Car trailer aerodynamics 1

[kenre]

I need to build a new trailer to cart my race things round the country. Size will be approx 1700mm high from ground, 1800 wide, 3500mm from drawbar to tailgate. single axle.
Fully enclosed with front section sloping down and in towards drawbar.

Im looking for simple ideas to reduce aero drag.

Would having the rear corners made with a radius of 100mm instead of square edges help? enclosed wheels? Rounded top edges? sloping front roof panel angles? I have metal working and fibreglassing equipment so nothing too difficuilt to manufacture.

Any practical ideas most welcome
Why tow a brick when it could be an aerodynamic brick

Ken

 

[rb1957]

i'd round the front corners, and leave the rear corners sharp (90 deg).

anyway you could reduce the rear end area ... slope the top down to the rear, angle the sides in. in plane view i picture a hexagon, widest at the axle ...

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(ideally fwd would be to the left, but practically i think it's to the right !)

side elveation would be ...


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fwd is to the right

good luck

 

[kenre]

Thanks, rb1957.

The shape you describe is basically what i was looking at, although to taper the roof down at the rear end would make entry difficuilt unless it was only 100mm or so. . Im interested in others opinion on square V rounded rear section. My view is rounded would let the air flow gather behind the trailer with less turbulence, but would it create more drag?

Ken

 

[rb1957]

i was thinking about vortex shedding ... a sharp edge separates the flow uniformly and predictably. yes, rounded corners would allow the flow to bend around the corner but i think you'd have problems with the wake shedding (it'd oscillate for side-to-side).

on the sloping roof, you could hinge that panel at the front to improve access.

 

[sprintcar]

From a construction and internal volume point, would it make better sense to just use a full height/length V-nose design? You don't need the downforce that the slope nose would generate - just the ability to cut thru the wake of the tow vehicle and whatever airflow is above it. This would allow you to use all of the tongue area as storage while providing a rather sharp V effect. Plus, it reduces the turbulent area gap between the trailer and tow vehicle. Properly designed, it will also give you a better balance against cross winds since the "panel area" in front of the axles would be more equal.

Regarding the rear, I would round the side corners somewhate. There are Vortex Generators available that go across the top edge to help reduce drag there.

Sometimes we have to shoot the engineer and build what is most practical. Yes, I'm full of bullet holes too!

"If A equals success, then the formula is: A = X + Y + Z, X is work. Y is play. Z is keep your mouth shut."
-- by Albert Einstein

 

[kenre]

From a construction and internal volume point, would it make better sense to just use a full height/length V-nose design?

Sprintcar, good thinking!! By doing this it would reduce the frontal area somewhat i imagine.

Not the simplest thing to design an aerodynamic trailer i know. The biggest drag must be from the turbulence directly behind the car. I have noticed the more aerodynamic the car
(well more modern anyway) the more you notice a trailer.

Would the vortex generator be a small eg 10mm lip?

Ken

 

[MikeHalloran]

I suspect that a rear edge radius between 10 and 500 mm would generate more drag than a sharp cornered Kamm-back. Yeah, the vortex street can cause the trailer to sway; that's what axle shock absorbers and hitch dampers are for.

Other alternatives include a large edge radius like an Airstream, or a long pointy tail, either of which would need to be hinged or removable to get a race vehicle in or out of the trailer.

I suspect that the Airstream's large side edge radius also helps a lot in crosswinds, at the expense of considerable interior space.

I'm thinking the least awful compromise is the boxiest of box trailers, with shrouds, panels, fabric curtains, brushes, inflatable seals, whatever, at the front, to smooth the transition between tow vehicle and trailer.

Another alternative: A long, nonarticulated two-axle transport vehicle could save you a bunch of money at tollgates. E.g. remodel a motorhome, or find a way to get your racer in and out of a bus.





Mike Halloran
Pembroke Pines, FL, USA

 

[kenre]

Another alternative: A long, nonarticulated two-axle transport vehicle could save you a bunch of money at tollgates. E.g. remodel a motorhome, or find a way to get your racer in and out of a bus.

I dont need anything that big. Trailer is for Model R/C Racing boats.

The floor will drop to ground level for easier access, to help keep the overall height reasonable.

I guess nobody has done any serious study in this area? With all the aero design that is done with modern cars have they overlooked this area?

Ken

 

[MikeHalloran]

In the back of "Wooden Boat", someone offers plans for a nice looking teardrop shaped camping trailer, near the dimensions you specified, made of plywood of course.

About dropping the floor: It sounds like a nice idea, but be sure to also include some actual suspension travel, i.e. springs and shocks, etc, so bumps in the road don't break your boats and your trailer. First timers often build trailers with 'rigid suspension'. Second timers don't.







Mike Halloran
Pembroke Pines, FL, USA

 

[berkshire]

Take a look at making a scaled down shortened version of a sailplane trailer.
The aerodynamics would be good and you could size it to fit your models.Here is a link to look at a full sized traler

http://www.wingsandwheels.com/Cobra.htm

B.E.

 

[kenre]

Nice trailers berkshire.

Mike, suspension it will certainly have. Single 'A' arm either side, coil springs and shock absorbers. much like the rear of the local Holden Commodore. I was in the suspension game for a while so nothing too difficuilt there.

Does anyone here remember the reverse wing used on Station Wagons years ago? Theory was they directed the airflow from across the roof and down to the rear of the car. Does anybody have any thoughts of this idea?

Ken

 

[MikeHalloran]

I thought the wing's purpose was to keep road goo from fouling the rear window.



Mike Halloran
Pembroke Pines, FL, USA

 

[berkshire]

kenre (Mechanical)
Mike is right. When you are towing a trailer you do not want that kind of spoiler.
If anything, you want an uplifting spoiler there, to keep the airflow out of the gap between the car and the trailer.
B.E.

 

[kenre]

berkshire,
I dont plan on using one of those spoilers! tow vehicle is a sedan anyway.

Mike, i can see how that would work, by creating a downdraft to stop road muck. I wonder if that was the general idea when first used?

I still havent been convinced of which way to go with the rear of the trailer. Rounded edges or standard square one.....

Ken

 

[MikeHalloran]

I suppose if you want to screw around with it, you could instrument the tongue, and make some interchangeable tails.



Mike Halloran
Pembroke Pines, FL, USA

 

[kenre]

Mike, I like that idea! Will see how i go when i get closer.
As im sure your all aware, there just isnt enough time to do everything. But i will certainly look into it. If i can get 20 MPG on the highway from the car/trailer unit i will be a happy traveller!

Thanks everyone for you input!

Ken

 

[WhiteRabbit]

I do not think a boundary control strip would work do to the obvious separation you would get in the aft section of the trailer.

But I still like the vortex generator idea. Have a slight sloping upper section and place vortex generators about 2/3 aft from the front of the trailer with a boundary layer control strip about 1/3 aft.