Some Thoughts on Aerodynamic Shapes
 

Back in September of 1976, Cycle magazine published an aerodynamics article, written by Kevin Cooper from the National Research Counsil in Canada.

While the jest of this article was written regarding road going motorcycles, many of the basic concepts for lowering air drag can be applied to any vehicle shape. With this in mind, data from this article will be presented here.

Figure 10 shows a typical 'elliptical' form and the resultant flow path around this shape.

http://ecomodder.com/forum/attachmen...1&d=1259337975

Figure 11 shows what happens to the efficiency of an airfoil shape when the tail is truncated to various lengths.

http://ecomodder.com/forum/attachmen...1&d=1259337975

Figure 12 is a very useful graphic. It shows two possible ways of truncating the shape of an aero device when the length is severely limited. Note how this shape can easily make our external mirrors more efficient.

http://ecomodder.com/forum/attachmen...1&d=1259337975

The next graphic shows how the overall drag in lowered as the length/width ratio approaches 4:1. For non-optimal shapes, the length/width ratio usually needs to be longer to achieve minimum drag.

http://ecomodder.com/forum/attachmen...1&d=1259337975

Figure 14 shows the affect of radiusing the leading or trailing edge of our aero shape.

http://ecomodder.com/forum/attachmen...1&d=1259337975

This last picture shows an actual application for the graphic in Figure 12, where the trailing edge of our aero shape is closing to reduce the flow separation to the minimum for this length.

Blue arrows highlight the spot on this fairing where the shape is narrowing past the ends of the handle bars.

http://ecomodder.com/forum/attachmen...1&d=1259337975

This fairing was made around 1990 using the principles of Kevin Coopers article, and is still valid today.

I hope you find this useful. :)

Jim.

Do you have an online source of the article? Was there more to it?

Interesting.

FYI, the article's author is an EcoModder member and has posted occasionally.

I ask because I have thought about the Mirror thing, reshaping the trailing line of the mirrors to close down the wake a little bit more, so that it reaches the "apex" of the shape and begins closing, rather than continuing to open past the aft edge termination. I dunno how much it would help compared to taking the mirrors off, but it might provide an option to those who aren't comfortable without mirrors?

Thanks for posting this! Do you have any more pictures of your fairing from the outside, etc?

It was good meeting you are hybridfest, by the way!

I'd love to read the full article if you can provide it. :thumbup:

Hi,

The only copy of the article that I have, is a scanned copy of the magazine in TIF compressed format. To get the graphics from the article and post here, I opened each TIF page and cropped the graphics, then converted to JPG, scaled down to 600 pixels in width, and posted.

For the article posted on-line, check...

http://picasaweb.google.com/threewhe...erAeroArticle#

Jim.

I think this is a great approach: The parts of the bike that have to stick out - mirrors, turnsignals - are incorporated into the fairing that covers the bars and levers. It's very smooth and comfortable, and with the grip warmers and warm gloves, my hands stay warm down into the 20s.

thatguitarguy -

I like that bike... it'd be a little better (minute amount) with a WindJammer type fairing, defeating the flow to the rider's legs. Also, fairing in the saddlebags to create a slight taper at the back would (in theory) help out with rear end aero, which is where the biggest gains are to be had. I'm thinking about putting an aero-ish tail on my 185, as well, since I'm doing a 1 up seat and incorporating a trunk into the back rest, and the sides of the bike are a point of concentration, due to the aspect ratio a bike has (width/height). The ideal shape is basically to turn your bike into a fish, but I'm not going quite that far. I'm just going to close up unnecessary holes, try to smooth out the sides some, and fair in the rear end for storage area and hopefully some decent aero. I hope to match theycallmebryan's numbers, at least.

Hi,

I was about to post a thought that I was having about ways to do boattails, related to their length, and it seems this thread is a good fit?

Since the top and the sides of a boattail can be up to 15 degrees, and the bottom is best if it is limited to 4 degrees, this means the car needs to have a height that is ~5/8 the width IF the boattail comes to a point.

But since we need to have the two tail lights spread apart by some reasonable distance, this increases the need to have a shorter height even than that.

So, what is a shade tree aerodynamics person supposed to do? Here's a possible solution: divide the trailing shape of the car into two parts!

Take the lower portion and taper the "top" and sides and bottom as much as possible to end the back in a "line" i.e. like the back of the Aptera.

And take the remaining top portion and taper the sides and the top at the maximum angles so it closes to a point -- but keep it joined to the lower portion.

We seen this in many cars: the 1939 Maybach is one (though it is quite different than my little xA). The original UrSaab is another example. The Volvo 3CC concept car is also similar to this idea; except it truncates it far too early to form anything like a boattail:
http://a332.g.akamai.net/f/332/936/1...oncept.500.jpg

Am I explaining it properly? I hope to photograph my xA from above to be able to better illustrate (and test) this idea.

Okay, I've done a quick and dirty 2D orthographic projection that attempts to show what I am proposing:

http://img.photobucket.com/albums/v7...rtBoattail.png

This is still ~6' off the back of the car, but it has a much smaller rear fascia, and should have a much smaller wake. The top and sides of the upper (red) part are ~15 degrees, and the sides of the lower (blue) part are ~9 degrees.

Sort of resembles the boat tail on the .....um.... Space Shuttle ?

Hi Neil,

Thanks for the picture of the Volvo. I love the shape of the roof line of this concept car. It sort of reminds me of the Insight, but with more aggressive taper toward to rear. To my eye, very organic.

Let us know how your design works out.

P.S. I've posted more motorcycle tunnel testing articles for those who are interested. There is also an article on how Cycle magazine used to do coast down testing to capture Cd's of various motorcycles. It's really interesting reading.

Jim.

Hi Neil,

Take a look at the car below...
http://ecomodder.com/forum/attachmen...1&d=1259532361

I found this photo on AutoSpeed. Notice how the smoke plumes move across the top of the car. And see how they drop away from the trailing edge, which are shown by the two red lines, and are at 8° and 15° from horizontal.

Then take a look at the plume from the bottom of the car, as it is very flat with hardly any angle at all for several feet past the bumper. It seems to reinforce your design angle of 4° or so.

Jim.

The smoke gives you an image of a "virtual" boattail.

Some people are using vortex generators to pull the wind down over steeply inclined rear windows in order to have a more effective rear wing.

I've been thinking of an experiment that I'd like to try combining vortex generators before a kammback that would be steeper than ideal in order to shorten the "virtual" boattail effect, and hopefully make a shorter kammback more effective and eliminate the need for a full boattail.

I don't have spreadsheets full of mathematical equations to back up my seat of the pants theory, but cardboard is free and I'm unemployed, so I've got some time...

Ran across two more interesting shapes in Automotive Engineering International magazine.

The first one is the new VW L1, which is being shown at the automotive show in Frankfurt.

Notice the air splitting device for the front wheels? It seems that they are attempting to divert more air *around* the outside of the car, than underneath. That car is tiny!!

http://ecomodder.com/forum/attachmen...1&d=1259691641

The same type of air splitter is shown on the new BMW as seen below. Looks similar to what Mike is doing on the AeroCivic.

http://ecomodder.com/forum/attachmen...1&d=1259691641

Based on this, I might have to change the shape of the front tire deflectors that are stock on the Insight, next year.

Jim.

I love that VW. I'd rock it.

Hi,

Well it took a while to find these old photos, but after scanning in copies from a photo album, you now have something to look at...

Picasa Web Albums - jsmosher - Motorcycle-Fa...

Jim.

angles
 

I would like to comment that,we might think in terms of how far along the ideal aerodynamic pathway a particular body is,and the included angle possible at that point.
I we toss out rules of thumb ( which I'm guilty of ) and angles,we run the risk that they'll be taken out of context.
The shaded template that Darin posted on his Permanent Kammback for the Metro,or the Aerodynamic Streamlining Template are tools designed to allow members to evaluate their vehicles potential for attached/separated flow,when compared to the "teardrop curve."
From Mair's research,it appears that 22-degrees is the maximum included angle that will support attached flow.You will notice that the "template" "finishes" at 22-degrees.
That angle cannot be realized if everything ahead of it is not a gentle sweeping curve of ever-decreasing radius of curvature.
Kamm was curving in the bodies of his K-cars as much as he was curving down the roof.Again,he was using very subtle and generous curves.
And if you study the photograph of Kamm's K-3 car pictured in the archive,you'll notice that the flow does separate over the last 1-foot of roofline,as the curvature is too steep for the air to follow.
Same for EV-1,same for Ultralite,same for Opel Calibra,etc..

theory
 

I think your theory is okay.Comparing how much Mitsubishi was able to "cheat" on their Lancer with the VGs may give you insight into how much you might be able to cheat on the tail.

Another interesting shape is logged today.

This is the second time this winter that this shape has appeared on the car, and this time a picture was taken before it melted and fell off.

The first picture shows a very interesting taper from the front tire, and is about the shape of something that I would expect to see from wind forces alone.

http://ecomodder.com/forum/attachmen...1&d=1266201029

The second image shows just how "narrow" the front edge of the ice really is, and this surprised me a little bit.

Although this "pointy" shape has been seen before, it has not been on a car, but on the leading edge of the Yamaha TZ750 road racer fairing at Daytona in 1977. The leading edge of the fairing looked almost exactly the same. Yamaha must have done some wind tunnel testing on that old fairing.

http://ecomodder.com/forum/attachmen...1&d=1266201029

This shape may make an excellent example of how to do this in foam/fiberglass when the time comes.

http://ecomodder.com/forum/attachmen...1&d=1266201902

Jim.

Jim -

Is there any way that the tire could have carved in that narrow shape? It looks pretty close, turning the steering may have cut into the shape that was originally there.

I see the same shapes on my car, and I hope to someday be able to make a pair of wheel farings in that shape.

Also for the rear wheels. As it is shaped by the wind, I belive it's the optimum shape for at leat your car.

Suspect I will be a slightly different shape from car to car - so go and take photograps of those Ice cones on different cars.

You know you're an EcoModder when "snuds" are fascinating (name I've heard for the snow/mud things that build up behind the tires).

EcoSnudder??
:D

I suppose it's possible that the tire would rub on the "sculpture" when turning, but certainly not when going straight forward.

There is almost the same gap between the tire and snow, as the normal wheel well has, and even when the wheel is turned, there are several inches of a gap to the liner.

As far as the back tire, the skirt prevented me from getting a good look, but substantial snow did accumulate in the cavity, and caused some noises when the car was first moving this morning. I sounded like something had broken loose under there until stopping to check things out.

Jim.

Two more data points

http://farm3.static.flickr.com/2772/...08b58727_m.jpg


http://farm3.static.flickr.com/2781/...a5a403d4_m.jpg

Wow, tires kick **** up and it sticks. May wonders never cease? :rolleyes:

It seems that Yamaha must have also kicked up some dodo, to come up with exactly the same shape back in 1976.

Maybe the bike ran off course, almost high-sided, ran through a cow-pie, came back on course, finished the race, and the crew chief Kel Carrithers came up to Kenny Roberts and said...

Holly crap Kenny, look at that work of art, right behind the front tire!!

Of course, the very next year at Daytona, guess what showed up as their latest "improvement" to the fairing, but a sculpture of dodo attached to leading portion of the fairing, and painted bumble-bee yellow to match the rest of the fairing. No need for foam and fiberglass here. Why not stick with the real thing.

It must have done *something*, because Kenny won the 200 miler that same day!! Was it crap that gave him the advantage? We'll never know.

Wonders never did cease, even back on that fateful day in 1976!!

Jim.

My type of luck, I'd carefully craft something like that shape, go drive in cold slushy weather, they'd get covered in a thin layer of "Snud", and some dam kid would walk up and try to kick them off! :mad:

Wow! Frank kicks s*** up and it stinks. May wonders never cease?;)






(Frank, that was wide open, how could I resist?)

LOL Chaz... I see you're another social pessimist, not unlike myself!

Several months back, Automotive Engineering had this picture of a Porsche Carrera.

It shows the air flow through the wheel well, similar to what AeroCivic has mentioned in his design.

http://ecomodder.com/forum/attachmen...1&d=1276622269

In the same magazine was shown a picture of the VW Up!

http://ecomodder.com/forum/attachmen...1&d=1276622269

Jim.

This is a picture of CFD analysis for the Chevy Volt.

The article went on to say that nowadays, the preferred method of measuring air drag is with CFD.

Building a clay model with the same level of detail that can be achieved with the CFD is cost prohibitive, and the CFD gives the results in a reasonable time period.

http://ecomodder.com/forum/attachmen...1&d=1278957176

The grill was detailed in such a way as to meet the cooling requirements on the hottest day in Arizona for example.

http://ecomodder.com/forum/attachmen...1&d=1278957176

Jim.

...looks like the "back-end" of the VOLT could definetly use more aero work!

Probably. Then again, when we only get to see data from the symmetry plane that offers us limited understanding of the 3D wake and overall drag of the vehicle. The head-on dispersion lines are helpful. Seeing the same from the rear would be really nice. :)

Cheers
KB

Yeah. Did you see the video of the Volt engineer bragging about how great the aero was and that the 1/2" lip "spoiler" on the back added 1/4 mpg? BWAHAHAHA!

back-end
 

A neighbor is planning to buy a VOLT.At some point we can run my trailer behind it to look for a difference.He's a pilot and a good friend of his is an ultralite designer so he's kinda 'gets it.'
Should be fun! West Texas is installing an enormous amount of windpower generating capacity on top of what it's already done.We have a shot at zero-carbon commuting staring us in the face.Not quite Santo Island,Denmark,but way out ahead of nothing.

Here are several pictures of the Nissan Leaf.

The first picture shows just how small this car really is.

http://ecomodder.com/forum/attachmen...1&d=1285262929

The second picture shows the adjustable inlet for cooling, presumably partially for controlling electric motor temperatures. I would imagine this inlet is computer controlled by the main computer.

http://ecomodder.com/forum/attachmen...1&d=1285262929

Jim.

If GM's cfd lofting is so great, why does the Volt have such sucky drag in this day and age?

I always look at the front corners of the vehicle, where air flow is fastest and therefore drag per square inch is greatest of anyplace on the vehicle. If there are lumps & bumps or nooks & crannies there (as Volt has), it tells me the designers are pikers.