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Old 01-12-2021, 01:35 AM   #1 (permalink)
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The hidden *real* drag coefficient

Most Cd figures are taken at zero yaw. This is why, on road cars, it's not a great idea to look only at that number. This video also has a lot of implications for testing in different wind conditions.


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Old 01-12-2021, 02:19 AM   #2 (permalink)
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Thanks. I recall driving my parent's Rambler station wagon on the Oregon Coast Highway in an 80mph 90° crosswind.
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Old 01-12-2021, 02:23 AM   #3 (permalink)
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Quote:
Originally Posted by freebeard View Post
Thanks. I recall driving my parent's Rambler station wagon on the Oregon Coast Highway in an 80mph 90° crosswind.
It wandered a bit? If so, that relates more to Cl and Cy than Cd.

So important in the real world, irrespective of what Aerohead argues here....
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Old 01-12-2021, 03:46 AM   #4 (permalink)
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I remember being there, but not the details. IIRC I was surprised that the steady crosswind wasn't hard to counter. It was an open road with no gusting.
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Old 01-12-2021, 04:25 AM   #5 (permalink)
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Quote:
Originally Posted by freebeard View Post
I remember being there, but not the details. IIRC I was surprised that the steady crosswind wasn't hard to counter. It was an open road with no gusting.
Yes 'steady' is relatively easy - a constant yaw moment correct by constant steering correction.

Gusty is much harder.

My experience is that moving the lateral centre of pressure backwards (i.e. rear fins) makes an absolutely startling improvement.

Rear fins will return, mark my words...
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Old 01-12-2021, 05:54 AM   #6 (permalink)
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Excellent video.
Thanks for covering the paper on trains.

When I added some vertical fins to the back of my box cavity, I intially thought of having them at a slant ( think SR-71 Blackbird ) for just this reason ( side wind drag )

They also present less side frontal area at a slant.

I had asked you before about this, but just to reiterate, how did you adress cross wind drag with your Insight fins ?
Also, what purpose do they serve ? I would guess for stability IN crosswinds ?

I added side skirts to my car, but was reluctant to do so due to the possibility of an increase in sidewind drag.

So in an ideal world, freight trucks would have rediused edges not only on the front behind the cab, but on the sides as well ?

Someday, I may take your advice and do a tuft test on a windy day with strong crosswinds. If and when I do so, I will compare the tufts on both sides of the car.
I'm interested to see the variations !
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Old 01-12-2021, 07:10 AM   #7 (permalink)
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Cd and Julian, I think this paper may be relevant to the side fins.

https://www.researchgate.net/publica...dy_using_flaps
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Old 01-12-2021, 07:39 AM   #8 (permalink)
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Thank you very much for the link AeroMcAeroface !
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Old 01-12-2021, 07:46 AM   #9 (permalink)
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Whenever there's a side wind I see my instant fuel economy rise whenever I pass something that blocks it, like a group of trees, buildings or a noise barrier.
This happens even when the wind is supposed to come from behind, so its relative speed in the wind shadow would be higher than out in the open. More friction just from hitting my car at an angle.

Likewise, if I come closer to a semi the turbulence in the wake may increase fuel consumption while on average the wake moves in the right direction; but the messy pattern kills the orderly flow over the car resulting in slightly higher friction.
When following a semi I may not have to adjust my CC settings for miles on end, because the added resistance slows my car a bit when I come within 40 meters or so. In economy mode the CC allows variations up to 3 km/h from the set speed before adjusting the throttle, which makes this possible.
Apparently 50 to 60 meters is the sweet spot where the wake still has some effect but the turbulence isn't strong enough to mess up the aerodynamics.
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Old 01-12-2021, 01:53 PM   #10 (permalink)
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Quote:
Originally Posted by The Fine Article
The two most efficient configurations are the two flaps on the side edges of the rear slant(-17.6% of drag reduction) and the flap on the top of the rear slant (-15% of drag reduction).
This suggests the Bertone BATs were on the right track.

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