Factors Affecting Maximum Attainable Forward Speed

Conventional helicopters are relatively low speed machines compared to their fixed-wing counterparts. The maximum flight speed will be determined by a combination of one or more of the following: 1. Installed engine power, 2. Airframe parasitic drag, 3. Gearbox (transmission) torque limits, and 4. Rotor stall and/or compressibility effects. Early helicopters were powered by reciprocating engines and were mostly limited in per­formance because of the lack of installed power. Reciprocating engines have relatively poor power-to-weight ratios and become extremely heavy when large amounts of power are re­quired. A reciprocating engine, on average, has a power-to-weight ratio of about 0.5 hp/lb (0.82 kW/kg), whereas a modem turboshaft engine has at least three times as much. Above a certain aircraft gross weight, it is inefficient to use reciprocating engines on helicopters.

As a result, on modern helicopters turboshaft engines are almost universally used because of their superior power-to-weight ratios. However, turboshaft engines have high acquisition and operating costs and are not usually found on small training helicopters. On turboshaft – powered helicopters, performance limits are dictated by allowable transmission torque. When it is necessary to transmit large amounts of torque to the rotor shaft, the transmission system becomes relatively heavy because of structural strength requirements, and so there is usually a torque limit imposed to minimize overall transmission weight. In this case, helicopter performance charts are usually presented in terms of indicated engine power or shaft torque versus indicated airspeed (e. g., Fig. 5.17).

Airframe drag constitutes a substantial impediment to high-speed flight. Therefore, the minimization of airframe drag has become a major issue in the design of a modern heli­copter. Over the past thirty years there have been progressive improvements toward reducing airframe drag and improving forward flight speeds and reducing fuel bum. A major drag producer at high forward speed is the rotor hub, especially because the blade hinges and controls are mostly all exposed to the airstream. Careful contouring of the fuselage in this region can significantly help reduce hub drag and control the extent and intensity of the separated wake behind the hub. More recently, there has been a shift to the use of hingeless or bearingless rotor hubs. Besides being mechanically simpler than conventional articulated rotor hubs, these types of hub designs are also aerodynamically cleaner and have a much lower equivalent flat-plate area.

On many helicopters, the maximum forward flight performance is limited by the aerody­namics of the rotor itself. This is because of the occurrence of one of two possible factors. First, high power (or torque) is required to overcome compressibility effects generated on the advancing side of the rotor disk. Second, retreating blade stall can produce sufficiently high blade loads and vibration levels to limit the flight speed. Compressibility effects man­ifest as wave drag as a result of the onset of transonic flow and the generation of shock waves. The intensity of the supercritical (transonic) flow may also progress to a point where the shock waves are sufficiently strong to promote rapid thickening of the local boundary layer, and it may even produce shock induced separation and stall. The approach of the rotor into these conditions is usually accompanied by a relatively gradual increase in power required or so-called power “creep” (see Section 5.4.3) with mild increases in vibration. However, the occurrence of retreating blade stall is often more sudden in its occurrence and is accompanied by high rotor vibration levels.

Finally, it will be apparent that an expansion of the flight boundary of helicopters to high flight speeds is limited by not only aerodynamic constraints, but also by aeroelastic and structural constraints as well. Usually, high stresses or intolerable fatigue loadings of the various structural components are limiting factors, particularly on the hub and pitch links. These vibratory stresses result from the generation of large unsteady aerodynamic loads on the rotor system, which is simply an undesirable outcome of pushing the rotor to its aerodynamic limits. The complex nature of these loads reflects the need to understand the highly unsteady aerodynamic flow field produced within the rotor disk, which is discussed in detail in Chapter 8.