Coefficient of Drag

[Tom Freeman]

Any one know the coefficient of drag on a C3? I have seen numbers for the late C3 (80-82) but do not remember them. But what I am interested in is the early C3s, 68-72 and then the 73 to see if the new nose made any difference.

Anybody aware of these numbers?

tom...

[Juliet P.]

Re: Coefficient of Drag

Tom, I do not have the Cd numbers you have requested, but I would like to mention that manufacturers on occasion vary the reference areas when calculating drag coefficients, so make sure you also find out the S so that you're not comparing apples and oranges. Perhaps someone who has followed the automotive aero development more closely can comment on the use of reference areas.

You probably know this already (but maybe someone out there reading doesn't), but it's the total drag in lbs on the vehicle which matters, (Drag = q * S * Cd) not just the coefficient, where q = dynamic pressure = 0.5*denity*velocity*velocity . :)

~Juliet ...who must now go off to class to get piled higher and deeper with equations in compuational aerodynamics / acoustics for rotorcraft (not corvettes unfortunately).




Juliet's 1970 Corvette

[Jerry Clark]

Re: Coefficient of Drag

Hi Tom:

I do not have the numbers you seek either but an interesting drag story you might, I hope,, enjoy regarding front end drag relating to new design aerodynamics. I worked with a former British Leland design engineer who was involved in the E type Jaguar project. Seems all the E Type pilot cars, when tested, would clock a consistent 155 mph top end. When released to the public, owners complained of a 150 maximum. Retesting netted the same 155. Engineers scratching their heads everywhere. Turned out to be the British standard license plate on the street cars, i,e, big flat front end surface.

jerry

[Duke W.]

How about some C2 numbers.

Juliet gave us the formula for total drag force, which is a function of the square of velocity. I might add that the required horsepower is a function of drag times velocity, so the horsepower requirement becomes a function of the CUBE of velocity.

I've got an article where Larry Shinoda admits that the C2 has a Cd of about 0.5. In addition, it has a high lift coefficient, and this along with the lack of rear suspension anti-squat increases the pitch angle with speed. My C2 is a pretty wild ride at 150 MPH. It is basically stable, but the front end lift reduces steering response. As long as there's no significant cross wind and the road is straight you're okay.

I heard years ago that the C3 Cd is worse than the C2 because the "coke bottle" shape is not Cd friendly, however, I expect is has less lift because of the lower front end and vestigal air dam.

A few years ago I interviewed Chuck Jordan as part of a reseach project for a book and discussed the situation with him. Chuck is a former GM VP Design and worked for Mitchell in the sixties, but he is also technically trained with a BSME from MIT. I recalled to Chuck that the C2 was tested in the Cal Tech wind tunnel by Dr Peter Kyropolis, a well known aerodynamacist of the era. Chuck said that back then there was no feedback loop back to design, and the models back then were crude solid blocks that did not simulate the air flowing THROUGH the car. (Ford had the same problem with the GT-40. Wind tunnel data indicated the original configuration should have run 205 MPH but it only did 197 in still air. They ultimately found that the internal ducting including a complex driver ventilation system was costing 40 BHP more that calculated, and the ventilation system was simplified.) Beyond that, Mitchell was the czar on design and wouldn't change it despite Duntov's effort. Duntov knew the car was aerodynamically flawed from the Stingray racer program, but Mitchell just wouldn't budge. I recall reading that the C3 air dam was added by Duntov because of the marginal cooling with big blocks.

Back in the era when our cars were designed very little was known about "low speed aerodyamics" of ground bound vehicles. It was the energy crisis of the seventies that motivated more detailed studies in the search for better fuel economy.

Today's C5 Coupe has a claimed Cd of 0.29. Testers report excellent stability at speed and since the Coupes all run at least 172 up top, I believe the number and reports.

Duke

[Everett Ogilvie]

Re: Coefficient of Drag

Lots of interesting stuff here in these posts. I have to admit that I have forgotten most of these things, even though fluid dynamics was my specialty in electives. (I do remember though, why the dimples are on a golf ball...) Anyway, the C2 items about drag and lift are interesting, especially since I have been reading lately about the Grand Sport cars in the 60's. Every photo of those beasts at speed looks like your typical MD80 angle of rotation at lift- off. Come to think of it, if you look at a C2 in side profile, it does look kind of like an Eppler 203 airfoil...

[Roberto L.]

Re: Coefficient of Drag

I remember to read that early C3 cars showed a poor 50 CX despite their fluid lines, with later front spoiler in 1980, they reached 40 to 45, still well over the present 30 coef. in modern cars. Well things like the rear portion of the doors are anything but aerodynamic, but when you see the entire car it doesn't matter. Roberto, NCRS #30019, RMC

[Jerry Clark]

Re: Coefficient of Drag

Not only is Roberto a gentleman and scholar but he has exceptional taste as well.

jerry

[Duke W.]

Golf ball dimples

Refresh my memory on those dimples, Everett. I was just thumbing through my newly arrived April R&T and noticed the new MR2 Spyder has them on the plastic insert in the rear quarter that forms the engine compartment air inlet. Something to do with breaking up the laminar boundary layer, perhaps?

Duke

[Juliet P.]

Re: Golf ball dimples

I don't know about the new car dimples shown in R&T, but the dimples on the golf ball provide for a Kutta condition, enabling circulation, so that the ball can create lift and actually fly. It's the same principle which stitches on baseballs allow those to fly too. Lift = rho*velocity*circulation Basically they trap a small amount of the boundary layer in rotation, which is the mechanism by which the circulation is created. The dimples only create the Kutta condition for rotational objects. My guess is that the new cars have them for possibly acoustic, weight, structural or possibly just styling reasons. :) ~Juliet


Juliet's 1970 Corvette

[Duke W.]

Laminar or turbulent

So I surmise from your post that the laminar boundary layer thickness is increased rather than decreased (?). Here's a related condition. A few years ago I was checking out a BMW 325is at Vasek Polak BMW in Hermosa Beach (CA). I noticed there were "bumps" ( the exact opposite of our "dimples")on the forward face of the outside rear view mirrors. I asked one of the saleman about them. He was from the old country. (Vasek liked to hire guys from the old country - got 'em cheap.) He replied: "Zer isss nusing wrong wit dis carrr." That's about as far as I got with him. So can any of your AEs out there help and aero challenged ME to explain the "bumps". I don't think they're there for structual or styling reasons. I alway figured that surface features such as dimples or bumps have something to do with managing the boundary layer transition from laminar to turbulent.

Duke

[Tom B.]

F/m = a .......

.......is what I always thought worked best. :-) TBarr #24014

[Wayne M.]

Re: How about some C2 numbers.

Duke --- I too was curious about the C2 drag coefficients; checked my library of road tests circa '63, and the best I could find was a "no comment", when asked by a magazine auto reviewer. Guess that with the open competition with Cobra at the time, it was considered classified info.

[Duke W.]

A car from another world

The article on the Cal Tech wind tunnel testing is in Corvette News Vol.5 No.6 It was published in late '62 and was the intro for the new Sting Ray. No numbers were quoted. The Shinoda interview was published in the March 1995 Sports Car International. It's worth typing the key paragraph.

Looking back, Shinoda admits that the aerodynamic aspects of this design weren't very good. "GM's windtunnel engineers, attempting to justify their theories, spend untold sums studying the 1963 Corvette. What they found was a drag coefficient of 0.53 and a vicious pitching moment [substantial lift in front, heavy downforce in back] In essense it was a flying machine!"

By our interest in vintage cars, we sacrifice modern engineering and technology, but sometimes it's worth it. I've owned my SWC for 37 years, but every time I see one I can't stop looking at it. It's like something from a parallel universe that I just can't reconcile with common experience. One things for sure, Wayne, nothing that ever rolled down the road on four wheels looks like that car, and nothing ever will, again.

[Roberto L.]

Re: F/m = a .......

Be quiet Tom... Newton is observing :) Roberto, NCRS #30019, RMC

[Bill Braun]

From the old country - almost

Duke,

Coincidentally, my BMW dealer sent me a brochure extolling the excellence of the BMW design team. The side mirror dimples you describe are actually mentioned in the brochure. The purpose of the dimples is to cause a slight amount of turbulence, thereby eliminating a persistent and highly annoying high-pitched whistle at Autobahn speeds. Autobahns, contrary to popular belief, are generally limited to 130 kmh, but there are long stretches where the speed limit is suspended (Germany only!) and it's hammer-down from that point on.

Bill Braun 33186