Some Thoughts on Aerodynamic Shapes

[NeilBlanchard]

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



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.

[Cd]

Originally Posted by NeilBlanchard

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



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 ?

[3-Wheeler]

Originally Posted by NeilBlanchard

...The Volvo 3CC concept car is also similar to this idea...

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.

[3-Wheeler]

Originally Posted by NeilBlanchard

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

Hi Neil,

Take a look at the car below...


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.

[thatguitarguy]

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...

[3-Wheeler]

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!!



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.



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

Jim.

[Christ]

I love that VW. I'd rock it.

[3-Wheeler]

Originally Posted by SVOboy

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

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.

[aerohead]

Originally Posted by 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.

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..

[aerohead]

Originally Posted by thatguitarguy

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...

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.