plywood+fiberglass blades / part 2

[kajs]

i was scared in previous post about weights the blade needs to cope with so i bought more fiberglass fabric for one more thin layer. this fabric is satin weave. should stretch less - reduce elasticity. not in the pictures yet.

if i would have been lucky to find c.r.jacobs patent 2,516,576 some time ago, i would definitely go for a tube in the blade. no luck for me this time. and that would be too much for the first project, wouldn't it? or can i catch up with this now somehow? or variable pitch for a blade like this is not a good idea at all? help appreciated, you lovely people!

i am still thinking about that root connection.

i thought that the wide root for the startup thrust is a great idea but actually, though it will look nice on the tower, such width and twist messes up real estate around hub.

The blades are sanded for the last fiberglass layer.

i started building up thickness in root part as i promised in previous post.

[Dave B]

 Tremendous job and thanks again for posting an update. I am sure there are many out here like myself following your progress on this project. You have extended the envelope on homebrew blades for sure, I applaude your effort.

 Keep us posted, I am anxious to see how you mount these and to what type of machine. There is no doubt they are going to really perform, as always the trick is in the matching of the load.  Dave B.

[kajs]

thanks

[kajs]

here is the last fiberglass layer and thickening at the root. i'll sand it and will look good.

blades are considerably less elastic with that satin weave fabric on.

[SparWeb]

Your blades look very nice.  I do hope you find a solid way to mount them.  If you are unsure about something, or run into trouble, I will try to help if I can.

Refresh my memory: are there nuts and washers on the threaded rods, tightened against the ribs?  I see none on the root rib face.

[kajs]

yes, there are nuts inside the blade at each side of each rib - so there are about 40 nuts inside every blade.

http://www.otherpower.com/images/scimages/12653/wing4.jpg

[kajs]

testing blades...

hugh's formula for betz thrust was.

F = (4/9) x (rho) x A x V x V

i should test my blades by putting some weight on one of them so i will need a kilogram-force number and divide it by n of blades.

so the formula would be

F = ((4/9) x (rho) x A x V x V / 9. / n

i calculate betz kilogram-force for r=2.5m at 12m/s

F = ((4/9) x 1.225 x 3.14 x 2.5^2 x 12^2 / 9. / 3 = 52kp

the force is evened out on that perfect blade proportionally to swept area of section of the blade. if i divide blade radius in 4 equal regions, then i should put these weights on the blade:

root section = 3kg

2. nd section = 10kg
   
3. rd = 16kg
   
   

tip section 23kg

------------------------------

10kg at tip gives me tip deflection 35cm.

i shall go for more fiberglass layers. what else?..

[kajs]

sorry for digging up this post, but something needs to be sorted out.
there was a discussion and my apparently wrong calculations about thrust on blades. i was suggested that 5m dia blades may get 100kg thrust at tip at high winds. that postponed my enthusiasm for a year or so..

now i entered my blade (http://fieldlines.com/board/index.php/topic,130087.msg845727.html) data in PROPID and it gives me a tower thrust at 22m/s 90kg (200lbs), well this is a COMPLETELY different talk. now the root needs to bear only 30kg. but of course, the machine must be loaded..
can I continue my project with this data in mind? or i got it wrong, again.

and by the way Piggott formulas for blade design look just perfect in PROPID. i see very high Cp.

[Janne]

Hello kajs,

How many layers of fiberglass do you have on the blades? As you indicate adding another, I'd suspect that the blades will work quite well(Don't take me for an expert though, as such I'm not). For a solid disc of the same diameter you would get around 600kg of force in 22m/s wind, so I'd be inclined to think the latter calculation is closer to the truth. In any case, I'd worry a little of those blades bending back too much, provided of course the furling / pitch control works and you have adequate tower clearance.

Hope you get the issues sorted, and get those terrific blades up and spinning in the air

[kajs]

Yeah in the air!
right now I'm re-reading all the pitch control pages i can find. nothing simple enough:( I keep the downwind option open.. it seems the blades should not be difficult to stall, but then again i did not want the batteries and controllers, just heat. confused.