Realistic TSR for this 12 Footer

[Baling Wire]

I read on this website  http://www.iesd.dmu.ac.uk/wind_energy/m32extex.html  that the "approximate" optimum TSR = 4(pi)/n .  "n" being the number of blades.  I believe that this formula was for "low solidity" props and for extracting the maximum amount of wind energy.  I've noticed that alot of 3-bladed props are said to run at tsrs of 6 & 7 .  Are THESE trading alternator efficiency for prop efficiency to get a better OVERALL?  I suspect?  The website did say that "for good airfoils", that the opt. TSR could go 30% higher than by the formula.  THEY didn't document that statement?

Anyway, I have this old prop made years ago that I'm trying to "figure out further"

It's 12 ft diameter 3 blades 14" chord 7.25" tip width and I believe twisted a little.  It was made from cooling tower blades, pretty heavy, and well balanced.  I thought it turned fairly easy in low winds.  I tested it some at the time; but lost most of my data.  After thinking about it some and doing some calculations; I believe that it wasn't turning enough RPM for the given wind speed.  Next thought was, I don't have the blades pitched correctly?  I'm looking for HELP in this area?

Since "a picture is worth a lot of words", I'm asking that those interested in helping to take a look at my pics if that would help them with their answer.

Thanks!

BW

[willib]

try it out

i mean , you have an adjustable pitch set of blades

so give it a try

a pitch of 4 to six degrees at the tip is a ballpark figure and may need adjusting.

less pitch they will go faster more pitch slower, but more torque.

[SamoaPower]

I agree with the TSR formula and is why I chose a TSR of 4.5 for my machine. I believe most choose a higher TSR to avoid building a larger alternator that can give the necessary cut-in speed at lower RPM. I don't think it's a trade-off of efficiencies.

Judging from your photos, I'm afraid those blades won't work very well for wind power because they are twisted the wrong way. Your photos indicate quite a bit of twist. Since you say they came from a cooling tower, they are fan blades which have the opposite twist direction compared to wind turbine rotors. The only way fan blades can be used for wind turbines is if they have zero twist and the blade angle can be reversed.

Sorry for the bad news.

[Ungrounded Lightning Rod]

Running at a "too high" TSR isn't a big deal.  It means the decelerated air from the previous blade hasn't cleared the active region of the following blade.  So the following blade interacts with air that had some of its power "stolen" by the predecessor.  But the predecessor USED that power, so it isn't lost.  The only "extra loss" is a bit of drag from moving faster than is necessary - a small thing.

Running at a "too low" TSR means some air slips between the blades without donating power either to the one that just went by or the one that follows.  So "too low" IS an issue.

If you don't have a max power point controller, but let the blade speed be regulated by the battery voltage and the wiring resistance, the blades will usually be running a something significantly off from their design TSR - too fast when the wind is low, too slow when it's high.  So deliberately going high by 50-75% means they not only will achieve cutin earlier (due to higher RPM) but they will remain efficient (at least as far as not letting energy slip between the blades) when the wind has risen until the genny's drag has them running about 57-66% of their design TSR.  (Not much point in going beyond that because they'll also start to become inefficient due to stalling and other angle-of-attack-error issues.)

[Ungrounded Lightning Rod]

Also:  Designing with a "too high" TSR for a battery/resistance-controlled mill can mean the efficiency peak is broad and corresponds to the wind speeds in the "hump" of the distribution.  Then the mill is running near peak efficiency for most of the time the wind is high enough to run it at all and low enough that it doesn't HAVE to get inefficient (by furling) to protect itself.

[Baling Wire]

Thanks! for the "good" news about the wrong twist.  Story of my life.  Hi!  Hi!

I know this prop will make some fair power as is.  Been there already.  But, is there anything that I can do to improve it?  Would there be a compromise pitch setting to offset the wrong twist?  If so, what pitch at what radius?  Just looking for More RPMs.  I thought about grinding off part of the trailing edge near the hub to reduce drag?, if I reset the pitch to a lower angle.  Also, thought about filling in the cambered windward side of blade to make it flat?  Making this alteration strong enough might be a problem.  Hey! guess I could just BUY a prebilt prop.  That would be no fun since I didn't "baling wire it up".  Any other suggestions for making this one better?

[Ungrounded Lightning Rod]

Actually they're "twisted" the right way but "curved" the wrong way.

 - Twist is the chord being almost flat to the rotation far from the hub, at a significant angle near the hub.

 - Curvature is the "cup" of the sheetmetal or "cup and/or hump" of the airfoil shape at any given distance from the hub.

Fans/propellors have the hump on the wrong side to serve as turbines.

 - Fans/props want to SPEED UP the air and put energy INTO it.  So the hump is on the upwind side, using lift to pull the prop upwind (and resist its rotation) and push the air downwind.

 - Turbines want to SLOW DOWN the air and pull energy OUT of it.  So the hump is on the downwind side, using lift to pull the prop downwind (and forward in its rotation) and pull the air upwind.

Using one as the other works, but very badly.

Of course if you flip the blades over to put the hump on the upwind side, THEN the twist will be backward.  B-)  You'll only be able to get the correct angle to the wind at one radius, and the twist will double your error, rather than compensate for it, at other radii.

[SamoaPower]

ULR describes the same thing but from a different perspective.

I would sure call this reversed twist for the fan blade. You might confirm this since your photos aren't really clear, particularly which is the leading edge. If I have it right, the trailing edge looks to be quite thick.

I assume these blades are made from cast aluminum.

Gee, I don't know there is much you can do to improve them without needing as much work as building new ones. Did these really turn in the right direction when you originally tested them - leading edge going toward the direction of rotation?

I certainly wouldn't fill in the concave surface. That would only reduce your lift coefficient.

You would have to remove a LOT of the trailing edge on the root end, perhaps for 30-40% of the length, to do much good. I would also sharpen the trailing edge for the remainder.

Changing the tip blade angle to 1 or 2 degrees may help speed them up.

I understand your desire to make use of them, but you might want to consider something else.

73 de AH8A

[Baling Wire]

U L R:

Thanks for explanation!  From what You and Samoa have said leads me to believe that what RPMs that I've been getting is basically as a result of the prop functioning as a "Drag Machine"?  I've seen 325 rpm in an est. 40mph plus wind.  From a couple calculations that I've made; the TSR appears to be running about 3.2 .  Do you think

turning the blades over and setting them at 2/3 to 3/4 chord at 4 degs. would pickup any more power/rpm?

[Baling Wire]

Samoa:

The prop is shown on the stands with the downwind side UP.  I'm pretty sure the rotation is with the thicker leading edge "leading".  The wind blows into the concave [what would be the Flat side on a "normal prop]; this side is Down on the stands.  The blades were made by Hartzell fan co. and are a synthetic resin-fiber mix that THEY call Hartzite.

BW