Airstream Travel Trailer aerodynamics
 

I spent a week and a half playing around with Dr.Frank Buckley's drag data from the late 1970s 'Sedan/Airstream' wind tunnel study at The University of Maryland.
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*I used a 1975 Chevrolet Impala 4-door Sedan as the tow vehicle:4,222-lbs,Cd 0.508,Af = 24.6 sq-ft (est.),CdA= 12.49 sq-ft.
*I used data for a 1975,31-foot,5,035-lb Airstream 'Sovereign',estimated at 86.66% gross frontal area,or, 58.66 sq-ft.
*As a tow vehicle,the Impala undergoes a 43.4% drag reduction,or, Cd 0.287,giving it's wake to the Airstream (D.M.Waters),and its power requirement drops to 12.52-hp,down from 16-hp.
*Using 94% of the rolling force coefficient estimated for the Impala (no differential,no propeller shaft,no transmission tail shaft) I estimated the power required to overcome the friction and tire losses of the Airstream,at 8.979-hp.
*The 34.6-hp of the 'rig',minus the power for the Impala leaves 20.07 hp to move the Airstream.
*Subtracting the 8.979 hp for it's rolling resistance leaves 13.090 hp to overcome aerodynamic drag.
*At Af=58.66 sq-ft,and solving for Cd,we get 0.196 for the Airstream in the shadow of the Impala.(equal to a trailing NASCAR in a 2-car draft,not quite as low as a Cd 0.167 bus,drafting another identical bus).
*Multiplying by it's frontal area to get it's CdA,adding the CdA of the Impala,and dividing by the Af of the Airstream,we get a coefficient of aerodynamic drag for the Sedan/Airstream combination of Cd 0.314.
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*Cd 0.314 is 40% lower than some Cd 0.53 combinations presented by Hucho.
*A 40% drag reduction would equal a 20% fuel economy advantage to the Airstream at 55-mph as tested.
*Airstream advertizes a 20% fuel economy advantage for it's trailers.
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*Based upon width and height,the Airstream has about 6.3% lower frontal area than the 'box' trailer it is compared to,which is part of it's lower drag.
*The bulbous nose,upper edge radii,and trailing surface radii,and belly pan would make up the difference in drag .
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*Testimonials from owners suggest that,on a frontal area basis,a longer,or shorter Airstream will require about the same amount of fuel to pull regardless of weight gain/loss,unless experiencing aggressive road grades.
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*Without the rest of Dr. Buckley's research,these values must be viewed as only an estimate.And of course,different tow vehicles will alter things.;)

They should combine the Hi-lo clamshell with the airstream upper and endcaps. It would cut probably 18sqft off the frontal area. I should try an airstream with a rotted lower floor and frame and put the upper part in my rotted top Hi-lo.

upper and endcaps
 

1960's California-think would have a few friends go in together to rent an Airstream/Argossy/Casita etc. travel trailer for a week,then take it to someone's shop where they could make a plaster splash of all the compound-curved parts from which they could create fiberglass reproductions for their own projects.
At week's end,pull off all the protective barrier films and return the T.T.,telling what a wonderful trip they'd had.;)

Thank you, Aerohead!!

The former head of A/S has a rental company on the West Coast that offers trailer and tow vehicle as a package.

In the mid thru late 1990s the trailer was modified to a wider body across the line. Different curvature at all junctions. Though I'd doubt any significant change in wind handling.

The real "problem" with A/S from about 1990 onwards has been weight gain. A trailer that could have used aircraft honeycomb floors and aluminum cabinetry instead went on a weight gain binge.

My 1990 Silver Streak -- 35' in length -- is lighter in all ways than a current 28' A/S. What this means is that the tongue weight is heavy enough in the trailers this size and larger to start to push out suitable tow vehicles which are also more economical in solo duty than a pickup.

While the lowered HP demand is great (it is) I would say that th combination of torsion axles and crosswind handling is what makes this iconic brand the real value it is. Andrew Thomson of Can Am RV outside Toronto writes of putting this trailer thru the slalom and being able to drift the trailer behind a properly hitched and well chosen tow vehicle. This, my friends, is what makes this brand desirable. It's just an excellent all around design.

Compare to a box trailer and it's completely unacceptable leaf suspension, the handling of heavy crosswinds with those rounded edges means that the wind does NOT pile up down the length of the trailer to start sway, but passes over and "pulls" as it tow by (a way of stating advantage in an earlier post by the OP). This is a lessened effect. It has not been unusual in my experience to get out at a stop and have the vehicle door ripped away by winds I was pretty well unaware of. Winds that cause tractor trailer and especially 5ers to have to pull over and wait out.

FE is great, but safety trumps it. Careful specification of the tow vehicle is what maximizes design advantage as to lowest fuel burn.

The typical towing penalty for a travel trailer is 40% from solo where the rest is the same. Up to 50% is the range. For an A/S it can trend down to 30% in the same scenario, but one must be painstaking to establish the best mechanical baseline.

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I "know" a man from the Airstream forum who pulls this same trailer as analyzed above. A 1972 29' Ambassador being pulled by a 2014 Dodge EcoDiesel 1500 4WD automatic. 8-speed & 3.92 rear gears. As the owner notes, this option-loaded truck would be the least fuel efficient example.

On a certified scale, the truck weighs 6,280-lbs loaded for travel. The trailer weighs 6,650-lbs. (He notes that he is shy by more than 1k on trade capacity used). The truck weight is not quite 1000-lbs over what that 1975 Impala would have been with a couple of people and some gear. And it would have seen maybe the same 7-8 mpg as my folks 1976 Cadillac saw back in the day with a heavier Silver Streak. His Ambassador is fully rebuilt right down to new axles and dead-nuts alignment on very good tires (has an Airstream restoration business).

The owner of this rig (at 12,930-lbs) sees mid 14's with a well sorted weight distribution hitch while traveling two laners across Central Texas at approximately 65-mph. No cruise control. A recent 900 mile in state trip was at a hair over 15. The same rig but with an earlier Toyota Sequoia was at 8-10/mpg.

Overall, his Fuelly report is showing right under a 21-mpg average for all miles (21k miles). Highs to 30-mpg.

Most RVers aren't terribly interested in mpg. This is "a vacation" after all. Setting cruise is about the limit of things, and only then if on the Interstate. So comps against other reports of this same tow vehicle with a boxy conventional trailer have to be done line by line as to weights, travel speed and terrain.

As I'm at 15-mpg or better (much better depending on winds and traffic) with a rig which weighs an additional 4,500-lbs, it should be noted that I travel more slowly and am willing to work for highest mpg. When I purchased my 2WD manual trans one ton in 2007 (2004 model 305/555 engine) it was a clean paper purchase of truck and trailer to maximize mpg, not just longevity and reliability.

I searched for and found (in the south central US) just over a dozen examples of men with the same truck pulling 28-34' trailers of this type, where weights were 7-11,000-lb) and travel speeds were from 55 to maybe 65-mph. The mpg range was 14-16 which told me I was in the ballpark. These numbers were from mid-1990s Dodge Rams up to about 2008 with the Cummins. Later models suffered from emissions controls, and we all lost a bit of mpg with the changeover to low sulfur diesel (some of which can be re-gained with proper fuel additive choice). Post-2013 models should be back to this range.

No Ford was ever in contention on pre-DEF engines and only the earliest GM Duramax engines were competitive in that twenty year period.

Another Ecodiesel owner who spec'd for economy (2WD) and has a shorter, lighter Argosy (a short-lived low priced Airstream model) has seen as much as twenty (but without supporting data as to hitch set and Fuelly report). For both those rigs it would be great to have some A-B data of the same course run at the same speed both solo and towing. It's hard to zero in on the percentage change.

Now, there are those towing with more fuel efficient vehicles. And regularly seeing above 20-mpg where the TT in question is 25' or less. I chose these examples as trucks are popular, but not necessarily the best for mpg. (One might be surprised at the number of those carrying loads of firewood for multiple nights of campfires, ha! Or, like me, where truck and trailer are home.

The overall average is the FE number that matters. From the average highway mpg to the average mpg while solo. What's the fuel burn penalty for towing? I'm at about 36%. This trailer design shrugs off the wind problems and maintains a higher average.

It isn't all just straight line concerns.

36%
 

That's a coup! With a RAM Ecodiesel,away from mountains and wind,an owner might glimpse 18-mpg while pulling.Then 28 running solo to and from campsite.
This is reverse-engineered alien technology sort of numbers for many.:D

2WD and a moderate load I believe he would AVERAGE 18-mpg towing a 25' or somewhat longer trailer given non-mountainous terrain and use of cruise control at a travel speed just below 60-mph with a moderate load in the truck. AND was painstaking in all the mechanical details. Far more than 90% are not.

I've seen solo numbers at 30 for the above. I just have not seen anyone do A-B testing with/without trailer, much less an aero all-aluminum one.

It on my list to do. The painstaking parts and the records establishment. Still digging out of the hole from divorce. When I can, I will. A thread here and elsewhere with pics and details. But don't hold your breath till it happens. Have to go back to work now replacing grey polybutylene plumbing with PEX in this trailer.

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A-B testing
 

For less than $1.00 per capita each year,we could fund hundreds of millions of Dollars of A-B testing at the Transportation Research Center in Ohio, to do test samples of available T.V.s and T.T.s each year and provide 'window stickers' to potential customers,as is done with passenger cars and light trucks.
Real estate agents wouldn't expect you to buy a home without knowing the square footage,or how much insulation was in the attic and yet EVERY RV salesperson IS selling motorhomes and travel trailers without divulging any information about how that RV will resist the clutches of the atmosphere,something which literally dictates for the most part how it will perform at the gas pump.
It's impossible for customers to make informed consumer purchase decisions when they're operating in a a complete information void.
(I can't afford therapy,so I appreciate the tolerance while I rant):p

A lot of meat on that bone. I going to suppose we've all heard of "regulatory capture" whereby Federal agencies give the fig leaf imprimatur of "safety" to a product used by citizens though the definitions of what constitutes "safe" are written by corporate attorneys in connivance with Federal administrators who plan to leave the public sector for a far higher paying private sector job as a result. Our latest Attorney General is case in point. This extends throughout Federal and State agencies. I wonder if it is even any longer considered what it is, and that is corruption.

As to travel trailers it appears little different. A group of industry captive engineers [SAE] has advised on towing standards for around fifty years [1965; Bundorf]. In the interim has addressed related concerns and made somewhat different recommendations. Ignores entire classes of vehicles as adequate tow vehicles by neither testing them (and the OEMs give them generically low numbers as to trailer weight; one hand washes the other), nor using more than one type of trailer to be representative. A high wind load, high COG travel trailer is considerably different than a low open trailer with very low COG. But the recommendations on how to set a hitch don't reflect this.

In fact, hitch set recommendations have been revised due to "more extensive testing" (my paraphrase), and that this has revealed something on the order of" yaw induced oversteer" when a weight distribution hitch is set to old standards. Handily, that helps eliminate lighter, more efficient vehicles from consideration. But a disinterested party , in simple testing, has demonstrated that these OEM recommendations result in worse, not better, handling. Increasing the chance of a loss of control accident. Funny, ain't it, how the experts took a half century to find the purported problem.

In turn, the acknowledged engineering experts are happy to take trailer industry consulting or speaking fees and what we have is no improvements to trailers since the Model A era, and worse as to what constitutes best practice. I could fill this post space with changes to cars in that time only noting, not explaining, them. But trailers, and the risk they pose, isn't a concern where ones bread is buttered. The low total miles and the total risk is borne by those politically invisible thus disposable Americans.

Towing standards are, essentially, a set of recommendations for a subset of vehicles with only a subset of trailers. And the recommendations of how to set a weight distribution hitch based on these -- against a half century of experience by owners -- upon experimentation turn out to be wrong. But fit nicely with the profit motive of pushing low cost, high profit pickups as the ONLY choice. Nicely done, boys. The least safe roadgoing vehicle coupled to industry standard trailers with suspensions unchanged since the dawn of time (not even As good as the leaf sprung designs on 19th century buggies; the science, like aerodynamics, is long understood).

Be careful of what you wish for. Something is not necessarily better than nothing.

IMO, the safety aspects outweigh the concerns over FE. FE, properly understood, is a part of overall better road performance. It trails in the wake. Better suspension, brakes, tires and then aero will achieve what you're asking. Crosswind handling means more. Minimizing risk. That it ALSO achieves lower fuel burn is the proper order of things.

But finding the disinterested party to test is what's gone missing in this civilization. Doctors or lawyers or plain old scientists (see the latest fun about so called peer review). There is no disinterested party as much as we might wish it that profit does not come first and foremost. Good luck finding monies to test. Tax monies are released to test that some private entity may benefit. Not otherwise. Not any more.

If it weren't for a very few willing to test their rigs as private individuals we would t have even this.

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I think the profit pushing "science" come from two sides. Not just those who want to get rich but those who will spend wealth to get power. For me it's easy to spot "science" put out by those who want your money. Worst case you are out some cash and life goes on. It those seeking power that really have effected lives.

testing
 

Well it looks like a few of us here at EcoModder will have to self-finance the free consumer information for the entire globe,just as we've been doing.
And I'm sure to make an absolutely fabulous contribution each year with my sub-poverty level pension income.
Dumpster-diving for cardboard and second-hand resin from Habitat for Humanity resale stores.Yeah,that 'll change the world alright.

I can attest to the smooth function of the torsion axles, our 1990 avion had them and it towed so much better than the current leaf sprung trailer.
My wife got a drink of water after we had secured for travel, and left the half full glass on the counter. We arrived home 100 miles later and I found it there, undisturbed. The next trailer will have torsion axles one way or another.

Thanks again, Aerohead.

Numbers do go a long way towards explanation. It's difficult to explain to others that a premium aero trailer pays for itself in longer life, reduced operating cost, and all around safety when underway. It ain't just fuel mileage, nice as that is.

And a ten year old Airstream isn't old. A conventional TT is ready for the scrapyard at that point ( get out the moisture meter. Those numbers don't lie, either). One needn't spend a fortune in acquisition, nor in operation when an ordinary car can be spec'd to be a good daily driver and tow vehicle. The current assumption is that a pickup is the default choice. And it needn't be.

pickup
 

There's a website that does unofficial mpg testing of RVs when they ferry them to their facility.
They claim that a VW Jetta TDI pulling up to around a 19-ft Airstream can average like 38-mpg,compared to 50+ mpg solo.I'd be willing to downsize expectations to enjoy such efficiencies while towing.
Beluga Caviar vs cat food with the fuel savings! Adam Smith would give it the nod.;)

Yes, I've seen claims of above 30-mpg from unimpeachable sources. But do not know details (travel speed, terrain, etc).

There are thousands of used Airstream trailers. For someone wanting to build a trailer, I'd strongly suggest rehabbing an old AS first. Will be done faster and better if original lay out is respected.

I've got a '96 F250 automatic turbodiesel that I bought new 20 years ago for towing my travel trailer. At the time, I had a 27-foot 1967 Airstream, and with my new F250 was averaging 16 mpgs towing on the highway. However, when the Airstream began showing its wrinkles at 39 years of age, with its floor developing some soft spots and the frame needing some attention and etc, I finally got tired of the idea of putting more work into it, so I gave it to one of my kids for their starter rig.

I wanted something new for when I retired that I would not have to work on. I considered a new Airstream, but choked at the $50K cost to play. So I bought a new 28-foot Jayco for $17K that has torsion bar suspension and weighs less than the equivalent Airstream. While it tows just as well on the road, due to the unfavorable aerodynamic differential, I did lose about 1.5 mpgs (now averaging 14.4mpg). On the other hand, I saved $34K in purchase cost. Even though I didn't like to lose the mileage, I figure I got my Jayco for free because the Airstream would have depreciated by what I paid for the Jayco in the 10 years since I bought it, and the Jayco is still in excellent shape requiring no repairs so far as long as I am diligent watching to assure the roof keeps properly sealed.

My old Ford used to be among the leaders in towing fuel economy. Nowadays, there are new choices on the market, such as those diesel Mercedes and VW cars and SUVs and those half-ton diesel pickups which regularly are seeing 18 and 19 mpgs while towing travel trailers. It's true that those rigs aren't as heavy as mine, but their efficiency is nonetheless amazing. On the other hand, considering that I bought my F250 new for about $26K using 1996 dollars and the Jayco new for $17K in 2006 dollars, the newest rigs can't touch me for relatively low overall investment, even with their increased fuel economy on the road. But they sure are nice.

That's my story and I'm sticking to it.

Quoted For Truth. I hope I don't push that off the top of the page.

My story... 50-year-old 35' R-license park model located next to a Grocery Outlet, with 5¢/kWh electricity. I got it for $900 cash, with a membrane roof and an airconditioner. No TV so gas mileage is infinitude. Why would I want to go anywhere, when the whole world comes at me.

A thought... They make these beautiful polished and clear-anodized shells with a more pedestrian welded steel chassis; then separate them with a thin, 5/8" layer of something not much better than sliced lunch meat.

Suppose one got a salvagable shell and cross-braced the gutted interior like when you're chopping the top on a car, then lift it and substitute a layer of swing-door material—aluminum skinned plywood. On a sand blasted and POR-15'd frame.

I'd put a perforated Gurney flap all around the rear like the tail of a shuttlecock. :thumbup: Mostly within the wind shadow of the shell as a whole, it would slow the air close to the shell and pull the flow inward. In fact, that might be a field expedient fix for 1930s boat-tailing in general like the V-S V2.

Having recently seen a few “I want to build an RV” threads (which takes years away from the object: going camping), figured I’d bump this back up.

HP demand.

On a typical RV day of travel (1968 or 2018) of around 300-350/miles, how much time is spent at that HP demand number? Five or six hours?

How many days per year as the average RV’er covers 5,000-miles? Fifteen?

I wouldn’t recommend fixating on those hours as the solution to less expensive vacation travel. As MeteorGray notes above, how much one has into the thing PLUS longevity, is more telling.

Airstream aerodynamics are such that pushing past them isn’t worth the effort.

The “less expensive“ route is an Airstream model that works great behind the family vehicle (most all of them). Is about ten years old so that depreciation is nil, and systems still close to new.

Both vehicles will spend 90% of their lives stationary. Reducing the number of stops (or cold starts from the campsite for a solo run) for supplies replenishment (water, ice, groceries, propane) NOT just tow vehicle fuel is how the game is played.

That’s the larger picture.

Next up is longevity. My vintage kin Silver Streak will be thirty years old next year. Looks close to new. Only now am having to replace some items and re-decorate. (Its contemporary square white plastic box travel trailers were tripped into the landfill years ago).

How long will you keep it?

(Let that question guide the rest).

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Here's what I was talking about then (from JACG a year later):

https://2.bp.blogspot.com/-rGpxD1p69...35048433_n.jpg
http://justacarguy.blogspot.com/2017/06/aspects-of-restoring-airstream.html

I think there is a way to make a modern airstream shape and style for much less that would perform just as well. I just wish a big company like Forest River or Jayco would try. The riveted aluminum is great but I think a SIP with a radius for the mid sections and molded endcaps. They kind of tried it with the Aviator but went off the deep end with cost and upgrades. I think there is something to be done somewhere between a $20,000 standard TT and a $75,000 Airstream.

Polymetal?

http://ecomodder.com/forum/member-fr...1-100-0866.jpg

It comes in 5x10 and 4x12 sheets. With an adequately large roller you could radius it lengthwise.

Your suggestion at http://ecomodder.com/forum/showthrea...tml#post568521 for SingCore is good. For interior partitions, etc.

There's also swing door material for the deck.

Getting away from aluminum, there would be basalt cloth in Super Sap epoxy, for the end caps.

The Airstream shell needs Faschenfeld tearing edges, something like this:

http://ecomodder.com/forum/member-fr...17-trdrp1a.jpg

That's actually closer to the Airstream Basecamp than the classic trailer.

I always thought the Saprtans were upscale to Airstream, but I just learned about Palace:

Just A Car Guy: The Palace Royale
https://3.bp.blogspot.com/-n0qJvfjMg...e-Royale-5.jpg
https://1.bp.blogspot.com/-aJIy-FM-P...944_palace.jpg

Spartan and Oalace aren’t “family car friendly”. Look at the shared post war Clipper both Airsyream and Silver Streak produced.

For today, the suspension AND aero on an AS are superior to the probably better built other lines. But they’ve been gone a half-century.

I won't disagree, I had a 1952 Silver Streak, and of course I now have an Airstream.

You know the Palace? It appears to be stamped steel. The raised center section may be to stiffen the shell.

I'd want a bobber truck for a tow vehicle anyway.

http://smclassiccars.com/uploads/pos...54-chevy-3.JPG
Rat Rod truck, GMC CoE, Diesel truck, Bobber truck, 1947-1954 Chevy - Classic GMC Truck 1949 for sale

That Rat Rod would be cool. Diesel turbo whine is the thing.

The steel Palace rusted away, I feel sure. I’d have noticed them in our family travels in the 1960s. Might have seen a few, but longevity wasn’t their point. The narrow inboard suspension was no help. Would have been rollover prone.

I agree about the stamping.

Have you ever saw somebody towing a pontoon boat down the highway with one if the narrow sizzor lift trailers? I can see moving them around the boat yard on those but they look scary on the highway.

I'd saw the pontoon boat in half lengthwise and hinge the halves on either side of a full-width trailer. Upside down on the trailer for the road, hinged and locked to make a 14ft wide platform to roll an Airstream onto.

It would have a jetski docked at the back of each pontoon that run in tandem to move the whole thing.