The hidden *real* drag coefficient

[JulianEdgar]

Originally Posted by AeroMcAeroFace

Cd and Julian, I think this paper may be relevant to the side fins.

https://www.researchgate.net/publica...dy_using_flaps

Wow, what a paper - on spoiler design on hatchbacks as well as side fins.

It's also the first paper I've seen that supports what I have measured on the road - that fins on a fastback measurably reduce drag. (And another piece of supporting evidence for throttle stop testing.)

The drag reduction they quote on the Ahmed body by the use of the side fins is much higher than I measured on my Insight, but put my measurements together with theirs and I am pretty confident in saying that, at minimum, people should test fins if they have a fastback shaped car.

[JulianEdgar]

Originally Posted by Cd

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 ?

My aero mods on the Insight are all to improve driving performance, not to reduce drag. So my undertray and diffuser are to give downforce, my rear spoiler is to give downforce - and my rear fins are to provide better straight-line stability.

That said, I have also tried to do aero modifications that are likely to reduce drag - and in the case of the rear spoiler / fins, they measurably do so (at least in light winds).

I haven't tested the rear fins / spoiler for drag in strong winds, so I can't answer the question about drag in these conditions.

[JulianEdgar]

Please note I have made a mistake in the video. The graph is not correct for the Calibra, that has a symmetrical increase in Cd with yaw.

Sorry about that.

Fun fact: the graph is actually for the Plymouth Superbird, that does have an asymmetric increase in Cd with yaw.

[freebeard]

The asymmetry seems odd.

I am pretty confident in saying that, at minimum, people should test fins if they have a fastback shaped car.

It's unfortunate that Porsche exploited the vortexes for engine cooling with the Beetle.

[AeroMcAeroFace]

Originally Posted by JulianEdgar

Wow, what a paper - on spoiler design on hatchbacks as well as side fins.

It's also the first paper I've seen that supports what I have measured on the road - that fins on a fastback measurably reduce drag. (And another piece of supporting evidence for throttle stop testing.)

I don't get involved in the throttle stop argument, probably not highly accurate but even if the numbers are out by +/-10% it is still a useful technique.

What about sailing, cars can be designed to get some thrust from crosswinds.
https://core.ac.uk/download/pdf/32425613.pdf figure 12
https://en.wikipedia.org/wiki/Midnig...olar_Race_Team
Is it ridiculous to envision some pop up roof sail or wheel fairings that reduce the effect of the crosswind? Or maybe a wheel fairing like the Daihatsu UFE, maybe road cars are too big, with too wide wheels etc.

[aerohead]

I was under the assumption that, all, reported, contemporary drag coefficients, are crosswind-averaged, as per E.P.A. certification protocols, and have been for decades.

[JulianEdgar]

Originally Posted by aerohead

I was under the assumption that, all, reported, contemporary drag coefficients, are crosswind-averaged, as per E.P.A. certification protocols, and have been for decades.

I don't think so, and nothing I can find today in a quick search supports that. But happy to look at any evidence you can cite that supports that.

[aerohead]

Originally Posted by JulianEdgar

I don't think so, and nothing I can find today in a quick search supports that. But happy to look at any evidence you can cite that supports that.

I'll look. Seems like it was Ford Motor, or SAE that mentioned it. They understood that driving in the REAL world involved a statistically-averaged, annual, 7-mph crosswind component and the wanted manufacturers to reflect that in their testing results submitted for new car certification.

[freebeard]

Please do. I'm being challenged for using that number.

edit: ....incorectly.

[jakobnev]

I keep thinking that rear-wheel steering could be used to turn a car a couple of degrees into the wind, but I wonder how comfortable it is driving a car that is a bit off.