Quote:
Originally Posted by JohnAh
Concerning the kammback I have read different figures about how to design it properly but Iīve got the impression that a slope of maximum 20 degrees on each surface is a good start. In some places I have read that the teardrop-shape should continue for at least 3/4 of the length of a full teardrop. At other places I have read that the area of the flat end should not be more than 50% of the maximum projected frontal area of the entire car.
If this is a good route to follow we can easily look at the streets to se that few car manufacturers follow this recipie. I guess this can be explained by avreage people not knowing what difference it can make and that most people still would find such a shape too extreme to feel comfortable with.
If I look at the Volkswagen L1 concept car instead I find it almost perfectly following the recipies of Kamm with a nice diffuser. Since they have obviously tried hard to get a one litre concept car (1l/100km) I guess they couldnīt afford to cheat with the body shape just to make it look cool. What we see is probably the naked truth and that is what I will try to copy!
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The 50% value is contextual in nature and has to do with model wind tunnel studies at FKFS,with Koenig-Fachsenfeld and Kamm involved.
*Their model was straight-sided,no camber,no tumblehome,no plan taper.
*At 100% boat tail,the model only achieved Cd 0.21.
*At 38%,Cd 0.25
*At 46%,Cd 0.24
*At 54%,Cd 0.24
*There was so much vorticity drag with their models you can see that continuing the tail all the way out would net them very little for their trouble.
*In contrast,Paul Jaray's 'pumpkin seed' model of 1921,which had body camber,tumblehome,plan taper,and generous edge radii achieved Cd 0.13 with a relatively steep aft-body slope of 26-degrees.
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*Fachsenfeld appreciated the low drag value of an extended tail and in his patent drawings you can see how he ended the 'rigid' portion of the car body at 50% (where it was going to strike the ground in an urban driving environment),then filled in the additional 'beneficial' portion of the tail with an inflatable 'extensible' boat-tailing device for use on the open road.
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Our cars have a chance to see coefficients of aerodynamic drag on the order of Cd 0.13-.12 with the long tail.It's up to the individual to determine what is 'practical' for their particular driving situation.