Aerodynamics - Trailers?

I think I'd spend the extra money on fuel! That is just too ugly!

Well, I don't know what prices are there, but I was thinking more along the lines of spending a day using a sheet or two of 12mm plywood and some tin or a sheet of 3mm plywood. And paint.

 

A "big bulb shape" would have very minimal benefit compared to rounding the front corners as proposed by dhutch above. As long as the flow is attached to the front end of a "bluff body" the drag is essentially the same. That's based on lots of wind tunnel testing.

 

I was just going by what page 6 on this site seemed to show (I'd post the illustrations directly but I can't figure out how to copy them)....

http://www.dieselmidatlantic.org/diesel/frieght/conferencecalls/9_27_06/MADCAeroOvw2.pdf

Looking at it all again, I'm beginning to think maybe the car is just too small to be hauling around that trailer or maybe any trailer. The problem might not be aerodynamics so much as a weight overload.

The stoplight holder/air brake is the easiest thing to reconcile.

 

The illustrations in that link are somewhat simplistic. Note that they show square front corners and a semi-circular front but not rounded corners. Sufficiently rounded corners will be very cloase to a semi-circular front.

Keep in mind that small cars rountinely tow trailers in Europe, and the combinations similar to what Dhutch has are not uncommon. Manufactuers provide trailer towing ratings on how heavy trailers can be towed.

 

Yes, and the trailer already does have the slanted front which has got to be a plus. The rounded corners would help, but a little difficult and involved to do. I do remember reading about scow hulls and the difference between a sharp, square chine and one with even a comparatively small radius (like 4" on a barge) offered a significant improvement in hydrodynamics.

The air brake board is an obvious negative. It wouldn't be hard or expensive to duplicate what was in the first two links I posted, the things at the back of the trailers getting some of the effect of what the thing on the back of the black car does, but without looking sort of obscene.

My travel trailer has a pretty much blunt front, the top is rounded where it joins the roof, but the sides are sharp/square. When I go down the road I can see the sides just behind the corner bulge out a few inches from the vacuum created (it's like vinyl siding but made of metal) to the point of I wonder if the siding is going to come out of the channel in the corner that holds it on, and just peel off the trailer in a flash. When driving through a heavy rain, the movement of air around the front of the trailer becomes visible and it is not pretty, it's looks just like what backing a boat up does.

If it was mine and I wanted to attempt something cheaply, which is always the case with me, I would replicate what was on the semis back ends. On the front, I would create a vertical center ridge in the center of both faces, with either a 12" or wider board set on edge, or a frame of metal or wood, done so the frame on the slanted part would be parallel and also slanted. Then some cheap ply would be either bowed around that and attached to the sides to create two rounded shapes or flat pieces attached to create two wedge shapes. If painting wasn't enough, and fiberglass is not liked, an old sheet embedded in wet paint with another coat or two to help smooth it out should work.

Other things can help. Following a semi leaving about 3 feet of space works wonders on your fuel economy.

 

The trailer is towards the top end of the weight the car is plated to tow and obviously smaller trailers of the same weight would produce less drag and therefore load. However it does tow perfectly well at 60mph, you just have to downshift for the steeper hills.

Daniel

 

I believe that there is a lot of commercial chicanery surrounding this subject. A fairly common scam( I believe it is a scam) is the so caled "cheat the wind" device. It is a large quarter cylinder that is attached to the towing vehicle. The inside of the cylinder faces forward. Looks something like the ramp things that skateboarders use..The claim is that the wind is deflected up and over the top of the trailer, resulting in improved fuel economy.

Seems to me that we are presenting another bluff body to the windstream. Said bluff body surely has a collosal drag inducing potential. The owners of these things almost always claim that it works. What else would they say? They have to vindicate their judgement for paying $2000 or more for such a gizmo. Some of them are quite attractive in beautifully polished aluminum,

Am I wrong for disdaining "cheat the wind" things of this particular sort?

 

Yes, you are wrong for assuming those devices never work. Such devices were developed in the 1970's for tractor-trailer combinations and were tested both in a wind tunnel and on-the-road. A properly sized, shaped and positioned device can reduce overall drag. The device itself has drag but the drag on the tailer can be reduced by lowering the pressure on the front of the trailer. The key is the size, shape and location of the device for the the particular tow vehicle and trailer combination.

But just adding such a device to a tow vehicle does not always produce a net reduction in overall drag. It has to be a good match for the particular tow vehicle and trailer.

 

The single, most effective thing you could do is slow down. A 10% reduction in speed will give you a 15 - 25% fuel savings at no cost, zero. Plus another 5 -10% fuel savings for the tow vehicle, again at no cost or labour other than time. You'll also enjoy increased safety by slowing down AND piss off everybody behind you unless you courtiously pull over occasionally. If you really must do something a bulbus stern protrution would be of the most benefit.

 

Petros and Cockey,

Now that you have revealed yourselves you are fair game!

Actually I just had a question about something I just saw. GasPods. Take a look here>

http://www.gizmag.com/gaspods-vehicle-airfoils/23899/

My aero is basically Popular Science level. Do you think these really work? One of the "testers" has the same pickup truck I have, which gets 15mph. Unfortunately they don't give his improvement.

These look very simple in shape. They might? work stuck on the back and sides of the trailer (without the light board) but everything said above seems to say no.

Back to the thread, wouldn't the fenders for the trailer be an ideal area to fair in? Possibly just a piece of plywood run forward and aft?

Marc

 

"you are fair game" in some contexts is sometimes used in an adverserial way. I assume that is not what you meant.

First, it is always interesting when a vendor claims an "independent" organization tested their device, but doesn't name the organization nor provide a copy of the test report.

For certain not-very good shapes such devices might lower drag. But for most shapes they would raise drag.

Very good suggestion.

 

David,

Fair Game just meant you established a reason for you to be an available expert in my world. Somehow all the ones I knew at work left, retired or just faded away. No one I can ask my silly questions of.

Actually it did not make you a target, just a person who's statements should be considered more seriously.

Marc

 

Understand, and I assumed that's what you meant. Feel free to ask questions.

 

wow, never thought I would see some selling vortex generators for your car! I might make some for mine. The idea with votex generators is you reduce the amount of separation off a surface by engeizing the boundary layer with a vortex. It can work, and the vortex generation also costs drag, but presumably their "cost" is less than the overall drag reduction.

This idea has been used and been around since at least WW2, to me it always indicates a mistake was made in the original design. If you fly on commercial aircraft you can see similar devices out on the wing usually right in front of the control surfaces. YOu will also see them on the tail. The idea is to improve low speed control (like at landing speeds), unfortuantly they do increase drag, but it is more important to land safely.

How they work is very complicated, and very difficult to guess at without a lot of trial and error. And if they work at all is highly dependant on the shape of the surface you are trying to improve the low over. they tend to help the aft end of bluff bodies, a rounded blunt aft end, the vortex generator will keep the flow attached further aft and reduce the size of the separtion. I would doubt they would help on a large square van or box shape, there is no way to keep the flow attached on the rear of flat surface perpendicular to the flow. Might help on something shapped like a VW bug.

The mistake I think these one make is they are too thick, they should not be blunt nubs, but little foils. they will make a cleaner vortex without adding as much drag.

As far as the fenders, I noted very early on in one of my posts that I thing noticeable drag reduction can be gained by fairing in the fenders, mostly on front and rear, not on the top edge. It would be an easy test to duck tape cardboard from the LE of the fender to blend forward and tape it to side of the trailer, same with the TE of the fender.

 

Still not had chance to do anything to my trailer but I was in northen Finland during the last week and all there general purpose trailers are covered. Ive simular once or twice in the UK but over there every has a lift up plastic cover almost.

Seems to be one of the more popular brands:
http://www.farmi.com/farmi_pro_75L_75T.htm

Daniel