Project: Geo Metro boat tail prototype - 15% MPG improvement @ 90 kph / 56 mph

[RobertSmalls]

Thank you for being rigorous as always. Hopefully, you'll see a big improvement with the belly pan.

The car is roughly symmetrical, so you might be able to either save time or increase the resolution (number of tufts) by testing only half of the car.

[Christ]

wagonman76 -

The few companies I looked into for the opportunity to use Cara for some ad space had waivers that the drivers had to sign, basically absolving them of all responsibility for damage to the vehicle whether due to advertising materials or otherwise. I'm pretty sure it's not a liability for them, really.

[MetroMPG]

Originally Posted by RobertSmalls

The car is roughly symmetrical, so you might be able to either save time or increase the resolution (number of tufts) by testing only half of the car.

Good point. That's my plan for tuft testing the top & side of the boat tail.

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Tidbit: I got out my undertray panels (coroplast & aluminum flashing) which I put away after testing them in September 2006. Will reinstall them tomorrow.

Other tidbit: I nearly wrecked the web cam today! Backing into the driveway after the last video run this afternoon I smacked the cam hard against the curb/lip of the driveway - it had been mounted low to the ground below the rear mud flap, looking up & backwards at the boat tail. Lucky me: despite some gouges & yanking the wire free from the housing, it still seems to work!

[JackMcCornack]

MetroMPG, honest, I scanned this whole thread first, I hate wasting bandwidth and I apologize if you've already told us, but what web cam are you using? In fact, I wouldn't mind seeing a pic of your webcam mounted as you did to observe the bottom of the boat tail.

PS--Majestic, that's a remarkable act of photoshopping you did there. I took it for real the first two times I saw it.

[MetroMPG]

Hi Jack - It's a Logitech Quickcam Pro 5000, I think. It came with a used laptop I got - didn't realize how useful it would be!

You can see it (the cam is the little sphere) duct taped to the end of a yardstick beneath the end of the boat tail in the 3rd photo in post 129 , above.

And you can see it in action in a previous

tuft test video I made of the Kammback

.

Edit: I spliced in an extension to the wire so I could mount the cam on the back corner of the vehicle, and still reach the laptop sitting on the passenger seat. Today's test with the longer boat tail required yet another USB extension.

Darin

[MetroMPG]

Got the belly pan installed this afternoon.

I'm convinced it's going to make a significant improvement to the flow on the bottom of the boat tail. The middle 25% or so of the belly pan is a straight shot between the rear control arms in line with the plane of the bottom of the boat tail.

One thing left to do: I'll do my best to blend/transition the control arms back to the boat tail as well.

All in all, I figure 50% of the boat tail bottom will have fully attached & relatively smooth flow, vs. 0% smooth flow now.

Unfortunately, it's rainy today, so I can't do tuft testing. Tomorrow looks decent though.

[Nerys]

how steep an angle can you go before losing laminar flow? I am wondering if you can make the angle higher and the tail SHORTER and get the same results. the flow seems awfully stable on top. it should be JUST on the verge of going unstable to be at optimum.

I ask because I am driving out to colorado next summer in my van. I am thinking of adding a giant boat tail to the back that will pivot UP onto the roof when I am parking etc.. (the van is already longer than some parking spaces so permanent tail would be a big nono :-)

The shorter I can make it the better!

[Cd]

Originally Posted by MetroMPG

One thing left to do: I'll do my best to blend/transition the control arms back to the boat tail as well.

All in all, I figure 50% of the boat tail bottom will have fully attached & relatively smooth flow, vs. 0% smooth flow now.

.

And the area behind those mudflaps ... ? ( ducks for cover )

[Cd]

It's too bad you can't create some sort of a flush NACA ducting on the bottom of the tail and have it exhaust some air around the dead zone.
That would be a waste of time, since you wouldn't produce any usable air pressure out of it, right ? ( Would it also create more drag ? )

It would look cool though.

[MetroMPG]

Originally Posted by Nerys

how steep an angle can you go before losing laminar flow?

Note the difference between attached flow and laminar flow. Very few automobiles (with the exception of solar cars, and maybe the Aptera) would have laminar flow over a significant portion of the body. For a discussion of the different flow types, see this thread: A Crash Course in Aerodynamics and an Introduction to Turbulence

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I've seen figures ranging from 10-15 degrees as the highest initial change you can do while retaining attached flow, and up to and beyond 20 if the transition is gradual (think curve, not sharp angle).

it should be JUST on the verge of going unstable to be at optimum.

Yup, exactly.

FYI, the top of my boat tail curves gradually from the roof to ~15.x degrees from horizontal. For the sides, I aimed for a transition of ~10 degrees, also taking advantage of the OEM curvature at the rear corners. (But since I accidentally made it off kilter, one side is actually less, the other more.)

I aimed for a smaller angle on the sides because I figure the boundary layer there (compared to the top of the car) is already more turbulent - from windows, mirror, front wheels, panel gaps, trim - and less likely to stay attached on a more aggressive taper. Only a guess though. I'm just making this up as I go along

It would take a fair amount of effort to keep refining the boat tail to find the "verge of going unstable" for a particular car. I'm going to be satisfied with my boat tail if the air likes it the way it is with these angles. I could keep refining until the cows come home, or make a best guess, and be happy with it if it appears to work (which it does so far).