I read in Standen’s 1999 thesis that when the wind is at a slight angle to the direction of travel a trailer will try to “line up with the wind” as is the case with a weather vane. However, in actual practise he pointed out that the trailer will swing further than this only stopping when the force of the wind is insufficient to maintain the position; the trailer will then swing back so setting up “snaking.”
It can be seen from my diagram that a side wind of 30mph and a head wind of 40 mph (due to the forward motion of the tow car/caravan) will give a resultant “apparent” wind speed of 50mph in the direction “R” shown in the digram.
ANEMOMETERS
I can use the above diagram to explain how my suggestion for revised software for the gill/biral anemometers will operate.
If R represents the apparent wind direction and speed of 50 mph as recorded by the anemometer fixed to a tow vehicle it can be seen from the diagram that the air speed of the vehicle is 40mph and the side wind component is 30 mph. The diagram above is drawn to scale using graph paper.
This method cannot be used for automatic calculation by a computer but the values obtained from the scale drawing could be calculated using mathematical formula and these could be programmed in so that the air speed and side wind component would appear on the anemometer display.
It must be remembered that a wind speed of 50 mph is fairly common. The met office will certainly not be putting out any warnings of buildings being damaged in these circumstances, but if your tow vehicle was equipped with an anemometer you can see from the above that you would have an air speed of 40 mph whilst parked.
It will be some time before the manufacturers of trailers will admit that there is a serious problem and follow the practise of the air craft industry, so in the meantime we can only judge for ourselves the critical air speed at which our trailer is likely to snake, and the minimum side wind component that will cause snaking.
