Quote:
Originally Posted by California98Civic
But it is not a CFD study. The author used pressure taps on a 1:4 scale model developed by BMW/Audi and tested it in a wind tunnel. The flow visualizations were done with paint on the body in the wind tunnel. Everything about the two shapes is standard except the slope of the roof/backlight/trailing edge.
That the modest change in the slope angle and curves on the fastback yields higher pressure readings on the roof is interesting. If the pressure readings along the backlight and decklid drop quickly to lower than the notchback, might it be possible that this particular fastback becomes too fast after the roof AND that the pillars are not optimized well?
If the vortices are the problem and the fastback shape is otherwise known to have more potential for reducing drag and lift when undertrays, diffusers, spoilers, and fins are used, then why not explore reducing the vortices off the pillars by tapering and radiusing the fastback's full rear body contours more? We might be able to get Cd possibly under 0.20 and lower lift too.
That kinda is a description of the XL1, which even narrowed the rear track. The XL1 was speed limited possibly because of higher lift over 100 MPH. But lift at high speeds could be dealt with through other optimizations or active aero. We'd have an ultra low drag body and no lift problems at 150 MPH.
I would love to find a contemporary study in automotive aerodynamics where some grad student tests an abductively reasoned thesis like this.
Here again is the link to the article (I previously posted it in post #58):
https://ecomodder.com/forum/showthre...tml#post630095 |
A properly contoured fastback would respect the degree of pressure regain tolerable to the turbulent boundary layer, so as to prevent triggering separation. That's it's job.Nothing more. And there'd be no attached longitudinal vortices.
Speed-limiting the XL1 may have had to do with what a major automaker had to say about their Cd 0.25 car of the 1960s , which never made it into production.
The argument was that, if you lower drag, the speed increases, then you need bigger brakes to stop it, better and bigger tires to withstand the braking, which increases loads on the chassis, which now needs to be beefed up to withstand the greater stresses, which adds weight, requiring more horsepower, in a never-ending spiral.
The first solution came in overdriving. By putting such tall gearing, say in 3rd, 4th, and 5th gear, the engine would never be allowed to reach a torque which would support an unacceptable speed. My CRX was engineered this way. The rpm splits between gears are so high, the engine just falls on its face shifting to a higher gear. My 1st-gen Insight is the same. Your stuck in 3rd gear.
Today, electronic fuel injection will just shut off fuel delivery at any predetermined set point a carmaker likes.
Somewhere there's an unregulated top speed published for the XL1. I want to say that it was 112-mph, but I'd want to check.
It's very easy to kill front lift with an airdam. Rear lift on the BEETLE was zero. I'd be very surprised if the XL1 had significant rear lift at all. Even if it were 'neutral' the diffuser could produce enough downforce for stability, as in the D-B, M-B C-111 III of 1978, which was running in the 250-mph range.