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Old 09-28-2010, 06:15 PM   #1 (permalink)
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Spoiler hypothesis

Looking at the testing of rear spoilers shows that they can reduce drag if properly used, such as the raised spoiler on Hot Rod Magazine's Bonneville-running Camaro.

From what I've read here, it's understood that having the spoiler will virtually decrease the angle of the rear end, tricking the airflow and resulting in smoother detaching.

This video shows Mercedes testing a dynamic rear spoiler:


I'm wondering if the spoiler would have an added effect. Vorticies are formed off a sedan's rear by air rushing to fill the low pressure area behind the rear window and over the trunk. If a spoiler was present, it would increase the air pressure on top of the trunk. Maybe this would help reduce the strength of the vorticies?

Thoughts?


Update: Visual aids

My idea is to turn this:


Into this:


Think of a Nascar-style spoiler, but not as large. Just enough to soften the virtual angle off the rear.

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Old 09-28-2010, 07:08 PM   #2 (permalink)
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AeroModder -

That's my theory too. I think the trailing edge just has to "meet" the point in the air where the boat tail would extend to :



Without teardrop overlay :



IMO this could be a compromise that gets *some* of the benefit of a kamm-spoiler-boattail without sacrificing rear visibility.

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Old 09-28-2010, 07:21 PM   #3 (permalink)
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I've got several tanks with the kammback on, so I'll pop it back off when I fill the tank later this afternoon and extend my lip spoiler up some.

Another idea I had was reversing the direction of delta-shaped VGs. When I was using them, they amplified the counter-rotating vorticies over my trunk, showing up on the rear glass. I'm wondering if you reverse the angles, and thus reverse the rotation direction of the vorticies, would they also cancel out the trailing vorticies over the trunk?

I have a 4'x8' sheet of red coroplast I was going to use to make a full kammback, so I might also use it to see which one works out best. My guess is that a full kammback would be better than having the gap with the trailing edge meet the slope, but the spoiler might be better than none at all (or at least better than a partial kamm on a sedan).
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Old 09-28-2010, 09:19 PM   #4 (permalink)
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You might be on to something. I drove a friends German Merkur on a 200 mile trip. It had two spoilers and grill block std probably 20 years ago. It had to be pretty aero as it would do over 125 with a 2.3 turbo 4.

Here is a photo of one that I found on the internet.
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Old 09-28-2010, 09:45 PM   #5 (permalink)
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Old 09-28-2010, 09:48 PM   #6 (permalink)
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Varn -

Yeah, here's a similar "monoplane" version :



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Old 09-29-2010, 12:17 AM   #7 (permalink)
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It was a race against the sun, but I got the kammback off and the spoiler on tonight with a full tank. I measured 4" above the trunk edge for the approximate 13 degree angle of the kammback, and my spoiler turned out to be just under 3", so I'm guessing the angle is rougly 16 degrees at the most.

The trunk and rear window are covered in dust because of the lack of airflow from the kamm, and I'm going to leave it alone and see what kind of surface flow I can get from it.
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Old 09-29-2010, 07:58 AM   #8 (permalink)
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Yup, that's how it works. Properly designed spoilers for drag reduction are kammbacks in the making. The more proper spoilers you have the closer to a kammback your results will be.
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Old 09-29-2010, 09:03 AM   #9 (permalink)
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Only diference with a NASCAR spoiler is its for downforce. The trunk spoiler gets adjusted for th balance between speed (laid back) and cornering downforce (stood up).
notice on the Mekur and the white car the spoiler is level not vertical.
pickups are adding a "step" on the top of the tailgate for the same reason.
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Old 09-29-2010, 10:28 AM   #10 (permalink)
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I'll have to experiment with adding a flat "step" to it then.

Update: This morning, the car was coated with dew, and on the usual commute the dew showed little signs of airflow over the rear window and trunklid. I'll probably do some tuft testing to better see what's going on and if the air is getting trapped and making that "virtual" angle.

I'm thinking of getting some kind of stick or something with tufts up in the airflow and see how the air is flowing above the surface, then measure the angle of airflow from there.

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