The DragBean

[Cd]

Originally Posted by DragBean

@CD

So why would rounding the front NOT decrease the overall drag? Pretty much in any sub-sonic application a rounded approach will allow air to slip around the object far better than a flat face.

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Air works in crazy ways. This car had a Cd of .19 :
The figure was later brought up to .20-.21 i believe.

This car tested out at .201 ...with the hood scoop :


Source : http://www.hotrod.com/how-to/paint-b...-aerodynamics/

Gm tested the EV-1 at their tunnel that gives a reading lower Cd reading than the example I just gave with the Camaro.
So ...That Camaro has a better drag coefficient than the EV-1.

Oh - and an interesting factoid : The guy that tests the Camaro in that article worked at GM on the aerodynamics of the EV-1.

Boggles the mind doesn't it ?

Rounding the corners does decrease drag, but only a slight amount is needed to retain attached flow.



Other times though, a sharp edge is better. You see a lot of new vehicles with fake scoops at the corners of the front ends. Air forms a pocket there and skips around the wheels of the vehicles creating an " air curtain"

It all makes a person just throw their hands up and say " whatevah ! "

[DragBean]

@Cd

Sweet article. I never would have guessed that the 362 HotRod has as low of a Cd as it does. Although it still has a higher Cd than the EV1 - I can see your point - it has a scoop which is most likely creating more drag (maybe 5% for 100 clicks and you get 0.19).

I think comparing the EV1 and the 362 is like comparing apples and oranges. The EV1 has side mirrors which if taken off the Cd for it would drop probably over 100 clicks. Also it doesn't have moon covers - another 50 clicks easily from the production Cd rating.

The 362 also is extremely low. It looks like it 3-6 inches lower than the EV1. There's no way you could run it on anything but salt flats and a flat track. I think that is having a considerable effect on bringing its Cd down to around the EV1.

One big thing worth noting is that Cd changes as your Reynolds number changes. I think comparing Cds of passenger cars works because the airflow that it was measured in is similar. The 362 got its at 250 mph - way different airflow than what the EV1 is operating at.

"The HRM Spl. walked in the door to A2 with a very good Cd of 0.229 with 762 pounds of front lift and 65 pounds of rear downforce at 250 mph. With all the tricks, we got a legal Cd of an incredible 0.201, the front lift reduced to 430 pounds, and the rear downforce increased to 180 pounds. Thats all in the right direction, and it helped us run 261 mph at Bonneville with about 1,000 STP-corrected flywheel horsepower. Now its off to Australia and racing at Lake Gairdner."

Also Cd goes down as speed increases

https://en.wikipedia.org/wiki/Filerag_sphere_nasa.svg

Another thing to note is that it gets harder to lower the Cd once it's already lower. For example a 100 click reduction in a car originally at 0.4 cd versus 0.2 is going to be way easier to do. This isn't called diminishing returns, but similar concept.

Also the 362's cD was measured head-on. If there's any crosswind or you are turning, there goes your magneficient Cd. I don't even think I need to supply evidence that a curved front is going to be better from different approach angles.

So although you might be able to design an aerodyanmic flat front, rounding it like a teardrop is going to be better.

PS When did Walter White get into Drag Racing?

[freebeard]

The aphorism on the Salt Flats is: "It's not how big a hole you punch in the air, it's how big a hole you leave in the air".

Rounding the nose reduces the size of the stagnation point and mitigates leeward separation in cross-wind conditions. But the maximum width on The Template is biased toward the front to give more time for things to settle out downstream.

In some cases features can affect airflow 'upstream' from themselves; for instance base pressure being transmitted forward in recirculating pockets.

[Fat Charlie]

I'd think that windshield would tuck conveniently in that hood scoop's wake.

[freebeard]

Plus it would keep the bugs off.

[DragBean]

I've been very demotivated to post any results after changing my tires and not getting improvements. In fact my mpg started going to low 30s. But then i realizedseafoamed my car and charged the sparkplugs. Now I'm back to upper 30s. Looks like there is hope afterall. I'll post a pic of the old plugs soon. (Well and gas prices are so low too saving money on gas hasnt been as high of a priority)