18 The helicopter main rotor generates vertical lifting force in against the aircraft weight and horizontal propulsive force (thrust) for forward flight. It provides a means of producing forces and moments to control the altitude and position of the helicopter 14. The knowledge of aerodynamics loads effects on the rotor blade dynamic response and environment in which the rotor operates is important. Helicopter has ability to hover and to perform forward flight.
19 During hover, the helicopter main rotor blades move considerable volumes of air in a downward direction. This process accelerates the air to relatively high velocities and requires lots of horsepower. To perform hovering flight, the helicopter main rotor must generate lift (L) equal to the total weight (W) of the helicopter. with an increase of blade pitch and high rotor blades speed, the necessary lift for a hover is induced and reach a state of stable stationary hover. The rotor tip vortex affects negatively the effectiveness of the outer blade portions in hover flight. The lift(L) of following blade is severely affected by vortex of preceding blade. For hovering the helicopter requires high power. This high-power requirement is effect of continuous creation of new vortexes and ingestion of existing vortexes. Unlike out of ground effect (OGE) operation, in ground effect (IGE) operation, the downward airflow patterns and outward airflow patterns tend to restrict vortex generation. Restriction of vortex generation results in increasing of efficiency of outboard part of the rotor blade and reduces overall turbulence of system produced by ingestion and recirculation of the vortex swirls 15.
20 In hovering flight, Collective pitch angle, tip Mach number and blade wake affect overall performance requirement for hover flight.
21 Collective pitch angle changes angle of attack of all rotor blades by an equal amount. The collective pitch is operated to control the average rotor blade pitch. Change in pitch angle changes the blade lift (L) and the average rotor trust (T) and increase the drag on blades. Increase in drag requires extra power 16.
22 A high rotor tip speed provides the rotor a high level rotational kinetic energy for a given radius. The high rotor tip speed reduces the rotor design weight. However, compressibility effects and noise are two factors that oppose the use of high rotor tip speed. The compressibility effects increase rotor power requirements. With increase in Mack number the rotor noise increases rapidly. For maximum hover flight performance lower tip Mach number is required 16.
23 The wake due to the rotating blade comprises, in part, a vertical vortex sheet, with formation of concentrated vortex at the blade tip. The vortex sheet has a vorticity with vectors aligned mainly normal to and parallel to the trailing edge of the blade. Experiments have shown that blade tip vortices are almost fully rolled up within only a few degrees of blade 16.
24 For forward flight the rotor is tilted forward, and total lift and thrust forces are also tilted forward and generate resultant lift-thrust force. The generated resultant force that can be resolved into two components: lift (L) acting vertically upward and thrust (T) acting horizontally in the direction of flight. In addition to lift and thrust forces, there is weight and drag. For steady forward flight, lift, thrust, drag, and weight must be in balance. When lift force exceeds weight, the helicopter accelerates vertically until both forces are balanced; if thrust is less than drag, the helicopter slows down until the forces are in balance. 15.
25 During forward flight, airflow moves opposite to the flightpath of rotorcraft. The velocity of air-flow equals the velocity of rotorcraft in forward flight. The velocity of air flow across the blade is determined by: the point location of the rotor blade in plane of rotation at a given time, blade rotational velocity, and airspeed of the helicopter determine the velocity of the airflow across the blades. The airflow on rotating blade varies continuously with rotation of blade. The highest airflow velocity occurs over the one side of plane of rotation for advancing blade in a rotor system and decreases to rotational velocity over the nose. It continues to decrease until the lowest velocity of airflow occurs over other side for retreating blade 15.
26 When the helicopter begins to accelerate from a hover, the rotor system becomes more efficient. Transitional lift results from improved rotor efficient due to acceleration of helicopter from hover flight to forward flight. As in-coming wind produced by helicopter movement enters the rotor system, vortices and turbulent stay behind and the airflow becomes more horizontal 15

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