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Old 04-10-2008, 01:38 AM   #13 (permalink)
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Originally Posted by trebuchet03 View Post

I say go for it - do a little research and see what you find... Look for cars with low cD values and start there....

But, I agree with SteveP. It's important, but look at the relative impact. A sailplane is, as Steve said, highly optimized in it's current condition. No doubt, much of their aero problem spots come from their leading edge. As cars are bluff bodies (exception for basjoos perhaps), the trailing surfaces have a much higher significance. Sailplanes are not bluff bodies - which is why their leading surfaces are more important (ditto for solar cars, velomobiles, submarines, etc.).

Now all that said - just because I agree with him... That's not me saying not to do it. If you've got an opportunity to clean something up - I say go for it. You'll no doubt make a difference
If you look at the nose of the Porsche, as seen in the first photo of my post just above, you will see that in front of the hood are the pop-up headlights, a header panel between the headlights, and the bumper cover below. These got whacked when my kid got crossways with a guard rail. If you look closely at the lower photo, you will see that the bumper cover has a sort of step before the contour goes up onto the header panel, rather than a smooth, continuous curve. Aesthetically, this is the only part of the car I don't like. It is sort of like Carmen Diaz' broken nose--just does not look right, or match the lines of the car. Aerodynamically, this can be improved, especially since the fog lights and turn signal lenses are recessed about 1/2" into the bumper cover. (They could be made flush by using spacers.) Those fog lights often are cracked by rocks, so this is not a good place for them to be. The inlet slots feed cooling air to the radiator and to the turbo intercooler. Ducting in is good, but the factory neglected the outlet ducting, so the cooling air is supposed to find its way back into the slipstream somehow.

So, I've considered gluing extruded foam onto the front bumper, then carve and contour it using the lines of the hood and side panels, down to a nice rounded nose at the stagnation point. Then, make an eliptical inlet (long axis of the elipse horizontal), so that the one inlet feeds both radiator and intercooler, as well as oil cooler below, including proper ducting to feed the used air into the front wheel wells. The intercooler sits under the header panel, a few inches in front of the hood. It has a very good diffuser inlet duct, fed from the upper slot on the nose, but has no particular outlet duct. So, I figure making a cowling flap (like in reciprocating engine aircraft such as B-17 or P-47) that is hinged about 4" in front of the hood. Due to air accelerating over the hood, this would be the lowest pressure spot on the car, whereas the inlet at the stagnation point on the nose is the highest pressure point. Fish gill--high pressure in, and Bernoulli suction out and over the hood. This would also bleed off underhood air, which gets heated by the exhaust manifold, etc., and keep underhood temperatures significantly lower. Bernoulli will probably open the cowl flap, but if not, a simple cable arrangement would work. This car also has ducting to cool the front brakes, and that would also be fed from the one eliptical inlet at the nose. Being smooth and with gentle contour, I think such new nose would have significantly less drag than the stock one. And, I would eliminate the pop-up headlights, which are turbulators and speed brakes when up. Instead, covered headlights in the fenders, like Jag XKE or Porsche Boxster.

Combined splitter/front wheel fairing at each front front corner, rather than a single piece type. The Calibra type would probably work and look good. Being two pieces, cheaper to replace if one hits a curb while parking.

Basjoos' car tells us a lot. Even though it has significantly more wetted area than stock, and even though it has some rough edges due to Coroplast, it gets radically better fuel economy than stock, and stock was good to begin with. This says to me that form drag is much improved by Basjoos, even though skin friction was more, for a gross net improvement. Some may say that most of his improvement is due to the extended tail, and maybe so. Still, I cannot help but think his nose job is also a major improvement. I wonder if Basjoos has tested his car with and without the new nose, but keeping the tail. That would be very instructive.

Note the wheels in the silver Porsche 944 Turbo in the second photo. That's what I have. ~30 of us did a 500 mile ratrace through British Columbia several years ago, partly in the rain and mist. You could see the spray and bow wave from each car, as well as its mist wake. Cars were all the same unmodified shape, but wheels were various Porsche types. Mine had the least wake and turbulence off the wheels, by far. Other open-spoke wheels acted as turbines and threw a lateral column of mist ~3-4' aout from the wheels and disrupted the slipstream.

The "flag" mirrors could stand a lot of improvement, and many Porsche guys replace them with "aero" mirrors off the newer 911 cars. I have a set, but have not put them on. I may design my own, using smaller aero mirrors from a motorcycle, and airfoil-cross section stalks, further back from the A pillar.

BTW, if the car is moving along at, say, 60 mph, that means the air flowing over the hood and past the mirrors is going a LOT faster than 60 mph. Sticking a mirror or other obstruction into that high speed air means exponentially increased drag at that location. The extra drag and turbulence makes noise, right next to your ear. Ungood.
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