I think getting wind tunnel time for a magazine test is going to be impossible for a long time. As I've stated on a couple other c3 aero threads, there's a lot simpler way to get a pretty good answer. Find a really long straight road, and go up to 100 mph or so and put the car in neutral. Have a passenger with a stop watch record your speed over time. The drag coefficient can be derived from that data. If you get to a low enough speed, drivetrain friction could also be estimated. Aerodynamic and (mechanical) friction forces will have a different dependence on speed, so if your data is consistent enough, the two things can be separated out. This is rather dangerous though. Maybe 80-50 or something would work, but its best to get more range, especially up high.

 

If most of the drag is created from under the car, can't we fabricate some kind of lightweight pan to close off the "open" areas?

If I ever do a frame up restoration, I may seriously consider this mod. Access for regular maintenance may be a problem, but if I'm doing a frame-up, I'm sure I'll have a big garage with a car hoist :yesnod:

 

My daily driver (Mercedes 300E) has underbody aerodynamic panels, and despite the top end of the car looking like a box, it has a 0.31 drag coefficient. They're made of plastic, and the only things not covered are the engine and transmission oil pans, and exhaust. The exhaust is tucked into the panels, which kind of puff out to integrate things smoothly. There are cutouts for oil pans. Its a really nice design, and not difficult to work on at all. None of the panels are very large, and they easily come off in a minute or two. I'd also like to see this on a C3 if its possible. I haven't looked under a vette in a long time to see if it is. I'd guess exhaust would be the major problem.

 

Methods other than a wind tunnel still provide too many other variables: Emergency brakes dragging, worn bearings, tail/head winds, variations in road surface, etc. What would be of great interest to me is the direct comparison using the same wind tunnel at the same velocity between a number of C2’s, C3’s shiny bumper, C3’s pillow ends, C4’s, and C5’s. I would imagine that this should be doable in just about any part of the country where a wind tunnel might exist.

There must be a number of wind tunnels across the country that have a lot of open dates on their calendars. It would take about the same amount of time to strap a car down as a chassis dyno, but all you would need is one run. I would gladly pay $50 or more for such a test on each of my cars. Such tests would be considered low speed runs in their book. The 150mph we brag about is little more than the approach speed for landing a jet.
There must be some engineers that are also car nuts at some of the aerospace or aircraft facilities that might inquire about arrangements. They would certainly want a hold harmless if a hardtop or hood blew off. But this would probably be less likely than a tire or engine blowing during a chassis dyno run. Still, better to find your hood would blow off during a wind tunnel run than on the track @ 140mph!

Chuck

 

Chuck, you're right of course that putting all the cars in the same wind tunnel under the same conditions would give the most consistent numbers for cross-comparison, but the problems you mention with a coastdown test aren't hard to (mostly) eliminate. To minimize tail/head winds, just don't do the test on a windy day. A couple mph here and there doesn't matter. Emergency brakes dragging, bearing problems, differential friction, tire rolling resistance etc all gives a nearly constant force with increasing speed. There is a small component proportional to speed as well. Aerodynamic drag force increases with speed squared. This means that (1) drag completely overwhelms everything else at high speed, and (2) one could formally fit a curve which separates out constant, linear, and quadratic force terms. Its a standard thing in data analysis. Its also fairly standard for auto engineers. I've seen graphs of these things reprinted from SAE journals which follow exactly what I just wrote. I have some papers I could look up where people have gone through coast-down tests very carefully eliminating errors.

Some comments on wind tunnels also: They're not magic devices. They can be made to give consistent, but not necessarily relevant numbers. A lot of factors such as ground-related effects are very difficult to model properly in a wind tunnel. Properly averaged coast down tests are more relevant to a vehicle's true performance characteristics.

Next, wind tunnels are very large, expensive facilities. Don't compare them to your local dyno. They are very rare, and very busy. Looking at a list of most of the automotive-capable wind tunnels in the world (there aren't many!), the ones in the US are at: Caltech, Chrysler, Ford, GM, Lockheed, Ohio State, Simtek Inc, and, um, that's it... Some of those aren't even large enough to fit a vette.

 

That list is only current to 1995 or so, btw.

 

aharte, while you and chuck debate the pros and cons of windtunnels and coast down testing, kick this idea around. coefficient of drag is a simple measurment of fluid dynamics, right? couldn't someone who really wanted to know do all corvette generations with scale models in liquid? seems to me there must be an equasion to convert force applied through a liquid media (water) against a scale model and convert that to real numbers on real cars with real air.
i seem to recall many moons ago, when i was a kid, i was watching a show on aerodynamics and i remember that ford was doing such testing. if i remember correctly, they were running about a 1/12 scale car through water at 4mph. they said that 4mph in water was roughly the equivalent resistance at 60mph through atmosphere. the amount of force required to maintain that 4mph was converted to real numbers. now this was before the 'proliferation' of full scale wind tunnels but for our purposes, models are relatively cheap and this should be fairly easy to perform. anybody wants to get frisky, i'll donate a model to the effort.


[Modified by clutchdust, 12:39 AM 6/1/2003]

 

clutchdust, you're right. That works too. I think It would take a lot of work to set up a fluid chamber in the basement. I'd be interested if you could come up with an easy design. I'm not too knowledgeable on what's available for water pumps and such.

 

what i was thinking was some type of trough and using a string to pull the models. something like a fishing reel. just fix a pulley off the bottom of the trough and weight the models to keep them down (of course, that might bring up the issue of lift :lol: ). there has to be a way to measure the resistance required to pull the model for our readings. hell, if we can figure out how to take the measurements, i'll do them in my pool. the shallow end has a smooth, flat surface and it's 12' across, that should be plenty to get a decent reading.

 

You can measure the force by attaching a spring to the fishing line, and pulling on that. You can get little springs with known force/deflection ratings. Or just buy a tension type scale.

Getting the velocity measurement down would be a little harder. I don't have a pool, but if I were doing this, I'd have the line wrap around another pulley attached to an electric motor with an optical tach (a hole in the pulley with an LED and photodiode on either side - measure the pulse frequency). Do you have much electronics experience?

If you can get a high torque motor, you can assume the velocity is constant most of the way, so you could use a stopwatch between known distances.

 

Hmm. I just looked up vette models. Most are 1/25 scale, with some in 1/18. That's bad. To get accurate data (same types of boundary layer separation and turbulence effects), you want similar Reynolds numbers. Given a characteristic length L and speed v, you want (L*v)_air=(L*v)_water*15 very roughly. So with these models, you'd have to go to around 120 mph to simulate 100 mph!

I found this graph giving the drag coefficient of a sphere over different Reynolds numbers . A vette at 100 mph has a Reynolds number around 10^7. You might be able to get your pool thing up to 10^6. Maybe it will work, and maybe not. That's pretty close to the crazy area in the graph. It would still be interesting to try it anyway.

 

ACI makes a cover for C3's that fits around the rear tire carrier and covers the opening there. It would seem that smoothing out this area would help with downforce in the rear and drag where the air gets caught up under the rear body. I don't have a rear tire carrier in my car, so I had planned to glass in the hole in their cover and make a complete cover.

Mark

 

I'm afraid there are still too many imperfections to such model testing. The functional vents on the car are generally not functional on the model. The underside detail is fair, but not the same. Remember my earlier post quoting the efect of a small spoiler and the side vents cut the required power for 120mph in half? Most scale models are just too imprecise.

Chuck

 

I wonder if the cad programs can be exported to a software thats similar to a wind tunnel? I think i've seen em... its been to long since i have though :sad:

 

If you wish to hear it from the master...Duntov wrote up an article in 1968 issue of Corvette News, volume 11, issue 6. Article is also reprinted in "The Best of Corvette News" by AQ.

He has graphs, illustrations of C3s with ink blots stretched from the wind tunnel. he states that in initial testing..."three factors needed further development, lift, engine compartment cooling and passenger compartment ventilation." Sound familiar?

These tests revealed that while they got it right and "clean aerodynamically" it was the forward area and front lift effect that needed further development.

The article doesn't state that these problems were fixed but the car was delayed from 67 to 68 due to the big block cooling problems.

 

68 delayed by Duntov who went to Pete Estes due to multiple reasons including aerodynamics. The very high ft. fenders were cut down & rear spoiler reduced to lip. Side vents were made larger for aerodynamics & cooling.
Ft. spoiler reduced power needed at 120 mph to 105HP.

 

I did. They had a pair up on lifts at a car show i went to this past weekend. What a difference between the two. Makes me glad i don't have to work on a C5.

 

You might not think thier cutting the air real good, But try turning on your head light while your going 80 mph. "DONT TRY THIS AT HOME". Wasn't thinking, did it once. Felt like the front was going to fly apart.

Steve

 

Speaking of headlights. Putting the headlamps up will lower the wind noise if you want to hear your g/f. Of course at speed you should lower for better aerodynamics ...

:cool:

 

That doesn't sound right, I have driven at speeds in excess of 80mph (and much faster too0 and never had a problem with headlights or rattling noises. Mine were very stable. The only problem I had when doing 130+ was that the raincoat canvas top blew off :lol: