Change of Initials

With effect from 1.1.09 I shall be “MInstP”

My blogs are for reference and are accessed at numerous points, each with a title. It will be some time before all the headings are changed.

When looking at weather forecasts it is better to seek out the speed of the strongest gusts forecast before deciding whether it is safe to be on the road.  

Notices on the back of a number of HGV's urge the public to phone and comment on the driving standards. However we do not know the standards prescribed. The DfT should consider updating their advice on safety when it is windy particularly on matters that relate to the safety of  HGV's especially high aspect trailers, caravans and below 3.5 tonne trailers.

I have previously pointed out that vehicle manufacturers ( as with air craft manufacturers) should be required to establish critical wind speeds with some degree of accuracy for each type of vehicle sold.

I have observed that road users are increasingly prepared to pay for extra safety devices such as electronic brakes and air bags, and feel certain that when they understand why knowledge of air speeds and side wind components is important for safety reasons, they will be prepared to pay extra for vehicles that are sold with the relevant information in the owner’s handbook.

As I have never driven an HGV or sailed a boat larger than my daughter’s 29ft Westerly (about 4000kg), my estimates for HGV critical air speeds and side wind components are not be very accurate. I am therefore pointing out how to make estimates so that those with relevant towing experience can do this for themselves.

I can be quite accurate concerning the fact that in the USA (for instance) when a Hurricane force wind is expected those able to do so move out of the immediate coastal area until the danger has passed. The remainder of the population go into buildings constructed so that they can withstand Hurricane force winds.

According to the Beaufort Wind Scale for Mariners a Hurricane is Force 12 ( about 78 mph) and above. If you are driving at an indicated 55 mph it will only take a head wind of 23 mph to bring the air speed that the vehicle/trailer is subjected to up to the bottom end of the dreaded Hurricane strength. The aerodynamics of trailers in particular needs to be taken very seriously.

In 1999 Bath University published “Towed Vehicle Aerodynamics” by Standen; the wind tunnel tests had proved that aerofoils to create down force as speed increases improved the stability of a caravan with a (approximately) central axle.

Aerodynamically a HGV trailer with approximately central axles/axle is  similar to a caravan.

Only a minority of HGV’s tow trailers with central axles, but there will also be the evidence of tyre marks on the road from all snaking accidents which may be similar to those which have been “leaked” to me concerning snaking caravan accidents.

The above Bath university results clearly stated that some trailer snaking is caused by the wind and explained how this happens.

As I feel that I have established that the maximum safe air speed for my small caravan is about 50 mph, and this is the stalling speed for a typical light aircraft of the same weight, I feel that I am justified in saying that the maximum safe air speed for a central axle HGV trailer is VERY ROUGHLY the same as the stalling speed of an aircraft of the same weight. In the absence at the moment of accurate figures from people with access to instrumentation that can measure the amount of lift created by the air under the vehicles, I suggest that HGV drivers cultivate the acquaintance of an airline pilot and try and obtain some confidential advice concerning stalling speeds to compare with their own experiences of the air speeds and side wind speeds when snaking/jack knifing has taken place. Starting about 30 years ago I always managed to avoid wind induced snaking by obtaining information from Radio 4 shipping forecasts and estimating the likely speeds inland. More recently I have been able to get a very good wind speed forecast from the BBC weather forecast on the web, but we really need to go direct to the met office so that we obtain the maximum speed of wind gusts.

As I started towing on the road when I purchased my first sailing cruiser I was always very aware of the power of the wind and those who doubt this should talk to people who sail some of the types of boat I list below. These sailors will be only too pleased to help as the relevant information concerning sailing cruisers is not confidential. I want the sailing community to tell people the area of sail they use to sail their boats in gale force winds (about 44mph) in sheltered waters where the sea is calm. If HGV drivers then compare the relevant sail area with the area of the side of their trailer of the same weight as a sailing cruiser, they will then understand why something needs to be done to improve road safety by using electronic braking systems and aerofoils to create down force on all trailers as well as paying greater attention to weather forecasts and "critical" speeds.

Aerofoils for HGV trailers would have to be quite large, and as this matter was not allowed for when bridges were built engineers will have a difficult problem to solve when designing the aerofoils for large trailers. For those who can afford the million euro VW sports car this problem has been solved by a computer controlled aerofoil over the rear wheels ( but I would not suggest that the latter is a good tow car!) Formula 1 racing cars, after years of development, now have two fixed aerofoils, one between the front wheels and one over the rear wheels. Rally cars are beginning to use fixed aerofoils and when the latter are above the level of the car roof they must be very effective.

Addendum Nov 2008

As I took a particular interest in Louis Hamiltons' performance in F1 racing this year I could not fail to notice that they have now added extra aerofoils.

A few Statistics:-

SAILING CRUISERS

Nicholson 55 (Weight 17.3 metric tonnes; area of main sail = 48.3 square metres ); Nicholson 42 (wt 10.16 tonnes)

Bowman 57 ( wt 19.05 tonnes); Ocean 71 (29.06 tonnes);

Source:- Boating World Guide to Sailing Cruisers; 1976

AIR LINERS

Boeing 737; weight 45.36 tonnes; take off air speed 150 mph.

My estimate of stalling air speed = 120 mph.

Fairchild Aviation C123B

Weight, 27 tonnes.  Stalling air speed, 96mph

Based on the above I would therefore estimate that a 27 tonne HGV trailer (if such a trailer is legal and exists) with approximately central axles would become liable to snaking when the air speed reached 96mph. The driver would be aware of the air speed if he/she had an appropriate indicator.

At 55mph road speed one only needs a head wind component of 41 mph (gale force is about 44 mph) to reach the estimated critical speed.

Tiger Moth Biplane.

As one of my main recreational activties over the last 32 years has been estuary and coastal sailing, I have never had the usual experience of flying off to wamer climates for my holidays.

I have only taken to the air twice, and that was as a passenger in a Tiger Moth Biplane in 1951. During the Summer holidays HMG "called me up" again to do an additional two weeks National Service in the RAF. During my 18 months national service I was an "Audiometrician," so it was deemed appropriate that I did some flying.

In 2003 when I started enquiring about the reasons for caravans snaking I recalled that the Tiger Moth take off speed was about 60 mph and that it must have weighed about the same as my small caravan.

You will see from my blogs that my memory had not failed as similar sized modern light aircraft have comparable statistics.

Peter W Jones AMInstP

Consultant for Towed Vehicle Accidents.

(charitable status to be applied for)