Your electrical figures are much as I would expect. It is a fast generator and even though it is a fast prop you will need something like 12mph to get cut in.
We used to use 12v dynamos on a 6 volt system, never used the 3rd brush and usually we left the field unmodified.It was possible to get a slower cut in with the fields modified for 6v but the field loss usually made the power available at cut in less than using the 12v field.
The weak field using the 12v winding limited the maximum current by armature reaction and did much the same thing as the 3rd brush. Even then the results were disappointing, a car dynamo really is not up to the job. Some larger bus and lorry dynamos gave better results but most of us ended up rewinding the Lucas A900R dynostarter, something which Lucas modified as the basis of their freelite dynamo.
As for the furling, it is something that some people follow easily and others never get to understand. With your machine you can regard the tail as a rudder that stays directly down wind. The dynamo shaft axis is offset from the pivot that carries it, so any thrust from the prop tries to turn the dynamo about the pivot ( not the yaw pivot in this case). It is balanced by the pull of the spring which holds it against the stop. The set up behaves as a rigid one and the tail steers the prop into the wind.
As the wind picks up the thrust causes a bigger moment turning it away from the wind than the moment from the spring holding it against the stop and the dynamo turns at an angle to the yaw axis and the prop turns from the wind, The tail stays directly down wind but the prop turns to a greater angle away from it as the wind picks up. In very strong winds the prop is at near 90 deg to the wind. Power is related to thrust so the power from the prop drops as the frontal area is reduced. In the perfect world the thing would run at an angle to provide constant power out and the thrust would be constant.
The scheme is the same as for conventional tail furling but different parts move in different ways. In the conventional scheme the thrust forces the yaw axis round the yaw pivot to come out of the wind and the secondary pivot is on the tailinstead of the generator.
I hope this makes sense, I don't know how to post drawings here.
Flux
