When a multirotor drone hovers within one rotor diameter of the ground, it experiences "ground effect," which results in:
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.SOURCE → PHAK Chapter 5, Aerodynamics of FlightCHECKED APR 21ACS IV.A.K1MED