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Old 12-28-2012, 05:18 PM   #72 (permalink)
aerohead
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optimal

Quote:
Originally Posted by diesel_john View Post
CarloSW2
I was thinking something similar, aka only lower the front end. Are you thinking that the "larger volume" open space at the rear would aid in air exiting faster at the rear? CarloSW2[/QUOTE]



good point. cfg83,
But, i didn't think of that until i started thinking about a smooth underbody.
I found lowering the back end increased the drag (with a stock underbody), so would raising the back end decrease drag (with a smooth underbody). So now that i have smooth under, I will try to raise the back.

Probably each car should be optimized for front to rear height.
We say the shape of the rear is more important, so does that mean I can raise the rear ?to enhance the tail and while blunting the windshield, and still reduce drag, inexpensively.
Angle of attack effects so many things.
the grill to hood flow
hood to windshield flow
windshield to roof flow
roof to rear deck flow
on the bottom lift or downforce
under flow exit

lowering the front would compromise my alignment.

aerohead, thanks for the explanation. It gave me the answer to my question. What is optimal?
""A symmetrical airfoil of 6-to-1 aspect ratio,cut in half lengthwise,would describe the lowest drag car body""[/QUOTE]

diesel_john,sorry for this very late response,I just now caught your question.
For airfoils in ground effect Abbott and von Doenhoff caution that none of the tabulated data they published will be valid.Ground effect violates the 'flight conditions' under which the wings were evaluated.Also,wings are tested at infinite span and aspect ratios impossible to use with a vehicle.Spanwise flow is off limits also,so we're s.o.l.on that one.Finally,free flight presumes laminar air and we don't get that either do to the viscous shearing of strata of air as you move from zero velocity at the road boundary upwards.
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As to sections themselves,I believe that a 4:1 cord/thickness ratio delivers the lowest drag per unit length.That's from Sighard Hoerner.(there's a pictorial drag table for this section at the Aerodynamic Streamlining Template thread.
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The research done for road vehicles has shown that derivatives of 3-D streamline bodies of revolution,'half-bodies' specifically,produce the lowest drag with conventional 'door-slammer' type designs.
We're cautioned to never exceed 22-23 degrees with the aft body roof/body contour,which puts it in the territory of the 'Template,'which is based on a 2.5:1 Length/Diameter fineness ratio streamline body which never exceeds 22-degrees.Hucho reports this body in free flight at Cd 0.04.As a half-body in ground effect it yields Cd 0.08.When 'skinny' tires are added,the drag goes to Cd 0.12.

Jaray,Lay,Fachsenfeld,Kamm,Heald,Elliot,Prandtl and others all leaned on the streamline body during development of their work.
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