This chapter has addressed some elementary analyses and predicted results that define the overall performance characteristics of the helicopter in hover, climb, forward flight, and during certain types of maneuvers. It has been shown that these performance characteristics can be derived, in part, by using relatively parsimonious mathematical models for the rotor aerodynamics that have their origin in the momentum and blade element theories given in previous chapters. Good estimates of rotor profile power can be made on the basis of blade element theory, perhaps allowing for radial, yawed, and reversed flow effects and also for compressibility losses at high speeds. Airframe drag has been discussed, and the modeling of these effects has been introduced through the ideas of an equivalent wetted or flat-plate parasitic area. Airframe drag increases rapidly on a helicopter in higher – speed forward flight, eventually becoming one limiting factor in defining its maximum speed capability. The resulting models give good approximations to the rotor power required and performance of the helicopter over the substantial part of the operational flight envelope. The results can be used to estimate performance as functions of helicopter weight and operational factors such as its density altitude. Performance issues such as the speed to fly for maximum range or endurance have been discussed, and it has been shown how these results follow directly from a knowledge of the power required curves.
However, the performance of modem helicopters is mostly limited by other aerodynamic factors, such as blade stall and compressibility effects on the rotor. While compressibility effects show up as progressive increases in power required in high-speed forward flight, the onset of blade stall is more critical because it is a source of high unsteady blade loads and increases in vibration. Both compressibility and stall effects are difficult to model without resorting to more thorough types of analyses that model the aerodynamics at a more fundamental level, such as using the blade element approach. These phenomena and methods for their approximation will be considered in the following chapters.
Helicopter performance when operating near the ground or in a wind tunnel has also been discussed. The complexity of the recirculating flow near the ground is such that this particular aerodynamic problem is not amenable to easy solution and the problem must be modeled semi-empirically. An introduction to the maneuvering flight capability of helicopters has been given based on simple kinematics for steady maneuvers and using an energy approach for the transient case. It is apparent that much can be demanded from the modem military helicopter, which must be able to maneuver effectively. Clearly the most important aspect of maneuverability is the ability of the helicopter to produce extra rotor thrust over and above that required for flight at the original equilibrium flight condition. The extra rotor thrust, however, may be limited because of installed power limitations, transmission torque limits, blade stall, rotor rpm limits, or structural loading limits. Finally, potential performance degradation effects from the effects of airframe and rotor icing has been reviewed.