Many times you will hear helicopter pilots refer to hovering in ground effect as resting on a cushion of air. Technically speaking, what they are referring to is the extra performance that hovering in-ground-effect (HIGE) provides versus hovering out-of-ground-effect (HOGE).
Ground effect is defined as a condition of improved performance that results from operating near a firm surface. A helicopter is normally considered to be in ground effect when it is hovering less than one-half of its rotor diameter from the ground. However, the amount of benefit varies as a function of height. A lower hover will generate more efficiency and as the helicopter climbs the advantage decreases reaching zero about one and one-quarter times the rotor diameter.
A helicopter requires less power to hover in ground effect for two reasons. The main reason is the reduced velocity of the induced airflow caused by the ground. (Induced flow is air flowing down through the rotor system and is also called downwash.) This reduced velocity results in less induced drag and a more vertical lift vector. As such, the lift needed to sustain a hover can now be generated with a lower angle of attack in rotor blades, which requires less power.
The second reason has to do with vortices generated at the rotor tips. The close proximity of the ground forces more air outward and restricts vortex generation. This reduces drag and increases the efficiency of the outer portion of the rotors.
The maximum benefit is achieved from hovering over a hard surface such as concrete. When a helicopter hovers over an area such as tall grass or water, energy is absorbed by displacing the surface, allowing the induced flow to increase, thus reducing the lift vector. This will require the pilot to add power to maintain that hover height.
When a helicopter is in a high hover, or out-of-ground-effect, it requires a lot more power because there is no obstruction to slow the induced flow or force it outward. This results in a more vertical downwash and also allows the formation of stronger rotor tip vortices, reducing efficiency.
Helicopter pilots need to consider this when making very steep approaches as it has caused accidents. Typically, what happens is while a pilot is attempting to land, they allow their airspeed to get too slow and their approach too steep. They then realize they do not have enough power to slow the descent rate. In this case, the helicopter begins settling from a lack of available power. This is not the same as an aerodynamic condition called “settling with power,” which involves the generation of a vortex ring state (subject of future blog).
The helicopter’s Pilot Operating Handbook (POH) has both In-Ground-Effect (IGE) and Out-of-Ground-Effect (OGE) hover charts. This allows the pilot to take the density altitude and gross weight into account to predict hover performance. It is very important for pilots performing some missions such as ENG (Electronic News Gathering) or external lift operations to know if their helicopter can hover out of ground effect. Safe helicopter operations depend on good performance planning.