Car Aerodynamics: Experimental trials not agreeing with lift theory, help

[rcgldr]

As to your statement about spoilers never reducing drag, here's a definition ...

Sorry, I sit corrected, apparently there are multiple definitions for "spoiler" in the automotive world (it's more consistent for aircraft, they spoil lift and increase drag slightly). Wiki's definition differs from the terminology used for race cars. In the link below, note that "spoiler" only refers to the angled wedge at the rear decklid of a car, and those "spoilers" increase both downforce and drag. A wing can produce the same downforce with less drag (as used on the "cars of tomorrow"). Air dam is the term for the wedge at the front bottom of a car. Other than Wiki, I've never heard the term "spoiler" used in reference to the "air dam" at the front of a car, or for wings at either end of a car.

http://www.stagefronttickets.com/nascar/nascar_terms.html

Deflector or turbulator would be a better term to use for what Wiki is including in it's definition of spoiler.

The additional friction on the upward side of the car contributes to drag

Aerodynamic forces are due to displacement of air, and friction only contributes a small component to the the overall displacement of air.

The surfaces on a car are pushing against the air.

Mostly pushing the air forwards. The forwards acceleration of air corresponds to drag. As a car passes through a volume of air, it leaves a void behind that the air accelerates towards from all directions, except the air can't accelerate backwards through the car, and the close proximity to the pavement means there's very little air to flow upwards from below the void created by a car (so ground effects have to occur at the car, not at the void behind the car). The net result is a mostly forwards (drag) and some downwards (lift) acceleration of air as a typical passenger car passes through the air.

The only vertical flow through a car is through the radiator and then down underneath a car, creating lift. Nascar race cars tape up the radiator based on the ambient temperature to minimize radiator air flow.

Even though air is deflected upwards by the forward facing surfaces of a car, the air will continue to follow the car's profile, and flow downwards past the rearward facing surfaces, and the net result is some lift. From the windshield and back, the profile of a typical sedan approximates an air foil that generates lift.

Since most sedan body shapes generate lift, the high powered ones are typically speed limited to 250kph = 155mph to prevent excessive rear end lift. This was an issue with the initial year release of the Audi TT, where several accidents occurred on the Autobahn because the rear end lifted and the cars spun out. They added a "spoiler" to the rear to increase rear end downforce (with the cost of some drag) and adjusted the suspension for more understeer. Here's a link to a video of rear end lift on a RX7 doing 215mph at Bonneville, the rear end lifts and the car starts yaw rotation and then flips over on its hood (driver was OK):

rx7215.wmv

Downforce can be created by pitching the overall body downwards, using air dams, spoilers, wings, ground effects (underbody tunneling). All this results in increased drag though, with the exception of ground effects combined with powerful fans at the rear of a car (like Chaparral race cars).

 

[Idjot]

Sorry, I sit corrected, apparently there are multiple definitions for "spoiler" in the automotive world (it's more consistent for aircraft, they spoil lift and increase drag slightly). Wiki's definition differs from the terminology used for race cars. In the link below, note that "spoiler" only refers to the angled wedge at the rear decklid of a car, and those "spoilers" increase both downforce and drag. A wing can produce the same downforce with less drag (as used on the "cars of tomorrow"). Air dam is the term for the wedge at the front bottom of a car. Other than Wiki, I've never heard the term "spoiler" used in reference to the "air dam" at the front of a car, or for wings at either end of a car.

http://www.stagefronttickets.com/nascar/nascar_terms.html

Deflector or turbulator would be a better term to use for what Wiki is including in it's definition of spoiler.

Aerodynamic forces are due to displacement of air, and friction only contributes a small component to the the overall displacement of air.

Mostly pushing the air forwards. The forwards acceleration of air corresponds to drag. As a car passes through a volume of air, it leaves a void behind that the air accelerates towards from all directions, except the air can't accelerate backwards through the car, and the close proximity to the pavement means there's very little air to flow upwards from below the void created by a car (so ground effects have to occur at the car, not at the void behind the car). The net result is a mostly forwards (drag) and some downwards (lift) acceleration of air as a typical passenger car passes through the air.

The only vertical flow through a car is through the radiator and then down underneath a car, creating lift. Nascar race cars tape up the radiator based on the ambient temperature to minimize radiator air flow.

Even though air is deflected upwards by the forward facing surfaces of a car, the air will continue to follow the car's profile, and flow downwards past the rearward facing surfaces, and the net result is some lift. From the windshield and back, the profile of a typical sedan approximates an air foil that generates lift.

Since most sedan body shapes generate lift, the high powered ones are typically speed limited to 250kph = 155mph to prevent excessive rear end lift. This was an issue with the initial year release of the Audi TT, where several accidents occurred on the Autobahn because the rear end lifted and the cars spun out. They added a "spoiler" to the rear to increase rear end downforce (with the cost of some drag) and adjusted the suspension for more understeer. Here's a link to a video of rear end lift on a RX7 doing 215mph at Bonneville, the rear end lifts and the car starts yaw rotation and then flips over on its hood (driver was OK):

rx7215.wmv

Downforce can be created by pitching the overall body downwards, using air dams, spoilers, wings, ground effects (underbody tunneling). All this results in increased drag though, with the exception of ground effects combined with powerful fans at the rear of a car (like Chaparral race cars).

You've definitely done your "homework" on this topic. Looks like we've both learned something here, me mostly. Now what about the person that started this thread... any of this stuff helping you ?