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

[Snowman9000]

Ron, you might also sign up for the forums at www.ecomodder.com

Your ideas would be welcome and evaluated there. I like them here too, but I'm just saying.

[gmw_photos]

Quality Johnson wrote:

gmw photos wrote:

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 ).

The shape of the Concorde is intended for supersonic flight. You will find that subsonic and supersonic aerodynamics are quite different.

....ok....not to start an argument here, but let's consider the fluid dynamics of slower speeds: look at how "fast fish" have evolved. Slim at the nose, tapered at the back.

The bottom line is, much could be done to make RV's more aerodynamic, by changing how air is managed at the front, side, bottom and rear. How many of these compromises the buying public is willing to accept is another question.
In my case, improving fuel economy towing my camper is not high on my list, simply because I only tow about 10K miles per year. The $$ savings would not be huge for me.

But, I applaud Ron's efforts in trying to come up with forward thinking solutions.

[gmw_photos]

Ron-C wrote:
....snip......

Another pet peeve is regarding the height of the unit. I find that some sit so high on the frame that they are sticking way above the TV. Not only do they look outright silly but that must move the center of gravity up considerably. Every square inch exposed above the TV increases frontal area and drag force. The big gap underneath probably doesn't help airflow either (pure speculation).

Ron,
My opinion is regarding the "high frames" now is that the market demands slide-out rooms ( I know I do ), and it's easier to make a slide out room with a "deck-over" frame design. It also allows for the slide-out room to be "flush floor", which is imperative in many floorplans. In the end, you wind up with a trailer that is higher off the ground.
As always, compromises. We trade away a potentially lower C-of-G, and potentially better aero, in order to get better live-ability when the trailer is stationary.
In my case, it was tradeoffs I was willing to accept.

[beemerphile1]

DutchmenSport wrote:
If they moved the air conditioner, where would they put it? To remove it off the roof mean either hanging from a wall or reset in the roof (which means the unit is actually inside the camper, maybe boxed in with a decorative cabinet, still, it would be a head bumper!

Under the trailer? Inside a jack knife sofa like some furnaces are installed. Then you have all the noise of the air conditioner actually inside the "box".

However, with today's technology and creativity, I WOULD think someone could design a more sleek shaped air conditioner, even if it had to sit on the roof.

Some manufacturers already put the A/C unit (or heat pump) inside or underneath but very few.

[Snowman9000]

I see all the usual suspects who can't wait to jump on anyone who wants better mpg. Don't ever change.

[Quality_Johnson]

gmw photos wrote:

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 ).

The shape of the Concorde is intended for supersonic flight. You will find that subsonic and supersonic aerodynamics are quite different.

[DutchmenSport]

If they moved the air conditioner, where would they put it? To remove it off the roof mean either hanging from a wall or reset in the roof (which means the unit is actually inside the camper, maybe boxed in with a decorative cabinet, still, it would be a head bumper!

Under the trailer? Inside a jack knife sofa like some furnaces are installed. Then you have all the noise of the air conditioner actually inside the "box".

However, with today's technology and creativity, I WOULD think someone could design a more sleek shaped air conditioner, even if it had to sit on the roof.

[Ron-C]

path1 wrote:

This old posting might help you...

http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/17575645/srt/pa/pging/1/page/1

Interesting read - lots to learn there. Thanks.

[Ron-C]

beemerphile1 wrote:
One (of the many) things that annoy me about the RV industry is the way they ignore basics of aerodynamics ...

Excellent points - Thanks! The a/c units have always been a sore spot (and an eye sore). I would like to see it a) reduced in size (by proper insulation) and b) moved off the top.

Another pet peeve is regarding the height of the unit. I find that some sit so high on the frame that they are sticking way above the TV. Not only do they look outright silly but that must move the center of gravity up considerably. Every square inch exposed above the TV increases frontal area and drag force. The big gap underneath probably doesn't help airflow either (pure speculation).

[beemerphile1]

One (of the many) things that annoy me about the RV industry is the way they ignore basics of aerodynamics that could be simply accomplished for minimal cost. Look at all modern cars/trucks, they have become very slippery in design. The headlamps, mirrors, and even door handles have been designed to reduce drag. Now look at the typical RV with multiple A/C units, vents, antennas, baggage doors, and windows all sticking out in the air stream.

Much could be gained at minimal cost by smoothing the body and components. Why can't those A/C units be mounted inside or underneath where they would be out of the flow? Why are all the other things just added to the surface with no consideration?

Most of what the RV manufacturers have done regarding aerodynamics is for marketing purposes, not real gains on the highway.

[Ron-C]

Old-Biscuit wrote:
Towing my 13"4" high 34' long 14K 5vr I get 11 mpg---22K combined weight

Now how are you going to improve on that and still give me all the room/space/storage etc that I currently have?

That is why I posted in the 'Small TT's' section. Sounds like you are doing fine with what you have.

---

I guess I should have clarified... This design is targeted to travelers with smaller TV's interested in a unit under 100 pounds per linear foot of dry weight. One element of this design is the aerodynamics which I thought was worth sharing my findings.

I would rather get 15-20 MPG and safely travelling with a lighter unit not as affected by cross winds and not killing my TV which is probably also my commuter vehicle and grocery getter.

Will that take some compromise? Definitely.
Ron

[path1]

Just tossing this out.

This old posting might help you. Had it booked marked for years now. Someday I'm going to do something with our class C. Don't know what to do with our Trailer without giving up interior room. Different style of RV but might help or you get ideas from. And there is another posting with some of same authors as link below that had charts and more detailed info. You'll have to search them out. All were over 12 months ago. One poster owned a couple patents, and had useful data.

http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/17575645/srt/pa/pging/1/page/1

[downtheroad]

Good discussion going here.
Doesn't or didn't Forrest River or someone make a V-nosed TT called V-lite or Ultra-V or Wind Jammer something like that.

I don't think it was a big seller. Very occasionally if ever see them in campgrounds....anyone else?

[Old-Biscuit]

Towing my 13"4" high 34' long 14K 5vr I get 11 mpg---22K combined weight

Now how are you going to improve on that and still give me all the room/space/storage etc that I currently have?

[the_bear_II]

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.

X-2

The way I look at it is I'm willing to pay whatever it takes to get away from the rat race for a few days. 6 MPG or 11 MPG just getting away is worth the price.