Aerocivic - how to drop your Cd from 0.31 to 0.17

[Stan]

Very cool project, basjoos! I have been playing with diesel engines in my street cars since th early 80's, though never with the body shape. My best was 72 mpg from Seattle to San Francisco (>800 miles) in my wife's new Golf TDI by doing 55 on the flats and 50 going uphill, plus the usual coasting, etc.

I studied aeronautical engineering in the Air Force (I'm a retired USAF pilot) and recognize most of the basic drag reduction features of your car. Oddly, Ford built a wonderful little FWD V-4 engine prototype aero car in the early 60's which incorporated many of the same features, though not nearly so comprehensively as you have.



You can search images.google.com for more photos with "1962 mustang prototype" (use the quotes).

Thanks for sharing!

Stan

[MetroMPG]

Welcome to the site, Stan.

basjoos is definitely an inspiration. Every time I look through this thread, or at his latest tweak/addition, I can't help but think, "here I am staring at the computer screen again instead of doing something!"

PS - that's great mileage in the Golf.

[basjoos]

MikeF, I don't know if the air in the front wheel wells is low pressure, but they are the main exhaust ports for my engine compartment and get the job done. I also exhaust a smaller amount of air out through the central hump (where it cools the exhaust pipe and catalyitc converter) to the right rear wheel well.

[MickF]

Thanks for the info, basjoos.

[basjoos]

I've been pondering and haven't decided yet which layout would be better. My current setup of air divider/front wheel/double side skirts/rear wheel/boattail, or alternatively to have a air divider/front wheel/boattail//air divider/rear wheel/boattail. The side skirts help to keep the air flow linear between the front and rear wheels, but has more wetted area than the alternative layout. Where's a full-sized backyard wind tunnel when you need one?

[Stan]

Based on extensive CFD studies we've done for racing customers I think having just the outside skirt between the front and rear tires is your best (lowest drag) option. That's because the outside skirts keep air from migrating down from the sides of the car to under the car. This reduces the disruption of the airflow and added turbulence under the car, and hence reduces drag. That's why nearly all racing rules prohibit skirts down below the bottom of the car frame.

Your "tire spats" do add a fair bit of extra wetted surface. A trick we use on race cars to reduce wheel opening drag is to add a small "Gurney Flap" to the leading edge and top of wheel openings. This photo shows a roughly trimmed left-rear fender we made recently for a customer's sports car.



You can see that the bodywork turns out 90-degrees right at the edge of the wheel opening. That turn-out needs only be a centimeter or so tall, but it does two things for you. First, it creates a low pressure area outside the wheel opening which helps evacuate air from the wheel well. Second, it diverts the freestream air away from the wheel and tire, thereby reducing the drag component of those pieces.

There's no free lunch, though, aa the flaps do add incremental drag, but possibly less than the spats they would replace.

Stan

[MetroMPG]

Stan - I know there are rarely simple answers in aerodynamics, but can you say whether skirts in racing are primarily meant to reduce overall drag, or are they there to retain a pressure differential between ambient, and the lower pressure underneath the vehicle (for more downforce)?

I haven't seen much written about skirt use on road cars, specifically for drag reduction - except a suggestion that in a crosswind situation, skirts will increase projected area in the "apparent" direction of travel through the wind.

Darin

[Darxus]

Stan: I think your racing customers are dealing with a lot less air flow under the body of the car.

[Stan]

The short answer is "yes"...the longer answer is..."it depends". Old aero joke...sorry.

Joking aside, many high performance techniques will transfer well to maximizing fuel economy. Not the particular solutions, mind you, but the techniques of, in this case, airflow management.

Racecars seeking downforce generally fall into one of two categories, flat-bottomed or tunneled. Flat-bottomed cars are just that. The underside of the car has to be built on one plane from side to side and front to rear except for specific exceptions, such as openings for the tires. Tunnel cars are permitted varying degrees of camber to their undersides. Both types of cars generate downforce by varying their ground height and "rake" (the difference in ground clearance between the front and rear). Both also seek to maximize downforce by employing side skirts whenever permitted to maintain nice linear flow under the car from front to rear.

Looking at the photos of basjoos' car, especially these two...





...it is clear to me that his car has considerable camber along its longitudinal axis. The leading edge of the nose appears to be about a foot off the ground. The bottom then dips down to within ~4" of the ground, and then gradually rises back up to the trailing edge of the car. In aero terms, basjoos has created a classic venturi, with a convergence zone at the front, a throat in the middle, and a diffuser at the rear. And since he writes that the car is stable up to 100 mph, my guess is that Aerocivic is producing modest net downforce. This added downforce comes at the cost an increase in induced drag. That said, the total induced drag on such a narrow-span "wing" (the car is only 5' wide or so) is slight. In other words, there is enough downforce to securely "plant" the car to the ground, but not enough to cost him much net fuel efficiency.

Basjoos, have you done any back-to-back fuel economy measurements with and without the side skirts? My guess is that they are giving you a measurable boost in FE. The question in my mind is whether you need both the inner and our skirts. I am fairly confident that if you remove the skirts and put boat tails behind the front tires that your fuel economy will suffer.

Be fun to find out, though!

Stan

[Stan]

Originally Posted by Darxus

Stan: I think your racing customers are dealing with a lot less air flow under the body of the car.

Yes, you are quite correct, which is why I noted in my most recent post that it is the techniques that will cross over, not necessarily the particular solution.