The flight of the shuttlecock:

by ALISON COOKE and JUSTIN
http://www.newscientist.com

More than forty years since the invention of the synthetic shuttlecock, players at next week’s All England Open Badminton Championship in Birmingham will still be using goose feather designs dating from the early part of the century. Top players and coaches say manufacturers have yet to produce a synthetic shuttle that’s as good as the real thing. But now manufacturers are hitting back. They’re beginning to work out why feathered flight is so difficult to copy and claim that within 18 months a new generation of shuttlecocks with carbon fibre ‘feathers’ will transform players’ attitudes.

The differences between feathered and nylon plastic shuttles are subtle. Players talk of a lack of control when playing with synthetic models. There is less ‘touch’, less ‘feel’. Badminton is a sport of cunning. Success depends on deceiving your opponent with changes in racket angle and wrist movement in the last fraction of a second before playing the shot. Shuttles fly at speeds in excess of 67 metres per second (150 miles per hour) during a game and must be deftly controlled to make the deception work. Anders Nielsen, Britain’s second ranked player, says: ‘With less control, plastic designs reduce the variety of shots available to the player and so lead to a less tactical game.’ And he points out that ‘synthetic models travel faster through the air and so favour the attacking player who likes to smash’. The difference in speed is due to differences in air resistance between the two types of shuttle.

Feather shuttlecocks are made of a hemispherical cork or plastic nose with 16 feathers attached to it, and weigh around 5 grams. The spines of the feathers are glued into holes in the cork or plastic and fan out behind the nose to form a cone. Synthetic shuttles have the same shape but replace feathers with a moulded nylon plastic skirt. But because the skirt cannot mimic feathered flight, synthetic shuttles are banned from top-level badminton competitions.

Badminton developed from the ancient game of battledore and shuttlecock, in which several players tried keep a shuttle in the air for as long as possible. One theory suggests the modern game was first played in the 1860s by the Duke of Beaufort’s family at Badminton House in Somerset. Another says the rules were developed earlier by British Army officers in Poona, India, and subsequently brought back to Badminton House.

The invention of a cheap and durable synthetic shuttlecock in 1952 gave the game wider appeal. Badminton is now played in over 123 countries and by an estimated 4 million people in England alone. Synthetic models now account for 60 per cent of shuttlecock sales and are used mostly by amateurs. They sell for about a £1 each, half the price of top quality feather models, and manufacturers claim they last three or four times as long. If a single spine breaks on a feather model, the whole shuttle must be replaced – although a recent version allows individual feathers to be replaced.

The International Badminton Federation (IBF), formed in 1934 and based in Cheltenham, issues guidelines on shuttlecock size, weight and shape. The feathers, a by-product of geese destined for the pot, must be between 64 and 70 millimetres long. When they are attached to the cork nose, they should form part of a cone between 58 and 68 millimetres across. The maximum diameter of the cone is important because it determines the drag on the shuttle and consequently its speed through the air. Manufacturers produce several ‘speeds’ by varying the weight of the shuttle between 4.74 and 5.50 grams and by using cones of different sizes. Players match the speed of the shuttle to playing conditions. At high temperatures, the air is less dense, for example, and shuttles fly faster. The players choose a slow speed to compensate.

Shuttlecocks must also spin. Because feathers differ slightly in size, weight and shape, shuttles are never perfectly symmetrical. The spin evens out the slight asymmetries which would otherwise cause the shuttle to veer off course. The feathers overlap like the blades of a turbine so that air flowing through them always produces an anti-clockwise rotation as viewed by the attacker as the shuttle leaves his or her racket.

The properties that give nylon the durability so treasured by amateurs, are, unfortunately, responsible for its shortfalls in professional play. Almost all the differences can be traced to the natural stiffness of feathers compared with plastic skirts. This affects the impact with the racket, the flight through the air, even the rate of spin. Feathers are ideally suited to badminton. Their circular cross section provides strength and, because they are hollow, feathers are also light. The plastic spines used in artificial designs, on the other hand, are solid but less rigid.

One important characteristic of shuttlecocks is how the skirt behaves when the shuttle comes into contact with the racket. Professional players describe the impact of synthetic shuttles with the racket as ‘heavy’ or ‘dull’ compared to the ‘crisp’ feel of feather models. High-speed photographs of shuttles striking a wall show that plastic skirts distort more than feather designs. The inward collapse of the skirt is a complex process that depends on how the shuttle is hit. In most shots, the racket hits the skirt as well as the nose of the shuttle. For example when the shuttle is hit high up in the air – a shot known as a high clear – it can descend almost vertically and if the next shot is an overhead smash the racket will hit the skirt as well as the nose.

COMPLEX COLLISIONS

Roy Buckland, a badminton coach and a mathematician specialising in flight dynamics, says that the more the skirt collapses, the more complex the collision between racket and shuttle becomes, making the shot more difficult to control. The fine-tuning players achieve with feather shuttles cannot be reproduced with a synthetic shuttle. Small variations in the way the racket and plastic skirt make contact lead to large differences in the way the skirt deforms, making control harder.

Plastic skirts also deform during flight. Wind-tunnel tests show that both feather and synthetic designs experience the same force at speeds of up to 23 metres per second. Any faster and the force on synthetic designs is less. During a typical smash a shuttle travels about a third of its trajectory at speeds greater than this. Video stills show plastic skirts starting to collapse at these speeds. The smaller cross section leads to less drag and explains why players such as Nielsen notice that plastic shuttles travel faster through the air.

Drag and speed differences affect trajectory. Computer simulations based on the wind-tunnel measurements show that plastic shuttles travel slightly farther in shots such as high clears and smashes, but a similar range for slow-speed drop shots.

The computer simulation could lead to improved designs. Manufacturers test new designs by building prototypes and asking top players to evaluate their performance. Computer models could calculate performance without the need for costly prototypes. But the programs are not yet sophisticated enough to reproduce all the effects of shuttle flight. The computer simulates the trajectory in two dimensions – in terms of height and distance. In top-level badminton, however, small three-dimensional effects such as a shuttle’s drift to one side during flight can make a crucial difference to the game. The drift is a gyroscopic effect caused by the rotation of the shuttle around its direction of travel – quite different to the curling shots achieved by tennis players, for example, who add top spin or side spin to the ball.

SHUTTLES IN PRECESSION

To understand such phenomena, imagine looking down onto a badminton court from the attacker’s end and watching a shuttle hit up towards you. At the top of its trajectory, the shuttle turns over to point down again. But as it turns, it angles itself always to the right, drifts a few centimetres before straightening out and dropping to the ground.

There are two processes at work. The first is a gyroscopic effect from the rotation of the shuttle around its direction of motion and the rotation as it turns over. Together these generate a force that turns the shuttle to the right, the same force that causes a spinning top to wobble, or precess, before falling over. The second process is a simple aerodynamic effect – with the shuttle angled to the right, it drifts to the right as it moves through the air. When the turnover is complete, the force disappears and the drift stops.

Top players hit high clears that seem to head out of the left side of the court (as seen by the attacker) but which then drift in. A smash, however, does not show this drift because there is no turnover in the middle of the trajectory. Players at the highest level must allow for drift, though many claim never to have noticed it. They even say they have more success with finely judged shots down the left-hand side of the court than down the right, but are unable to explain why.

The amount of drift depends on the amount of spin. Synthetic shuttles rotate at only half the speed of feather designs. Manufacturers can increase this speed with aerodynamic foils on the plastic spines but this makes the shuttles heavier or less rigid. Problems with rigidity affect an alternative skirt design which has a set of ridges like a paper fan. Manufacturers make large holes on one side of each ridge and smaller holes on the other. The air flows more easily through the larger holes creating a turning force. But wind-tunnel tests show that the skirts collapse more easily at high speeds because the ridges fold up like a concertina.

But synthetic shuttle design could be revolutionised by carbon fibre, which is much stronger than plastic – the reason for its use in rackets for tennis, squash and badminton. New artificial feathers will use a central core of carbon fibre surrounded by plastic. Manufacturers claim that shuttles made with carbon fibre feathers will look, feel and behave like the real thing and last longer too. But on the basis of current designs, they would be twice as expensive as feather shuttles and four times the cost of plastic models. Nevertheless, the race is on to find a new design that could produce reasonably priced carbon fibre shuttlecocks, perhaps within 15 months.

While some manufacturers regard this schedule as too optimistic, players at the 1996 All England Open may be able to judge whether carbon fibre feathers designed by scientists are better for badminton than the natural variety.

Alison Cooke is a mechanical engineer at the University of Cambridge and scientific adviser to Carlton Sports.
From issue 1916 of New Scientist magazine, 12 March 1994, page 40