The rotary-wing aircraft and modern materials.
A Long-Awaited Happy Marriage
"Vertical flight, with its capacity to be transformed into horizontal flight, has been one of the earliest dreams of man. Yet, it is also the youngest art in aeronautics - in practical terms scarcely half a century old."
This statement, particularly the second sentence, by Sergei Sikorsky in the foreword to his book entitled "Straight Up" is probably the shortest explanation of why helicopters - until recently - have appeared to be lagging behind fixed-wing aircraft as true military weapons systems. After all, did it not take the powered aeroplane more than twenty years after the first flight of Flyer 1 to reach a sufficient degree of maturity to be perceived by the military as a potentially useful machine? The Wright Brother's aircraft had profited extensively from the experience gathered by Otto Lilienthal's experiments with gliders years before. As for First World War bombers, they were merely "ordinary" machines from which bombs were tossed overboard.
The very first enemy of the helicopter is Weight. On fixed-wing aircraft, this constraint can be compensated to a certain extent by increasing the speed of the air over the airfoil and hence the lift. On helicopters there is a limit to this because increasing the rotational speed of the rotor means that the outer part of the blades (which are, in actual fact, wings) will very quickly end up hitting the sound barrier, when all sorts of strange and adverse phenomena start appearing.
The second characteristic required from a helicopter is to be able to move forward at a reasonably high speed, but then another odd problem crops up: the blade which moves forwards into the stream gets more lift than the retreating one. These are just two examples of the numerous brain-teasing problems associated with rotary-wing technology. Right from the outset, therefore, it became very clear in the minds of the military that helicopters were never going to be armoured maid-of-all-work flying machines. For the designer, there is no way out: the development of the helicopter was - and still is - dependent on the discovery of new materials and technologies.
Plastics and Elastomers
The first significant improvement - if one excepts the advent of the turbine - was probably achieved by France's SNIAS company (now Aerospatiale) who exploited to the full the properties of elastomers to reduce the number of parts of the rotor head. The resulting Starflex rotor was actually killing several birds with one stone: not only were the number of parts and hence the manufacturing and maintenance costs as well as weights cut down, but most important for military applications, the high reliability thus achieved permitted a virtual all-time readiness of the rotor thanks to the "on-condition" maintenance concept. This means that the components are replaced when they are deemed to have had their day, whether worn out or not, rather than exchanged at regular flight hours intervals as per maintenance book instructions. Aerospatiale then brought the helicopter one step closer to the dream machine required by the military by adopting the "plastic blade" concept, as used on the German MBB BO 105. These blades are made of several plies of resin-impregnated glass and carbon fibre fabric wrapped around a Moltoprene foam filler. Unlike their conventional metallic counterparts, plastic blades can sustain a bullet hole, and thus vastly improve the survivability of the helicopter on the battlefield. In fact, the strength of the new blades was demonstrated quite by accident when a pilot landing too close to a tree cleanly chopped down the trunk with his rotor. Although the resulting dent in the leading edge of the blade did require the replacement of the blade, the helicopter was otherwise undamaged. Metal blades would have literally disintegrated, with disastrous consequences for the airframe and its crew. In 1976, Aerospatiale unveiled its AS 350 AStar helicopter which, in addition to the three-bladed Starflex rotor (a four-bladed version of which was developed for the AS 365 N Dauphin 2) and the plastic blades mentioned above, also made extensive use of plastics and foam-sandwich composites in the panel work. Light alloys were only used for the structural parts of the airframe. France had made a decisive breakthrough in the field of survivability and agility, and chalked up an impressive string of records.
Simplifying the design of the rotor and thereby reducing the number of its components and, as a consequence, maintenance costs has been major concern of helicopter designers ever since the intricacies of the rotary-wing principle were fully mastered. Early helicopters had to be equipped with fully articulated rotor hubs, i.e with lead-lag and flapping hinges.
One of the very first steps towards simplification was probably made by Bell, who adopted a two-blade design called the teetering rotor. This "see-saw" rotor equipped all Bell helicopters from the Model 47 (with the stabilizing bar) through all the Hueys to the 222. However, the demand for increased performance (speed and capacity) had pushed the two-blade rotor to the limits of feasibility: on the Model 214ST, for example, the blades have a chord of 85 cm! The first departure from the two-blade design came with the four-bladed Model 412 which made its first flight in August 1979.
Another remarkable milestone was reached by MBB in 1967 with the BO 105. Research on this model was partly (60%) funded by the Federal Government. The German company decided to use plastics, fibres and foam fillers to manufacture the blades because it realised that their inherent flexibility was sufficient enough to take the strain of the flapping and lead-lag motion which allowed a rigid rotor design to be adopted. Exactly the same rotor head is used on the BK-117 helicopter developed together with Kawasaki.
A novel design, which for the first time transferred the elastic element to the interior of the rotor hub itself, took to the air in March 1971. This was the WG13 Lynx developed by Westland and Aerospatiale. The four-bladed rotor is of the semi-rigid type and uses a single-piece titanium flexible-arm star to carry the blades. The inner part of each arm of the star takes most of the strain of the flapping movement, while lag motion is absorbed by the outer portion.
Today, the vast improvements made in the field of elastomers, composite materials and metallurgy allow one to dipense with conventional bearings, and only the larger designs like the EH-101 or the NH90 still require separate lead-lag dampers. In addition, the rotors have not only become lighter but also smaller and more aerodynamic. Composite blades, for their part, are commonplace as they can now be manufactured at high production rates with a very high degree of accuracy, thus permitting total interchangeability of blades of the same type (little or no matching required). Aerospatiale, for example, has recently celebrated the manufacture of its 100 000th composite blade. Incidentally, the French manufacturer has simultaneously been breaking a few records which, for once, are not related to purely physical helicopter performance: not only did it manage to sell a record 77 helicopters to Japan in 1988, but it also succeeded the same year, through its American subsidiary Aerospatiale Helicopter Corporation, in taking a leading position (in terms of value of aircraft delivered) on the US market.
The Narrow Fuselage
In the United States, meanwhile, developments had followed another tack. Manufacturers had been kept busy churning out helicopters for use by the Army in the Vietnam war and their minds were not purely concerned with new materials, which would in any case have complicated the logistic problems. The war, however, soon showed that since attack helicopters could not be heavily armoured, the only way of giving them a better chance to see another day was to make them as discreet as possible (the term "stealth" was not yet in fashion). This is how the venerable UH-C Huey (Bell 205) workhorse was turned into the AH-1G HueyCobra (Bell 209) gunship. The AH-1G incorporated all the UH-1C dynamic components, including the engine (Lycoming T53), transmission and rotors, inside a narrow airframe featuring tandem seating for the two-man crew. The low silhouette and narrow profile made for more effective ground camouflage and presented a more difficult frontal target. Production started in 1966 and had reached 1127 units by 1973. The standard armament of the HueyCobra consisted of a 7.62 mm chin-mounted Mini-gun firing at a rate of 6000 rounds per minute and of 2.75" rockets carried under side-mounted stub-wings.
The narrow fuselage, tandem-seat concept for dedicated attack helicopters has since been investigated and adopted by a majority of manufacturers. These even include Westland, who worked on a project with VFW and Fokker, but, oddly enough, not Aerospatiale. The latter was nevertheless forced to follow the trend when the agreement on the design requirements of the Eurocopter PAH/HAC were finally signed. The following list (in chronological order) gives an idea of how the design has spread over the continents: * Bell 209 HueyCobra series: first delivery in 1967; over 1400 units, including the Sea Cobra series, have been produced. * Mil Mi-24 D (Hind) Series; introduced in 1976 and, with the Hind A, B, C series, extensively used in Afghanistan; clearly an afterthought design, the Hind D is largely a cross-bred helicopter using the A's centreframe on which a new nose section was grafted; it is thus not truly a fully dedicated attack helicopter since it can still carry around 16 men. High losses in Afghanistan tend to prove that it was too cumbersome to be used as an effective strike machine. * McDonnell Douglas AH-64A Apache: first delivery on 21 January 1985; deliveries to date reach about 400 units out of a total requirement of 1200. * Agusta A 129 Mangusta: first flight on 13 September 1985; 70% of the wetted area is made of composite materials; avionics are built around a Harris computer-based integrated multiplex system which integrates and manages the electronic systems and the flight control systems. Now used as a basis for the Tonal programme (Italy, Great Britain, Spain and the Netherlands). * CNIAR Iar-317 Airfox: first flight in April 1984; like the Alpha, this Romanian design is also based on the Alouette III's dynamic components (CNIAR manufacturers the SA 316 under licence); statically displayed at the Paris Air Show in 1985; * Armscor Atlas Alpha: little is known about this South African development based on the Alouette III; first flight took place on 3 February 1985; armament includes a 20 mm chin-mounted gun slaved to the gunner's helmet. * Mil Mi-28 (Havoc); development of this dedicated combat helicopter, believed to be the first Soviet effort in this field, was confirmed by the US Department of Defense's "Soviet Military Power" in 1984; armament includes a 23 mm chin-mounted gun, up to 16 AT-6 - type missiles; believed to have air-to-air missile capability. * Eurocopter PAH/HAC: probably the helicopter programme that has had the longest pre-development history, as first formal discussions took place as long ago as the mid-seventies and finally led to the long-awaited agreement on the aircraft's general configuration on 13 November 1987. Contrary to further preliminary agreements signed in 1984, there will be one common EUROMEP-equipped (European mission equipment package) anti-tank helicopter for both France and Germany, respectively designated HAC (Helicoptere Anti-Char) and PAH-2 (Second-generation Panzerabwehrhubschrauber); EUROMEP includes 8 anti-tank missiles (initially HOT-2s and later AC3G-LPs of the TRIGAT programme), four Mistral (HAC) or Stinger (PAH-2) air-to-air missiles, a mast-mounted sight with thermal camera, two helmet-mounted sights for the gunner and the pilot. Using the same base, France will derive an HAP (Helicoptere d'Appui-Protection) equipped with a GIAT chin-mounted gun, rockets and air-to-air missiles. * Kamov "Hokum"; little information on this new Soviet anti-helicopter helicopter is available; it uses a contrarotating six-bladed rotor system - a typical feature of the Kamov design bureau - which obviates the need for a anti-torque tail rotor.
Noise and IR Emissions
Back to the Vietnam war years. While Bell was working on its Cobras, Hughes Helicopters (now McDonnell Douglas Helicopters) won the US Army Light Observation Helicopter contract in 1965 and was to gain considerable experience with its OH-6A (Model 500). It took part in the US Army's Advanced Research Projects Agency's R&D programme which was aimed at reducing noise levels. The programme gave birth to the "Quiet One" which was basically a Model 500 featuring a five-bladed main rotor carrying specially-designed blades, a four-bladed tail rotor, fine-toothed gears, an engine exhaust muffler and engine-bay soundproofing which resulted in a fifty percent reduction in the overall noise level. Since the 500 became very popular in the United States with the police for highway and urban patrol duties as well as with TV companies (it was the official Los Angeles Olympic Games helicopter), the reduced noise level was much appreciated by the local inhabitants, but it also greatly contributed to the quest for the silent approach. Hughes also took part in the NASA/US Army Higher Harmonic Control (HHC) project. The micro-computer-controlled HHC system was aimed at increasing the level of stability (and alternatively comfort) for precision delivery of air-to-surface missiles, and a dramatic drop of 95 percent in vibration levels was achieved when the HHC system was switched on. Finally, and to conclude with the major improvements achieved using the 500 as a working base, Hughes developed the NOTAR concept under contract from both the US Army and DARPA. NOTAR stands for NO Tail Rotor.
There are three important reasons for trying to get rid of the tail rotor. The first one is that when the helicopter is on the ground with its rotors turning while embarking or disembarking troops, the tail rotor is an invisible disc that has caused many casualties. Aerospatiale of France has contributed to solving the problem with the Fenestron shrouded tail rotor. The second one is that, during quick manoeuvres close to the ground, tail rotors have been known to hit bushes and even the ground. The third one largely results from recent developments in thermal imaging devices: while all other heat-radiating spots are relatively easy to conceal, it is almost impossible to shield the heat produced by the tail bevel-gearbox. This unit has become a favourite target for gunners, particularly at night, as one well-aimed shot into that heat spot is sure to down the aircraft.
With the NOTAR, anti-torque and directional control is provided by a stream of air blown by the engine-driven fan through to the end and out of the port side of the tail boom. The airflow is controlled by the fan's variable pitch blades.
Oddly enough, the pioneers in the field of anti-tank missile-armed helicopters were neither of the two Superpowers. The concept was developed by the French during the war in Algeria at the turn of the sixties. The Alouette IIs were initially armed with AS 10 anti-tank missiles and soon afterwards with the more powerful Nord Aviation AS 11 (it is worth noting that the AS 11 round was strictly identical to the SS 11 - AS and SS stand for Air-Sol and Sol-Sol respectively).
The Alouette III soon followed in this new anti-tank role and was also apparently very effective against the jebel caves where the rebels hid. The missile was subsequently adopted by the British army to equip its Lynx helicopters, and by Germany. Command of the missile was very simple, not to say rudimentary: the steering signals acting on the rocket motor nozzles were sent by the operator's joystick via a cable, and "target acquisition" was by eyesight (a flare in the tail of the round helped the operator to see where he was directing his round).
Accuracy improved vastly when the gyro-stabilized APX260 sight was introduced in conjunction with the AS 12 missile (basically a scaled-up version of the AS 11). However, the primitive guidance system of these first-generation helicopterborne missiles imposed a relatively slow flight speed; this meant that the helicopter had to remain exposed to enemy fire for a period of time that became quickly unacceptable with the fielding of hand-carried anti-aircraft weapons. In addition - and quite paradoxically - these missiles had a very limited short-range capability because the rounds required some 400 to 500 metres to stabilize their flight after launch before the gunner could guide them effectively.
Another ten years elapsed before the next major milestone in helicopter anti-tank warfare could be reached, thanks to the advances made in electronics. The breakthrough was eventually made by Hughes Aircraft and Bell Helicopters following the US Army's decision to arm its combat helicopters with TOW missiles, but only after a considerable amount of sweat and tears. The first trials started in 1966 using a UH-1B and the XM26 guidance and launch installation. The programme was then dropped for economic reasons, but thanks to intensive trials by the German armed forces in 1971 with the AH-1G, the US Army was able to deploy a number of TOW-equipped UH-1Bs in Vietnam in 1972. However, the US Army only received its first production AH-1Q helicopter equipped with a suitable XM65 launch and guidance equipment in June 1975. But the Army then faced another frustrating problem: now that the TOW was operating satisfactorily, it was realized that the helicopter was not powerful enough to carry its prescribed armament (amongst which eight TOWs) and fuel load in nap-of-the-earth flights (agility) or out of ground effect. As a result, only 93 AH-1Qs were built, and a new Huey Cobra - the AH-1S - appeared, finally putting an end to the teething problems of the US Army's anti-tank helicopter programme. This newcomer was powered by a 1340 kW engine mated to the higher rated transmission of the twin-engine AH-1J Sea Cobra.
The most significant advance of this second-generation missile system compared with the original AS-11 is its automatic guidance: the gunner only needs to keep the target centered in the crosshairs of his reticule. This has been greatly facilitated by the development of a monocular eyepiece mounted on the helmet which is slaved to the nose-mounted sight via the computer unit. The missile is automatically steered to impact on the target which the gunner keeps in his sights.
Thanks to the tremendous advances made in the field of plastics, robotics, electronics, aerodynamics and weapon systems, the helicopter, and particularly the attack/anti-tank variety, is one of the systems that has been subjected to the sharpest growth curve during the past decade. However, the world outside the cockpit has not stopped turning either, and it is interesting to see how many different systems (radars, missiles, decoys, automatic gun systems, ECM and ECCM systems, IR sensors and even anti-helicopter helicopters) have been devised to stop the helicopter. For the pilot and his co-pilot/gunner, the workload during a mission has thus become incredibly stressful. During a night mission, not only does the crew need carry out its navigation work, fly as low and as fast as possible avoiding natural obstacles as well as man-built ones such as power lines (this is a serious problem in West Germany), find the target and destroy it, it also needs to watch out constantly for a possible hostile element, which means keeping one eye on the cockpit and the other looking outside, without forgetting, of course, to give an occasional glance at the aircraft's instruments. A few years ago, a simulator at the AUSA show in Washington provided the interested visitor with both a vivid experience of the nightmare of such a mission... and a wet shirt.
The major problem now facing the designers of modern attack helicopters is how to enable the crew to cope with so many duties and commands. Even if the human brain can absorb and perform a vast number of operations, there comes a moment when the crew-member's physical movements inevitably reach saturation point. Since "plugging" the helicopter's systems into the pilot's brain is in the realm of fiction (but for how long?), engineers are now exploiting a hitherto unused command interface - the human voice. Initially, voice will be used as an additional finger to flip mode selection switches such as radio frequencies and weapon systems.
The Next Generation of Helicopters
Another field where things are moving fast is visionics, and particularly helmet-mounted devices which allow the crew-member to see outside the cockpit "through" a symbology display extending the full width of his visor. Today, the Integrated Helmet Display and Sight System (IHADSS), for example, provides the Apache pilot with a television-type image which is displayed on a small combining lens (similar to a monocle) placed a couple of centimetres in front of his eye. Because the system is mounted on the helmet and linked to the helicopter's PNVS (which is slaved to the movements of the helmet), the night-flying pilot permanently gets a display of the terrain ahead of his line-of-sight. Naturally, the gun can automatically follow the same movements.
Quite clearly, on a helicopter like the LHX, a certain amount of decisions will have to be made by the system to reduce crew workload. In other terms, some sort of "intelligent selector" will have to be integrated so that only the information required is presented to the pilot at the right moment so that he is not distracted by an element of secondary importance during a delicate operation. The machine will have to work out some priorities, particularly in view of the "single-crew option". The number of high-technology firms in each of the two current LHX teams gives some idea of the complexity of what will be the next generation of helicopters. The McDonnell Douglas/Bell helicopter "Super Team" includes electronics specialists like Honeywell, Litton, Northrop and Texas Instruments, while the Boeing/Sikorsky "First Team" lists Boeing Electronics, Collins, Harris, Kaiser Electronics, Martin Marietta and Westinghouse. As a light attack helicopter, the LHX will have to be able to destroy armour and mechanized forces, suppress and destroy enemy air defences, destroy second-echelon forces, provide rapid rear-area protection and conduct joint air-attack-team operations. It will also be asked to fly armed reconnaissance missions during which it may have to destroy high-value targets, but its air-to-air combat capability, thanks to its high manoeuvrability, will also allow it to attack enemy helicopters and evade hostile fire. In 1983, Jack Real, then President of Hughes Helicopters, put the objectives in a nutshell: "The LHX will replace four famed workhorses - the UH-1 Iroquois, the AH-1 Cobra, the OH-58 Kiowa and the OH-6A Cayuse - and will team with the AH-64 Apaches and UH-60 Black Hawks to pave the way on the battlefields of the 21st century".
Leonardo da Vinci must be turning in his grave.
PHOTO : Atmosphere inside the Sikorsky Boeing LHX: through a wide-field-of-view visor the pilot
PHOTO : sees a read-out constantly displaying target and flight parameters.
PHOTO : The Aerospatiale SA 365 Dauphin (here in the Saudi Arabian "F" AS 15-equipped version)
PHOTO : uses the four-bladed version of the revolutionary Starflex rotor.
PHOTO : The new Spheriflex rotor, seen here mounted on an Aerospatiale Super Pumpa (insert), will
PHOTO : be used on the all-composite structure and fly-by-wire NH90 helicopter.
PHOTO : The Anglo-Italian EH-101 features three engines, BERP-based composite main rotor blades,
PHOTO : and one of the largest glass "cockpits" ever seen on a helicopter to date.
PHOTO : The Agusta A 129 Mangusta tank-buster makes an extensive use of composites and is built
PHOTO : around a Harris integrated flight and weapons control system.
PHOTO : The proposed McDonnell Douglas/Bell version of the LHX adopts the NOTAR tail-rotor-less
PHOTO : concept originally developed by Hughes under a DARPA contract.
PHOTO : The Agusta A 109, in a Helitow configuration, recently took the helicopter world by
PHOTO : surprise by winning the Belgian anti-tank helicopter contract.
PHOTO : With the new BERP blade design allowing high blade-tip speeds, a Westland Lynx broke the
PHOTO : helicopter speed record by achieving a top speed of 249.1 mph (400 kph).
PHOTO : The Sikorsky/Boeing LHX contender adopts a "Fenestron"-like shrouded tail rotor and
PHOTO : T-shaped fin tail configuration.
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|Author:||Biass, Eric H.|
|Date:||Jun 1, 1989|
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