Quote:
Originally Posted by ERTW
I'm quoting Antonio Morelli's 1976 SAE technical paper #760186
"The historical development seems to indicate that an optimization of the body shape should be sought for through the reduction to a minimum of the sum of the profile and the induced drag, the third term of the sum expressing th total drag, the friction drag, being practically not variable and negligible (7).
In fact, till now, either a low profile drag was obtained (but not a low induced drag) as with the Jaray body, or a low induced drag (but not a low profile drag) as with the "Kamm" body or with the adoption of spoilers and dams.
...The scope of this work is to determine a shape, suitable for motor cars, having a low profile drag and zero induced drag.
The following conditions were imposed:
A) total aerodynamic lift equal to zero;
B) gradual variation of the area of the transverse sections of the body
C) gradual shape variation of the transverse section of the body.
In order to meet other minor requirements, the following additional conditions were also imposed:
a) aerodynamic pitching moment equal to zero, for stability reasons
b) an oval contour of the transverse sections of the body (mainly of the "basic body" which will be defined further on) in order to reduce the "wetted" area and, as a consequence, the friction drag."
I think it's most important to note that Morelli got close, but didn't achieve his goals. The final shape had a Cd of 0.23.
Figure 5 shows that a "drop shape" in free air has a Cd of ~0.09, while in close proximity to the ground (9-14% of the body thickness) it rises to ~0.14.
A beneficial shape is a "banana" with ~9% camber - the mass centroid peaks at 9% of the length of the body.
His shape is NOT "reverse aerodynamics". He states that a gradual pressure gradient is beneficial on the front and back of the body. You can mess up the air flow in a short distance, and spend the next 10 ft to settle it down, and Morelli believed that this method "increased the car length without contributing much to the usable space."
I learned that you can super impose various bodies and sum the Cd of the individual shapes.
Jaray's aero philosophies are 80 years old. Even this paper is 35 years old. Stay tuned.
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*The banana car achieved Cd 0.161 without wheels in ground proximity.
*23% of the tail was chopped off with no measurable drag increase.
*When wheels were added the Cd jumped to 0.35.
*The rest of the time at Pininfarina's wind tunnel was spent anguishing over detail optimization to shave the drag finally achieving a Cd equal to Kamm's K-3 of 1939.
*And I believe it is 'Alberto' Morelli.