When a multirotor drone hovers within one rotor diameter of the ground, it experiences "ground effect," which results in:
AIncreased power requirement due to turbulent air recirculating into the rotors.
BReduced power requirement because the compressed air cushion between the rotors and surface increases rotor efficiency.✓
CNo significant change: ground effect only applies to fixed-wing aircraft.
Why →Ground effect occurs when a rotor or wing is within approximately one wingspan/rotor diameter of the surface. Compressed air between the rotor and ground reduces induced downwash angle and increases rotor efficiency, allowing the aircraft to produce the same lift with less power, typically 5–15% power savings in ground effect.
The trap →Vortex ring state (turbulent recirculation) can occur in ground effect under specific conditions but is not the primary effect. Ground effect applies to rotorcraft just as much as fixed-wing aircraft.
Field note →Ground effect is why a drone often feels steadier and draws less current in the first few feet of a takeoff hover than it does at altitude.
SOURCE → PHAK Chapter 5, Aerodynamics of FlightCHECKED JUL 16ACS IV.A.K1MED