The act of hovering in mid-air involves a series of complex wing movements. Bees hover in a slightly upright position, and their wing movement involves a forward and rearward wing stroke. This motion is similar to how humans tread water by moving their arms forward and backwards to help maintain buoyancy. The bee’s full stroke consists of three areas: mid-stroke, rearward-stroke, and forward-stroke, each contributing specific lift characteristics.
At mid-stroke, the bee’s wing resembles an airplane wing reaching with steady-state aerodynamic forces develop at the front edge of the wing. An airplane wing must maintain less than 9-degree angle of attack (a wing’s orientation relative to the direction of motion) to prevent stall (turbulent air interrupting lift). The bee wing angle of attack, however, averages between 41.1 – 50.5 degrees. To prevent stall, a vortex (a swirl of air) moving along the bee’s wing from the abdomen delays the onset of turbulent air at higher angles of attack.
At the end of both the rearward and forward strokes, the bee’s wing rotates along its lengthwise axis to orient the leading edge into the new direction of wing travel. This rotation resembles how a human with hands stretched out to their side, rotates their hands from palms up to palms down. This rotation at stroke reversal creates three different lift forces to enhance the overall lift: rotational, acceleration, and wake capture. Additionally, at the rearward stroke when both wings can almost touch, a jet of air shoots out to enhance thrust and creates a low-pressure zone that jump starts lift forces during the forward stroke.
During low-load hovering, the bee’s wing operates within a forward-to-rearward stroke range of 90 degrees at around 230 Hz. This stroke range creates an imaginary plane called the stroke plane. To move forward and in reverse, the stroke plane is tilted forward or rearward similarly to how helicopters fly. The bee’s indirect flight muscles are tuned for this higher 230 Hz frequency, which is atypical of other insects. To increase the bee’s speed or its ability to carry a heavy pollen load, the wings maintain a 230 Hz stroke frequency by increasing the forward-to-rearward stroke range beyond 90 degrees. At a maximum stroke, the bee can fly at 3/4 mph.Edit Summary
“Honeybees hover using a shallow stroke amplitude and high wingbeat frequency that produces multiple force peaks during each wingbeat (Fig. 1). The presence of high-magnitude force transients at the onset and termination of each stroke suggests that rotational, acceleration-reaction, and wing-wake interaction forces are more important for bees…” (Altschuler 2005: 18216)
Short-amplitude high-frequency wing strokes determine aerodynamics of honeybee flightPNAS
“even at high angles of attack, a prominent leading edge vortex remains stably attached on the insect wing and does not shed into an unsteady wake, as would be expected from non-flapping 2-D wings. Its presence greatly enhances the forces generated by the wing, thus enabling insects to hover or maneuver. In addition, flight forces are further enhanced by other mechanisms acting during changes in angle of attack, especially at stroke reversal, the mutual interaction of the two wings at dorsal stroke reversal or wing–wake interactions following stroke reversal.” (Sane 2003: 4191)
The Aerodynamics of Insect FlightThe Journal of Experimental Biology
“To meet the aerodynamic demands of ascending flight in air and hovering in heliox, honeybees modulate wing tip velocity by increasing stroke amplitude while maintaining wingbeat frequency.” (Vance 2014: 875)
Hovering Flight in the Honeybee Apis mellifera: Kinematic Mechanisms for Varying Aerodynamic Forces.Physiological and Biochemical Zoology
“…normal hovering. The motion consists of relatively constant translational velocity in the mid-stroke and rapid accelerations and wing pitch near wing reversal. Thus, during the mid-stroke, the force is dominated by the translational force in dynamic stall regime, whereas near the wing reversal, the force is affected by wing rotation and acceleration. By varying the timing of the rotation with respect to translation, they identified the sources of the force peaks near the wing reversal. One peak depends on the phase between rotation and translation, which is called the rotational force.” (Wang 2005: 201)