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In early May of 2006, I was fortunate enough
to spend a day in the
San Diego Air and
Space Technology Center’s Low Speed Wind Tunnel
(www.lswt.com)
working with weekend warriors on their
positioning and also investigating the
aerodynamics of the Zipp 808 and the Specialized
Tri-Spoke (currently known as the HED3).
The wind tunnel time for the wheel testing and
the samples were all purchased by me or my
immediate family in order to eliminate any
perceived financial obligations or connections.
Wind Tunnel Test Facility
In a simple, discrete building just off the
runway of San Diego’s Lindbergh International
Airport lays the San Diego Low Speed Wind
Tunnel. Judging from the exterior, one
wouldn’t realize that this facility has stood
there since the mid 1940’s and has been a part
of the development of military (F-16, F111, B58,
Global Hawk UAV, Tomahawk cruise missiles) and
civilian (Boeing 7XX series, Cessna, etc.)
aircraft ever since.
More recently, the wind tunnel folks in San
Diego have begun transferring some of their
extensive wind tunnel knowledge gained over the
last 60 years into measuring the aerodynamics of
sports - cycling in particular. In 2003
they designed and manufactured a dedicated wind
tunnel balance (the aerodynamic force
measurement system) and elevated ground plane
from the ground up. The highly accurate
and repeatable balance is one of the features
that separate San Diego’s facility from the rest
of the cycling wind tunnels in America (Texas
A&M, UWAL, MIT, etc…).
The balance is the heart of a wind tunnel.
It is arguably the most important thing in
making an expensive wind tunnel entry a success
– especially when the force one is trying to
measure is extremely small. Most other
facilities use the same balance that was
designed to measure loads on 1 meter wing
sections with the wind blowing at 160 kph – in
other words, loads on the order of hundreds of
pounds. The use of this type of tunnel
balance is not necessarily a problem for bike
related testing since adequate results can be
had at many facilities – someone had to raise
the bar, though. Trying to do experimental
work at some of the existing cycling facilities
has been described by people as similar to
“Trying to weigh a dollar bill with a
truck-scale”.
In an attempt to see if the balance was as good
as claimed, the folks in San Diego were
challenged to weigh fifty cents with their
tunnel balance during an entry on April 4th,
2004. A lab quality scale measured the
average weight of two quarters to be 0.0248 lbs.
The wind tunnel balance weighed the quarters to
be 0.0265 lbs – a difference of 0.0017 pounds,
or about 0.75 grams. Don’t believe I tried
it? Here’s the proof:
Figure 1. Weighing a dollar bill with a
truck scale…?
www.lswt.com
pony’s up to the fifty-cent challenge.
How’d they get a balance this sensitive?
The force measurements group of Allied Aerospace
(former owners of the wind tunnel facility) has
a dedicated department that specializes in
designing, fabricating, gaging, and calibrating
precision force measurement systems for both
their internal use and for outside customers.
The Allied crew has been doing this kind of
stuff for years and has therefore become
extremely competent – the end result of this
sports specific force measurement project was an
external wind tunnel balance calibrated to an
accuracy of 0.02 lbs (less than 10 grams).
The walls of the San Diego Low Speed Wind Tunnel
are solid concrete, so not only are they
extremely stable (insignificant
dilation/vibration during tunnel operation)
which creates an extremely low turbulence flow,
but the tunnel is nearly sound-proof. One
can't hear the tunnel running when the power is
on and the huge 20 foot diameter blades are
spinning on the other side of the tunnel.
Figure 2. Lots of concrete and the 20-foot
diameter wooden fan blades.
Figure 3. The prop tips spin less than a
quarter of an inch from the tunnel walls.
Another feature of the facility is the
elevated ground plane, or splitter plate.
This raised platform helps put the rider/test
sample in the lowest turbulence and most uniform
air flow of the tunnel – right in the middle of
the section.
Figure 4. The elevated ground
plane/splitter plate in the wind tunnel test
section.
The control room at the San Diego Low Speed Wind
Tunnel is top-notch as well. Using a
custom developed LabView based data acquisition
system, all the relevant tunnel and data
monitoring parameters are displayed in
real-time.
Figure 5. Real-time data display (right)
and tunnel control panel (left).
While these tunnel features may seem like inane
details to some, it is these details that gives
one the best chance of reliably and accurately
documenting the aerodynamic differences between
the two aero wheels in question.
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