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. 

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. 

Hi Plendo

I went to a roofrack mob in Adelaide and asked for the deflector. Ended up with a roof trunk (75KGs) that has a good aerodynamic shape to it.

Now I have no clue as to the before and after fuel consumption however I enjoyed 13.9L per 100KMs over 2500KMs last trip. We went to far west Eyre peninsular both inland and coast, suffered head, side and the occasional tail wind.

I think the good mileage may be a result of our coffin (we call it) may also be the fact I have a 2.2L 4 cyl CR turbo diesel with snorkel.



Take care

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

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

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. 

When the price of fuel was rising dramatically through the late 80's & early 90's I was managing a truck engine repair shop. There was a lot of testing done on air drag. One finding was, that once the gap between the cab & trailer was more  than 1.2 meters any effect of the cab breaking through the air was lost & the trailer then had to punch its way through the air.  This is why we see skirts between the cab & trailer. The rear trailer on B doubles is as close as possible to the front trailer.

I'd imagine this gap effect would apply to the distances between any deflector on the tug & van.

It was also found that side winds were as bad as head winds, if not worse.

Cheers Pete 

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

David the problems with drag are widely known by people interested in ballistics (as in firearms etc.) It does indeed increase with the square of the velocity. The idea is to streamline the air flow and to reduce turbulence as much as possible. A deflector on the tug may be worthwhile but reducing the drag caused by turbulent air flow behind the van would be more beneficial. Something like the deflectors used on station wagons to prevent dust collecting on the back window comes to mind. Keep us posted with any future experiments as any increase in economy is worthwhile.

Regards Alan  

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

Im fascinated by the discussion on the vacuum at the rear of the moving van. Perhaps a line of small deflectors on the roof at the very rear of the van may reduce that vacuum??? I would be interested to hear what else is out there as far as ideas go. Next trip I may try a few strips of aluminium angled upwards on the trailing edge of the van just to disturb that airflow.

Then again I may be barking up the wrong tree...

Be good, great discussion.

EDIT: I have just taken a peek at some interesting stuff on youtube.

www.youtube.com/watch?v=sweZltfD-no

www.youtube.com/watch?v=karGqjCZOV8

That is why golf balls have dimples and are not smooth...

Aussie Paul. 

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

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

You can mess around at the edges, but having a big hole between the tug and the van will always be bad.
All other things being equal, a single vehicle (motorhome) will always have better fuel economy that 2 vehicles (tug and van).

A fifth wheeler is a much better starting point.

Closing that gap has always been the first priority of long distance road transports.
The second area of interest is usually ground sealing the front and sides.
The third major area is the rear turbulence. This is the hardest to fix because it requires a long tapered solid cone behind the load. The maximum angle to the airflow for the sides of that cone is 15 degrees. Steeper taper than that and the air breaks away into turbulent flow again and all is lost.

As much as it seems attractive, transports typically do not got past filling the gap between the cab and the load. They have great incentives to do as well as they can and the fact they don't suggests to me (at least) that the results are not worth the effort to achieve them.

"Lift 'um foot" will give better results every time.

Cheers,
Peter

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 

Vanderee wrote:

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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.

 

Hi Vanderee

A few comments. Racing cars do not have a big spoiler on their cars for the aerodynamics. They use them for downforce to increase grip so they can corner faster and use their power without wheelspin. In fact they actually reduce the speed to some degree because of the drag they cause. In practice they set them up for best lap times depending on the track conditions. They can also cause great stability problems in some circumstances. Not very applicable to our use unless you intend to get up into the superspeed range.

Regarding the Vortex insert into the intake manifold. This has recently been discussed in another thread if you wish to look for it. I made my comments there.

Jaahn

 

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