GRAHAM WARWICK / WASHINGTON DC
Can technology advances and new configurations lift rotorcraft out of their niche and into the mainstream of aviation?
People have dreamed of taking off and landing vertically for as long as they have dreamed about flying. But today the two classes of air vehicle remain fundamentally distinct: those that hover efficiently and those that fly efficiently. Closing the gap between rotary-wing and fixed-wing aircraft is the dream of vertical flight proponents, and designers are pursuing two paths: improving the efficiency of helicopters and perfecting new configurations.
The first "free" vertical flight was made by Frenchman Paul Cornu in 1907, barely four years after the Wright brother's first flight, but the machine was impractical. Autogyros became quite sophisticated between the wars, but they were not true vertical take-off and landing (VTOL) machines. The first practical helicopters were the German side-by-side rotor Focke-Angelis Fa61 of 1936 and intermeshing-rotor Flettner Fl282 of 1941. But Igor Sikorsky's VS300 defined the modern single-rotor helicopter when it first flew in 1939.
Despite all the technological progress since then, helicopters remain niche players in aviation. Compared with equivalent fixed-wing aircraft, they are still more expensive to buy and costly to operate; they are too noisy inside and out; and they vibrate too much for the comfort of their occupants or the longevity of their components. Yet helicopters are arguably the most versatile and useful of air vehicles.
Room for improvement
There is still considerable development potential in the conventional helicopter, albeit incremental. Higher speeds, heavier payloads, less noise and vibration, and lower production and operating costs are possible. Typical industry goals for a 2020-timeframe helicopter include a 200kt (370km/h) cruise speed; 30% reductions in empty weight and fuel consumption; 60% lower external noise; fixed-wing levels of vibration and safety; 30-50% lower development, production, operation and maintenance costs; and all-weather operability.
The latest helicopters can cruise at up to 160kt, but this is an economical, rather than physical barrier. At 160kt the power required in forward flight is close to the power required in hover; to increase speed the power required in level flight has to be reduced. This will require lower-drag airframes, active rotor control and new anti-torque concepts. Eurocopter's Dauphin-based DGV200 demonstrator has cruised at 195kt, and exceeded 200kt, proving that faster helicopters are possible.
More important than higher speed are lower noise and vibration, as both are barriers to the wider acceptance of helicopters. External noise is being tackled with rotor designs and operating procedures. The latest high-thrust blades allow the main rotor to be slowed in the cruise, reducing fly-over noise, and both passive and active means to reduce approach noise are being evaluated.
The main source of noise on the descent is blade vortex interaction (BVI) - the main rotor blades hitting the air shed by preceding blades. Among the mitigating technologies NASA has evaluated is the low-noise planform rotor. This has a "wavy" blade that distributes the shed air and reduces BVI noise. Another is the modulated rotor, in which the blades are spaced unevenly to generate a more random, less annoying noise.
A third concept for reducing BVI noise is the active twist rotor, in which the load distribution and spatial position of each blade is controlled individually. This reduces the strength of the wake and allows the blade to be "flown" away from the air shed by the preceding blade. The active twist rotor has shown substantial reductions in noise and vibration in NASA windtunnel testing.
Active rotor control is a feature of most advanced low-noise, high-speed helicopter designs, with advances in materials and electronics making individual blade control practical. Manufacturers are testing main rotor blades with active servo flaps driven by piezo-electric actuators. These are precursors to smart-material "morphing" blades that would allow elimination of the mechanical swashplate used to control blade pitch.
Smart, or active, structures also promise to reduce internal noise, as well as vibration. Passive vibration reduction has reached its limits, with the trend towards variable rotor RPM to reduce external noise requiring an adaptive anti-vibration system. Approaches being tested include acoustically active gearbox struts and cabin ceiling panels fitted with piezo-electric actuators that oscillate to cancel out noise and vibration.
Pushing helicopter speeds higher may require a new approach. One concept receiving attention is the reverse velocity rotor. This tackles the fundamental limit on the forward speed of a conventional helicopter, which is a result of the rotor flying sideways. As forward speed increases, airflow over the advancing blade gets faster while that over the retreating blade gets slower. Eventually the retreating blade begins to stall, setting the speed limit.
The reverse-velocity rotor (RVR) has a double-ended aerofoil that generates lift whichever way the air is flowing over the blade. As forward speed increases, the rotor is slowed until the retreating blade is immersed in reverse flow, but still producing lift. This requires a variable-speed transmission and auxiliary propulsion, as at high speed the rotor is autorotating and pitch and yaw control is provided by thrust vectoring.
Windtunnel testing indicates the reverse-velocity rotor is capable of cruise speeds exceeding 300kt, but it retains the simplicity of a helicopter with no reconfiguration required to transition from vertical to forward flight. Under NASA contract, Sikorsky has studied an 80-passenger RVR runway-independent aircraft, with three engines, an eight-blade rotor and ducted-fan propulsor on the tail. The baseline RVR has a 340kt cruise and 1,000km (540nm) range. Compounding - adding a wing to offload the rotor in the cruise - results in a smaller aircraft for the same mission, but increases empty weight and hover download.
Tilting forward
The RVR rivals the speed of a tiltrotor, widely regarded as the next step in rotorcraft evolution. The tiltrotor combines the hover capability of a helicopter with the cruise efficiency of a turboprop. The first example flew in 1954, and the first conversion from helicopter to aeroplane mode was accomplished in 1958, but the first production tiltrotor has yet to enter service.
Today's tiltrotors, the Bell Boeing V-22 military transport and civil Bell/Agusta BA609, are designed to cruise at 275kt, while NASA's 2020-timeframe technology goals call for a 350-400kt cruise to make the tiltrotor competitive with short-haul jets.
The tiltrotor has speed and range advantages, but the configuration presents challenges. Weight is one: an airliner can carry 120% of its empty weight and a helicopter 80%, but today's tiltrotors can lift only 40%. Hover capability is reduced by the compromise proprotor design - thrust in the hover is 10 times that needed in the cruise - and by the download on the airframe, which can equal 10% of aircraft weight.
Led by AgustaWestland, Europe is working on a second-generation tiltrotor to address some of these issues. Key innovations in the Erica - a 350kt-cruise, 1,100km-range, 20-passenger tiltrotor - are the reduced-diameter rotors and tiltable wing. Smaller proprotors improve cruise performance, while a tilting wing offsets the loss in hover efficiency by reducing download to around 1% of aircraft weight. The smaller rotors allow for take-off and landing in aeroplane mode, enhancing safety, while tilting the wing independently of the nacelles widens the conversion corridor.
Future tiltrotors
Bell and Boeing independently are looking at future tiltrotors and improvements to the current generation. The latter include a variable-geometry rotor with slotted blade providing higher lift in the hover without incurring a power penalty in forward flight. Others address the download issue, and include movable overwing vanes to deflect the rotor downwash and active synthetic jets to delay flow separation over the wing.
Bell is studying a larger quad tiltrotor (QTR) with four proprotors. The military version has a C-130-size fuselage, 20t payload and uses V-22-size dynamics. A 120-passenger civil QTR would cruise at 340kt. Boeing's advanced tiltrotor concept - designed to meet the same NASA runway-independent aircraft needs as Bell's civil QTR and Sikorsky's RVR - is a tailless canard configuration with two large-diameter, five-blade proprotors at the tips of a W-planform wing designed to minimise download.
Faster rotorcraft have yet to be flight tested even experimentally, but Boeing hopes to fly the unmanned X-50 canard rotor/wing (CRW) demonstrator this year. Previous stoppable rotor/wing designs had problems with conversion between rotor- and wing-borne flight, but the CRW is different. Earlier designs used the rotor/wing to provide lift in all modes, whereas the CRW unloads the rotor/wing during conversion, with lift being provided by the foreplane and tailplane.
Boeing believes this will ease the transition between helicopter mode, where the rotor/wing acts as a two-blade reaction-drive rotor, and aeroplane mode, where the rotor/ wing is locked perpendicular to the fuselage. The concept simplifies the powerplant design, with warm turbofan exhaust gases ducted to nozzles in the rotor tips for vertical flight, then redirected rearwards to provide jet propulsion for forward flight.
Although it promises to be the first concept to combine the best attributes of rotary- and fixed-wing aircraft, the canard rotor/wing will have to fly successfully to be taken seriously. Until it does, the gap between helicopters and jets will remain.
Source: Flight International
