Trampolines are a 'wing', here's how to control them when it gets windy
When New Zealand is hit by gale force storms, it's always the trampolines that take to the skies. As wild weather hits the country once again, we re-visit how to keep your trampolines safe.
FROM THE ARCHIVES: Once again New Zealand has been hit by gale force winds, and yet again trampolines have taken to the skies.
On Thursday March 24, there were several reports of trampolines turning up in unusual places.
But why do these heavy objects always seem to go missing? The reason behind it's not all that complicated.
"From a purely aerodynamics point of view, these things are a wing," Canterbury University mechanical engineering professor Dr Keith Alexander says.
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Alexander invented the Springfree trampoline, which has won awards around the world.
If a trampoline was standing flat and the wind was horizontal, nothing much happened, he said.
"But if it tilts it at all, then it has an angle of attack and it will want to fly."
Nets put up around trampolines to help keep users safe could add to the problem. Even though most of the air went through the holes, a net was still a bit of a sail and could be enough to allow a trampoline to tilt, he said.
Once that happened the wind was able to hit the mat, which typically was only about 12 per cent holes.
If the flat area ended up being at the right angle to the wind "that will make it fly, that's how a wing works".
Alexander is on the standards committee for trampoline safety in the US where wind warnings on trampolines have been strengthened after the death of a small girl.
"It was windy, but not strong winds. There was a localised gust. It was strong enough to pick up the trampoline, she fell of and was killed," he said.
He once saw a trampoline in someone's yard get lifted by the wind before it rolled over their fence and halfway over the roof of a neighbour's house. Then the wind eased and the trampoline rolled back, falling between the house and a hedge.
In the US, a Springfree trampoline turned up, essentially undamaged, in the middle of a motorway.
"Apparently it went nearly 2km. It was picked up by a twister ... it might have rolled some of the way," Alexander said.
Hang gliders could take off at speeds around 28kmh (15 knots).
"A hang glider's got a bit more area than a trampoline but not all that much."
Wind speeds didn't usually get up to 28kmh around houses in many areas because of the presence of obstructions such as buildings and hedges. It was more likely in exposed places, he said.
Hang gliders weighed about 40kg and some basic round trampolines would be a similar weight.
"They are the ones most likely to be flying around because they are the lightest."
Rectangular trampolines, because they needed thicker steel, were heavier, as were Springfree trampolines, which weighted between 70kg and 130kg because of their design.
Regardless of the weight, "there's always a wind that's going to be strong enough", Alexander said.
If strong winds were forecast the best thing to do was to put the trampoline away but people didn't want to do that.
"One thing that's pretty sensible, if you could rope it to something. If there's a tree and you put a rope around it. The wind might blow but it won't go anywhere," he said.
If a trampoline had a net around it the net should be taken down, and obviously if it was windy children should not be allowed on the trampoline.
Kits were available that used pegs to try to anchor trampolines to the ground but their effectiveness could vary depending on the type of ground and how well they were put in place.
Sand bags could also be put over the legs of trampolines to try to hold them down, but they might not work in exposed places, Alexander said.
"What I would be recommending is that people are aware that when there's going to be high winds the trampoline might go somewhere and it's probably going to be their responsibility. If it hits someone's Mercedes Benz across the road, whose insurance company is going to pay?
"You want to tie it up like a dog that's likely to stray."
Even then he couldn't guarantee it wouldn't blow away, but it was much better than doing nothing.
The above story was first published on March 24, 2016.