Spin the teardrop around

[printer]

The sharp leading edge at the top front sure probably does not help it. But rolling it would be a lot of work.

Speaking of lot of work, this time I took the line drawing of my car and drew over it in Paint. Boy I miss a good drafting program. Not that the trailer was any easier. I was wondering what this configuration would give.



It does do a good job of keeping the air on the car on the car. Mind you it all gets sucked under the trailer and it ends up oscillating in the back with the top air.

[gudmund]

my trailer sits 4inches higher than the canopy on my PU truck - two years ago I built the spoiler seen in the photo below on the back of the the canopy and after 12000++ miles so far, I have realized a overall gain in my gas mileage of 1.3mpg ..... had been 16.0mpg before and now after the 2+ years of use and just over 12000 miles of towing = 17.3mpg on long steady runs. Cost me right at $300 to build it (used Yakima rack parts, Yakima rack mount stripes, Plexiglas panel, etc) With the $$ of gas at $3 per gal and with the 12000 miles driven so far at 17mpg (+1mpg increase) that ends up with a savings of about $135 so far = this Friday I am leaving on another run from here (Wash. State) to Minn. and back = hope to end up with some more gain on the payback of the $300 originally spent. (that's as long as the roof rake and side awning I just added doesn't "screw" things up too much now with a roof rake and awning added could end up losing what had been gained?? starting tomorrow heading east to Minn., should be finding out soon enough.....................

[tony.latham]

Here are a couple of real-world wind tunnel tests.





I've watched my 'drops a lot in the dust and don't think I get a bobble between them and the truck. It's not the best test since I don't have dust pouring over the top of the truck, but it's as good as it'll get in the sticks.



Tony

[Capebuild]

Interesting thread.
I'm currently building what might be considered a reverse teardrop.
When I was going through the design process I too was concerned about extra drag this shape might cause.
I had my reasons for going with this shape and I figured if it used up a bit more gas in our travels, so be it.
When I get it finished and on the road, that will be the real test; I suppose.

John

[printer]

I am guessing the truck is the dominant one on the battle, the TD is more in its wake. The car outline really does have a good profile in keeping the air over it until the trunk, even with the simulation of the car itself. Here is some shots of me playing around.
1

2 - thoughts are using an air shield at the axle, sort of like a broom. Helpd a little from the lower vortex kicking up.

3

4 - Air still gets sucked under the TD

5 - Less underside air feeds the lower vortex less.

6 -

7 - high turbulence after the wing but controlled vortexes downstream.

8 - Fairly wild fluctuations of the vortexes.

9 - A lot of turbulence right after the wing but downstream the vortexes do not shed any higher than the lower edge of the wing.

10

11

12 - vortexes swirl around below the lip but generally stay below the lip height when shedding, this one is not bad.

13 The bottom and top vortexes mainly swirl around close to the back and occasionally let a vortex loose.

14

15

16

17 - Big vortexes but oscillate slowly between the two.


Hard to say which is that much better, less length of the trailer the more turbulence, which makes sense. #13 seems pretty good watching the video. A skirt around the bottom of the trailer would be advantageous.

Did a quick Nano-styled shape. The vortexes sure whip around behind it. The lower one still will push the top one a little higher.

[bartek]

I've asked similar question to a friend who's an engineer working on commercial air plane designs.
His instinct was to worry about how sharp the back would curve down, creating turbulence and negative pressure in the back of the trailer. He thought that generally this negative pressure will likely contribute more to the drag than the leading edge (which is rounded and in the wake of a vehicle to some extent for smaller trailers)

Personally I have no experience here - mine is definietly lowering my gas mileage, I drop from 24-ish MPG on a minivan alone to about 14MPG on a typical trip...

[printer]

I've asked similar question to a friend who's an engineer working on commercial air plane designs.
His instinct was to worry about how sharp the back would curve down, creating turbulence and negative pressure in the back of the trailer. He thought that generally this negative pressure will likely contribute more to the drag than the leading edge (which is rounded and in the wake of a vehicle to some extent for smaller trailers)

Personally I have no experience here - mine is definietly lowering my gas mileage, I drop from 24-ish MPG on a minivan alone to about 14MPG on a typical trip...

Messing about with this program I found the front end doesn't matter too much as long as you don't do anything real stupid. The back is complex. You can have the same area of drag between one or the other profile but one is dragging the air along with you and the other is buffeting wildly. I will have to play with it some more. Mind you minor tweaks may not matter much as the trailer is really narrow as compared to the length and the air around the sides will get involved. But it is worthwhile looking at gross differences and seeing what fluid dynamics has to say rather than guessing. I am leaning to the top profiles with the battery/propane box and possibly round the side edges. But for the time being I want to redo my frame, should be out grinding away right now as the weather is cooler.

Oh, and seeing the picture of your TD I need to check out your build thread now.

[MickinOz]

Power boats use the flat back because they intentionally want the flow separation.

That's a good way to put it.
Further to that, that's how they deal with one of the fluids they are interacting with,
The squared off transom that allows them to cleanly separate from the water sure creates air turbulence, as well illustrated by the engine exhaust fumes swirling back into the boat on a calm day, or the way power cats suck salt spray into the cockpit on any damned day.

[MickinOz]

I've asked similar question to a friend who's an engineer working on commercial air plane designs.
His instinct was to worry about how sharp the back would curve down, creating turbulence and negative pressure in the back of the trailer. He thought that generally this negative pressure will likely contribute more to the drag than the leading edge (which is rounded and in the wake of a vehicle to some extent for smaller trailers)

Personally I have no experience here - mine is definietly lowering my gas mileage, I drop from 24-ish MPG on a minivan alone to about 14MPG on a typical trip...

That's a pretty big change. What's the vehicle?
Edit: Just looked at your build journal. It's a work of art mate, very impressive cabinet making skills there, but it's a big work of art - seems size has a price. It was always going to require more fuel to move than a 700lb 8 x 4, I guess.

And that's the thing - now matter how small it is or what shape it is, it will always require more fuel to move a teardrop and tow vehicle than moving the tow vehicle alone.

My teardrop / tow vehicle combination seems to result in about 15% less mileage.
i.e. the truck regularly returns a touch over 10 km/L - about 24 mpg US.
With the teardrop connected - its about 8.5 km/L - about 20 mpg.
When I take a decent length trip (not 60 miles each way as is about standard at the moment) I will experiment with lower speeds.
I'll pick a less busy period on the roads and try 80-90 km/hr (50-56 mph) instead of sticking close to the speed limit (110km/hr / 68 mph).
My truck is a light weight 4wd with a 2.5L turbo 4 cylinder diesel. A ford courier. Might have been sold in the states as a Ranger.

In light of your friend's educated and experienced first reaction as an aircraft designer, might the square drop style be considered to be the sharpest curve down and therefore the extreme end of the drag/turbulence equation?

I see the teardrop shape, even the fairly blunt back end of the classic Benroy which will forever remain my favourite profile, as a reasonable compromise between the massive region of negative pressure behind a squaredrop, and the near perfect aerodynamics of a tapered air foil such as an aircraft wing or a propellor blade.

We can't get it perfect, too many design considerations.
Such as how close to the road surface is it, do we get enough air flow underneath, etc?
Should it be dead flat and smooth underneath, and just about scraping the surface, like a formula one car?
Or so high in the air it is more comparable to an aircraft fuselage?

Big ships "squat" and lose speed in narrow channels as the shallow water restricts the flow of water rushing in to fill the void created at the stern by the forward motion.

Seems to come down to how much taper can we get v how much room do we want, etc?

[friz]

I feel like mine is pretty efficient. I get 26 to 30 towing. 32 to 35 not.

Sent from my SM-G965U using Tapatalk

[Pmullen503]

One thing to remember about the simulations is that it doesn't account for what's happening at the sides of the TV and trailer. Towing an 8 foot wide trailer will have much more drag than a 4 foot wide trailer that is completely within the slipstream of the TV.

In my case (4' wide trailer towed with a minivan) I go from 21 mph to 18 with the trailer. My trailer has a vertical square back for toy hauling, so the worst possible configuration. But here's something I've observed for years: the effect on mileage is greater at 55mph than at 70mph. I believe this is due to less turbulence at the higher speed between the TV and trailer.

When towing in the rain, you can "see" the turbulence by what the rain is doing on the back window. That turbulence peaks around 50-55 mph and smooths out once you get over 70 mph.

In my case at least, what's going on between the TV and trailer, presumably turbulence, has as much to do with aerodynamics as anything else.

[tony.latham]

I get 22 MPG from my Tacoma without the teardrop. With her, it's 18 MPG. I'm packing for a camping trip today that's about 250 miles. If I've done my math correctly, pulling the teardrop will jump my fuel costs from $34 to $42.

An $8 increase to pull the teardrop.

If I chose to build the sleekest possible camper --shaped like the cross-section of a 747's fuselage-- I suppose I could chop that down to $4. But I'd probably have a camper that was a pain to camp with.

I'll spend that eight bucks today. There's a point where function eclipses increased gas mileage.





Tony

[twisted lines]

Tony's tail light design may give you some very good results to play with!
A very small hump across, similar too the hurricane hinge on a flat surface can affect swirls in a good way

[Pmullen503]

I agree on the costs. I just got back from 900 mi round trip. It probably cost $25 more in gas for 5 days worth of shelter and comfort vs a tent. Totally worth it.

The next trailer will be much more aerodynamic: lower, curved sides,corners and roof and lighter than my current foamy. But I want to be able to tow it with an EV. "Mileage" has implications in terms of range and time to recharge so it's not just a question of money.

[printer]

I wish it were just an issue of a couple of bucks. The car has a load rating of passengers and cargo of 920 lbs. WTF? So I should be making a little 4x8 foamy. I need the space though if I intend to have someone sleeping along side me. So that adds a few inches. Lowering the ride height as much as practical. I am debating going to an 8" wheel, between the pair and the spare it will save 30 lbs and the trailer will bounce around less due to the weight on the road side of the spring being lighter than the body side. Or the smaller tire with its greater air pressure will make for a more harder ride? Not much experience here. That is what I figure I will learn with this trailer and my second will be better. I am thinking of keeping the front section of the body kind of traditional to give a reasonable door height and toward the back I am thinking slicing of a section of the top corners and have a 45 degree triangle shape toward the back. I will not have a hatch in the back and my feet should not mind the reduced headroom. Sort of like Friz's trailer (can't remember the name of it). It is only a thought for now though, I will see how things go.

And thank you for the replies. I always think experience beats ten theories.