Spiel

May 2022

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ALLAN CALDWELL Technical Editor RESTORING PORSCHE AERODYNAMICS A fter the last three years of COVID-19 health limits and related travel modifications, there are signs that our Porsche activities are starting to get back to normal this year. It may be time to get that stored Porsche back onto the road again. Production Porsches have always included above average attention to aerodynamic details for good performance and economy of operation with modest engine power. Design considerations for efficient aerodynamic performance include the basic body size and shape, protuberances, body and fender openings, and under-body sealing and shaping. After the 911 was introduced in the mid-1960s, increases in engine power, cornering ability, and car speed brought on an increased sensitivity to aerodynamics in road holding and cooling requirements. Since then, Porsche has systematically offered carefully matched and balanced aerodynamic aids where required. Aerodynamic aids, such as flow deflectors, spoilers, and airfoils have been used in production cars whenever there is a substantial payoff in stability, cooling, or noise reduction. It is important for the owner of a given model to recognize the need for maintaining the correct balance between the various aerodynamic surfaces or devices originally designed for their car. Some of the effects are subtle and can be negated or made worse by careless maintenance or modification. The purpose of this note is to review some of the aerodynamic features and considerations for Porsches as an initial guide for owners. OVERALL DRAG The most common aerodynamic parameter quoted in the automotive press and advertising is a car's drag coefficient. Drag is of interest because it is a major contributor to fuel consumption and the effect of speed on fuel consumption. As used in the ads, auto drag coefficients can be misleading because they are a non-dimensional parameter that is independent of size, whereas the actual drag force on the car is proportional to the car's frontal area. The drag coefficient, Cd, is defined as follows: Cd = d/qS, where: d = drag (lb) S = frontal area (sq. ft.) q = dynamic pressure = ½ ρ V2 (lb/ft2 ) ρ = air density = 0.002378 (lb-sec2/ft4 at sea level) V = car speed (ft/sec) To compare the drag of two cars, the Cd value must be multiplied by the frontal area for each in order to include the size effect. For convenient calculations, with all the conversion factors and constants included, drag in pounds at sea level = (Cd) x (S, sq. ft) x (V2 ÷391, with V in mph). Porsche is one of the few manufacturers who have published both the drag coefficient and the frontal area for their cars. With a few exceptions, drag coefficients for the various Porsche production models have remained primarily in the 0.30 to 0.40 range and have consistently been near the lowest of any production cars, even in the early models, as well as having low frontal areas. Major contributors to the drag include the protuberances, underside openings, and bluff areas at the rear of the car which cause flow separation and base drag. Basic skin friction is usually not a major drag item on a car. 38 Spiel – May 2022 APRIL TECH NOTES

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