Chainsaw blades tutorial

[imsmooth]

For those interested in carving blades with a chainsaw, instead of chiseling, I have put together a photo and video tutorial at http://www.mindchallenger.com/wind/bladecarving.html

Hopefully, this will offer others another option for carving blades.

[birdhouse]

nice post!

hoping to be doing the same in a week or so(waiting for lumber to dry before i lam it up) got mostly clear fir 2X3's for $1.68 per 8 foot, but pond boards

looks like that pouland doesn't like what your putting it through.  maybe it's time for for big stihl or echo or husky???

thanks for the info!

wonderful tutorial!

[Ungrounded Lightning Rod]

If I understand this right (and I MAY NOT):

 - The leading edge should be rounded - nearly a half cylinder.

 - Not the chord but the bisector of the angle between the upwind and downwind surfaces just behind the leading edge rounding should be angled at the TSR (at the end of the blade and scaled inversely with radius as you come toward the hub.)

 - The trailing edge should be sharp and at a narrow angle.

 - The bisector of the angle of the trailing edge should be at an angle of about 1/3 the TSR (again at the end of the blade and scaled inversely with radius.)

The idea is that:

 - The leading edge must split the air even if it's not coming toward it at the bisector of the angle and must make it "attach" to both sides of the blade.  You pay some air friction to increase the range of angles-of-attach over which the air attaches.

 - The leading edge encounters the essentially undisturbed apparent wind and you want to split it evenly between the upper and lower surfaces.

 - At the trailing edge you can rejoin the flows and form a jet going essentially the way the edge is pointed.

 - The Betz ideal occurs when you slow the air to 1/3 its incoming speed, which dictates your trailing edge angle for a given operating TSR.

That doesn't take into account some other stuff (like pre-slowing of the air upwind of the blades by pressure from the blades' resistance propagating at sonic speeds, compensating for the energy left in the wind as spin and energy lost to surface drag, etc.)  But I'm guessing this arrangement should be reasonably close to ideal.

[imsmooth]

I can answer some of these.  I have seen many profiles for blades. Some have the leading edge like a half-cylinder, where the top surface and bottom surface are rounded at the leading edge.  I have seen some where the top surface is rounded to a quarter circle and joins the flat surface of the bottom.  My last set had the latter; this set I am trying the former leading edge profile.  The trailing edge comes to a sharp, nearly flat edge to allow the two air streams to join with the least amount of turbulence.  You also want the surfaces smooth to lower drag, and will most likely result in lower noise levels.  My last set were baby-ass smooth and were silent until they hit about 400-500 rpm at which point you could here the rush of air.

[HenryVG]

Love the webpage descriptions! They make it very clear. Except the second cut isn't obvious to me, is this just the same blank turned over with the other 'guides' used to support the sleds of the chainsaw? Or is it a second jig?

And how did you get the profiles calculated? Just a standard multi-station blade calculator program with the marks transferred to the blocks you initially put on the jig?

The pictures of the laminated blade being put together and glued is terrific. It makes that SO clear (especially with the partial shaping already done at the tip).

Thanks for the detail.

[imsmooth]

Thanks for the compliment.  I thought the section on the jig fabrication was a little light.  I will add another page describing how I came to fabricate my jigs over the next few days.

Basically, I used a blade calculator that gave me the cord length, drop and thickness.  I fabricated the rails so the chainsaw would drop by that amount to give that pitch and thickness.   I used the maximum thickness for each station, knowing that I will power sand the board down to get the profile later.

The windward and leeward sides have their own pair of rails.

[Ungrounded Lightning Rod]

I have seen some where the top surface is rounded to a quarter circle and joins the flat surface of the bottom.

That makes a kind of sense for a wind turbine - and I've thought of doing that way if I ever do a set of blades cut from PVC pipe, to simplify construction.

The curvature is about getting the incoming air to attach for a number of angles of attack.  A mill is going to be going either about the right speed or too-slow-to-stopped - and you don't need efficiency if it's already spinning above the design speed (i.e. the wind is dropping momentarily or the turbine is not adequately loaded).  So the wind will be between "forward" and "upward from the flat side" but not "downward from the rounded side" - at least when it counts.

A quarter-round on the front of the bulgy downwind side should keep air attached to it about as well as a full half-round, while the flat upwind side doesn't need the curve.

Or at least that's what I think is the case.

(Getting this wrong might end up with a blade that whistles when the air detaches intermittently along the sharp corner.)