another blade question

[rien]

hi
i am building a12 volt hugh piggott 1800 design with 2 magnet rotors instead of one and a set of Dans 7 foot blades
i think the start up rpm  of the blades is around 200 rpm
i have not tested the blades yet but it might be that the start up is to low for the alternator
i can not put anymore than 120 turns of 1,5 mm in the coils
so a new set of blades that run faster might be the answer.
the blades have a strait taper, the pitch at the tip is 3 degrees and halve board 6 degrees
to increase the tsr  will it be enough to go to a pitch of 2.5 degrees at the tip and 5 degrees  halve board. to get a cut in rpm of 215- 220 ?

       

[SparWeb]

If you are starting with a single-rotor alt, and changing it to a dual-mag rotor alt, yes, you should be concerned that you will stall the blades at start-up.
If I take your word that Piggott's alternator has been matched to a 1800mm diameter rotor already, then you will probably need more than a 7-foot rotor to make up for the much more powerful alternator.  How much more is a difficult question to answer.
The extra magnets will drastically reduce the cut-in speed for 12-volts.  You may find it better to operate a 24-volt battery system.  All other factors may be easier to deal with if you do that.  The effect of adding the extra magnets on the rotor, and the doubling of the system voltage may come out in balance.
What coil size is specified in the original Piggott design?
A degree here or there of blade pitch will not be a significant factor if the blades are over- or under-sized, so let's focus on finding the right diameter first.

[scoraigwind]

We had a fair bit of email discussion about the best winding to use for this job.  The speed can also be fine tuned with the air gap to avoid stall.

I think what is happening here is that you are focusing too much on the cut-in speed.  You can make blades that work best at cut in, and I can understand the desire to do this, but they will not work well over the whole range of windspeeds. A set of high tsr blades that work well at cut-in will tend to stall as the wind gets stronger.

Hugh