Some Thoughts on Aerodynamic Shapes

[3-Wheeler]

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.



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



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.



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.



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



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.



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.

[Christ]

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

[MetroMPG]

Interesting.

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

[Christ]

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?

[SVOboy]

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!

[theycallmeebryan]

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

[3-Wheeler]

Originally Posted by Christ

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

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.

[thatguitarguy]

Originally Posted by Christ

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?

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.

[Christ]

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.

[NeilBlanchard]

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:


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