Inflatable boattail
 

I just found an article on semi-truck aerodynamics located here.

Scroll down a bit and it mentions the concept of an INFLATABLE BOAT-TAIL.

Why not? I have seen giant inflatable movie projection screens and even heard of an inflatable car.

An inflated boat tail sure would be light.


Who wants to be the first to make one of these things!?

Inflated boattail worked on T-100
 

Back in the 90s I did a inflatable boattail for the T-100 ,when it was John Gilkison's.We took it on a camping trip near the Very Large Array,in New Mexico,and did a roundabout long way home,just to get more miles.For about 600 miles of driving,the inflated tail gave over 30-mpg.We were quite pleased with the performance.It was constructed of sewn naughahyde,tailored over patterns drawn for a 12-degree convergence angle (top and sides),stapled to an airtight box constructed to fit the tailgate region of the truck.A 12-VDC heater fan out of a Volkswagen Westfalia camper van provided the inflation pressure,which was powered through a simple electrical umbilical cord, alligator-clamped onto the truck's battery,and strung underneath the truck to the rear.A larger unit would no doubt help a 18-wheeler,although thats outside the scope of what I can take on.A fellow in Lubbock,Texas has a U.S.Patent on such a device for 18-wheelers.I've seen photographs of it.He has a lot to learn about aerodynamics,and could look to the German designs of the 1920s,where the concept originated.Hucho's book,"Aerodynamics of Road Vehicles",published by Butterworth's has a beautiful image of an ideal design.

It would be neat to inflate for highway travel and deflate ot park in the cities, too.

Nuts. And here I thought I'd come up with a Great Idea (tm).

I've been noodling on this for a couple of weeks since I read the thesis from the guy at the Naval Post-Graduate School. There was a picture in there about research NASA did in the 80's (IIRC) on the optimal shape for a large truck/bus design.

A sloped bedcap + an inflatable boattail would be great for my monster F-350. When the weather improves, I plan to have another go at grille block mods, as the bed cap is a larger project and/or expense.

Plus in a rear collision you would be in the privileged position of having air bags outside your car! It would good to have one with a semi rigid bottom section that allowed the tail to curl upwards as it deflated into the pick up bed. I'm imagining two hollow tubes on top of each other with a cord or strap through the middle of each of them (sort of like those poles in modern tents). By pulling the top string as the tail deflated you could have it curl up like a monkey's tail rather than sagging down to the ground. Then you might have the possibility of doing while driving as you entered the city and needed a shorter length. I am considering buying an inflatable kiddies pool or large beach ball to carry out some experiments hehehe.

A shop vac is a great way to inflate anything.

I made the mistake once of inflating a queen-size airmattress in my livingroom with the shop vac. Took about a second and a half. Almost smashed a couple of lamps with the mattress.

Shop vacs can also be setup to suck or blow - so you could inflate or deflate your boattail.

How about using the exhaust to inflate it? After all an engine is an air pump. Many moons ago, in design school we "invented" a low cost shelter . Take an igloo shaped "balloon", blow it up with exhaust, keep inflated and spray water on it in fine droplets. In sub zero weather. You can tint the water with food coloring. Spray till you have a 1 -2 inch thickness of ice. Pull out deflated igloo and enjoy.
I was looking for a low cost shelter for the ice fishermen here on the Richelieu river, then I proposed the idea to the national capital Commission in Ottawa. We have after all the world's longest skating rink, 7 miles of it, and a few of these shelters could be enjoyed. At night a candle light placed inside would transform them in giant lanterns. Anyway, I used the exhaust from the water pump to inflate....I got off track....

Years ago, I saw an infomercial for a car jack that was just a big plastic bag you stick under your car, then use your exhaust pipe to inflate it.

I wouldn't trust that to hold up my car, but might work great for inflatable aeromods.

I saw that one too. Very rugged. Made in England, second world war technology. Same stuff as the curtains around a hovercraft.

I had one of those. It worked good. The major drawback was it was large and combersome.

Remember though, this itself will take a lot of extra fuel. As the bag inflates, pressure head increases.

You don't have any pictures or diagrams perchance?

Subscribed:
I Want one :-)
Bad

I think I posted a pic in the Misc.Photos section over at maxmpg.I've got over 200 photos over there .I'll check when I get a chance.If I didn't,I'll try an dig up the image and re-post it.

I wonder if the Fun Jumps (bouncy buddies; etc) people have a forum?
I tried to get a local place that sews all of their own jumps to build me an arch (few years back) and they flat out refused on grounds that it might not work.
Anyway; those guys sew up some AWESOME inflatable stuff; I can't see why someone in that field (so Inclined) couldn't sew something up.

Any Body know Someone in the field?
Incase you guys haven't figured it out I'm really psyched on this idea.

Heck, all you really need is an inflatable "bed" mattress and a big jar of contact-adhesive.
Get one of the inflatable beds with a built-in pump (that inflates and deflates) and cut up the mattress to make your boattail. There's plenty of material there to work with, and its rubber, so it'll glue up with contact cement wonderfully. And for tips on how to properly glue up rubber with contact cement, just watch the Mythbusters episode where they escape from Alcatraz on their homemade inflatable raft.

Here's the pic, I think.

The plan is to set up a gallery here so we can host those photos on EcoModder. It's on the massive to-do list (which we're slowly chipping away at)!

http://forum.ecomodder.com/attachmen...1&d=1203131237

I see you've also got the half-tonneau in that image.

I've been thinking about this a lot lately.

My biggest question at the moment is how many/ much internal braces would be required so it did not turn in to a bubble, but stayed shaped more how I'd like it to look. Do you guys think it woud be posable to have all of the braces run paralel to airflow? (reduced drag)

I am considering using pictures of Basjos's car from side, top and using that as a pattern for the top/ sides (scaled to golf size) (and ofcouse if he would give permission).

Considering convertible plastic rear window material for the upper portion to retain visibility(in a final version) Maybe visquine to start.

As far as stiffness goes, I'm not sure where/ how, but I believe that the structure could be greatly enhanced with plywood panels and / or tent poles glued/ sewn in.

And a final thought, wouldn't it be great if it could be collapsed entirely just by sucking the air out? How about using the tension from the top to hold the boat tail from hitting the ground, then having tent poles (or plywood)hinged in 2 (folding so the "V" is inverted) collapse in such a manner that the entire assembly sits between the bumper and the back window?

The old Subaru Brumby (BRAT, I think they were called in other parts of the world) were quite aerodynamic with their canvas cover on (which was partly transparent so you could see out the back). One of those would only need a smallish wedge-shaped tail on the tailgate!


This would also be a great idea if you're towing a heavily-laden trailer on long trips. When I drove across the country I was lucky to get 350km to a tank (64L). Not because of my car (which was full of stuff + surf ski on the roof), but because of the heavy box-shaped trailer in back.

An inflatable tapering tail and some kind of curtain to join the car and trailer would have worked wonders.

Here's an idea for turbocharged engines, you could basically bleed unused boost to the boattail instead of just opening the wastegate.

And, shoot the bleed air out the back of the car either for rocket boost, or for boundary layer control .

Not so sure about bleeding boost off, I believe the boost gives a fuel economy boost, Not at all sure on gassers, but on diesels 4 sure.
However if it was just to inflate, it might not be an issue . .. . . .
I'm banking on a 12 volt inflator from an air mattress, or equivalent.

Hi Dremd,

Yes, of course. The pressure that makes it to the engine from the air pump is controlled however. Because at certain running conditions the pressure could be very very high. So high, that if allowed into the engine the fuel octange would not support a controlled burn, but detonation. So on a turbo there is a device called a waste-gate. It shunts some of the pressurized air either out into the atmosphere, or back to the pump input stream. The energy to pump up that waste air is wasted. The idea would be to use it to help push the vehicle, or reduces its air drag.

The mileage figure you achieved with the boattail and half-tonneau was great. I did not see a pre-mod mileage figure though; do you have that data?

I need to add my other vehicles to my ecomodder garage

1 87 Supra T77 Drag only Tial wg (open) sprung at 18, Boost controller set at 22 Liquid 2 air intercooler (8.4/1 compression) soon to be E-85 powered R154
1 89 Supra 60-1 ct26 High compression GTE AE340 (9.5/1 compression) Pump Gas (scca Solo2/ blast on the street with the targa off)

I've wrenched a lot on friends DSM's, WRX (just 1 of them) I've turboed an 07 focus (from scratch), Worked on a 9 second turbo 5 liter (10.5 class), farted around with a turbo caravan, had a tt z32 for a spell, turboed a few civics (bolt on affairs), farted around a little with supercharged fords (not much) done an LS7Rx7, worked a little on ls1 TransAms, and lots of fun with SBC's. (By the way the new BMW TT 3.0 is AWESOME I got to do a little stunt driving in one (and I secretly want one))
On the 89 I once towed a 99 Surburan which was towing a camaro on a trailer for 25 miles (off of the basin bridge) at 55mph I ran a sustained 5~6 psi, I don't want to know what the fuel economy was, made mental note to not check mileage on that trip.
With the boost controller set at 0 the wasegate will open at 6.9 psi.



By far the most efficient boost control setup is VGT. The TDI has one and am totally in awe how much better of a setup it is than a waste gate . . . .

Cliff note, yes I know what a wast gate is, at constant speed they are very rarely open Find me a gas car that has the wastgage open at cruise and I'll do, . . .. well whatever you say.

Could you please elaborate on that? I have and A3 TDI with a wastegated kkk03 and never heard the comment that a vnt-15 on the A4 TDI was "more efficient".

Geese, I guess I don't know how to explain it; other than to say that the turbine Map is somewhat alterable by the vgt, so as to move peak torque around; closer to your current rpm.
Not much here->
http://www.osti.gov/bridge/servlets/...ive/771164.pdf
http://www.patentstorm.us/patents/65...scription.html

Hi dremd,

Sorry if you felt I was being simplistic. But your comments were phrased simplistically. You apparently have more practical knowledge of Turbos than I.

But the method being proposed has been researched by NASA, although not used in production practice. They put little holes on the downstream side of a wing, and injected bleed air from the jet engine through the holes. The goal was prevent flow seperation at low speeds. Inflating an air bag at the back of the car would be alont the same lines. One might actually be able to combine the two, and put small holes in the air bag.

If there is no waste gate operation at highways speeds, there is no bleed air available for this.

If you bleed air off the TDI turbo, will the variable geometry control be altered to bring the pressure back to what it supposed to be ? As you alluded to, there will be effective comprssion ratio lost however. Maybe the VGT control is just shooting for as high an ouput pressure it can achieve, all the time in the TDI ? As the fuel is going to detonate anyway...

If I take my TDI as an example, I agree that at cruising speed on a flat road the turbo does not produce enough boost for the wastegate to open.

In this case a better option might be a 2nd turbo dedicated as a compressed air source.

Donee, the "electrical bleed" that some people do to the VGT 4th gen TDI turbo will only up the boost limit. I'm pretty sure the turbo is shooting for as high as it can for any given operating state below the limit.

First off I need to apologize for my attitude, I was been dealing with a family member with mental illness all day yesterday, That family member didn't drive me nuts, but others did, by constantly defending their (patient) actions. I got to a point where where I was forced to disagre with everything, and that has carried over to this thread.

My position on bleeding off turbo air does still stand, however I still don't know about gasoline cars. I did a quick quasi experiment in the 89 last night (driving the car was like therapy for me) by blipping the throttle at 75 the manifold pressure will jump up to about 2 psi, this will obviously vary from vehicle to vehicle, but there is some pressure available. I almost wish I had FE instrumentation (and a map based ecu) to see what affect bleeding that air off would have.

As far as the venting of gasses out of the tail goes, that is definitely a proven method at high speeds, I'd be interested to see some results at highway speeds.

And another idea for a blower; A/C blower from junk yard.

That is a good question, I think we would have to discuss that with one of the chip tuners. One of their claims is quicker spool, weather that is due to more fuel (and thus more flow), or some N75 programing, I don't know.

Another bit of data may be the fact that when the N75 goes bad the ecu will throw a code before you reach peak boost (around 5 psi) Ask me how I know.

Diesels and detonation
 

It's a common misconseption that there's detonation in a properly tuned diesel engine. Actually the fuel burns as it is injected into the cylinders. There is a small delay after the injection first starts and this is what makes the typical "diesel knock" you hear. The better your cetane number the less knock. The
low and varying cetane numbers and generally low quality of US diesel fuel is a big reason why it is difficult for manufacturers to introduce modern diesels to the USA.

No Joke on the variable quality.
I put some D2 in at a wallmart a while back, Car sounded like a 12 valve cummins, just smaller, added a double dose of Powerservice and a few gallons of bio then it was back to normal. I wish that the Cetane ratine was easier to access at the pump.

T-100 inflateable boattail
 

I think I did post a pic of it over at maxmpg,in the misc.photos.It will be a week before I can check.Also,If you go to maxmpg,and go to "links" I left a link to NASA's research on boattails.The shape is what works,although its not inflated.Crude patterns can be used to tailor the envelope,and I have purchased rip-stop nylon for my next go-around with inflatables.I invented a mechanical 3-way reversing valve which allows the fan to empty and evacuate the envelope for non-highway driving.Simple cruise-control logic can make the whole thing automatic.

the diesel knock could be caused by high cylinder pressure before TDC. the actual impact generating the knock, could be the piston swinging over center from rubbing the compression side skirt to the thrust side skirt.

i have measured about 750 psi right before TDC.

Thats what I've been told.
Knocking noise in a diesel is created when the piston skirt hits the inside wall of the sleeve on the down stroke.
And I've been working on them since 1982.
You want a quiet diesel, make it a Gas Turbine.
in a gasoline motor the pistons are installed so the skirt lays flat on the upstroke.
diesel pistons are 180* out so they lay flat on the down stroke.
so you are not really hearing detonation, only the slap of piston against cylinder.
BTW the Cetane #s are supposed to be clearly visible from the pump nozzle.

inflateables
 

Goodyear Aerospace (the blimp people in Dayton,Ohio) came up with an inflatable airplane for the air force.It never saw service,however,there may be de-classified patent images that would illustrate the internal construction used to prevent shedding eddies from setting up wild cyclic deformations.I think inflatables would be a multi-billion dollar/euro/yen/piaster/peso/etc.,idea for the ground vehicle industry.I'm prior-art,so I don't worry about patents,and any one of you entrepreneurial-spirited modders,with access to venture capital could be drowning in cash if your first to market.You can get the patent later.

You guys are all making this way too complicated: Just look to those self-inflating parasails or parachutes for the answer, where the leading edge rams air into the internal channels to inflate and stiffen the sail. Made of inexpensive Dacron, one could taylor a very aerodynamic bed cap for a pickup truck, a boat tail for a car, truck, or motorcycle, or whatever.

Deflates when you slow down and falls back into the bed of the truck.

That would be a major source of drag.