Factoring drag into design

[asianflava]

The 6ft width is gonna hurt you also.

[myoung]

Has anyone on these forums made a spherical front design like the ironhorse trailers I listed above or an airstream, but out of wood.

We have a 25-foot Airstream and typically get 13.5-14 mpg towing its 6,000+ pounds with a Dodge Durango at 55-60 mph. At the same speeds without the trailer, the Dodge gets 23 mpg.

Many people think that the Airstream has a spherical front an rear, but in fact it has rounded edges with a radius of about a foot. But, compared to TTTs, it is also 8.5 feet wide and over 8 feet high and has all sorts of drag-inducing contraptions on top.

In general, rounding the four front edges and, if you like, the two side roof edges probably is as good as you can get. Also, it is better the have a square back on a tapered body, sort of like a truncated teardrop. See "Kamm effect" for more info.

[GPW]

There's a lot more to it than just the profile shape ... Frontal area presented to the relative wind (cross section) , and parasitic drag... all the little stuff sticking out , vents , fenders , handles, lights ,etc. Skin drag ... rough or smooth surface makes a difference... Rounded edges vs vortices forming sharp edges... Lots to consider ... and what seems logical sometimes isn't always ...
And Drag increases with Speed!!! Want to decrease drag? ... slow down ... Do we Need to haul TDs at 85mph ... Seen folks passing me on the hwy ... blistering speeds , towing trailers ...

[kennyrayandersen]

Yeah, that's not easy -- you'd probably want to do that part out of foam and fiberglass, or get access to an English wheel if you want to do it out of metal.



One other thing to consider is that if you look at really early airstreams and aero trailers rather than have an exact curve they use a series of flat segments to approximate a doubly curved surface. If you had three singly rounded surfaces in the corner, aerodynamically speaking you'd probably already get the same approximate drag as the quarter spherical one.



However, less well known is that the front matters some, but the back is where it's at. The back has much more influence regarding the overall CD than the front. In the front the air builds up and makes it's own pseudo shape, but a flat in the back causes tons of turbulent flow (drag).

Well at first it seems a bit counter-intuitive, but a cone at the end is different than a sphere. Spheres are kind of funky relative to drag -- that's why they put dimples on a golf ball so that the boundary layer is tripped earlier, but it's kind of a special case.

Anyway, if you do a little searching on the internet for drag reduction of tractor trailers (which is probably the most similar in configuration to a teardrop relative to the research that is out there, there has been a LOT of research done on trailers. Basically it boils down to it's really important to have a rear taper (not just a half circle like a ball). And, it's also better to round the front, but it's actually more critical to taper the rear (bigger difference in total drag). As the air piles up on the front of almost any shape, it makes it's own aero shape, but at the rear, you get a lot of turbulence from the squared off edges. When the aft is tapered you essential decrease the aft area of turbulence and therefore total drag.

Unfortunately, most tear taper faster than the air can flow in a laminar fashion anyway, so most of the discussion is for fun and is purely theoretical. The big deal is to keep it small and as close to the tow vehicle as possible. Practically, that what you can do to decrease the tear drag.

Also, as someone mentioned, you are kind of already pulling the tear in some pretty dirty air, so the biggest thing is whether the tow vehicle is as big or bigger than the tear. If the tear is smaller, then the overall drag of the car doesn't changes so much, but if the tear (or other trailer) is larger, as has been noted, the it makes a big difference in overall drag since it changes the area in the drag equation.

[angib]

To illustrate kennyray's point about taper, this British semi-trailer is shaped like this to reduce drag



You didn't just look because of the woman, did you?

[jdarkoregon]

Sounds to me like you should think about a classic "canned ham" shape.

Have a good time, make it lite!

John

[kennyrayandersen]

To illustrate kennyray's point about taper, this British semi-trailer is shaped like this to reduce drag

Note also the side skirts which make a big difference. There were a lot of experiments with cone-like things on the rear as well, which are pretty effective but would be a bit of a challenge to design into the tear.

[Wolffarmer]

I would also build over the wheels and put skirts on them. Use rims that do not beat the air like an egg beater. They will also probably be cheaper. And if you want something flashy there. Paint something on them.





build the hitch to keep it close to the tow vehicle.

I know there are stories out there about people reporting they had no loss of millage and even some claiming they gained some. This I would have to see myself.

I tow with a 1994 Suzuki Sidekick and a 2000 Suzuki Vitara and I loose about 33% or a bit more. Larger motors seem to lose less. Winds are a killer and in my motorcycling and towing experience, unless they are coming from behind at less than a 45 degree angle from straight on they hurt millage. Just my observations.

Randy

[eamarquardt]

To illustrate kennyray's point about taper, this British semi-trailer is shaped like this to reduce drag



You didn't just look because of the woman, did you?

I like the curves on the woman better than the curves on the trailer!

Cheers,

Gus

[GPW]

How streamline could we get ??? This one looks pretty sleek...

[Nobody]

May not be applicable to your situation at all but, here's some 'real world' figures you may be able to use -

My TV is 2008 Toyota Tacoma, 4dr, 4WD, V-6, auto trans, w/Hi-rise fiberglass camper shell. Fuel mileage truck only 20-22mpg depending on speed, terrain, etc.

My TD is 5'x9.5' (cabin), approx 6' overall height, & 13' hitch to rear, weighs in neighborhood of 1250+ lbs loaded for travel.

Just bought a 1984 Scamp, 13' hitch to rear, approx 12-1300 lbs, 6.5x10' body, approx 7'+ overall height.

The TD tongue is approx 44" from closest curve on the body to coupler while the Scamp body is approx 35" from body to coupler.

I've towed the TD more'n 5K miles with this TV, over all kinds of terrain from 400' to 10,000' elevation, with tailwind, against headwind, cross winds, etc., at speeds from 55-75mph. Have gotten as high as 21mpg & as low as 15 (in mountains or facing high winds). Under normal conditions I average 17.5-19mpg, again depending on terrain, road conditions, etc.

I picked up the Scamp near Memphis last Saturday & towed it home 168mi at 65mph, avg 17.3mpg with a 15-20mph cross wind.

Both trailers are approx same width as the TV, neither stands much higher than the rear 'up-sweep' of the camper shell, & tongue lengths are close enough that TV 'wind shadow' should be pretty close. The Scamp has NO sharp corners/edges while the TD has curved top & bottom, a tapered rear, & square edges at the flat sides.

While I haven't yet towed the Scamp enough to completely evaluate its towing characteristics, it appears the two while similar in weight & size with quite different profiles, yet may have similar enough CD that my TV achieves similar fuel economy towing either?? Both appear to result in a 20% or less drop in mileage.

With Scamp last week


With TD in Colorado last summer

[eamarquardt]

May not be applicable to your situation at all but, here's some 'real world' figures you may be able to use .................

Kinda in line with my experience.

Cheers,

Gus

[happy_camper]

its why most teardrops are 4 wide and 4 high. and under a 1000 pounds.

this is to take atvantage of less drag. less weight and as much gas milage as possible.

Slowcowboy.

I've always thought the "4 foot" dimension(s) of many teardrop trailers (width and height) were more a function of readily-available sheet stock than advantages of less drag, weight and gas mileage. Seems to me those perceived advantages are a result of the size, not the reasons for them. If not, then if "4 feet" is good, "3 feet" should be better.

The teardrop desing most people forget is to have a step up from a tent and the bare nessistys not the luxury of a bigger normal rv.

the returns on a teardrop gas milage lesss drag resulting in faster towing speeds with a smaller compact car that delivers good gas milage makes up for the more space more luxury of the bigger RVs and standies.

the bigger you get the more drag the less gas milage you will get.

and the bigger tow vehicle you will have to have to pull the bigger camper.

so its a desingen with high gas prices this year that a lot of people will have to make.

larger cost more more comfterble. or size down and not break the wallet and live simpler.

Slowcowboy.

Not sure how these comments are germane to Jon's situation as he stated in his intial post the trailer was intended to "live in for extended periods" and his current tow vehicle is "a 2000 Cadillac STS". Hardly a "smaller compact car". In any event, unique and interesting opinions on aerodynamics, physics and economics.

Like Harvey ("nobody"), I tow a boxy, molded fiberglass "egg" (six feet wide and eleven feet long, weighing ~1,300 loaded) with a similarly equipped Tacoma that costs about 20% more fuel, IF I stay at ~62MPH. Worth it for the creature comforts. YMMV.

[PaulC]

http://en.wikipedia.org/wiki/Bernoulli's_principle
If you want to learn about the principles of drag, and the effect it has, I suggest you all have a quiet read at the link shown.
Cheers
Paul

[Wolffarmer]

http://en.wikipedia.org/wiki/Bernoulli's_principle
If you want to learn about the principles of drag, and the effect it has, I suggest you all have a quiet read at the link shown.
Cheers
Paul

Paul, you trying to learn us something?



Randy