Partial Boattail/Kammback on a sedan?

[aerohead]

Originally Posted by mpg_numbers_guy

I know there have been myriads of threads related to kammbacks and boattails for hatchbacks, but how about sedans? Could a partial boattail or kammback be implemented on a sedan as well, and would it have the same FE benefit? I'm considering doing a kammback on my Civic sedan and was wondering about the following templates (color code interpretations below the images):



The white indicates the kammback and the black indicates a possible partial boattail.



The blue and green indicate the pieces for the kammback, and the purple and red indicate the pieces for the lower part of the partial boattail.

Any thoughts on this? Location, possible FE gains, durability (coroplast w/ zipties, tape, and clamps)?

I know it would be better if the sides extended to the outside, but this would require taping to the exterior paint, and I'd rather not risk the car's paint job. I might have to relocate the license plate as well so it remains visible.

Any tips/suggestions welcome! Or if it's just a bad idea just let me know!

The Kamm-back car is based upon a 1/2-streamline body of revolution.These bodies cannot produce separated flow due to their perfect sectional density.Drag coefficient is simply a function of body elongation.
The notchback car is a mutilation of the ideal form and technically,is 'challenged',as far as drag goes,and will not (cannot) react to boat-tailing as will a K-form.
It is true,that up to a point,the notchback can have little penalty as far as drag goes,compared to the same car with a K-form greenhouse,but after that,you'd need the K-form to get the drag lower.
It may have been MetroMPG who modified a Honda similar to your's with a K-form addition,only to see no real drag reduction after all his effort.
You might want to select a Cd target,then inspect vehicles(especially Honda's) which already meet that target,and see if your car might benefit from some of their features.
Research by Audi,on their Audi-100 III demonstrated 'saturation' points for specific mods,whereas some part would respond positively to a certain alteration,after which,no additional shape change could produce additional benefit.This is exactly what 'optimization' is all about.
There are production Honda's of Cd 0.27,and Honda concepts of Cd 0.20.Honda solar racers as low as Cd 0.10.

[19bonestock88]

I did vortex generators on the Malibu that I had and water tested the back glass before and after... I copied the formula from the Evo, and had them about 4” above the glass... beforehand, there was flow separation in the center of the glass, that showed itself in water(dew in the morning) taking forever to run off if at all on my commute... after I added the vortex generators all the water run off as soon as I got up to 50mph, but I couldn’t detect any net improvement in drag... it took the same(or within a window of error) throttle/load to hold 55mph on flat ground as it did beforehand... now I never did a true coastdown test to produce more accurate data than just engine load at cruise, but eventually I removed them(after two tanks I think) both because I didn’t think they were helping and because my then wife was starting to refuse to ride in the car...

The visor isn’t a half bad idea(might use on my build), and they can be found for $30-50 for the Civic... a shame I can’t get one made specifically for my car, I’d have to cut up one for a Corolla or something... like mentioned earlier, cut a slot near the leading edge(about 2” wide(est.) and almost as long as the whole visor) to allow an air fence and I’d really think of using thin Plexiglass to extend the trailing edge to about where the trunk lid comes up to when open... if you follow the template shape, it *should* help

[freebeard]

One would have to basically rebuild the Honda for those...

Which I proposed at Permalink #5?

[aerohead]

Mercedes-Benz used a combination backlight-header deflector,combined with a rear wing on their 190 Evo.I don't know if the two created a drag reduction,or just dealt with lift issues on the Autobahn.

[19bonestock88]

It’s safe to assume that the giant wing was to reduce lift/add downforce but the rear window deflector I’m not sure about... maybe to increase the effectiveness of the wing? That’s a lot of why Mitsubishi put the vortex generators on the Evo...

[kach22i]

Originally Posted by mpg_numbers_guy

How about something like this that I posted earlier?

That item in the "this" link is what I call an air flow "disruptor", more commonly called a spoiler because it spoils the air flow. This is in the Gurney Flap or Wickerbill family of aerodynamic devices, it creates drag because of the swirling vortex of air behind it, but that same swirling mass of air also pulls the air back down faster than it would without it.

So yes it would do something, something aggressive but it's main function is to create down-force on it's front face via build up of air pressure, and this also results in a lowering of air velocity at that point by the air hitting this blunt plane.

This as an analogy is a hammer, and I think you may want to use a knife. The right tool for the job as it were.

Have you seen some of the things forum user Sven posted?

http://ecomodder.com/forum/showthrea...car-20303.html


He also put together a PDF that shows the roof wing on my pick-up truck (still cannot find it - sorry), the intent on that design originally was to have a clean sharp edge for the roof air to release from. However by leaving an air gap it became some sort of weird Venturi channel generator and helped pull the air down on to the tonneau bed cover (a good thing).

Here is an idea, using clear acrylic make a Funny "U" looking similar to the below image.

http://ecomodder.com/forum/showthrea...n-22468-5.html


The F-U (standing for Funny-U) as we will now be calling it is only attached at the bottom two sides, the back/rear is open, and the front leading edge is beveled or rounded but not touching the roof - there is a air gap 1/4" - 1".

Just a concept.

Many people want to go with the tried and true for a known results. Others like sticking their head out a little and taking chances, the thrill of discovery and all that.

Make your choice.

If you are a "if it's not broken - then don't fix it" type of guy, then why are you posting here?

I don't know what's best for you.

Just have fun doing some thing.

Building a cardboard and duct tape mock-up is a good way to start.

Enough talk, just build something.

FYI: I think your car is currently in the Cd .30 range.

http://ecomodder.com/wiki/index.php/...t_of_Drag_List

EDIT:
It should come out a bit like this.

http://www.sarona.com/single_product.php?id=4973


Or like this:
https://www.carid.com/aero-function/...f-spoiler.html

[Ecky]

Originally Posted by mpg_numbers_guy

Maybe I'm missing a point that's obvious to you. If the idea of a boattail/kammback on the end of a sedan won't work because the airflow goes over it and doesn't reattach, wouldn't it do the same thing with a spoiler, regardless of it having an aero shape or not?

It really depends on the spoiler. I see aerodynamics as a black art.




A spoiler won't reattach flow, but it can reduce turbulent wake if it's very carefully placed. It's not nearly as effective as a kamm or boat tail, which can keep laminar flow, but it does affect wake.

Here's an article you might enjoy:

https://tylerlinner1.kinja.com/a-pra...=1437153595522

A good example is a pickup truck bed. It's usually the case that drag is lower with the tailgate up, rather than down.

[kach22i]

Originally Posted by Ecky

https://tylerlinner1.kinja.com/a-pra...=1437153595522

EM user kach22i’s Chevy S10

That's the PDF (link), thanks for posting it.

Hey, good post overall, hit on a lot of good points.

[mpg_numbers_guy]

Originally Posted by aerohead

The Kamm-back car is based upon a 1/2-streamline body of revolution.These bodies cannot produce separated flow due to their perfect sectional density.Drag coefficient is simply a function of body elongation.
The notchback car is a mutilation of the ideal form and technically,is 'challenged',as far as drag goes,and will not (cannot) react to boat-tailing as will a K-form.
It is true,that up to a point,the notchback can have little penalty as far as drag goes,compared to the same car with a K-form greenhouse,but after that,you'd need the K-form to get the drag lower.
It may have been MetroMPG who modified a Honda similar to your's with a K-form addition,only to see no real drag reduction after all his effort.
You might want to select a Cd target,then inspect vehicles(especially Honda's) which already meet that target,and see if your car might benefit from some of their features.
Research by Audi,on their Audi-100 III demonstrated 'saturation' points for specific mods,whereas some part would respond positively to a certain alteration,after which,no additional shape change could produce additional benefit.This is exactly what 'optimization' is all about.
There are production Honda's of Cd 0.27,and Honda concepts of Cd 0.20.Honda solar racers as low as Cd 0.10.

Makes me wonder why people react negatively to kammback shaped vehicles such as the CRX and Prius when they're so much more streamlined...after all it isn't that hard to produce an aesthetically pleasing fastback car.

Originally Posted by 19bonestock88

I did vortex generators on the Malibu that I had and water tested the back glass before and after... I copied the formula from the Evo, and had them about 4” above the glass... beforehand, there was flow separation in the center of the glass, that showed itself in water(dew in the morning) taking forever to run off if at all on my commute... after I added the vortex generators all the water run off as soon as I got up to 50mph, but I couldn’t detect any net improvement in drag... it took the same(or within a window of error) throttle/load to hold 55mph on flat ground as it did beforehand... now I never did a true coastdown test to produce more accurate data than just engine load at cruise, but eventually I removed them(after two tanks I think) both because I didn’t think they were helping and because my then wife was starting to refuse to ride in the car...

The visor isn’t a half bad idea(might use on my build), and they can be found for $30-50 for the Civic... a shame I can’t get one made specifically for my car, I’d have to cut up one for a Corolla or something... like mentioned earlier, cut a slot near the leading edge(about 2” wide(est.) and almost as long as the whole visor) to allow an air fence and I’d really think of using thin Plexiglass to extend the trailing edge to about where the trunk lid comes up to when open... if you follow the template shape, it *should* help

The description of the ebay visor SAYS it helps with FE....but then again practically every exterior vehicle accessory claims to help with FE, even the ones that obviously don't.

Originally Posted by freebeard

Which I proposed at Permalink #5?

cringes Yes, lol.

Originally Posted by kach22i

Have you seen some of the things forum user Sven posted?

http://ecomodder.com/forum/showthrea...car-20303.html

That is an AMAZING chart. Saved it for reference!

Originally Posted by kach22i

If you are a "if it's not broken - then don't fix it" type of guy, then why are you posting here?

A Civic is broken if it can't easily beat 40 MPG on a tank, and my Civic is down to 36-37 even with hypermiling (due to short, cold engine trips). Therefore my Civic is broken and I need to fix it.

Originally Posted by kach22i

Enough talk, just build something.

Just waiting to get my timing belt replaced before modding.

Originally Posted by Ecky

A good example is a pickup truck bed. It's usually the case that drag is lower with the tailgate up, rather than down.

I know my Civic isn't a pickup truck, but just curious, is this because having the tailgate up creates an enclosed area that contains the airflow more?


Long post, I know..hopefully y'all can find all my responses to your posts in here.

[freebeard]

cringes Yes, lol.

S'okay. I come to Ecomodder because people pay more attention to what I say here than in real life, in general. Sometimes I even get thanked. Unheard of.

I know my Civic isn't a pickup truck, but just curious, is this because having the tailgate up creates an enclosed area that contains the airflow more?

Yes, tailgate up is a weak version of the half-tonneau, which is close to equivalent of an aerocap.

Originally Posted by aerohead

Mercedes-Benz used a combination backlight-header deflector...

I'd stopped using backlight in favor of window. I now feel justified in reverting. (It's all DayLight Opening)