Is Golf More Difficult
Than Flying?
Playing golf and “driving” a ball with some degree
of proficiency is difficult, and almost as time-consuming to perfect
as “driving” a plane. For one, the rules of golf do
not permit “Artificial Devices and Unusual Equipment which
might assist [the player] in making a stroke in his play,”
which essentially means that golfers don’t get the benefit
of autopilot. Second, golf requires a person to stroke an object
(the ball) with another object (the club head), which is attached
to the end of a 40-inch shaft (+/-5 inches). This action must be
performed so precisely that the ball will fly approximately 280
yards in the air, within a horizontal window of 4 degrees from the
launch pad, and stay in the fairway (short grass)…only so
you can find it and repeat the process with another club, resulting
in a completely different trajectory.
Swinging the club with some degree of consistency is enough of
a challenge. Never mind making contact with the ball at precisely
the right point on the clubface in order to achieve maximum distance.
But these challenges are just part of what makes the game so addictive.
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How the Ball Flies
The complex science of aerodynamics can help us understand precisely
how the air flows over the surface of the ball, allowing it to fly
through the air like a plane rather than take on the trajectory
of a bullet. In fact, a golf ball can travel farther than any other
round object of the same size and weight launched under the same
conditions.
A lot is known about how air flows over a wing, but less about
how it flows around the wing tips, where it is believed the trailing
vortices begin. The type of turbulence and aerodynamic forces created
in this area are complex. Some have even compared the airflow surrounding
a spinning golf ball to the dynamic reaction of two wing tips meeting.
Does this mean that a golf ball is in part more complex aerodynamically
than the Falcon 7X, Boeing 777, or even the Concorde? The only thing
that makes the golf ball a little easier to study is that it doesn’t
get into the supersonic range, although it may seem like it does
when John Daly or Tiger Woods hit the ball.
It is the dimples (surface treatment) on the golf ball that are
responsible for its flight characteristics. Their design (i.e.,
size, shape and pattern on the surface of the ball) will help dictate
the ball’s trajectory.
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Discovery of
the Dimple
Some 600 years ago, someone observed that an older, used ball with
a roughened surface performed better than a new, smooth one –
hence the origin of “dimples.” As ball construction
evolved from a stitched pouch of skin stuffed with feathers to the
molded, solid, natural rubber (gutta percha) ball introduced about
150 years ago, a roughened surface was recognized as an integral
part of the design, even though it was not understood why or how
it worked.
In 1672, Newton recognized that transverse forces existed when
spinning tennis balls flew through the air. Magnus explained these
forces in the 1740s, and their effect bears his name. Then, in 1890,
Professor Guthrie Tait of Edinburgh University was the first to
study and describe his basic understanding of the aerodynamic principles
of a golf ball. He, too, recognized that the rough surface, in combination
with backspin (underspin), created lift (Magnus force). This allowed
the ball to travel farther than a smooth ball launched at the same
speed and launch angle without spin.
 
Fig 1 and 2: A comparison of the airflow around a smooth ball compared
to a dimpled ball.
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Flight Conditions
As soon as a golf ball is launched off a driver, it starts its
trajectory straight down the runway (fairway) with lift forces greater
than the weight of the ball and drag forces that will immediately
start slowing it down. Yet, it is still able to cover a distance
of approximately 265 yards in the air and then bounce and roll on
to about 290 yards in total. A smooth ball with otherwise identical
physical properties, launched in the same manner, will only travel
about 140 yards.
The reason for this is that when the golf ball passes through the
air, the roughened surface creates a layer of turbulence at the
surface of the ball. As it spins, the surface air is dragged around
the ball, creating a profile of disturbed air similar to an airfoil.
This then works the same way as a very short (front to back), stocky
wing, creating the lift force which allows the ball to stay in flight
for longer periods of time than a smooth ball. It is unfortunate
that these same forces, which create lift, also create a hook and
slice (an undesirable deviation left or right of the target) when
the axis of spin is tilted to the left or right.
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Drag Force Barrier
Wind tunnel experiments show that the simples on a nonspinning
golf ball actually decrease the drag force, allowing the ball to
slip through the air with less resistance than a smooth ball, but
only at speeds between about 55 mph and 300 mph. Below 55 mph, both
the smooth and dimpled balls slip through the air with about the
same resistance (drag force).
It is at the critical speed of about 55 mph that the nonspinning,
dimpled golf ball passed the critical “Reynolds number”
(aerodynamic jargon for “force barrier”), reducing the
drag force significantly. The smooth ball goes through a similar
force barrier, but at a critical speed of about 300 mph, and at
this speed has less drag than the dimpled ball. It is at these critical
speeds that the drag tail (turbulence behind the ball) suddenly
decreases in size. The separation point of air causing the turbulent
tail or wake rapidly moves from a point about 80 degrees from the
air flow direction to about 110 degrees around the back of the ball.
This is sometimes referred to as “delayed separation”
(see illustration). When this happens, the drag force decreases
by almost 40% of that just prior to this critical speed.
The drag force on the golf ball will then slowly increase as the
speed increases. A golf ball well-struck off a driver is launched
at about 160 mph and lands at about 70 mph. Thus, the ball would
be in this decreased drag zone for most of its flight, taking advantage
of the phenomenon.
Unfortunately, golf balls are required to spin to achieve the Magnus
effect, allowing them to stay in the air for six to seven seconds
(average drive hang time). In addition, spinning balls do not have
these clean and well-defined force barriers because the air flows
over different parts of the ball at different speeds, which complicates
airflow patterns. However, the dimpled ball, even in its spinning
mode, has less drag resistance than a smooth, spinning ball.
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Design by Trial
and Error
Designers have taken an experimental approach and have analyzed
the cause and effect of a variety of designs in search of the “perfect
dimple,” which will result in an optimum trajectory.
The formal trial-and-error method examines variables such as dimple
shape, number, pattern and surface-area coverage, as well as simple
depth, ratio of depth-to-surface area and even the smoothness of
the dimple surface. However, most designers believe that there are
only + / - 4 yards left in optimizing the aerodynamics of the surface.
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The Ball is
Going too Far?
For the last 100 years, golf’s governing bodies have been
concerned by the continual improvement in distance that balls have
gained through technological research.
By 1976 the “Overall Distance Standard” (ODS) was adopted.
The standards were based on launching a ball using a mechanical
golfer to simulate real field conditions and setting a limit on
the distance under specified test conditions. Individual properties
of the ball, which contributed to the overall distance, were not
isolated or limited for two reasons: first, they were not understood;
nor were they able to be measured at the time.
Things have changed since, and the USGA has perfected the “Indoor
Test Range” (ITR) which supersedes a wind tunnel and force-balance
system to measure the aerodynamic properties of a ball. The ITR
is a 70-foot-long open area with a series of stations along its
length. At each station, the exact ball position and speed are measured
as it passed by. This method of firing a spinning ball through a
still body of air has proven to produce far more accurate and reliable
data than trying to support a spinning ball in a laminar stream
of air in a wind tunnel. From the information collected in the ITR,
the coefficients of lift and drag for a number of different speeds
and spin rates are calculated and combined for use in a simulation
to describe the complete trajectory of any golf ball tested.
To further match the optimum launch conditions with the golfer,
club design is changing in order to reduce the spin rate, launch
the ball higher and launch it with more speed. The spring-like effect
has been a major advance in club design. Titanium clubs with shell-like
hollow heads and thin faces, which deform and recover during impact
(impact time, while the ball and club are in contact, is about .000450
of a second), enhance ball speed by increasing the “Coefficient
of Restitution” (COR).
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The Laws of
Physics Will Govern Distance
Unfortunately for many golfers looking for an extra thirty yards,
the laws of physics will limit the distance balls can travel to
only a 10- to 15-yard improvement – even without existing
performance standards! In other words, equipment is reaching its
limit.
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