Author Topic: How wide should my blades be at the tip?  (Read 1665 times)

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alibro

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How wide should my blades be at the tip?
« on: December 27, 2006, 09:46:02 PM »
Hi Guys

I have finally got to the stage of carving blades and am a little confused. I have an alternater very similar to the old Otherpower 10 footers with 24 2" x 1/2" magnets, 9 coils with 60 turns and I am getting a cutin rpm of around 150 for 24V in Star. If I need to increase the cutin speed I will either widen the air gap or make another stator with a couple fewer turns as I may need to remake it anyway. More details are in my Diary


My question is this

I have bought some timber (just cheap softwood for £20) which is 1800mm (5'11") long x 220mm (8 5/8") wide by 45mm (1 3/4") thick

How wide should I make it at the tip and how far out from the centre should I start the taper?

 If I follow the blade plans in Hugh Piggots book "How to Build a Wind Turbine" it would be 100mm (4") at the tip whereas in the blade theory page at Otherpower the tips (for a 3600mm, 10 foot blade) are 75mm (3") which I guess would be 90mm (3.5") for 3600mm (11'10") diameter blade. I know 1/2 an inch isn't much but it is near enough 22% of the width at the tip (I think) which makes it sound a lot more when looked at like that.


The second point probably makes more of a difference. Where to start the cut? In Hugh Piggots book the cut starts 600mm (24") from the centre whereas The Otherpower blades start  at 300mm (12") for a 3600mm, 10 foot blade so I guess that would be 365mm (14.4"). Or could it remain at 300mm (12")?


It seems to me that the Hugh Piggot blades are wider and heavier with a higher drag and therefore slower  which means a lower TSR. Is that correct? He says in the book they have a TSR of 6. Did I read somewhere that the DanB blades have a TSR of 7?

If everything I have said above is correct then I have finally hammered into my brain what TSR really means and how to change it. If not could someone please try to explain it again.


I want my turbine to be on the safe side of ideal as I have seen blades spinning very fast and it was a scary sight, so should I make it more like Hughs or more like Dans?


Or will it really matter?


Many thanks

AliBro

« Last Edit: December 27, 2006, 09:46:02 PM by (unknown) »

Flux

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Re: How wide should my blades be at the tip?
« Reply #1 on: December 28, 2006, 01:51:03 AM »
I don't think you will find that it makes much difference. With either type of blade you will be running at about tsr 8 at cut in and likely below 4 at full power.


Your alternator will determine the speed not the exact shape of the blade.


Hugh tends to stick to the more exact angles and profiles, whereas Dan does straight line taper from root to tip and uses 3 deg angle at the tip, 6 deg at the middle and whatever the wood gives at the root.


If you want things slow and quiet with that alternator I wouldn't let the rotor diameter exceed 10 ft. If you go with the 5ft 11" you will be rather powerful for that alternator.


If you are in a modest wind area want maximum power and don't mind higher speed and you use a bigger alternator offset and furl early then the nearly 12 ft blade may work.


You seem to be worried about a few % change in chord width that has no real effect but you seem happy to regard it as a 10 ft machine with a prop that is nearer 12 ft. That is likely a good reason for many of the things that don't really furl.

Flux

« Last Edit: December 28, 2006, 01:51:03 AM by Flux »

luv2weld

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Re: How wide should my blades be at the tip?
« Reply #2 on: December 28, 2006, 09:10:38 AM »
Personally, I think you need to answer a question here. Are you willing to experiment or do you want to stay with something that is proven to be safe. (OK, safe is questionable.) Both blade designs have been proven to work. But if you start mixing them, you move into the experimental area and that can get scary real fast. Come to think of it, all of this is experiMENTAL, with heavy emphasis on the mental. You won't be disappointed with either design. The only way to know for sure which design is better for your site and alternator is to build them both and compare them in actual flight.
« Last Edit: December 28, 2006, 09:10:38 AM by luv2weld »
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alibro

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Re: How wide should my blades be at the tip?
« Reply #3 on: December 28, 2006, 10:35:36 AM »
Thanks Flux

DanB always said these machines tended stall and suggested a slightly bigger blade. Maybe I didn't take into account the alternater has already been changed to allow for that. I will go with a 10 foot blade and see how it works. If the exact profile isn't so important I will just go with the Otherpower blades as they were the original match to this alternater.


Cheers

AliBro

« Last Edit: December 28, 2006, 10:35:36 AM by alibro »

alibro

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Re: How wide should my blades be at the tip?
« Reply #4 on: December 28, 2006, 10:43:12 AM »
I agree with you however I think as soon as you take anything out of the hands of an expert and let a novice build it then everything is experimental. My missus would do like you and put the emphises on the mental part.

Anyway as I said to Flux since the alternater is a DanB design I think you are right that I stick with his design for blades. I always loved the look of his earlier blades anyway. With something like this how it looks is very important when trying to keep sceptical spouses happy.


Cheers

AliBro

« Last Edit: December 28, 2006, 10:43:12 AM by alibro »

hvirtane

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Re: How wide should my blades be at the tip?
« Reply #5 on: December 28, 2006, 02:43:43 PM »
I think that the important thing is to make the furling system working well. If that works well and safe, you might use even a bit bigger blades. The important thing is anyway to harvest well low winds, and for that the thing, which matters is the size of the blades.


- Hannu

« Last Edit: December 28, 2006, 02:43:43 PM by hvirtane »

picmacmillan

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Re: How wide should my blades be at the tip?
« Reply #6 on: December 28, 2006, 03:41:47 PM »
hello there..i have experimented with both types of blades you mention, and i have used the exact same length as both hugh and dan...hughs worked real well but i found they were too small at the tips for my liking, i then made a set of dans blades and they worked fantastic..take note that hughs blades are beautiful, but for my low wind area, i needed more blade width to extract any useful power from the wind..hope this helps...personally i like having both sets around, hughs are real light and turn easily in higher winds where dans turn in real low winds..good luck with your project...pickster

« Last Edit: December 28, 2006, 03:41:47 PM by picmacmillan »

alibro

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Re: How wide should my blades be at the tip?
« Reply #7 on: December 28, 2006, 04:19:51 PM »
Thanks for that.

I am drawing the blades out ready for cutting tomorrow as I type. They will be 10 foot DanB blades, but as my timber is a little wider than his there will be a small change. I am going to make them identical to his from the tip to 12" from where the cut starts at the root then the last 12" of cut will take it out to the full width (8 3/4") with a bit of a curve. It is only a small change which probably won't make much difference but I want to make the best use of the wood I can, while keeping them more less the same, I love the idea of using what you have and adjusting it to fit.

I never really tested the wind strength here but there is a gale blowing outside now and we seem to have had a lot of strong winds this winter.


Cheers

AliBro

« Last Edit: December 28, 2006, 04:19:51 PM by alibro »