Blade help

[fabricator]

Ok, I have the blades for my 17' mill all blocked out, but, I have to admit I have a serious mental block when it comes to blades and direction of rotation, I know the face the wind side is flat, and the airfoil is on the downwind side, I'm going by the blade carving 101 directions so the leading edge is straight and the trailing edge is tapered  the thing I have trouble seeing is the rotation direction.

Looking at the mill from the face (Upwind) the offset is to the left, so as per Dave B I need blades that rotate clockwise, I guess my question is are the blades 101 clockwise blades?

[Perry1]

Yes, If carved as per the book the blades will spin clockwise.

Perry

[Dave B]

Fabricator,

 My experience with the blade rotation vs. the offset side certainly corrected my (anti furling) machine. Other's may argue it shouldn't have made any difference.

 Hugh's comments a while back indicating he changed and was now using the same configuration whether it was CCW rotation and offset to the right or CW rotation and offset to the left also strengthened my thoughts on this.

 Observing also that the Dan's machines have seemed to morph into the same relationship also made me think. Finally, realizing that Bergey was running their 10 KW machine this way also just told me not to question it but to just keep doing it this way.

 Many variables between all the home built machines as well as location etc. will never etch a perfect design or the same performance for all. Ultimately it's what works best for you and the "standard plan" may get you close enough to satisfy your expectations.

 Tweaking a little here and there could make a huge difference in performance, good or bad. My experience with the offset vs. blade direction first hindering and then after the change assisting the furling is a perfect example.

 Have fun and go with what works for you, it won't always be what the numbers say it should be. Last I saw, bumble bees seem to fly pretty well.

Dave B.      

[fabricator]

Thanks guys, Dave, your several posts on the subject have me convinced, that link you posted with the bicycle wheel on a string was quite enlightening.

[DanB]

Regarding the side on which the alternator is offset, and blade rotation... I think you have it backwards.

If the alternator is off set to the right side of the machine, then ideally the blades ought to rotate counter clockwise.  (this is assuming you are looking straight on at the front of the machine)

and of course, if its offset to the left, the blades should run clockwise!

This doesn't have anything to do with furling.  These machines with the common furling system will yaw more quickly one direction than the other in gusty winds.  

Consider my 20' turbine.  The alternator is offset to the left of the yaw bearing.  Imagine the wind is mostly from the west and the machine is pointing west.. if a gust suddenly comes from the South, the tail will swing up first and the machine will slowly come round to point to the south and 'catch up' to the tail (this kind of looks like its furling but its not).  If instead, a strong gust comes from the North, the tail cannot swing that direction so the machine will quickly yaw around to face north. In other words, if Im looking Down at the top of the wind turbine -it will usually yaw more quickly in a clockwise direction than it will in a counter clockwise direction.

So in the case of that machine - if its got counterclockwise rotating blades, the direction in which it yaws more quickly will cause the blades to be pulled in towards the tower.  Better is to have the blades rotating clockwise so that in that direction of 'faster yaw' they'll be pulled away from the tower.

My 20' turbine originally had blades that rotated counter clockwise and they struck the tower twice on windy days.  

So looking at our 10' machines, our book, and every single page I've made up about building a wind turbine you probably notice I have it wrong every single time ;-)

If you have enough clearance between the tower and the blade tips it doesn't matter anyhow.  Hugh always tell me that he finds counter clockwise turning blades to be bothersome for some reason...  but other than that it's not an issue so far as I know.

[Dave B]

Hi Dan,

  I think you might want to read at least what I wrote again, it agrees with what you say. Offset to the right, CCW rotation, offset to the left CW rotation looking at the front from upwind.

  I agree with your explanation of YAWING faster one direction because the furling action of the tail is not damping the yaw rotation in one direction. Yes, this in fact can have an adverse affect on blades flexing and crashing the tower when you have gusty turbulent winds. All true and if this is the reason that both Hugh and yourself have changed this relationship then great, it has helped what you both have seen to possibly be a problem and especially with the longer blades and minimal tower clearances.

  For my machine changing this rotation relationship completely solved my anti-furling, blade seeking problem 100%. I wouldn't say in your book you are doing it wrong, for the smaller machines it in fact may make no observable difference at all in regards to assisting or hindering the furling action. Knowing you had your larger blades crash your tower a couple times and this is the reason why both you and Hugh have made the change is helpful. This in regards to YAWING and realizing that this will help to flex the blades away from the tower when the wind direction pushes against the lower tail stop quickly yawing the machine.

  The Furling action itself can be a seperate issue. I don't pretend to understand it all but I am only stating that for me when I changed the rotation / offset relationship it solved my anti FURLING machine 100%. If I'm getting the anti-blade srike YAWING also in the same deal it's a bonus.

  We all agree there are many variables in each individual system. Some things work for some other things work for others. It's all a big part of the fun of it all.

  Happy Holidays everyone !  Dave B.  

[fabricator]

I'm pretty sure I have it straight, looking from upwind at the face of the machine, my offset is to the left, so my rotation should be clockwise.

[Ungrounded Lightning Rod]

The offset versus rotation is about not hitting the mast when furling.

When swinging a spinning object the gyroscopic effect causes the tilt from the applied force to be "carried along with the rotation" for a quarter turn.  So when your mill yaws one way it also tends to pitch nose-down and the other way to pitch nose-up.

Furling is abrupt, and occurs when the blades are spinning fast and the gyroscopic effect is strong.  Unfurling is more gradual and occurs when the blades are spinning a bit slower.  So you want your mill to pitch UP on furling - if it pitches DOWN the blades may hit the mast.  (Remember that the blades can pitch by flexing even if the yaw bearing doesn't let the shaft pitch appreciably.)  You accept the much smaller down-pitch on unfurling as the lesser of two evils.

This means the blades should spin so they're going up on the side away from the mast.

[Ungrounded Lightning Rod]

yep

[Dave B]

 "The offset versus rotation is about not hitting the mast when furling"

 Yes, this can be very true. Just as with the smaller machines this relationship may not come into play at all either. If the blades are stiff enough, the furling works well enough, the blade clearance to the tower is wide enough etc. etc. etc.

 Those experienced with the larger machines have other stories to tell as well. Offset versus rotation for my machine also is about the ability of the machine to furl (not just the reaction after it furls) Nobody has to listen but if anyone has a larger machine and you are having problems with the furling maybe this will work for you as well.

 I went through it. I built my base machine from the "standard" plan and layout. My offset was (is) to the right and my blades then rotated clockwise. (this was an 18' machine not 10') Blade crashes and stator burnout and other fun stuff lead to hours and hours of tweaking to dial things in. Fortunately I had the resources for new blades to be carved opposite rotation and this was actually easier than changing the whole main assembly offset to the other side.

  This solved my anti-furling machine, period. Yes, it is a lot of work to either change the blades or change the offset to the other side from the original (book assembly) and again for the smaller machines as drawn it may never come into play.

  Just a word that might save someone a lot of time by changing the offset versus rotation from the start (such as the Dans and Hugh are doing now). What ever the reason, avoiding blade strikes during rapid YAWING or as in my case a more effective FURLING function. The larger machines ceratinly seem to favor this change.

  Dave B.

[TomW]

Dave;

I am not a builder but this is interesting information to have.

Thanks for posting your experience with it! Theory is all well and good but nothing beats hands on data.

Tom

[DanB]

yes thats right and my posting was a bit confusing and wrong.  I get dislexic about this stuff sometimes ;-)

[Dave B]

Thanks Tom,

  I'm just trying to provide information based on my own experiences. It may help someone it may not, my machine is not an exact scaled up version of the 10' machine.

 Many of us have found out that when building larger machines not everything reacts in a predictable manner such as so much is known about the 10' machine.

 Expect the unexpected, this is fun stuff. Thanks again for your comment,  Dave B.

 

[fabricator]

Man, keeping you guys straight is giving me a headache.

[dlenox]

Guys,

Thanks for your comments concerning the larger machines.

For myself It is easier for me to switch around the stator and tail mount to the opposite side of the yaw bearing.

I concur with both of your findings and will be making the switch, my blades rotate CCW, currently mounted to the left of bearing (from the front). And I will be switching so it is mounted to the right of the yaw bearing.

I have to admit that when I originally built the machine this thought was a flash in the pan, but then I forgot once I got committed one way or the other...

Dan Lenox