Author Topic: Heavy Tail = Trouble (sort of) (Read 1631 times)

Dave B · « **on:** December 04, 2007, 07:06:19 AM »

We have had strong winds for the past few days of 30-40 MPH with gusts to near 50 MPH here in Western NY. My 18' axial has been cranking out the power to my 3 2000 watt 120 volt hot water heating elements. A constant wind of 30 MPH can hold my machine fully furled which is faster than I wanted, the RPMs can accelerate up to and above 250 before furling starts. I wasn't concerned about my 85' tower or burning out the stator but my curiousity and excitement got the best of me in such fine consistent high winds to let it run to find any limitations. After several hours of 3-4000+ watts output in winds of 20-35 MPH the winds increased to over 40 MPH with my Weather Wizard showing peak gusts of 45 MPH. I actually recorded a peak output of just over 5000 watts ! I could feel that I was pushing it but at this time I was hesitant to try to wait for a lull to short it out and then apply the mechanical brake. (there really was no lull in the wind) Then CRACK ! and a different sounding machine, this was 3:00 AM. At probably 250-300 RPM a major gust blowing in the down direction of the fully furled tail slammed the tail breaking loose the low stop chain and to the normal low stop of pipe over pipe arrangement. This tremendous accelerated stop immediately yanked the alternator to yaw and ...... It's hard to believe that a laminated wooden blade could flex more than 11 inches over about 7' in length and not break. The very end of one blade tip hit the tower and broke off the 2' piece shown. (sorry about the low res. photos) I shorted it out and pulled on the mechanical brake, I was amazed I was able to slow and then stop it at these speeds. I learned much and my friend Dave Moller and I will lower it sometime soon to lighten the tail and probably replace the entire blade set with an extension to the hub. We are also working on some type of damper for the tail returning to normal run position. Comments welcome. Dan, any thoughts on how many lbs to reduce the total tail weight ? I think the boom and tail are approx. 40-45 lbs total, it is 9' long, offset is 9". Thanks, Dave B.

Flux · « **Reply #1 on:** December 04, 2007, 01:50:41 AM »

Dave

Don't forget that your method of loading is not the same as Dan's stall controlled battery charging mode. Your matching will let the blades run at a more variable speed and you will get more power at a lower wind speed.

It is also likely that although you are getting more power your furling speed may be lower in real terms. You will need to establish your furling to suit the new loading scheme.

I do wonder whether you are up to speeds where the prop seeking force is starting to dominate things. I am not sure how you are loading it now, but if you are not adding extra resistors as the load comes on then your speeds are going to be above the peak power point when the stall dominated ones are well below peak.

It seems to be more difficult to get things furling reliably at low wind speeds.

At such a tower height I am a little surprised that you are seeing bad turbulence but that is always a problem with furl controlled machines. Damping the tail motion may certainly help ( at least to prevent damage to the stops) but reducing the tail weight will also help.

I can't offer any direct help as I have never built anything as big as this but with my method of loading into batteries with a boost converter I can't use tails as heavy as those suggested here.

Flux

Dave B · « **Reply #2 on:** December 04, 2007, 11:43:53 AM »

Thanks Flux,

Very good point about the loading. I am doing things a bit different than many here in that I am not charging batteries but running resistive loads direct. These Wincharger blades seem to want to run at about a TSR of 6 and when they break stall they take off with amazing acceleration. What I thought to be rather simple to design a variable load controller has not been easy, the power curve rises so fast it's either hold the machine stalled and squeak what I can out of it or try to protect it in overspeed very soon after it breaks stall. I know 250 RPM is not really considered fast for 18' but I have wound my coils for approx. 100 VAC @ 200 RPM unloaded. I am getting 300+ watts in 10 MPH winds with the right load combination to allow start up and over 3000 watts in 25 MPH winds quite easily. Basically I've created a monster that I need to tame. Different loading combinations most definetly makes a difference in the furling action. I think a much lighter tail with some type of return damping is the way to start this testing process over again. I hate to sound like a commercial but these Wincharger blades my friend Dave is carving are amazing, but then I guess Wincharger knew what they had turning gear boxes to generators with 8 and 10' machines and still needing their "air brake". I guess I knew I'd be in for a challenge with 18'. Great fun, any comments and suggestions appreciated. Thank you, Dave B.

DanB · « **Reply #3 on:** December 04, 2007, 11:55:17 AM »

Hi Dave - interesting story, Im happy for you that more damage was not done. I figure my 20' blades flexed close to 16" last time they hit the tower...

Regarding furling - like flux said, it all changes depending on how the machine is loaded and how fast the blades are running. My 20' turbine starts to furl with about 65 pounds of thrust against the spindle. Tail weight is important - but my figures probably wouldnt apply to yours - it depends on the angles involved, the distribution of weight along the tail - loading, blade rpm etc etc.

My 20' turbine was very well behaved last winter - then in late march I opened the airgap just a touch (because the rotors were hitting the stator). Now it's a different animal - its very quick to produce 5kw and I've seen close to 9kW (8.6 actually) at times. A bit scary actually - I shut it down in high winds now after two blade failures in 6 months. I am inclined to close the airgap again - or shorten the blades - or knock off all the N35 magnets and replace them with higher grade ones. For now - I just try to be careful with it.

Gary D · « **Reply #4 on:** December 04, 2007, 01:09:29 PM »

I'm curious if increasing the rotor offset would affect furling? On Hugh's 8 footer, wasn't there something like the option of from 4 to 6 inches? If Dave desired to move out the stator to 12 inches, would it help things a bit(furl sooner) with the same tail weight? Or am I just more confused than normal(whatever that means to you)? Gary D.

Flux · « **Reply #5 on:** December 04, 2007, 01:27:06 PM »

Yes Gary increasing the offset will let it furl earlier for the same tail weight.

With bigger offsets the effect of the prop seeking force is reduced in relation to the thrust and things are less dependent on it.

There is unfortunately a price to pay and so things need to be a compromise. Large offsets need larger tail vane area to keep the tracking reasonable with normal operation below furling. To keep things directly into the wind with a finite vane area you need to offset the tail stop away from the furling direction so that the force on the tail vane balances the force from alternator offset. The larger the offset the more you will need to offset the tail. It doesn't seem to have ill effects on operation but it does look silly when you need to offset it 45 deg.

What you have is a force balance between the tail and alternator thrust. There is no restoring force in a vane trailing directly down wind.

There is a way round this but it is messy and needs two pivots and is mechanically more complex and has more to go wrong so it is not often used. The later type Freelite furling scheme used this and the SWWP angle furl scheme uses the same idea.

This is all complicated by the fact that the prop seeking force is dependent on the tsr and blade profile. The Wincharger profile was used with an air brake rather than a furling scheme and it may be that this profile has a higher seeking force. For some reason the Bergey blades seem to have little tendency to seek the wind ( don't ask me why).

For those that love computer simulation here is your chance to do something really useful and model this furling problem.

Flux

Dave B · « **Reply #6 on:** December 04, 2007, 01:36:42 PM »

Hi Dan,

Thanks for the reply. Another point you make is the airgap. I am running (40) N42 1 x 2 x 3/4 with a stator of 5/8". Probably 1/8" to the coils from the magnets off each rotor, wouldn't surprise me if the total is close to 1" between magnets. My 1/2" rotors were plasma cut and are not perfectly flat but close so I allowed a safe clearance. I don't think I'll change this to closer, stronger magnets and a wider gap kind of equals out. It will be good to check everything out when we drop it down, it's been running pretty consistant since late July. Have you had any thoughts or concerns with your hub bearing(s)? I think we are running the same unit, I believe your blades and hub are significantly heavier than mine. Just wondered if you needed to re-adjust the spindle nut or regreased or anything, you have got more miles on your machine also. Do you have an estimate of your RPM when as you say it gets scarey ? Thank you Dan for all of your help. Dave B.

harrie · « **Reply #7 on:** December 04, 2007, 05:54:21 PM »

what the heck, In a previous post by Dave, I asked how much his blades would bend back at high RPM, and you all said that the blades dont bend!! Is there a difference in bending and flexing?? Just thought I would bring this up again, to get clarification!!!

Ungrounded Lightning Rod · « **Reply #8 on:** December 04, 2007, 06:40:49 PM »

In a previous post by Dave, I asked how much his blades would bend back at high RPM, and you all said that the blades dont bend!

NOt all of us. I didn't see it or would have taken issue with such an assertion.

Think "laminated wood bow" as in bow-and-arrow. Wood is all about millions of years of evolution with a target of at bending in the wind repeatedly for more than a century without fatigue cracking.

Dave B · « **Reply #9 on:** December 04, 2007, 09:16:49 PM »

Hi Harrie,

I knew they would flex (bend) that's why I let it go. I know now at least how much one blade flexed to reach the tower. I never planned on running them that fast but took the opportunity to find any failures in the high winds and pretty radical yawing in that turbulance. It's amazing the forces generated, these blades are six laminations face glued so that the board edges are the front and rear surfaces of the blade (like the originals). Just over 7' from the outer most part of the root where the profile starts to the tip and it had to flex just over 11 inches to hit the tower. The blade did not de-laminate or fail, the impact cracked through the boards breaking eack off clean on this piece. We had previously clamped a blade at the root to Dave's welding table just to roughly test the flex at the tip and figured no way without completely breaking off could the blade ever hit the tower. Never say never when it comes to high speed and quick yawing. Incredible forces going on up there and anyone would be hard pressed to try to convince me to try to build anything other than wooden blades. Thanks for the comments. Dave B.

DanB · « **Reply #10 on:** December 04, 2007, 10:15:41 PM »

Dave - Ive done similar work bench tests and am quite amazed as well - how far my blades flexed to hit the tower (without breaking first).

You might consider repairing yours, the damage doesn't look too bad. 2.5 years ago my 17' turbine really took some serious damage to two blades and we repaired that one - it's been fine since. (I also gave it lots more clearance by tilting the alternator back a couple more deg)

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