Kammback or rear spoiler?

[kach22i]

Originally Posted by Viturro

So in your opinion, the aileron could have less lift and similar drag reduction?

I'm not the forum's local guru aerodynamicist (Aerohead is), just voicing my opinion.

On my own roof wing (see link in signature below), I've found that an air-slot flush with the roof plane will still draw air down into it, there need not be a "Ram Air" effect.

I recently posted an article about the new full sized GM Pick-Up truck, they slotted the side joint between the cab and bed which reduced drag from air being sucked into it. I think there is a correlation here.

In my opinion the aileron will have less lift than the Kamback.

Beware, less lift and more down force typically results in more drag. This rule of thumb of course has many exceptions. The clarifyer here would be how the sides of each section shown are actually treated.

[Viturro]

Thanks for your opinion!

now waiting for aerohead's reply

[Sven7]

Personally, I think that aileron is going to create lift when it forces a bunch of air down.

The kammback works by gently reducing the roof's high air pressure before it hits the low pressure wake. It allows the air to do what it likes, in an organized manner.

The aileron is forcing high pressure air directly into the wake. It forces the air into doing what it wants the air to do.

Not to mention leaving the sides open... just look at old wind tunnel studies of airplane wings and you'll see this is a bad idea. If you try to build a test prototype of this it's going to be hard enough to get it right, and if you want to have stabilizing fins on the ends, or bend the thing down with the C-pillar angle, you're definitely going to need a wind tunnel and a Master's in aerodynamics.

I predict the aileron will produce big, unpredictable vortexes and probably increase drag. Feel free to go prove me wrong, though!

[Viturro]

Thanks! ,i should put the sides of the airfoil in the example.
Im don't agree with you in this ''The aileron is forcing high pressure air directly into the wake'' becouse, the angle on the wing and the angle of the kammback are the same, if the air separates from the kammback it will make some drag, like the upper part of the airfoil, then they sould be similar.
And the lift should be higher at certain speed when in the air starts to sepparate from the kammback, making drag, in the airfoil should make lift at the same time that reduces drag... maybe (?
also the upper part of the airfoil should make drag as the kammback or similar (more). so idk!

[aerohead]

Originally Posted by Viturro

yes, in the example i just want to know about the roof (the top) of the car. want to know about the difference about the kammback and the aileron.

*The drag you're trying to reduce is pressure drag.
*The pressure drag is dependent upon where the flow separates from the body,as the entire wake will have THIS pressure.
*If the Kamm-Back is properly contoured,the air will stay attached to it until it ends,slowly decelerating (Bernoulli's Theorem) and gaining static pressure as it passes down the 'back.'
When the air does separate,it's at a higher overall pressure,and this higher pressure in communicated throughout the wake,increasing the base pressure behind the car,hence lowering the pressure drag.All good!
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*The 'aileron' is aspirated all around and can only act in the capacity of a deflector.
*The aileron has it's own frontal area and drag coefficient minimum (not to mention it's support structure) at zero-angle-of-attack,but when it is angled,as in your diagram it will produce lift,which is induced drag which the engine must overcome.
*The aileron may direct cambered airflow towards the wake,which is in itself a good thing,but it does nothing to slow the air and build static pressure,and again it's acting at the expense of power robbed from the engine for an overall net increase in drag.
*I suspect that the aileron would put a motorist in jeopardy,especially during wet road driving,where the car would be prone to hydroplaning.
*I've never seen a wing added to a car which did not increase its drag.

[Viturro]

Thanks aerohead for your opinion! i was waiting for you : D
but i still not conviced about some-things.
I did some Flow ilustrator jobs (i know that the program isnt a guide at all, but its something)



the setup of the program is stock but with 200 on velocity. it was tested with and without floor with similar results on the upper part of the cars.
in the image you can see that the lowest drag of the ''CARS' is the kammback with airfoil one ,and looks like it have just a little drag on the airfoil.

Repeat: i know that the program isnt a guide at all,it have bugs and other unreal things but its ''something'' to see what i mean a little better.

[ChazInMT]

Viturro, the Flow Illustrator is just what it says it is, it "Illustrates" what you think the flow should look like. It is explained by its creator as this. It in no way can predict what the flow will look like because it is much too simple for that. You adjust the controls until the flow looks right to you.

Also, Aerohead has said that the foil spoiler will create lift and drag, and not really help in any way make your car more efficient aerodynamically. I think you are assuming the goal is to pack as much air in behind the vehicle as possible to reduce drag, but it is not the goal.

The goal in aerodynamics is to make the air move as little as possible from where it started out. Remember, you are driving through still air, the air passes over the car only because the car is moving....the air was just sitting there before you showed up. So if the air can move over the car with the least amount of disturbance, this will create the lowest drag. The Kamm is really intended to create a smoother transition of air from back of the roof on a sedan down to the trunk area. I'm pretty certain it was never meant to be a fix for the back of a hatchback or wagon without a trunk.

The goal is to let the air return to where it was before you rammed through it as gently as possible, the wing spoiler does not help with this goal.

[Frank Lee]

Beware, less lift and more down force typically results in more drag.

"Lift"- no matter the vector (including down) requires power to create. Zero "lift" results in the least drag.

[Viturro]

''Viturro, the Flow Illustrator is just what it says it is, it "Illustrates" what you think the flow should look like. It is explained by its creator as this. It in no way can predict what the flow will look like because it is much too simple for that. You adjust the controls until the flow looks right to you.''
YES YES, OF COURSE, i know, but it's something. i put the velocity to 200 cos then you can see the ''air'' separating from the car, and no more cos i would need more space (large) of the drawn making the ilustrator slower (old pc) it's a balance.


''I think you are assuming the goal is to pack as much air in behind the vehicle as possible to reduce drag'' No, i know it has to flow, little by little, gently , that's why i dindn't made a 90º wing, i made a similar to a kammback...but with air flowing under it... i think basically is that, that's why i dont think that there could be so much difference.

thank you, and thanks to you Frank Lee for the info, i didn't know that the lift was a vector!

[Frank Lee]

Really, lift for us land lubbers is either straight up or straight down (perpendicular to the road). Anything else is drag or, conceivably, thrust. It's the combination of these that makes vectors.