If you are curious about the method, it is best
to follow the reference links in the article:
http://exploration.grc.nasa.gov/education/rocket/rktcp.html
The cardboard cutout Cp estimation method has
been used routinely for years simply because it works very well.
Ideally, one would put the full sized vehicle in
a wind tunnel and perform a rigorous matrix of
tests at varying speeds and varying angles. This
would take 100s of hours of wind tunnel time
costing countless dollars. This is far beyond the
resources of the typical Bonneville racer. No one
would spend the money to do this,
There is only one assumption that the cardboard
cutout method makes; The Cd of the vehicle,
perpendicular to its normal direction of travel,
is approximately uniform along its length.
Basically, we are assuming that the weight of any
slice of the cardboard cutout proportionately
represents the drag of that slice of the vehicle.
If the perpendicular Cd is approximately uniform, then this assumption is true.
Since a vehicle is typically bilaterally
symmetric, and not purposefully streamlined
perpendicular to the wind, the perpendicular Cd
is in a range of 0.5 to 1.0 and does not vary
much. In every vehicle I can think of, the rear
of the vehicle would be more angular and less
rounded (and have a concomitant greater Cd) than
the front. This automatically puts a safety
margin in any Cp you estimate using the cardboard cutout method.
The cardboard cutout method is nothing terribly
new. It is just a method more typically used for
rockets and model airplane and model submarines,
rather than for race cars. Air works the same, no
matter what the application. :-)
Bill D.
At 08:07 AM 4/8/2015, you wrote:
Really? Why does this balancing of a 2 D
projection work? The force per unit area, or
pressure, on a vehicle due to air drag depends
on the projected area of the vehicle with a
normal anti-parallel to the air velocity vector.
It seems for a vehicle with fairly constant
height as a function of length if the front of
the vehicle is much wider than the rear, it will
have much greater pressure than the rear. I
donât see how finding the balance point of a 2
D projection from the vehicle side, or finding
the centroid of the area of this projection
captures this since it contains no information
on width, only length and height. It seems you
have to assume the width of the vehicle as a
function of length is constant. -- View this
message in context:
http://electric-vehicle-discussion-list.413529.n4.nabble.com/Science-Envy-magazine-tp4674833p4674877.html
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