No Twist, No Taper

[solarengineer]

I'm preparing to make a test set of blades like those used on the Berkley XL1

I was wondering what would the angle be looking straight on down the blade tword the hub?

If you look straight down the blade the leading edge is farther away from the hub than the trailing edge. Is this about 6 degrees?

Thanks

Jamie

[TomW]

Jamie;

Well, you essentially want the prop to screw itself into the wind so the leading edge [blunt edge] should be set so that it is "ahead" of the trailing edge [knife edge]. I think of it as the blade is set up to slice a hunk out of the air as it turns with the blunt edge cutting the air.

Hope thats only mildly confusing. And I got it right.

Cheers.

TomW

[electronbaby]

I thought that hand looked familiar! :-)

Dont forget, the XL.1 hub that the blades bolt on to also adds to the angle the blade rests at. The alternator hub is a casting and has three spots where the blade root can rest inside of and this is where the bolts pass through the blade to hold it onto the hub. This "saddle" is what sets the attack angle of each blade. It is perfectly contoured to the exact curvature of the top (or back side) of the airfoil. I have the info somewhere.. but Im sure it is also available online if you were to do a search.

Im interested to see how your test blades come out. The bergey blades on the XL.1 are quite silent even at a high RPM (550). If yours are noisy and have a constant pitch, you might want to try my fishing line trick. Bergey calls this "tripping" the blade and it was mentioned at the end of my previous post. I also know bergey gets around $340.00 for a new set of XL.1 blades. I guess you have to offset the cost of R&D somewhere.  :-P

Have fun!!

Roy R

KB2UHF

[scorman]

Not quite as simple as just picking an angle and going with it, but you will probably have anywhere from 2 to 12 degrees to choose from.

The piece of information you have left out is what avg WS you are working with and what  speed in rpm you desire which is dictated by TSR which then determines the AOA.

Once you know the AOA, then the pitch angle you desire is defined.

BTW, there are simplier blade profiles than Bergey's to build that would perform equally as well ie flat bottom like the ones used on all Jacobs:

http://www.ae.uiuc.edu/m-selig/ads/afplots/usnps4.gif

from another thread, I have posted the performance data for the above Jacobs blade:

http://www.otherpower.com/images/scimages/7526/usnps4polar.txt

first column is AOA and max Lift/Drag on left is the key data column (higher is better)

I have created a handy Excel calculator for above variables , send me an email if you want it.

Stew Corman from sunny Endicott

scorman@stny.rr.com