Wind Turbine Tail Furling
Tail furling is a simple and affordable way to help protect a wind turbine and charging system from damage in high winds. More effective braking means would involve automatic electric brakes controlled by an MPPT charge controller like the Midnite Classic. Blade pitching, hydraulic, or centrifugal brakes are other options, but their extra cost and complexity make them impractical to incorporate onto small residential machines.
A furling mechanism is a hinge that’s positioned at a compound angle on the turbine frame, to which the tail boom attaches. The hinge can be constructed in any fashion that works, but the pivot point must be at a compound angle to work properly, as shown below.
A furling mechanism is a hinge that’s positioned at a compound angle on the turbine frame, to which the tail boom attaches. The hinge can be constructed in any fashion that works, but the pivot point must be at a compound angle to work properly, as shown below.
The furling hinge allows the tail boom to swing independent of the turbine frame, but only when the wind reaches a certain speed, which is determined by the offset distance, tail boom length and weight. Notice how the rotor shaft above is offset from the yaw/tower. When the wind speed gets too high (ideally between 13-16 mps, or 30-35 mph), the force it generates will push the rotor and blades out of the wind due to the offset. The angled tail hinge will allow the wind to lift the tail boom and vane as the rotor turns so that they remain tracking and don’t get turned around and broadsided when the rotor turns. This would turn the rotor back into the wind and render the offset pointless as the rotor won’t have enough time out of the wind to slow down to a safe speed. So as the rotor turns, the tail is lifted by the wind and angled hinge, but remains tracking. When the wind speed lowers to an acceptable level, then gravity will pull the tail back down and force the rotor to turn back and face the wind again. An example of tail furling in action can be seen below.
A couple of examples of furling in action can be seen in the videos below
The table below presents a guideline to follow for constructing furling tails for various size turbines. Keep in mind that this is a guideline - depending on what you’re using your turbine for and the input power ratings of any gear that you couple with it, you may want to adjust the furling moment after seeing it operate. The furling moment refers to the moment, or more specifically the wind speed that the tail starts to furl. The parameters below will generate a furling moment somewhere between 13-16 mps. The furling moment can be fine tuned later by simply adding weight and/or adjusting the length of the tail, or adjusting the size of the tail vane. For example, to make the tail furl later, add weight to it, or make the vane smaller. To make it furl earlier, make the vane larger.
Another braking mechanism that should be included is an emergency switch brake. This is just a simple high voltage and high current rated switch that, when turned on, will short the leads from the turbine and effectively bring it to an immediate stop. This is beneficial to have in case the turbine becomes damaged, or gale/hurricane force winds are expected, or simply just to stop the turbine for maintenance purposes. Here's and example of such a switch that's available from Windy Nation.