Any discourse on the future of helicopters necessarily entails despair over serious limitations of forward speed of rotary-wing craft. Speed, agility, manoeuvrability, firepower and self-protection are the prominent desirable attributes of a combat platform. In this context, helicopters have been somewhat of a disappointment to the military inasmuch as their speed limitation overshadows the immense advantage their capability to take-off and land vertically offers. Understandably, fixed-wing combat aircraft with higher speeds and offensive roles have been given more importance than slower, less potent rotary-wing craft. Indeed, the first production helicopter (Sikorsky R-4, 1942) came three decades after the first massproduced fixed-wing aircraft (Wright Model B, 1910).

Of course, the potential of rotary-wing flight was recognised early and in the US National Aeronautics and Space Administration (NASA) has had its Langley Research Centre in Hampton since 1917, where it has been carrying out research on helicopters and other vertical flight aircraft for almost a century. Nonetheless, at the outset, it may be postulated that the potential for future developments in helicopter design and development will always take second place to fixed-wing aircraft. While advances in fixed-wing aircraft are dramatically projected as newer, consecutive generations, aviation analysts view progressions on the rotary-wing front as having reached a plateau and argue that future developments are unlikely to demonstrate spectacular advances in speed, performance or design.

Speed

Forward speed remains the biggest bug for helicopter design and, despite fervent efforts to increase speed, it has reached a plateau. The rotor disc represents an insurmountable problem as far as drag reduction is concerned. A rotor system has an inherent aerodynamic challenge which increases with forward speed. The blades in a rotor disc travel at a velocity determined by its length and the speed of rotation when the helicopter is in a hover. However, as the helicopter moves forward, the velocity of the blades relative to the air depends on the velocity of the helicopter as well as the rotor speed. The airspeed of the advancing rotor blade is much higher than that of the helicopter itself; it tends to approach the speed of sound with resultant increased drag and vibration. Also, because the advancing blade has higher velocity than the retreating blade, there is asymmetry of lift between the two. Rotor blades are designed to ‘flap’ – lift and twist in such a way that the advancing blade flaps up and develops a smaller angle of attack. Conversely, the retreating blade flaps down, developing higher angle of attack and generates more lift. At high speeds, the force on the rotors is such that they ‘flap’ excessively and the retreating blade can stall. The forward speed is thus limited and any endeavour to exceed 130 knots involves special materials and technologies.

The leaders in the race for faster helicopters are the Sikorsky X2 and the Eurocopter X3. In June 2013, the X3 broke the X2 record for level flight and dive, recording 255 knots and 263 knots respectively. However, both X2 and X3, are technology demonstrators and are not designed as classic helicopters. Their speed records are not recognised by the World Air Sports Federation as yet. Russia is also working on a high speed design to beat the existing record of 249 knots. The prototype based on a Mi-24 airframe is expected to fly by 2020. However, these speed-oriented rotorcraft have limitations in use and affordability. Having despaired of achieving any breakthrough in the basic rotor disc design, military and civil designers are courting other templates referred to as vertical take-off and landing (VTOL).

APACHE & CHINOOK, SIGNED FOR IAF

Even before Prime Minister Narendra Modi ushered into the meeting room of President Barack Obama in the Headquarters of the United Nations, the officials of the Indian Ministry of Defence put final signatures to the two most significant deals with the US Department of Defense and Boeing, the manufacturers of the two helicopters Chinook and the Apache. Significantly, the $3.1-billion deal was inked at breakneck speed, rarely seen in Indian defence establishment. This was the first big-ticket defence deal signed during the 16 months of the Modi regime. The deal inked on September 28 envisions the supply of 22 Apache attack helicopters and 15 Chinook heavy-lift helicopters.

AH-64 Apache - Quick Facts

Boeing produced 937 AH-64A Apaches between 1984 and 1997. Some remain in service today, and have:

• Two high-performance turboshaft engines and maximum cruise speed of 284 kph
• Laser, infrared, and other systems (including target acquisition designation sight/pilot night vision sensor) to locate, track, and attack targets
• A combination of laser-guided precision Hellfire missiles, 70mm rockets, and a 30mm automatic cannon with up to 1,200 high-explosive, dual-purpose ammunition rounds

Boeing’s AH-64D Apache and the AH-64D Apache Longbow have numerous enhancements, including:

• Longer-range weapons accuracy and all-weather/night fighting
• Detection of objects (moving or stationary) without being detected
• Classification and threat-prioritization of up to 128 targets in less than a minute
• Integrated sensors, networking, and digital communications for situational awareness, management of the combat arena in real time, and digital transmission of images and target locations to joint operations battlefield commanders

CH-47 Chinok - Quick Facts

• The Chinook is a multi-role, vertical-lift platform. Its primary mission is transport of troops, artillery, equipment, and fuel.
• The current CH-47F/MH-47G modernization programs will ensure this tandem rotor helicopter remains in the Army fleet through the 2030s.
• Chinook is the helicopter of choice for humanitarian disasterrelief operations, in missions such as transportation of relief supplies and mass evacuation of refugees.
• Chinooks serve the armed forces of 19 countries around the world.

VTOL

The rotor disc helicopter has retained its basic shape since its inception and has several disadvantages such as high vibration levels, loss of power required to run the anti-torque rotor, loss of tail rotor effectiveness, ground resonance, mast bumping, loss of control during negative ‘g’ conditions, power settling, dynamic roll-over, anti-torque rotor failures, the requirement for unusually quick response in case of engine failure and the non-feasibility of an ejection system for the crew. Thus it was realised that there was a need to move away from the original shape retaining the VTOL feature. Developing a hybrid platform with the performance of a fixed-wing aircraft in forward flight has proven to be a difficult task. The engineering challenge consisted of achieving two main goals. The first was to accomplish controllable vertical flight using the very same mechanisms required for forward flight. The second was achieving ‘power matching’, i.e. a VTOL design that requires the same power in vertical flight as in forward flight. Numerous approaches to VTOL aircraft have been explored over the years; the prominent ones are tilt-rotors, tilt-props, tilt-wings, deflected-slipstreams, deflected-thrust, thrust augmenters, ducted fans, tilt-ducted rotors and tail sitters.

As the name implies, a tilt-rotor aircraft uses tiltable propellers or prop-rotors for lift and propulsion. For vertical flight, the prop-rotors are angled to direct their thrust downwards, providing lift. In this mode, the aircraft is essentially identical to a helicopter. As it gains speed, the prop-rotors are slowly tilted forward through 90 degrees. In this mode the wing provides the lift and the wing’s greater efficiency helps the tilt-rotor achieve high speed like a turboprop aircraft. Bell Helicopter has been dominant in tilt-rotor development with several major designs since the 1950s. Bell is currently partnered with Boeing on the first production tilt-rotor aircraft, the V-22 Osprey. Tilt-rotor prop-rotors require all the fundamental parts of a twin-rotor helicopter. They also have a full set of airplane controls and a tilt mechanism that rotates the lifting rotors. Bell and Boeing are working on larger Quad Tilt Rotor (QTR) military models for possible use by the US Army. The QTR has two sets of fixed wings and four tilting rotors mounted at the tips of the wings. The programme has been nicknamed the V-44 tilt-rotor for the four tilt-rotor version and V-66 tilt-rotor for the six tilt-rotor version. These poly-tiltrotor aircraft have also been called ‘The Flying Freight Train’. These can carry 100 passengers or cargo over 22,700 kg.

The Bell V-280 Valor is a tilt-rotor concept being developed by Bell and Lockheed Martin for the US Army. It is different from the V-22 Osprey in that the engines remain in place while the rotors and drive shafts tilt. A driveshaft runs through the straight wing, allowing both prop-rotors to be driven by a single engine in the event of failure of one engine.

NASA’s Greased Lightning or GL-10 deserves a mention here. It is a battery-powered, ten-engine remotely-piloted tilt-rotor and the prototype has a ten feet wingspan and can take-off vertically like a helicopter as also fly efficiently in forward flight. It is in the testing phase having undertaken test flights early May 2015.

Getting Bigger

Besides speed, size has also fascinated the designer and the user. V-12 was the largest helicopter ever flown with a maximum take-off weight (MTOW) of 105 tonnes. Built in the late 1960s by the Mil Moscow Helicopter Plant, it still holds four world records for payload/altitude combinations. However, only two prototypes were built. The Mil Mi-26, with an MTOW of 56 tonnes is the largest, heaviest and most powerful helicopter to go into series production. While the Mi-26 had a unique design with a single disc having eight rotors, the closest Western heavy-lift helicopter is the CH-53 Super Stallion (33.3 tonnes) followed by CH-47 Chinook (22.7 tonnes). The CH-53 has a seven-rotor single disc and the CH-47 has a tandem rotor system. The foregoing figures have been mentioned to assert that, as far as designer’s skill and operator’s ambition are concerned, big can keep getting bigger. However, the heavier the helicopter, the more restricted is its operational envelope on account of the limitations of ground infrastructure.

Unmanned Craft

The most notable developments in rotary-wing designs are manifest in the Unmanned Aircraft System (UAS) regime. The GL-10 mentioned earlier is a small tilt-rotor; but there is a spectacularly successful drone helicopter that has pioneered its way to fame. The K-MAX is a UAS transformed by Lockheed Martin Corporation and Kaman Aerospace Corporation from an existing helicopter model to enable US Marines to deliver supplies by day or night to precise locations without risking lives. The aircraft can fly at higher altitudes with payload larger than any other rotary-wing UAS. It can lift 2,700 kg at sea level and 1,800 kg at 15,000 feet. The K-MAX rendered laudable service in Afghanistan from 2011 onwards and has proven the concept beyond doubt. The future can be expected to see variations of the concept in optionally manned versions as well.

Conclusion

Due to scarce possibility of increasing speed beyond the current levels except in hybrid/VTOL models, future developments are veering away from the basic rotor-wing design. The future vertical lift (FVL) programme of the US Department of Defense (DOD) adopts a focused approach towards the problem of new types of rotorcraft for US Army beyond 2030. In October 2011, DOD issued the FVL Strategic Plan outlining a joint approach for the next-generation vertical lift aircraft for all military services. FVL concept is to create new rotorcraft that use new technology, materials and designs that are faster, have longer range, better payload, are more reliable, easier to maintain and operate, have lower operating costs and can reduce logistical footprints. FVL is to create a family of systems to replace most army helicopters. A precursor for FVL is the joint multi-role helicopter programme which will provide technology demonstrations planned for 2017.The use of the term ‘Vertical Lift’ instead of helicopter or rotary-wing is an indicator of things to come.

With significant advances in carbon composites, lighter weight with increasing strength has become possible. Helicopter manufacturers are eyeing carbon composites although cost factor is inhibiting. 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. As a single piece composite structure, there are no rivets or bolts resulting in substantial resistance to corrosion, fatigue and impact.

Advances in technology have led to significant improvements in rotorcraft. The classic helicopter design has mutated into newer types of rotorcraft or hybrid VTOL ones; but the intrinsic limitations have not been eliminated. Nonetheless, the majority of helicopters retain the original tadpolelike shape with a main and tail-rotor. Due to the high costs of alternative designs, these are likely to continue in service for the next three decades. Thus while technology has not been able to generate affordable alternative designs, it has served as a handmaiden for the purpose of providing upgrades to existing models so as to enhance performance, safety, role capability and utilisation for new missions. Thus the future of helicopters appears to be a mix of upgraded existing models and emerging designs. The former appear set to remain the cheaper option for at least another two decades.