Author Topic: simple blade question? (Read 6968 times)

picmacmillan · « **on:** October 28, 2010, 11:19:36 AM »

starting to finally put my 17' machine together..hit a snag on blade construction....it has to do with the 6 degree part of it.(danb's type).what i read is the tip is 3 degrees and at halfway it is 6 degrees...what i have always done is figure the width of my blade at the tip..and then draw a straight line to the root part..thats it..
i cant comprehend what the 6 degrees is??

the way i see it is if you started at the tip and made a line at 3 degrees..then at halfway point made a line at 6 degrees you would be making the blade less wide at the root>> unless you made the 6 degrees outwards , so the blade is wider at the root???i am thinking that is what it is?

because the weight of the blades at the tip is more critical than the weight closer to the root? plus? it would catch more wind if it was wider near the root??

Or is it in blade thickness?? or say on the leading edge..any help to clarify would be appreciated.i cant see what it is in all the photo's i have looked at..thanks in advance ....pickster

Flux · « **Reply #1 on:** October 28, 2010, 11:50:04 AM »

I can't follow your description or your problem

If you follow the accepted theory the calculators use then you will have a tip width to suit your tsr and typically the 3deg is about right.

At the root you will have something crazy wide and thick and it really doesn't work any better for all the waste of timber.

You seem to be implying that you decide the width and angle at the tip and keep this width and angle to the root. It will work quite well but with larger blades I suspect it will be weak at the root.

What Dan does is make a compromise, choosing the correct figures at the tip and choosing a practical widthand thickness for the root.

If you use his board dimensions and start with the drop to give 3 deg at the tip and run this line out to drop equals full board thickness at the root you will get a straight line at the trailing edge that gives a drop to equal 6 deg at half radius and the width will be half way between tip width and full root board width.

This gives a satisfactory compromise over the outer 2/3 diameter where you do all the work, the root only provides strength to hold the thing together.

I don't think in degrees, just drop and width at each point but if you do it Dan's way the tangent of TE drop over width at the tip will give you 3 deg and when you measure the centre of the blade the tangent of TE drop over width at mid point should give you 6 deg.

If you have protractors and like measuring angles then work in degrees, otherwise use your Tan figures to work from width and drop at the trailing edge.

I haven't looked at Dan's drawings for a long time but they were pretty clear and easy to follow as far as I can remember as long as you start with the same size board.

If working in degrees confuses you then take a look at Hugh's web sit and look at his material on blade construction. He uses the width and drop method and his calculator programme gives the widths and drop at each station.

Have another look at Dan's drawing with this in mind and see if it makes sense, if not come back with more detail about what is confusing you and I will have another look. ( preferably link me to the drawings or description that is confusing you as it takes a bit of finding sometimes)

Flux

tanner0441 · « **Reply #2 on:** October 28, 2010, 01:29:38 PM »

Hi

I wonder if he is mixing blade twist angle with trailing edge taper....

Brian.

freejuice · « **Reply #3 on:** October 28, 2010, 08:29:48 PM »

Quote from: picmacmillan on October 28, 2010, 11:19:36 AM

starting to finally put my 17' machine together..hit a snag on blade construction....it has to do with the 6 degree part of it.(danb's type).what i read is the tip is 3 degrees and at halfway it is 6 degrees...what i have always done is figure the width of my blade at the tip..and then draw a straight line to the root part..thats it..
i cant comprehend what the 6 degrees is??
If not mistaken by your question, its the angle across the board from the leading edge to the trailing edge, at any give crosss section of the blade[/b] the way i see it is if you started at the tip and made a line at 3 degrees..then at halfway point made a line at 6 degrees you would be making the blade less wide at the root>> unless you made the 6 degrees outwards , so the blade is wider at the root???i am thinking that is what it is? Once again if I'm reading you right, keep in mind the blade is flat on the backside ( for the most part except the 1/3 leading edge) but tapers in thickness towards the tip....let me see if I can attach a few photos....I cant function without photos.

because the weight of the blades at the tip is more critical than the weight closer to the root? plus? it would catch more wind if it was wider near the root??

Or is it in blade thickness?? or say on the leading edge..any help to clarify would be appreciated.i cant see what it is in all the photo's i have looked at..thanks in advance ....pickster

some more:

I hope this helps but I feel , I might have missed your questions all together.
All the best,
Gavin

picmacmillan · « **Reply #4 on:** October 29, 2010, 09:25:46 AM »

thanks men for the help..i can use gavins last picture to figure it out...

if you just did 3 degrees from the tip to the root, the actual fin(or the piece of the blade that protrudes out at the root),the fin would be smaller than it is in his photo..

and what i had read was at the halfway point towards the root, you change the angle to 6 degrees which would make the root wider, and catch more air where the weight isnt as critical....

the blades i am building are identical to gavins last picture, except i am carving one set out of pure cedar, not laminated. i am also going to build a laminated set too...only exception is that i build 4 or 5 blades and keep 1 or 2 of them for spares if anything happens to any of the ones on the turbine ....i have found that if a blade breaks for whatever reason it is almost impossible to make another one that weighs approximately the same, especially over a period of time..so it is good to have an "extra laying around that was carved and dried the same amount of time as the originals for the sake of balance....and thanks flux, and gavin for you're time....its appreciated..now if i can figure how to get my old pictures out of my old files i might be able to be a little more clear when i ask a question

..pickster

http://www.otherpower.com/images/scimages/1135/blade_root_as_per_hughs_book.jpg

http://www.otherpower.com/images/scimages/1135/windmill_on_a_sunny_day.jpg

picmacmillan · « **Reply #5 on:** October 29, 2010, 03:06:08 PM »

question for gavin if you read the post again?? or anyone who has done this for that matter..its kind of a sidebar question...i am ordering my blade material...wanted 2x4 - 10 foot cedar..all they have in stock is 12 foot lengths of it? and the crucial aspect is they only have 27 of them?? now to the question?? is that enough..it is hard to count how many you used in the picture?? if you could let me know..it would be appreciated ( thinking i can use some of the waste off the 12 footers for the root/fin on the blade)..thanks again ...pickster

DanG · « **Reply #6 on:** October 29, 2010, 03:27:04 PM »

Mr Pic - those 27 are probably mostly ones not selected by other shoppers.... you need to pick through fresh pallet loads, six, eight, twelve at a time... I've moved 300 boards to select eight before.

freejuice · « **Reply #7 on:** October 29, 2010, 09:05:26 PM »

Pic, Dan is correct.
I actually used 2x6 lumber which I snapped a chalk line ripped them down the middle with a skill saw.
The cost was a bit better that way and the chances of getting two boards from a good piece of lumber was even better...noyt so much picking through a pile of lumber to do.
I made them from white pine, I can get western cedar but white pine is so plentiful and cheaper here in the east. I looked for quarter sawn lumber, but was not to fickle in that aspect. But I looked for wood with few knots, no splits and as straight as possible.

Using 2x6 lumber I think this is my board count for three blades:

each blade needed 5 pieces from10 foot long lumber...so that makes 8, 2x6's at 10 feet
Then I think I bought about about 4 of 5 more 2x6's at 8 foot long. Thats about it for 3 blades...the left over cuts I used for the shorter runs...this was plently of lumber to build all three blades...and I didnt do any splicing.
I would suggest you get Hugh Piggott or the Dan's's manuals...I think the Dan's warn against butt splicing blades together and show an example of where one came apart.

I think I had 10 boards of the 2x material side by side, to make a complete blade, which is roughly 15 inches wide, but trimmed it to 14 inches wide upon finishing.
I used titebond 3 and worked fast with a 2 inch brush laying the glue down...I layed all the boards flat, poured the glue from a gallon jug in a zig zag fashion and then spreadd it out as fast as possible with the brush, then flipped them up on their edges and clamped it together. you have about 15 mins working time.
Dont put too much titebond 3 down, it takes longer to dry, but dont cheat it either....just a good moderate to heavy coat, dont go "sloppy" on the glue, it will take quite a while to dry if you do.

fabricator · « **Reply #8 on:** October 29, 2010, 09:09:55 PM »

When I got my white pine from menards I actually went to three area stores and sorted through all the 1x4 lumber they had.

picmacmillan · « **Reply #9 on:** October 30, 2010, 07:44:38 AM »

i have a whole whack of white pine logs thats a good idea..huge ones already down and peeled...ready to put on a log house...i could use that.some are over 40 feet long and 30 inches at the butt...i dont have a mill, but there are plenty around.....i guess looking at it i have tons of options with no expense...but i like the idea of using cedar..they just look cool, as well as being light and long lasting...and youre right about the quarter sawn ones..ill try and use that, my supply is almost endless.i am thinking that would be the way to go unless of course the weight of them hurts my output in light winds, cuz thats typically what we have here...but man

that put a smile on my face....you guys saved me 350 bucks already

...wish girlfriend thought like you

thanks again pic

p.s. if anyones around my (ontario, quebec) area i can donate pine for youre projects ..as stated above i have lots..and i am always on the road so might even be able to deliver it to ya..that portion will cost you a coffee

freejuice · « **Reply #10 on:** October 30, 2010, 04:12:24 PM »

Pic,
I'm not too sure about it but even heavy baldes which are balanced correctly and on good bearings will spin fairly easy.
I like the idea of western ceder too...it is light weight no doubt, but the baldes I made from white pine only weighed in at 20 lbs a piece, and mine are kind of bulky...now some of the balde pros around here could probably turn them into a lean and slender, race horse and probably knock another lb or more off of them.

If youre into the "gotta cut a blade from a whole log" thing, then check into the chainsaw blade making videos on youtube.
As to long lasting, I dont think its deterioration you have to worry about as much as the leading edges taking a beating. After a year, my 11 footer blades....especially for the last foot or so towards the tips were chewed up,....probably from smacking hail stones like a Nolan Ryan high-insider curve balls.

I think Fabricator has covered the leading edges of his blades with some kind of metal sheeting

SparWeb · « **Reply #11 on:** October 30, 2010, 08:31:58 PM »

freejuice,
I put on a plastic "leading edge tape" which is meant exactly for this. A 3M thick clear poly tape and it sticks well.

freejuice · « **Reply #12 on:** October 30, 2010, 08:36:46 PM »

Thanks Spar!
I will look into getting some of that stuff!
Does it have a particular name?
Thanks,
Gavin

Flux · « **Reply #13 on:** October 31, 2010, 03:30:58 AM »

Here in UK it's called 3M prop tape or sometimes helicopter tape.

Very expensive here and doesn't last all that long before it starts bubbling up and delaminating. I only tried on varnish, may be better directly on wood.

Flux

SparWeb · « **Reply #14 on:** October 31, 2010, 11:04:20 AM »

Search under "Abrasion tape" or "leading edge tape" and the 3m product number is 8674.

You can spend a lot of money getting a roll that's larger than you need (which I guess I did) but here's a supplier that can get you much smaller rolls.

http://www.aircraftspruce.com/catalog/cspages/8674.php

As for durability, I haven't had any pieces come off or show signs of letting go. I do lower my turbine and deliberately replace off the tape once a year. And my climate is dry and cool. Considering the erosion of the wood that I saw after one year without the tape, I've made my choice. My blades are bare, soaked with linseed oil and given at least 2 days to dry in warm air before the tape goes on.

If it's a cool day (say you want to put the tape on without removing the prop and bringing it indoors) when you put the tape on, get a hair-dryer and that will warm up everything.

For even greater protection, you can always look for thin brass sheeting, or shim stock in a roll. Much more expensive though, but that you can form to the blade contour, bond it permanently, and even put a tiny nail on some of the edges. This is the way large wooden aeroplane props were protected in days of old, and it worked very well.

Flux · « **Reply #15 on:** October 31, 2010, 03:50:25 PM »

That is probably the best approach, change it every year. I found that much beyond a year it started coming away and you get quite considerable changes in moisture absorption that affects balance once it starts lifting. The flapping edges cause lots of noise also.

The metal foil protection works well, I believe there is some form of stainless foil tape available but I have never found it. Not sure how it compares with the polyurethane.

Flux

SparWeb · « **Reply #16 on:** November 01, 2010, 01:58:29 AM »

Quote from: Flux on October 31, 2010, 03:50:25 PM

... I believe there is some form of stainless foil tape available but I have never found it....
Flux

The same supplier... http://www.aircraftspruce.com/catalog/mepages/0330800.php

But that helps you little in the UK, I know. There's nothing very special about this AL sheet except that it's packaged for shipping. It really wouldn't difficult to find a flying club or a few sport pilots that would know where to get it. Of course you run the risk of getting many visitors in the back garden, once you mention what you're up to!

Ungrounded Lightning Rod · « **Reply #17 on:** November 01, 2010, 05:48:38 PM »

Quote from: Flux on October 28, 2010, 11:50:04 AM

What Dan does is make a compromise, choosing the correct figures at the tip and choosing a practical widthand thickness for the root.

If you use his board dimensions and start with the drop to give 3 deg at the tip and run this line out to drop equals full board thickness at the root you will get a straight line at the trailing edge that gives a drop to equal 6 deg at half radius and the width will be half way between tip width and full root board width.

This gives a satisfactory compromise over the outer 2/3 diameter where you do all the work, the root only provides strength to hold the thing together.

I don't think in degrees, just drop and width at each point but if you do it Dan's way the tangent of TE drop over width at the tip will give you 3 deg and when you measure the centre of the blade the tangent of TE drop over width at mid point should give you 6 deg.

The unloaded speed ratio of a given patch of the blade is the inverse of the slope of the chord at that radius. The loaded speed ratio is a constant times that (about 1/2), presuming you're trying to achieve the same angle-of-attack regardless of radius (which you probably are).

Since the speed of the blade goes with the radius and the wind speed is the same regardless of radius, the slope (tangent of the chord angle) has to double every time the radius halves if you want to keep the above relationship true.

If the arctangent of the angle at the tip is 3 degrees, the arctangent of twice that slope (the midpoint of the blades) will be just a hair under 6 degrees (and while the ideal change in slope is a curve, a straight line isn't all that far from it). And the arctangent of four times that slope (a quarter of the radius out) will be nontrivially under 12 degrees (and the straight-line approximation of the curve will be a worse approximation). It approaches 90 degrees as you approach the center of the shaft, and gets ridiculously steep near the hub. This makes the "ideal" shape VERY sloped. It also widens, because with more slope it has to be wider to present a given cross-section to the wind. At the hub it would be infinitely long, which is obviously silly.

But a patch at the tip with some given delta-radius has twice the swept area of a patch with the same delta-radius at the midpoint, four times the swept area of a patch a quarter of the way out, and so on. The inner 10% of the blade has 1% of the swept area. So as you get near the hub, and the "ideal" shape starts becoming enormous, it doesn't really cost you much power to just say "phooey on it" and stop trying to collect power at all.

Dan's approximation is really close to the ideal for the outer half of the blade, which sweeps 75% of the swept area, not too far off for the next quarter, which gets you up to 93 3/4%, and then fouls up progressively worse for the last 6 1/4%.

Well, lah-de-dah. If we're only getting 50% of the ideal on the inner quarter of the blade we lose 3 1/8% of the available power. If we're just a flat plate we lose about 6 1/4% (or maybe a little more from fouling up the airflow on the next patch out.) So let's just make the inner quarter of the blade strong - the bending stress gets very high there - and maybe give it a shape that gets us some starting torque without being a horrible air brake, a big weight that makes raising the tower difficult, or an over-optimized tree trunk that breaks the wood budget.

picmacmillan · « **Reply #18 on:** November 04, 2010, 12:22:12 PM »

one more item that may be of interest here...i had the opportunity to watch those guys that fly those miniature airplanes by remote control...their blade balancing equipment, leading edge issues, and construction are very, very similar to what we do, except they do it on a much smaller scale...but to anyone who has the opportunity its worth looking into how they conquer all their respective propeller issues...it may give some a few ideas on how to build really good props...pickster

galeforce jones · « **Reply #19 on:** November 04, 2010, 02:55:37 PM »

I wrapped my blades with fiberglass tape from station 2 up to the tip and put three coats of epoxy paint.
Its important to cut tape down to about 1" wide and wrap from the root towards the tip

picmacmillan · « **Reply #20 on:** November 12, 2010, 03:02:21 PM »

one more question concerning the edge tape, brass whatever ...anyone noticed more noise off the blades because of it??..not that it matters in my scenario, but even without tape when the blades got whistling you could here them...in fact someone i have met someone who bought a store bought turbine and the neighbors near him complained about the high pitch noise..and the neighbors must be 1000 yards from his place??..thanks ...pickster

SparWeb · « **Reply #21 on:** November 12, 2010, 09:59:23 PM »

Yes it changes the noise. More / Less noise? Just different... hard to explain...
It will depend on the thickness of the tape (the 3M stuff is 2mm thick, brass you can get less than 1mm) and how much of the LE it covers, how close to the tip you laminate, and then you can also check the change in noise at different speeds, meaning a change may have less effect at low speed but more when faster. Sorry there's no science to it like 1+1=2.

TomW · « **Reply #22 on:** November 13, 2010, 01:52:04 PM »

Quote from: SparWeb on November 12, 2010, 09:59:23 PM

Sorry there's no science to it like 1+1=2.

Every geek knows 1+1=10

Couldn't help it.

Tom

freejuice · « **Reply #23 on:** November 13, 2010, 10:15:16 PM »

Hi,
FWIW, you can also buy brass , aluminum or stainless steel shim stock in 6 inch wide rolls, from about .001 thick to a thicknes which would require something other than hand fitting... such as a metal brake.
When I lower my 17' again I will use either the aircraft prop tape or use a roll of brass shim stock about .010 thick on the leading edges: http://travers.com/product.asp?RequestData=CA_Search&navPath=All+Products%2F%2F%2F%2FUserSearch%3Dshim+stock%2F%2F%2F%2FUserSearch1%3Dblock+id+113248+and+class+level3+id+244833&eaprodid=113248%2D71%2D201%2D100

scoraigwind · « **Reply #24 on:** November 14, 2010, 04:49:39 PM »

Leading edge tape works well if you need it, but to be honest if you keep the speed down it's not a problem. I was recently looking at a set of 6 footer blades that have been running over a year on wood that had only linseed oil on it, and they showed very little wear.

It's a bit like the issue with overheating stators. Push it a bit too hard and you have a lot of issues and a lot of hot air discussing solutions, but if you run it nice and slow then you have reliable quiet power for a year and no problems.

Ungrounded Lightning Rod · « **Reply #25 on:** November 15, 2010, 09:25:58 PM »

Quote from: scoraigwind on November 14, 2010, 04:49:39 PM

Leading edge tape works well if you need it, but to be honest if you keep the speed down it's not a problem.

But you want a high TSR for efficiency and a TSR of 6 means that in a 50 MPH storm the tips of the blades are spinning at 300 MPH and the middle of the leading edge at 150. That can make quite a ding if it hits a bug or a bit of flying debris.

scoraigwind · « **Reply #26 on:** November 18, 2010, 06:08:46 PM »

Quote from: Ungrounded Lightning Rod on November 15, 2010, 09:25:58 PM

Quote from: scoraigwind on November 14, 2010, 04:49:39 PM
Leading edge tape works well if you need it, but to be honest if you keep the speed down it's not a problem.

But you want a high TSR for efficiency and a TSR of 6 means that in a 50 MPH storm the tips of the blades are spinning at 300 MPH and the middle of the leading edge at 150. That can make quite a ding if it hits a bug or a bit of flying debris.

Yeah sure efficiency is great. But first of all keep them turning. Kwh is kW and h. If h doesn't work then kWh doesn't either. In a 50 mph storm you ought ot be lying low in my opinion. But I live off grid. Maybe on grid with attitude the leading edge tape is the way to go.

fabricator · « **Reply #27 on:** November 18, 2010, 07:37:56 PM »

I have 24 gauge stainless on my leading edge and .030 aluminum on my trailing edge, 17' machine, I also have a 3' radius on leading edge tip, this noise issue is hard for me to understand, my machine is very close to completely silent, it's only on a 25 foot tower right now but even standing right under it in a 20 mph wind al you hear is a slight swish, swish.

divemaster1963 · « **Reply #28 on:** November 18, 2010, 10:00:12 PM »

I have a question for everyone. Has anyone tried using Bass wood? I just got done painting my set this week. I have a three blade set that is 48" long per blade 5in wide at the base and 3 at the tip. each one balanced out at 400 grams each ( i tink that just over a pound). I even added a 3 degree base angle using matching shimes. I'll get photos up this comming week if anyone wants to see them.

thanks
divemaster

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Author Topic: simple blade question? (Read 6968 times)