blade carving question

[defed]

i'm using Hugh's 4' plans.  in the blade section, it states minimum blank of 3 3/8" (i think, i left the book in garage) wide.  it also says that it can be wider, it just makes the root beefier.  ok good, i'm using a piece that is 5 1/2" wide.

this is hard to explain, but here goes:

when laying out the stations, i have:  1 (tip, 24" from root), 2 (16" from root), 3 (8" from root), 4 (6" from root).  so far so good.  the question comes in when it says to draw a line 45 degrees from the leading edge of the last station (6" from root) to the trailing edge, and then connect that from the trailing edge last station (where you take the depth to the maximum).  this creates the steep angle from maximum depth back to the root.

if i had a 3 3/8" blank, this sloped area is not that large but w/ a 5 1/2" blank, this sloped area is quite large.  is this a problem?  also, due to the extra width, the 3 blades don't match up perfect when putting them together.  is there a bit of extra root sticking out.  again, not sure if this is a problem since it will be covered by the hubs anyway.

thanks, hope this is clear!

Dave

[scoraigwind]

This is a good question because it does puzzle people that the corners of the blade roots do not meet using my system.  In fact that doesn't matter.  bear in mid that not much does really matter at the root of the blades but it's nice if they are wide and steep and all the same as each other.

Make sure that the pointed ends all mesh together at the exact centre (the 120 degree angled corners meet in the middle) but it doesn't matter how wide the piece of wood is, nor whether the corners of the blade root meet each other.

I hope this helps

[oztules]

It doesn't get much better than that Defed...... from the horses mouth so to speak

[defed]

yea, thanks Hugh!  now i just need to learn to carve them...it's not going as easy as i thought it would.

speaking of the steep angle that goes from the deepest point back to the root, what is the best way to cut this?  doesn't chisel worth a darn because of the grain, spoke shave doesn't fit too well, and my drawknife is waiting for some sharpening supplies to arrive!

[defed]

i think i have most of it figured out.  one last thing is the air foil on the back.

it says to take it down to 1mm on the trailing edge.  how fast do you get to this from the root end?  there is the area at the root that is called the 'thickness cut guideline'...should the edge be 1mm RIGHT at this line?  or does/can it taper down from this line until a 1mm edge?

same goes for the leading edge.  there is alot of material to get the leading edge down and rounded over at the root end.  i can't see a good picture of it, so not sure what the allowances are - if it should be right up to the guideline or if a tapering is allowed.

thanks again,

[Flux]

If you mark out and cut the front driving face you can then mark the thickness of the blade at each section.

Try this link to see a typical aerofoil section and you can aim for something like that on the back.

http://www.google.co.uk/imgres?imgurl=http://www.aerospaceweb.org/question/aerodynamics/wing/clarky.gif&imgrefurl=http://www.aerospaceweb.org/question/aerodynamics/q0005.shtml&h=500&w=500&sz=4&tbnid=zwPJzuF7Sc-veM:&tbnh=130&tbnw=130&prev=/images%3Fq%3Dclark%2By%2Bairfoil&hl=en&usg=__6EdV3h31uPQ9qPA-B4JPBj4v50A=&ei=gNr3S_aEHaKI0wTi9tzpBw&sa=X&oi=image_result&resnum=4&ct=image&ved=0CC8Q9QEwAw

The fattest part is about 30% or a bit more back from the leading edge. Make the front section from the leading edge to the thick part something like that curve, most people don't round the leading edge and most treat the front face as flat whereas that section shows it to go slightly below the flat at the front.

From the thick point back to the trailing edge again try to keep like that diagram but this section is much less curved and you wouldn't have a disaster if you just made it more or less triangular. Ideally the trailing edge should be razor sharp but in this case it won't really matter as long as it is not more than 1mm thick.

I suspect virtually all these hand cut aerofoils are different and within reason as long as you are somewhere near the mark they work.I really do wonder how drastically far from the mark some folks get and it may be bad enough sometimes to mess up the results.

If you attempt the high performance aerofoil sections then they need to be very exact and probably they can't be hand carved without many templates and more care than the job is worth but if you aim for something like the Clark Y that i linked to or some of the early NACA sections it will work fine. One commercial manufacturer used a back section that was a section of a circle, obviously so that it could be produced on a spindle moulder, it worked fine and it was on a faster and more critical prop than this.

Flux

[defed]

thanks flux.  i'm reasonably happy w/ the air foil on the back as a whole, just not sure how to transition it into the root.  i will get some pics to illustrate what i mean exactly.

figured out several techniques that work for me on the 1st one, so hopefully the other 2 come out better!

[Flux]

I look forward to the pictures but I blend the front face to the root with a semicircular curve cut with a spokeshave, others use a flat 45deg transition which you can cut with a saw.

On the back again I use the spokeshave to blend the aerofoil to the rectangular section over about 2" on that size of blade.

I seem to remember seeing somewhere that you are using a wide board. If I did this I would run the driving face full width at the last station even though it will reduce the angle, the inner part does virtually nothing. Alternatively you could cut the board width to the correct value near the point where the hub reinforcing discs go and mark the main blade section as for the correct size board, this would give greater root strength and keep the blade shape.  In fact this particular blade shape is so strong that there really is no need to keep the larger timber and it is most probably easier to saw it to size than carve away much timber with planes and spokeshaves.

If I have missed the point then I await your pictures.

Flux

[defed]

instead of taking the pics like i was supposed to, i carved a 2nd blade.  mostly came out the same, is a bit thinner at the tip, tho.  i'll have to take some thickness measurements to see how the rest compares.  i think i might have taken a little too much off the back.

i have been cutting them by making saw cuts down to my guide lines, chiseling them out, then spokeshaving the rest.  i have a drawknife, but it needs to be sharpened and i have to get some new stones.  still need some sanding.  they are far from perfect, but each one is getting faster and less errors.  just need to get them more consistent.  one thing i do 'wrong' is shaving it down to the line.  i should leave them high and sand to the line.  would be more accurate i think.

instead of making the 3rd blade, i'll go get some pics.  anxious to get the 3rd one done, and a hub, so i can put it up in the wind and see if it even spins!

[defed]

#1:  airfoil
#2:  blank
#3:  front of carved blade
#4:  back of carved blade
#5:  this is the extra width to the root vs the "minimum width" spec.  you can barely see the line i drew.
#6:  here is the leading edge portion of the airfoil.  not sure how far back i should round it out.
#7:  closer pic of leading edge airfoil
#8:  trailing edge airfoil.  again, how far back should i take the 1mm thickness?
#9:  closer pic of trailing edge airfoil.

[Flux]

That's coming on fine.

Just a few comments that may help. In future I wouldn't take the ramp at the root of the drive face into the mounting part where the discs go ( picture 3). I like to leave the part under the disc at full thickness but this time make them all the same, this prop is very strong and it will be no problem.

When you finish sand it, take the trailing edge down to a virtual knife edge.

Also on the last two pictures you show a big lump on the trailing edge near the root, just blend this from the thickest point down to the line of the trailing edge from the outer part of the blade.

I think this is where the width changes rapidly in the marking out of the stations, there are a lots of changes going on at this point and it may be confusing, but just try to maintain the basic aerofoil section at each point and bring the trailing edge to the knife edge, lumps here are not going to help performance, keeping the trailing edge very thin is one of the essentials.

Otherwise you are doing fine, with a bit of sanding it will be great, I have seen worse than this flying ok and you still have the sanding to do.

Flux

[defed]

thanks again flux.  the ramp part was part of my initial confusion.  if the blade was the 'minimum' width, that line wouldn't go into the mounting area of the root so much.  being a wider blade, and the spec calling for a 45 degree angle, i just let it go where it went...keeping the angle at 45 degrees.  i've found a few pics of 4' blades, and they actually look wider than mine!  i will have to check out how they did the ramp angle.

that big lump also looked strange and wasn't sure if it should be rounded out.  my biggest concern is weight/balance matching.  the 2nd one came out a tad thinner, at least at the very tip where you can visually compare them.

sure hope they spin!

[defed]

on the ramp to root...on the backside, you draw out the circle and this is where the root is untouched...on the front, you don't, just the 45 degrees from the last station.  i will make the last one the same, but before i cut it, i will see what would happen if i make the circle on the front too.  if i recall, Hugh calls for a triangular front on the hub (round on back)...maybe due to this ramp angle?  i will look at this as well.

[Fused]

I've made 3 sets of these same blades, it looks like your doing a fine job. I've made them anywhere from 3ft diameter to 5ft 7 inch diameter and they all came out the same at the root end on the back side. If I can figure out how to post a picture Ill show you where I blended out the back side to the trailing edge. My blades fly well and are quite high TSR, the cut in on my axial flux is right at 7mph and will spin up to a blur in higher (15 mph) winds.

[defed]

got the 3rd one done and i did blend them back.  not quite that far, but maybe 1/2 that much.

i weighed them, and 2 are almost the same (between 0 and 5 grams difference) and the 3rd is about 25 grams (1 ounce) lighter.  i did a non exact weighing w/ the root supported by some pins and the tip resting on the scale, and they share the same differences...2 almost exact and one 15-25 grams lighter.

if this is the case after i get a more precise root/tip weight, where should i trim the 2 heavier ones down?  oddly, the one that looks the thinnest at the tip is one of the heavier ones!  i'm thinking down where the root and blade face meet (where the blending occurs) is where the most discrepancy lies.

as always, thanks for all of the advice.

[Fused]

I've always weighed the tip and then the root and then total weight of each blade. After I determine the heaviest blade, I start marking the back side with a pencil where I found weight differences and I use a belt sander to remove wood from the back side only. Make sure they are the same length first before weighing. Once I get the blades very close to each other in weight its time to go ahead and mount them in your hub, I like my blades to be mounted on the generator for the final balancing to get them perfect and working with any slight unbalance that may be built into the generator disks. This has worked for me, but may not actually be the right way to do it. Practice makes perfect, but slight differences in the blades just add character (or so my wife tells me). I've rarely used counter weights on my blades, I'd rather sand the back side to match. Belt sanding just helps it go quicker for me.

Good luck and have fun.
Fused

[defed]

thanks.

i think i am going to leave them for now.  i forgot that the light one had some edge damage (missing a cpl of small chunks) in the uncut root area which was there on the board when i started.  don't think it's missing 1 ounce worth of wood from these damaged areas, but i bet it's at least 1/2 of the difference.  i want to get it on a hub so i can see if it even spins unloaded.  then, since i suspect alot of the total weight difference is from the root area, i can add a little weight there instead of trying to get the other ones lighter.  but, we'll see how it goes when i start getting it together.

[Flux]

For future attempts I would mark the area where the mounting discs go and make sure you don't cut into it. There is no rule about that 45deg bit where it drops down to the front face, you could make a vertical cut but it would be weak. You can go down at a steeper angle to get to the last station . I prefer to go in with a spokeshave and that starts near vertical but leaves a nice strong radius at the bottom.

I have never weighed blades, it may have some virtues if building from fibreglass but for wood it seems pointless. Just keep them looking as near as you can and assemble them. Sometimes if you use different bits of wood one is obviously heavier, you can plane it down a little and if all made from the same board you can usually balance them this way.

If they are too different to balance this way without one being obviously thinner than the rest then just resort to balance weights.

Weighing the whole blade is virtually useless, weighing the tips is more helpful but even so it is not likely to be more than a very rough guide and the spacing of the 3 blades is unlikely to be perfect and it is far better to balance the assembled prop.

You could remove weight from the root section of the heavy one in this case if it is quite near ( doesn't matter if the roots are identical) but weights are simpler and easier.

Flux

[defed]

after i studied the instructions and my blades, it looks like my angles on the root are correct, to Hugh's specs.  for the front, he calls for a 10" triangle, which when i put my blades in position, it looks like a 10" triangle fits perfectly on my 45 degree lines.  so i guess i'll give what i have a whirl and hope it whirls.

[jlt]

Make sure when trying to test your blades to have them mounted to something solid,and not held in your hands. I Tried that before ,kind of like holding a wildcat by the tail.

[Fused]

Im glad to hear Flux says the 45 degree angle is not set in stone. I cut mine closer to a 30 degree angle off vertical because I felt it may increase the strength at the root.  I did not use the triangle front hub but I put one round plywood disk in front and one on the back. Its been up since Christmas and all is well. Don't forget to re tighten your hub bolts after a month or two as the wood will compress slightly. I would not let these blades free wheel either as they can get out of control and be dangerous.

Fused

[Fused]

I forgot to mention, fiberglass resin works great to fill small voids in the wood. It bonds great to wood as it is also for boat repair, but I paint my blades so Im not sure how it would work with linseed oil.

Fused

[defed]

Im glad to hear Flux says the 45 degree angle is not set in stone. I cut mine closer to a 30 degree angle off vertical because I felt it may increase the strength at the root.

that was my initial thought when i saw where the 45 degree line went.  but, for my 1st try, i just went with it.

[Ungrounded Lightning Rod]

after i get a more precise root/tip weight, where should i trim the 2 heavier ones down?  oddly, the one that looks the thinnest at the tip is one of the heavier ones!  i'm thinking down where the root and blade face meet (where the blending occurs) is where the most discrepancy lies.

I'd take a few measurements and try to figure out which section is off before deciding where to cut.

One measurement to take is to balance them on a raised edge of something and find the center of gravity.  In addition to being the same weight you want the center of gravity the same distance out to get the total assembly to balance - because the effect of a little extra or missing weight on the assembly's center of gravity is its mass multiplied by its radius.

Remember:  All that matters for static balance is that the center of gravity is at the center of rotation.  You don't need the blades to be the same weight (though that may make it easier to get the centering to happen.

Also:  A very slight error in the angle of one of the blades (with respect to the others) can move the CoG a bunch.

[imsmooth]

See http://www.mindchallenger.com/wind/axial13.html for a tutorial on static blade balancing.  My blades spin perfectly as all speeds.

[defed]

ok, after trying to decide how to ensure a proper alignment, i finally got up the nerve to screw the blades to the rear half of the hub.  blade tip to blade is equal all the way around with 120 degree angles in between...so far so good.

next problem i can see coming, tho i can't verify it until i get it mounted on the spindle....the blades don't appear to be traveling in the same plane (runout?  i forget the proper term).  best i can figure right now is 2 are really close and one may be 1/8" off.  how do you correct this?  shim between the blade and the hub?

thanks, as always.
Dave

[TomW]

The blades don't appear to be traveling in the same plane (runout?  i forget the proper term).  best i can figure right now is 2 are really close and one may be 1/8" off.  how do you correct this?  shim between the blade and the hub?

thanks, as always.
Dave

Dave;

The term is "tracking"

I think most folks use some shims to adjust it but I have been lucky and never had one not run within reasonable limits.  1/8" may be perfectly acceptible.

My most recent 12 footer was within a finger width and DanB thought it was fine. I have somewhat large fingers that are between a half inch and three quarters of an inch wide.

1/8" may be perfectly fine but that will depend on the blade length. Perfect is good but 1/8" may be "good enough".

What is the diameter of the prop?

Tom

[defed]

it's only a 4 footer.  when i get it mounted, it will be easier to tell.  i have a 5 bolt wheel hub.  i had the idea that i might be able to adjust this w/ the wheel hub bolts (shimming between the wooden hub and the wheel hub)...or shim between the blade and wood hub.  hopefully get it all mocked up on the rotor in the next day or 3.

[Flux]

The tips are only a small fraction of the total blade weight in this particular case and I would imagine that 1/8" tracking error would have negligible effect in introducing a couple.

You can probably get some adjustment by tightening some bolts more than others, you could also use thin shims between the hub and prop, I wouldn't try messing with the individual blades.

If it is being used with the intended alternator the speed will be fairly modest anyway. I wouldn't mess unless you find you have a problem, my guess anything under 1/2" will be fine.

Flux

[defed]

ok, i thought it would be a little more sensitive than that.  we'll see how it goes.

[defed]

i haven't gotten the thing up in good wind yet, but it seems to be a slow starter, ie needs some pretty decent wind to get spinning at all.  i was just looking at some older plans of Hugh's (2003) and it looks like his 4' blades were bigger then.  if i recall, in the newer book, the tip is 1 1/2" wide.  the older plans were something like  2 3/16" wide at the tip.  i have to compare station measurements on both, but i think this would give the blade more surface area overall...which seems like it would catch more air.

from looking at various pics, my blades do seem leaner than some...which i'm guessing were made to the older specs.