Difference in drag from flat front to curved top front?

[qwerty11]

If you have a flat front on a camper measuring roughly 7' x 7', how much reduction in drag would you have if the top 3' of the front was either a. slopped to the roof at a 45 degree angle, or b. curve instead of a flat slope? Trying to decide if it is worth the effort to add more steps to my build.

Thanks!

[Bogo]

I'd use a curve.

NASA research says you will get almost as much as possible by making the forward edges rounded to a 2 foot radius, but this was on a semi cab sized frontal area. For simplicity, do you round the top or sides? For interior cabinetry it likely is easier to round the top.

Another thing to remember is aerodynamics isn't just the frontal shape. How the air is brought back together behind is nearly as important. A 15 degree slope to the top and side walls are around the maximum they can brought together without the air separating from the surface. This sloping in gives the air more impetus to come together smoother and quicker and thus the overall drag is lowered.

There are many little tricks that can be used to reduce a few percent drag that are easy to do. Extending the rear wall edges and top a couple inches beyond the rear wall will shave off a few percent of the drag.

You may want to read the Foamie Aerodynamics thread: viewtopic.php?f=55&t=49036

[qwerty11]

I'd use a curve.

NASA research says you will get almost as much as possible by making the forward edges rounded to a 2 foot radius, but this was on a semi cab sized frontal area. For simplicity, do you round the top or sides? For interior cabinetry it likely is easier to round the top.

Another thing to remember is aerodynamics isn't just the frontal shape. How the air is brought back together behind is nearly as important. A 15 degree slope to the top and side walls are around the maximum they can brought together without the air separating from the surface. This sloping in gives the air more impetus to come together smoother and quicker and thus the overall drag is lowered.

There are many little tricks that can be used to reduce a few percent drag that are easy to do. Extending the rear wall edges and top a couple inches beyond the rear wall will shave off a few percent of the drag.

You may want to read the Foamie Aerodynamics thread: viewtopic.php?f=55&t=49036

You wouldn't happen to have the link to the NASA research would you? I'd be interested in reading it.

[Bogo]

I don't know if the research I read is on the web. I read it decades ago. I'd use Google Scholar to do literature searches on truck aerodynamics. The Foamie Aerodynamics thread had some photos and links to some NASA research with vans and trucks. Here is a link from it that shows some pictures and briefly summarizes the research: http://spinoff.nasa.gov/Spinoff2008/t_3.html

[Corwin C]

Drag reduction is the Holy Grail of transportation design. Engineers and scientists have struggled with its complexity for centuries. The ideal shape would be one that smoothly transitions the air around the object and returns it into place with no net air movement after the object has gone by. In reality, minimizing that movement is the best we can hope for.

Since we are dealing with a trailer, the biggest factor in drag reduction is the drag characteristics of the tow vehicle. In what condition does the TV leave the airstream in its wake and how can the trailer be shaped to take advantage of that airflow configuration. If you're towing with boxy shaped car, the ideal trailer aerodynamics are going to be very different than if your towing with a bullet shaped car even if the frontal size is the same.

In general, curved surfaces are better. Transitions need to be as smooth and even as you can make them. Internal and external corners will generate vortices and should be avoided. If possible, "hide" the frontal profile of your trailer within the frontal profile of the tow vehicle. The back of an object will affect the aerodynamics just as much as the front. AND there are exceptions to almost every rule.

Judging by the sizes that you've given and making the assumption that your TV is a truck shaped vehicle I would suggest the curve for the biggest reduction in drag. Also remember that you have two other corners (at the front/side transition) that could be curved or sloped for an even further reduction in drag. It really is an exercise in compromise to come up with an "ideal" shape.

[Strop]

What about the width of the trailer? If your car is 5' wide and the trailer is of equal width or wider?

I am thinking about that now. I have come to the guess that as long as the body of the trailer is not wider than the car it should have minimum impact. Wheels under nice aerodynamic guards outside that width should cause minimal drag - I hope anyway.

It is my hope to pull a teardrop behind a VERY aerodynamic car (when I get the car back together).

Laurie

[bc toys]

I have a cargo about 7 ' high and wide it took my gas milage from 18 to 10 i would suggest you find rounded front end or a v nose

[Shadow Catcher]

I have thought about this a good bit and have come to the conclusion that unless you design the trailer to match the tow vehicle profile any add on is probably not going to pay off the effort i.e. filling the space between the TV and trailer.
One story I remember from many years ago was a man with a TT and the TV was a pickup who found that having his boat on top of the truck improved gas mileage towing. We will be taking our canoe with us to Canada this year and I will be interested to see if it makes a difference.

[Roo Dog]

SC,

We did a long trip of about 5 months with two Kayaks on top of a transit van with a caravan in tow.. They really did make an improvement to the fuel consumption.
You are very right about the tow and tug needing to be matched from an aerodynamic viewpoint if you want to improve your fuel consumption and even handling for that matter.
The Teardrop shape is fine but if it is being towed by something with the aerodynamics of a brick and not matched to the Teardrop not much will be achieved. Even two aerodynamic shapes in tandem can have detrimental effects between them.
Also well thought out shapes can lose their effectiveness when exposed to crosswinds. There are so many variables in play.

Take it easy.

RD

[DMcCam]

We seem to have hit upon a good balance ourselves with our TD and TV combo. The teardrop is lower than the roof of the Element and sits in the slipstream. We're only down about 2 miles per gallon while towing getting about 24 at 55 mph. My guess is that the closer you are to an airfoil match the better. Many of the Vardo builders have mentioned that they suffer poor mileage with the design.

All the Best,

Dave

[jonw]

Dave - lucky you!

I have a new (2012) Honda CRV. One of the ways they've increased the gas mileage is by curving the top and sides to direct the airflow right behind the vehicle.

Unfortunately this directs it right into the front of the TD. I regularly lose 10+ MPG when towing, more if there is a headwind. I'm hoping that by extending the length of my tongue (which is on the short side) I'll get better air management and get back some of those lost MPGs.

[DMcCam]

Hi Jon, Your combination sounds like it supports the reasons why I'm doing pretty well with the mileage. Our trailer is a 4.5 wide so I wonder if that isn't another aspect to consider?


You can see in this early image how the two were shaping up. It would be very interesting if everyone who can posts their set up and mileage.

All the Best,

Dave

[Bogo]

Wheels under nice aerodynamic guards outside that width should cause minimal drag - I hope anyway.

They help allot, and it isn't to hard to design them in from the beginning. They can be quite simple if they are just flat plates bolted on. A set of nuts can be built into a frame that borders the wheel well. Then a set of short bolts or domed head machine screws can be used to hold the cover in place. Press in captive nuts can be used for the machine screws to be screwed into, both in stainless steel so they don't corrode. Alternatively regular nuts can be welded onto the frame, but only if the metals match.

Captive nuts at McMaster-Carr: http://www.mcmaster.com/#press-in-captive-nuts/=mwal2y Hum, I notice they have nylon insert ones, but I wish they came in 1/4". For captive nuts you drill an undersized hole of the right size for the captive nut where you want it to be. Then you press the captive nut into that undersized hole using a press, and the deforming of the metal holds it in place and keeps it from turning.

Also I'm thinking of modifying my wheels to have two or three valve stems to make it easier to check the tire air pressure with the covers on. It's a mod I've decided will be done when my rear tractor tires are replaced in a year or two.

It is my hope to pull a teardrop behind a VERY aerodynamic car (when I get the car back together).

The size of TTT I want likely will make that impossible, but I still look at the possibility every now and then. A recent conversation on another board revived the interest in getting a Porsche 550 Spyder kit car. Low and very sleek. Being so low, small and sleek they don't even have room for two suitcases... I'd need a trailer to take anything with me on a trip.

[Fyddler]

That's what trailer mirrors are for!

[Strop]

You don't want trailer mirrors. You want to look sleek and sweet as you drive down the road.

It is real balancing act and point you keep going over and over during the planning stage. At least I am. Do you go an extra 6" high or 6" wider. I just looked at some of the boxes on the front of the teardrops and they could obviously add to the drag or lessen it depending on their shape and size.

At the end of the day I have to say that I think camping comfort and needs is the first priority. Perhaps a slower drive will help wind you down before you totally wind down camping.

All the best

Laurie