Introduction: A History of Helicopter Flight

The idea of a vehicle that could lift itself vertically from the ground and hover motionless in the air was probably bom at the same time that man first dreamed of flying.

Igor Ivanovitch Sikorsky (1955)

LI Rising Vertically

Aerodynamics is the science of all flight. The role of aerodynamics in the engi­neering analysis and design of rotating-wing vertical lift aircraft, such as the helicopter, is the primary subject of this book. A helicopter can be defined as any flying machine using rotating wings (i. e., rotors) to provide lift, propulsion, and control forces. The rotor pro­duces a lift force equal to the weight of the helicopter and because the generation of this lift force does not require any forward flight speed, the helicopter can rise vertically from the ground and hover. A simpler definition, therefore, is that a helicopter is an aircraft using a rotor (or rotors) that can hover – see Hafner (1954). Tilting the orientation of the rotor disk(s) provides the forces and moments to control the helicopter in flight. Tilting the rotor disk fore and aft provides pitch control, and tilting it left and right gives roll control. If a single main lifting rotor is used, then a sideward thrusting tail rotor provides anti-torque and directional (yaw) control. If the rotor disk is tilted progressively forward, the rotor provides a propulsive force, accelerating the helicopter into forward flight.

Although the helicopter has been described as an “ungainly, aerodynamic maverick” [Carlson (2002)], the modem helicopter is indeed a machine of considerable engineering sophistication and refinement (Fig. 1.1) and plays a unique role in modern aviation provided by no other aircraft. It is truly a unique form of aircraft and a mastery of modem aeronautical engineering. The helicopter can take off, fly forward or backward, climb and descend, and move in almost any direction at the whim of the pilot. This is the form of tme flight that inspired humankind literally hundreds of years before the helicopter became a reality. Igor Sikorsky’s vision of a rotating-wing aircraft that could “lift itself vertically” and safely perform all these desirable flight maneuvers under full control of a pilot was ultimately only to be achieved in the mid-1930s, some thirty years after fixed-wing aircraft (airplanes) were flying successfully. In the last seventy years, the helicopter has matured from a cumbersome, vibrating contraption that could barely lift its own weight, into a modem and efficient aircraft that has become an indispensable part of modem life. Its modem civilian roles are almost limitless and encompass sea and land rescue, police surveillance, oil rig servicing, homeland defense, and other important missions. Without question, helicopters are an essential part of any modem military.

Rotating-wing aircraft are far more complicated than they might first appear. Aerody – namically, the airflow through the helicopter rotor is extremely difficult to define and even after many years of intense study it still defies a fully adequate description. The ability to define and predict the rotor aerodynamics, however, is key to the prediction of the perfor­mance of the helicopter as a whole. Mechanically, the helicopter is complicated as well. The long slender rotor blades twist and bend, flap up and down, and lead and lag about

hinges that attach them to the rotor shaft. The need to control the aerodynamic forces on the rotor requires that the pitch of each blade be changed individually as the blades rotate about the shaft. Despite the relatively high aerodynamic and mechanical complexity of the rotor system and the helicopter as a whole, there are still many parallels in their devel­opment when compared to fixed-wing aircraft. However, the longer and more tumultuous

Year of first flight

Figure 1.3 Types of helicopter, (a) Single main rotor/tail rotor (conventional) configura­tion. (b) Tandem rotors, (c) Coaxial rotors, (d) Side-by-side rotors, (e) Intermeshing rotors.

gestation period of the helicopter is clearly attributable to the greater depth of scientific and aeronautical knowledge that was required before all the various technical problems could be understood and overcome. Along with the need to understand the basic aerodynamics of vertical flight and improve upon the aerodynamic efficiency of the helicopter, technical barriers included the need to develop suitable high power-to-weight ratio engines and high strength-to-weight ratio materials for the rotor blades, hub, fuselage, and transmission.

Compared to airplanes – the development of which can be clearly traced to Liiientnai in Germany, Pilcher in Britain, Langley in the United States, and the first controlled flight of a piloted powered aircraft by the Wright Brothers in 1903 – the origins of successful helicopter flight are less clear. It may seem surprising that until the middle of the nineteenth century there had been more attempts to build helicopters than fixed-wing aircraft (see Fig. 1.2). Yet the early preference of helicopters over airplanes is perhaps not so surprising given the rapid adoption of the marine propeller during the same time period. Therefore it would seem that the preferred means of vertical-rising locomotion through a fluid would be a propeller of some type. Yet, other than making short hops off the ground, none of these early machines were successful in demonstrating sustained, fully controlled vertical and hovering flight.

Many problems plagued the early attempts at powered vertical flight with rotating wings. This included the relatively poor understanding of rotating-wing aeromechanics[1] to allow for efficient rotors, the lack of suitable engines, counteracting torque reaction from the shaft driven rotor(s), and providing the machine with enough stability and control. Many of the early machines were of the coaxial or side-by-side (lateral) rotor configuration – see Fig. 1.3. Contrarotating rotors – one rotor above the other on a concentric shaft – automatically balance torque reaction on the airframe, despite the greater mechanical complexity involved in gearing and controlling the two rotors. Side-by-side rotors, especially if the shafts were inclined inwards,, gave the early machines somewhat better lateral stability, but again there was a greater level of mechanical complexity associated with this type of design. The intermeshing rotor design has outward tilted contrarotating shafts. The simplest idea of using a single rotor with a sideward thrusting tail rotor to compensate for torque reaction was not used until much later in the initial development of the helicopter.