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

[travelnutz]

Ron-C,

What results in your trailer nose model example is disrupting the suction cone created by the flat rear wall of the trailer. Nose up compacts the air under the trailer as the air goes from front to back which then releases it's compression upward directly behind the trailer's rear wall and greatly reduces and disrupts the true suction form cone. Aerodynamic entry going into the wind is only half the drag force created. The suction created by the end of an object going thru the wind is nearly as strong of a drag force. Look at your example model and you can easily see the rear suction differences.

Parting the air with an adequate configured Vee shape entry reduces the parted air created vortices (spinning tornados of air) along the sides of the object and smoothens the return at the rear which also reduces rear suction. Airliners create very strong "wing tip vortices" when taking off down the runway and why there's a few minute delay between large plane takeoffs. Enough time for the vortices to lose enough effective force. Been many plane crashes over the years occur with small planes taking off right behind large planes before this powerful force was recognized and dealt with.

[westend]

Images and dimensions of a couple commercial offerings:Aeroshield
Aeroplus

I plan to fabricate my own out of aluminum plate and integrate it into a cargo rail system on the top of my truck cap. Assuming it will be as far back as possible to get wind above the trailer. I can make an estimation of the spans of the wind shield as 24" X 72". Deployed height might be 16".

Truck dimensions: L- 245.8" W-79.9" H-79.8" Trailer dimensions: L-238" W-90" H101"

The only things interrupting the roof line on the trailer are two typical air vents, a 3" plumbing stack cap, and a 4" exhaust stack for the heater. There is no AC on the roof.

Thanks for taking the time to measure. There are a couple of guys on the Forum that are using them and they do help with mileage. I'd like to know the drag differences, too!

[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!

Hi Westend, I am set up to do this now if you are still interested. Could you give me a little more detail of what you have in mind and where it should be located (aligned with TV windshield, on top of cab, on top of cap, height, etc).

[Ron-C]

Oops, trying again...

[Ron-C]

Thank You Barney!

Images of Flat vs Nose Up vs Nose Down

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[Ron-C]

rbpru wrote:
Several years ago a friend tried adding the various ground affect devices used by NASCAR to increase his cars mpg. He found that the effects at 75 and 80 mph were far less than those claimed at 180 to 200 mph.

I am glad to see you are having results at lower speed.

Got me thinking... and testing more. Here is what I found:

Towing a box TT requires 60% MORE power at 80mph vs 60mph
Towing my optimized TT design at 80mph requires 15% LESS power than the box trailer at 60mph!!

I think I am on to something 🙂
Ron

[BarneyS]

Ron-C wrote:
Snip...
Apparently I am barely smart enough to figure out how to use the CFD software but not quite enough to figure out how to post a picture of it here!
Ron

Take a look at this thread.
It makes it as simple as:
1. upload picture
2. Copy URL
3. Paste URL into post.

Even a caveman can do it! :B
Barney

[Ron-C]

pasusan wrote:
I was reading an article about how the dimples in golf balls affect lift and drag and thought about this thread. Ever thought of adding dimples? :B


If you notice my sig pic we have a low profile supposedly aerodynamic trailer (wind tunnel tests done in England) and we get a whopping 8.4 mpg. Then again our tow vehicle has a carburetor and no computer.

Hmmm. Dimples, eh? I wonder...

Seeing your truck and trailer combo reminds me of a colleague who would have salad for lunch... and then down 3 beer :B

[Ron-C]

rbpru wrote:
...
Now if you can get a boost of 3 or 4 miles to the gallon, them you have a winner.

We're pulling for you.:)

Working on it!! By cutting the weight in half and reducing drag by half, I might just get there 🙂 Thanks for the encouragement!
Ron

[Ron-C]

93Cobra2771 wrote:

Cup fan wrote:
...
Something I would be curious to know would be how the attitude of the trailer behind the TV affects mileage. I see a lot of trailers going down the interstate that aren't level. The nose
of the trailer is often much lower than the rear, perhaps due to improper hitch ball height, or improper springbar adjustment. I know for me, proper hitch set-up is huge as far as handling is concerned.
...

Nose up or nose down also affects mpg, as you are now exposing either more roof area (towing nose down) or more bottom area (towing nose up) to the air when towing. If perfectly level, you only have the nose and whatever objects are above the roofline.

On the flip side of that, being slightly nose down gives more stability while towing.

I was curious also so I ran a simulation with the nose up and nose down by 2 degrees on the trailer only. Results compared to the same box trailer were as follows...
Nose Down - 1.5% decrease in force required (Interesting to note that the down force doubled)
Nose Up - 10.9% decrease in force required
Conclusion: Nose down doesn't make a difference but nose up is actually better. You will see that in the images posted later. Hmmm. Not what I expected.

Apparently I am barely smart enough to figure out how to use the CFD software but not quite enough to figure out how to post a picture of it here!
Ron

[rbpru]

All of this is fine and doing what ever is necessary to reduce drag and improve stability is a good thing.

But, a 1 or 1 1/2 mile to the gallon increase means little unless you are a trucking company with several million fleet miles per year.

I have seen my 10 mpg go to 11.4 with a stout tail wind but more commonly drop to 8.4 to 9 with any amount of head or cross wind.

Now if you can get a boost of 3 or 4 miles to the gallon, them you have a winner.

We're pulling for you.:)

[travelnutz]

GrandpaKip,

Your post says/reveals the very same thing/results we have experienced including MPG's towing our flat with rounded corners 6 X 10 X 6' high inside enclosed trailer and our 6 X 12+ the front Vee floor length X 6' high inside 90 degree Vee nosed enclosed trailer behind the same '05 I-6 Chevy Trailblazer with the same GVW (3200-3300 lbs) on the same road trips and same speeds. The only difference was the Vee nose vs flat front on the trailer. Thus, the aerodynamic efficiency effect.

I'm a retired automotive engineer having owned a engineering operation for nearly 40 years and already knew approx what the difference should have been by calculations, and it was! It's a fact not a fantasy!

FWIW: A smoothly covered trailer underbelly also creates much less turbulence and air drag than an un-enclosed trailer underbelly because each crossmember is like dragging it flat ways to/thru the wind and creates lots of turbulence and when added together, they really count up in air drag! Obviously, neither of our enclosed cargo trailers had enclosed underbellies.

[93Cobra2771]

Cup fan wrote:
...
Something I would be curious to know would be how the attitude of the trailer behind the TV affects mileage. I see a lot of trailers going down the interstate that aren't level. The nose
of the trailer is often much lower than the rear, perhaps due to improper hitch ball height, or improper springbar adjustment. I know for me, proper hitch set-up is huge as far as handling is concerned.
...

Nose up or nose down also affects mpg, as you are now exposing either more roof area (towing nose down) or more bottom area (towing nose up) to the air when towing. If perfectly level, you only have the nose and whatever objects are above the roofline.

On the flip side of that, being slightly nose down gives more stability while towing.

[Snowman9000]

Slowmover wrote:
Aero trailers have curved radius of 12-22 degrees where walls meet.

You mean inches of radius rather than degrees, right? I'm no engineer, so if it is degrees, I need to figure out what that means.

[GrandpaKip]

I've towed a 6x12 v-nose converted cargo trailer (2400 lbs loaded), a 23' sloped, flat front camper (3800 lbs loaded), and a 21' curved front camper (4300 lbs loaded) with the same truck. I got about 12mpg with the v-nose and 10 mpg with the other two. The v-nose was 90 degrees if I remember correctly. The only difference is the cargo trailer did not have a W/D hitch. I keep to 65mph on the interstates.