Not just for the birds

FROM Leonardo da Vinci to the Wright brothers and among today’s aircraft designers, there is a fascination in studying the wings of birds, better to understand the mysteries of flight. But there are also more than 60 species of fish that have the ability to take to the air, and new research shows they could have a trick or two to help make aircraft fly more efficiently.

Flying fish can whip their tails back and forth with tremendous speed to propel themselves out of the water. Once airborne, they use long pectoral fins and shorter pelvic fins on the sides of their bodies to create lift, much like the wings of an aircraft. Sometimes the fish fly to escape fast-swimming predators like dolphins and tuna, but they may also take to the air because it is an efficient way to speed up their movement. Whatever the reason, once aloft they can glide for more than 40 seconds, cover 400 metres (1,312 feet) and move at about 70kph (43mph).

It was when he was reading about flying fish to his children that Haecheon Choi, a mechanical engineer at Seoul National University in South Korea, realised just how amazing these flying feats are. But when he looked at the scientific literature he found that the aerodynamics of their fins had not been fully explored. So, together with Hyungmin Park, a colleague at Seoul National University, Dr Choi set out to discover more.

The scientists caught a number of darkedged-wing flying fish and had them stuffed with their wings fixed in different flying positions. Then they popped the fish into a wind tunnel to calculate their lift-to-drag ratios, sometimes called the glide ratio. When the shape of a wing creates more lift from the air passing around it than it does drag (air resistance), an aircraft will fly. And the higher the ratio, the farther the aircraft will glide. This means if you cut the engine on a small Cessna with a lift-to-drag ratio of 7:1 it would fly seven metres forward for each metre of descent.

As the researchers report in the Journal of Experimental Biology, compared with other flying animals the fish score well at 4.4:1. This makes them more efficient than swallowtail butterflies (3.6), fruit flies (1.8) and bumble bees (2.5). Flying fish are just as effective at gliding as birds that are known for being strong flyers, like red-shouldered hawks (3.8) and petrels (4). Nighthawks (9) and black vultures (17) make more impressive gliders.

In trying to explain why the fish are so good at gliding, Dr Choi theorises that the positioning and shape of their large and flat pectoral fins at the front and the smaller pelvic fins at the back create a flow of air that accelerates towards the tail of the fish. This faster-moving air provides the pelvic fin with additional lift.

The experiments also revealed that the fish could glide even farther if they were just 4-5cm from the bottom of the wind tunnel. When the experiment was repeated with a tank full of water in the bottom, the fishes’ lift-to-drag ratio rose from 4.4 to 5.5. This, the researchers believe, is caused by the surface of the water smoothing out a vortex of air coming off the flying fish and reducing drag. A similar thing, called ground effect, can happen when an aircraft gets very close to the ground. Flying fish, it seems, use this effect to their advantage as they skim across the sea.

Dr Choi and Dr Park believe there are still more interesting aerodynamic tricks to be learnt from flying fish. And, as engineers, they wonder if any of these features could be incorporated into aircraft. In years to come, if you see aircraft with strange fin-type wings then you will know where that idea was fished up from.