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

[Bicycle Bob]

That would depend upon ground clearance and other issues. If departure angle is a problem, turbulators can be added just as the angle is increasing.

[Piwoslaw]

Originally Posted by RobertSmalls

Okay, I suppose it would interfere with your ability to load akward cargo and parallel park. But if you drop the requirement that it be able to be stored inside the Metro, it becomes easier to design and build, since you can use a single rigid frame. Storable in the garage is almost as good. You'd use the boattail on road trips, and leave it at home for commuting.

You're right about a rigid frame being easier to construct and more stable, but being able to fold a boattail and stow it in the trunk is very useful. Not all road trips are of the 'drive to grandma's ranch' type. This summer we drove to Romania and a boattail would have been awsome on the 1500+km trip each way, but once there we were driving around, sightseeing, city driving, parking, etc. The boattail would only be in the way, and we couldn't just take it off and leave it at the border station for two weeks. But if it deflated and packed into a size not much bigger than the rear light assembly in Botsapper's pic, then it could just get stuffed anywhere until it was time to return.

[basjoos]

Originally Posted by 3-Wheeler

Nice to hear from you again!

If I am reading what you have said about the relative lack of air on underside of the vehicle compared to the sides and top, it would seem that you could conclude that the taper on the underside of the aerocivic is too steep.

I remember reading in that thread somewhere, that you asked Mike if he had performed tuft testing in that location. How much higher can the 4° go before air detachment would be a concern in this location?

Jim.

The underside of my boattail is steeper than I would have liked it, but I have a sharp approach to my driveway and didn't want to risk dragging the bottom of the boattail. I haven't tuff tested the underside of my boattail. I can't see it when I'm driving and don't have any aeronut neighbors to follow me, observe, and make sense of what they were seeing. But I have noticed that sections on the rear of vehicles that have turbulent flow tend to collect dust and dried deposits of dirt from water/salt spray. The underside of my boattail doesn't collect get dirty, so it can't be too turbulent under there. The side taper helps in allowing air to fill in the space between the road the underside of the boattail.

[NeilBlanchard]

Hi,

I'll post the image I just made for my thread here as well, because I think it illustrates the issue.

1st possibility is relatively short and curved to a point
2nd possibility extends out to nearly the point where a 15 degree top slope intersects with the 4 degree bottom slope
3rd is a truncated version of the 2nd

[MetroMPG]

Originally Posted by botsapper

At the end of the day, it could also serve as the children's slide on the front lawn.

And a gardening hothouse!

Another amazingly good image, botsapper - thanks.

---

I have nothing to report other than when not occupied with a few unrelated tasks last weekend, I sat on my duff. Haven't done any construction yet (aside from the virtual kind in my head - done lots of that!).

[moorecomp]

And you can call it the BubbleButt

[Wonderboy]

And you can call it the BubbleButt

hahaha brilliant!

[aerohead]

Originally Posted by basjoos

The underside of my boattail is steeper than I would have liked it, but I have a sharp approach to my driveway and didn't want to risk dragging the bottom of the boattail. I haven't tuff tested the underside of my boattail. I can't see it when I'm driving and don't have any aeronut neighbors to follow me, observe, and make sense of what they were seeing. But I have noticed that sections on the rear of vehicles that have turbulent flow tend to collect dust and dried deposits of dirt from water/salt spray. The underside of my boattail doesn't collect get dirty, so it can't be too turbulent under there. The side taper helps in allowing air to fill in the space between the road the underside of the boattail.

If the underside of the tail were a little dirty it might have an advantage with high speed stability.
Some race car designs have purposely slanted tails which communicate the low base pressure under the shelf of the tail to the cars underside.
The distance out from the rear wheels acts as a moment arm creating a torque at speed,creating useful downforce albeit with some drag penalty.

[wagonman76]

Originally Posted by Old Tele man

...pardon my simplistic analogy, but it sure "looks" like a really bad case of the "...tail waggging the dog..." situation to me.

I remember back in 1989 when a Yugo got blown off our Mackinac Bridge. Hard to say what really happened, everybody's stories were contradicting, some say she was speeding, some say she stopped, some say she started swerving... but one thing was for sure, it was a tiny car driving in high winds. Since then they installed anemometers and close the bridge in high winds.

I can understand one might want to remove the tail. Taking a trip to the city, it can be a lot harder to maneuver in tight spots with a tail.

[Christ]

So what about an umbrella frame deal?

Couple pieces of tubing that slip inside each other to make the main "handle" section, a ring that slides up the tubing to open the "chute" section, could be made of thin rods and fabric, would fold up in seconds and could be stuffed in the trunk with no problems. A quick PCV tube frame could be attached to the hatch to connect the loose ends of the fabric.