Cargo trailer drag estimating

[slowmover]

Glad you brought up those points before someone searches one out. Trailers that age demand a high level of skill. Aerowood for skills guru on AIR.

By the mid 1960's this is not quite so given a trailer stored under cover. 1983 and later is fairly conventional in all respects with fewer frame problems (being able to use standard RV suppliers past Airstream-only minor items); 1996 and later for the "wide body" thus new in A/S terms .

There are those rigs at 20+mpg with the latest (and not the smallest) Airstreams behind TD SUVs.

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[slowmover]

Originally Posted by ennored

Do we know the drag coeffcient of a typical (any?) Airstream trailer? Google hasn't yielded one for me yet.

A little better than a sphere (≈0.47) maybe? Worse due to awnings, A/C, and other stuff?

I think we tried an estimate earlier. Search posts by aerohead and Airstream.

The gap between tow vehicle and trailer is more to the point. And, that crosswinds do not pile up and push against the trailer so much as that they flow over the radiused edges and exert a "pull". Much easier to live with and definitely has less of an affect at the steering wheel.

THe Airstream supplied figure is that it takes 20% less fuel to pull one.

HI-LO is another brand to look at.



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[slowmover]

Originally Posted by aerohead

A home-built Pappose would give standing room with minimized frontal area,nice drag characteristics and hard sides (Grizzly bears at Yellowstone).
http://cdn.c.photoshelter.com/img-ge...mm0062mod4.jpg

I'm in favor of hard-sided trailers, but a black bear -- god forbid a grizzly -- can tear open the much tougher sheet metal of a truck. Bring the lever-action .358 Winchester along.

[slowmover]

Originally Posted by Fourbtgait

The 30% fuel mileage loss held true for the F150 with a topper pulling an 18' travel trailer, 8' wide, 10'6 tall at 60 mph. Frontal area killed the mileage as it was 4' higher than the tow vehicle, as did mountain roads.
Hence looking at a trailer whose frontal area does not exceed the tow vehicle, would have thought the loss would be less than 30%.
I have thought of building my own box to go on a small flatbed trailer, wheels enclosed in the body, not extending out past the body as in a cargo trailer. If I did so, design would be like the previously mentioned airstream, giving me 6' headroom inside, so then extending above the tow vehicle.

My TT is considerably taller and wider than my pickup. Yet 16-mpg was not hard to achieve with the last one that was one foot shorter and no lighter than the present one (see link in revised signature; pic of rig about post #25. Pic can be expanded several times). Turbodiesel is the answer to trailer towing mpg. 30% doesn't hurt in this instance. And these smaller vehicles (light duty trucks) can comfortably tow TT's with power to spare.

As to building ones own, having the trailer wheels outside the body means lower ground clearance and wider stance in re roll center. Much better stability. UHaul does this for this/these reason. Torsion axle is the way to go, not leaf spring.

My trailer has interior headroom of 6'4'. Exterior clearance with A/C on roof is 9'8". Width is 96". Height is the killer, not overall frontal area. I would much, MUCH rather have a trailer six feet longer than one foot taller.

Look to pics/spec of 1960's Streamline Duke for dimensions.

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[freebeard]

THe Airstream supplied figure is that it takes 20% less fuel to pull one.

It has been estimated that a 10 percent reduction in Cd can lead to a 5 - 6 percent improvement in fuel economy at highway speeds and a 1 - 2 percent improvement at urban speeds. (source: http://72.14.203.104/search?q=cache:...22miles+per+ga llon%22&hl=en&ct=clnk&cd=2)

http://ecomodder.com/forum/showthread.php/side-mirror-drag-effect-fuel-economy-quantified-95.html#post936

This suggested 40% improvement, but over what? 0.50 ==> 0.30, or 1.0 ==> 0.60?

[Hersbird]

The problem with a modern airstream and a modern diesel to pull it is you will never, ever, save enough on gas to offset the price compared to a $8000 10 year old common Chevy 5.3 Tahoe or pickup and a new $15,000 Jayco mid 20 footer. The first 250,000 miles is basically free. Personally I think the $15,000 Jayco charges is robbery compared to what you get let alone a $80,000+ airstream. Why I figure I will just build one. And if you are going to build one you might as well improve the shape a bit.
I get what you are saying about size and weight, but to me 30 feet is just wayyy more then we want. With all the mountains and curvy roads around here the weight does kill mileage. No matter how you try to drive you will be burning energy with the brakes. The reason I was talking about cutting down a big airstream was because and old 20' is still to much money while and old 30' is pretty reasonable. I also don't really care about, "man that looks amazing inside, like a fine yacht!" I just want warm and dry with a place you can sleep and stand upright in, a place to heat up some soup, and a place for my wife and daughter to pee in the middle of the night. Otherwise we aren't hardly in the thing from 9am to 9 pm. Our last setup was a big 12' lance slide in in a duramax crew cab. We could get 11-14 mpg (although with diesel $.70/gal more here not much better then a 454 getting 8-10 mpg) and also take our pontoon boat but the pontoon needed a new outboard and the truck sat in the yard all but 3 weeks out of the year and a couple weekends so we sold it all, about a $20,000 total. That $20,000 is what I want to keep the new setup (camper and puller) under but get the 14ish mpg on less expensive unleaded.

[freebeard]

I say go for it. Salvage a trailer frame and construct the shell out of rolled 5x10' sheets. Two sheets per 5' of length and a single seam down the middle of the top. 20' needs 8 sheets, so at $125/sheet you have a single-layered shell for $1000.

[aerohead]

I've never seen numbers published for Airstream.Hucho shows a few caravan Cds from the late 1950s in his first book.
Since the tow vehicle/trailer combination is what we're after,we'd have to have numbers normalized to some specific reference tow vehicle.And the composite Cd is based on the trailers frontal area.
For a clue about the Airstream,look at the radiused bus body of Cd 0.314,and then think about that Cd being lower,as it is partially shielded while drafting the tow vehicle.

The gap affects Cd

And if the tow vehicle is sufficiently large,the trailer drag can be cut in half.

It's complicated

[freebeard]

Since the closest fit is at 0.314 Cd, it appears that the tapered Airstream might have 0.263/0.314 or 84% the drag of the donor vehicle. Replacing the back window with a door would create about the right amount of taper.

I never intended to keep re-posting that manila folder and masking tape model. It was based on a 4x12' sheet of plywood with 4', 3' and 5' bottom edges.

I shall revise it into a 20'er with 4ea 5' bays. The first a wedge, the 2nd a straight barrel section, a third with the [single-axle] wheelwell and taper in plan but not elevation, and the 4th as a transition to a Gothic arch.

aeerohead can confirm I'm heavily into Gothic arches.

[ennored]

Something in that series of "buses" doesn't add up.

.88 Cd to .36 by just rounding the front? That doesn't agree with other known Cd's I've seen (NASA and others).

I think the claims of an Airstream being about 30% better than a "normal" trailer are probably close, some from a better Cd, and some from a smaller frontal area. Which is also in the same ballpark as one of the NASA studies where they rounded the front of a boxtruck. 30% better than .88 is .62, way off .36.

Which is all just speculation. No real wind tunnel tests = no real number. Seems like there isn't one out there.