Need some boattail advice for the CRX-hf

[ttoyoda]

AM i crazy enough to make this thing??



"Crazy" is what they called this guy, too.

[ebacherville]

Originally Posted by ttoyoda

ebacherville,
What do you envision for the top view of the tail? Do you see it coming to a point in the top view? Or just getting narrower?

...

If you make your tail go down to the ground, you might worry about it scraping. But if you let it be flexible or hinged at the car, with spring preload pulling it down, (like a clamshell idea) and have a caster or two under the back (which normally does not touch the ground) then you will not damage the tail when you go up a ramp.

Yeah the tail would narrow down width wise.. and yes the MB c-111 was the idea that Aerohead pointed me at.. As for ramps etc.. I live in a rural area thats fairly flat .. there are not any parking garages and the worst hazard I have to worry about are wild animals in the roads.. as for the tail yeah I could make it like a break away mirror.. hinged etc.. .. however that will add lots of weight.. the construction methods on this tail will be much like the cloth constructed planes... very lightweight.. however it will be rigid via a coating of fiberglass resin over the cloth. And I don't see much need for it as there are not may hazards here to constitute the need for it.

[Cd]

Originally Posted by ebacherville

With the hatch at the needed angle you wont be able to see out the rear glass anyway.. no need to keep it there if its useless... ill retail the opening hatch it will just be re-angled to fit aerodynamics.

The 1992-96 Mazda MX3 had a much 'faster' back window than your car, yet it was still possible to see out the back alright. The Insight also has a similar angle.
The '71 Mach 1 backlight angle was almost horizontal !

[Cd]

Originally Posted by ttoyoda

.Cd,
do you have any pics or links of that camaro from the front showing more of the air dam details? And is that a reverse scoop on the hood?

When tested at the Texas Mile, it had a cowl induction hood. Later at Bonneville it sported that mega scoop ( which actually did not alter the .Cd !! )

( Note the brick like front end. They went so far as to create a huge foam windshield extention at a much steeper rake - it did nothing to alter the .Cd )

[TheDon]

Originally Posted by ebacherville

Yeah the tail would narrow down width wise.. and yes the MB c-111 was the idea that Aerohead pointed me at..

yes! I have a die cast of that car somewhere



I love gullwing cars... except the Bricklin... Bricklins suck.

[ebacherville]

notice the rear spoiler.. draw a line from the top of the rear window to the top of the spoiler.. that angle is far slower than the angle on the stock CRX.. the CRX had a spoiler to cheat a bit also, however actually having body panels in that open void would be better as not vortexes of air can form..

That imaginary line on the camaro.. is just about what angle is pictured in the mockup for my CRX

As fo the insight you have to look at the entire length of the car.. not just one particular angle.. the mx3 is the same shape as my concept.. and insight have concaved window. if you look at them..

MX3



My concept is lessening the flat portion at the rear by extending he tail longer.. making for less area for the air to create draft.

[Cd]

Are you going to cover the wheels ?

[ebacherville]

Originally Posted by .Cd

Are you going to cover the wheels ?

yes

[aerohead]

Originally Posted by ebacherville

I think this would be the ideal shape for a full boat tail on the CRX.. just going by eye.. however has no mathematics behind it..


AM i crazy enough to make this thing??

aerodynamically your new image is way better and since "a picture is worth a thousand words" I'd like you to take a look at something.Go to GOOGLE Images and do a search for gentoo penguin.When the images come up,go to page three,and there you'll find the image of a single penguin swimming underwater.I printed this image and did some biometrics on it.The back 53% of the bird is "boat-tail".the aftbody of the bird is 2-body-lengths long.In the first half of the body,the angle never exceeds 10-degrees (as recommended by Kamm).In the last half of the boat-tail,the slope-angle of the penguin's body never exceeds 20-degrees ( as recommended by Mair ).All this makes for a long tail,and for fish,its no big deal.By the way,this penguin has a drag coefficient of 0.07,the lowest ever recorded for an emmersed structure of it's size and speed.On our cars,its a different ball game.What Kamm and Korff and Fachsenfeld,and the others have recommended,is to "truncate" ( chop-off ) the tail at any length that is convenient.Flow will be attached right up to the chop,and then the wake forms kind of a "phantom" boat-tail of turbulence which in theory,the surrounding air flows over like a solid body.It doesn't actually work as good as they say,but its a compromise for parking and turning and pedestrian safety,etc..Take a look and then think maybe about a shorter tail.Korff figures that if the chop occurs at about 50-% of frontal area,you're pretty much home free with respect to drag.