Aerodynamics Matter !!

[Ron-C]

Hello All. First post here... wanted to share some data on aerodynamics.

I am designing an ultralight (under 100 lbs/ft) small travel trailer and wanted to find out the effect of shape on aerodynamic drag. I modeled up 2 trailer designs using SolidWorks CAD software then applied CFD (Computational Fluid Dynamics - a virtual wind tunnel) to test them.

The baseline trailer was a box shaped design with slightly rounded corners - essentially flat nosed cargo trailer. Then I created 4 versions of a swept profile trailer steeper at the front and with a longer sweeping tail in different widths.

Results showed that a swept profile trailer 7" wider than the baseline box trailer at the same height would have 45% less drag force. Looking at it the other way around, it would take almost twice as much force to pull the box trailer!

My conclusion, the shape of the trailer is way more important than the size of it. This means that a larger trailer would require less fuel to pull if it is the right shape.

There are lots of other factors including the shape of the tow vehicle, speed, temperature, etc. but this is at least some food for thought.

Now I am working on a developing an aerodynamic shape that is functional 🙂
Ron

[gmw_photos]

Ron-C wrote:

gmw photos wrote:
regarding real world aero effects on fuel mileage. ... Rounded front end, rounded corners similar to an Airstream design ( except the back, which is square ). Approx same weight as the camper. I get approx 1.5 mpg more pulling the horse trailer with same truck, same speeds.

I suspect that the back shape has more influence on the drag force than the front although everyone seems to focus on making the front nice and round. Your TV has already punched the hole. It is more likely the back creating suction with the wall behind you.

Ever see an airplane or fish with a flat tail section?

Ron, I suspect you are very much correct on the comment about the flat back of my horse trailer. That's why I mentioned it in my post, because for instance, comparing it to an Airstream, the rest of the horse trailer is similar. The A/S has the advantage of both the trailing roof and side walls being rounded.

Reduced frontal area and a nice long tapering tail would of course be ideal from and aero standpoint. Looking at pictures of the Concorde is a perfect example of optimum shape ( for aero ).

[rbpru]

I have to agree, when we travel and sight see the gas cost was a small portion of the total cost, even last years trip to Montana at $3.80 /gal.

[sch911]

The better shape can only affect the Drag Coefficient and it's affects are minimal in the overall calculated amount of drag. The only significant improvement would be if you reduce the frontal area. Which as we all know makes the TT much smaller. Who want's that?

[Ron-C]

westend wrote:
Ron,
Would it be possible for you to set up the CFD program using a full shell on the truck bed and add in a top side wind deflector? Some users report better mileage with the deflector but it would be interesting to see the actual loss of drag. This may be too time intrusive in your schedule, I have no idea how much work it would entail.

Thanks for posting your findings so far. Good luck on the build!

It is definitely possible but would require models of the truck, trailer and wind deflector. It would be influenced greatly by the length of vehicles, size, angle and position of deflector etc. That would be a pretty big task but interesting work.

I see that a few transport trailers are incorporating the skirting and even a back door wind deflectors now. I don't imagine they are doing that for any aesthetic reasons. Maybe fuel mileage does matter 🙂

[Lynnmor]

downtheroad wrote:
Same old discussion....fuel mileage.
While good fuel mileage is a good thing, anyone who worries about it and makes it a prime concern - should not be RV'ing.

The guy is trying to do some good here, and I think it's great. Just like managing the wind, he is making an effort to improve the flow and not be a drag.

What I would like to see is how much help the fancy molded fronts are, and also making some improvement to the flat rear.

[Ron-C]

downtheroad wrote:
Same old discussion....fuel mileage.
While good fuel mileage is a good thing, anyone who worries about it and makes it a prime concern - should not be RV'ing.

Interesting opinion...

[Ron-C]

gmw photos wrote:
regarding real world aero effects on fuel mileage. ... Rounded front end, rounded corners similar to an Airstream design ( except the back, which is square ). Approx same weight as the camper. I get approx 1.5 mpg more pulling the horse trailer with same truck, same speeds.

I suspect that the back shape has more influence on the drag force than the front although everyone seems to focus on making the front nice and round. Your TV has already punched the hole. It is more likely the back creating suction with the wall behind you.

Ever see an airplane or fish with a flat tail section?

[Ron-C]

Thanks for the comments...

I agree, functionality and livability is the most important factor today and I am not ignoring it. I purely wanted to test the limits of what is possible. When gas prices double (which they will), we will be prioritizing a little differently, I suspect.

Hopefully I can do more testing with different tow vehicles, speeds, etc to make for real world simulation.
Ron

[westend]

Ron,
Would it be possible for you to set up the CFD program using a full shell on the truck bed and add in a top side wind deflector? Some users report better mileage with the deflector but it would be interesting to see the actual loss of drag. This may be too time intrusive in your schedule, I have no idea how much work it would entail.

Thanks for posting your findings so far. Good luck on the build!

[downtheroad]

Same old discussion....fuel mileage.
While good fuel mileage is a good thing, anyone who worries about it and makes it a prime concern - should not be RV'ing.

[rbpru]

Yes aerodynamic do matter. But livability inside the shell, eye appeal, ease of construction and weight are also prime considerations.

I think you will find the airstream tube about optimum shape for the TT market.

Also you are working with drag forces at 70 mph not 170 mph.

Building a TT for max pulling efficiency would be a fun project however.

Good luck

[gmw_photos]

regarding real world aero effects on fuel mileage. I tow this box shaped camper I am in right now ( funfinder 189 FDS ) ....7 feet wide, 10 feet tall, 19 feet long including tongue. Sloped back front, the rest of it sq edges, with my Nissan Frontier. Overall fuel average over approx 25K miles has been right at 11.4 mpg.
I also tow a two horse trailer with this truck. Lower roof height ( whole trailer is lower to the ground due to torsion axles, and wheels being set outside the box. Rounded front end, rounded corners similar to an Airstream design ( except the back, which is square ). Approx same weight as the camper. I get approx 1.5 mpg more pulling the horse trailer with same truck, same speeds.

[DutchmenSport]

gmw photos wrote:

downtheroad wrote:
....snip.....

Compromise. No one wants a trailer that gets marginally better fuel mileage and then gives up livability, cabinet and storage space, head room, etc...

Well, some do ! Those who buy an Airstream, or some of the smaller fiberglass egg campers for example.

Everyone has their own reasons for purchasing the RV they finally get, but I've not encountered too many (if any), that purchases an egg style or an Airstream because of the airodynamics. Everyone I've run into has them because, Airstream is a better built trailer than the average, and Egg shaped trailers are just smaller and can be towed with a smaller vehicle, quite possible to get one in a tall garage or easily behind the house so the HOA snoop won't see it!

[gmw_photos]

downtheroad wrote:
....snip.....

Compromise. No one wants a trailer that gets marginally better fuel mileage and then gives up livability, cabinet and storage space, head room, etc...

Well, some do ! Those who buy an Airstream, or some of the smaller fiberglass egg campers for example.

[DutchmenSport]

But! You are leaving off the airodynamics of the tow vehicle and how the wind rushing off the tow vehicle impacts the front sail of the tow vehicle.

Depending on the tow vehicle and how the wind deflects around it, the same trailer can give considerable resistance with one tow vehicle, and simply sweet with another.

You have to take into account the tow vehicle. Trailers don't tow themselves.

Welcome to the forums!

Independent engineering is what makes this country great! Keep working on it!

Edit: An open bed pick-up truck will cause a vastly different airodyanamic flow than one covered with a full shell. A van will have a different airodynamic drag too. Wide duly have a different dynamic than single rear wheel trucks, and the height of the truck makes a difference too.

Lots to think about.