Aerocivic - how to drop your Cd from 0.34 to 0.17

[trebuchet03]

Originally Posted by SVOboy

I do believe newton has had that thought (of making aero body kits for ecomodders), and I definitely think it's worth a go. If I knew anything about molding and all that jazz I might play with it myself...

If someone made a boattail for a prius I'm sure that would get the business going.

Molding isn't too hard - just requires several pretty pennies of start up capitol

[UfoTofU]

Looks good basjoos!

I found this via Hack-A-Day actually.

[basjoos]

Originally Posted by Spyral

Wow, do you think I should do this to my GTO? I get about 26mpg on the highway and about 12mpg around town. Depending if I am on the gas or not. When I activate the nitrous oxide system I get even worse gas mileage. However, the car is very fast. I have no catalytic converters on it either.

The car looks great!
.

Cd reduction would work to improve the high speed performance of any car and is what they do on the Bonneville Salt Flats to increase the speeds of their speed record cars. Since aerodynamic drag is the dominant load factor at higher speeds, the faster you go, the more you'll notice the effects of a low Cd, reducing the amount of power you need to travel at that speed and in effect acting as the equivalent of a horsepower boost at the higher speed. As far as fuel efficiency goes, a low Cd extends the range of speeds where you can still get good fuel economy into the higher speed brackets and a low Cd car will get better fuel economy than an otherwise identical high Cd car at any speed above 30mph, the difference becoming more and more noticable the faster you travel.

[basjoos]

Based on what I saw in the road salt deposition patterns, the largest recirculating eddys remaining are those produced downwind of each rear wheel,so I built and installed a mini boattail behind each rear wheel.

[voxelman]

Basjoos,

You are an inspiration to us all!

[Coyote X]

Is there a deflector on the front of the rear wheels?

[MickF]

Basjoos - could you do some pics of how the radiator exit air leaves via the wheel wells? And how efficient is this (relatively) to bonnet vents? I know many of the recent Porsche 911 models vent the rad air into the rear wheel wells, but they appear to use shaped spokes on the rear wheels to act as a fan.

[Darxus]

Originally Posted by Coyote X

Is there a deflector on the front of the rear wheels?

No, better:

Originally Posted by basjoos

there are 2 side skirts on each side, one even with the inner edge of the tires, the other even with the outer edge of the tires

[basjoos]

The radiator air exits through the stock openings to the front wheel wells, which have the OEM wheels. The advantage of this method over bonnet vents is that I am dumping my radiator exhaust into an area that is already turbulent, so it isn't adding to the total amount of turbulent air created by the vehicle. But the air exhausted from bonnet vents, depending on how it is handled, could add turbulence to the car's airflow as it merges with it.

There are deflectors in front of the rear wheels in between the two side skirts. I started out with a wheel spoiler in front of each wheel using the spoiler design found on many hondas and toyotas that deflects the air downward in front of the wheel. I later replaced the front wheel spoiler with the current air splitter and added the side skirts, but left the rear spoilers in place to deflect any air that got up inside the side skirts before it encounters the rear wheel.

[MickF]

Originally Posted by basjoos

The radiator air exits through the stock openings to the front wheel wells, which have the OEM wheels. The advantage of this method over bonnet vents is that I am dumping my radiator exhaust into an area that is already turbulent, so it isn't adding to the total amount of turbulent air created by the vehicle. But the air exhausted from bonnet vents, depending on how it is handled, could add turbulence to the car's airflow as it merges with it."

So, is the air in the wheel wells at low pressure? That seems to be a contentious issue on the net, seeing as how many race cars have vents over/behind the front wheels to vent air from the wheel wells (although on some drag VW Beetles they put vents over/behind the wheels to stop the fenders from generating lift). I know the air in front of the tire is at high pressure, but I would assume the air behind the tire is at low pressure.

I'm trying to solve a cooling issue with my car (1971 Datsun 240Z with intercooled 6cyl 2lt turbo) due to horrific airflow issues with the engine bay. I can (and will) reduce the air entry through the radiator support panel, but I'm trying to work out how best to get the air out from the engine bay. The air past the turbo gets to ~ 230C, and heats the firewall & trans tunnel to ~ 70C which makes it rather uncomfortable in Summer.

I could use the trans tunnel as a duct to the rear wheel wells (undertray and with insulation on the tunnel to try and keep the heat out of the car) - not sure how well this would work. Or possibly duct from the engine bay to the front wheel wells (through or under the inner guards) with vents in the guards behind the wheels.

I'm also looked at the 2lt Super Touring cars - the ones I looked at (BMW RWD and some FWD's) had the air through two small radiators and then the outlet air is ducted to vents in the ends of the front spoiler (side of the body) ahead of the front wheels.

[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.