> This is an interesting web site, it doesn't list the Berlina or GTV >>>> but it list the '71 Spider at .380cd and a '90 Spider at .410cd -I >>>> guess that big rubber spoiler was just there for "looks"! >>>> >>>> Or--perhaps to trade off a bit of drag against less uplift at >>> speed--which is generally what spoilers are intended to do. >>> >> >> With that reduction in lift comes a reduction in induced drag. A car >> with an effective spoiler will have a lower drag coefficient than the >> same car without the spoiler. If an aerodynamic device creates >> downforce at the expense of additional induced drag then it's a wing, >> not a spoiler. >> > > Spoilers "spoil" or disrupt the flow over a surface to reduce the amount of > lift made--without regard to their affect on drag. A spoiler's sole purpose > is to create a region of "separated" flow downstream. The separated flow > region has low momentum--low velocity--and that means that the pressure > inside of that region will generally plateau, or remain constant. Without > the spoiler, the flow in that region would maintain higher velocity and > hence lower pressure, generating lift, if it's an upper surface. Spoilers > found on the back of cars, like the spoilers on Spiders, rarely do anything > at all. On the Spider, the spoiler is operating in a huge region of > separated and recirculating flow--with the top up or down. It has no clean > flow going over it. It's strictly ornamental. On a car like a NASCAR, the > bottom of the car is rather clean and allows a significant flow to pass > between it and the ground. Looking at the car from the side, you'll see > that the front end is low, with a small gap between the chin and the road. > The back end is higher, with a significantly larger gap to the road. In > cross-section, it's a crude, upside down airfoil, and generates > ground-effect downforce. The spoiler helps to increase the "camber" or > upward concavity of the whole body. Picture an airplane wing with a plain > flap turned upside down. And so in that application, the spoiler helps to > increase the ground effect downforce that the body makes. But it's also not > operating as a spoiler. It's operating as a flap. True spoilers are found > on jet wings. They're located inboard, toward the fuselage, and are used to > both control the airplane in roll and to reduce the lift that the wing > generates, when needed. Even in that application, the spoiler, when > deployed, increases drag--dramatically. The reduction in lift does reduce > induced drag. But the creation of large regions of separated flow more than > makes up for it. Separation consumes huge amounts of energy. On cars, it's > responsible for about 95 percent of the drag and it's where 95 percent of > the fuel goes. Generally, "chin spoilers" on cars aren't spoilers at all. > Instead, they operate like the Kruger flap on the leading edge of a 727's > wing, providing a solid landing spot for the leading stagnation point. In > the process, they reduce the separated flow over the leading edge of the > hood, and reduce drag. On a race car, they also help to reduce separation > of the flow as it enters the gap between the car and the ground, smoothing > the flow under the car and aiding in making the most ground-effect downforce > possible. Yes, even cars that *aren't* designed as ground-effect cars can > generate ground-effect downforce. > > Anyway, the spoiler on the Spider is just for looks, like the little wings > on the Honda CB900F and CBX, and the whale tale on the Porsche 930. Well, > actually on the 930 it served a purpose. It provided a space to mount the > intercooler. > > The spoilers on an airplane wing will only increase drag if they're deployed beyond some critical threshold. Certainly if a spoiler goes to 30 degrees as a speedbrake or 60 degrees as a ground spoiler, there will be a lot of drag. But a big reason for using spoilers (rather than more aggressive aileron deflections) for roll control is because a few degrees of spoiler will reduce lift on that wing and induce a bank, without the additional drag you'd get from additional aileron deflection. To look at it another way, if an airplane suffers a disconnection of a spoiler actuator, the spoiler won't just lie flat on the wing, it will "float" up a few degrees. That position is where drag is minimized.
For a well-documented automotive example of spoilers reducing drag, we don't need to look any further than my own '87-91 Porsche 928, which boasts a drag coefficient of 0.34 and a pretty obnoxious looking spoiler. Contrast that to the original 928 of 1978, which was aerodynamically identical aside from the lack of a spoiler, and demonstrated an unimpressive 0.41 drag coefficient. (Intermediate model years had a small lip spoiler, not unlike that on a Milano Verde, which reduced the drag coefficient to < .4, although I don't remember the exact value.) -Joe -- to be removed from alfa, see http://www.digest.net/bin/digest-subs.cgi or email "unsubscribe alfa" to [email protected]
