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Originally Posted by blue2000s
I'm not sure where you're getting your definitions, but lift and downforce are both descriptions of the same force. The difference being the direction of the positive reference. Wings, spoilers, venturis, and whatever else are used to influence this force, but their use has nothing to do with the terminology.
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Well, I think we're going to have to agree to disagree here.
Think of a car in terms of an airfoil or wing. It splits the air as it passes through it. Some air is forced under the car, while the rest flows over the car. The path the air must take over the car is longer than the air passing beneath and so the air must flow faster over the car than under it. Air flowing faster, drops in pressure.
So, now, you have the air rushing under the car, and because of the lower pressure on top, you actually have a partial vacuum. 'Lift' is actually the difference between these two pressures - the air flowing over and the air flowing under the car. But, more than the car being lifted upwards, it's actually more like it's being
sucked upwards - into this partial vacuum.
Downforce is created when the energy of the oncoming air is redirected upward by a Wing. This imparts a force upon the Wing. Newton's 3rd Law (every action has an equal and opposite reaction) forces the Wing (and the car) downward. But, the Wing operates above the laminar flow of air over the body of the car - air passes underneath it, and so
the low pressure area is still being created. It is just overcome by the
separate and
distinct downward energy on the Wing.
A Spoiler does not allow this Laminar Flow to pass underneath it. It is really an Air Dam which allows the onrushing Laminar Flow to pile up in front of it. This slows it down and so it's pressure rises.
Because there is now no difference between the air above and the air below the car, lift has simply been eliminated, not just overcome as in the use of a Wing. More simply,
there is no Low pressure, so no 'Lift' and so Downforce is not necessary.
Again, we may have to agree to disagree. And in practical terms, it is just semantics to many. But, there is a distinct difference between the existance of a Low Pressure area above the car, and a counteracting force produced to overcome it.
So again, Downforce, produced by rushing air and a Wing is used to overcome
existing Lift. But, Downforce is not necessary when a Spoiler is used, because
no lift exists to overcome.