In theory the ideal cut in is not related to stall, this situation only comes about when you try to pick the best characteristics to cover a reasonable wind speed range with a simple directly connected alternator.
The ideal cut in speed is when the rotor can supply the losses for the available wind speed. There is little energy in low winds and until you can supply the associated losses you will get no output whatever speed you choose.Iron cored machines have iron losses at cut in as well as friction, so they need a higher wind speed to produce the energy to supply theses losses. By the time that you are just beyond cut in, the ideal speed would be that which brings your prop to the peak of its power curve ( design tsr).
With the normal method of connection the prop power curve slope is very low near cut in and the alternator curve is far steeper so you go rapidly towards stall almost beyond cut in. Normally we make two compromises to reduce this effect. We raise the cut in speed so that the prop is running fast at cut in and has a chance to get some way up the wind speed range before its power has fallen to the peak of the curve ( it will be below peak ,on the fast side to start with).You are also shifting your working range more towards the steeper part of the power curve by doing this.
The other compromise we are forced to make is to lower the slope of the alternator power curve so that it is much more parallel to the prop power curve over the most useful wind speed range.
The final consequence of all this fiddling is that we can choose the lowest possible cut in and have good low wind performance but with a lower operating efficiency in higher winds or we can choose a higher cut in that will worsen the very low wind performance but will let us run more efficiently in the sort of winds that produce most of the power.
If you aim for the lowest possible cut in for the very best performance at cut in, you will produce a few watts on occasions where you may otherwise get none, but the trade off is that you will need to make the alternator so inefficient to get the power curve slope low enough that you will be running fairly inefficiently in mid winds and by the time you reach higher winds you may not be able to load the prop effectively and you will be relying heavily on your furling to prevent the thing running away.
Things tend to work fairly well with a prop intended for tsr6 if you choose cut in with the prop running at tsr8 or a bit more at 7mph wind speed for air gap machines. With iron cored machines then you may not get down to 7mph cut in if you can't supply the extra iron loss so many of these machines cut in about 8 to 10 mph and are far less influenced by stall.
Fortunately the blades ( at least with my methods of construction) do seem happy to run faster than the design tsr in low winds and they don't seem to mind running slower than the theoretical in high winds. This obviously makes the compromises less damaging than it would otherwise be.
When matched throughout the speed range the ideal prop speed would directly track the wind speed. I find that even with correct matching the prop wants to run slower than the ideal in high winds ( presumably something to do with excess drag). This does seem to limit the maximum gain from a mppt scheme in very high winds, but perhaps it does have the advantage that we are not too tempted to run into the sort of speeds that cause serious noise and blade leading edge erosion.
I have no reason to believe that increasing rotor inertia has any merit, it will smooth the power out but I can't see it doing anything to minimise stall and as the cf and gyroscopic forces rise with increased mass I think that taking this to extremes is a bad idea. Power out is determined by the power available in the wind and if you extract short term power from the inertia of the blades then you have to wait to replace the energy during the next gust whereas the light rotor would give a little power blip in that gust.
Flux
