why hasnt someone made the inflatable boat tail yet?

[ChazInMT]

Something else to think on. The boat tail could be used mainly for long highway trips where it would be most useful. You could even block the rear window since on the open road side rear views work pretty well. Then, when you're back in town, stow it. This way you could make it out of much less durable materials since it only gets used once in a while. Think "Air Mattress" tech instead of "Rubber Raft"

But I suppose if you drive on the highway a bunch everyday for your routine commute, this is a crappy idea.

[Otto]

Originally Posted by euromodder

Done that way, it'd add drag.

Nope.

NASA et al have already done this with fabric, self-inflating. Although not a perfect shape, it is pretty good and gets most of the aero benefit at very little cost or complexity. Makes the draggy squared off shape of a truck trailer into a nice rounded shape, for better aero.

Using Dacron fabric (as with sails and ultralight aircraft wing skins), you can use an iron to heat shrink the material to adjust the shape. Very light, and cheap to do. PVC pipe would work to make a light, simple, and cheap frame, if needed.

It would make a nice protective envelope for bicycles carried on a rack at the back of the car or van, if the front wheels of the bicycles were removed to make the package smaller.

[miket]

One problem with using self inflation only is that it could deflate and drag on the ground when you stop or it could get burnt by the exhaust. Also if you drive in reverse it might get stuck under the rear tires.

Either the boattail need some way to stow automatically or it needs to stay inflated till you can stop and stow it manually on the street outside of the parking lot. A one way valve could keep it from deflating but a release mechanism would still be needed. The only completely passive way i could think of that would work would involve some sort of spring that would stow it away at low air speeds and let it self inflate at higher speeds. I haven't figured out how such a stowing mechanism would work.

For rear visibility i would install a camera on the back of the boatail. Of course i plan on having a backup camera and no windows on the utility cap anyways so its much less an issue for me than others.

[euromodder]

Originally Posted by Otto

Nope.

NASA et al have already done this with fabric, self-inflating. Although not a perfect shape, it is pretty good and gets most of the aero benefit at very little cost or complexity.

Essentially, it works on the very same principle as a parachute: catch air and inflate, then generate a lot of drag.

While you may see an overall improvement because the appropriately-shaped parachute improves the lousy aerodynamics of the vehicle, you'll still lose a fair deal of the potential improvement to the drag the parachute needs to keep inflated.

[Otto]

Originally Posted by euromodder

Essentially, it works on the very same principle as a parachute: catch air and inflate, then generate a lot of drag.

While you may see an overall improvement because the appropriately-shaped parachute improves the lousy aerodynamics of the vehicle, you'll still lose a fair deal of the potential improvement to the drag the parachute needs to keep inflated.

Nope.

Billows out because internal pressure in the bag is a bit more than external pressure.

Watch a motorycyclist's shirt billow out at speed--not because he has a parachute or uber-ventilated arm pits.

The NASA truck thing was simplicity itself, and was not so long as to droop and tangle in the wheels or gear when stopped or in reverse.

Somebody here on Ecomodders as I recall within the past month or two had pics of a stowage bag on the back of an SUV or station wagon, which partly filled in the draggy zone just aft of the back bumper. A self-inflating bag of somewhat larger size is what I have in mind, and would make a nice, streamlined protective envelope for, say, a bike rack (including bikes).

Also, hinged panels at the rear edges of box trailers have been reported on this website. On an SUV or Volvo wagon, such could cheaply and easily be made of Coroplast, with a Dacron panel to fill in, making a semi-rigid and rounded rear cap for the vehicle, with better aerodynamics and fuel savings.

[miket]

Does that motorcycle shirt stay completely solid or does it flap around side to side turbulently? How would you remove that dacron panel so you can fold the tapered boxtail closed.

One advantage of inflating it with a pump is that it can be completely deflated with a pump.

One idea i had for stowing it is add eyes to either the inside or outside of the inflatable tail and run a cable around the tail in spiral pattern and then connect it to the end of the hinged flat bottom panel. A mini winch could pull on the spiraled cable narrowing or squishing the tail flat and then lifting the bottom panel(s) up against the vehicle.

[Bicycle Bob]

Euromodder - the "parachute drag" you worry about is just a small inlet in a natural high-pressure zone. If the parachute does not leak, there is no penalty, except minor turbulence as the stagnant air passes the edge of the opening. Self inflating wing kites are very fast.

If I were to try this scheme, I'd use mountain tent material, and a spring to roll it up at low speed, like a New Year's noisemaker.

[miket]

Those noisemakers get wider as they flatten and retract.

[miket]

Oh and a simple drum and helical torsion spring could replace the mini-winch for auto retraction in my previous idea.

I don't think its a great idea for the boatail to be inflating and retracting randomly around town. Other people might freak out. I think it should either be inflated and deflated just outside the parking area or it should inflate and deflate automatically but only on the highway and not around town.

[miket]

Also you could use a spring loaded vent next to the high pressure inlet that would allow the air to bypass the tail untill the pressure was high enough to trip the vent closed and send the air to the tail

I can imagine some issues with self inflation. What happens if your waiting at a stoplight and a motorcyclist is parked behind you when a strong gust of wind comes and inflates the tail??