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

[Nevyn]

Could you make it so that you open the hatch to be the "top" of the boattail, and the rest of it extends/unfurls from inside the car?

[MetroMPG]

Unfortunately not. The top of the hatch is actually about 6 inches of the end of the roof. Opening it even part way increases frontal area and hammers Cd.



Semi-related: Video: Illustrating the Vast Importance of Aerodynamics (open hatchback experiment) - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com

[botsapper]

Here's another view of the removeable tail. The clear/body color PVC panels welded into a rear-body conforming airfoil. The inflatable would have multiple high pressure tube spars either longitudinally or transverse (interestingly the transverse 'bumps' offer some aero benefits, Mythbuster testing?)
The temporary structure could be held down with straps & there is a rigid spar coming from the trailer hitch, holding the signal harness, plate & rear light assemblies. It also tucks in & tidy up the inflatable tube ends.
At the end of the day, it could also serve as the children's slide on the front lawn.

[aerohead]

Originally Posted by botsapper

Here's another view of the removeable tail. The clear/body color PVC panels welded into a rear-body conforming airfoil. The inflatable would have multiple high pressure tube spars either longitudinally or transverse (interestingly the transverse 'bumps' offer some aero benefits, Mythbuster testing?)
The temporary structure could be held down with straps & there is a rigid spar coming from the trailer hitch, holding the signal harness, plate & rear light assemblies. It also tucks in & tidy up the inflatable tube ends.
At the end of the day, it could also serve as the children's slide on the front lawn.

I'd set up bowling pins on the curb and then see how many can be knocked over with a left turn!
Wow,the imagery is just too good!
I do think maybe Kamm has a point with "practical" length issues.Something this long might want to be half car/half trailer.I'm to squeemish to attempt something that radical in pure cantilever.Not sure what a strong momentary sidewind gust might do.
We do need an EcoModder wind tunnel!

[Bicycle Bob]

The area swept by a turning car is symmetrical fore and aft of the rear axle, so overhang, especially if tapered, will not hit stuff.
Most cars can use more tail area for crosswind stability. The center of "lift" is usually about 25% of the way back, since the front is redirecting the air, while the center of mass falls naturally around 50% back. The tail does generate more side force, but it is on the back axle, which has traction that otherwise makes the front end wander. Even with long tails, several streamliners have added a tail fin for stability. More side force, within the ability of the chassis to support it, may even give a boost by working as a wing-sail.

[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.

[MadisonMPG]

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.

Not if you ran some strings to opposing sides of the car, that hooked on the inside of the balloon.

i.e. top left hooks to bottom right

[RobertSmalls]

With something as useful and beautiful as this, why would you want it removable?

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.

@Botsapper: That's a very nice render. But if you taper the sides, bottom, and top at the same angle, you preserve attached flow with shorter overall length. Actually, with the Metro taillamps being almost half as wide as the rear of the car (based on counting driveway tiles), you probably want a square rear truncation, with the license plate mounted above or below the taillamps.

MetroMPG: How long do you think you're going to make the boattail?

[Bicycle Bob]

The bottom is not so well supplied with air, even with a great belly pan and rounded. 4 degrees on the belly pan is usually as conservative as 15 deg on the top and sides. If the tapers were continued to a point at the back, it should be only slightly higher than the stagnation point at the front. A bit of rake will help cancel the lift and induced drag if the air goes over the top faster than underneath.

[3-Wheeler]

Originally Posted by Bicycle Bob

The bottom is not so well supplied with air, even with a great belly pan and rounded. 4 degrees on the belly pan is usually as conservative as 15 deg on the top and sides.

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.