Analysis compiled by NASA’s Ames Investigation Centre experts, who specialise in the field of aerodynamics, revealed that the Jabulani becomes unpredictable at speeds in excess 44mph.
Further criticisms voiced by a host of World Cup players, before and during the tournament, of the ball's inconsistent flight patterns have been attributed to it's ultra light weight of just 440 grams which makes it vulnerable to a force known as the ‘knuckle effect’.
NASA also believe that the effects of altitude could have increased the unpredictability of the ball with most stadiums in South Africa at least 1,000 metres above sea level.
Weird spelling/grammar sic in the original.
The knuckle effect, as baseball fans can probably tell you, occurs when a lack of spin causes irregular, oscillating drag forces to push and pull the ball in different, unpredictable directions in flight, based on surface features like seams, scuff marks, etc. A rapidly spinning ball, by contrast, basically evens out the drag coefficient of any difference in surface features, creating a nice smooth laminar boundary layer. Here's how one aerodynamics engineer explains it:
When the boundary layer of a sports ball undergoes the transition from laminar to turbulent flow, a drag crisis occurs whereby the drag coefficient (Cd) rapidly decreases. Especially, we can observe the strange swaying on non-spinning type soccer balls by lateral force fluctuations, which can cause the ‘knuckle effect’.
Now every object will knuckle at a certain speed range, given its weight, shape, surface texture, and overall balance. The Ames researchers estimate that your classic soccer ball experiences knuckling at much lower speeds than the frisky Jabulani. Heavier, with a rougher surface and more seams mean that at striking speeds...penalty kicks, face-crunching field goals, etc....the old checkered hexagonal ball most pro players are used to has basically punched through its knuckle effect. It's heavier, so inertia and drag coefficients have stabilized at 45-60 mph and above.
But that speed range – the striker's speed range – is precisely where the Jabulani begins to do its little dance. Its smoother surface means a non-spinning ball attains a higher velocity before drag coefficients begin to 'flutter'. At the same time, the lower weight means that even at the higher speeds it has a lower inertial mass to overcome flutter effects.
And voila! Pound the Jabulani hard enough in the relatively thin air of South Africa, and if you don't get enough spin, you got yourself a knuckleball.
Well. Not all players hate the ball. Arguably a ball that gets hinky at high speeds could be an advantage for strikers, leading to higher-scoring matches. (Doesn't explain the goal drought very well, though it might explain some of the more bizarre near-misses.) As Adelaide University's Derek Leinweber remarks, even while criticizing the Jab,
"If you want the goalkeeper to design the ball, they will give you an
iron ball that sits at the centre of the field."