So - here are some random bits from the conversation with Gary Eaker :
When asked about how a car with a flat front end, or flat windshield can still have good aero, he replied that you can have good aero if you curve the edges on a "barn door" at 80% flat to 10% curved.
You would end up "Almost as good as a nice round hemispherical nose. It's not the ideal though ".
"A soft plan view desn't really buy you much as long as attached flow is retained ."
- On the roaster image I posted with attached flow, there is indeed a pressure bubble at the base of the flat windshield that is causing air to flow up and over the roof.
The most important factor in that car for good attached flow are the radiused edges. ( The "critical radius" )
He mentioned that when designing the GM Impact ( early EV-1 ) they found that :
- Giving the car a 'sportier' windshield for styling caused more drag, since it sent air over the sides of the car.
- Fender covers ( wheel covers ) - " More wasn't that much better "
- The EV-1 backlight ( rear window ) taper is "too steep" for good attached flow, but only in 2D
- Plan taper angles result in good flow. ( EV-1 ) But the two angles have to be matched just right, or you get trailing vortices, which can really kill your drag.
- Besides the boattail, "fender flares" were added in front of the front tires on the Ft.Stockton 'Impact' car that went 183 mph, and got to .137 cD
- An underbody panel came loose and fell off on the Impact / EV-1 and the drag went down, much the same way a 3/4 toneau cover helps on a pickup, or a box cavity works.
- Gary is most proud of two cars - the Impact at .137, and Bobby Alison's '87 Buick Le Sabre - as raced, the car had a .237 cD
- Don't go too long on a design, or viscus drag results in separation
- You want the pressure coefficient at the rear of the car to be as low as possible.
- When the Citation IV was created, the 'shell' tested at .14 - G.M. produced a drivable car, and the cD went to .28 ( " cooling flow,wider tires, exposed wipers, moudings, less efficient wheel covers, etc
The G.M. team then did a coast down test on the driveable car and concluded that wind tunnel testing was not a good way to predict real world cD ( When the opposite was true, and in reality the car now had a .28 cD.
)
- Notch angle is just as important as backlight angle on a design.
- A rear window with perfect attached flow will show a pressure bubble at the base of the window when a rear deck is added. This would have people saying " Oh that must be bad for drag" , but it's not due to the Adverse Pressure Gradient
When trying to get the record for the stock bodied car class at Bonneville on a 1988 Firebird, they couldn't do much besides remove mirrors and lower the car.
Since the car is a "bottom breather" they didn't have to do any grille blocking, but they still got good results by adding ultra fine screen at the entry to the radiator inlet under the nose.
The car ended up with a .23 cD
