Biggest gains are at the back???

[aerohead]

Originally Posted by Sven7

1987 GM Sunraycer by Tyler Linner, on Flickr


Oldsmobile Aerotech by Tyler Linner, on Flickr


1988 Pontiac Banshee Concept by Tyler Linner, on Flickr

Notice that these all have smooth, rounded noses.

I taped the gaps on the Probe for awhile and found no noticeable improvement. However if the nose were reworked I'm sure it could do better. It's just that the time/money would be better spent making a full, smooth belly pan.

there's no doubt that the nose helped on Sunraycer,although it was a consequence of shrink-wrapping the occupant while integrating into the solar array.Sunraycer would be nothing without its tail.
On the AEROTECH,I have no Cd for the short -tail depicted,although a member posted Cd 0.19 for the Long-tail version.No better than Mercedes Boxfish,although great compared with the Cd 0.57 March INDYCAR hidden under the body (downforce really kills you at the pump).
As to The Banshee,I've never seen drag data published for that car so we learn nothing from GM on that one.

[aerohead]

*Hucho credits the wheel-less Aptera with Cd 0.05
*Sailplanes are governed by skin friction drag
*Automobiles are governed by separation drag
*Cd 0.12 and less are routine numbers for streamliners on 'real' tires at Bonneville.
*Sunraycer scored Cd 0.089 with wheels fairings
*the 1957 MG EX 181 registered Cd 0.12 on 'real' tires
*Sharp edges are to mitigate shockwave drag for ballistic vehicles.
*Ogival noses can begin to play an important role with automobiles at 250 mph as the car enters the compressibility effects of transonic flow
*NUNA is a purpose-built,body-in-white design for the specific purpose of winning a closed-course,limited velocity competition
*the 'Template' is a streamlining tool to correct flow separation with production automobiles operating on public thoroughfares.Wind tunnel investigations have demonstrated Cd 0.12 with vehicles with the 'Template' aft-body.
*Sub-0.12 coefficients of aerodynamic drag can result from wheel streamlining.

[Ken Fry]

Hi Aerohead,
Good observations. I imagine you are thinking about more than you are actually writing. For example, you mention the wheel-less Aptera/Morelli shape which (several sources agree) is a .05 -.06 shape, as is a typical bomb, torpedo, etc. But what you did not mention is that the actual CD with wheels, suspension, ground proximity and drive shafts is at least .15, and the X Prize test results suggest a little higher than that -- although I don't have an accurate frontal area.

*Sailplanes are governed by skin friction drag

Sailplanes are all about lift/drag, and fuselage drag is a small part of the overall drag. Induced drag is the big part. If you were writing only about the fuselage, then there is a lot more than just skin drag. It would be simple to make a sailplane with a cylinder for a fuselage, but such a shape produces, (for the fuselage alone) a Cd close to 1. So I think your statement is a little shorter than it needs to be to be valid.

*the 1957 MG EX 181 registered Cd 0.12 on 'real' tires

[COcyclist]

Originally Posted by Ken Fry

My old Citroen SM was a reasonably good fit for the template, but gained very little from extending the rear into a full boat tail, vs the existing Kamm back. Its Cd was around .30 either way.

Ken, did you test the Citroen with a full boat tail? Was there a difference in mpg with or without?

[3-Wheeler]

Originally Posted by Ken Fry

.....There are many reasons sailplanes look the way they do. There are many reasons solar racers look they way they do. It is not coincidence that they look very "clean" at the front, as well as at the rear. It is no coincidence that solar racers have impossibly skinny tires......

I agree.

It's interesting, but when you think about it, the tail of the vehicle does not matter one hoot without attached air at the front....

This is important, so I will say it again in a different way.

On a properly designed vehicle, the tail "will" give the greatest benefit aerodynamically, but not without a properly designed front.

Sounds simple and somewhat trite, but very true non-the-less.

So when it comes time to start aero improvements to a vehicle, the best place to start is at the front....

The tail will come, in it's own time!

Jim.

[Ken Fry]

Originally Posted by COcyclist

Ken, did you test the Citroen with a full boat tail? Was there a difference in mpg with or without?

Yes. I followed the slope of the trunk floor and the hatch, and the taper of the sides. (The SM body was already on its way to a boat tail, with rear track 10" narrower than front.) I'd say the fairing in of the tail with the body was "pretty much OK". The surface was insulation board. I'd tufted the rear window and flow there was pretty well attached in both cases.

At the time (late 1970's) this was a pretty casual experiment. I didn't have access to really accurate fuel flow equipment, so used two 50 mile runs, filled at the same pump location at the same station, etc. and didn't observe any difference. At the time, I was half expecting to see a slight increase in drag from the tail, being, at the time, a half believer in the Kamm effect, but also thought that the flow was well enough attached (at least to the rear window to benefit from a boat tail. There is a little bump at the center of the trunk lid that I faired in crudely. Perhaps removing the bump (not something I would do for a test, of course) would have helped.

Far to many variables at play to say what its real effect might have been. I picked a pretty still day, but could have had a slight tailwind on one trial and a slight headwind on the other. (Although both trials were out and back, but on a widely divided highway.) I think there was very little effect on this car that was already pretty clean and close to being boat-tailed... But I don't have a scientific leg to stand on.

[COcyclist]

Cool car Ken!

File:Citroen SM in Stockholm rear.jpg - Wikipedia, the free encyclopedia

I do admit thinking they were a little odd looking back in the day. Now, thanks to Ecomodder, I understand what they were doing with that design.

[jime57]

I have always admired them and thought them attractive for the form which follows function. But then again, I think the Gen 1 Insight is one of the prettiest cars ever built ;-)

[Ken Fry]

Originally Posted by COcyclist

Cool car Ken!

File:Citroen SM in Stockholm rear.jpg - Wikipedia, the free encyclopedia

I do admit thinking they were a little odd looking back in the day. Now, thanks to Ecomodder, I understand what they were doing with that design.

At the time, they were often called the world's most aerodynamic production car. A slightly later model, the CX, was named for coefficient of drag, Cx. The SM's Cx (or Cd) is sometimes quoted as .26, although I remember .30 being usually seen. (But this article shows it higher. The article makes the good point that the value depends strongly on the wind tunnel used.)
Aerodynamics


A glutton for punishment, I had the SM and a DS21 at the same time. Both were fun to drive and own. Almost everything was different than in other cars. The SM had self-centering steering with just two turns lock to lock. The assist would vary with road speed, a new idea at the time. The self- centering worked even at a standstill, so you could crank the wheel over to full lock, release the wheel, and it would center itself.

The ride height could be set anywhere from about 4" clearance to 12" clearance ( and it self-levelled to adjust for any load). The extremes were used for jacking the car: you raised it all the way, put a special jack stand on a boss on the chassis, and then lowered the car all the way, and the support-side wheel would be off the ground (lifted by the antisway bar).

I used to lower the car when I parked. Then I'd come out, and if there were people around, I'd get in, start the car, set the ride height back to normal, and then get out. The back, being lighter always came up first. So I'd walk around to the back and pretend to lift that end. I'd have enough time to then walk to the front and pretend to lift that end. Got some great looks!

The brakes were operated by a servo valve instead of a master cylinder. (Brakes, steering and suspension were all operated by the central hydraulic system.) There was a mushroom-shaped button on the floor that moved a total of about 1/4 - 1/2 inch. This had a feedback loop that pressed the button toward your foot according to brake pressure. Worked very well but took a few stops to get used to the quite powerful brakes -- which recorded shorter stopping distances than all other cars of the time in some tests. It was fun to have friends drive the car: most would weave all over the road, over-correcting steering, and many would come to a few quite abrupt stops.

Then there was the great looking four cam Maserati engine, which performed pretty well and didn't catch on fire with every start... just a few. Three dual throat Webers were used with velocity stacks hidden by the aircleaner plenum. No chokes, just mixture enrichers -- which I think may have been manually operated. Lot's of overlap and low velocity through big throats lead to some spitting back and an occasional backfire. But what's an Italian engine without some drama?

Mine had US headlights: just 4 ordinary fixed-mount sealed beams. You could not have covers over headlights back then in the US. The European lights were covered, so the whole front of the car was glass. There were 6 lights and the middle two steered and self-levelled, so that under hard braking, you could still see out ahead.

[aerohead]

Originally Posted by Ken Fry

Hi Aerohead,
Good observations. I imagine you are thinking about more than you are actually writing. For example, you mention the wheel-less Aptera/Morelli shape which (several sources agree) is a .05 -.06 shape, as is a typical bomb, torpedo, etc. But what you did not mention is that the actual CD with wheels, suspension, ground proximity and drive shafts is at least .15, and the X Prize test results suggest a little higher than that -- although I don't have an accurate frontal area.



Sailplanes are all about lift/drag, and fuselage drag is a small part of the overall drag. Induced drag is the big part. If you were writing only about the fuselage, then there is a lot more than just skin drag. It would be simple to make a sailplane with a cylinder for a fuselage, but such a shape produces, (for the fuselage alone) a Cd close to 1. So I think your statement is a little shorter than it needs to be to be valid.

The MG, Bonneville streamliners, the .143 Sunraycer, etc all taken together show that there is something about them that is dramatically different than a Malibu -- it takes more than tapering the back to make a really low Cd.



I think you meant to say, perhaps, that sub .12 Cd's can result from wheel streamlining if everything else has been optimized. The MG's wheels are already quite well streamlined, and the body is extremely well streamlined, front and rear, but still it is not sub .12. The Aptera had well streamlined wheels but is a long way from sub .12. The VLC has well-streamlined wheels but is also a long way from .12

As of 2010, the .143 number for the Sunraycer still stood as the lowest recorded at GM.



This makes it sound as if one merely puts a boat tail on a car according to the template, and a Cd of .12 results. I imagine that's not what you intended. My old Citroen SM was a reasonably good fit for the template, but gained very little from extending the rear into a full boat tail, vs the existing Kamm back. Its Cd was around .30 either way.