Hood vent design.. –
I figure we can continue this here. This is an interesting post i found in response to the Ferrari F50 hood scoops/design..
"Gentlemen,
I do not profess to know all about everything, but I can say with most certainty, that my statement regarding the F50 front fans is absolutely true. I have a degree in Aerodynamic Engineering and have been employing that knowledge to great effectiveness on such vehicles as privateer race cars to classified aircraft. I am well versed in this field.
To be specific, the F50 employs air vents seen by the black mesh in the front center of the car (the vents on either side at the front actually vent the front disc brakes). This vent extracts high pressure air that essentially "builds up" at the nose of the car. The air in this location has higher pressure than any other spot around the car. It is like when you stick your hand outside the window of your car. You feel the air pushing your hand backwards. There is high pressure on the front of your hand and low pressure at the back of your hand (when your hand is oriented with your palm facing forward in the direction of travel).
Although this is the area in which the air is at its highest pressure around the car, it is not stagnated (or brought to a complete stop as in the case with your hand outside the window or the front of a Mack truck). You will note that the nose of the F50 is slightly sloped back and not competely vertical. If the nose were completely vertical, the air in this area would be competely stagnated, or stopped. As a result of stopping the air, it "spills" over the nose at the expense of higher drag (higher pressure). The slight slope of the nose allows for reduced drag.
While the air may be at a higher pressure, it is also at a slower speed relative to the oncoming flow of air not affected by the car. For example, if the car was going 60mph, think of this as though the car were completely still and the air were doing 60mph (the effect is the same, but easier to visualize here), the air at the nose of the car would be reduced to little more than approximately 20-30mph. The slower air is more effective for cooling purposes. Slower air actually extracts heat more effectively from a radiator, in this case two radiators.
In the case of the F50, the now warmer air as a result of heat extracted from the radiators is vented along the nostrals of the hood. This warmer air is less dense than cooler air (think of a sponge rather than a brick). Aerodynamically for automobiles, less dense (warmer) air is desirable and reduces drag, making the car faster. An airplane wing is just the opposite. A wing performs better with more dense (cooler) air.
For a car to produce downforce, one must minimize the amount of air that travels under the vehicle. That is why performance cars are always extremely low to the ground, why they have large front spoilers and large diffusers under the rear of the car to extract any air that does travel under the car. By the use of aerodynamics and the strategic placement of body panels, flares, spoilers, etc a low pressure region is created under the entire profile of the car. This low pressure sucks the car to the ground, also known as downforce. It is actually more of a suction force rather than downforce, but the desired effect is the same.
True downforce is achieved by deflecting air upwards by the use of an inclined surface such as the gentle slope of the hood and the slightly more inclined slope of the windshield. These two surfaces provide most of the downforce for the F50. There are also other minor design features that further contribute to the effeciency of the car. However, if all of the downforce for the car occurred only at the front, the car would be aerodynamically unstable and would be disasterous at speed as it would quickly lose control. That is why Ferrari required the use of the rear wing on both the F50 and the F40. Without this rear wing to help balance out the downforce, the car would not be stable at the high speeds it can achieve.
If indeed the fans were intended to provide downforce, the source of air from which the vents would draw from would be from underneath to further provide the much needed vacuum or suction pressure under the car and not the front. As you will note from pictures of the underside fo the F50, there are no vents whatsoever under the front of the car. The only vents on the underside of the car are towards the rear on either side which are used for oil coolers and to provide some cool air for the engine bay that exits through the rear black mesh.
By removing the high pressure air at the front of the car, Ferrari has achieved further reduction of drag, but has not enhanced or instigated downforce with the use of the fans. To further make this clear, I have personally had the hood and front fan shrouds off of an F50. The fans do not turn fast enough to generate a sufficient amount of "suction" other than to simply enhance air flow over the radiators at slow city speeds. In fact, they do not even come on unless the car is above a certain temperature. One can hear them on startup even when cold, but they do shut off if the engine temperature is not high enough to merit the added cooling requirement.
As for Top Gear Magazine, the claims that the fans are for downforce are entirely false. Despite the fact that a journalist, who more than likely has zero background in aerodynamics, may have been directly told this by someone either by Ferrari or the dealer that lent the magazine the car for testing, that information is completely untrue. I have witnessed many mistakes by various magazines when stating information about various cars.
As for the McLaren, the Suction System was deemed unneccessary. It was too costly, too heavy, and too complicated to be a reliable benefit. Aerodynamically, the McLaren is exceptionally stable even at high speeds. In wind tunnel testing, the car was found to be of exceptional performance for just the right balance of sufficient downforce, low drag, and overall stability.
Suction is not used in any production car on the market to enhance downforce. Such a system is called an Active system, whereas a spoiler or a wing are called a Passive system. Not to mention the fact that all racing body sanctions do not allow any Active Aerodynamic systems.
I don't mean to ridicule anyone. I only state as fact, how aerodynamics truely affect how a car behaves and what the intent was when the car was designed. My appologies for the long post."
Interesting..
