For 300 million years dragonflies have maintained their independently controllable, four winged form – whilst other flying insects have repeatedly modified or reduced one pair of wings or mechanically coupled their fore and hind wings. Previous studies into the flight of four-winged animals have suggested that the four winged form is not as efficient as using two wings.
In order to hover a dragonfly, just like a helicopter, has to push air downwards – this is referred to as the ‘wake’ or ‘downwash’. Any air motion that isn’t downward fails to support the body and is wasteful. This study shows that dragonflies are able to use their lower wings to recover energy wasted in side to side air motion in the wake – if they flap their wings at right time.
“I’ve been repeatedly struck by how inefficient dragonflies seem to be when they fly and I wondered whether they were using any additional tricks to become more efficient,” said Jim Usherwood, Wellcome Trust funded researcher at the Royal Veterinary College and co-author on the paper. “By working with Fritz-Olaf Lehmann, who has been developing flapping robots, we were able to simulate dragonfly flight and measure the aerodynamic forces. We found that two pairs of wings can allow the dragonfly to produce higher forces, allowing acceleration and climbing, whilst the lower wings are able to reduce energy wasted if the wings flap at the right time.”
Working with robot dragonflies allowed the researchers to look at what would happen if the front and back wings flapped with different timings – which would not have been possible to simulate with real dragonflies. Whilst most of the flapping timings were less efficient than hovering with one pair of wings, the scientists discovered that there were some instances when flapping with two pairs of wings was more efficient because it required less power to lift the same weight as just one pair of wings.
Whilst this research demonstrates that two pairs of wings have some aerodynamic advantages which have been used in insects for more than 300 million years, it could also be used to aid the development of micro air vehicles based on flapping designs. Such aerodynamic mechanisms used by dragonflies could, if technically feasible, be applied to allow micro air vehicles to fly a little longer or carry a larger load.
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