Aerocivic - how to drop your Cd from 0.31 to 0.17

[alohaspirit]

nice video!


are there any videos while on the inside

also its been a while since new pictures were up too

[Cd]

Thanks 3-Wheeler.
My Civic hatch is a 1993 model, so the ScanGuage will not work on my car.
Since I have a SuperMID, I will use Mikes advice "I used the 2nd km/l line on the SuperMID (the line that averages over the past 5 km) to get the mpg reading over the 2 miles."

Thanks for the advice guys !

Mike - I have another question that I'm too dull to figure out : In the video that was just posted above, It seems as though you stated that the caulking on the gaps doesn't need to be smooth because there is a thick boundary layer around the car that won't even blow sand off the top of the car.
( Perhaps I misunderstood what you were saying )
How do tufts ( tuft testing ) work if they are underneath this boundary layer ?
Also, if the boundary layer is so thick that it won't even blow sand off the car, why go through all the trouble of filling the gaps with globs of messy caulking if the air doesn't even reach this area ?
By looking at how far that your caulking protrudes, it seems as though it would actually cause more drag than just leaving the gaps unfilled.
Finally, was there some reason that you chose to not smooth the caulking as you applied it ?
No offense intended. I'm just confused.

Im really impressed with the strength of that Lexan that you used on the back window. How does it do compare to a glass window when the windows fog up ?

One more last question : While looking at your photo album, I noticed the vents that are inside the cabin and exit around the license plate. Can you please tell us a little more about these and their function ?

Thanks !

[basjoos]

The boundary layer doesn't have a distinct demarcation line to it, but is a velocity gradient that becomes thinner with increasing speed. The air down right next to surface (where the loose sand grains would lie) has zero velocity relative to the surface. As you get farther from the surface you encounter faster moving air until at the edge of the boundary layer the air is moving at the velocity of the vehicle. At lower speeds (30 mph) the boundary layer can be up to 1/8" thick, but it gets thinner as the airspeed increases. Those sand grains would be blown off at 130 mph and at 250mph you would want to have a wax smooth surface for minimum drag. Yarn tufts work because the tufts stick up high enough into the moving air to be influenced by it even if they are still partially within the boundary layer at low speed. If you tufted using fine nylon thread you would not see them streaming with the air flow until you reached a much higher air speed than with the much thicker yarn.

The Lexan fogs up about as easily as glass. You can't use Rain-ex on it, but rather one of the Lexan compatible defoggers that Motorcylists use such as Rain Magic.

The two vents at the back of the cabin exit into the small low pressure pocket produced at the back of my boat tail (because it ends in a flat area with the license plate instead of a point). This low pressure exhausts air from the back of my cabin.

[Cd]

So what was the reason that you applied caulking to the body gaps ?

[basjoos]

Many body gaps are wider than 1/8", so they are affecting the shape of the boundary layer on the surrounding sheet metal. Also some body gaps, such as those around parts of the doors and the top of the hatch, open up into spaces large enough that air currents could flow through them, increasing drag as outside air is diverted into the gap, flows through the spaces inside the gap, and then exits back into the air flow somewhere else. If you look at how much dirt gets into the area inside the front of the door where the door hinges hide, its obvious that outside air and spray are getting into that area. Gap sealing won't produce a large drop in drag, but every bit counts, especially at higher speeds.

[Cd]

Regarding your bellypan, I have an aluminum cover that I made which comes to the end of the cataylic converter. I have a large piece of Coroplast and plan to re-do the rest of my 'pan.
Please give some more details about the areas around the exhaust pipe running the length of the car. From what i understand, you used sheetmetal there, and i was wondering how you secured it to the Coroplast.
It also looks like you have the Coroplast just sitting on top of the rear suspension arm.
How does this work when the car hits a pot hole and the suspension has to flex ?

Also, I seem to recall you stating that the 'tail on your car gave you around 4MPG.
( I'm not seeing all your data on your fuel log BTW )

If you chopped the tail at 1/3 its current length* and had it open ended ( versus capped ), how much would you lose in F.E. ?

I remember you had this configuration before when you built your original half boattail out of clear vinyl. What was your mileage then compared to now ( minus the new engine )

* about 3ft from the back bumper

[basjoos]

My bellypan consists of 3 pieces of coroplast, one each running from the front bumper down the driver's and passenger's side, covering the space between the central tunnel and the edge (underside of rocker panels, up inside of the wheel wells). The 3rd piece continues on back down the driver's side of the central tunnel and widens out to cover the underside of the gas tank going back to the rear bumper with the underside of the muffler exposed with a 1" space between the muffler and coroplast. 24" wide aluminum flashing runs from the front bumper down the length of the central tunnel and a 2nd piece follows the jog of the exhaust tunnel toward the right rear wheel well. The coroplast is secured with 6-32 screws/washers going into holes drilled and tapped into the underside of the car along the front/rear bumpers, the plastic rocker panel, and along the edge of the central tunnel. I used either tapped holes or clearance holes with screw/washer/nut on the parts of the coroplast that went up into the wheel wells depending on whether I was going into either metal or rubber sheet inside the wheel well. I used 1/8"X1" aluminum bar to provide attachment points for the coroplast spanning the space underneath the engine compartment and under the gas tank. The flashing is attached by screws into tapped holes into the underside of the car along the edge of the central tunnel. I positioned and cut the coroplast so the parts of the coroplast sitting under the suspension arms can flex as the suspension moves. Its been in place for over 150,000 miles without any fatique problems on the part of the coroplast.

I didn't have a SuperMID installed until I had already installed most of my aero mods, so I had to use at the pump calculations and changes in coasting performance to determine the effects of the aero mods, but the boattail improved mileage by about 4 mpg.

I only ran the partial boattail for less than one tank before completing it. I was working on the complete boattail, but ran out of time and had to drive with the partial boattail for a couple of days before I had time to complete it. I got a slight coasting improvement going from the partial to the complete boattail, so I figure about a 1 mpg improvement from completing the boattail. Assuming attached airflow along the entire boattail, the F.E. lost by a partial boattail would be proportional to the cross sectional area (sq. ft.) at the back of the cut off boattail compared to the original cross sectional area of recirculation eddy produced by the car.

[Cd]

Thanks for the very detailed description Mike.

[Smokingwheels]

Very interesting car, but I wonder If I ported your engine and put a carby on it I might be able to get you up to 126 mpg if my references are correct.
see Where I store my results
This forum is where I have started logging my results for my engine.
The reason I say use a carby is my methods really mess up the amount of air and fuel going into the engine eg I have it on video my idling engine using half the normal requirement of fuel an EFI system is going to have a hard time with that, then there is the other end at high rpm its about 50% less efficient but I had the extra power until I cooked my engine.

Mythbusters did a aero test on a car by putting golf ball dimples over the surface of the car I think it went from 26 to 29 mpg, imagine the looks you would get then if your car looked like golf ball surface. I haven't a clue on how you would press the dimples into your body panels though.

[Joenavy85]

Originally Posted by Smokingwheels

Mythbusters did a aero test on a car by putting golf ball dimples over the surface of the car I think it went from 26 to 29 mpg, imagine the looks you would get then if your car looked like golf ball surface. I haven't a clue on how you would press the dimples into your body panels though.

a thin fiberglass layer could be laid down, heck you could use a golf ball to make the dimples, how appropriate would that be?