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Old 05-28-2008, 02:54 PM   #1 (permalink)
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Need some boattail advice for the CRX-hf

Ive made the nose for my CRX.. thet talk about the boat tail..

My main question is the stock rake on the CRX's hatch acceptable for the boat tail top angle.. if so ill just continue it as I planned on my concept drawing..


Now for the bottom angel whats ideal, I have seen some tails concaved, for instance on the Aptere.


Also I was looking under the car and I can start pulling the rear end angle up right after the rear tires, its very empty back there..

Any particular science behind designing the boat tail I should know about?

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Old 05-28-2008, 03:55 PM   #2 (permalink)
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Here's the patterns I used to make my boattail. I pasted this tailcone shape onto a photo of my Civic and then worked out the dimensions. The Questair Venture is a 350mph kitplane.


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Old 05-28-2008, 04:04 PM   #3 (permalink)
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CRX boat-tail

The CRX hatch rake is not acceptable,is too "fast",and you'll need to extend the back of the car at least 12-inches (300-mm )to clean it up.Also,the space in between your "new" back end,and the top of the backlight will be dirty.I extended my C-pillars all the way back,and stuck a cantilevered wing spanning out to the rear to re-configure the air there and added a symmetrical airfoil across the span at mid-height between the C-pillar extension,running very close to the glass to stop circulation.As for the bottom upsweep,technically it can have no more than a 2 and 1/2-degree upsweep without flow separation,but if you run straight up and down driveway ramps,you could run into scrubbing ( or worse! ).Darin posted a pic of my CRX on page 7 of the aerodynamics section.I just looked at it and its 10 lines down,entitled"The Phil Knox fleet----------.Like your Photoshop work,good luck with the project.I think your in for some fun at the gas pump.P.S.,a movable bottom diffuser would allow you to configure it for straight highway travel,where ground clearance is less an issue.
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Old 05-28-2008, 10:23 PM   #4 (permalink)
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Hmm yeah I've seen the photos of your CRX.. My thought was the large flat space on the rear is a great place for air eddy currents to get you.. extending out the rear by a foot or more should not be a issue.. however I'd like to get the rear section to a smaller size to reduce eddy currents.. Like the airplane drawing above it seems to be a more flowing design...

I assume anything will be a improvement.
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Old 05-28-2008, 10:34 PM   #5 (permalink)
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how about this..?? its extreme but what the hell..
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Old 05-29-2008, 05:21 PM   #6 (permalink)
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advice for CRX-hf

Quote:
Originally Posted by ebacherville View Post
how about this..?? its extreme but what the hell..
ebacherville,read your post and will touch on your points mentioned (all good!) (1) The flat portion on my CRX moves the the back of the car to the point where it intersects the ideal roof curvature.And yes,there is a captured-vortex over that portion,contained within the C-Pillar extensions.And as in a "bird-bath" rear spoiler,air above, scoots over the vortex as if its a solid body.Yes,there is a small penalty to drag,however it allows uninterrupted rear-view vision.Should you attempt to angle the glass up to the ideal roof slope,light refraction would destroy your vision,as all you would see is internal reflections of your car.(2) By running the slope at a steeper angle to achieve a smaller rear section,you shoot yourself in the foot,as the flow will separate,and your new tail will be in a region of separated flow,with no lower drag than before.Please look at my installment on "roofline" I posted a few days back.(3) The bottom of your boat-tail begins above the line defined by the bottom of the car.I'm putting an installment together for "underbodies",however I'll go ahead and mention if I haven't,that the flow under your can not follow an upsweep of more than 2-1/2 degrees.With what you've illustrated(and I'm not trying to beat up on you),your under car flow will never touch the under side of your boat-tail,and will just be surrounded in turbulence.(3) The aft body of the airplane that basjoos posted,you must remember,is in free air,high above ground-effect.For that tail to work with as low as drag close to the ground,it would have to be stretched to twice it's length,and all those curves would have to be twice as gentle,to prevent separation do to the "mirroring" effect of the road surface.(4) basjoos' CIVIC is a 3-door hatchback and has distinctly different aerodynamics than our CRX's.His roofline is further from the road,is less effected by ground-effect,and because the sides are also converging as the roof is dropping,he gets away with no longitudinal attached vortices,which can be the highest drag component for a "fast-back" car like the CRX.Take a look at the 1978 Mercedes-Benz C-111 record car.You can GOOGLE it,and there's even a plan view of the car showing the boattail detail from above.I'm excited about your project and want to steer you around any obvious snafu's if I can.Ive tried to force the atmosphere's hand before, only to have it laugh at me good and loud.You can't cheat the sky.
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Old 05-29-2008, 05:49 PM   #7 (permalink)
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ebacherville -

Quote:
Originally Posted by ebacherville View Post
Ive made the nose for my CRX.. thet talk about the boat tail..

My main question is the stock rake on the CRX's hatch acceptable for the boat tail top angle.. if so ill just continue it as I planned on my concept drawing..


...
This sooooo looks like a Datsun 240Z :



Curb weight of 2350 lbs :

Nissan S30
http://en.wikipedia.org/wiki/Datsun_240Z

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Old 05-29-2008, 06:49 PM   #8 (permalink)
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Areohead..

Thanks for the info.. My plan for the rear windows was a camera like in a RV for rear view.. I was totally going to make it sealed off.

If you were going to make something "drastic" that was the ideal form what would it look like, could you draw up a quick profile for it? I'm up for a 8 foot tail.. if need be.. since the car is so small already.

I looked at the MB c111 looks more of a "bump in the road " than a tear drop shape.. a.k.a. lift the air up at the nose of the car and drop it back to the ground at the rear..
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Old 05-29-2008, 07:40 PM   #9 (permalink)
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I looked at the MB c111 looks more of a "bump in the road " than a tear drop shape.. a.k.a. lift the air up at the nose of the car and drop it back to the ground at the rear..

Half a teardrop. The other half below the road surface has been omitted.
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Old 05-29-2008, 07:43 PM   #10 (permalink)
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I would like to see any sort of diagram too, to help visualize it, but it sounds from aeroheads description (and I could be very wrong) that your design looks (as mentioned) sort of like a 240Z, but aerohead is describing something a bit more like the shape of an Audi A2 or a Prius, with the difference being instead of a solid rear the CRXs rear window angle is retained with side plates and a wing to cause the air to act as if the full rear window was there but with a bit of a tradeoff for drag. Am I even in the ballpark?

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