Blade Testing
We had a problem with one set of blades snapping near the blade root in high
winds. This turned out to be a problem with the electrical system - the rectifier
had overheated and broke which allowed the blades to over-speed until they
broke.
Even so, we decided to perform tests upon the replacement set of blades to
prove their durability. Included here is a basic overview of the very simple
tests performed. This is for information only – it is not intended as a guide to
testing as the tests must be greatly improved.
The blades were tested using a very simple test rig. This is shown here:
The blades are clamped at the root. A large container is then hung from the
end of the blade. The container was filled with water to simulate a wind load.
This was hung using a sling so that load is applied at two points, one 10cm
from the tip, the other 70cm from the tip. The wind would, in reality, apply a
force across the whole surface of the blade. Greater force would act at the
root of the blade. This is very difficult to simulate, especially with the available
resources.
Water was added in 1kg steps (1litre = 1kg water, so only a measuring jug is
required). The deflection of the blade from its starting position was noted at
each step. This allowed samples of slightly different design to be compared.
These very simple tests help to show how the blade design will perform,
although nothing can quite simulate real life and the very high forces to which
the blade will be subjected.
We found that the strap concentrates the force to two locations (where the
strap is wrapped around the blade). It would be better if the force was more
evenly distributed, perhaps by using a ‘sling’ wrapped around the whole blade.
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