Bruce, List,
I must respectfully make a couple of comments about the post saying the
"kamm effect" or bobbed real has a lower drag than a teardrop shape.
Automotive Aerodynamics Handbook page T 8-10 Says:
" Bobbing the tail causes the air flow to change from laminar to turbulent,
but the difference in total drag is not great. The reason is that skin
friction, even though it may be in laminar flow condition is reduced by
removing some of the tail. The turbulence introduced by the bobbing
modufication is not very great. A substantial reduction in weight is also an
effect. In a race or sports car situation, the weight reduction and lift
reduction produce more gain than the increase in drag can effect"
Race Car Aerodynamics page 44:
"If the flow seperates behind a bluff (bobbed) body, then such a wake will
will result, and in the wake area the flow seems to be dragging behind the
vehicle. The energy of dragging this wake behind results in drag, whereas an
ideal body with completely attached flow may not have such a wake ( and also
form drag)".
Aerodynbamics for Racing and Performance Cars pages 31 & 32:
"There are endless examples from the world of racing that confirm that the
design of every race car is a compromise. Whether or not the shape is
dictated (or at least limited) by the rules, there will be some practical
conflict involved in virtually every design decision. This especially the
case when it comes to attempts to reduce drag. Apart from the the problem of
added weight, as mentioned in the previous chapter, any vehicle whose length
is several times its maximum cross-section dimension will wind up with a
tail so long and unweildy as to make it completely impractical virtually
everywhere but on the salt flats. (sound familiar?)
When considering this last problem as it is applied to passenger cars, the
german researcher Dr. Wunibald Kamm reasoned that some flow seperation was
inevitable no matter how closely a vehicles shape might resemble a teardrop
(or more correctly, a teardrop modified to take account of the presence of
the ground), because unavoidable details like gaps between panels would
"trip" the boundry layer. At the same time, it was known that the largest
source of drag for well streamlined shapes is skin drag, which is
proportional to surface area. Kamm turned vice into virtue by proposing that
the pointy tail be abruptly chopped off short, paying a little more,
perhaps, in in pressure drag, but gaining something in skin friction,
because of the reduced area of body wetted by the air stream. The optimum
point turned out to be where the cross-sectional area of the body was about
half of the maximum."
No where in any of these references is it stated or implied that the "kamm"
effect reduces drag over the optimum teardrop shape.
If it did, why are suspension parts on F-1 and Indy cars made from teardrop
shaped stock? Airplane struts? Why are the cars built for LeMans long tailed
and not bobbed like Can Am cars? The Old's Aerotech that AJ drove was built
with a short tail and long tailed bodywork and the long tail was used for
the high speed runs.
Why is the cd of a round nose bullet .23 and a teardrop .035? A secant ogive
bullet would be better but I don't have the numbers.
Draw your own conclusions but to my feeble mind for OUR application, the
bobbed rear is not the optimum shape and does not exhibit lower drag.
I am not doing this to stir up another hornets nest because I will be
signing off here in a couple of days for my pilgrimage to the City of Angels
to work on the engine program and it's installation. I just feel there are
some misconceptions about the Kamm theory and why it was developed in the
first place. It was not developed to reduce drag, but rather to minimize
some of the compromises that go along with a properly shaped body in most
applications. (non Bonneville)
Howard Nafzger
448 Streamliner
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