Know your gear

How do balls fly?

February 2008

Anyone who believes that the sports industry is not rocket science has not looked closely at the sports ball industry. It is no coincidence that one of the world experts on sports ball flight is also a rocket scientist working for NASA: the things that affect the flight of a ball are, after all, quite similar to the things that affect the flight of a spacecraft.

To put it most simply, these are the things that make the difference between the flight of a table tennis and a golf ball, even though both are (mostly) white balls of roughly the same size. The same principles will apply to inflatable balls.

First a bit of history

At first, manufacturers believed that the smoother the surface of a ball, the further it would travel. Their reasoning made sense: the smoother the ball, the less wind resistance (or drag on the ball). But had it been true, then surely the smooth table tennis ball would travel further than a golf ball?

The first golf balls consisted of leather pouches filled with wet goose feathers that were stitched on the inside, dried, oiled to make the surface smoother and painted a shiny white.

Had there been no technological advances, selling golf balls would have been an exceptionally lucrative business as these original golf balls (called featheries) were simply thrown away when they got wet.

Then, in 1845, the gutta-percha ball appeared on the scene. Made of heated tree gum moulded into a very smooth round shape, water did not affect them at all.

The surface often became chipped during play and a lecturer at St Andrews University in Scotland realised that these chipped balls travelled further than the new smooth ones. He worked out that this happens because the uneven surface helped to convert the spin into lift, allowing the ball to travel farther.

Soon gutta percha balls were being sold with all kinds of dimples, pimples and furrows on the surface. By the 1930s golf balls made by winding a rubber thread around a rubber core, and covered with dimpled enamel paint, was standard.

Today golf ball manufacturers know that different dimple patterns result in different ball trajectories and therefore make balls with dimple patterns to suit the needs of different game styles and players.

Dimples and pimples

What does it actually mean when manufacturers say a ball has:

» An outer dimpled casing that redu- ces drag and improve aerodynamics;
» Aerodynamic pimples or
» Deep pebble outer

Strikers and goal kickers will know that balls travel further at high altitude because of the reduced resistance from the thinner air. Conversely, a ball travels less in thicker air.

But, it is not only the wind and air pressure in the atmosphere that has an influence — if that had been the case, all balls would have travelled equally far under similar conditions.

As you will know from sticking a hand out of a moving car, a moving object creates its own air flow. While it feels as if the air is moving from the front to the back past the car, the wind resistance (drag) is actually created by the car travelling in the opposite direction, while the air outside remains constant.

If it had been possible to see air flow with the naked eye, one would have seen the air separate and form a thin layer of airflow, a boundary layer, very close to the surface of the car and your hand. As the air flows around your hand and the car, it produces a feeling of drag.

Scientists working with wind tunnel experiments actually illustrate this with a mathematical formula that calculates the impact of air drag on the surface of an object. They call this the Reynold’s number, which indicates the relationship of a solid object to resistance forces.

The more drag on a moving object, the more its speed will drop and the less far it will travel. This is indicated with a higher Reynold’s number. Lower drag has a lower number.

Most of the drag on a ball comes from the separation of the flow as the ball sails through the air. When the air flows smoothly past a ball, the flow separates very early, causing a larger drag in its wake (high Reynold’s number).

When there is a turbulent flow past a ball, caused by an uneven or dimpled surface, the air flow clings to the ball, forming a boundary layer that separates later. The pressure drag is thus lower (low Reynold’s number).

Tripping this boundary layer into action makes the flight of the ball more consistent and predictable.

The faster a smooth ball moves, the more pressure drag is produced and the faster it will slow down. But when the outer surface is rough, speed does not increase the drag by much.

Golf balls, which are heavily dimpled, have quite a high surface roughness and the wind drag therefore drops at a relatively low Reynold’s number. A football, however, is smoother than a golf ball and the separation is reached at a much higher Reynold’s number. A volleyball surface is even smoother than a football.

A golf ball therefore travels further than a football and a football further than a volleyball because the outer casings in each case offer less surface turbulence.

A dimpled surface also increases lift in the ball as they help keep the air flow attached to the ball for longer (see illustration). Dimples also cause the flow to be focused into the flow of the wake.

Although round dimples became the standard on golf balls, many other shapes were tried.

Hexagons (six sided) dimples, for example, resulted in lower drag than round dimples, because they cover more of the ball surface than round dimples and therefore create a more turbulent surface. Tests of golf balls with hexagonal dimples showed that they not only travelled further, but were also less affected by cross-winds.

Some golf ball manufacturers have even started including dimples with sharp corners rather than circular dimples, since research indicates that these shapes reduce drag even more. This same principle of sharp, pointed pimples was used by Gilbert in their Xact match ball.

What about the panels?

In inflatable balls the seams of the panels will affect the turbulent flow of air around the ball in a similar way as the dimples on the outer casing. As the ball spins, the seams trigger the boundary layer of air to attach itself longer during flight, which keeps the flight of the ball more predictable.

As can seen above, experiments have shown that hexagonal dimples cause even more turbulence than round dimples. Therefore the shape of the panels on a standard football — 12 pentagons and 20 hexagons — would contribute to the predictable flight of the ball.

Every major international ball sport competition has its own specially designed ball and every new ball design results in complaints from at least one goalkeeper or goal kicker. Is this justified or just an excuse from losing goalkeepers or incompetent kickers?

While complaints from the Bafana Bafana goalkeeper during the recent African Cup of Nations might be met with scepticism, the many concerns about the flight of the adidas Teamgeist ball used in the 2006 FIFA World Cup were taken seriously enough for more than one aerodynamic expert to comment.

Goalkeepers complained that when kicked without spin, the ball swerved much more unpredictably than any other ball they had encountered. They compared this zig-zag pattern to a knuckleball in baseball.

Baseball batters hate these knuckleballs that fly at them from every which way and are impossible to predict. It happens when pitchers throw the ball with very little spin and as the ball rotates lazily in the air, the seam disrupts the air flow around the ball at certain points on the surface, causing an unpredictable deflection.

World Cup 2006 goalkeepers blamed the unpredictable flight of the Teamgeist ball on the fact that it was made of 14 curved panels, instead of the usual 32 hexagons and pentagons. The Teamgeist panels were also bonded together, instead of stitched on the inside, as most match balls. These two factors would have made the outer casing of the ball smoother than a normal football ball.

Could this smoother surface account cause such an unpredictable flight?

Dr Rabi Mehta, an aerodynamics expert working for NASA who has a special interest in the aerodynamics of sports balls, explained it as follows in Sports Trader of August 2006:

"A football will tend to produce this unpredictable flight or knuckle (a baseball term for the same effect) when it is kicked without much spin at a critical speed. In simple terms, at this critical speed the air flow very close to the ball surface around the apex region can be affected by the seams between the panels. This can produce a lateral force which makes the ball knuckle.

"The critical speed for a traditional football is about 20 mph. With the smoother surface, the Teamgeist ball will have a much higher critical speed, perhaps closer to 40-50 mph."

Because free kicks are likely to be executed at about 40-50 mph the probability of the ball knuckling has increased, and the knuckling is exacerbated at the higher critical speed.

Another sport scientist who studies the aerodynamics of balls at the University of Bath, Dr. Ken Bray, agreed that the construction of the Teamgeist could result in uneven flight.

"Due to the fewer seams on the Teamgeist, they may move in and out of the right location to trip the boundary layer, so the ball will veer between stable and unstable movement in the air," he says.

Even before the 2006 World Cup started Bray said:. "Watch the slow motion replays to spot the rare occasions where the ball produces little or no rotations and where goalkeepers will frantically attempt to keep up with the ball’s chaotic flight path."

It will be interesting to see if the adidas Europass ball designed for the UEFA Euro 2008 is met with the same criticism:

» It again has 14 panels
» It is again thermal bonded, not stitched

They have added a revolutionary PSC-Texture surface structure to give it perfect handling characteristics that will allow perfect ball control.

Good news for the goalies: this outer skin is supposed to provide a better grip between glove and ball, if and when they catch it.

According to Dr. Mehta a lighter ball will also increase the knuckling effect and the swerve of a spinning ball.

It’s what’s inside!

But, its not only the outside that influences how a ball behaves. What goes inside is equally important.

The rounder the ball, the greater its balance in flight. A ball that loses shape during a match, is obviously not going to perform as well as when the match started.

The amount of air pressure in a ball will affect how far the ball will travel when struck by a boot (head, or hand). The higher the air pressure, the better a ball will rebound as more energy is transferred to a stiff ball. If a ball deforms during impact, energy will be lost to deformation.

Inflatable balls will lose air pressure over time, especially those with latex bladders, which lose air pressure faster than balls that use butyl bladders.

Natural latex rubber bladders offer the softest feel and response, but do not provide the best air retention as micro pores slowly allow air to escape. Balls with natural rubber bladders therefore need to be re-inflated more often than balls with butyl bladders.

The latex bladder will leak enough air to become necessary to inflate the ball back to recommended pressure every day or two. Some balls use carbon-latex bladders — the carbon powder helps to close the micro pores — which can increase air retention to about a week.

Some manufacturers have addressed the problem by using more than one bladder.

Balls with butyl or PU bladders can keep air for weeks and months. They are usually used in most middle to upper priced balls.

Rugby ball shape

The rounder a football, the better it performs. So, what about the shape of a rugby ball?

The oval shape and rounded ends of a rugby ball is not a very aerodynamic design and the ball will threfore have more drag than, for example, a round ball or the pointed ends of an American football.

But, this is ideal for the play requirements of a rugby game, as more drag is better suited to the short passes that are characteristic of the game. Had the ends of the rugby ball been more pointed, passes would have travelled across the length of the field like in American football.

From the above description of the effect of drag on the flight of a ball, it is clear that any irregularity on the surface of a ball will affect the air flow, and hence the flight of a ball.

These could be scuffs and abrasions on the soft leather outer of a football — that is why durable nylon wound balls are so popular on our hard fields — or the placement of the valve, especially in a rugby ball, will influence flight.

While a round football does not have different sides, an oval rugby ball will have a top and bottom, which would need to be balanced if a truer flight is required.

Ball manufacturers have therefore overcome the problem of valve placement affecting flight by placing the valve in a seam, or placing valves on opposite sides of a ball to improve balance.

In rugby it is also very important that the player should be able to catch and hold on to a ball even in foul weather — but also release it quickly. The outer casing of a rugby ball is therefore a crucial component of the ball manufacturing process. It must be sticky enough not to slip through the fingers, but not tacky so that the player can not release it fast enough.