Insects are remarkably agile fliers capable of complex aerial manoeuvres and hovering in turbulent environments. In fact, conventional ways of generating lift, such as those used by airplanes, would not be sufficient to keep them airborne, so they rely on mechanisms such as bound vortices on the leading edge of their wings to help them fly. Despite these exotic mechanisms, for years it was thought that insects were unstable while hovering. Previous analyses of how they fly relied on averaging the forces produced by their flapping wings. Based on these methods, it was thought that flapping fliers such as hawk moths would be vulnerable to toppling head over heels because of this instability. It was assumed that averaging the forces over time was a fine approach to study insect flight, as the frequency of the flapping wings was much higher than the time scales of the body movements. Therefore, it was thought that insects must use sensory feedback – either perception of the body's position in space or vision – to stabilize their flight. However, a team of researchers based in the USA recently reinvestigated the flight of a hawk moth and found that they may be more stable than previously thought.
Haithem Taha and colleagues from the University of California Irvine, the University of North Carolina and the Pacific Northwest National Laboratory used a method called chronological calculus, which allowed them to study the time-varying effects inherent to the flapping of the wings during flight. The new technique allowed the team to identify a novel way for moths to prevent themselves from toppling by precisely synchronizing the oscillations of the body and the wings. This synchronization generates turning forces, which are used for correction when they tip unexpectedly after encountering turbulence or colliding with another object.
Next, the team tested their revised model of hawk moth flight to find out how well the real insects coped when tipped during a collision by firing a pellet at individuals as they approached an artificial flower. Filming the disturbances on high-speed 3D videos, the team then tracked the wing and body motions as the moths recovered, revealing that the predicted synchronizations occurred in real flight tests as hawk moths stabilized their flight.
Finally, the researchers used chronological calculus to compare how stability changes in seven other flying organisms, from hummingbirds to fruit flies, whose flapping frequencies varied from tens to hundreds of wing beats per second. For animals such as hawk moths and hummingbirds, which flap their wings at approximately 20 to 30 wing beats per second, the effect of the stabilization provided by the synchronized body and wing oscillations is stronger than for fliers operating at higher frequencies, such as the fruit fly. This demonstrates that sensory feedback still likely plays a role in stabilizing flight for insects that really buzz about.
Insects and other flapping fliers continue to amaze us with new revelations about how they stay aloft. It turns out that the very act of flapping helps them stabilize against pitch disturbances that are likely present in the windy skies they fly through. These findings could completely revise our understanding of how flapping fliers control and stabilize their flight. While insects still likely require active control and sensory feedback to fly, perhaps their passive stabilization mechanisms will inspire the mechanical design of robotic fliers and relax their computational requirements.
