Dave, (A2WindTunnel)
Katz is talking about just aero shaped bodies, for the "round" body he is talking about an aero shape, i.e. symetrical wing section, for the side view and an oval shape in cross section. The flat bottom shape is 1/2 of an aero shape in side view with a flat bottom. Neither is encumbered with wheels, tires, axles, etc the things that a typical Bonneville streamliner or lakester would have. I would speculate that a lakester designed with the wheels far enough away from the body could come pretty close to this configuration. (See pictures of the Aussie lakester)
I do agree with you that individual cars, especially NASCAR shaped cars, probably like to be ran as low as possible as they have pretty "dirty" undersides and lowering effectively reduces frontal area. I am not sure that I agree you can have free down force with out drag, I would wonder if you have seen these results that it might be more a function of your tunnel design, i.e. no moving floor, no wheel/tire rotation and less than optimum ratio between the car cross section and the tunnel test area cross section (blockage).
One additional note regarding generating down force with a body that is wing shape in side view, similar to the NACA airfoil shapes both symmetrical and also cambered, will generate down force simply by running them close to the ground. No angle of attach is required. This is because as you lower the shape the air flowing on the bottom has to gain velocity because it is required to travel through a restricted space, this increase in velocity will cause the pressure on the bottom to be reduced (Bernoulli strike again!!) and this generates down force. Again if you lower it to much, such that the boundary layer of the body begins to interfere with the ground plane it will block the flow and the down force will go away.
Sparky, the comment "More than half the HP is inertial - you still have to accelerate the lead! You don't have to accelerate down force! " is mostly BS, when you get to the speeds you are looking for, 300 mph+, the inertial loads are very small compared to the aero loads. The basic equation for force to accelerate is F=MA which is a linear equation, the equation for aero force is a function of the velocity squared and the required horse power is a function of the velocity cubed!! These forces rapidly pass the inertial forces. If you have huge horse power and are looking to run a big number, 400+, then you may be concerned about inertial acceleration because if you can accelerate faster it will give you a longer time to overcome the aero forces at the target speeds. This would only pertain to Bonneville and not El Mo or the eastern mile tracks as acceleration is important at those tracks.
Rex
