2011 Prius to get full boat tail w/ faired trailer

[aerohead]

Originally Posted by jimepting

Phil, would you please give me a reference to the appropriate section in Hucho's book. I haven't been able to find it, though I have little doubt that it is somewhere in that vast book

I'll dig 'em out as time affords.

[jime57]

This is the sort of thing which makes me really suspicious of Cd data measured and published by people who have vested interests, as most have through time:

Aerodynamics and Efficiency Through the Air – Honda Insight, Toyota Prius, and Chevy Volt :: Auto Upkeep Blog

I strongly suggest that we really don't know much about the precise Cd of many cars past the theoretical objects such as Lange's theoretical car. (Which I think I recall that Hucho verified with only slight difference.) Folks with vested interests produce biased results. Some of the data is too old and too unreliable and some of the data is pretty clearly biased. Chevy certainly doesn't have a very credible record when it comes to dishing out numbers. I trust Hucho's VW tunnel data, but any derivative data is questionable IMHO.

This is not to say that our reasonably proven techniques, such as the template, and tuft testing, and the obvious such as grill blocks and mirror removal won't get us better MPG, but we won't necessary know what Cd we started with and we won't know what Cd we gain.

Sorry for the rant, but I just can't contain myself when I see this sort of thing published.

[KamperBob]

In all science and engineering there is inherent risk of over simplifying something complex like aero down to a scalar number like Cd, especially without any tolerance such as measurement variation or confidence bounds. Reporting Cd = 0.25 +/- 0.1, for example, would be more useful. How many fellow EMers have spanked their EPA rating big time?

[jime57]

Originally Posted by KamperBob

In all science and engineering there is inherent risk of over simplifying something complex like aero down to a scalar number like Cd, especially without any tolerance such as measurement variation or confidence bounds. Reporting Cd = 0.25 +/- 0.1, for example, would be more useful. How many fellow EMers have spanked their EPA rating big time?

I couldn't have said it better, and I am an engineer! In addition to the confidence bounds issue, you also have to question the choice of measurement method. GM talks about a "new" method of measuring Cd. I think it is pretty safe to say that they will not have chosen a method which doesn't help separate them from their competition. I'm glad to have found at least one soul who agrees that published Cd numbers might not be as reliable as we have thought Some of the "old" numbers have been proven wrong with more modern technology. How they got "wrong" we can all speculate, but I have my own personal explanation

[aerohead]

Originally Posted by jimepting

This is the sort of thing which makes me really suspicious of Cd data measured and published by people who have vested interests, as most have through time:

Aerodynamics and Efficiency Through the Air – Honda Insight, Toyota Prius, and Chevy Volt :: Auto Upkeep Blog

I strongly suggest that we really don't know much about the precise Cd of many cars past the theoretical objects such as Lange's theoretical car. (Which I think I recall that Hucho verified with only slight difference.) Folks with vested interests produce biased results. Some of the data is too old and too unreliable and some of the data is pretty clearly biased. Chevy certainly doesn't have a very credible record when it comes to dishing out numbers. I trust Hucho's VW tunnel data, but any derivative data is questionable IMHO.

This is not to say that our reasonably proven techniques, such as the template, and tuft testing, and the obvious such as grill blocks and mirror removal won't get us better MPG, but we won't necessary know what Cd we started with and we won't know what Cd we gain.

Sorry for the rant, but I just can't contain myself when I see this sort of thing published.

there is an SAE Paper in which wind tunnel calibration models were carried form continent to continent and run in the various tunnels,with published results and explanations for the differences.I would suggest that there is a fairly full accounting for the quanta developed.
Every year these quanta are peer-reviewed by representatives of all the worlds auto makers.They can't just pull numbers out of thin air and be able to hold their corporate head up.

[jime57]

Originally Posted by aerohead

there is an SAE Paper in which wind tunnel calibration models were carried form continent to continent and run in the various tunnels,with published results and explanations for the differences.I would suggest that there is a fairly full accounting for the quanta developed.
Every year these quanta are peer-reviewed by representatives of all the worlds auto makers.They can't just pull numbers out of thin air and be able to hold their corporate head up.

Phil, would you be willing to copy the paper and send me a copy? I'd reinburse you for copy and mailing costs. Or provide me a link where I can purchase the paper?

I'd love to read the paper.

Jim

[aerohead]

Originally Posted by jimepting

Phil, would you be willing to copy the paper and send me a copy? I'd reinburse you for copy and mailing costs. Or provide me a link where I can purchase the paper?

I'd love to read the paper.

Jim

Jim,I'd be happy to.Send me a PM with address and I'll copy and mail.Email is problematic 'cause I rely on Al to do this sort of thing and he's plenty busy.

[jime57]

I just "discovered" this earlier thread on the subject of Chevy potentially fudging Cd numbers. I wish I had found it before I let loose on Phil. Much of what I tried to discuss has been pretty thoroughly covered by others. Sorry Phil.

http://ecomodder.com/forum/showthrea...0-a-11313.html

Now that the Volt number have been adequately cast into doubt, I want to make a small case on the EV1. From the one Cd ref I found, it appears that they claimed .21 for the car. I saw one of the cars at the Green Drive Expo in Madison a month ago. (As I understand it, someone managed to retain this one, less drive train.) I have to say that again I don't trust GM's numbers. I try to relate what I see on the EV1 to the Insight 1. I see considerable similarity as far as Cd development goes. They both have mirrors, and closed rear wheels. Their front wheel openings are well rounded, with perhaps a small advantage to the Insight. They both use aero wheels. They have rather similar stagnation points at the front.

Certainly the EV1 benefits by having no front radiator opening, but this benefit, according to Hucho, is only about .02 over the Insight 1 setup. (See Hucho. edition 4, figure 4.97)

The EV1 is handicapped by being too fast at the rear, while the Insight 1 is pretty darn ideal, IMO.

Bottom line is that I don't think GM can justify the .21 for the EV1, not that it really matters is a practical sense. It only matters to those who are interested in theory and modding.

[aerohead]

I did a workup with the plan view photo of the EV-1.I'll post it when I can.I think it will explain a lot about GM's numbers.
I have a plan view of the 1st-gen Insight also,there's really no comparison.
Also,when you compare windshields between the two cars you'll see a distinct advantage with the Impact/EV-1 architecture.
PS,EV-1 has a pretty sweet underside.

[dremd]

So much win in this thread.