Windmill Blade Balancing
It is essential to balance the blades carefully. The aim is to ensure that the centre of gravity of the assembled rotor is exactly at i the centre of rotation, i.e. the centre of the shaft. This is known as ‘static balancing’. Dynamic balancing is not necessary, provided that you ensure that the tips of the blades ‘track’ each other. Rotor blades are axially thin, so static balancing is quite sufficient.
Balancing should be done indoors, in a large open space, free of draughts.
Pieces of lead flashing (from the scrapyard) make ideal balance 1 weights. If very heavy weights are required, they can be shaped (from steel or lead), and tucked into the recesses between the three 1 blades.
Here is one method of checking the static balance. The blade is poised on a sharp spike (Fig. 4.9), perhaps made from a 100 mm nail or similar, driven into a wooden support, and sharpened with a grinder. The spike engages with a punch mark at the exact centre of the rotor.
Make a jig from a small piece of aluminium sheet (Fig. 4.9) with a punch mark at the centre, between two screw holes. Position the holes precisely, at (say) a 25 mm radius from the mark. Make two corresponding holes in the back of the rotor hub, on an exact line through the centre of rotation. Screw on the jig, with the punch mark at the exact centre of the rotor.
Engage the spike with the punch mark and sit the whole thing on its stand. It will be unstable. Set the rotor level, using a spirit level. Lift off the spirit level gently, and observe in which direction the rotor falls. Add weights to the opposite side, until the rotor is capable of balancing momentarily on the spike, with no preferred direction to fall in. You need to place the spirit level both ‘north-south’ and ‘east-west’.