The marvellous mechanics of flying

The wings of birds and insects must fulfil two functions simultaneously: give them lift (to get them off the ground) and thrust — to power them forward. With fixed-wing aircraft, lift is provided by the wings and thrust by the engines, be they jets or propellers. Helicopters and drones use vanes and propellers to provide both these forces simultaneously. Working against these forces are the weight of the bird and gravity, pulling it down, and drag — the natural resistance of air against forward motion.

A bird’s wing is shaped so that it automatically creates lift when pushed through the air. Take a fallen flight pinion, say of a black kite, hold it loosely between thumb and forefinger and move your hand rapidly forward. You will find the feather literally trying to ‘lift off’ taking your arm along with it! Thrust for forward motion is supplied by mighty flight muscles — some powered by glycogen, a carbohydrate for instant blastoffs, like what pheasants and francolin perform — and others by a rich constant supply of blood to the muscles for long-distance endurance flying. Some birds, like eagles, have a combination of both kinds. It’s the up and down ‘flapping’ motion of the wings that provides the thrust, pushing the bird through the air: the down-stroke is responsible for this, with the tips of the flight pinions twisting like propellers, and little tornadoes of air called vortices creating pressure differences which assist lift.

When a bird glides, the convex shape of its wing makes the air passing smoothly over its upper surface move faster than the air passing below, creating a low-pressure area, which keeps the bird aloft. Seabirds like albatrosses have long narrow wings for this purpose — they use the wind moving off the waves to give them thrust and can glide for many a mile with minimum expenditure of energy. (That’s why gliders have similar wings.) Fast-flying birds have broader wings to enable them to push away the air over a larger surface area, which makes it easier to manoeuvre and gives them more thrust.

Feathers are one of Mother Nature’s most ingenious creations: made of keratin, they are light, strong, easy to maintain with a zip-like ‘tidy up’ mechanism, and can regenerate when past their prime — as when the bird moults.

If the wings of birds are a marvel, those of insects are truly astonishing. Thin as cling film, strutted with veins for support, they are made of, often transparent membranous chitin or cuticle and are flexible; able to twist and turn about their axis, describing a figure of eight in every wing-beat. This enables the insect to create lift and thrust with both the up and down strokes, unlike birds, where the upstroke is more of a ‘reloading’ stroke for the following downstroke. Insects like dragonflies have two pairs of wings, whereas those like flies have just two. Each wing may be powered by its own custom muscle which is either directly or indirectly attached to the insects’ thorax. A dragonfly for example can beat each wing completely independently of the others, (and even fly with just three wings), which enables it to make all those lightning-fast changes in direction as it pursues its prey. Insects too create tornadoes of air that blow off their wings to obtain lift — using what has been called unsteady aerodynamics — as against the ‘steady state’ aerodynamics used by birds. The only bird that flies like an insect is the hummingbird, which can twist its wings around after every beat, enabling it to fly forwards, backwards, sideways, up and down without a problem. (But this is very energy intensive).

What’s also mind-boggling is the computing power of the insects’ minuscule brain: how a butterfly precision lands on a bloom, how a dragonfly darts after a dodgy mosquito, and how a fly lands upside down on the ceiling. The math involved must be incredible! Just think of all the adjustments aircraft or helicopter pilots (or even auto-pilots) have to make to enable them to touch down safely aided as they are by batteries of computers and floor-to-ceiling gauges!

If we were to fulfil our dream of personal flight, we would need very strong and large pectoral muscles that would stick out four feet in front of our chests! We have imitated bats and flying squirrels with our ‘flying suits’ but just watch a crow, have fun with updrafts along a mountainside and well, we’re not about to do that anytime soon, are we?