Room for improvement

D. There is still considerable development potential in 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.

E. 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 antitorque concepts. Eurocopter’s Dauphin-based DGV200 demonstrator has cruised at 195kt, and exceeded 200kt, proving that faster helicopters are possible.

F. 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.

G. 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.

H. 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 reduction in noise and vibration in NASA windtunnel testing.

I. 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.

J. 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 antivibration 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.

K. 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 retracting blade gets slower. Eventually the retreating blade begins to stall, setting the speed limit.

L. 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.

M. 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 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.