Quote:
Originally Posted by Lee Stewart
(Post 1570549)
https://i.postimg.cc/Fs1fvWnD/5.jpg
Chrysler used a wind tunnel to design the 1969 Charger Daytona. They were able to experiment on what would yield the least resistance for the back wing resulting in the highest top speed.
When Plymouth designed the 1970 Superbird, it shared most, but not all, of the Daytona's speed features. And the one feature that they changed, to make it look more aesthetically pleasing was to slant the rear wing back. This caused more turbulence at super speedway speeds.
The top speed of the Charger Daytona was 200 MPH. The top speed of the Superbird was 185 MPH. The loss of 15 MPH . . . to make it look pretty. :eek2:
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Aerodynamics is a crazy blend of art and science:
a) Airflow over a vehicle is not intuitive. You don't really know what's going to happen 'til you smoke it. Example: We introduced a duct at the front of the vehicle one time, looking for a ram air effect. It wasn't working like we expected so we brought it into the tunnel for evaluation. With the smoke, you could see the air was actually hitting the inside corner of the inlet, scooting across the opening and shooting out the outboard side of the inlet and around the vehicle. No air was actually going into the duct and in the hose. We would never had known (or even guessed this) without the smoke and video. The whole pickup truck "tailgate up vs down" and bedcover debates that just won't die (in spite of objective data) are a symptom of this.
b) You need to be able to focus on one small area. On the road testing incorporates the overall vehicle, which is fine for a final "capping run" evaluation, but for development work you need to isolate one area and gage the effects that can be attributable to those specific changes. Sure - the NASCAR and Bonneville guys to some testing on site with duct tape and cardboard - but if they're smart it's after spending hours and hours iterating and developing inside a tunnel like GM has.
c) Things change while you are looking somewhere else. So - the bad news is while you are focus on one area of the vehicle the other end can be changing without you knowing it. Hopefully you find out and can nuetralize the effects but if not it can generate a lot of confusion.
We had been working for years on Chevy Volt prototype and preproduction vehicles so we were quite anxious to do a "sense check" on the first saleable vehicles off the line. We put them in the tunnel and the results were .289 - not good because it was higher than expected but also because the media reports the values rounded to two decimal places = .29. We very much wanted the .28 value reported, so we had to be below .285 to achieve this.
After much angst and diagnosis it turns out the cars were building 5mm too high. This change in trim height was affecting our test scores, the advertising numbers and our fuel economy certification. Dropping the cars down 5mm brought everything back into line. Note that's 1 count of aero drag for every 1 mm of trim height, in this case (your results may vary).
With my duct example above, we could've changed something at the rear of the vehicle, changed the flow under or around the truck and suddenly our front duct could've started working. You never know.
d) Small changes have big results and big changes can have small results, like the example above. My boss used to joke that he would give his finger for .001 CdA (one "count" of drag). Little scallops and skegs and flanges that look like styling features can actually be aero enablers. Things like mirrors and door handles and grille openings are fairly significant aero disenablers, and can skew your early predictions of performance.
K
