Plendo wrote:

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Has anyone tried either of the common types of air flow modification to reduce drag?

The two I am aware of are:

1. Rooftop mounted deflectors, designed to push the fast moving air stream higher thus reducing the air pressure on the front of the caravan. An example of this would be Aeroplus, which is designed for use with caravans.

2. Attachments designed to disrupt the airflow at the back of the vehicle, thus creating turbulence, and disrupting the vacuum that forms behind the vehicle.  An example of this would be Airtab, designed and marketed to the transport industry.

Analysis of the fuel consumption on my recent trip indicates an 9% increase in consumption as I increased the towing speed from 95 to 100 kph. To my simple mind this has to be all about drag, and thus I am thinking about how I could improve it.

Interesting to hear others experience or views. 

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 The thing with drag is that it squares compared to speed. So, double the speed and the drag quadruples..

Aussie Paul. 

They were a common thing on cars towing vans 20-30 years ago, but they seem to have disappeared of the face of the earth.

I had one on my statesman when towing a 30' Franklin, it was mounted on a pair of roof bars, you adjusted the angle of the deflector by moving the rear bar forward or back.

We attached a couple of long ribbons to the front bar and let them flow over the face of the deflector towards the van, had a mate drive the car with the van attached on a straight length of road a varying speeds to see what the best angle was.

I'm not 100% sure we got it perfectly right, from memory the ribbons weren't the best solution, in hindsight if we had used smoke or a powder it may have given a better result, now with digital cameras how much accurate information would we have gleaned.


Crikey you have sown food for thought, oh well I will be on the lookout at markets and garage sales in the future.

Gday...

I'm like Phil C and have this -

I used to have a 'wire basket on rails' ... and there has been a slight improvement in fuel consumption - particularly in head/side winds. 

I did not fit it for the 'aerodynamic' affect - just to keep stuff up there out of the weather

cheers - John

www.purpleline.com.au/products-aeroplus.php - got a stack of interesting info and some user feedback (positive of course but that's to be expected)

Dave

Thanks Folks,

I am enjoying reading the feedback thus far, and it is interesting to see what people are doing.

Clearly speed has a direct impact on drag, sitting still no drag, and the faster you move through the air the more drag you get, up to a point it is probably to low to notice, but I suspect anywhere above 40 or 50 k it will be having a noticeable impact. It is also clear that the relationship is logarithmic as oppose to linear, whether it increases with the square or a higher power.

Hmm, we could drive around at ridiculously low speeds and make a real nus since of ourselves, or we could forget about economy and travel at normal traffic speeds, or a compromise in between. What I want to do here is to swing the balance a bit so we can sit at the speeds we are comfortable to drive at, and where I feel other road users will regard as as mobile road blocks.

Interestingly so far everyone has focused on reducing the frontal drag, the effort required to punch a hole in the air, no one has mentioned anything about reducing the drag back effect of the vacuum that forms behind a vehicle moving at speed. Most vans will be prone to this because of the large flat backs.

I will continue my research on both, and once I am finished the coupling changes we are making, and changing our solar configuration I will move on to experimenting with drag.

I am thinking of making a roof top wind deflector adjustable, and monitoring the impact (the tufts of wool trick).

I am also trying to think of a way to test airflow disturbers at the back of the van, the only thing I can think of is by measuring the amount of vacuum behind the van.

I love having a project to play with.

 

Peter_n_Margaret wrote:

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aussie_paul wrote:

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The thing with drag is that it squares compared to speed. So, double the speed and the drag quadruples..

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I think it is a cube actually, so doubling speed = 8 times the drag.

By far the most effective modification is how far you depress the accelerator. 

Choosing lower rolling resistance tyres is also a useful change. Michelin have done a lot of work on this. Most AT tyres are poor. In Europe, all tyres sold include a rating for noise, wet weather grip and fuel consumption.

 

Cheers,

Peter

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 Nope Peter, definately x 4. Pilot BG will tell us..

Aussie Paul. 

Hi,

The trucking industry OS has done a fair bit of research on this. Google there. They certainly have looked at fuel consumption as it is a large cost. However it is not as simple as just putting on a device here or there. Vortex generators can be purchased in Australia for the rear edges. I thought the payback time, if they worked, was a bit tenuous.

Certainly the trucking industry OS has got good improvements in fuel economy reducing speeds by governing at less than the speed limit. I observe here in Australia that does not seem to be used and neither do I see many air flow devices used Just putting curved corners on the square trailers or vans has been shown to reduce drag quite a bit. Never seen a van like that, perhaps it dosen't look macho enough !

Cheers jaahn

-- Edited by Jaahn on Monday 3rd of November 2014 09:20:41 AM

-- Edited by Jaahn on Tuesday 4th of November 2014 07:40:00 AM

http://en.wikipedia.org/wiki/Drag_(physics)
The power required to overcome the aerodynamic drag is given by:
P_d = \mathbf{F}_d \cdot \mathbf{v} = \tfrac12
ho v^3 A C_d
Note that the power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Exerting four times the force over a fixed distance produces four times as much work. At twice the speed the work (resulting in displacement over a fixed distance) is done twice as fast. Since power is the rate of doing work, four times the work done in half the time requires eight times the power.

Cheers,

Peter

 

Don't forget the boundary layer.. Your better off having air follow your vehicle than splitting it..
this causes much more turbulence..
Notice airplane wings are rounded at the front..

Get yourself the biggest ball peen hammer available and cut loose on peening the entire surface area of your tug n van.

Should help according to this; www.youtube.com/watch start a the 4.40 mark.

G.

If you are interested in disrupting the drag behind the vehicle this link may interest you.

 

http://www.airtab.com/how.html

Not sure I want to turn my ute or van into a golf ball, however, at the moment we enjoy 13.8L per 100KMs with 2500KGs van. Im interested in the vacuum and vertices between the car and van that buffet the front of the van, and how to reduce those, we have the "box" on top of the ute which has a nice aerodynamic shape and looks like it pushes the air upwards.

Maybe the mind trust in here can come up with something simple or they have seen something on the road that could be interesting.

Im not sure there is a decisive answer..

Cheers

They claim great results for better fuel consumption by installing a Vortex insert

into the tug engine's air intake, causing the air to spin,for better airflow like air induction. 

I will try one in my motor home. A mate has one fitted and he claims a big improvement.

RE: wind deflectors on a tug to change turbulence; I believe more air flow resistance(on the tug)

creates higher fuel consumption and less stability. A large deflector on the back of the towed is the answer

less turbulence more stability should also improve anti-sway properties. Racing cars have a big rear spoiler

for that purpose. Interesting to experiment all the same.

Cheers