An Investigation into Wind Deflectors

[kevinG]

Hello everyone,

My name is Kevin and I'm a final year Aerospace Eng student at the University of Hertfordshire. For my final year project, I'm looking into roof mounted wind deflectors and their impacts on stability, fuel economy and service life of the car. I've read mixed reviews on these and hope to get to the bottom of it with computational fluid dynamics. I've researched Purpleline's AeroPlus, which seems to be the most popular on the market at the moment. Do any of you have experience with it? Or perhaps other commercially available wind deflectors? I'd love to hear from you and will share my research if anyone is interested.

Thanks in advance and cheers,
Kevin G

 

[emmerson]

Kevin, good luck with your research, but I haven't seen a wind deflector in use for many years now.When caravan makers started making vans a little more aerodynamic in the early nineties they were no longer necessary.
I think I might have one rusting away somewhere.................

 

[Craigyoung]

Only ever seen one in the five years we've been going out, was on the back of a Mercedes Estate looked good, we looked into getting one but since then we have changed our car to higher car so no longer need one

 

[camel]

Kevin,
I think you should direct your research towards the road haulage industry, this research was done years ago in the haulage industry, deflectors do work in saving on fuel economy, pointless asking on here because everyone will give you a different answer to you enquiry without any knowledge of the subject,

 

[kevinG]

That's fine, I'm open to hearing opinions from anyone with experience with air deflectors, I want to hear public opinion.

 

[ProfJohnL]

Hello Kevin,
One of the ways manufacturers have been improving economy has been by keeping the cars Cd values as low as possible, and this has been aided by CAD modelling and some dedicated air flow modelling packages, so I suspect there isn't a lot that can be done for solo cars by adding after market aerodynamic products.

But as soon as you attach a caravan, the aerodynamic package changes considerably, and for the worse. Obviously there is the additional mass of the caravan, passengers and luggage, which has a very obvious effect on the ability of the engine to haul the outfit from stand still, but in addition is the physical size of the caravan's body, which has more resemblance to a brick than arrow, and that has an increasing effect as the speed builds up.

Where as most family cars that are capable of towing an average caravan are mechanically able to reach 100MPH or more and their aerodynamics and mechanical components should be rated accordingly, they are of course limited to 70 MPH in the UK and when towing they are further restricted to a maximum speed of 60MPH on de-restricted dual carriageways and motorways. So the higher speed aerodynamics of the tow vehicle may not be explored when towing.

Caravan manufacturers are not unaware of the aerodynamic effects of their products which is why since the 1980's most new products have gone away from the vertical slab fronted designs and now include some more front rake angle, and detailing of the rear panels, usually created by moulded panels.

The problem is neither the car manufacture or the caravan manufacturer knows whose product their own will be paired with by the caravanner. Consequently any particular feature either manufacture creates for their own car or caravan may be negated by the design of the other manufacturers product.

However clean the air flow is over a solo car, as soon as a trailer or a caravan is coupled the recombining airflow off the car is presented with a new obstruction of significant size and effect. What this means in detail will be different for every possible combination of car and caravan.

There have been after market aero packs for caravanners which basically consist of an angled board or scoop designed to be fitted to the roof of the towing car. The marketing gave the impression these are designed to interrupt the airflow passing over the roof of the car and to deflect it upwards in the belief it will then be high enough to pass cleanly over the towed caravan.

It is questionable if these devices do provide the suggested improvements they claim. What I am convinced is they would need to be placed and tuned very carefully, and if miss placed are likely to make matters worse, and as there are no detailed instructions on how to check the position of them, its down to trial and error, and the differences will so easily be swamped by other factors which will easily mask small incremental improvements.

I do hope your research will prove fruitful, but do make sure the methods and results stand up to fine scrutiny.

 

[Martin24]

Kevin,
I think you should direct your research towards the road haulage industry, this research was done years ago in the haulage industry, deflectors do work in saving on fuel economy, pointless asking on here because everyone will give you a different answer to you enquiry without any knowledge of the subject,

Thanks for the implication that we don't have any knowledge on the subject, especially when you have no knowledge about the knowledge of others

 

[Jaydug]

The boom years for wind deflectors was during the late 70's early 80's and many caravanners were using them - me included. It's doubtful if there was any real improvement in fuel consumption when towing but there certainly was a rise in consumption if you left it fitted when driving solo.

 

[Raywood]

We did buy a wind deflector when we started caravanning some 34 years ago. Then the caravans were a lot less streamlined and the cars had a gutter on the roof edge which meant you could fit the deflector right at the back if the car which was an Escort hatch back. At that time the deflector did appear to have some effect in reducing fuel consumption but it was not very great.
When we changed the caravan to a more streamlined design and the car was a saloon with a lot larger gap between the deflector and the caravan no noticeable reduction was achieved although it did still have some aerodynamic effect as it did not leave dead flies on the front of the caravan except at the very top. The difference was however never apparent.
It did also have the effect of pushing the rear wheels down which could make the steering light which is not good.
To sum up the deflector needs to be at the very back of the tow car which means it only works with estates and hatchbacks and only those that allow it to be mounted at the very rear but modern caravans are more streamlined so the effect is now reduced as against older designs. The weight transfer to the back wheels is undesirable though.

 

[kevinG]

Thank you very much for your detailed reply, appreciate it. It would be perfectly reasonable to dismiss air deflectors if I find them to be ineffective (which seems to be the general user consensus). I'm more interested in the real engineering processes of CFD and CAD that goes into them, because it's the same processes that apply in the aerospace industry.

 

[kevinG]

Hello Kevin,

It is questionable if these devices do provide the suggested improvements they claim. What I am convinced is they would need to be placed and tuned very carefully, and if miss placed are likely to make matters worse, and as there are no detailed instructions on how to check the position of them, its down to trial and error, and the differences will so easily be swamped by other factors which will easily mask small incremental improvements.

I do hope your research will prove fruitful, but do make sure the methods and results stand up to fine scrutiny.

Thank you very much for your detailed reply, appreciate it. It would be perfectly reasonable to dismiss air deflectors if I find them to be ineffective (which seems to be the general user consensus). I'm more interested in the real engineering processes of CFD and CAD that goes into them, because it's the same processes that apply in the aerospace industry.

 

[otherclive]

Given the period when you did see more being used I suspect that the makers did precious little research into their true properties. Probably less research or development testing than went into the ridiculous front air spoiler that I fitted to my Morris Marina estate. A real runway scraper. After all CFD wasn’t being used except in research establishments, and then principally for sytems used in water borne environments. The computing power available to most businesses wasn’t anywhere near what is available today via the integrated software suites. But with what is now available you should be able to look at a number of modelling scenarios and investigate the performance and sensitivity aspects of the roof spoilers.

 

[ProfJohnL]

Thank you very much for your detailed reply, appreciate it. It would be perfectly reasonable to dismiss air deflectors if I find them to be ineffective (which seems to be the general user consensus). I'm more interested in the real engineering processes of CFD and CAD that goes into them, because it's the same processes that apply in the aerospace industry.

I didn't say they couldn't work, its more likely there is no way the ordinary driver could calculate the optimal position, shape or size for their outfit, there are simply far too many interactions.

I encourage you to research the topic, and if you are lucky enough to find viable solution - more power to your elbow. Do bear in mind that sometimes research can throw up compelling reasons not to do something, and that can sometimes be as valuable as finding positives.

 

[camel]

When the transport company I worked for had the fleet fitted fitted deflectors , the method of calculation was the height of the vehicle cab along with the height of the trailer and the gap between the back of the lorry cab and the front of the trailer, the deflectors we had fitted had on the back of the deflector frame a scale which you then referred to the manufactures chart along with your measurements and you then set the deflector at the correct angle,

 

[ProfJohnL]

When the transport company I worked for had the fleet fitted fitted deflectors , the method of calculation was the height of the vehicle cab along with the height of the trailer and the gap between the back of the lorry cab and the front of the trailer, the deflectors we had fitted had on the back of the deflector frame a scale which you then referred to the manufactures chart along with your measurements and you then set the deflector at the correct angle,

There are some crucial differences between the arrangement for an HGV lorry and the arrangements for cars and caravans. Generally speaking HGV's load bodies (solid and artic) are close coupled to the cab, and the goods body is usually a vertical wall, the geometric relationship between the cab and body are quite constrained. Consequently it is easier to produce a general fitting guide such as you have described.

Look at the variety of car shapes, from coupe;s saloons and estates, and the variety of front wall profiles used by the caravan manufacturers, and suddenly the dimensions and interactions multiply to a much wider range. it is conceivable that in some cases the air flow over the car and the towing caravan may already be optimal, and the addition of a deflector would actually be counter productive. Similarly put a deflector in the wrong place and again the results could be negative.

I know that aero packages for goods vehicles can very important, as fuel costs are such a significant part of the operating costs. even a 1 , 2% or more saving can be so important so it may be worth investing in deflectors, in this context the the means justifies teh end, but for caravanners, whilst economy will always be a consideration, most caravanners will take the loss of economy on the nose when towing, becasue its the end that justifies the means, so it may be questionable if the cost of packages can be justified.

 

[camel]

Personally I don't think I would even bother with a wind deflector when I'm towing a caravan it's just more equipment to carry and set up, then take off and put back on, no thanks

 

[Craigyoung]

Camel , Different people have different circumstances for deflectors as we had a lower car before and we were getting problems with front vibration on our window whilst towing and went through 3 or 4 in a matter of a few years we needed something to push the wind up and over the caravan and not directly hit the front window .

 

[Hutch]

Kevin , good work for you on your project it, is neither to support or to negate the use of an aero foill. The idea to originally pass the airflow over the caravan, worked weĺ but the downwards force drained the power from cars.
Anyway good luck . Wish you many years in the aviation industry,
Hutch. AME 17116.

 

[Raywood]

The deflectors are made by small companies who I doubt are able to put a lot of research in to the effectiveness of them, so although they may work for some there is never going to be any real evidence unless someone does a lot of research. They do work in the HGV market, but the gap between the trailer and the back of the cab is usually quite tight and easier to adjust, and the trailer front is vertical rather than sloping. I do gather many operators did get the correct adjustment by trial and error though. They do far greater mileages so any saving is easier to calculate and far bigger than for a car and caravan.
The laws of physics would confirm though that a very short gap is needed between the deflector and the trailer front for them to work. A saloon boot, a long a frame and the more streamlined front of most caravans will probably mean there is no useful saving.

 

[KeefySher]

Hello Kevin,

It is questionable if these devices do provide the suggested improvements they claim. What I am convinced is they would need to be placed and tuned very carefully, and if miss placed are likely to make matters worse, and as there are no detailed instructions on how to check the position of them, its down to trial and error, and the differences will so easily be swamped by other factors which will easily mask small incremental improvements.

I do hope your research will prove fruitful, but do make sure the methods and results stand up to fine scrutiny.

Thank you very much for your detailed reply, appreciate it. It would be perfectly reasonable to dismiss air deflectors if I find them to be ineffective (which seems to be the general user consensus). I'm more interested in the real engineering processes of CFD and CAD that goes into them, because it's the same processes that apply in the aerospace industry.

I think your challenge in any modelling will be the vast range of boundary conditions you will need to establish to make any meaningful analysis, to be frank.

Any modelling will then need to be subjected to empirical testing, which in turn will ratify or otherwise the boundary conditions and assumptions the modelling made. The range of variables is simply vast.

A massive project I suggest. Have you the time and computing power to do this?

The caravan fraternity can't determine conditions to load to measure nose weight :evil: :evil: let alone actually measure it.

 

[ProfJohnL]

Kevin , good work for you on your project it, is neither to support or to negate the use of an aero foill. The idea to originally pass the airflow over the caravan, worked weĺ but the downwards force drained the power from cars.
Anyway good luck . Wish you many years in the aviation industry,
Hutch. AME 17116.

Hello Hutch,

The concept behind the early deflectors was to try and resolve the problem of air flow having negotiated the tow vehicle finding the solid front wall of the trailer (Caravan) which cause steep pressure changes as the air met the wall, and subsequent eddies as the air was redirected across the front wall and around the sides and roof, and to a lesser extent underneath the trailer. It was these pressure changes and eddies which increased the air drag and sapped the tow vehicles power. It was envisaged that a deflector on the tow vehicle would help to easy the air flow over the trailer and reduce the air drag.

If in practice the deflector was raising the air drag and sapping even more power, then it was clearly not working, possibly because it was the wrong size, or incorrectly positioned.

I think Camel may have also hit part of the nail on the head as its something else to do, for what may only be a marginal benefit.

This also plays into the Mileage thread, becasue caravanners have to accept poorer towing consumption (Circa 10 to 15% more fuel). If real solo fuel costs are in the order of £0.10 per mile solo (40mpg ish) towing it would rise to £0.115 per mile a difference of £ 0.015pm.

Its likely a deflector will cost in the order of £100 (Wishful thinking :lol: ) so 100 / 0.015 = 6,666 Miles of towing.

 

[Hutch]

Prof, Kevins thing is to do a thesis into a wing deflector, not to see if it works or not. Hopefully he will get to his Masters degree as my son did, and maybe get to be a fully licenced engineer as I am
I just like sitting in my driptray and playing with my nuts.

 

[Craigyoung]

To much information hutch :lol:

 

[Parksy]

I've never used a wind deflector, my approach has been to use a relatively powerful towing vehicle which uses brute force delivered via engine power to overcome headwinds and weight to help with stability.
To look at this from a different angle, I wonder if some sort of semi-disposable plastic aerodynamic deflector could be fitted to the front of caravans, with a possibility of it being cut to the correct size so as to form a more effective aerodynamic piece of equipment than the old type roof mounted deflectors?

 

[Craigyoung]

Parksy , my situation, it was the other way round for me, I wanted it on the car to stop it hitting the front of the caravan window .
If I was to get a caravan mounted one , it woulda still hit the front window .
All changed now so I'm not bothered !