Quote:
Originally Posted by COcyclist
Ken, did you test the Citroen with a full boat tail? Was there a difference in mpg with or without?
|
Yes. I followed the slope of the trunk floor and the hatch, and the taper of the sides. (The SM body was already on its way to a boat tail, with rear track 10" narrower than front.) I'd say the fairing in of the tail with the body was "pretty much OK". The surface was insulation board. I'd tufted the rear window and flow there was pretty well attached in both cases.
At the time (late 1970's) this was a pretty casual experiment. I didn't have access to really accurate fuel flow equipment, so used two 50 mile runs, filled at the same pump location at the same station, etc. and didn't observe any difference. At the time, I was half expecting to see a slight increase in drag from the tail, being, at the time, a half believer in the Kamm effect, but also thought that the flow was well enough attached (at least to the rear window to benefit from a boat tail. There is a little bump at the center of the trunk lid that I faired in crudely. Perhaps removing the bump (not something I would do for a test, of course) would have helped.
Far to many variables at play to say what its real effect might have been. I picked a pretty still day, but could have had a slight tailwind on one trial and a slight headwind on the other. (Although both trials were out and back, but on a widely divided highway.) I think there was very little effect on this car that was already pretty clean and close to being boat-tailed... But I don't have a scientific leg to stand on.