Help me understand aero: rear corner radii, yes or no?

[MetroMPG]

I'd seen that PDF before, but forgot about the rear radii reference. (How's that for alliteration )

Comparing tested Cd values for configurations E & F, all else being equal:

0.350 (rounded rear corners)
0.365 (square rear corners)

So: if you've got a square shape, and no boattailing, rounded rear corners are better.

But boattailing is better than rounded rear corners:

0.238 - 0.242 (tests with full vs. partial boattail - although this isn't directly comparable to E & F because they were using a 3/4 length undertray on E & F vs. full-length undertrays on the boattail runs).

What it doesn't answer is whether rounding the ends of an otherwise properly boat-tailed or Kammback shape would net a similar reduction in Cd (I think not).

BTW, welcome to the site NewsFlash.

[NewsFlash]

Thanks MetroMPG.

Originally Posted by MetroMPG

This is one question I've never been able to answer to my satisfaction:

Regarding transitions from the side of the rear quarter panels of a vehicle to the rear-facing areas (ie the plane that the licence plate is on), what's better for lower Cd: transitions constructed with generous radii, or transitions that are sharp?

Another archived NASA file (1.61 MB) shows the details of that same boattail seen in the previous pdf file. It doesn't give a degrees radius, but does show the lengths at the base of each curved surface as well as the overall length.

http://www.nasa.gov/centers/dryden/p...ain_163113.pdf

Obviously, the truncated boattail is the way to go as the Cd ratio of the truncated isn't much different than the full boattail. Hope this helps.

[NewsFlash]

MetroMPG,
I've thought about it and agree that the sides of the boattail do not need to be curved when the height of the boattail is reduced from being as tall as the one on the van referenced to above. Then you would wind up with a boattail like on a boattail nozzle on a jet. File is 400 kb.

http://techreports.larc.nasa.gov/ltr...aa-99-2670.pdf

I reference this file mainly for the boattail's shape, they put air pressure through the open nozzle, and airspeeds around the nozzle are at least 0.6 Mach and greater (supersonic).

Imagine a boattail nozzle shape on the back of a pickup truck!

[MetroMPG]

Another good link, thanks.

It seems to me one reason their full boattail didn't work as well as it could have (you could see flow separation in the telltales) is they continued to curve the planes away from horizontal/vertical as they were extended; the tip of the tail was dramatically more than the often-quoted 10 degree optimum angle.

Their results are consistent with what Kamm discovered though: a properly shaped, truncated boattail will be nearly as effective as a full boattail.


You should post a thread in the "introduction" forum to say hello and tell us a bit about yourself. Obviously aero is one of your interests.

[broodlinger]

I'm new here, but I just wanted to say something that should cut through all the confusion. A famous quote: "Most cars would be more aerodynamic if driven backwards." That is, a big flat rear surface (such as a hatchback) creates the most drag.

Question, what is the most aerodynamic object in the world? An airplane wing. How is it designed? BIG in front and TAPERED in back. It doesn't matter how you split the air, but you have to *put it back together* properly.



Let's use the Geo Metro as an example. The air going over the top suddenly gets sucked down by the flat hatchback and swirls around. That's drag.



Now let's drive the car backwards. The sloped engine body forms a smooth airflow at the "rear" of the car.



Thus, the rear of the car should ALWAYS be tapered. I guess you call this boat-tailing?

With the caveat that generously rounded side-rear transitions are also less stable in cross-winds than sharp rear angles.

Well, that makes sense, if you taper the rear, you're making the car longer, and thus the side larger, so crosswinds would have a bigger area to hit. But how often are crosswinds an issue in everyday driving?

What I see on a large number of ultra-sleek concept cars are side/rear transitions which look literally "chopped" - sharp 90 degree corners

Market forces. A "sharp" trunk is what customers *expect* to see, but it's not aerodynamic.

Ford Probe V:



Probe driven backwards:

[Dynamically Aero]

I can't vouch for the validity of this, but more than once I read that the Pro Stock Pintos of the early '70s were more aerodynamic when going backwards. Given how high they sat in the rear, I don't know. Does it sound plausible?

Maybe you Pinto fans can find some better photos. I wonder if the same aero peculiarity applied to Gremlins and Vegas, those other members of the economy car elite.

[ELF]

Is the AMC pacer the ultimate drive backward aero car ?

/

[broodlinger]

Originally Posted by theclencher

There's nothing I love more than a pic or drawing of an object with "airflow" lines going around it!

Well, here's a quick reply:

http://www.av8n.com/how/htm/4forces.html

"A non-streamlined object (such as the flat plate in figure 4.9) can have ten times more form drag than a streamlined object of comparable frontal area (such as the one shown in figure 4.10). The peak pressure in front of the two shapes will be the same, but (1) the streamlined shape causes the air to accelerate, so the region of highest pressure is smaller, and more importantly, (2) the streamlined shape cultivates high pressure behind the object that pushes it forward, canceling most of the pressure drag, as shown in figure 4.10. This is called pressure recovery."

"Any object moving through the air will have a high-pressure region in front, but a properly streamlined object will have a high-pressure region in back as well, resulting in pressure recovery."


Another quote from the same article:

"At high airspeeds, parasite drag is dominant and induced drag becomes almost negligible."

[GasSavers_BluEyes]

I'm pretty sure that everyone here already accepts that a boat tailed rear is the best for low drag, but that's not always practical in the real world. Things like the need to fit four people and luggage in a car that will also fit in a garage get in the way...

The main point if the thread topic is that when the ideal shape cannot be used, what should be done with the trailing edge. Since tapering the rear at the ideal angle is usually not possible, a hatchback is actually a very good shape for a car of a given length and useful interior volume but should one chop the edges off sharply or with an asthetic (but not always aerodynamic) rounded edge.
I find it interesting that you use pics of one of the Ford Probe concept cars. It was designed specifically as an exercise in aerodynamic design and has one of the lowest (if not the lowest) Cd values for a car. I'm also not sure that it would better driving that car backward. Given that it has a nice sharp Kammback cutoff I would expect to see some disturbance caused by driving that tail into the wind. That disturbance would then lead to 'dirty' airflow over all the rest of the car causing excess drag. I'm also curious about your example for the Metro. The angle from the roof to the windshield isn't exactly an ideal boattail and might cause turbulence over the entire hood area. Are there any Metro owners intrepid enough to try coastdown tests forward and reverse?

[broodlinger]

>I find it interesting that you use pics of one of the Ford Probe concept cars. It was designed specifically as an exercise in aerodynamic design

I used the Ford Probe because GeoMetro mentioned it in his article, and because the pics were easy to find. The Probe is an extremely aerodynamic car, but the chopped-off rear is disturbing to me, and noteworthy to make a point.

>I'm also curious about your example for the Metro. The angle from the roof to the windshield isn't exactly an ideal boattail

That's true, it isn't ideal. There's a sharp angle between the hood and the windshield that could be a problem.

After all, there's more to mileage than just aerodynamics, such as weight and number of cylinders. On the latter two points, hatchbacks win.