Aerodynamics of forward fliglit

MOMENTUM AND ENERGY CONSIDERATIONS

Just as in hover and vertical flight, there are two basic methods for analyzing the characteristics of a rotor in forward flight: the momentum, or energy, method; and the blade element method. The blade element method is necessary for accurate performance estimation and for establishing the limits of rotor performance, but the momentum method provides a rapid means of obtaining a first estimate of the performance as well as a valuable insight into the physics of the system. The balance of forces that governs the helicopter in forward flight is shown in Figure 3.1. The rotor thrust vector must balance not only the gross weight, as it does in hover, but also the horizontal drag of the rotor blades and the lift and drag of the fuselage, hub, landing gear, and other necessary—but draggy—items by which helicopters achieve their role as useful vehicles. The drag of the rotor blades is known as the rotor horizontal force, or H-force, and all other drag items are classified as parasite drag.