Split boat tail = less wake area ?

[cdx]

I am trying to reduce the wake area on a Honda Civic hatchback. ( '93 )
From what I understand, the best way to do this would be to create a full boat tail that extends out a good eight feet or so and comes to a point.

Such a design would have several disadvantages such as not being able to use the hatch, poor visibility, weight, and even legal issues.
I know the best solution is to improve upon the Kamm back design by extending the existing spoiler out further.

What I was wondering is if there would be any advantage to creating -two- seperate partial boat tailed extentions that extend from the back of the car.

If the car had two boat tailed extentions for the back of the car, I would assume that the air would seperate into two wakes. The air would then have less area in which it would be turbulent vs. a traditional Kamm back of the same length.

This would have the advantage of allowing the hatch to open and be fully functional, good visibility, less weight, and .... well ..it would still be ugly .

I suppose another solution would be to channel air from the grille opening to exit behind the back of the car, but I suppose the air would loose velocity too quickly to be of any use. ( Yes ? )
Also, this would mean a clever way of channeling the air to the back of the car and having a nozzle setup - as well as converting the car to be a 'bottom breather'.

Next question : I was also wondering what effect that compressing air into a ventauri shape ( on the belly pan, or on a rear diffuser ) has on the drag. I would assume that there would be a large amount of drag produced since the air molecules are being squeezed together.
( If you take a cylinder vs. a cone with a hole in the bottom and place both face first in a wind tunnel,I'm assuming that the cone will have far more drag. )

Thanks for any help !

 

[carla00]

Hi,

I'm probably being a thicko here but what exactly is a boat tail ?!

Regarding a diffuser, yes it will increase drag because you're producing a wider wake. however you can significantly reduce drag by making the underside of the car smoother. having a venturi underneach the car speeds the air up underneath the car which reduces pressure and hence sucks the car the the road which helps on cornering. Drag wise, perhaps it does reduce turbulance slightly but I'm not sure.

If you had laminar flow underneath the car, you could direct the streamline upwards at the rear which would reduce the wake. I'm just speaking theoretically though as I imagine it's not possible to get laminar flow on the underneath of the car !

If you have a look on the question I put on here, there are loads of suggestions of books that I bet you will find useful

 

[IRstuff]

The back end of a boat, e.g., its tapered shape

TTFN

 

[cdx]

A diffuser creates a wider wake ? I must be confused as to what a diffuser is.
I am creating a full belly pan for the car. At the rear of the car just below thw bumper I planned to have several upright sections which will channel the air and 'pinch' it into a much tighter wake.
( Imagine a shape like what I mentioned in my previous post - a cone with a hole at the back. The air would get 'pinched' into a tighter wake.
My question is if this results in much higher drag due to the friction of the air being compressed. )

BTW, Why would it not be possible to have laminal airflow on the underside of a car ? ( with a smooth belly pan )

As far as the split boat tailed idea, Think of the old Lola race cars from the '60s ( or a more extreme example being the 'Mach5' from the old 'Speedracer' TV show.)
Both cars had a central 'pod' that contained the driver and two 'pods' which flanked the driver and contained the wheels .
Now imagine the reverse of this and have the back of the car shaped in this way.
What I'm wondering is if the airflow would follow each of the 'boat tailed pods' and be channeled into a much less turbulent and controlled wake.

Thanks.

( Sorry.
Did I confuse you all even more ? )

 

[carla00]

Hi Again, sorry for not replying sooner. my only access to the internet is at work and hence I can't use it too often !

I think a belly pan will be a good idea, it will result in a significant reduction in drag. However, you can't avoid turbulence under the car due to the road surface and wheels.

Diffuser wise, I've always thought a diffuser was something that has a small cross section where the air is entering and a larger one where the air is exiting, hence the wider wake (but please correct me if I'm wrong !)

I'm not sure about the cone shaped sections at the rear of the car. Yes they will channel the air which will reduce drag. A cone shape like you described will cause the velocity of air to increase through the channels which will result in a reduction of pressure. I'll have to look into it as I can't remember what effect it would have. I'm sure there's a lot on the internet about different shapes in airflow that would be useful. I'll also have a look at the old racing car shape you described above.

Another thing you could consider is using the belly pan to channel the air and direct it into streams at the rear of the car. You could also look at redirecting the airflow around the car instead of underneath (again, this will help with downforce)

 

[Kapitan]

cdx mentioned a 1960's Lola, this is a reasonably up-to-date one;

http://www.mulsannescorner.com/lolab2k40-2.html

It is an aerodynamically successful car, although the design didn't have twin boat-tails the rear wings shape tends towards that. A lot of air came in from the sides, to exit at the rear in order to reduce drag.

 

[cdx]

Thanks for the replies, as well as the link.

I have read that an alternate method of decreasing the size of a cars wake would be to channel the airstream into a smaller area using air jets at the back of the car.
Any clues as to how this could be done ?

( My original idea of channeling the air from the grill opening to the back of the car wouldn't work very well. )

Much to my surprise,I found a study on semi trailers that showed that using angled spoilers ( similar to those seen on the back of minivans and SUVs to keep dirt off the back glass ) actually -increased- the drag even when not angled vertically.
I would have thought that a spoiler similar to this could be angled at a less steep angle so as to channel the air into a smaller wake, thereby reducing the drag.

Does anyone here know why that angled spoilers such as these actually increase the drag ? Other than the slight increase in frontal area ( they have to protrude above the boundary layer of the vehicle ) I can't see why they would have a negative effect.

--Thanks !

 

[cdx]

Think of it this way : Take a spoiler on a car and turn it upside down.
Instead of increasing the size of the wake, it seems to me that the size of the wake would decrease since the air would now be channeled into a smaller area.( but with the expense of added lift )


........................Anyone ?

 

[Tmoose]

Aerodynamics is stubbornly 3D.
Massaging the profile/side view and ignoring the plan/bird's-eye view is kind of like rowing with one oar.
Some makers laid their windshields back 40 years ago, but left big rain gutters (also a body manufacturing convenience) way out there. Look at the rain-gutter treatment (and fender front edge, and rear-fendeer-trunk-tail) on any car in the parking lot today.)

Studebaker Avanti set several speed records at Bonneville. The claim was in part they had an aerodynamic advantage.
Compare the profile view to the fender edge here.

http://www.bobsclassics.com/sold/84a1.jpg

Look at the sharp edged rocker panels on modern race cars and mimicked in "aero-kits." The purpose is keep under car air flow from spilling laterally across the car, or maybe the other way.
One thing deck spoilers do is simply keep low pressure from "migrating" from the license plate and tail light panel (where it makes drag) up to the trunk lid where it would make lift.