Bill writes,
> By the way, you techies out there, does aerodynamic design
> actually save on gas and propulsive energy to the point
> that it matters in a car not driven faster than 87mph
> (139.2kph)?
A couple of interesting points before a discussion:
(1) At idle, 100% of the engine's <available> power is being
used just to overcome the internal friction of the engine.
I have always thought that that was cool.
(2) At maximum speed, 100% of the engine's <available> power
is being used to overcome internal friction, rolling
resistance and to overcome aerodynamic drag.
Aerodynamics is a VERY complex subject involving so many
variables that it is often studied as an empirical science,
hence the continued need for wind tunnels. Yet, good math
exists for realistic calculations.
Coefficient of drag varies with the frontal area of the body
but 3-D design shape enters into actual drag. A tear drop is
about the most aerodynamic shape and a flat plate perpendicular
to the direction of fluid flow, is about the worst. Some
math:
Cd = D / (0.5 * R * V^2 * A)
where,
Cd is the coefficient of drag
D is drag, as measured
R is fluid (air) density or about 1.22 kg/m^3
V is velocity in m/s
A is frontal area
Rearranging, one can solve for drag. Notice that drag
increases with the SQUARE of velocity (e.g. a doubling in
velocity causes a four fold increase in drag, a tripling in
velocity causes an eight fold increase, etc.!)
Here are a few Cd values for a few cars. Sorry, the MGB
is not in my reference material but given a drawing of the
frontal area, an estimate is included below;
Circular plate: Cd = 1.17
Tear drop: Cd = 0.04
Porsche 911: Cd = 0.38
VW Beetle: Cd = 0.48
3-series BMW: Cd = 0.38
7-series BMW: Cd = 0.43
MGB: Cd ~ 0.42
March INDY race car: Cd = 1.06
The INDY car is a high Cd car! Why? Because increased
drag, caused by the wings, is traded for increased downforce.
Slower on the straights, faster in the turns.
So, to answer the question, finally; AurAerodynamicape
matters at slow (sub 80mph) speed ONLY if the aerodynamic
design is VERY poor or vehicle is very big (read: moving
van or perhaps a 4x8 sheet of plywood handled up on a roof!).
Review the Cd numbers above. They are not very different
for all practical designs. HOWEVER, fuel economy is a hot
topic because it is right in our faces all the time. As I
posted earlier, I know people who have spent $20,000+ to
achieve 8mpg improvement in mileage! In reality, they
just wanted a new car but couldn't live with the honesty of
admitting that they "just wanted it."
Best
Rick Geek Lindsay
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