[aerohead]

The December, 2022 MOTORTREND, page 65, contains a vignette on the 1972 911 Carrera RS 2.7, by Aaron Gold.
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Aerodynamicist Hermann Burst was charged with addressing the rear lift/high-speed instability of the 911, associated with the drawbacks of the sloping rear.
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Burst was a graduate of the FKFS at the University of Stuttgart, where the 'Kamm-back' was developed from Fachsenfeld's patents, with Dr. Wunibald Kamm riding shotgun on the research.
And Burst already had success in Porsche's Le Mans-winning long-tail kammtail racing cars. The 1968 907 long-tail was Cd 0.27. The 1969 908 long tail was below the 907's.
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Burst's 'solution' for the 911 was an airdam, along with a kammtail roof.
Porsche management was okay with the front, but shot down the K-form roofline. The silhouette of the 911 was not negotiable.
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Over three days, three rear spoiler designs were hashed out. The 'ducktail' is the one-out-of-three designs approved for homologation and production.
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The spoiler was incrementally increased in height until lift and drag reached a 'reversal point,' after which performance degraded again. ( Sweet-spot ).
Drag dropped from Cd 0.4239, to Cd 0.40.
Top speed improved from 149.2-mph, to 152-mph.
Separation-induced lift was killed off.
Engine cooling was improved.
Taillight back-soiling was mitigated.
As Wolf H. Hucho had commented in 1986, the spoiler performed almost as well as simply lofting the roofline up to where the 'tearing edge' of the ducktail ended.
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In 1973, the kammtailed ROVOMOBIL debuted, with Cd 0.23.
Presently, Porsche is using the 1930s K-form roofline. The lower performance variant of the Taycan is Cd 0.22, close to ROVOMOBIL.