Square shapes for aerodynamics ?
 

How is it that cars such as the Opel g90 ( point 22 .Cd ) the Ford Prodigy, and others such as Cadillacs can have a front end that literally has a completely flat front end with no curvature , and huge gapping grills as well, yet superb drag coefficients ?
Also, there are no wheel covers on their wheels ( which are also open spoked versus smooth. )

I don't understand.

The simple answer would be it's not how much you disturb the flow up front, it's how smooth you leave it behind.

The complete answer can be found by reading the aero forum and the references you'll find in it.

http://www.carstyling.ru/resources/c...opel_g90_1.jpg

I don't think the Opel is as bad as it looks, at first glance. There's quite a bit of roundness to the front in plan view (to my eye), and the front/side transition doesn't look horrible.

On top of that, the designers undoubtedly sweated the "detail optimization" portion of the design to compensate for some styling compromises. EG - partial rear skirts, wheels fill the arches, the wheel covers are fairly smooth, low ride height... there doesn't even appear to be the usual rain channels on the leading edge of the A pillars.

The low cD is probably due to windtunnel data. Surprisingly minor changes to a non-aerodynamic shape can create significant differences in drag. These minor changes are pretty much impossible to find outside of the wind tunnel.

Food for thought: the lowest cD of a wheeled vehicle in ground effect - ~0.15

- LostCause

I'd like to add that .15 is assuming a completely smooth body.

I thought the lowest Cd for a ground vehicle was 0.11, which is what they claim is the Cd for the Aptera.

Rain channels are one of the biggest culprits to poor aerodynamics, especially the way they did them in the 60's.

The back end of a vehicle is what needs to be relatively smooth for good aero. Pressure drag is a killer...

Ground effect is the key part. The Aptera isn't exactly your average ground vehicule. The 0.15 reference is for a basic bluff body with wheels at regular car height. The higher from the ground you go, the lower the Cd will be for a given shape.

Interesting to note how the mirrors are not at the base of the A pillars.

I think that's because the car has quite a bit of "cab forward" design to it - ie. the mirrors are probably the same position relative to the driver as they are in most cars. Just a guess.

I would have guessed the mirrors are back farther for aero reasons - seems to my very non-professional eye like they'd cause far less drag there...

.137 Cd
 

1985 Ford concept with a Cd of .137

All the texts I've read (Hoerner, Hucho, an SAE symposium, and another) claim that 0.15 appears to be the lowest cD for a blunt body, w/ wheels, in ground effect. In the symposium, one professor argued that cD could be lowered further by removing vehicle lift, but the issue was contentious. This was all before CFD, though (1970's).

The lowest theoretical drag shape near ground effect (w/ out wheels) was a shape very similar to the Aptera (~0.09-0.11 mark). The Aptera's wheels are far from aerodynamically clean (two control arms in the breeze). Solar cars supposedly have a cD of 0.09, but they don't seem like blunt bodies to me.

I should add that these might have been 2d shapes (i.e. infinite span). 3d shapes can have lower drag. If that is the case, sorry for being misleading. :o

I believe a slippery slope some might be falling into is claiming cD*A as cD, which is often done and easily confused.

- LostCause

That car looks more similar to the ideal 0.15 shape than a real car. Either the art has progressed (likely), cD*A is being claimed (possible), or we are getting into an issue of semantics (likely). I assume a body is defined as being blunt @ a certain fineness ratio. Also, I don't believe 0.15 was ever believed to be an impenetrable barrier, just a likely limit.

From memory, the ideal 0.15 car and ideal 0.09 GE shape:
http://i264.photobucket.com/albums/i...asicshapes.jpg

Good contributions.

- LostCause

http://www.bikefix.co.uk/i/vector.jpg
Vector-One
cD = .1 even
Frontal Area = .42 m^2

http://thelongestlistofthelongeststu...s4/bicycle.JPG
Varna Diablo
cD = .11
Frontal Area = .183 m^2

http://www.adventuresofgreg.com/hpvl...alRushSide.jpg
Virtual Edge
cDA = .011m^2 (don't have the actual cD value)

All of these are wheeled land based and close enough to experience the ground effect (at least, they are designed to work with that - except for the Diablo which was just sculpted - tested later). I'm only showing human powered ones because a great deal of effort has been put into achieving ridiculously low cDA values :p


Generally, it's not a good idea to place limits on "best" :D 400 years from now, when the star trekian force field is perfected - we'll have the ability to have cD values of .0001 and under etc. :thumbup:

Larry Niven's "sonic fold" from Ringworld.

notice the angled view, the photos never show an x,y,& z view. and you all know why, cause we would have it on our cars faster than you can say, (alt. print screen, paste).

They don't now?

http://www.bernardiparts.com/images/...oto_Fr_Bck.gif

http://www.autoclub.com.au/uploaded_...008-799408.jpg

http://www.autounleashed.com/images/mazda_6.jpg

http://z.about.com/d/cars/1/7/F/b/gm_08malibu_frt.jpg

http://z.about.com/d/cars/1/7/C/b/fm...cuscpe_frt.jpg

http://www.blowupmyride.com/gallery/...serialNumber=1

Sure, they have their subtle differences... Just like the streamliners I posted above... But, they look very much the same (other than the Mazda perhaps - it has a smile sort of look from it's grille) :p

I see no reason why we can't have variation and low drag - for instance

http://oneworldnews.files.wordpress....10/aptera1.jpg
and
http://www.motorauthority.com/wp-con...1Liter_new.jpg



-Images ripped for all over the internets :p

That VW is Sweet!

I've long had an idea for front suspension that would work ideal with highly streamlined cars.

I found some more images of the G90 :[IMG]
http://farm4.static.flickr.com/3237/...21aa27af_o.jpg

I superimposed an image of the Audi A2 over the G90.
The roof angle is identical :
http://farm4.static.flickr.com/3148/...64c7fd1d_o.jpg

I'm completely baffled by the .25 .Cd that the A2 has, since it not only lacks wheel covers, but it also has wheel flares and a huge wake.

I would love to see an image of the underside of an A2. It must be totally smooth.

Just to the right of the tire in this image is a good view of the flat underside on the G90 :http://farm4.static.flickr.com/3240/...bc99bc6f_o.jpg

A few more :http://farm4.static.flickr.com/3255/...b350b664_o.jpg
http://farm4.static.flickr.com/3277/...39aac487_o.jpg

Very, very true. I suppose I shouldn't have claimed 0.15 to be the lowest possible value ever, but I still stand by that it is a realistic (not unbreakable) lower limit for conventional automobiles.

I intended the number to be used more as a guideline, to have people who are at 0.25 realize that the curve is shallowing out as you get closer to practical limits. 0.35 to 0.25 is relatively easy. 0.25 to 0.15 will be much, much harder for any conventional car to acheive. 0.15 and less will either require one-off innovation or the loss of conventional characteristics (i.e. tandem seating, severely reduced occupant space).

My eyes are opened a little wider, though. Thank you. In 400 years, I'll be hoping Scotty will be around to beam me places. cD 0.0001 will be horribly high...:)

Just wondering, how do you guys calculate cD for your HPV? Scale model, moving road surface windtunnels? CFD? Road tests? I know at our school we have a supersonic wind tunnel with a subsonic in the works...definately nothing full scale, though. Do you guys have time at nearby windtunnels?

- LostCause

I guess I'm 'beating a dead horse'by asking this once more, but I keep reading differing information.
One of the aerodynamicists that worked on the EV1 claims that there is nothing to be gained by having the windshield on a car lay back any more than 45 degrees. Also, pointed noses on cars have no real aero effect. As long as the flow transitions smoothly to the sides and top of the vehicle, you can have a bus with excellent aerodynamics.

So why then do I sometimes see comments about laying back a winshield for better aerodynamics ? And why are you guys putting pointed noses on your cars if it doesn't do anything but add weight and get stares ? The cars are not going Mach 1 !

( But then I see comments about a low front end being better for aero than a high front end .......... I'm really confused . )

Square shapes
 

When you consider that Cd 0.11 has been achieved in an actual road-going vehicle,then Cd0.22 doesn't look so good.If GM will get rid of the square front,then they can cut drag by half again,and boost fuel economy by another 25%.

The Aptera ?

What I mean is something with four wheels that looks like a normal car .

The best figure that I have seen for an semi-normal looking car was .16 for the Precept.

( Which to me looks a lot like a Nissan Maxima with wheel covers )
Hmmmm.


So Phil, it sure would be interesting to see some results on a top speed test run at the Texas Mile. ( Down near Houston ) It's expensive though - around $ 250 but it sure beats driving to Utah and getting salt on your chassis .
Can you imagine though if you could see what difference your current aero mods make on a top speed run ?

about G90: from ople source

G90 — Opel’s Three-Liter Car
Concept vehicles such as the G90,
first presented at the Frankfurt IAA
auto show in 1999, play an important
role in the effort to further reduce
fleet fuel consumption. They represent
the state of technology at a
given moment in time and constitute
an important experimental platform
from which new solutions are developed
for large-scale production.
The vehicle’s name reflects its purpose:
“90” refers to the emission
value of 90 g CO2/km. This corresponds
to a gasoline consumption of
exactly 3.88 l/100 km for the fourseater
of the Astra-class.
ITDC engineers were thorough in
taking account of every parameter
which impacts significantly fuel consumption:
air and rolling resistance,
vehicle mass, and efficiency of the
drivetrain. With a drag coefficient of
cD = 0.22, the G90 sets a new
benchmark. Its body is similar to a
teardrop shape with a distinctly sloping
roof and sharply cropped rear.
The smooth, rear-rising underbody,
with its completely integrated
exhaust system, has proved particularly
efficient, allowing the air to pass
the body and merge behind the vehicle
more quickly.
Comprehensive lightweight construction
and the targeted use of aluminum and
magnesium result in an exemplary curb
weight of 750 kg. For the powertrain,
Opel chose a three-cylinder ECOTEC
gasoline engine with inlet port deactivation
and a manual transmission auto
shift (MTA). It is clear that the power
plant specialists in Rüsselsheim consider
the potential of the conventional gasoline
engine to be far from exhausted.
Weighing just over 80 kg, the G90’s
engine puts out 44 kW/60 hp and can
take the full-scale four-seater (with a
further 350 l of space for luggage) to a
top speed of 180 km/h.

Thanks Fabrio.
What frustrates me is that they ( GM /Opel ) could easily make ( and sell ) this car, yet choose not to.

Good find, fabrio!

when I have marked the opel G90 at the "tigrafans" (forum oriented to opel), they have refused the aeshtetic one, beautyful for me.!

I'd love to have a powered version of that Vector-One for commuting. Something like the Alé would be a close second, you know, so I could carpool, but they want to price it like a sports car.
http://www.fuelvaporcar.com/Feb._25_..._bound_011.jpg

The HONDA Dream,solar car achieved Cd 0.10.

Hucho's book addresses windshield angles and diminishing returns.Also,light defraction limits the angle that a windshield can be laid back,as at some point,all you get is reflected interior images and inability to see out.I favor convex windshields,as they divide the airstream for the roof equally,allow for homogenous velocities over top and sides,erasing C-pillar vortices.Expensive to build,however low drag impossible to achieve without.

Cd 0.25 isn't low anymore and is relatively to achieve,has been for 40-years.