[EVDL] Cd is mostly a marketing figure

[Paul Compton via EV]

Drag is usually measured in a wind tunnel and is the force needed to
force a vehicle through the air at a given speed. From this drag force
you can calculate the required torque at the driving wheels and derive
the power required.

Cd is a figure derived from the actual drag. It uses air density and a
reference area. The reference are is usually frontal area for
vehicles, which might be gross width times gross height, or might be
the silhouette.

Comparing published figures for Cd, engine power and top speed, it
appears that most manufacturers are using gross frontal area to
generate a lower Cd for marketing purposes.

When you get into airfoil sections the frontal area is quite small, so
it's common to use plan area, or 'wetted' area, ie the total area
exposed to the air flow. Airfoil sections should keep their airflow
attatched over much of their surface area, so a plan or wetted areas
are relevant.

The solar racers are probably using plan area to generate such low Cd
figures, so you can't compare them to conventional cars.

If only people would actually publish the drag force measured in the
wind tunnel.

--
Paul Compton
www.morini-mania.co.uk
www.paulcompton.co.uk (YouTube channel)
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