Questions about this whole downforce spoiler issue...
02-28-2003, 10:40 AM
#1
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Questions about this whole downforce spoiler issue...
I've been thinking about how the TT now has a spoiler because of the butt tending to lift up. What exactly is this caused by, the body shape design or something to do with the design on the underbody? (I'm sure both contribute at least a little)
My main thought about this, is that if the problem is the overall shape of the car, does the Roadster need the spoiler to the same degree as the Coupe does? Audi puts the spoiler on both, but I'm wondering if the difference in the bodies are enough that the spoiler may not actually be needed on the Roadster as much as the Coupe, or maybe it's needed even more so?
Thoughts?
My main thought about this, is that if the problem is the overall shape of the car, does the Roadster need the spoiler to the same degree as the Coupe does? Audi puts the spoiler on both, but I'm wondering if the difference in the bodies are enough that the spoiler may not actually be needed on the Roadster as much as the Coupe, or maybe it's needed even more so?
Thoughts?
02-28-2003, 10:50 AM
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the overall shape of the car.
The car acts like an airplane wing: the air travels longer over the car than under, so it creates a vacuum that pulls the car upward. I'm not sure what the aerodynamic characteristics of the roadster are, but I would think it has about as much lift as the coupe (the air bounces off the windshield over the cockpit). Except I don't know if it flows down to the wing as well as on the coupe. Would be interesting to see the roadseter in a wind tunnel...
HTH - I'm not expert in aerodynamics...
HTH - I'm not expert in aerodynamics...
02-28-2003, 11:12 AM
#7
Aerodynamics of a TT (brief explanation)
Bernoulli's principle helps explain the lift issue with the rear of the Audi TT. Consider two air molecules next to each other before the TT approaches. Now consider that one of the air molecules travel over the top of the car while the other travels under the car. Both molecules of air wish to meet up again after the car passes by. In order for this to happen the air molecule on top must travel faster than the molecule on the bottom. The distances on the upper surface is much greater than the virtually straight path on the underside. For air to increase in velocity the pressure must drop. Therefore a pressure differential forms; higher pressure underneath versus lower pressure topside. Hence this is lift.
The TT is a bit more complicated than this simple example the the principle is the same. The curvature of the rear causes air to "stick" for a greater period of time. The path is increased so the air must move faster, lowering the pressure at this point. This creates a pressure gradient and consequently lift at the rear section of the car. The spoiler actually causes the air to separate from the surface of the car soon, reducing the pressure differential and hence reduces lift. The underside of the TT is also designed to manage airflow and to stablise the car.
The TT Mk I was unique given that the lift at the tail assisted with the handling characteristics of the car. The original TT had very sharp handling characteristics caused by lightened tail that help neutralize the forward-leaning balance of the car. Unfortunately, not too many drivers had the necessary skills or experience to handle a sudden application of oversteer. This is why Audi added the spoiler and revised the suspension. The original was great in the hands of highly skilled driver but potentially dangerous in the hands of the inexperienced or the surprised.
Cordially,
John J. Szobocsan
NM TT #11
p.s. I designed aircraft in a former life.
The TT is a bit more complicated than this simple example the the principle is the same. The curvature of the rear causes air to "stick" for a greater period of time. The path is increased so the air must move faster, lowering the pressure at this point. This creates a pressure gradient and consequently lift at the rear section of the car. The spoiler actually causes the air to separate from the surface of the car soon, reducing the pressure differential and hence reduces lift. The underside of the TT is also designed to manage airflow and to stablise the car.
The TT Mk I was unique given that the lift at the tail assisted with the handling characteristics of the car. The original TT had very sharp handling characteristics caused by lightened tail that help neutralize the forward-leaning balance of the car. Unfortunately, not too many drivers had the necessary skills or experience to handle a sudden application of oversteer. This is why Audi added the spoiler and revised the suspension. The original was great in the hands of highly skilled driver but potentially dangerous in the hands of the inexperienced or the surprised.
Cordially,
John J. Szobocsan
NM TT #11
p.s. I designed aircraft in a former life.
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02-28-2003, 12:01 PM
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Brief explanation is right...
I don't mean to jump on your s**t here, but...if you designed aircraft "in a former life" then I would expect you to be the last person to use the "Bernoulli Oversimplification" to explain the way an airfoil, lift, and/or downforce work. Even if stated metaphorically, molecules of air do not have intelligence, emotions, or feelings, so they sure as hell can't "wish to meet up again after the car passes by". This "principle of equal transit times" has been shown to be totally inadequite.
If you don't consider all of the factors involved...i.e. the aerodynamic (Newtonian) vector forces, center of aerodynamic pressure, and a bunch of things beyond my limited knowledge of the subject, you are falling into the same oversimplification trap which ultimately caused BMW to flip two cars on the Mulsanne Straight at LeMans a few years ago, and withdraw entirely from the race because of it.
Read: http://aeroman.de/html/the_popular_explanation_of_lif.html
and: http://hallphysic.com/index.php/Mode/product/AsinSearch/0964680629/name/Stop%2520Abusing%2520Bernoulli%2521%253A%2520How%2 520Airplanes%2520Really%2520Fly/browse/14554/page/1
Cheers
If you don't consider all of the factors involved...i.e. the aerodynamic (Newtonian) vector forces, center of aerodynamic pressure, and a bunch of things beyond my limited knowledge of the subject, you are falling into the same oversimplification trap which ultimately caused BMW to flip two cars on the Mulsanne Straight at LeMans a few years ago, and withdraw entirely from the race because of it.
Read: http://aeroman.de/html/the_popular_explanation_of_lif.html
and: http://hallphysic.com/index.php/Mode/product/AsinSearch/0964680629/name/Stop%2520Abusing%2520Bernoulli%2521%253A%2520How%2 520Airplanes%2520Really%2520Fly/browse/14554/page/1
Cheers