Question on Boattail taper angles
 

Hello all, I am currently planning and designing a boattail for my Echo, and am using the template for the top, 10 degrees for the bottom, and am not sure what to use for the sides. I am guessing 10 degrees, as people bring 10 up a lot, but I didnt see anyone say the angle for the sides specifically. I just want to make sure before constructing it.

This is from California98Civic:
http://ecomodder.com/forum/attachmen...9&d=1386051195

plan-view 'angle'
 

We're really looking at curvature so an angle would be subject to a location along that curve.
I'll try 'n put up some images to compliment the 'Template'
Here is a plan-view of the AST Part-C gen-II.It is based upon a the trailing portion of a 3.92:symmetrical wing section.This contour has been used successfully on some of the lowest drag vehicles tested and is my current 'buy' recommendation for modding.
http://i1271.photobucket.com/albums/...ad2/Bottom.jpg
Here (in the center image) is the actual wing section I'm recommending.It is the aft portion,adjusted to the body width of your car which you'd use.
http://i1271.photobucket.com/albums/...rohead2/-2.jpg

I'm confused about the wheel fairings/spoilers on the car in the upper picture.
They are tapered along a straight center line, which would be great if there were nothing else around there.

But then there is the car body, pushing the air aside at the front and pulling it back in at the back. Imho the center line should follow the direction of air flow, or even slightly more slanted than that to help push away or draw in the air.
So I would think the fairings ahead of the front wheels should need strong 'toe in', and on the rear wheels some 'toe out' would be beneficial.
By directing the air under the tail that could rise slightly steeper, allowing for a shorter and lighter tail with no added drag.

I also wonder why they are that long. The wing profiles in the second picture suggest that causes unnecessary drag.

I'd shape them like this:
http://imageshack.us/a/img845/7370/v8dj.jpg
Excuse me for my poor drawing skills - just wanted to show my idea.

Said the aero layman... sorry if I have missed the obvious.

Thanks everyone for all the input! Curve for the sides, it is. That'll make the build more..interesting, I suppose.

wheel fairings
 

Red Devil,these are all good questions and observations,and I'll attempt a reasonable and rational explanation,as I'm certainly no expert when it come to such things.
*The premise of the simple canoe fairings,is that the front airdam is shearing most of the air away from the underside,and the flow which remains is traveling for the most part,longitudinally towards the rear.
*The smooth underside is a 'wall' with the wheels erupting out of it,contributing interference drag from these Cd 0.488 rotating wheels with 'MOONs' on both sides.
*The 'wall' affects the boundary layer thickness around the fairings,and by the time the horseshoe annulus at max. cross section displaces to the tail of the fairing,the semi-circular cross section of the boundary layer is quite thick geometrically by comparison,significantly altering the effect of the fairings performance.
*Hoerner reported that due to this interaction,it was beneficial to lengthen not only the aft-body of the fairing,but also the forebody,with minimum drag occuring for fineness ratios between Length/Height = 10 and 15.
*A marginal fillet at the fairing/wall intersection also was found to be efficacious in reducing drag (as you see on all aircraft wing roots).
*Since underbody drag can constitute 50% of total drag on a low-drag car,the fairings are critical.
*The long fineness provides minimum pressure spikes and respects the peculiar boundary layer conditions present.
*The symmetry of the fairings provides uniform accelerations,velocities,and pressures,which help prevent transverse contamination of the boundary layer which might trigger vortex formation.(This is not to say that in this simplistic 'model' of the 'Template' underbody that flow is actually as is theorized,but to know would require actual wind tunnel testing and verification which is beyond the scope of a simple 'one-size-fits-all' solution).
*Historically,very fine results have been achieved with 'straight centerline fairings,as we see today with Cambridge University's CUER eco racer of Cd 0.11.
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*Looking at your illustration,I suspect that there's a chance for separation to occur on the leeward face of the lead fairing.I can't prove it,but I'm certain that this surface,without suction, would be be beyond burble point and into full stall.
*As to the asymmetry of the tail fairing,again,the steep contour appears to invite separation,as well as asymmetrical opposing pressure regimes which could induce vorticity as the flow fields attempt to re-combine.
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*Neil Blanchard has Goro Tamai's 'The Leading Edge' and I hope that at some point,Neil may be able to shed some light on these fairing issues.
*In the past,many World Solar Challenge university teams granted access to GM's Hughes Electronics' VSAERO CFD program,ran the simulations without wheels as they are too difficult to model mathematically in a 3-D flow domain.
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*With the looming CAFE standards and such,and production vehicle Cds approaching 0.20,I think there will be more and more interest in these underbody issues.
*That's about all I can think of for now.We should dedicate a thread to this issue,where all the members can have an opportunity to contribute.
*Another year is passing and Santa Claus has failed again,to deliver EcoModder's full-scale wind tunnel.:p

^ I must be picking up some stuff because for once i understood the whole thing.

Those fairings reminded me of the VW Xl1. I remember thinking the fairing were pretty long. I suppose the reason why the back of the rear fairing is shorter is space issues. They just extended the whole thing forward of the rear bumper.
http://2.bp.blogspot.com/-1haYiPcIUM...-xl1-views.jpg

Then thers the fairings of the Edison2 VLC. I see your point RedDevil. The rear fairing tapers toward the car at the end.
http://blog.industrysoftware.automat...Edison2Top.png

Edison II
 

Perhaps the new book on the X-Prize will shed some light on the Edison II car's rear fairings.
Since this is monoposto design,the influence of the four wheel pods will be unlike that of the wheel fairings on the fully-enclosed saloon body of the XL1.
Thanks for the images!:)

I don't believe the book discusses the "whys and wherefores" of most of the specific pieces on the car. It is more general than that, and concentrates more on personalities.

-soD

I see. I might have been too obsessed with my own car.
On this car, with the air dam and flat bottom, I agree the fairings need to follow the air flow under the car - which is, be straight.

I like to think that the toe-in front wheel fairings may serve as an aid to or even replacing the front air dam when it is not possible to make that low, forcing the air aside; keeping them short (merging upwards into the body) for practical rather than aerodynamical purposes.
As you pointed out this should be put in another thread though.