Producing Thrust

In its simplest terms, the thrust on the helicopter rotor is generated by the aerody­namic lift forces created on the spinning blades. To turn the rotor, power from an engine must be transmitted to the rotor shaft. It is the relatively low amount of power required to lift the machine compared to other vertical take off and landing (VTOL) aircraft that makes the helicopter unique. Efficient hovering flight with low power requirements comes about by accelerating a large mass of air at a relatively low velocity; hence we have the large diameter rotors that are one obvious characteristic of helicopters. In addition, the helicopter must be able to fly forward, climb, cruise at speed, and then descend and come back into a hover for landing. This demanding flight capability comes at a price, including mechanical and aerodynamic complexity and higher power requirements than for a fixed-wing aircraft of the same gross weight. All of these factors influence the design, acquisition, and operational costs of the helicopter. It is clear that much has been accomplished over the last sixty years in improving the capabilities and efficiency of the helicopter, but one must wonder what would really be possible with future helicopters if these complex aerodynamics could be fully understood and controlled to harness its maximum efficiency!

Besides generating all of the vertical lift, the rotor is also the primary source of control and propulsion for the helicopter, whereas these functions are separated on a fixed-wing aircraft. For forward flight, the rotor-disk plane must be tilted so that the rotor-thrust vector is inclined forward to provide a propulsive component to overcome both rotor and airframe drag. The orientation of the rotor disk to the airflow also provides the forces and moments to control the attitude and position of the helicopter. The pilot controls the magnitude and direction of the rotor thrust vector by changing the blade pitch angles (using collective and cyclic pitch inputs through the controls), which changes the blade lift and the distribution of lift forces over the rotor disk. By incorporating articulation into the rotor design through the use of mechanical flapping and lead-lag hinges that are situated near the root of each blade, the rotor disk can be tilted in any direction in response to these blade pitch inputs. However, the mechanical complexity of the rotor hub required to allow for articulation and pitch control leads to high design and maintenance costs. As the helicopter begins to move into forward flight, the blades on the side of the rotor disk that advance into the relative wind will experience a higher dynamic pressure and lift than the blades on the retreating side of the disk, and as a result asymmetric aerodynamic forces and moments will be

produced on the rotor. Articulation helps allow the blades to naturally flap and lag so as to help balance out these asymmetric aerodynamic effects. With the inherently asymmetric airflow environment and the flapping and pitching blades, the aerodynamics of the rotor become relatively complicated and lead to unsteady forces. These forces are transmitted from the rotor to the airframe and are a source of vibrations, resulting in not only crew and passenger discomfort, but also considerably reduced airframe component lives and higher maintenance costs. However, with a thorough knowledge of the aerodynamics and careful design, all these adverse factors can be minimized or overcome to produce a highly reliable and versatile aircraft.