From: dr@teal.csn.org (Dan Ruparel)
Subject: AERODYNAMICS OF THE CRICKET BALL - D.Lillee says...
Followup-To: pdy@sun.soe.clarkson.edu(Yapa,,,)
Keywords: cricket ball, aerodynamics, cricket, ball, flight
Organization: Colorado SuperNet, Inc.
Date: Sat, 2 Jan 1993 02:45:32 GMT

From: pdy@sun.soe.clarkson.edu (Yapa,,,) on the
AERODYNAMICS OF A CRICKET BALL
I remembered reading what DENNIS LILLEE wrote about the 
aerodynamics of the cricket ball. I have taken the liberty 
of attaching the following paragraphs from his book, "The 
Art of Fast Bowling" by Dennis Lillee, published by Rupa & Co., 
Calcutta, India. (1987); Pages 68-9.


"What makes a ball swing? It is all a matter of aerodynamic. Quite 
a small difference in the air pressures on the two sides of the 
ball will create an appreciable swing in flight. With a new ball, which 
is shiny on both sides, the proudness of the seam, angled to first slip 
for the out-swinger and to the leg slip for the in-swinger, introduces 
a roughness on one side of the ball. This causes an air flow which is 
different on the two sides of the ball and produces a sideways force 
which results in the ball swinging, but the ball must be bowled so that 
the seam points in the same direction throughout the flight. As the ball 
becomes rough through use, the air flow becomes identical on both sides 
and it ceases to swing. When the ball loses its natural shine the bowler 
should polish only one side of it, and bowl with the seam in a vertical 
position. The smooth layer of air on the shiny side now breaks away from 
the ball faster that the turbulent layer formed on the rough side, and the 
ball swings in the direction of the rough side.

Tests have shown that a cricket ball will not swing when it is travelling 
above a certain critical velocity, and this may be why some fast bowlers 
have the ability to make the ball swing very late in its trajectory. In the 
initial stages of its flight the ball is travelling above the critical 
velocity of both the smooth and rough (seam) side of the ball; and as it 
slows down it reaches a speed which is below the critical velocity of the 
smooth side whilst still above that of the rough side, and the ball then 
swings in the direction of the rough side.

The new ball swings more readily in humid conditions, on a moist morning 
or by the sea, and the most likely explanation of this is that the 
moist air raises the seam a little and this small difference in roughness 
makes a great deal of difference to the air flow around the ball in flight.

The most critical part of swing bowling is the way you let the ball go 
from the hand. If this is not done with a high degree of precision, the 
ball either will not swing at all or will swing only a little too early 
in its flight to be any great danger to the batsman. It starts with 
the grip of the ball..."


=========
Sounds like a reasonable explanation from one of the worlds' best...
Any thoughts?

Wishing all of you accessing r.c.s and r.c.s.c. a Happy New
Year!!

Dan Ruparel




-- 
================================
Dan M. Ruparel                          eMail: dr@cricketusa.com
4130 East Peakview Circle                      dmruparel@mcimail.com
Littleton, Colorado 80121, USA  Tel: O:+(303)290-0053/F:+(303) 290-8469