Credit: SPL

Dragonflies can out-manoeuvre other flying insects — a great benefit given their diet of mosquitoes and other flying pests. They are unique among four-winged flyers: their two sets of wings, known as the forewings and hindwings, can move independently of each other. When taking off from standstill, the wings beat in phase, but while hovering, they are out of phase. Despite many studies using photography, film and computer simulations, the wing interactions have yet to be fully understood. To this end, Z. Jane Wang and David Russell have used a high-speed camera to record the tethered flight of the dragonfly Libellula Pulchella (Phys. Rev. Lett. 99, 148101; 2007).

The authors painted three points on each wing so that, as well as wing velocity, they also recorded the three-dimensional motion of the hovering wings — including the time-dependent angle of attack. They then entered a two-dimensional projection of their data into their calculation of the aerodynamic force and power as a function of the phase lag between the forewings and hindwings, which are close enough to interact hydrodynamically. Unsurprisingly, the vertical force and power are maximal when the wings are in phase, as at takeoff. But the force drops quickly with increasing phase difference, whereas the power falls more slowly, reaching a minimum at 160°. The phase difference helps reduce the required power while generating the minimal force required to balance the weight. Moreover, the relative flatness of the power minimum for hovering, between 100° and 220°, means that precise control of the phase is not necessary.

For a microscopic explanation, Wang and Russell use the analogy of two moving cylinders. For cylinders moving in opposite directions, the net drag on each is reduced. But the reduced drag points in opposite directions and cancel, so the net force is nearly the same in hovering mode. However, the net power is reduced, as it is an additive effect.

Such a detailed understanding of the wing–wing interactions not only helps us appreciate the flight of the dragonfly, but it may also lead to improved aircraft designs for increased stability and energy efficiency.