It is all very confusing until you actually sort out what is going on.
There are 2 forces, the thrust of the prop acting at the offset radius gives a torque trying to rotate it out of the wind. This is balanced by the force on the tail vane acting on the boom length.
For the tail to produce any balancing torque it must be at an angle to the wind. The larger the vane and the longer the boom then the smaller this angle needs to be.
Theses forces balance up to furling speed. Beyond this you need the tail controlling force to become constant so that the prop acting on the alternator offset can rotate the prop to a suitable angle to the wind to re balance things and maintain more or less constant output.
The simplest tail would be pivoted somewhere just behind the yaw pivot and would be held against a stop with a spring. Up to furling speed the whole thing would act as a rigid bar. Beyond furling the tail would come away from the stop and stretch the spring. The tail vane is effectively anchored in space ( more or less down wind), so the prop would move towards the tail direction as the force stretched the spring.
Springs are not ideal and usually they are replaced by an inclined hinge. In normal winds the tail is held against a stop and part of the weight component is resolved into a horizontal plane and holds the tail against the stop just like the spring. The controlling force comes from the need for the tail to rise as it turns against the angled hinge. The force needed to move it from the stop depends on the moment of the tail ( weight at c of g a length from hinge to c of g) and the angle of the hinge from the vertical.
The second angle is more difficult to understand. If the hinge was directly behind the yaw pivot, the lowest point of the tail would be straight back and there would be no force holding it against the stop. To solve this the pivot is set at about 45 deg from straight back so that without the stop the tail would again try to fall to the lowest point at this 45 deg angle, but it is prevented from going there by the stop and it now exerts a force on the stop just as the spring would.
It is perfectly possible to combine these two angles and mount the hinge straight back Bergey, Sencenbaugh etc) but construction and understanding is easier if you keep these angles separate.
It is almost certain that if you keep this hinge point well back from the yaw pivot it will effect the results but some designs do exactly that. Not to worry, calculations to put it mildly are very inexact so it's not a big issue. There is another force involved ( not often considered) that is not subject to reasonable calculation and that is the force of the prop seeking the wind. This is the thing that trips you up when the alternator offset is too small and the seeking force dominates and the thing fails to furl completely. This seeking force seems very much influenced by the blade design and the operating speed.
I hope this makes sense, it really needs lots of drawings to explain all this.
Flux
