Teardop aerodynamics

[LandCommander]

I just had this thought...

teardrops are intrinsically aerodynamic, but in truth, they should be much more enlongated for optimal effeciency. I don't know if anyone here has done this yet, but ideally the shape would be the usual in the front (with as much corners rounded off as possible) and from there it would decline 11 degrees towards the rear until you get to the end at which it would be best to have a sharp truncation of the teardrop shape to cleanly detach airflow, think the back of a Prius.

Anything steeper than 13 degrees is not maintaining attached airflow and will not be as aerodynamic.

Thoughts?
Eric

[tony.latham]

There's a point where the advantages of a more efficient aerodynamic shape start cutting into function. One could build a camp trailer that looks like a high ballistic-coefficient bullet, but how funcional would it be? It likely would cause access problems with the galley. When my brother in-law insisted I build his 'drop with a curved front bottom, he lost 8" of bed space in his eight-footer which brought the mattress length down to about his height. I think it was more of a visual issue for him, I doubt it reduced drag behind his tow vehicle.

Here's a pic of my 1300# ten-footer. You can see where dust is deposited behind the low-profile fan. I get a bit behind the hurricane hinge too. Obviously both are creating turbulence and thus drag. I sure wouldn't want to lose my fan nor shift over to a flat-plastic, more aerodynamic hinge (I've got one on my tongue box and don't care for it). If I keep my towing speed at about 55, I lose about 1.5 MPG –I suspect a lot of that is the weight. When I go above sixty it really starts to drop off. As you well know, drag increases exponentially with speed.



Of course there are other factors such as tow vehicle shape/height and gap between it and the trailer. I think the rear of the trailer may be more important that the front, as long as there is a semi-smooth transition of airflow over the top/sides of the two objects. (I don't think the sides have much effect on drag) It'd be interesting to have someone play with a TV(s) and teardrop(s) in a wind tunnel. How much of a roll do tires and fender shape play? How about my truck topper?

Tony

[LandCommander]

Tony, good observations on the turbulence behind the fan and hinge. I've done a whole lot of research on this topic as I was very much into hypermiling techniques. I found one thread in particular (on ecomodder.com) that had some amazingly informative pages scanned out of a text book.

The truck camper cap is definitely GOOD for aerodynamics. The wheels would ideally have flat saucer-like caps on them and no through holes. The gap between vehicle and trailer also has a big effect, the closer the better.

My idea for a more aerodynamic teardrop actually INCREASES the useable space and could potentially be EASIER to build! The truncated teardrop shape could be considered a "Kamback."

You are right though about the trade off between practicality and aerodynamics, but I think theres a middle ground that I haven't yet seen…. still thinking the front is a normal teardrop (much less important than the back like you said, because its drafting the tow vehicle) and the rear has a truncated teardrop or kamback like a prius (or insight, or any one of the new high mpg vehicles)

ideally, it would be some 17' long with a 10-13 degree down slope, and I suspect, depending on weight, that this ideal length and slope could actually INCREASE the MPGs compared to no trailer at all; but who would want to tow such a long, non-practical trailer?


The problem with the usual teardrop design (not that its a big problem) is that the rear slope is far to steep to keep the air attached and creates a vortex of low pressure behind the trailer. yay for aerodynamics! And lets not forget the underbelly… nice and smooth is best!

BTW your trailer is beautiful and pretty close to an ideal height compared to your truck

[Vedette]

From what I have read here on the Forum the last couple of days, I am considering more "Lucky" than "Smart" .
In that when I built "Miss Piggy" I just wanted round fat corners for her pig nose. I was unaware that I was illiminating "Dirty Air"??
The smooth "boat like" underside was done to make cleaning easier. But, apparently also decreases drag.
And no vent on top? I just didn't want to cut a hole in here beautiful roof after I had spent so much time making it blend together smoothly.
The only thing that may be getting in the way of Great gas mileage is my lead foot. As we did manage to get our towing speed into the 3 digit range this past trip, as I was tired of telling people that we have towed her up to 96 mph.........so when the opportunity arose to pass a long string of traffic on a road I could see ahead on for about 3 miles, I let Sandi's Hyundai Santa Fe stretch her legs a little and get over that 100 mph barrier.
Now, that I have started to design "Kermit" our new TD project, I will be looking to make it even more slippery! Chevrolet gave me a good head start with the aerodynamics they designed into their 1947 Fleetline Aero Sedan, which I have chosen as the main donor car for Kermit.
You can bet that there will be no sharp leading edges on this project!!
Good Roads
Brian & Sandi

[Waksupi]

This is the same topic I had asked about a day or two ago. At the time, I have a piece of plywood with various arcs and ellipses drawn on it. I've been studying aircraft wing design, and throwing in my experience in drawing engraving patterns, hoping to find the Golden Mean.

[citylights]

Low hanging fruit and biggest offenders.

If you want to improve teardrop wind drag efficiency you should go for the easiest fixes and biggest offenders.

Fixating on the smaller items doesn't get you much gain.

Smooth bottom is an easy fix with a good gain. This would be a low hanging fruit.

The biggest offender is the space between tow vehicle and teardrop. It is my opinion that all the others combined do not approach the extra drag created in that location.

Here is the solution. Spring loaded rollers recessed in the teardrop with neoprine sheets rolled up on them. Snaps mounted on the tow vehicle to receive them. Extend the neoprine sheets from teardrop to tow vehicle, close the space for smooth transition between. The key is that the material is flexible and retracting to accommodate turns.

Another added benefit is stones from the tow vehicle would be deflected from the teardrop.

Seems like a lot of work for very little gain. Easier to just keep your speed down at 55 mph to keep the drag down.

I need to shoot a video of my teardrop in a wind tunnel... Going down a dusty dirt road.

[lrrowe]

I would love to see "wind tunnels" results whether they be the real McCoy or one driving down a dusty road. For TD's and CT's.

[citylights]

I would love to see "wind tunnels" results whether they be the real McCoy or one driving down a dusty road. For TD's and CT's.

Haha, I should have done that 2 weeks ago. 90 miles of gravel roads one way, Colorado City, Utah to Tuweep north rim Grand Canyon. When I looked back, the boiling dust cloud was pretty awesome.

[LandCommander]

I like the idea of using neoprene to bridge the gap, I may try something like this...
And you are very right about fixating on all the little things when in the end, they won't amount to a whole lot.
But if you the little things are quick and easy, why not?

Another place to look is the tow vehicle itself, a front air damn and smooth hub caps can net you an extra 5% if done correctly

Waksupi, it is my understanding that airplane design is actually very much different from land vehicle design in principle. The proximity to the ground creates a totally different effect than being in the air. You be better off looking for inspiration from Prius, Insight, Chevy volt, Nissan leaf, tesla etc.
I do believe the tesla Is the most aerodynamic production vehicle ever made

[MtnDon]

Tape 5 or 6 inch lengths of yarn (contrasting color to paint job) to the side of a trailer in a grid pattern; maybe an 8 to 10 inch grid. Drive it at 40 mph or so and have someone on the side of the road shoot pictures. Or do it from another vehicle at something like 55 mph, if that can be done safely. A digital SLR with a high speed (5 frames per second or so) setting will make it easy. Then you can see the flow and play with adding air dams, spoilers, whatever.

Autospeed magazine ( autospeed.com ) did several articles a few years ago. Here's a link to one of them
http://www.autospeed.com/cms/article.html?&A=108656

[mezmo]

Here's a thread from the Foamie section on the topic,

Foamie Aerodynamics.
http://tnttt.com/viewtopic.php?f=55&t=49036

Cheers,
Norm/mezmo

[LandCommander]

Great forum you linked too Norm, they also linked to an aerospace article about golf ball dimples and aerodynamics, how interesting!!

I've been considering doing some sort of lightweight kamback on my TT that also acts as an awning... a fixed, short, downward sloped, rigid, awning coming off the back.

[ChasCABQ]

I like your reasoning. I would love to know about where you found 11 degree to seem to be optimum angle. I'm getting ready to cut my sidewalls and have been trying different designs in Sketchup. Many included 1/4 ellipses similar to generic Benroy. Here's latest design incorporating the 11 degree break (other dimensions are trial only). Let me know what you all think:

Teardrop 11 degree slope sidewall.jpg (38.45 KiB) Viewed 1048 times

[Corwin C]

Trying to reduce aerodynamic drag of a trailer is so dependant upon the tow vechicle that it is impractical to compare shapes without using IDENTICAL tow vehicles ... right down to tire size, radio antennas and number/size of passengers. Air flow around the tow vehicle in almost every case puts the trailer in a turbulent wake that will defy even the most dedicated supercomputer or wind tunnel to analyze and isolate real potential improvements. And those improvements would be specific to a tow vehicle/trailer/loading combination.

My suggestion is to take care of the big obvious issues, i.e. high speeds (probably the easiest), flat surfaces perpendicular to airflow both leading and trailing, trailer frontal area being larger than tow vehicle frontal area, and so on ... beyond those, the return on effort diminishes really quickly.

Perfect aerodynamics is a golden coin that people have been seeking since the Wright Brothers ... before that, very few items moved fast enough to make any difference (perhaps a train). Seek the coin if you must, but realize that your results will most likely be unique to your situation.

[Martiangod]

aero is an interesting and complex issue to attack. My build started as utility trailer for a cross country move but my trade got the best of me and I built a camper, so areo was not on my mind when I started utility was.
Anyhow....skip to yesterday
My wifes health makes the cross bed and climbing over an issue so thinkin of a real TT again.
My red Hotrod is a heavy little sucker at over 3000 lbs loaded, 6 1/2 wide, I average 10 miles per gallon towing at 65 miles per hour.
Brought a 29 footer home for a test drive weighing in at 7500 lbs
I have a 40 mile to and from work at 60 -65 miles per hour, I was getting 12.5 mpg with the big trailer, 2 mpg better then with little trailer on same stretch of hiway.
So a bigger wall, more frontal area behind truck, double the weight, triple the length and 2.5 mpg better.

#1 big trailer is closer to truck then little trailer
#2 big trailer is a blunter nose then little trailer, Hotrod breaks to a 45 degree angle at halfway up the front
#3 big trailer has rounded leading edges, little trailer sharp edges.
#4 big trailer has narrower tires then Hotrod does

Very curiouse that 3 times as big, more then twice the weight and it gets better milage??????
I think the 4 points I made are the big culprits, but then the tow vehicle has to come into play.
That 5 liter F150, sounds like its working harder with the big trailer, not in a bad way but a nice throaty sound
Does the extra load move the performance into a sweet spot for my particular TV?

I know I'm not a TD but am a TTT, just posted from my observance,
i'm starting to think the 45 degree cut on the front is the biggest culprit and
that a blunt nose would have been better.
The TD follows the blunt nose concept