This chapter has reviewed many of the aerodynamic issues important in the design of the modern helicopter. It has been noted that there are several trade-offs in the basic sizing and overall optimization methodology of helicopter main rotors. The final design is always a compromise to meet the needs of a particular set of customer or mission requirements. The use of improved airfoil sections and advanced tip shapes generally helps to improve overall rotor performance, allowing higher figures of merit and better cruise efficiency. The computational tools for rotor design are now at a high level of maturity, although significant empiricism must generally still be relied on.
The aerodynamics of the helicopter fuselage and empennage are complicated in themselves, mainly because of the extensive regions of separated flow that can exist. This is complicated further by the aerodynamic interactions that exist between the main rotor and the fuselage, a subject considered in detail in Chapter 11. Predictions of airframe drag is still beyond the state of the art, but can be reliably estimated through component testing of the fuselage and rotor in the wind tunnel and verified by flight testing experience. Besides the main rotor and the airframe, the design of the empennage and tail rotor are key elements in the successful design of the helicopter. Because of the various aerodynamic interactions and the trade-offs in weight and stability, the sizing and positioning of the horizontal stabilizer on the tail has proven to be one of the most difficult challenges facing helicopter designers. The special issues associated with tail rotors have been shown to be extremely important to the design of the modem helicopter. The tail rotor operates in a complicated flow environment, with its operation being affected strongly by the main rotor wake, an issue also discussed in Chapter 11. Many other factors need to be considered to ensure that the tail rotor operates effectively as an anti-torque and directional control device over the full operational flight envelope of the helicopter. Other anti-torque devices such as the fenestron and NOTAR have proved viable alternatives to the conventional tail rotor.
Some concepts for compounds and “high-speed” helicopters have been reviewed. While many ideas have been put forth over the past fifty years, there are no high-speed rotorcraft other than tilt-rotors currently flying. The aerodynamic and aeroelastic problems of highspeed helicopters have proved difficult to solve cost effectively. However, with the advent of new technologies such as smart structures to help control aerodynamic forces and vibration levels on the rotor, it is likely that a further expansion in the operational flight envelope of conventional helicopters will occur.
The unique problem of designing a human-powered helicopter has also been discussed. While perhaps feasible, albeit at the expense and practical difficulties of building a truly enormous and lightweight rotor, it does not seem likely that even the most athletic human has the energy or endurance that can power a helicopter rotor for any significant time. This does not mean, however, the the task is impossible. Finally, the emerging area of hovering micro air vehicles has been mentioned. Current MAVs fall short of anticipated performance because of the low Reynolds numbers and relatively high viscous drag forces. However, this is a relatively new area of research and further developments in aerodynamic understanding will likely lead to much improved MAV designs.