Spoiler hypothesis

[robchalmers]

the two sierra 's you'v feature we're spoilers design as homologation specials for racing and to produce Downforce. The cosworth (white mono plane one) i know is a NACA profile across it section which is one towards the rear, from the glass to about 3/4 out there is nothing there so the flow is just the same as a normal hatch. The XR4i (black one) could well work on more of a lid extension basis, but a nascar wing is likely to create more 'churn' and therefore form drag.

[AeroModder]

Measuring the angle with GIMP, it's about a 16 degree angle from the roof to the tip of the spoiler. With no spoiler, it's roughly 19 degrees to the trunk edge. The window is about 35 degrees.

To mount a kammback at a 13 degree average, I'd have to extend the spoiler another 3 inches.

I've driven 130 miles so far on this tank and I'm about a gallon or so above half, so we'll see what the tank average looks like around the 7th.

[AeroModder]

Update with some flow simulator videos:

Stock:

http://www.youtube.com/watch?v=RCb47DxCYHc

Short spoiler:

http://www.youtube.com/watch?v=-d_dw2UqByE

Large spoiler:

http://www.youtube.com/watch?v=RNpeyzfUUpY

Partial kamm:

http://www.youtube.com/watch?v=t-jaMvex8RI

Full kamm:

http://www.youtube.com/watch?v=D9XxvIotTVU

[gone-ot]

...be able to predict the natural oscillation frequency and you'd be able to devise a proper "damping" device.

[aerohead]

Originally Posted by AeroModder

Measuring the angle with GIMP, it's about a 16 degree angle from the roof to the tip of the spoiler. With no spoiler, it's roughly 19 degrees to the trunk edge. The window is about 35 degrees.

To mount a kammback at a 13 degree average, I'd have to extend the spoiler another 3 inches.

I've driven 130 miles so far on this tank and I'm about a gallon or so above half, so we'll see what the tank average looks like around the 7th.

CAR and DRIVER's experiments revealed that for drag reduction,a spoiler should never angle up beyond 30-degrees or there will be a concomitant growth in drag.
The spoiler is creating a locked vortex over the backlight/boot area with the free stream above,skipping over the vortex and re-attaching on the spoiler.
The low pressure above the backlight/boot is 'contained' by the spoiler. The impacting flow on top of the spoiler,as well as the lower pressure of the walled-off wake behind it provide the downforce.
I think the Kamm extension was allowing what Hucho refers to as 'overshoot' and this prevented any chance of re-attachment onto the trunklid,leading to the drag increase.
The Mercedes EVO 190 suffers the same situation,but their rear high mounted spoiler does allow for re-attachment on the wing,although not on the boot.

[AeroModder]

Fuel economy turned out to be around 27, but that included some adverse conditions, so it doesn't seem to be any different than with the kamm on.

Do you think adding a piece of coroplast to the trunklid to lay down the spoiler angle would be more effective?

Edit: Taking some advice from here, it seems like adding coroplast to lay the spoiler down to a 30* angle and meeting the roofline at a 12*-13* angle seems ideal. I think I'm pretty close to that, after some quick Gorilla tape/coroplast work. We'll see what happens.

Update: Okay, dropping the spoiler angle to 30* and extending it to meet the roofline at 12* seemed to make things worse. The needle dropped much faster than it usually does. There wasn't any wind, so that wasn't a factor. I'll pull it off and try the rest of the tank without a spoiler.

[aerohead]

Originally Posted by AeroModder

Fuel economy turned out to be around 27, but that included some adverse conditions, so it doesn't seem to be any different than with the kamm on.

Do you think adding a piece of coroplast to the trunklid to lay down the spoiler angle would be more effective?

Edit: Taking some advice from here, it seems like adding coroplast to lay the spoiler down to a 30* angle and meeting the roofline at a 12*-13* angle seems ideal. I think I'm pretty close to that, after some quick Gorilla tape/coroplast work. We'll see what happens.

Update: Okay, dropping the spoiler angle to 30* and extending it to meet the roofline at 12* seemed to make things worse. The needle dropped much faster than it usually does. There wasn't any wind, so that wasn't a factor. I'll pull it off and try the rest of the tank without a spoiler.

I would be very reluctant to gauge any performance on the position of the fuel gauge.After 320,000 miles on the T-100 it remains 'unreliable' as an indicator.

[cfg83]

AeroModder -

The full kamm YouTube is a good fit for the teardrop/boattail overlay :



CarloSW2

[AeroModder]

My fuel gauge has been pretty consistent, actually. I have both spoilers disconnected right now, and I'm keeping them aside for testing later on.