Idea fer an inflatable boat tail made from mylar space blankets...

[Bicycle Bob]

Mylar lacks not strength, but toughness. Because it stretches so little, it is prone to tearing. However, if it is protected from stress concentrations, it is useful to increase rigidity in a structure.

[max_frontal_area]

even ripstop mylar would not be very suitable - in more than one regard.
nylon material used in tents would not hold up long (physically and UV xposure). once again a ripstop variant would be a gross improvement. preferable to that and a lil more expensive dupont *cordura*.
tops would be modern sailcloth very ridgid, ripstop, good uv holdout but also very $$$

ultrathin pvc would be inexpensive and not impossible to weld (seam), flexible
for forming curves. to control unwanted bulging like convertible tops
do on the highway just add some slats or similar stiffeners such as the ones in use for sails. alternatively individual chambers could be used, think motionless waterbed, to add structural integrity and or by varying inflation pressures to help form its general shape. even non UV stabilized PVC is decent in the sun, improved grades should be source-able

if money is no issue, polyurethane - think modern condoms or speedsuits
however durability would have to be determined...

i would be very interested in anything happening along these lines.
project groovy, see my thread in the intro section, will be utilizing
cordura, nylon ribbons for reinforcements and thin wall 1" 1.5" PVC tubing
- quick and dirty ez on ez off.
the fuzz will love it i am sure

[stevet47]

ok, so as I was going to bed last night a got another idea.

I am new to aerodynamics so bare with me here...
The whole point of a boattail is to keep attached flow and 'shape' the air down to a smaller wake. But at some point the boattail has to stop, of the few I have seen I think only aerocivic's comes to a point, the others have flat backs which still leaves some wake.

What if instead of taking the traditional boattail concept and altering it to make it inflatable, you build a boattail-shaped windsock. Filled via air from a scoop or whatever like others have said, but with an opening at the end of the boattail. The wind would blow out, essentially lengthening the boattail giving the air flowing on the outside of the boattail something to attach to at the back.
The main issue would be having enough cfm flowing through to fill it, but ignoring that, would this be any better than an inflatable boattail?

[MadisonMPG]

It's a definite maybe.

[thatguitarguy]

"Inflatable" as in "windsock" was more what I was thinking about than "inflatable" as in "air mattress". Basically a purpose specific windsock.

I don't think it needs to be "flat on top", I think that as it transitions from the shape of the rear of your vehicle to something more conical (not comical) is what is desired.

The lightness and cheapness of the mylar is what makes it desirable, as well as the fact that it doesn't stretch out of shape. I don't know if you've had these "Kettle" brand potato chips, but I'd love to get my hands on some of the material they use in their bags. It's nearly impossible to tear, and here in Leadville, where the atmospheric pressure is much less than where the chips are bagged, by the time they get here they are lined up on the store shelf like tightly inflated balloons.

These ideas are great, but some of them are straying from easy and economically feasible. The idea of fashioning a boattail out of cardboard is perfect because all it costs is your time. When you multiply your time with tent fabrics, Spandex, raft fabric, even Coroplast, you need to have a budget of money you would just throw away anyway, in the case of failure. Personally, I'm not in that position. And even if it works great, you are easily treading into the area of diminishing returns if you spend a hundred bucks on a boattail in order to save .0037 cents per mile driven with the contraption on your back...

Keep thinking, but think "cheap"...

[thatguitarguy]

Originally Posted by max_frontal_area

if money is no issue, polyurethane - think modern condoms or speedsuits
however durability would have to be determined...

If your neighbors are wondering about you now, wait till they see you trying to stretch a giant condom over the rear of your vehicle...

[max_frontal_area]

Originally Posted by thatguitarguy

If your neighbors are wondering about you now, wait till they see you trying to stretch a giant condom over the rear of your vehicle...

MetroMPG would love one for his firefly, for rainy days - ya know.

back to mylar. static electricity would be another concern unless of course
your ride is EV and you can figger out how to capture it.
recent research suggests that is how the hindenburg caught on fire.
the hydrogen only fueled the secondary combustion.
.................................................. ..................
steve, how long is your concept cone? will it be dragging and being consumed by crushed toilets and shredded tires when not receiving the min cfm required?
how heavy is it, will it need guy wires to keep its shape when vehicle is not movining @ like early windturbines and double and tripple decker airplanes once used?

i developed drag cutes for gravity racers years ago i can tell you it is not as cut-n-dry as it might seem.

[max_frontal_area]

oh, the DOT would never allow it because of the before mentioned electro-static problems and errand gasoline/methanol fumes, also DOT is picky about highly reflective objects therefore it could not be shiny both sides and a dye or
heat print would need to be added..

[stevet47]

Originally Posted by max_frontal_area

steve, how long is your concept cone? will it be dragging and being consumed by crushed toilets and shredded tires when not receiving the min cfm required?
how heavy is it, will it need guy wires to keep its shape when vehicle is not movining @ like early windturbines and double and tripple decker airplanes once used?

It would be very light. I was thinking ripstop nylon.
In order to prevent it from dragging the ground when coming to a stop I was thinking along these lines... try to follow along....
Say the basic shape is a cone with corners, or a rectangular cone. Along each seam/corner there would be a small ring or loop on the inside of the material down the length of the seam every 8" or so. A fishing line runs down the through these loops, one line per seam, starting at the large end down the seam to the small end. It will be attached at the large end. At the small opening (end) of the boattail the 4 lines get tied together and attached to a single line that runs straight back to the car through the middle of the boattail, not through loops or anything, just straight from the end of the tail to the trunk of the car. Here, there is a retracting mechanism that is constantly trying to pull on this string, and therefore "close" or retract the boattail. I was thinking something along the lines of a tapemeasure.
This way the boattail will be retracted completely at low speeds and extend as speeds increase. And using the loops and strings for each corner will make sure you arent just turning it inside out, it will actually retract closed in a somewhat orderly fashion.

[thatguitarguy]

The retractable idea is nice, if it could be made to work. A retractable dog leash might be able to be used for this, but the tension on these isn't very high. And you might want a pole through the center so it retracts along the pole instead of trying to droop.