For starters you need to look at Cedric Lynch's streamliner
http://www.bikeweb.com/node/1412
Cedric could achieve 80 miles at a constant 40mph on four Optima Yellowtops.
It's pulling about 1.5Kw from the battery at 60mph.
Secondly forget Cd.
To measure the Cd (coefficient of drag) of an object, you place it in an
airstream of known speed and measure the FORCE exerted on the object. The Cd
is then the force divided by half of the reference area (more later) times
the air density times the velocity squared.
Of course what you actually wanted to know for your design calculations was
the original force value. Instead you have to try and recreate it from
dubious data.
Cd was only ever intended to campare the relative aerodynamic efficiency of
simple shapes. It has however become a marketing tool and stated figures are
very often distorted. All you have to do is exagerate the frontal area and
the Cd goes down.
The reference area is usually the frontal area (but was it gross width by
gross height or an actual outline figure?), but not always. If you're
dealing with slim airfoil shapes then the frontal area is very small and in
any case you're trying to keep the airflow attatched over the whole surface,
so plan or total surface area is more likely to be used. This has applied to
some of the solar racing cars where very low Cd figures have been quoted.
There is a great deal of total bollocks talked about aerodynamics, much of
it in centres of learning. They still spout the obvious nonsense about
airofoils generating lift because the air has further to flow over the top
surface and is therefore travelling faster and according to Bernouli's
principal at lower pressure.
This ignores:
Paper planes with no airfoil shape.
Chuck Gliders, ditto.
Planes flying upside down.
The number of planes with symetrical airfoil profiles.
Quoted aircraft wing loadings that would require a DIFFERENCE in speed
between top and bottom surface exceeding the total speed.
I threw a paper plane at my Physics teacher when he tried this crap out on
my class!
Airfoils generate lift primarily by changing the direction of airflow. The
trick is to do this with minimal drag and that's where the shape comes in.
Borrow a copy of 'The Leading Edge (aerodymanic design of ultra-streamlined
land vehicles)' by Goro Tamai; published by Robert Bently Inc. The maths is
tough going, but the discussions, descriptions, and pictures are invaluable.
Paul Compton
www.evguru.co.uk
www.sciroccoev.co.uk
www.batteryvehiclesociety.org.uk
www.morini-mania.co.uk
www.compton.vispa.com/the_named
