Aerodynamic drag help

[Tigris99]

Its me again.

Have a few trips on the trailer, wish funds would allow me to start a new build but not there yet.

I replaced my POS van with a 2003 CR-V (ya we still have the odyssey, i was driving a 1999 caravan till I bought this). Used it to tow the camper this last weekend for the first time, see how she would do.

Surprisingly well!!! Didnt loose much fuel mileage. Through town she wasnt terribly bothered by the weight. I have done some mods to increase torque (well free up wasted torque). ****, passed a truck and trailer on 2 lane without too much problem. Also installed a large and better version trans cooler which after the drive there I could put my hand on and hold it, so trans didnt get hot at all.

Tows WONDERFULLY even in a minor cross wind. Sits beautifully in line with CRVs profile.

But as has been discussed here plenty. The draw back of square rear box of a trailer. The drag..... my understanding is its actually more important than the front of the trailer if the trailer doesnt protrude past the TVs body on any side.

So I was thinking:

We all see the semis going to the highway with those add on units that are supposed to help with rear drag. Do they actually work at a noticeable level and thoughts on doing a mod to my square drop to basically create that effect. Something that is mounted to galley door on the sides and hinged on top so it doesnt effect opening and using the galley?

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[Dan242]

I did this to mimic the big rigs, Going on a 2000 mile trip this weekend, will see how it does, haven't done much highway travel yet

 

[coyote-TNT]

There used to be a couple peeps around here who were in college and had programs/studying along those lines. Maybe, if they're around, they could chime in.

 

[Andrew Herrick]

Aerodynamics is a gray science. Hard to predict the exact usefulness of any mods just by eyeballing. You can try to talk to an engineering department at a local university. They might let you run mini wind tunnel tests using your camper shapes as a class project.

 

[Tony Latham]

I have not played with this, but you can run your own wind tunnel tests online. It's free.

http://www.flowillustrator.com/

I'd make sure you upload a photo that includes your tow vehicle.

Let us know what your pickup on.

Tony

 

[working on it]

• Do you really need aero help? It seems to me that you have a nicely-shaped (looks streamlined to me) tow vehicle, and your trailer has an angled front (top and sides), both of which will certainly lessen drag, and aid your MPG. It also appears to me that your trailer is lightly-built (small-ish tires, and lightweight frame), nd just the right height to follow in the wind-shadow of the CR-V. Why bother with aero mods or add-ons, for marginal gains?
• Conversely, my TTT is overweight, with much less aero-worthiness than your trailer, and pulled by a gas-guzzling heavy pick-up, with a full bed of gear in it. Absolutely no possible aerodynamic advantage anywhere, that I can see, that I can make to it. My truck gets about the same MPG with or without the trailer (it doesn't seem to know it's there-I frequently check my center rear-view mirror to confirm its' presence; I can't see it in my side mirrors!). I'd be happy to be your situation; my MPG is 11-14, on trips. Luckily, I have a 36 gallon tank, and I always take along 5-15 gallons extra, in the truck bed (varies with the camping-gear load-up), a habit I started on long hauls to drag racing venues. Can you say over-built, over-weight, over-packed? Ha,Ha,Ha !!

 

[Tigris99]

Lol thanks guys.

Mine us overbuilt for its size. Had I had more faith in how tough it would be when completed I could have shaved a couple hundred pounds fairly easily.

In my case aerodynamic drag can help a fair bit at higher speeds. Once I get above about 45 mph she starts to take notice of the load. Seeing that in town speeds shes barely bothered by the weight, higher speeds mean aerodynamics are playing a bigger part. Obviously reducing weight would help but new build is low on the list right now.

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[dancam]

Aerodynamics is more important than weight( to a reasonable extent) at 65-75mph. Could you post more photos of your trailer and some photos of the mods your considering doing?
I think what i did is similar to what you want to do but im not sure. Havent been on the highway really yet, still building.

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[aguyfrom ohio]

Those tail fairings are proven to work when tested by groups like the Trucking Research Institute and the US Dept of Energy.
For big class 8 trucks at highway speeds they improve fuel economy at least 1%, maybe as much as 4%.

Notice in the photo that they taper just a bit - that's what does the magic to reduce drag.
Some of the photos above look like the extensions are straight back parallel to the walls and roof.
A straight tail fairing like that would not help at all, it has to taper to create a sort of boat tail.

 

[dancam]

Those tail fairings are proven to work when tested by groups like the Trucking Research Institute and the US Dept of Energy.
For big class 8 trucks at highway speeds they improve fuel economy at least 1%, maybe as much as 4%.

Notice in the photo that they taper just a bit - that's what does the magic to reduce drag.
Some of the photos above look like the extensions are straight back parallel to the walls and roof.
A straight tail fairing like that would not help at all, it has to taper to create a sort of boat tail.

What you posted a photo of is proven to work like you said. Tilting in like that would not have been practical on my build. My understanding is that the point is to break the vaccum created by the flat back. I believe mine will still do that, just less well. A boat tail shape would be a lot better, but to really be a boat tail all sides would have to meet at a point and have a less steep angle. I think this is just to break the vaccum
Mine also doubles as a place to get out of the rain when your trying to get into the trailer though. It was a big pain to make so i really hope its worth it :/ have yet to test.
I believe there is sort of a minimum length for it to work as well. Airtabs are another thing to look into but are more for crosswinds i believe

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[GuitarPhotog]

Remember that 1% improvement is a big deal to big truckers who put thousands of miles and hundreds of thousands of gallons of fuel in their trucks. You won't notice a 1% or even a 5% improvement in your mileage unless you log every mile and every type of terrain.

<Chas>

 

[djunglew]

Here is a study that showed significant improvements by rounding edges and corners. May be the least impactful change.
https://www.nasa.gov/centers/armstrong/ ... -DFRC.html

 

[Tigris99]

Those tail fairings are proven to work when tested by groups like the Trucking Research Institute and the US Dept of Energy.
For big class 8 trucks at highway speeds they improve fuel economy at least 1%, maybe as much as 4%.

Notice in the photo that they taper just a bit - that's what does the magic to reduce drag.
Some of the photos above look like the extensions are straight back parallel to the walls and roof.
A straight tail fairing like that would not help at all, it has to taper to create a sort of boat tail.

What you posted a photo of is proven to work like you said. Tilting in like that would not have been practical on my build. My understanding is that the point is to break the vaccum created by the flat back. I believe mine will still do that, just less well. A boat tail shape would be a lot better, but to really be a boat tail all sides would have to meet at a point and have a less steep angle. I think this is just to break the vaccum
Mine also doubles as a place to get out of the rain when your trying to get into the trailer though. It was a big pain to make so i really hope its worth it :/ have yet to test.
I believe there is sort of a minimum length for it to work as well. Airtabs are another thing to look into but are more for crosswinds i believe

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Actually hes right, your system actually would do nothing if not have a negative effect. Your not breaking up the rear pressures at all, actually creating a void. Yes the angles and rounded corners are hugely important up front. But at best you currently have nothing more than a square back with a big void for turbulence to form.

I did do some research since I started this thread, as said above those pieces dont create a "big" difference.

Seems I may be better off fully modifying to just redoing my rear hatch so it is angled. Not opposed to a complete make over of my galley hatch.

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[Andrew Herrick]

I have not played with this, but you can run your own wind tunnel tests online. It's free.

http://www.flowillustrator.com/

I'd make sure you upload a photo that includes your tow vehicle.

Let us know what your pickup on.

Tony

Thanks for sharing the URL link to this online software, Tony. I hadn't heard of Flow Illustrator, and it has piqued my interest.

But I might compare Flow Illustrator to actual wind tunnel testing the same way you'd compare SketchUp to Autodesk Revit. You wouldn't try to manufacture aerospace parts from a Sketchup file, and in the same way, you wouldn't try to calculate percentage gains based on Flow Illustrator. The company itself is very open and honest about the accuracy of the tool: http://www.flowillustrator.com/flow-ill ... curacy.php

I guess what I'm trying to say - and you probably already know this Tony, but not everyone will - is that any free (heck, even a lot of expensive) software won't be able to do much besides compare overall shapes. As one poster pointed out, merely rounding the corners can have an incredible effect on efficiency. Just look at the design of any windshield wiper, mirror, door handle or headlight on an automobile!

I don't know if Flow Illustrator can deal with the complexities introduced by the trailer traveling within the turbulent airflow created by the tow vehicle, either.

So, to the OP, this sort of a software is a great tool! But don't expect it to give real-world answers. But hopefully, it'll point you in the right direction.

 

[dancam]

Remember that 1% improvement is a big deal to big truckers who put thousands of miles and hundreds of thousands of gallons of fuel in their trucks. You won't notice a 1% or even a 5% improvement in your mileage unless you log every mile and every type of terrain.

<Chas>
[emoji481]

It is a bit relative. For example in post #6 working on it said he sees no milage difference with his big truck between pulling his trailer and not pulling one. No amount of aero mods to just the trailer will change that. However if you put the exact same trailer behind a smaller and more fuel efficent tow vehicle that (to make an extreme example) that can barely pull it aero mods will make a huge difference.

So, on average semis loose 30-36% on fuel economy bobtailing to towing an empty trailer and around another 35% empty to loaded. (Common numbers being 11,7,4.5mpg ; 13,9,5.5mpg... depends on loads and trailers and terrain, but the numbers come out fairly close percent wise). Thats around 60% bobtailing to full load.


So lets go with 33% increase from pulling an empty trailer to no trailer on the semi. Random conservative guess is 1/3 of that is weight and rolling resistance increase so were at 22% now. A tail fin improves your overall milage by 5%. Thats 23% of your total potential.

My car from no trailer to to towing loaded drops 43% on the fuel economy but im not fully loaded like a semi would be. My trailer empty vs loaded doesnt seem to affect fuel milage much (not enough data yet).
Doing the same calculation as with the semi where the tail fin gets you 23% of your potential thats 6.6% instead of 5% which is 2mpg for me.

Big difference number wise but then calculate the cost. 5% increase over the 20k miles i plan to do this summer saves me one tank of gas. $43. Doesnt pay for much modifications.
However if drivability is a concern its a big deal. Having a more comefortable drive when towing is always nice.

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[Tony Latham]

I have not played with this, but you can run your own wind tunnel tests online. It's free.

http://www.flowillustrator.com/

I'd make sure you upload a photo that includes your tow vehicle.

Let us know what your pickup on.

Tony

Thanks for sharing the URL link to this online software, Tony. I hadn't heard of Flow Illustrator, and it has piqued my interest.

But I might compare Flow Illustrator to actual wind tunnel testing the same way you'd compare SketchUp to Autodesk Revit. You wouldn't try to manufacture aerospace parts from a Sketchup file, and in the same way, you wouldn't try to calculate percentage gains based on Flow Illustrator. The company itself is very open and honest about the accuracy of the tool: http://www.flowillustrator.com/flow-ill ... curacy.php

I guess what I'm trying to say - and you probably already know this Tony, but not everyone will - is that any free (heck, even a lot of expensive) software won't be able to do much besides compare overall shapes. As one poster pointed out, merely rounding the corners can have an incredible effect on efficiency. Just look at the design of any windshield wiper, mirror, door handle or headlight on an automobile!

I don't know if Flow Illustrator can deal with the complexities introduced by the trailer traveling within the turbulent airflow created by the tow vehicle, either.

So, to the OP, this sort of a software is a great tool! But don't expect it to give real-world answers. But hopefully, it'll point you in the right direction.

Nor will it pick up on the hurricane hinge.



T

 

[KCStudly]

You say hurricane hinge, I say wickerbill or Gurney Flap!

 

[aguyfrom ohio]

Aerodynamics get complicated pretty quickly- Reynolds numbers to indicate the transition from laminar flow to turbulent flow, and boundary layer growth, and vortex shedding...
Here's a cool image from a paper that relates to those tail extension fairings.
The whole paper is here, it's from Transport Canada http://www.tc.gc.ca/en/programs-policie ... buses.html
It's very readable; I only spotted one mathematical formula, a basic expression of the aerodynamic drag force. You can get a good feel for vehicle aerodynamics from the paper.

Without becoming an aerodynamics expert we can see the practical solutions by looking at products in the marketplace.
Nobody bothers to streamline a travel trailer more than an Airstream or a teardrop.
Boats and aircraft get even more streamlined because it pays off for them. But not for road vehicles.

 

[Tom&Shelly]

You wouldn't try to manufacture aerospace parts from a Sketchup file, and in the same way, you wouldn't try to calculate percentage gains based on Flow Illustrator. The company itself is very open and honest about the accuracy of the tool: http://www.flowillustrator.com/flow-ill ... curacy.php

I've worked with aero-engineers who use Navier-Stokes equation solvers to analyze fluid flow problems. It's one of the most difficult numerical computation problems and, to do it for real, takes the power of a super computer.

I hate to disappoint, but agree with Andrew. This is an interesting way of making fun looking pictures, but, as the creators say on the web site, it isn't intended to be used for analyzing real world effects. First, it is a two dimensional version of an analysis that has to be done in three dimensions (which requires much more computation than can be done on a personal computer). Then, a large part of using these sorts of tools correctly is setting the grid size fine enough to capture the effects you are looking for, but not so fine that the computer never reaches a solution. That's where engineering expertise and experience comes in.

Sounds like the intent of the people who did the web site (besides allowing the creation of pretty pictures) is to give students a chance to try (2D) Navier-Stokes equations.

Tom

 

[elcam84]

The research I have done I have come to the conclusion to do your best from what you can gather then it's all wind tunnel testing to see what happens. Just too many variables and the real thing is cheaper easier and faster than an iffy simulation.


I will say that using our Sorento to tow the 5x10 utility trailer empty It was getting 30 mpg going from Fort Worth to Wichita,ks cruising 80-85(not bad for a 290hp 6 cyl SUV). Coming back I had to have the ramp vertical due to the load(light and short but awkward). I got an average of 17mpg on the trip home and much of that was way under the speed limit at around 60 ish and finding the odd slow truck to draft behind. Needles to say the trip home took much much longer and was very frustrating not keeping up with traffic. I could just fine but at the expense of really guzzling gas.

If I had a tools I would have cut that ramp off and left it on the side of the road. The morons builders put a hinge on backwards so you can't remove it. Need to fix that as well as build a two piece hinged ramp.

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