Blade pattern required

[windyknight]

Where could i get a pattern (or measurements of drop, chord etc) to best utilise 6 ft 6x2s I have.Thanks keith

[Speo]

Check here.
The page has a link inside to an Excel spreadsheet that has all the stuff you are looking for.
I built a set of blades using that spreadsheet and I was very happy with the results.

Good luck,

Speo

[windyknight]

Thanks Speo

Bit complex for my simple soul though. I have too many unknowns. All I do know is that i have a pmg which runs well at 3-400rpm, a windspeed of say 8m/s and lots of 6ft lengths of 6x2. So.....how do i extract max power from wind given those parameters? On assumption that max diameter will give max potential power I should get at least a 3.5m rotor (but this assumes a long thin blade unless blade width is the limiting factor) My understanding is that rpm is determined by tsr but is it better to get max torque by having max no. of blades or short fat blades or what?

[Speo]

I think that you should start the calculations from the generator.

Let's say your generator output has to be 300W at 250 RPM.

Go tho that spreadsheet and input 300 in the cell "C6" and 250 RPM in the cell "C7".

All the other cells will be updated automatticaly with values calculated based on your input.

C9 suggests you the diameter value. Insert this value in "C10" and you just got a set of values for your blades in rows 26 to 45. You don't have to modify any of the other values, but, if you want you can play around with them.

You can modify the TSR, the number of blades, or gearing ratio to see how that affects the size of the blades, but I sugest to stick with their default values unless you know exactly what you are doing.

So, find the output and RPM for your generator, insert them, get the suggested diameter, insert that value, and you got your blades pattern.

To find out your generator output, spin it at different rpm's and measure the volts and amps with some load, then find the power P=U*I for each RPM measurement.

Have Fun,

Speo

[Speo]

I "guessed" a value of 200W at 350 rpm for your generator.

By inserting that into the spreadsheet, for 10 stations, i got :

number of blades: 3           

No. of stations    STN:10               

station    radius    angle    Chord    A chord    thickness(m)    drop

1.     0.12    44.0    0.344    0.344    0.041            .239
   
2.     0.24    25.1    0.293    0.293    0.035            .124
   
3.     0.36    16.3    0.225    0.225    0.027            .063
   
4.     0.47    11.5    0.178    0.178    0.021            .036
   
5.     0.59    8.5    0.146    0.146    0.018            .022
   
6.     0.71    6.5    0.124        0.015   
   
7.     0.83    5.0    0.107        0.013   
   
8.     0.95    3.9    0.094        0.011
   
9.     1.07    3.0    0.084        0.010
   
10.     1.19    2.3    0.076        0.009
    
    

[windyknight]

Thanks, I've managed to work the spreadsheet ok. Can you just clarify something for me - the measurement are in metres? so in your eg. at station 1 blade width is 344mm and 41 mm thick? Am i correct in thinking drop is distance down from horizontal to give blade angle? if so how can drop be 239mm! Apologies if i'm being real thick here!

[Speo]

"the measurement are in metres?" - yes

"at station 1 blade width is 344mm and 41 mm thick?" - yes

"drop is distance down from horizontal to give blade angle?" - yes

"how can drop be 239mm!" - That's the ideal drop that is generated by the formula.

That drop will help the rotor to start in very low winds, but once started it won't improve to much the performance in higher winds. So, that's an ideal drop, but you will drop whatever you can. For example I built a set of blades where I had to drop 121 mm out of a 50 mm piece of wood: I dropped as much as I could and...that's it.

The spreadsheet gave me 174 mm chord and I had only 150 available....and it's still working fine and starting in low winds.

Remember that the root is not as important as the rest of the blade. I would say you can cheat on the first quarter of the blade starting from the root, but not on the rest of it.

Here is a link to the blades that I was talking about.

Speo.