Blades and tracking issues

[Shadow]

I've got a tracking issue I'm trying to cure on my 14 foot wind turbine. It has a noticeable wobble at slow speeds, you can watch the tail 'rock' slightly in light winds. I thought it was magnets discs so I replaced them making sure they were flat and balanced. The blades are very balanced, I've been over them a half dozen times. But I have two blades that track very close about an 1/8 of an inch difference but the third blade is about 1/2 out from the other two. So I thought I should remove the plywood discs and sand down the root of the one blade to bring it in line, but that would no doubt ruin my good balance. So I went ahead and mounted it and tightened 3 of the 5 locking nuts more than the other two to suck in that errant blade. So now the tracking is very close but I think thats where the wobble is coming from.So How important is perfect tracking? Is it better to have equal and even balance at the hub and let the blades be out of track a bit? I know ideally you want both to be near perfect, but what are the consequences of out of track blade tips?

[willib]

i believe that a good balance is more important than perfect tracking.

[Flux]

Ideally you need perfect tracking, but if you get to within 1/4" that is usually good enough.

If you are only 1/2" out on one blade, a thin shim of plastic or something may be enough to correct it, depends on where the blade is in relation to the bolt holes.

A single plane can be static balanced. Once you have things in more than one plane you have dynamic balance and you can't correct it without dynamic angle measurement.

If you correct the tracking then static balance is easy to deal with by adding a little weight especially if your bearings are free enough to balance the whole assembly.

With motor conversions or with stiff bearings it may not be possible to balance on its own bearings, then you have more trouble. The prop needs to be mounted almost exactly on the test bearing. 2mm difference in the mounting centres between the test shaft and the alternator shaft will make a huge difference.

There are other factors that can cause vibration other than balance, such as small differences in angle or profile between the blades.

If your yaw is something such as a bearing rather than pipe on pipe it will be very sensitive to very small twisting forces and also you will almost certainly hit some resonance at low speed so even if you are very careful with balance and tracking you may still see a wag at some speed.

If you know the balance is good and tracking is better than 1/4" you may have to live with it. The exciting force will be very small and will have no effect.

When it occurs at all speeds and you can see and feel it on the tower and parts of the tail structure then is the time to worry.

Flux

[johnnythefox]

HI Shadow

 I'm not too smart on all this stuff but have you looked at the tips to see if they are 120 degreas from each other?

 I don't know  how close they have to be but if they are off very much I am shure it would make a diffrence.

 this is just a shot in the dark,something to look at anyway.

Good Luck

John

[Scotth]

Had the same problem myself except the tail was bobbing at all speeds-pretty scary at high RPMs.My tracking was not perfect by any means and I thought maybe that was it,however Dan B and other members have stated that tracking can be out a little and not cause any serious problems.In my case the mounting holes were off-center by a millimeter-not much but enough to offset the weight of the blades just a bit.Impossible to see with the eye when turning the blades by hand but once it gets spinning up on the tower things get amplified quite a lot.Although I balanced the blades nearly perfectly the offset threw everything out when the mill was actually operating.I removed the blades and took some very precice measurements and then elongated the mounting holes very slightly to bring the whole blade/hub assembly to center,then re-balanced the whole works on the alternator.No more bobbing!I would look at that if I were you.It doesn't have to be out by much to get the bobbing effect you are talking about and it will drive you nuts trying to balance it away.

[Shadow]

Thanks for all the pointers. I'll be bringing it down in next few days to solve it one way or another. It just seems to do it at slow speeds, faster it goes smoother it runs.