Mechanical Design
The model was designed using Autodesk Fusion 360 to be accessible for everyone to download and work with as Fusion 360 is available for free for hobbyists to use. It is also widely supported online and plenty of guides are available to allow for the model parts to be manufactured using almost any method.
The inspiration for this design comes from the way helicopter blade pitch mechanisms are designed. Helicopters are controlled almost entirely by changing the orientation of the main rotor. To go forward and backward, the whole assembly is tilted to direct the generated thrust as required. This can also be done to fly sideways, although turns are coordinated by using both the main and the rear rotors. The amount of overall lift is adjusted using the collective, which changes the pitch of the blades while keeping the throttle application unchanged. This has the effect of pushing a larger volume of air through the rotor 'disk'.
To use this idea on a wind turbine, we need to simply change our frame of reference. Let us examine the requirements for maintaining a hover with a helicopter. If the air is perfectly still and uniform, all we need to do at a certain throttle setting (a fixed throttle setting assumes a constant speed of rotation regardless of the pitch setting for simplicity) is to find the blade pitch that pushes enough air to counteract the weight of the helicopter. If we were to increase the throttle, thus increasing the rotor speed, the veolume of the air pushed through the disk would increase as well. To keep the helicopter in a hover, the pitch would have to be decreased to keep the volume of air per unit time constant.
In a wind turbine, the speed of the air comes from the wind, not the rotor, and the turbine is fixed so it obviously cannot hover. But pitch control of the blades is still crucial, particularly at high winds, when the turbine is at the limit of its generator capacity. At the limit, if the generator spins any faster, it could lead to the failure of the turbine. Thus, the speed of rotation has to be kept constant. With the turbine fixed, the only was to do this is by changing the blade pitch.
Another important task of the pitch mechanism is to reduce stress on the turbine tower if the wind speed gets too high. In this case, the rotor brake is engaged to stop the turbine, while the blade pitch is adjusted to provide the least possible resistance to the wind (lowest possible lift), thus minimising the forces exerted on the tower structure.
The most crucial reguirement for the rotor is that it must be able to adjust the pitch while still for the rotation of the blades. In helicopters, this is solved by using a swash plate, which is essentially a bearings, poth parts of which are independent from the main rotor shaft. One part rotates at the same speed as the shaft, but crucially, can slide along it to adjust pitch. The other part does not rotate with respect to the main helicopted body and is connected to the servos that actuate the pitch mechanism. This is a requirement as there would be no sensible way to have the servos rotate with the blades.
