# Hovering **Understanding Hovering Flight** Comprehending hovering flight is vital for safe aircraft operation. Pre-mission planning must consider aircraft weight, engine power parameters, and aerodynamic forces. Hover checks are conducted to verify performance planning and ensure mission parameters are correct. ## **Airflow in Hovering Flight** Increasing blade pitch (via collective application) raises the angle of attack (AOA), generating the additional lift required for hovering. To maintain a hover, the lift produced by the rotor system must equal the total weight of the helicopter. In a no-wind condition, the tip-path plane remains horizontal. Lift and weight forces are balanced during stationary hover, requiring adjustment—via collective application—to ascend or descend vertically.
At a hover, the rotor-tip vortex (air swirl at the tip of the rotor blades) reduces effectiveness of the outer blade portions. Vortices of the preceding blade affect the lift of any other blade in the rotor system. When maintaining a stationery hover, this continuous creation of vortices—combined with the ingestion of existing vortices—is the primary cause of high power requirements for hovering. Rotor-tip vortices are part of the induced flow and increase induced drag. During hover, rotor blades move large amounts of air through the rotor system in a downward direction. This movement of air also introduces another element—induced flow—into relative wind, which alters the AOA of the airfoil. If there is no induced flow, relative wind is opposite and parallel to the flight path of the airfoil. With a downward airflow altering the relative wind, the AOA is decreased so less aerodynamic force is produced. This change requires the aviator to increase collective pitch to produce enough aerodynamic force to hover. ## Ground Effect The difference in rotor efficiency while operating in-ground effect versus out-of-ground effect is substantial. Knowing what affects the efficiency of the rotor system is foundational knowledge for a number of mission sets. Some mission parameters require the aerodynamic aid of ground effect for successful flight. ### **Induced Flow** Proximity to the ground alters induced flow velocity, reducing induced drag and increasing rotor system efficiency. This alteration leads to a more vertical lift vector and a higher angle of attack (AOA), resulting in reduced induced drag. ### **Vortex Generation** Operating close to a surface for ground effect diminishes vortex generation, as the downward and outward flow of air restricts vortex formation. This reduction in vortex size enhances the efficiency of the outboard portion of each blade and reduces overall system turbulence. ### **Categories** Ground effect is categorized as in-ground effect (IGE) and out of ground effect (OGE), both crucial elements in rotary-wing performance planning. In-ground effect hovering increases rotor efficiency up to a height of approximately one rotor diameter from the ground, as depicted in Figure 1-50, where induced flow is reduced, necessitating a reduced blade pitch angle and decreasing the power required to hover IGE. **In-Ground Effect** Ground effect enhances rotor efficiency up to a height of approximately one rotor diameter, measured from the ground to the rotor disk, for most helicopters. In the depicted IGE hover in Figure 1-50, induced flow is reduced. This reduction increases the angle of attack (AOA), necessitating a decreased blade pitch angle. Consequently, the power required to hover in-ground effect (IGE) is reduced.
### **Out-of-Ground Effect** The advantages gained from positioning the helicopter close to the ground are diminished above the in-ground effect (IGE) altitude. Beyond this altitude, the power required to hover remains relatively constant under similar conditions (e.g., wind). Illustrated in Figure 1-51, out-of-ground effect (OGE) hover exhibits increased induced flow velocity, leading to a reduction in angle of attack (AOA). To maintain the same AOA as in IGE hover, a higher blade pitch angle is necessary. However, this elevated pitch angle also generates more drag, resulting in a greater power requirement for OGE hover compared to IGE hover.
*** ## **Translating Tendency** During hovering flight, single-rotor helicopters with counterclockwise rotation have a natural inclination to drift laterally to the right. This phenomenon, known as translating tendency (illustrated in Figure 1-52), arises from the right lateral thrust generated by the tail rotor to counteract main rotor torque, which rotates the main rotor counterclockwise. To counteract this rightward drift, the pilot must offset by tilting the main rotor disk slightly to the left. This lateral tilt creates a leftward force from the main rotor to balance the rightward thrust from the tail rotor. Helicopter designs often incorporate features to assist pilots in compensating for translating tendency: * **Flight control rigging**: Designed to tilt the rotor disk slightly left when the cyclic control is centered. * **Transmission mounting**: Arranged so that the mast is tilted slightly left when the helicopter fuselage is laterally level. * **Collective pitch control system**: Designed to tilt the rotor disk slightly left as collective pitch is increased. * **Programmed mechanical inputs/automatic flight-control systems/stabilization augmentation systems**: These systems may provide automated compensation for translating tendency. * ```
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