Aerocivic - how to drop your Cd from 0.31 to 0.17

[Cd]

Yes - "skid plate" was the first thing that came to mind. :-)
It would be a flat piece curved up at the end facing the rear of the car.
Coroplast should be tough and flexible enough to withstand being banged up - just look at the ones used on the catamaran channels behind the front wheels on the car.
It would get scuffed up - much like a 'skid plate' .

[freebeard]

Oh, OK. But were it I, the skidplate would want to be titanium. It's light and has other advantages:

[Fat Charlie]

Not a skidplate so much as a tailskid.

Just a reinforcement to protect against glancing impacts:

[basjoos]

That's just what I need. Scrape the underside on the road and start a bush fire.

Starting to cogitate a few ideas on how to do this. Thinking hinged at bottom front of existing boattail. Metal bar frame skinned with coroplast for the underside. Frame will be positioned about a foot below the existing boattail at the back end to produce the reduced departure angle and will extend about a foot beyond the existing boattail. Two compression springs at back end to keep frame in down position when not in contact with road. Possibly small wheels under frame at back (wait to see where the coroplast shows road rash). Lower sides made of aluminum sheet with hinged seam (piano hinge)running length of boattail to maintain rigid shape in wind yet fold out when frame contacts road. Hinged aluminum bars to attach back of frame to back of boattail. Don't want to go with fabric sides to the boattail extension as that would likely flutter in the wind unless kept tensioned just right. Any thoughts? It would be illuminating to do a CFD image of the aerocivic with a 10 degree departure angle to see what the flow improvement would be.

[BamZipPow]

I would lose the small wheels and go with a lower profile wear/scuff plate. Plastic cutting board material works well. Use bolts that have been countersunk into the plastic to secure it.

Never been a fan of piano hinges especially fer things that have a potential flex as the piano hinge flexes, it will start to bind. I've used normal hinges and taped the gap seams with Tyvek tape to keep the gaps covered up while it still allows flexing/pivoting.

When I was building my belly pan and was in the early stages of development of my rear diffuser, I had the diffuser on hinges and a few bungee cords to allow it to hang and thinking that gravity would overcome the air pressure underneath the truck. I was wrong and the diffuser ended up rising/falling based on the air pressure building up which would pushed the diffuser up until the pressure would bleed off causing the diffuser to drop. This banging cycle started around 30mph. I finally set the diffuser in a fixed position and angle but I left the hinges in place as I have a rear wheel drive and needed the flex in the rear. I believe you will probably discover this if you go with a gravity based solution.

You could go with an inner tube/air bladder between the rear diffuser and the bottom of the car body so as you contact the few driveways, it would compress enough to allow clearance and then extend back out to the proper angle. You'll have to experiment with the amount of air pressure that will meet yer needs. You will still have to install some wear skid strips so it doesn't beat up the diffuser too badly.

[freebeard]

It was a joke.

Since the plate has a single degree of movement, the sides could ride up into slots in the boattail or be made in overlapping halves. No need for a flexible cover.

Forget the self-optimizing behavior, make it stout and require 25% of the vehicle (rear axle) weight to move it. That should dampen the aerodynamic oscillations.

[3-Wheeler]

Originally Posted by Cd

Some CFD flow images of the car, and some of a standard car without the aero modifications...

Nice CFD work!

I'm surprised that there is this much flow separation next to the red arrow?? I would expect the flow to stay attached longer in that spot.



Jim.

[aerohead]

Originally Posted by basjoos

That's just what I need. Scrape the underside on the road and start a bush fire.

Starting to cogitate a few ideas on how to do this. Thinking hinged at bottom front of existing boattail. Metal bar frame skinned with coroplast for the underside. Frame will be positioned about a foot below the existing boattail at the back end to produce the reduced departure angle and will extend about a foot beyond the existing boattail. Two compression springs at back end to keep frame in down position when not in contact with road. Possibly small wheels under frame at back (wait to see where the coroplast shows road rash). Lower sides made of aluminum sheet with hinged seam (piano hinge)running length of boattail to maintain rigid shape in wind yet fold out when frame contacts road. Hinged aluminum bars to attach back of frame to back of boattail. Don't want to go with fabric sides to the boattail extension as that would likely flutter in the wind unless kept tensioned just right. Any thoughts? It would be illuminating to do a CFD image of the aerocivic with a 10 degree departure angle to see what the flow improvement would be.

I wish you could have been with me in 1984 at Epcot Center.
At the GM Pavilion,they had their Aero 2000 concept car on display.And they had a video depicting the airflow under the car,with an active diffuser deployed,and with it stowed.
The flow looked really good with the diffuser down.After Darko,I know I need to do something with the T-100.

[Cd]

As I mentioned previous : "I should add that the CFD images may not be totally accurate, and this entirely due to my lack of understanding of how to use the program.
Sometimes artifacts appeared, and the tests contradicted themselves.

I fully encourage others here that are CFD pros to run professional level testing on the car."

To give you some idea of some of the odd things going on with the CFD, just take a look at this image of the top rear of the car that shows flow separation where we know that there is none.
I would like to see a redo of the CFD done by a professional - there are a few on this site that work in CFD.

I never posted these images due to the inconsistencies in the results, and especially the Cd value.
I tested the CarBEN and ran into the same issues.



( The flow along the bottom appears much less turbulent here as well )

[basjoos]

Originally Posted by BamZipPow

Never been a fan of piano hinges especially fer things that have a potential flex as the piano hinge flexes, it will start to bind. I've used normal hinges and taped the gap seams with Tyvek tape to keep the gaps covered up while it still allows flexing/pivoting.

You're thinking of the cheap, steel, light duty piano hinges sold at the big box home improvement stores. If you look at the range of piano hinges available at McMaster-Carr, you will find mil-spec, stainless, aluminum, titanium, load-rated piano hinges in many thicknesses and widths. I have piano hinges mounted as the hinge point on the top of my wheel well covers where they have been serving for years without trouble.

For the boattail bottom extension, I'm thinking about using coroplast for the flexible sides rather than aluminum sheet. This dispenses with the need for piano hinges and, as my side skirts have proven, coroplast can flex numerous times without fatiguing. Just need to keep paint on it to keep solar UV from damaging it. Although I might want to experiment with using white elastomeric coating rather than paint to protect from UV. Around the farm I've been using it successfully on those gray tarps to extend their life. They normally last about 6 months in the summer sun, but I have a coated tarp in use that's lasted a year so far.

I've got a design worked out. Now just need to source the parts and find the time and weather (I'm working outside) to implement it.