Airstream Travel Trailer aerodynamics

[aerohead]

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.

[Hersbird]

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.

[aerohead]

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.

[slowmover]

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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[slowmover]

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.

[aerohead]

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.

[slowmover]

Originally Posted by aerohead

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.

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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[aerohead]

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)

[slowmover]

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.

.

[Hersbird]

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.