Blade Specs

[pisces00797]

First I'd like to say to Flux for the comments. This is kind of a follow up to a previous post.

I did some more 'surfing' and found a blade carving calculator and did some more research.

I believe there is a trade off of higher TSR vs Starting Torque (also obviously RPM). The higher the TSR the lower torque applied at given wind speed. I would assume it is the torque that is needed to 'break' free from bearing friction and  get a windmill spinning at first. Not cuttin speed on the generator but to at least get it turning. Perhaps this is another topic.

Anyway I plugged numbers for a 3 meter diameter (10 foot prop) and come up with these numbers. TSR of 4 and 'attack' angle 5 degrees, 3 blade prop.

Here is what it gave me for 3 out of the 10 stations on a blade

Station 1 - 27 1/2 inches wide at the root

            21 inch drop (for angle of 54 degrees)

            4 1/2 inches   thick.

Station 5 - 19 inches wide

            4 1/4 inch drop (for angle of 13.4 degrees)

            2 3/4 inches thick

Station 10 - 12 inches wide

           - 7/8 inch drop (for angle of 4.5 degrees)

           - 1 1/4 inches thick

The recommendations at this site are to cover 10 percent of the sweep area with blades.

Are these numbers believable and correct?

[DanOpto]

Go to http://windstuffnow.com/main/formulas.htm

There is a blade calulator there which gives much narrower blades and the angles are not as steep.

Also try

http://www.ceere.org/rerl/projects/software/mini-code-overview.html#RotorAero

Both of the calulators linked here produce similar results to each other.

It is worth looking at these alternate calculators.

Please post the location of the calulator you are using. I would be interested in looking at the calculations and why they are choosing wide blades.

Cheers

[pisces00797]

The calculator was here

http://www.alton-moore.net/wind_calculations.html

[zubbly]

Hi pisces00797!

I use Alton's calculator almost exclusively and with great success.

What is making your blades so wide in chord width is the use of TSR-4. I usually use tsr of 7 or 6. Not sure of what specs you need for your type of genny, but 6 and 7 seems to work quite well with my induction conversions.

Also, keep one thing in mind. In my opinion, computer programs are "stupid" and don't take into account the size of stock that it says to use, it just creates numbers for you. This is sort of where you have to read between the lines. What i am finding is that if i use a wood stock no wider than the chord of the halfway point in the specs (station 5 in a 10 station design) you should have a very good blade as most of the power in the prop is created in the outer portion of the prop anyways. You will have to re-calculate the "drop" for stations 1-4 to keep the same original twist in the blade.

I have made approx 10 blade sets using the program and find that it does work well.

hope this helps ya!  have fun  

zubbly

[pisces00797]

Thanks ... i did print out for TSR or 2,3,4,5,7 ... I have been thinking exactly what you have mentioned ... being concerned about cords 5 to 10 more than the root. With TSR of 7 it still works out to a tip 3 3/8 inches wide, 3/8 thick and almost flat (.4 degrees). Station 5 is 6 1/2 inches wide, 1 inch thick. So if i try another set a need to be wider than a 2 x 6 (5 1/2 wide)

Dry, clear (knot free) cedar is not a problem for me ... I have 4 x 4, 2 x 6 and 2 x 4 ... and yes it is easy and nice to work with. Waiting on magnets, but i might get out the planer, glue and sander again. :-0

[Flux]

Yes Zubbly is right, if you choose tsr 4 you will get a very slow blade. If you are thinking to use an air gap alternator you should not have starting problems so it would make sense to use at least tsr6 and 7 should be fine.

Lower tsr is needed with iron cored generators with iron loss and with small props.

Starting at 10 ft should not even be a problem with iron cored alternators such as Zubbly's motor conversions.

I think that your present prop based on Dan's ideas should be fine, As Zubbly said, you need to treat calculators with a bit of care, there is no point in taking the inner third to the theoretical dimensions, you will end up with an enormous waste of timber for no real gain.

Flux

[whiskey]

I propose we drop the word 'Drop' in favor correct terms. It is leading to mass confusion.

Who seconds the motion?

Whiskey

[TomW]

Whiskey;

I second the emotion!

Terminology is confusing enough without mixing them up.

Cheers.

TomW