Aerotab

[GD485]

In the States there is an aero-dynamic add-on being sold for RV's and the like that is supposed to produce multiple air vortex and reduce the drag, improve stability and make the sunshine everyday ( Maybe not the last one).
Has anyone had any experience with this simple add-on? I have some doubts about its effectiveness. In my opinion it should go the same way as exhaust extractor tail pipes and Snake oil.

 

[otherclive]

In the States there is an aero-dynamic add-on being sold for RV's and the like that is supposed to produce multiple air vortex and reduce the drag, improve stability and make the sunshine everyday ( Maybe not the last one).
Has anyone had any experience with this simple add-on? I have some doubts about its effectiveness. In my opinion it should go the same way as exhaust extractor tail pipes and Snake oil.

Where does it fit on the RV? However the airflow over a caravan outfit is totally different cf an RV so I'm not sure there would be any advantage in even considering it for a caravan. But like the aero foils that fit onto the back roof of the car and flit the air over the van.

 

[ProfJohnL]

In the States there is an aero-dynamic add-on being sold for RV's and the like that is supposed to produce multiple air vortex and reduce the drag, improve stability and make the sunshine everyday ( Maybe not the last one).
Has anyone had any experience with this simple add-on? I have some doubts about its effectiveness. In my opinion it should go the same way as exhaust extractor tail pipes and Snake oil.

Aircraft manufacturers and formula 1 teams got to extraordinary lengths to avoid vortexes, preferring as near laminar flow as possible to reduce drag, so a device that produces "multiple Vortexes" seems to go against the leading aerodynamic practice.

There used to be several car roof mounted sold for caravanning, but its so difficult to find the optimum position as each combination of car and caravan.

An incorrectly fitted device will not provide any practical benefits, and could make fuel consumption and stability worse.

 

[GD485]

I tend to agree with the Prof and Clive as to the effectiveness of these devices.
The theory is they are supposed to work like the little vertical tabs on aircraft wings and control surfaces.
As air passes over a surface there is a critical speed where the air no longer stay in contact and becomes turbulent (Bernoulli principle). This change in speed can be caused by the change in the surface shape. Like the top surface of an aerofoil or the curve of the back panel/ roof junction on a caravan. The separation can cause the air near the surface to reverse it's flow and create a low pressure zone and drag.
This reverse flow forms a vortex across the width of the rear of the caravan. So, from my limited knowledge there should be vertical version at the trailing edge of the side panels too. This could explain the strange pattern of rain drops and dirt streaks on the back of my caravan in wet weather.
The little vortex tabs are supposed to produce streamers of air forcing the air to separate cleanly breaking up the formation the harmful surface reverse flow turbulence.
The Mitsubishi Evo road car uses them on the rear of the roof to stop the air flow detaching from the rear window and becoming turbulent before passing of the enormous rear wing/aerofoil.
With the abrupt change between the sides and rear panel on a caravan the air separating must be causing drag. Surly when a truck approaches with a huge high pressure bow wave hits the back of the caravan more on the offside than the nearside the airflow off the caravan is altered. Could this be the dragging feeling when a truck is about to pass and possibly the change to the drag force on one side could trigger instability.
However, would placing little bits of plastic near the trailing edges have any positive effect at the speeds we trundle along at is another matter.
I just thought it might be something to gets The Prof's keyboard busy!

 

[ProfJohnL]

...I just thought it might be something to gets The Prof's keyboard busy!

You win

To gain any benefit from this type of device, their positioning would be critical, as I pointed out above, unless each possible car and caravan size and shape were identified and tested to derive the most effective positioning of the devices, I seriously doubt they will have any discernible effect.

It's likely that any benefits they may have will be swamped by the the variety and scale of changeable conditions we experience on the road. Unlike aircraft which when in flight in free air, the air flows will be far more consistent than the gusts of wind we get a ground level from mother nature, exposed roads and gaps between buildings as well as the HGV effects

However I wouldn't lump them in with chocolate tea pots, Tyre bands and calibrated compression spring nose load gauges whose claims are unjustified, these tabs may have some small benefits provide they are fitted correctly.

Perhaps PCM should undertake a test of the devices.

 

[Dustydog]

Following on from the Prof’s comments this link is well worth a read although a few years old.

http://assets.swiftgroup.co.uk/swift-group/miscellaneous/CFD_study.pdf

 

[GD485]

Hi,

I’ve been doing a bit of online research regarding the aerodynamics around vortex tabs on F1 cars and the like.
Small aerodynamic tweaks can make massive jumps in performance mainly due to the high speeds but the engineers have to produce a car that still works well at low speeds More in the caravan range.
Although Swift and Bailey have published their findings regarding caravan stability and drag the question still remains could caravans be made safer to tow through aerodynamic means.
I feel the caravan manufacturers are far more interested in suppling a box to live in and make some profit than performance improvements when towing.
Look how long it’s taken for the industry to get its head around water ingress.
I still think a chocolate frying pan tastes nice!

 

[otherclive]

Following on from the Prof’s comments this link is well worth a read although a few years old.

]http://assets.swiftgroup.co.uk/swift-group/miscellaneous/CFD_study.pdf

http://assets.swiftgroup.co.uk/swift-group/miscellaneous/CFD_study.pdf[/quote
Thank you for the link. An interesting piece of work by Swift and all credit for their efforts. I find though that dropping from 60mph to 55mph gives good fuel saving increased further by the reduced need to pass so many HGV s on their governors. A double bonus. However if I get a clear section of motorway I move back to 60mph if conditions allow.

 

[ProfJohnL]

Well done Dusty,

I was unaware of the Swift study, But I'm please to see the subject has been looked at by a manufacture.

Let me state now I have no doubt that Swift has the evidence to support the findings of this study, the snippet of teh results does not actually tell us very much, and I suspect that most caravanners would not see anything like the efficiency savings Swift suggest.

For a start The report is not comparing a new swift with and older model swift, but an unnamed competitors model. I would presume the model chosen for the comparison would be most likely to emphasise the differences. We also do not know if the competitors model was a current or really old design. So by removing makes and model identifiers in this short report prevents any meaningful conclusions to be drawn by readers.

The testing has been carried out under Lab conditions, and simulations, so this is considering ideal conditions. Now fundamentally this is the way to derive repeatable and verifiable data, and the results should believable. Unfortunately real life conditions are highly variable. I looked for evidence of real life road tests in the report but I found none, So I am sceptical about the scale of the claimed benefits being repeatable in real life."

However one sentence did catch my eye, and is relevant to this thread.

"Again, the results were surprising with the smaller features not having as great an impact as first thought."

Bearing in mind the report is about towing efficiencies rather than stability, its still likely the underlying meaning of teh quote holds true in relation of stuck on tabs affecting stability.

More validateable evidence please Swift.

 

[GD485]

Hi Prof
I certainly agree with your last point.

It’s about time manufacturers got around to some development work to make Caravan more stable, reduce emmisions and improve fuel economy.

The idea of some fancy splash back or monbloc tap pales into insignificance when compared to better handling and improved fuel consumption.
As cars become lighter the towing ratio is going to creep towards 100% or more so better safer ‘vans become necessary and a good selling point.

 

[Hutch]

If when you go on a holiday without your caravan, "Naughty People" look out at the aircraft wing and towards the front you will see small maybe 2 inches high vertical "bits" these are what were are talking about . They are not in line with the wing , maybe 15 degrees from center. They cause the air to move about at low speed , 250 knots, toenhance, the Bernollies effect, by making the airflow move further, and give better lift to the aircraft wing.,at low speed . How this would enhance a caravan or RV truck I dont know. But on F1 racing cars they would enhance the airflow for brake cooling , engine cooling etc, wish i knew more about that..

 

[GD485]

I found another site offering a similar product http://airtab.com/application-rv.htm
These add-ons look a bit like plastic wishbones!
The idea is they stop the reverse surface airflow caused by the air becoming turbulent instead of a nice smooth laminar flow. The blurb says the vortex produced by the tabs is inline with the airflow and this “fills” the void area created by the reversed surface flow. This reversed surface flow is referred to as a bubble of low pressure air that is at right angles to the direction of travel.
My best guess at the tabs on the surface of an plane wing maybe there to counter act turbulent vortex produced by the wing. Smoothing the airflow and reducing drag (????).
My view of these tabs on caravans or truck will have very little effect at the speeds they travel at but if it does have a good effect on airflow/stability/fuel consumption then it should be investigated.