While lagging behind fixed-wing combat aircraft that are stretching out to their sixth generation, military helicopters have yet cornered substantial attention and funding to have made notable advances in technology. Their immense utility to the land forces by dint of the ‘high ground’ benefit they present and the speed and flexibility advantage which they afford over all land-borne fighting weapon systems, make them invaluable. Military use of helicopters started with medical evacuation, but has evolved over the years to include roles like attack, anti-tank, suppression of enemy air defences, reconnaissance and observation, airlift of troops and cargo resupply, fire fighting, surveillance, scout, heavy-lift, troop induction and de-induction and Special Operations. As can be seen from these roles, the helicopter uses the medium of the air to move around, but is umbilically connected to land forces.
Progress in helicopter design and development has lagged behind fixed-wing because of high costs of technology as also by limitations inherent to the vertical lift-off capability of the classic shaped helicopter design. However, helicopter technology is making impressive advances some of which are moving away from the traditional tadpole-like rotary wing shape into diverse models possessing Vertical Take Off and Landing (VTOL) and hover capabilities that characterise a helicopter. Higher speeds, precise and incisive fire power, superlative agility and manoeuverability, stealth and self-protection, high survivability and low vulnerability, with the capability to perform designated roles in bad weather and night conditions, are the concerns that occupy the rotary wing designers’ minds. Major users of battlefield helicopters (US and Europe) have taken on these challenges and are working on designs to produce technological enhancements for battlefield helicopters.
Military Driven Programmes
Inarguably, the US military has had the most diverse and wide ranging experience with helicopters notably in Vietnam, Korea, Iraq and Afghanistan while the ongoing US war against terrorism continues to drive initiatives in helicopter technology. The US Army currently operates over 4,000 helicopters in various roles. In 2009, it started conceptualising its future requirement and evolved the strategic Future Vertical Lift (FVL) programme – a combination of rotor and fixed wing as the possible future solution – with the target date set as 2030. Towards this objective, a US Army-led programme, the Joint Multi-Role (JMR) Demonstrator, drew up an ambitious wish-list with inputs from the Coast Guard, Special Operations Command and NASA.
The final objective was a design with vastly improved avionics, electronics and range, a speed of 200 knots, survivability, operating altitudes and payloads. The ‘multi-role’ part of the programme alludes to several roles ranging from attack configurations to cargo, medevac, search and rescue and anti-submarine warfare. Originally, three Capability Sets were planned in 2009 – JMR Light (Scout version to replace the OH-58 Kiowa), JMR Medium-Light, JMR Medium (Utility and Attack versions). Later on two more sets were added – the JMR Heavy (Cargo version to replace the CH-47 Chinook) and the JMR Ultra (Ultra-sized version for vertical lift aircraft with performance similar to fixed-wing tactical transport aircraft). Eventually, these new designs are expected to replace 25 current rotorcraft types in use with the US military.
The first focus was on FVL Medium, seen as the replacement for the US Army’s Sikorsky UH-60 Black Hawks and Boeing AH-64 Apaches. In 2014, the Army selected two firms to develop FVL demonstrators. These were Textron Inc’s Bell Helicopter and a joint initiative between Boeing and Sikorsky – Lockheed Martin’s rotary wing aircraft expert division. Bell is offering next-generation tilt-rotor, V-280, evolved from its V-22 Osprey, which tilts its huge rotor blades between vertical and horizontal positions to gain both the best features of a helicopter (vertical take-off and landing) and those of a propeller plane (fuel-efficient speed for long range flight). The V-280 is expected to integrate cabin armour, fly-by-wire component redundancy, airborne battle boards that track mission updates in real time and an equipment package that will enable en route situational awareness through digitally fused reconnaissance, surveillance, intelligence and friendly force information. It first flew in December 2017, and reached its target speed of 280 knots in January this year. The Sikorsky-Boeing SB-1 Defiant looks more like a traditional helicopter, but is based on Sikorsky’s “X2” technique of combining two counter-rotating rotors, one on top of the other, with a pusher propeller at the tail. On September 15, 2010, the X2 Technology Demonstrator had unofficially broken the rotorcraft speed record of 250 knots. It first flew in March this year. In a related development, in July this year, the US Army approved a draft capabilities development document for the Future Long-Range Assault Aircraft (FLRAA), the next generation of affordable vertical lift utility aircraft to the Army to replace some Army’s UH-60 Black Hawk helicopters.
Next in line is the JMR Light category under which a Future Attack Reconnaissance Aircraft Competitive Prototype (FARA CP) programme has been designed to produce a new armed scout aircraft. Five contenders – an AVX Aircraft Company/L-3 Communications team, Bell, Boeing, Karem and Sikorsky – have been shortlisted by the US Army for Prototype Agreements from the Army for the design, build and testing of their FARA proposals. The first flight of a demonstrator aircraft is slated for 2024, with initial operational capability by 2028. Interestingly, the Army’s Armed Aerial Scout programme was shelved due to budget cuts in 2013 and its entire fleet of Bell OH-58D Kiowa Warriors has retired in 2014. It has been using Boeing AH-64E Apaches as an interim measure in combination with UAVs since phasing out use of the OH-58Ds in 2014.
In Europe, Airbus Helicopters’ X-3, “high-speed long-range hybrid helicopter” has a set of propellers for forward motion instead of a tail rotor and is one of the aircraft that have flown under “hot and high” conditions (6,000 feet at 95 degrees Fahrenheit) like a rotorcraft with airplane-like speed. The X-3 demonstrated a speed of 255 knots in level flight and 263 knots in a shallow dive on June 07, 2013, beating the Sikorsky X2’s unofficial record set in September 2010, thus becoming the world’s fastest non-jet augmented compound helicopter. The design was retired in 2014, although in 2016, Airbus filed a patent for X-3, claiming it as the world’s fastest rotary wing aircraft. However, it did not bid for the JMR due to cost and the X-3 remained a museum piece till recently when Airbus Helicopters pitched a design based on its technology for the US Army armed scout helicopter (under FARA).
Rotory Wing Technology Challenges
Let us now address some of the challenges for helicopter technology. To state the obvious, speed remains the single major limitation of a helicopter and the biggest concern for rotary wing designers. In adversarial situations, whether involving predators or modern contending aerial machines, speed has an inverse relationship with vulnerability; the higher the speed, the lower the vulnerability to ground fire. Technology has evolved solutions to the problem of airframe drag, but the helicopter’s rotor disc poses an intractable problem as its very shape presents an inherent aerodynamic challenge with increases in forward speed.
At high speeds, the force on the rotors is such that they ‘flap’ excessively and the retreating blade can reach too high an angle and stall. Consequently, conventional helicopter design has remained predicated to the rotor system which restricts maximum speed to around 180 knots. The basic helicopter design has other limitations as well, essentially because the rotor system is not as efficient for forward travel as a fixed wing one and consumes approximately 15 percent of its total engine power available to run the essential tail anti-torque rotor which is an essential part of the design to keep the helicopter from spinning. The helicopter must also deal with high vibration levels and uses more fuel and requires more maintenance. In acknowledgement of the fact that a single rotor system with a complementing tail rotor, has reached a performance plateau and is unlikely to ever achieve a spectacular breakthrough in forward speed, designers are exploring other avenues which are tending to metamorphose not just the shape and silhouette of the helicopter, but also its name to Vertical Take-Off and Landing (VTOL) craft.
Vertical Take-Off and Landing (VTOL)
VTOL technologies aim at retaining the hover and take-off characteristics of a helicopter, but transform the machine into one more resembling a fixed wing one with all its speed advantages, after the takeoff from a restricted space and without the need for a runway. Numerous approaches to VTOL aircraft have been explored over the years and VTOL is as old as helicopters. Indeed, as far back as 1922, Henry Berliner demonstrated to the US Navy a fixed wing aircraft with two horizontal rotors mounted on it. However, the machine was not a practical one and was never operationalised. Since then, multifarious VTOL designs have evolved; some of 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.
As the name implies, a tilt rotor aircraft uses tilt-able propellers, or prop rotors, for lift and propulsion. For vertical flight, the prop rotors are angled to direct their thrust downwards, providing lift. Once airborne, the prop rotors are slowly tilted forward, eventually becoming perpendicular to the ground. In this mode, the aircraft is essentially a turboprop aircraft. Bell Helicopter has been dominant in tilt rotor development and partnered with Boeing on the first production tilt rotor aircraft, the jointly developed and manufactured Bell/Boeing V-22 Osprey. The V-280 design mentioned earlier on is a successor of the V-22 Osprey, but is different inasmuch as its engines remain in place while the rotors and drive shafts tilt. It has a maximum speed of 280 knots at sea level achieved in January this year and 305 knots at 15,000 feet. While these speeds are much higher than those of helicopters with rotor discs, they do not compare well with fixed wing aircraft.
A variation is the VTOL proposal for a six-engine craft called the Hexplane which, it is hoped by the developer, would cross 400 knots and is seen more as an airplane that hovers rather than a helicopter. The developer claims that it can carry out full operations with any of its rotors inoperative. The military uses of such a machine are self-evident. The Bell Boeing Quad Tilt Rotor (QTR) is a proposed four-rotor derivative of the V-22 Osprey developed jointly by Bell Helicopter and Boeing. The concept is a contender in the US Army’s Joint Heavy Lift programme. It would have a cargo capacity roughly equivalent to the C-130 Hercules, cruise at 250 knots and land at sites vertically like a helicopter. It will also have a Short Take-Off and Landing (STOL) capability.
The VTOL experimental plane or VTOL X-Plane programme of US Defence Advanced Research Projects Agency (DARPA), is worth a mention here. It sought to cross-pollinate fixed wing and rotary wing designs to attain sustained flight speed of 300-400 knots. Aurora’s Phase 2 design for VTOL X-Plane envisioned an unmanned aircraft with two large rear wings and two smaller front canards – short winglets mounted near the nose of the aircraft. A turbo-shaft engine – the one used in V-22 Osprey tilt rotor aircraft – mounted in the fuselage would provide three megawatts (4,000 horsepower) of electrical power which would drive 24 ducted fans. It was hoped the XV-24A would be able to achieve flight speeds in excess of 300 knots while carrying a load of at least 40 percent of the aircraft’s projected gross weight of around 5,443 kg. Flight tests were to commence last year but DARPA, during a programme review, while stating that the major objectives of the VTOL X-plane had been achieved with a subscale demonstrator, cancelled the programme for the time being. It may resurface at an appropriate time in the future.
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 goal of the DARPA-funded Disc Rotor Compound Helicopter programme is to design and demonstrate the enabling technologies required to develop a new type of compound helicopter capable of high-efficiency hover, high-speed flight and seamless transition between these flight states. An aircraft capable of long range high speed (300-400 knots) and VTOL/hover is expected to be the end product although currently, the programme is on the back burner.
Unmanned Battlefield Helicopters
With spectacular advances in Artificial Intelligence (AI), unmanned and optionally manned, battlefield helicopters are gaining attention. They offer the significant advantage over manned ones especially in roles and missions where manned helicopters would expose their pilots to high risk and probable loss of life including flights over enemy territory, in very poor visibility and weather conditions, above or in close proximity of enemy troops, Nuclear, Biological and Chemical (NBC) environments and Suppression of Enemy Air Defenses (SEAD) tasks. They also eliminate the need for air conditioning and pressurisation of the cockpit, oxygen systems, highly expensive avionics, voice communication equipment, and navigational displays while reducing the size of or eliminating altogether, the cockpit-equivalent. Moreover, the unmanned machine can be designed for high manoeuverability and agility and long operating hours without human physiology becoming a design constraint. Finally, if an unmanned helicopter gets shot down over hostile territory, the public uproar over a pilot being taken a prisoner is absent. However, while a human brain can assess all inputs from the cockpit and the environment around and take mission and survival critical decisions even when some of the inputs are ambivalent, the unmanned helicopter’s situational awareness is limited by the artificial intelligence that it houses.
The most prominent role for unmanned helicopters is logistics. The K-MAX, flying to the drop locations through known routes and involving minimal risk to life or limb, has been providing an excellent platform for re-supply of Marines deployed in Afghanistan. It can deliver a full 2,700kg of cargo at sea level and over 1,800kg at 15,000 feet and has helped to prove the concept beyond any reasonable doubt. The K-MAX was used in a demonstration of a medical person with a hand-held tablet guiding an unmanned helicopter to locate, evaluate and evacuate a simulated casualty. The idea is even more alluring when one considers the possibility of the same machine delivering cargo to troops engaged in operations and, on the return leg, evacuating the wounded or the sick. In May this year, the US Marine Corps (USMC) announced plans to retrofit its two K-Max helicopters with additional autonomous capabilities, thus validating the service’s immense faith in its battlefield prowess.
Bell was toying with a tail sitter model to cultivate a cargo carrier (Hydra) which, although technically not a helicopter, will operate like an unmanned load-carrying helicopter. In another initiative, Aurora Flight Sciences now acquired by Boeing, is working on Autonomous Aerial Cargo/Utility System (AACUS) – an autonomous helicopter programme designed to meet requests for supply delivery via helicopter. An autonomous logistic operational demonstration was given in November 2017. Other similar successes have been the Sikorsky UH-60 Black Hawk in a fully autonomous mode, deploying a fully autonomous ground vehicle and Russia’s BPV-500 unmanned helicopter with a takeoff weight of 500kg and payload of 180kg in autonomous and semi-autonomous modes, demonstrated at International Maritime Defence Show, St Petersburg in 2017. In 2017, Israel Aerospace Industries (IAI) made a successful proof of concept demonstration of Air Hopper – an unmanned helicopter capable of cargo carriage and casualty evacuation.
Unmanned helicopters have been serving as eyes over the battlefield and technologies are constantly evolving to make them more capable. Boeing’s Little Bird H-6U is the unmanned version of AH-6i manned scout helicopter; with autonomous flight capability, networked payloads and communications, it provides over-the-horizon search, re-supply, communications relay and surveillance capabilities. Its useful payload is 635kg and has an endurance of six hours and a flight ceiling of 20,000ft. The American Unmanned Systems’ Guardian unmanned helicopter UAS provides full use of multiple onboard sensors and payloads up to 23kg, resulting in continuous ISR engagement and quick delivery to operational and tactical forces. It can locate and accurately detect, identify and track targets for five hours on station.
One significant trend in unmanned helicopters is the miniaturisation of machines. Pulse Aerospace produces VTOL Unmanned Aerial Systems (UAS) produces small unmanned craft with payloads varying from a kg to 11.3kg and is ideally suited for carrying sensors including in dual sensor configurations such as infrared, multi-spectral and high resolution electro-optical camera systems. Honeywell’s RQ-16A T-Hawk Micro Air Vehicle (MAV) has been in service since 2007 with the US Army and Navy for explosive ordnance disposal. The PD-100 Black Hornet developed by Prox Dynamics recently bought over by Flir, a Norwegian drone maker, offers ISR support and is deployed in Afghanistan by UK armed forces. It has a length of ten centimetres and a rotor diameter of just 12 cm, with a weight of 16gm including the surveillance camera. According to Flir, PD-100 systems have been sold to operators in 23 countries.
The Fancopter is another micro UAV designed and manufactured by German company EMT Penzberg for the German armed forces. The UAV performs Intelligence, Surveillance and Reconnaissance (ISR) operations in urban environments in all lighting conditions. The UAV has a height of 44cm and a diameter of 73cm. The takeoff weight of the aerial system is 1.5kg and it can collect reconnaissance data including video images, still pictures and GPS coordinates. Spyball-B is a micro-electrical rotary wing UAS designed and developed by Selex ES (merged into Leonardo), for use by the Italian Armed Forces and NATO Forces. It has a height of 0.55 metre, a diameter of 0.48 metre and a maximum take-off weight of 2 kg. It can be carried in backpacks and operated by a single person.
Design of unmanned helicopters for more offensive roles (attack, anti-tank, air-to-air combat, close air supported) has been slow largely due to the technologies related to instantaneous decision-making based on many inputs from the battlefield milieu. As AI becomes more and more refined, it can be expected that these roles will also mature.
From January 01, 2016, Leonardo’s Helicopter Division absorbed the activities of AgustaWestland which is working on the world’s first electric tilt-rotor aircraft, as a technology demonstrator termed ‘Project Zero’. The aircraft incorporates more than 80 percent composites, including 100 percent of the skins, rotor blades, shroud and spokes. The structure is nearly all aluminium and carbon with hardly any steel used. The only version being envisaged is an unmanned one. NASA’s Greased Lightning (GL-10) deserves a mention here. It is a battery-powered, ten-engine remotely piloted tilt rotor and the prototype has a ten-foot wing span, and can take off vertically like a helicopter as also fly efficiently in forward flight. The final target is to have an autonomous 20-foot span surveillance aircraft that would cruise at 100 knots and loiter at 21,000 feet for 20 hours. A report was published by NASA about its flight testing in 2018, but not much has been heard thereafter.
In conclusion, let us look at the trends in advances being made in military helicopters. The most prominent trend is the gradual shift from the tadpole-like helicopter to the more versatile VTOL models most of which look like fixed-wing aircraft, but retain vertical take-off and hover capabilities. AI promises to smarten up the ongoing advances in military helicopters mainly through the unmanned option by itself or as a hybrid model wherein a pilot is onboard; but the machine is capable of totally autonomous operations in the battlefield. Increasingly, evidence is pointing to the unmanned option being more useful than the manned one in the military context. As far as significant leaps or ‘generation’ jumps in technologies go, it does not appear that advances will produce a markedly superior helicopter design with speeds matching fixed wing although incremental changes will continue to emerge with passage of time. More significant would be the advances that accrue as innovative changes enabled by emerging technologies are made to existing helicopter designs in order to enhance performance, survivability and utility as military machines. Notwithstanding the FVL or any other programme introduced in the future, the trend to improve existing models is likely to endure at least until the end of the next decade.