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While designers pursue technologies related to speed, range, performance and survivability of rotary-wing platforms, the military looks at designs that provide battlefield survivability and mission accomplishment
Advances in aerospace technologies have delivered some improvements in rotary-wing platforms, but the majority of helicopters retain the original tadpole-like shape with a main and a tail rotor. The classic helicopter design is mutating into newer types of rotorcraft or hybrid vertical take-off and landing (VTOL) ones. However, inherent limitations especially of speed, continue to defy rotary-wing designers the world over. Nonetheless, technological progress holds out two related promises. Firstly, innovative changes to rotorcraft design are clearly manifest the world over. Secondly, technological advances are proving to be expedients for the helicopter industry by offering increasing capability to upgrade existing models so as to enhance performance, role capability and mission utilisation. Technological updates also help in prolonging life of an existing helicopter and rendering it safer by increase in crash survivability and reduction in vibration levels. Obviously, upgrades cost a fraction of the expense on a new helicopter and so, looking into the near future, upgrades of existing, tried-and-tested models are likely to be at least as prominent as the emergence of new designs.
Future Vertical Lift
Unarguably, the leading edge of rotary-wing design development is under way in the US. In October 2011, the US Department of Defense (DOD) launched the future vertical lift (FVL) programme as a demonstration of its focused approach towards the problem of new types of rotorcraft for US military beyond 2030. The DOD issued an FVL Strategic Plan to outline a joint approach for the next-generation vertical lift aircraft for the military. FVL is based on the concept of creating new rotorcraft that use new technology, materials and designs that are quicker, have further range, better payload, are more reliable, easier to maintain and operate, have lower operating costs and reduced logistical footprints. The idea is to develop a family of systems to replace most army helicopters and a precursor for FVL is the joint multi-role (JMR) helicopter programme, which will provide technology demonstrations planned for 2017.
It is expected that the first FVL platforms to fly would be in the medium-lift category which brackets the attack and cargo roles. The US Army’s perspective of FVL is a family of helicopters to replace its current fleet into five “capability sets.” The first set is the lightest variant while the fifth is the heaviest. Medium-lift lies in the middle as Capability Set 3. As this category has been of interest not only to the US Army; but also the US Air Force and Marines, this has become the focal point for the first helicopters to emerge from FVL, possibly to fly in the early 2030s.
THE ROTOR SYSTEM IS NOT AS EFFICIENT FOR FORWARD TRAVEL AS A FIXED-WING ONE AND USES MORE FUEL AND REQUIRES MORE MAINTENANCE
The US military and the world eagerly await the JMR Technical Demonstration in 2017 which will define the full requirements of the FVL programme. A Bell Helicopter and Lockheed Martin team is working on the demonstration with its advanced tiltrotor concept, the V-280 Valor, while Sikorsky and Boeing are fabricating its Defiant coaxial helicopter. Thus two disparate rotary-wing platforms are competing in the FVL programme. Sikorsky’s Defiant will weigh 30,000 pounds and is based on the smaller X2 whose technology was demonstrated in 2010 and further upscaled to the Raider weighing 11,000 pounds. The Bell V-280 Valor is a tilt-rotor being developed by Bell Helicopter and Lockheed Martin for the US Army. In one major difference from the earlier V-22 Osprey tilt-rotor, the engines remain in place while the rotors and driveshafts tilt. A driveshaft runs through the straight wing, allowing both proprotors to be driven by single engine in the event of engine loss. Bell is also confident that the tilt-rotor design is ‘eminently scalable’.
While these two programmes are funded by JMR, Piasecki Aircraft has secured army science and technology funding to revive its Piasecki X-49 SpeedHawk programme, an advanced winged compound helicopter design. Interestingly, Piasecki sees this technology not only in the context of the FVL, but also as an insertable technology to keep existing fleets of AH-64s and Black Hawks relevant and affordable. Piasecki’s compound Apache design adds wings to the AH-64 gunship to achieve an 11 per cent increase in speed from 180 kts to 200 kts and a 39 per cent increase in lift. The company’s most ambitious configuration would be the CH-47 ‘Tilt Duck’ compound cargo helicopter that modifies the heavy-lift Chinook with long wings and two electric powered ducted fans and increases its speed by 18 per cent from 170 kts to approximately 200 kts, its lift by 12 per cent and its range by 115 per cent.
AVX Aircraft, a Texas-based company, is working towards the lightest FVL category with two 7.5-tonne winged coaxial compound helicopter configurations described by the company as “vertical take-off fighter aircraft” for light reconnaissance, attack, assault and medical evacuation missions. The new design includes a swept horizontal tail with two centreline ducted fans. AVX is also working on a 27.2-tonne tiltrotor replacement for the Chinook with 13.4 m propellors as it is convinced that a tilt-rotor aircraft makes the most sense for a future heavy-lift cargo platform.
Russia has pioneered the compound coaxial helicopter with the Kamov design bureau producing popular operational types like Ka-52 gunship. However, that technology is gaining popularity and acceptability in the US as can be seen from the JMR technology demonstrator phase.
The US Defense Advanced Research Projects Agency (DARPA) is endeavouring through the ‘VTOL X-Plane’ programme to make a significant jump ahead in the area of vertical take-off, hover and flight. The future X-plane must be able to act like a helicopter, but also be able to execute high-speed flight like an airplane. DARPA aims at a technology demonstrator that will fly at speeds of 550 km to 735 km per hour. Four companies have offered designs to build this technology demonstrator. The designs include new types of tilt-rotors, compound helicopters with innovative mechanisms for vertical and horizontal thrust and innovative systems of electrical fans. The fans are powered by distributed electric systems using conventional gas turbine engines which in turn produce electricity for the powerful fans that will propel the vehicle vertically and horizontally. DARPA will choose one company to build the VTOL X-Plane and flight test it in 2017-18. Meanwhile Aurora Flight Sciences completed a much advertised first flight on a subscale vehicle demonstrator named Lightning Strike using a path-breaking distributed electric propulsion system through 24 rotors. The final version is expected to produce a sustained speed of 400 knots. The design combines tilting tandem wings with hybrid-electric distributed propulsion derived from a single Rolls-Royce AE1107 turboshaft engine driving three one-megawatt electrical generators that power 24 variable pitch ducted fans, 18 in the wing and six in the canard.
The tadpole profile of the helicopter is evolving into various rotorcraft designs and moving from the original single main and tail rotor to tandem rotors, synchopter (intermeshing rotors), coaxial rotors, tip-jet driven rotorcraft, NOTAR (No Tail Rotor), tilt-wing aircraft, tilt-rotor aircraft and compound helicopters.
The most irksome limitation of a helicopter is its forward speed. Drag reduction, as applicable to fixed-wing airframes, applies to a large degree to a rotary-wing platform as well.
However, the problem is the inherent aerodynamics of the geometry of a typical helicopter rotor. The forward speed is thus limited and any endeavour to go beyond 130 knots involves special materials, designs and technologies. In a tandem rotor system, like the Chinook, the effect of retreating blade lift asymmetry on one rotor is countered to some extent by the other, but not wholly.
The rotor system is not as efficient for forward travel as a fixed-wing one and uses more fuel and requires more maintenance. Thus the need to move away from the original shape, but retain the VTOL characteristic of a helicopter. Developing a practical, hybrid aircraft with the performance of a fixed-wing aircraft in forward flight, is a huge challenge with two aims: accomplishing controllable vertical flight using the very same mechanisms required for forward flight and achieving ‘power matching’, i.e. a VTOL design that requires the same power in vertical flight as in forward flight. Any mismatch would represent excess capacity which corresponds to excess weight in one mode of flight. Numerous approaches to VTOL aircraft have been explored over the years. The prominent ones are tilt-rotors, tilt-props and tilt-wings, as well as deflected-slipstreams, deflected-thrust, thrust augmenters, ducted fans, tilt ducted rotors and tail sitters.
WHILE DESIGNERS PURSUE TECHNOLOGIES RELATED TO SPEED, RANGE, PERFORMANCE AND SURVIVABILITY, THE MILITARY LOOKS AT DESIGNS THAT PROVIDE BATTLEFIELD SURVIVABILITY AND MISSION ACCOMPLISHMENT
As the name implies, a tilt-rotor aircraft uses tiltable propellers, or proprotors, for lift and propulsion. For vertical flight, the proprotors are angled to direct thrust downwards, providing lift. In this mode, the aircraft is like a helicopter. As it gains speed, the proprotors are slowly tilted forward, eventually becoming perpendicular to the ground. In this mode the wing provides the lift and the wing’s greater efficiency helps the tilt-rotor achieve high speed. In this mode, it is a turboprop aircraft. Bell Helicopter has been dominant in tilt-rotor development with major designs from almost every decade since the 1950s. They are currently partnered with Boeing on the first production tilt-rotor aircraft, the jointly developed Bell/Boeing V-22 Osprey. Tilt-rotor proprotors require all the fundamental parts of a twin-rotor helicopter and are perceived as the most attractive solution to the speed problem of rotary-wing design. Bell and Boeing are working on larger Quad Tilt Rotor (QTR) military models for possible use by the US Army to carry as many as 100 passengers or troops or heavy cargo over 50,000 pounds. They would use uprated versions of the tilt-rotor engines used for the V-22 Osprey.
NASA’s Greased Lightning or GL-10 deserves a mention here. It is a battery-powered, ten-engine remotely piloted tiltrotor and the prototype has a ten feet wingspan and can takeoff vertically like a helicopter as also operate efficiently in forward flight. It is in the testing phase and flew a series of test flights during May 2015. The final version is expected to have a 20 feet wingspan.
Besides tilt-rotors, VTOL aircraft could have other designs like ducted fans (Bell X 22A, Ryan XV 5A/B), hovering platforms (UrbanAero X-Hawk), or the Elytron design which combines three sets of wings: one pair of rotary-wings called ‘proprotors’, mounted on a single tilt-wing in central position and two pairs of fixed wings, or the Disc Rotor in which for hover, a set of blades are extended from the periphery of the disc, much like a helicopter, but forward flight is like a fixed-wing aircraft with the blades either fully retracted into the disc or with two of the rotors sticking out like conventional lift producing wings.
The helicopter is a complex machine which uses a substantial part of the lift its rotor system generates to just stay away from the ground even when not using its engine power to move forward. Thus weight becomes a crucial design factor. With significant advances in carbon composites, lighter weight with ever-increasing strength, has become possible and every helicopter manufacturer is increasingly eyeing carbon composites. However, the cost factor has bridled that trend a bit. New Zealand’s Composite Helicopters International has been developing the KC-518 Adventourer, an all-composite, frameless, six-seat helicopter constructed from Carbon and Kevlar using EvoStrength technology. It is the world’s first helicopter with a monocoque fuselage made entirely from composite materials and is a change from the use of aluminium and steel tube framing used in helicopter manufacture. As a single piece composite structure, there are no rivets or bolts used in the assembly with the result that there is substantial resistance to corrosion, fatigue and impact.
The K-MAX made a significant contribution for US marines deployed in Afghanistan by way of unmanned cargo delivery into the battlefield thus generating huge interest in unmanned and hybrid rotarywing platforms. According to some estimates, global vertical take-off and landing UAV market is expected to attain a CAGR of over ten per cent until 2020 due to suitability of VTOL UAVs for urban applications, utilisation of VTOL UAVs for military applications and increased demand for civil-commercial applications like surveying, scanning, aerial photography, 3D mapping, oil and gas pipeline monitoring, wind turbine blade inspection, real estate survey, etc.
The FVL programme is also considering whether the way ahead lies with ‘Optionally Piloted Vehicles’ which would have a provision to carry a pilot for complex combat missions while routine supply runs would be unmanned. Technology demonstrations on this aspect are expected in 2019 and may not be part of the first machines for JMR.
While designers pursue technologies related to speed, range, performance and survivability of rotary-wing platforms, the military looks at designs that provide battlefield survivability and mission accomplishment. To the constant chagrin of designers, the pace of development is agonisingly slow. According to Jane’s Defence Weekly, funding is already a challenge. While fixed-wing domain negotiates fifth- and sixthgeneration design definitions, it does not appear that there is a possibility of a major revolution in rotary-wing design in the next few years. Ideally, doctrine should have dictated technological trends to meet military requirements, but the slow progression in rotary-wing design indicates that, at least in the next decade or so, doctrine would be impacted by limitations of rotary-wing platforms.