engine produces no smoke, being a turbofan. Its IR signature is low due to the low temperature exhaust, masked underwing.

"Viffing" and Ski-jump Take-off

Surprisingly and significantly enough, two important operating procedures of the Harrier which are today its two main selling points were initially developed by those that flew them and not by the designer or offical research bodies. It just goes to prove that computers are not about to replace the human brain and that the craftsman's skill can still challenge the best designer. * The first of these is "Viffing" (<<Vectoring In Forward Flight>>) i.e. using

the thrust vectoring control in flight. The development of this technique owes much to the pioneering work done by the US Marine Corps, in particular by the then Major Harold W. Blot (now Brigadier-General and V-22 programe manager) who, in flight at 500 knots on an AV-8A, slammed the vectoring lever to the hover stop, discovering that the deceleration effect was more powerful than any airbrake. (Some RAF pilots are said to have "played" with it before, but kept quiet about it.) Subsequent trials gave this phenomenon the official seal of approval.

"Viffing" has several advantages whose cumulative effects greatly enhance the aircraft's air combat capabilities. * It increases total lift, thus permitting tighter turns. * It generates (even with as little as 20 [degrees] of vectoring) a powerful nose-up trim change, enabling the pilot to bring into his sights an enemy at which he would otherwise have no hope of shooting. * The Reaction Control System, which starts to operate automatically at 20 [degrees] of vectoring, greatly enhances the manoeuvring capabilities in a dogfight. * It produces an extremely powerful deceleration (-2g), enabling the pilot swiftly to shake off a pursuer or missile. * While "viffing", the engine remains at full power, allowing the pilot instantly to reaccelerate when he brings back the vectoring lever to the full back stop.

These various factors combine to give the Harrier a decisive advantage in a dog-fight. Because his flight path is unpredictable, the Harrier pilot is liable to open fire at any moment. In a ground attack, the increased rate of turn through "viffing" enhances survivability and increases the chances of hitting the target on the first run. "Viffing" also provides for easier speed control in a dive and shortens the reaction time in attacking a target of opportunity.

"Viffing" so enhances manoeuvrability in air combat that irrespective of the STO/VL performance, this capability would certainly spin off on conventional fighters if it could be afforded without incurring too heavy a weight penalty. The vectoring mechanism weighs a mere 45 kg. Together with the RCS, the total weight of the systems is in the order of 160 kg, less than 3% of the operational weight empty. Peacetime dummy engagements against various fighter types (F-14, F-15, F-16, F-4, F-5E) showed that the Harrier/AV-8B outperformed them all in "visual initial encounters" by 3:1. Aircraft on both sides were flown by experienced pilots of equivalent training levels. In the contest, the F-16 was the runner-up. * The second technique unforeseen when the Harrier was developed is the ski-jump take-off. Lt. Cdr. Doug Taylor, RN, first proposed this technique in 1973. It seems that his initial concern had been to make a rolling take-off safer on board ships, particularly on a pitching deck. In a large conventional carrier, pitching is quite moderate even in heavy seas. Moreover, a catapult launch is so fast that the flight deck officer can adjust his timing to the pitching of the ship and launch when the deck comes up so as to be sure not to "shoot" on a downwards trajectory. However, the Sea Harrier is designed to operate from relatively small ships, more sensitive to sea states and with shorter pitching periods, and when performing a rolling take-off from a downwards pitching deck it might come dangerously low over water. A ski-jump guarantees that regardless of the pitching angle the initial flight path will be upwards.

The ski-jump take-off procedure is similar to that of a rolling take-off on a short field. Before applying full power, the pilot sets the thrust vectoring lever stop to about 50 [degrees]. The nozzles are vectored fully aft during the deck run but as the aircraft reaches the top of the curved ramp, the pilot slams the vectoring lever to the preselected stop. At this point, the lift is split about one third between the wings and two-thirds the vertical component of the engine thrust. The airspeed is still too low for the aircraft to "fly" but as it arches up and levels off, the forward thrust component builds up speed while the pilot progressively brings the nozzles aft. Typically, the transition takes about 10 seconds to reach 180 knots in normal flight.

Another advantage of the ski-jump is that, should anything go wrong, it gives the pilot more time to eject.

The ski-jump can also greatly reduce the take-off run or, using the same available strip length with a ramp at the end, greatly increase the maximum take-off weight, and hence the payload. Alternatively, the same payload can be flown off from a much shorter field. The gains are roughly of the order of 50% (of load or length). During trials at maximum weight, astonishing end-speeds of 75 knots were recorded on a 12 [degrees] ramp (65 knots less than a "flat" short take-off) and even as low as 42 knots on a 17,5 [degrees] ramp. Ramp settings in excess of 20 [degrees] were not tested for at 20 [degrees] the aircraft sustained a 4 g vertical acceleration and the wheel's oleos just bottomed.

These gains are such that studies are now being made to transfer the ski-jump technique ashore in the form of grid matting strips equipped with a mobile ramp. At sea, the ski-jump is now standard on all new STO/VL carriers.

Jump Take-off and "2D" Nozzles

Ski-jump take-off and "viffing" have now gained so much favor that some aviation circles are anxious to extend their benefits to conventional fighters. Within the framework of the painful A-6 replacement programme for instance, McDonnell Douglas is studying a Super Hornet F/A-18 equipped with "2D" nozzles (i.e. vectorable through an are below the fuselage axis) with reheat and thrust reversers. This would give the F/A-18 some STOL capability, enhance its manoeuvrability and increase its payload and/or endurance. Of even greater interest on that same aircraft, the three legs of the undercarriage would be fitted with powerful actuators which would play a role similar to that of pre-loaded springs. On take-off, at a given speed, they would suddenly expand (nosewheel first for rotation), literally thrusting the aircraft off the ground without having to wait for airspeed to build up and the stick to come into play.

Last May, a US Air Force/McDonnell Douglas team began test-flying an F-15 STOL Maneuver Technical Demonstrator (S/MTD) fitted with "2D" nozzles and thrust reversers, a modified "rough field" landing gear and an integrated flight controls/propulsion system (involving some kind of dynamic attitude control at slow speed). The study contract, awarded in 1984, specifies that the demonstrator should be able to operate from a 450/15 metres strip. Both initiatives confirm that the main purpose of the ski-jump take-off is to get the aircraft airborne sooner than it would otherwise, and this is only possible if control on the three axes is achieved at speeds lower than normal take-off speed.

At sea, arrested landing at high speed has apparently been stretched to its ultimate technical limit, and the ability of designers to invent new aerodynamic gimmicks intended to slow down the approach speed is reaching the point of exhaustion. But the catapult is most likely to survive. It may even gain more favor and see its use extended to land operation.

Performance

The raw performance of the Harrier in terms of speed, payload and endurance does not of course match the F-14, F-15, F-16 and F/A-18. A fair comparison must however take into account their respective capabilities.

The Harrier is often accused of being incapable of lifting its maximum payload in the VTO mode. True. But the Harrier is basically a STO/VL aircraft and the diehards are invited to name a single conventional fighter able to lift its maximum payload on its shortest take-off run. * As for its performance endurance, * the Harrier only burns some 50 kg of fuel in a typical take-off sequence versus 250 kg for a modern twin jet fighter; * the fuel cost of a typical landing sequence is only 70 kg, with no extra allowance necessary for a missed approach; * in a dogfight the Harrier forces his opponent to go over to reheat, without increasing his own fuel consumption; * above all, an almost total disregard of weather conditions at the time of returning to base or to ship allows pilots to draw much deeper on their fuel reserves and thus perform their mission with much greater peace of mind. The typical fuel reserve of a Harrier at the break is in the order of 100 to 300 kg versus 800 to 1200 kg for a conventional fighter. * Payload

In ISA + 15 [degrees], an AV-8B taking-off from a flat 300 metre strip carries 4 tons of bombs with a radius of action of 350 km. It should also be borne in mind that the forward basing capability further reduces the actual range and reaction time to reach the target zone.

At sea, the so-called performance gap between the Harrier and conventional fighters dwindles to such a point as to turn to the advantage of the former, except in interception beyond visual range. Anyone with carrier experience can remember how suddenly the casual routine of launching and recovering aircraft becomes an emergency whenever a pilot reports low on fuel or if the deck is unexpectedly fouled, even in fair weather. In peacetime, captains usually operate within gliding distance of an emergency airbase ashore, which of course breeds bad habits. When the ship is actually way out at sea, safety requires that a ship-based tanker aircraft be kept overhead round-the-clock to help any plane short of fuel.

The Soviet Yak-38F

Spotted for the first time on the "Kiev" in 1976, the Soviet VTOL Yak-38 Forger belongs to the hybrid family. In the hover, the lift is roughly split evenly between its cruise, vectored thrust engine (a pure jet) and two vertical thrust jets situated aft of the pilot, which leads to several inevitable complications.

When the Russians decided to develop fixed-wing shipborne aviation starting from scratch, they deliberately preferred to deploy a rather poor VTO to start with, rather than try to copy the Western saga of catapults and arresting wires. The price paid for this conservative approach is that their four "Kievs" are hybrids, more helicopter-carriers than full-fledged aircraft carriers. It should be noted, however, that the latest Soviet aircraft carrier the 65 000t "Tbilisi" (ex-"Leonid Brezhnev") has a 12 [degrees] ski-jump at the bow and an 8 [degrees] angled deck. "Belt and braces", some will mutter. Though the ship has no catapult, her configuration seems to confirm that the Russians are developing a conventional naval fighter (a derivative of the Su-27 Flanker?) and an improved derivative of the Forger, the STO/VL Yak-41.

The Supersonic Harrier

The Harrier design team very early on undertook the development of a supersonic derivative of the basic aircraft. In 1965, only five years after the subsonic prototype (P.1127) first flew, a supersonic prototype (P.1154) was about one-third completed at Kingston when the British government of the day cancelled the programme.

The main challenge of course was to increase the engine thrust while keeping the original well-tried configuration of four vectoring nozzles. Merely adding an afterburner to the existing engine was out of the question as it would have unbalanced the longitudinal thrust split of the four nozzles in the hover. The original solution for the P.1154 was Plenum Chamber Burning (PCB) i. e. heating the relatively cool fan flow directed through the front nozzles. While the PCB principle was extensively tested on the ground, it never flew. Several other concepts were also investigated including those of a "tandem fan" and RALS (Remote Augmented Lift System), which diverts part of the compressed air flow to the front nozzles.

Whatever configuration may eventually emerge on some future supersonic Harrier, one thing is certain: because the speed of the ejected flow in the hover will be significantly higher, two factors will be of a major concern that are not critical on the existing Harrier: the recirculation of burned hot gases (reducing engine efficiency) and the ingestion of ground debris in the air intakes.

While the development of a supersonic Harrier has officially been shelved for about twenty years, the British have never actually closed the file. In line with their practice, they have quietly, inventively and tenaciously kept on working, particularly on the engine side. The signature in January 1986 of a MoU with the USA for the joint development of an Advanced [i.e. supersonic] STOL aircraft is an indication that they did not come to the negotiation table empty-handed. Nothing has leaked of what is being done (the programme is classified) but the ultimate outcome is fairly certain: the future supersonic Harrier will be Anglo-American.

PHOTO : The Harrier GR5 - the latest version - can carry seven BLU 755s (including one under the

PHOTO : centreline pylon) and two AIM-9s.

PHOTO : This Sea Harrier offers a clear view of its two 30 mm Adens housed in blister pods on each

PHOTO : side of the centreline pylon.

PHOTO : In Spain, the Armada's Harriers are called Matadores. This AV-8S is seen here above the

PHOTO : deck of the Principe de Asturias.

PHOTO : The Rolls-Royce 11-61 - the latest version of the Pegasus-produces 23 800 lb of thrust. It

PHOTO : is designated F404-RR-408 by its first customer, the US Marine Corps.

PHOTO : This picture taken during proving trials in 1977 shows the vertical columns of thrust

PHOTO : breaking the air as the Harrier lifts off from the deck of HMS Hermes.

PHOTO : FRS. Mk. 1: note rectractable emergency power ram air turbine ahead of tail root and

PHOTO : systems cooling air intake just above.