Aerodynamics and TD Design

[troubleScottie]

My knowledge of aerodynamics is really quite small. I am attempting to piece together things that I have been reading here.

I understand the big issue with towing anything is drag. My belief was basically the head on cross-sectional area of TD is the concern. Smaller is better. To a large extent, the major improvement is to have a towing vehicle that had a bigger, overlapping cross-sectional area or to re-state, minimize the surface area extending away from the towing vehicle.

After that, I was unaware anything in the designs that actually could improve the aerodynamics. The difference between towing a brick eg a big rectangular box and a "wing" is really minor given the same cross sectional area (correct??)

However, several people have stated rounding the front improves performance/reduces drag. Similar statements have been made to modifying the rears of the TD to improve air flow.

What is the bases of these design claims? Did anyone do a wind tunnel test or mathematical modeling ? Or is this derived from empirical testing? Using a "n" of how many? Or it a gut feeling?

I have not seen anything that describes the differences of a towing vehicles eg sedan, van, truck. Should one assume the air flow over the top of each vehicles and onto/over the TD is the same? Or what happens if the distance of the tongue is altered? Again, no matter the distance, there is no affect?

Signed

Engineering challenged.

[GPW]

There seems to be a difference between theoretical perfect aerodynamics and that practically applied to trailers ( always good for an argument here) ... The one point is .... Drag increases with speed ... So if you plan on towing at 85 mph , then you’d want a more bullet (actual Teardrop) shaped trailer (theoretically ) For those that like to like the ride , enjoy the scenery , and not traveling at high speed , drag is Not so important and the theoretical ideal can be compromised ...
Naturally any flat surface to the wind creates the most drag .... rounded shapes less so ...
Without an Engineering Degree we can test Drag , by sticking our hand out the window while driving , seeing /feeling what positions have the most drag ...

Good luck ...

[warnmar10]

Pay attention to modern 18 wheel trucks going down the Interstate. It all started with simple wings on the roof of the cab to divert air over the trailer. It evolved into very elaborate aerodynamic designs. Old and new have about the same frontal area.
If the frontal area of the teardrop falls completely behind the frontal area of the TV then the teardrop's aerodynamics are far less critical but some shapes just look better than others.

[vigilant1]

To really know the answers requires some very complex computer modelling or some real modelling (using wind tunnels, correct wind speed for the scale of the model so that the Reynolds numbers are correct, etc), or even a mock-up full-sized trailer pulled behind a tow vehicle with a strain-gauge on the coupler. So, it is hard. Here are some rules of thumb:

1) Getting the frontal area of the trailer "shielded" by the tow vehicle can have big impact. Sure, there's going to be turbulence in the "gap", but in general terms, you've already "paid the price" to part the air with the tow vehicle, so the major frontal drag from the trailer comes from the parts that stick out above/outside of the "shadow" of the tow vehicle. This is particularly true if the tow vehicle has a blunt rear (i.e. a panel van)--a sedan with a trunk and a more "streamlined" rear end will not provide as much effective shadow area.

2) If air is forced to turn a sharp corner,it will become turbulent and often will increase drag. This is why even a small (e.g. 2" radius) corner at the top of a trailer is preferable to two sheets coming together at 90 degrees.

3) The rear of the trailer can contribute as much or even more drag than the front. In the link below, they say that, for a typical tractor-trailer rig, the front of the truck accounts for 20% of the drag, the skin friction is 5%, and the rear of the trailer accounts for 25% of the drag--so, more than the front of the truck. (The remaining 50% comes from drag underneath the truck/trailer, which is why we now see front and side "skirting"). In simple terms, when the air flows past a sharp edge at the back of the trailer, it creates a low pressure zone there (a "partial vacuum"), and this "pulls" the trailer back. If we could have a true teardrop shape (a streamlined airfoil) and keep the air attached the whole way to a sharp tail, this would significantly reduce drag. Unfortunately, that usually makes for a very impractical trailer shape. Still, even giving the rear of a flat-back trailer a minor "boat tail" using the actual shape of the body or with panels that can fold out of the way can significantly reduce drag. We see these sorts of panels on commercial trailers, and they do reduce fuel use. For details on various configurations that have been tested, see: http://people.clarkson.edu/~kvisser/res ... AE_365.pdf. The details of these panels (size, angles, inset from the edge, etc) don't appear to be very critical, so just an eyeball approach is probably good enough for our purposes, and might produce observable improvements in fuel economy for a square-back standy trailer.

4) If our vehicles are at least somewhat similar to commercial tractor/trailers, then there might be a lot of drag reduction to be gained by aerodynamically cleaning up the underbody area (as noted above--older trucks had 50% of their drag here). But, there are obviously practical limitations--skirts, air dams, or anything else that reduces ground clearance will also reduce the number of places you can take the trailer, etc.

5) It's important to keep all this in perspective. For most of us, our trailers aren't being pulled all day, 98+% of the time they aren't moving. And when we are using them to camp, they need to be practical and we need to be able to us the space inside them. Even a super job of reducing drag might improve our fuel economy by, at best, a few MPG, which really doesn't amount to a great deal to most of us over the course of a year--obviously things might be different if the trailer is pulled at 85 MPH for 12 hours per day and it is just a place for sleeping on rapid cross-country cannonball runs. But if we assume a trailer is pulled 2000 miles per year, fuel is $3/gal, and mileage goes from 20 MPG to 22 MPG, the difference is just $28 per year. So, the payback period for any drag reduction expenses might be a very long time for the typical user.

Mark W.

[vigilant1]

. . . .

[WizardOfOdds]

For a readable intro to the classic simple physical model you might give this link a try
http://physics.info/drag/.
In brief you will find the simple model tells you drag in a uniform media (behind a tow vehicle is any thing but uniform) is proportional to frontal area, the square of speed and a shape factor, but do not depend on simple equations for drag calculations, it just is not a simple thing.

Pay attention to modern 18 wheel trucks going down the Interstate. It all started with simple wings on the roof of the cab to divert air over the trailer. It evolved into very elaborate aerodynamic designs. Old and new have about the same frontal area.

I find this to be good advise. Also, if you already have a trailer, you can try attaching a lot of little ribbon tapes to observe where the air flow is more turbulant and work to stream line those areas. Change the tow vehicle, don't expect the same results.

[Tomterrific]

It is still fun to think about. :-) Lowering the trailer will make a difference too. Those Dexter axles might pay for themselves in time.

T

[Alan_H]

Also keep in mind, it's not only frontal area that affects drag. How the air spills off the back also affects your drag. Think about the things we are seeing on the back of semi trailers lately. They funnel the air down to a smaller rear cross section, much like the traditional teardrop shape does as well. A flat back creates a void in the airstream, with acts like a vacuum holding you back. Reducing the rear area reduces the drag.

[Cosmo]

This site has the poor mans wind tunnel test. Feel free to post your results!
http://americanarv.com/aerodynamics_07.asp

Extract from above link.

"Tests, such as "roll down tests", help prove the effectiveness of aerodynamic improvements (Roll down tests are the poor man's wind tunnel). Here is how it works. First, find an old WWII airport in the Western Desert that is very flat, has very long runways, and (this is an important one) no air traffic. Next, get a tow vehicle and it's RV up to 70 MPH and at a predetermined point on the runway, put the tow vehicle in "neutral" and see how far it will coast. Note how far you got. Now make a change to the RV shape and repeat the test."

[Vedette]

Sharp edges cause "Dirty Air" which leads to "Drag"
Keep as many edges as round as you can, stay inside the shadow of your TV.
Good Roads
Brian & Sandi