Coefficient of Drag

[Tom Freeman]

Let's recap here....

We have .52 for the C2 but nothing yet for the C3 and way too much info on golf balls.

tom...

[Roberto L.]

Re: Let's recap here....

I found something in my archives from another post in a mail list. Concerning Cd: 1963=.402 1979=.503 1980=.443 1984=.341 So, from 79 to 80 the effect of new bumper is visible here, assuming this data is correct. All my best, Roberto, NCRS #30019, RMC

[Everett Ogilvie]

Toilet Paper Rolls

My memory from when we had to solve this problem in class is that the dimples "cause early separation of the boundary layer". This in turn reduces the component of drag called pressure drag which is a low pressure area at the rear of the forward moving object. Now Juliet says they may increase or otherwise mangage the boundary layer, which may be true, but I also remember the demonstration of the rotating toilet paper roll, which if done correctly can give lift to the roll as you spin it and propel it forward, without any dimple type of structures. I think that rotation itself can cause lift, and the dimples main effect is to reduce pressure drag, so we can all drive like Tiger. Disclaimer: it sounds like Juliet is currently studying these things, so I defer to her. Besides, I just love these lively discussions when we get them going.

[Everett Ogilvie]

Re: A car from another world

Hey Duke, SWC? Is that a Cobra? Is the S for Shelby? No one commented about the Grand Sports that I mentioned, but I have been having the best time reading the recently re-issued Grand Sport book (available now from our own Eric in Cincinatti - free ad plug for NCRS here), and I think that the Cobras and the Grand Sports duking it out at Nassau Speed Week was incredible. Anyone on this board have direct memories or experiences with either of these cars?

[Everett Ogilvie]

Correction !!! (before Juliet catches me)

The discussion here caused some discussion with some of my old school buds (rather than digging out the text books) and now I am set straight. The dimples cause DELAYED separation of the boundary layer which then reduces the pressure drag (at least I had that part right). I now remember the side profiles of the boundary layer separation with and without dimples and the dimples cause the boundary layer to extend down farther around the back of the ball, reducing the size of the low pressure area and reducing the pressure drag. Sorry Tom, about continuing this discussion about golf balls and toilet paper rolls.

[Tom Freeman]

Ain't this board fun...

This has got to be the best discussion board out there. Can surely learn a lot of interesting stuff here. Will have to try some of these this with the TP, but think will wait until I get home. They may think I have lost it if I start playing with the TP here at the office.

tom...

[Jerry Clark]

Dimples VS Pimples

In an attempt to solve this dimple vs pimple comparison I have emptied six toilet paper rolls, three plain, three with dimples, installed by using a nail file and rotating it to form a divot, (golfbaslllike), for the life of me I can't determine a method to form a pimple while sitting here, (the nailfile was handy). I can't arrive at any conclusions without exhausting, (pardon the term), all possibilities. I will accept any suggestions that will enable to resolve this so I may leave this room.

jerry

[Duke W.]

Re: A car from another world

SWC stands for Split Window Coupe. The car magazines of the era covered the Nausau Speed Weeks, and so did Corvette News. Bob Patterson from Woodside CA was the first to resurrect a GS (#5) and vintage race it in the mid-seventies. As you can probably imagine, the car was utterly awesome, and he invariably won his class. With a thousand pounds less weight and nearly double the power of a production Sting Ray the lift problem was exacerbated. All five GSs are accounted for, but at well over a million dollars valuation, they have been mostly locked up. The last time I saw a GS run was at the Palm Springs Vintage Races several years ago. It was the number two car in '66 Sebring trim as driven by Dick Guldstrand - a roadster conversion with a Mark IV. Doug Hooper was the driver. Doug was the guy who won the production Stingray's inaugural outing at the Riverside six-hour in October 1962, but it was clear that the Cobras were faster. The handwritting was on the wall. Duntov's objective was to build at least 100 Grand Sports so they could be homologated as GT cars, but then management ordered all racing activities to cease. Zora eventually got the five completed cars out to serious racers such as Grady Davis, but they had to run as prototypes. If you ever get a chance to watch the Sebring Lemans start from '64 to '66, the Grand Sports were always the first away from the grid and usually led the first lap. I remember watching them live on Wide World of Sports.

Duke

[Duke W.]

Now we're making sense!

It's beginning to come back to me. Actually I shouldn't be that aero challenged as my graduate work was an attempt to mathematically model multiphase non-steady flow in a carburetor. I'll make this short because I'm concerned that we are about to be admonished by higher authority for going off topic. I'm not sure at this point if a bunch of engineers discussing the aero aspects of dimples and pimples has anything to do with vintage Corvettes.

Duke

[Everett Ogilvie]

Don't Try This At Home !!!!!

Don't want any home injuries due to irresponsible toilet paper roll cuts, etc. Leave these tests to a professional. I tried to get away with our off topic discussions by doing the free ad bit for NCRS and then trying to tie aero discussions to C2's by bringing in the Grand Sports... Do you think they bought it?

[Duke W.]

Fudging the numbers

Like advertised horsepower numbers, advertised aero numbers are subject to manipulation. It's difficult to directly measure a car's frontal area. It should include the outside mirrors, but if you test the car without mirrors then include the mirror area in the calculations, the computed Cd will be lower. Ride height is a significant player too. If you load the car to gross weight, this will lower the ride height and yield a lower Cd.

Duke

[Everett Ogilvie]

P-51

I completely agree with you about the car from another world. Nothing else looks like it. I sort of think of the mid-year coupes as the P-51's of the car world. Nothing ever has or ever will look as good as a P-51 Mustang. The car and the plane are Classics for all time.

[Juliet P.]

Windshield Rake angle effect on drag....

Everett, Good catch... That's what I get for not checking the board in the AM! :D I missed my opportunity! LOL! The idea is to delay separation for minimum drag, unless you're in a dynamic field where under some circumstances you might want to "trip" the boundary layer for flowfield stability. Almost all unsteady aero exhibits some form of hysteresis, and under certain conditions flow can oscillate between attached & detached states, possibly accompanied by vortex shedding at particular frequencies. This can cause some aero-structural dynamics which if it occurs at structural resonant frequencies with the forcing and the response out of phase things can start to fatigue or fail... but I digress (again)....

This thread is absolutely hysterical! And I can just picture you all raiding the stalls at work for empty toilet paper rolls ...BTW a rotating cylinder does create lift so feel free to carry on with your experimentation....

OK, and now to keep it about Corvettes so I don't get myself in too much trouble here.... Has anyone noticed the dramatic change in windshield rake angles since the 60's? I've think that the windshield angle is a strong influence on the corvette Cd, probably as much as the other body changes. Has anyone done any measurements or seen any data on the windshield angles on our beloved corvettes?

And driving with the top down is a MAJOR increase in drag (large separation bubble behind the windshield).... just watch the gas tank & mileage difference! I ran out of gas when I got my first convertible (95 Mustang GT) because my hot rodder "we don't need no stinkin' gas gague" husband had me trained to go by mileage..... (Standing family joke now....) The first 2 weeks of ownership it rained so I calibrated the mileage in my head with the top up... then the sun came out and I thought I had at least another 75 miles to go... There's about a 25%-35% reduction in range with the top down. Hee Hee Hee.... There's nothing like experience to make the lessons sink in. ~Juliet


Juliet's 1970 Corvette

[Juliet P.]

Applied Aero-acoustics...

Great info! I love to see Applied Aero-Acoustics! However I think on my C3 I can go a long way towards eliminating high speed whistles just by getting my windows & ragtop to seal properly! There's a book out about reducing wind noise & sealing water leaks... I'll bet NCRS sells it... Anyone have it? Is it any good? ~Juliet


Juliet's 1970 Corvette

[Jerry Clark]

Hook , Line ...

and Stinker