Staunch and steadfast rotary-wing pilots may be forgiven for grudging the fixed-wing domain all the limelight despite the fact that the helicopter was imagined by man before a fixed-wing craft was. The hand-spun Chinese soaring toy and the helical machine design of Leonardo de Vinci preceded the first drawings of fixed-wing planes by centuries. Alas, development in rotary-wing design has remained lamentably slow in contrast to fixed-wing. This is so because the latter had more potential as a military platform, as well as for expeditious transportation of cargo and passengers. Ironically, the foundation of modern rotary-wing design was laid in pursuit of a fixed-wing design problem. Juan de la Cierva, a Spaniard and an aviation enthusiast, built—in 1918—a three-engined bomber aircraft which stalled and crashed during its test flight. It was his persevering genius which invented the autogiro as the solution. The rest, as they say, is history. Paul Cornu’s first flight without ground assistance came in 1907, but the first practical helicopter, the German Focke-Wulf Fw 61 flew only in 1936. Subsequent progress has also been slow and fruits of labour invested in rotary-wing design research have not been rewarded with the same degree of spectacular success that parallel endeavours in fixed-wing arena have produced. While the fifth-generation fighter is in its testing stage, helicopter development is still struggling to demonstrate a progression over initial designs, significant and adequate to be termed a generation leap.

Need for Speed

Be it in the military domain or the civil one, the need to attain higher speeds is an inescapable preoccupation of designers and users alike. So how does one go about making a helicopter fly faster? More powerful engines, sleeker designs and minimised drag in flight are logics common to fixed- and rotary-wing design. However, the single major factor impeding higher speeds for rotary-wing is a phenomenon called the retreating blade stall which affects—during each rotation of the rotor system in flight—the blade travelling in the direction opposite to the flight path (thus termed ‘retreating’ blade). The effect of this blade movement contrary to the forward flight leads to the effective airflow over the retreating blade slowing down considerably. As lift on an aerofoil is proportional to the angle of attack and forward speed, the result of the reduced airflow speed over the retreating blade is a stall of that blade. Technologies to change the angle of attack cyclically to compensate for the lower airflow speed have peaked out and it does not look likely that there will be a significant speed enhancement to the conventional rotor disc design which situates the rotor disc essentially in the horizontal plane. Attention has thus shifted to a design which uses one or more rotating discs as conventional rotor systems to generate lift for a vertical take-off but once in the air, tilts the rotor systems forward to transform progressively into propeller systems producing thrust in the forward direction of flight.

Defence research and development (R&D) plays a prominent part in development of any design with a potential for use as a military platform. The Osprey CV22 is a tilt-rotor aircraft in service with the US Air Force (the MV 22 flies with the US Marine Corps). Its remarkable feature is its tilt-rotor design which allows it to hover, take-off and land vertically while permitting a transfer to high speed horizontal flight once in the air. This design gives the CV-22 the flexibility of the helicopter with the speed and range of a turboprop cargo plane. The CV-22’s rotors can be folded in order to reduce the space needed to park the aircraft, especially in the close confines of a hangar. It can fly at 240 knots, carry 32 troops over a range of 2,100 nm and has a ceiling of 25,000 feet. These impressive statistics, especially the speed, possibly qualify it as a ‘generation’ leap.

Following in the wake of the military tilt rotor programme was a joint one for a civil one with AgustaWestland and Bell Helicopters as the two partners. However, in November 2010, AgustaWestand took over the programme completely and the result was the AW609 tilt-rotor. It has a maximum take-off weight (MTOW) of 7,620 kg, an unrefuelled range of 700 nm (boosted to 950 nm with auxiliary fuel tanks), a service ceiling of 25,000 feet, and can carry 12 passengers/crew. More importantly, it has a cruise speed of 275 knots and a maximum forward speed of 310 knots. Development in the civil tilt-rotor regime has not been as rapid as one would expect (given the apparent advantages of tilt rotor). However, the appeal of tilt-rotor technology remains strong, and more designs may be expected to appear in the future.

An allied technology is represented by the Eurocopter X3, a hybrid design using a five-bladed (conventional) rotor system and two forward facing propellers installed on stub-like wings. The X3 has demonstrated a forward speed of 230 knots and Eurocopter sees for it a future in a wide range of missions including military roles, long-distance search and rescue (SAR), coast guard, border patrol, passenger transport, offshore airlift and inter-city travel. A competitor is the Sikorsky X2. Both the Sikorsky X2 and the Eurocopter X3 have propellers but are based on different design philosophies. The X2 has contra-rotating main rotors and one aft propeller, while the X3 is a true compound with a single main rotor, wing stubs and two side-mounted propellers. Also, their designers target different markets. Sikorsky has demonstrated a 250-knot cruise on X2 and is now applying the technology to the military X2 Raider, an armed reconnaissance prototype. Eurocopter prefers to limit the speed to around 220 knots for economical reasons, as it focuses on commercial uses. Also, the X2 has fly-bywire controls, while the X3 has conventional controls. According to Eurocopter, the X3 technology will be implemented in a new helicopter during the next six to seven years, with the new type being specified by the end of 2013. The Sikorsky X2 Technology Demonstrator is being projected in partnership with Boeing, for US Army’s Joint Multi-Role Technology Demonstrator Phase 1 programme. Sikorsky will take the lead role in Phase 1 proposal and Boeing will take a lead role for Phase 2, the mission systems demonstrator programme. The US Army is expected to announce its selection of one or more wining bids in late 2013. Demonstrator aircraft are expected to fly in 2017.

The Exotic

Some of us would recall Little Nellie—the autogyro flown by James Bond in the film You Only Live Twice. A look alike, but much scaled down, version—the SQ-4 Recon—one of the smallest rotary-wing, unmanned aerial vehicles in the world is all set to be used for battlefield (and anti-terrorism) reconnaissance tasks. Its 23 cm diameter and its 200 gm weight make it well-nigh invisible from a distance. It contains two cameras which allow operators to look over hills and inside enemy bunkers or terrorist hideouts without the risk of being killed or injured. It can be controlled remotely by an operator sitting in a control room thousands of miles away or by soldiers on patrol using a tablet computer. It can fly and hover for 30 minutes or switch off its engines and perch like a bird on the ledge of a building, and without being spotted, zoom in on suspicious activities for up to eight hours. Its cameras can transmit live images or take still photos or video footage using day or night vision. The merging of VTOL and information technologies render this type of application invaluable.

Atlanta-based Oliver VTOL is developing the Hexplane based on a Piaggio P180 Avanti airframe; the design incorporates six conventional propellers and its efficiency is expected to lie somewhere between the V-22 and a conventional fixed-wing aircraft. Another concept is a civil VTOL fan-in-wing design—reminiscent of the craft seen in the sci-fi thriller Avatar. Switzerland-based Ray Research is a small entrepreneurial company working on the concept and it believes that this design could be cheaper to operate than helicopters or tilt rotors. The Ray has four low disc-loading fans that are mounted in a low aspect-ratio wing to generate lift in vertical flight, and two tilting ducted fans on the tail that provide vertical thrust and pitch control in hover mode and propulsion in forward flight. Longitudinal vanes under the fans provide sideways thrust vectoring. These vanes and roll shutters on top close off the wing fans in forward flight. A long way from prototype flight, the Ray holds alluring promise.

American Dynamics Flight Systems (ADFS), a small US firm based in Maryland, is developing a VTOL unmanned aircraft, the AD-150, in the hope of interesting the US Marine Corps in its design. The AD-150 is very similar in concept to the Doak VZ-4, the first tilt-duct VTOL aircraft, which first flew in February 1958; it has a design gross weight of 1,065 kg and a turboshaft power plant, driving a pair of fans about a metre in diameter inside 1.14-metre-diameter nacelles. The design speed is 300 kt, with a 228 kg payload. ADFS has patented “high-torque aerial lift” ducted fans which can independently tilt laterally to provide directional control.