Inflatable boattail

[Otto]

Originally Posted by tasdrouille

That would be a major source of drag.

Depends on how you do it. A largemouth intake would be draggy, offset somewhat by the fairing effect of the airfoil that follows in its wake.

OTOH, you could feed the inflatable airfoil from, say, a duct at the nose stagnation point.

Or, it may well be self-inflating by billowing. Ever notice how the shirt or jacket of a motorcyclist billows out behind his back? This even though his jacket cuffs are not air intakes. Why would this not work?

Seems to make convertible car tops billow at highway speed, due to pressure differential and Bernoulli effect.

Here's a cheap and easy experiment for one of you pickup truck guys: Duct tape a piece of cheap plastic tarp to the top rear of your cab, then down the sides of the rear cab corners, then along the lip of the cargo bed. Make it moderately taught, but with just enough slack to billow up into a rounded bubble. Drive down the road at speed. Observe fuel economy. Report back.

I'll bet this works pretty good, esp. with a bit of tinkering.

[Cd]

How would you guys go about making sure that the taillights are visible ?
Also, if you have this huge 'airbag' that falls out the back of your truck everytime that you drive up to speed, you are really going to freak out a lot of drivers behind you.

[OfficeLinebacker]

Yeah I think the only way to go is to sacrifice a little bit of energy initially to inflate the thing and then seal it off. To use a continuous stream of air, whether from the airstream or a fan, is counterproductive.

[dremd]

A flap 4 inflation could be made, then closed once inflated.

Mine should be totally out of the slip stream and inflated/ deflated by a mattress pump (a pretty serious one that I happen to have)

[Otto]

Originally Posted by dremd

A flap 4 inflation could be made, then closed once inflated.

Mine should be totally out of the slip stream and inflated/ deflated by a mattress pump (a pretty serious one that I happen to have)

Well, using the billowing shirt of a motorcyclist example, it would be self-inflating.

Per my duct tape and Dacron or tarp cover idea as noted above, maybe air from under the truck would come up in that gap between cab and bed, and fill the bag from beneath, with no extra effort.

Worth trying, methinks.

Actually, this mike work even better with the tailigate down, and the tarp affixed to its trailing edge--longer, more gentle slope from the cab roof that way.

[jeremy128]

Couldn't you rig up something that the flow of air over the car would inflate the tail?
EDIT: missed the last page before posting this. Oops!

[aerohead]

Originally Posted by jeremy128

Couldn't you rig up something that the flow of air over the car would inflate the tail?
EDIT: missed the last page before posting this. Oops!

Even with the pressure of an inflation pump my boattail wanted to dance around behind the truck.I had to add a cantilevered strut and band around the envelope to capture it in order to "quiet"it down.The periodic shedding of vortices create significant oscillatory deformations without a lot off rigidity.Goodyear's inflated airplane can give you a good working example of good (no pun intended ) design.

[Bicycle Bob]

I have considerable experience with inflatable displays, and they always are assumed to leak and require a continuous air supply. Some idea of the possibilities can be found in the hot-air balloon contests. Given a chance, an inflated structure will look as much like a bubble as possible. Cones are easy to produce, but cubes are hard. For a flat panel such as a fastback rear window, some extra structure is needed. Ribs can be added by using separate small-diameter tubes inflated to fairly high pressure. The nicest realization of this is made with two tailored layers separated by deep ribs, much as a really high-quality down sleeping bag is made.

I would favour a lower structure that would curl up like a New-Year's noisemaker to stow the whole inflatable tail in a neat roll at the bumper. It might be necessary to mount the taillights and licence plate on tubes that telescope from the belly pan or roof, and follow the tail in and out.

[dremd]

OH wow, new years noise maker is a great idea!
What is that made out of, spring steel?
I had folding plywood on the mind.

[slopemeno]

The TV show "Wings" did a segment on the Goodyear inflateable airplane, and showed the interior of the air cells. The interior has a series of hundreds of flat "threads" that bridge the top and bottom skins and help shape the exterior by limiting the maximum shape of each cell. This allows you to pump up the cell without it looking like the Michelin man.

I would think the easy way to do the "ballute" inflateable nose and tails would be to have a small onboard compressor that was more tuned to volume (I can't imaging needing more than 4 psi in one of these things) than pressure. As needed, the pump could turn on and inflate the structure. That or maybe you could use an EGR valve and program it to turn on when needed to pump the structures up with exhaust.

Joanne Fabrics has a pretty good selection of non-permiable fabrics for doing experiments. I'd probably use a bead of Goop or E-6000 to hold everything together, and use some of the Tap Plastics "Marine Mounting System" to attach the structures but keep them removeable for tweaking. Just guessing, but the front structure would probably need more stiffness, since one problem blimps and dirigibles run into is their skins rippling on the front when thier reach higher speeds. The low pressure region on the rear would probably help shape the rear structure. Maybe some "stays" built into the nose would help it maintain shape.