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News Archives: 1999 * 2002 * 2005 * Current

MAY '99

Venus Bay machine throws blade!

After some nuisance-tripping of the electronics (which turned out to be caused by a faulty temperature switch) Ross restarted the machine, was irritated that the thermal trip operated again, and then was alarmed as the brake came on and failed to stop the machine!

He retreated to a safer distance as the machine ran very quickly, eventually shaking off its tail fin and then losing the wind. He was about to approach and connect the "storm load" when the wind changed direction and the machine sped up again to terrifying rpm. Several fascinated eye witnesses described it as like a heavy helicopter, or cargo plane trying to take off!

The wind was gusting to about 20 m/s, and as there was no load on the turbine its runaway TSR of 18 would have sent the tips to near supersonic, explaining the great noise. Professor Wood suggests that shock stall would be setting in by the speed of 800 rpm, and laughingly suggests it as a potential control mechanism for a wind turbine!

The blades on the machine were the ones made by Transform Composites, and while showing some fretting due to flexing from load, had excellent tensile strength and low mass. When a blade eventually tore free of the mounting bolts and shot off into the sky it went nearly 100 metres in an erratic trajectory before crashing into the malley.

The real tragedy happened in the next half cycle of the machine as the unsymmetrical radial force pulled the prop shaft sideways enough for the blade stump to collide with the brake actuating lever, only ever a millimetre away. This pulled on the brake very hard, bending the brake drum, ripping out the band rivets, bending the brake lever, bending the prop shaft, breaking off a gearbox foot, snapping several braces in the tower, snapping the brake reset pipe in the turntable, bashing the brake reset quadrant into the nacelle cover, but failing to snap off the other blade!

Newcastle University now have the broken blade pieces, and are examining the tortured fibre glass. The machine has been repaired and is running again nicely, this time with blades made of Hoop Pine by Tony Meggs, a Lutenist from Nimbin. The first week of operation saw the power never drop below the 500 watt level! The summer wind usually hissed in from the Antarctic Ocean keeping the temperature as low as 16 degrees C but sometimes it turned northerly from the desert and the temperature shot to 47. This site will be a good test of the resilience of timber in very unsympathetic extremes of temperature and humidity.

The cause of the disaster was a simple lack of brake travel caused by the band being cut too long, making the take up with lining wear minimal. After cutting the band 15 mm shorter and relining it the machine stopped alarmingly quickly - 1 1/2 revolutions!

MORE DISASTERS

Two prototype machines in the Nimbin area have fallen down recently. Both failed from inadequate tower anchorages which fatigued from shaking by wind turbines. One was an A frame tower with a badly designed hold down bracket which failed from repeated uplift force, and one a guyed tower which failed after its cable eye broke in half allowing the foundation plate to cut the cable. It is depressing how many links in the chain of wind energy conversion can be critical.....

News Archives: 1999 * 2002 * 2005 * Current

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