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

[aerohead]

I probably got my usual pace at the same store as you did Darin!

[Christ]

My usual pace hasn't arrived yet.... must've bought it from one of you guys.

[Piwoslaw]

Originally Posted by BamZipPow

How about this train of thought...

Something like one of those collapsible cups but in a bigger format to fit the back of the vehicle. It could be made out of coroplast quite easily fer shape and low weight. Attach it to the back of the vehicle, pull it out and lock it in place when you need the aero capabilities and unlock/collapse it back down when you don't need it.

Do you really need to go to the very edge to git all of the aero effects? Can't you just kinda step it down from the edges to git some effect? Sort of a step/chop of the extension...a compromise? Just a thought...

From an old, forgotten post:

Originally Posted by Piwoslaw

Another idea is that the boattail is in segments, like descibed above, only they're not inflatable, but stiff and they retract one into the other like a telescope. The first segment is a kammback, the others fit inside of it. This reminds me of a collapsible cup:

[BamZipPow]

Exactly...now who's up to making one? I think it would be more of gitting the calculations correctly to git them to nest and expand without being too loose. Although...it could be augmented with some rubber sheeting...

[wagonman76]

That's an idea. At work we build retractable loading spouts with cones that have to collapse similar to that without jamming. Calculations are pretty easy.

[Christ]

Was that a volunteer post? ^^

[MetroMPG]

This week I unbent the aluminum bows I picked up.

Set up a stool on the driveway, got out the duct tape and taped one of the tubes between the stool and the car, extending the existing angle of the Kammback (~15 degrees). Spent a long time staring at things.

Fired up Photoshop. Drew some lines.



Then I chose 130 cm / 52 inches as the length of the boat tail (roughly to the vertical green line. Two reasons: partly for aesthetic proportions because it's close to length of the car's existing "sections" (hood / door / rear), and 2) partly because the longest aluminum tubes will fit crosswise in the back of the car.

I think I'm going to aim for a 6 degree bottom angle, approximately what's shown there (don't do the photoshop measurement to check my work on this image - it's 5 degrees there.)

Bicycle Bob advocates for no more than a 4 degree angle. Unfortunately, that will leave the end of the tail a bit lower than I'd like for departing slope angles from the back tire.

I've since made a cardboard "transom", and four struts. Mounted on the car.

Plan is to make an aluminum + cardboard skinned prototype tail based on these angles and tuft test it.

Should have a couple of photos of construction this weekend.

[Bicycle Bob]

It takes me a long time to stare at stuff that isn't there yet, too. Looks good so far. When it is done, you can tape a camera on a lower rear corner to make a video of some tell-tales taped on underneath. If they are flapping, a nice line of vortex generators will probably save the day. Being a bit sloppy on the belly pan would get somewhat the same result. I'm sure a lot of people would be interested in that bottom side data.
Another approach to departure angle is to leave the attachments a bit loose at the bottom, so that the whole assembly can ground out without damage and slip back into position, if anybody really wants to get fussy.

[MetroMPG]

I probably won't bother tufting the bottom yet, because I have no belly pan on the car - it'll be messy even with a shallower angle.

Interesting idea for a possible "tail strike". Could put a little wheel under the bottom of the tip, too.

[Bicycle Bob]

Wheels are the 1st thing that come to mind, but they create point loads I try to stay away from. Race cars use skid plates on the bottom instead of bump stops, saving space, weight and complication. They can be very cheap to replace, and protect a wide area. Some helicopters land with forward motion so often, they have carbide skids.