When does the spoiler go down?

[Lil bastard]

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.

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.

[blue2000s]

Originally Posted by Lil bastard

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.

Your description of the airflow and resulting pressure is essentially correct, although your use of the term laminar is a bit inaccurate.

Nevertheless, you're referring to lift as an inherent characteristic of a car, and downforce as a measure to counter it. These terms are not specific to cars, they apply to the forces acting on any body moving through a fluid and therefore need to be understood in a more general sense. If you refer to these terms without forcing them to be applied to a car (which they aren't) you end up with my description.

[Lil bastard]

Originally Posted by blue2000s

...although your use of the term laminar is a bit inaccurate...

Well, yes and no. Laminar Flow of air does exist generally on several points on the top of a car. It is disrupted by such things as the windscreen and such, but often re-established on the rear deck.

As you know, laminar flow really gets the air moving and so it is at these points where the greatest low pressure areas exist.

Generally (and I'm being general because not all cars are the same), laminar flow is re-established on the rear panel of the car (trunk area behind the windscreen) and so it is an area of particular low pressure. This is where the spoiler comes into it's own, breaking up this clean laminar flow and restoring high pressure to that area. It also has the by-product of moving the very turbulent, drag producing air which accumulates right at the rear bumper further back, beyond the car's bodywork.

In essence, I really think we're saying the same thing, but from a different perspective.

[blue2000s]

Originally Posted by Lil bastard

Well, yes and no. Laminar Flow of air does exist generally on several points on the top of a car. It is disrupted by such things as the windscreen and such, but often re-established on the rear deck.

As you know, laminar flow really gets the air moving and so it is at these points where the greatest low pressure araeas exist.

Generally (and I'm being general because not all cars are the same), laminar flow is re-established on the rear panel of the car (trunk area behind the windscreen) and so it is an area of particular low pressure. This is where the spoiler comes into it's own, breaking up this clean laminar flow and restoring high pressure to that area. It also has the by-product of moving the very turbulent, drag producing air which accumulates right at the rear bumper further back, beyond the car's bodywork.

In essence, I really think we're saying the same thing, but from a different perspective.

Laminar is not really the word you're looking for. Laminar flow is a type of airflow where small eddy disturbances are non-existant. The bulk flow over a body can and will be both laminar and turbulent in different areas, but it's actually mostly turbulent in the case of a car. The terms you really want to use are attached flow and separated flow.

If you have a wing moving through air that forces the air to take a longer path above it than beneath, this will will produce lift (negative downforce), if you turn the wing up side down and move through the air at exactly the same speed, it will produce negative lift (downforce) in the same amount but in the opposite direction. The wing didn't change dimensionally, and neither did the force, just it's direction, which is the only difference between downforce and lift.

Now, depending on how fast that wing is moving, the air flowing over it will either be laminar or mostly turbulent.

[Lil bastard]

Originally Posted by blue2000s

Laminar is not really the word you're looking for. Laminar flow is a type of airflow where small eddy disturbances are non-existant. The bulk flow over a body can and will be both laminar and turbulent in different areas, but it's actually mostly turbulent in the case of a car. The terms you really want to use are attached flow and separated flow.

OK - Fair enough. I'm far from, nor do I claim to be, an expert on the subject. I'll definitely look into the points you've made.