Carve a wooden blade from a straight-grained, knot-free plank 8 to 10 cm thick, cut to the blade’s plan, with a flat underside and a curved top shaped to templates at each station, then seal it with oil, varnish or paint and balance the set to within 1 to 2% by weight. A 1.5 kg blade on a 381 RPM rotor pulls on its root bolts with about 1,430 N, and four times that if the rotor runs away to twice the speed.
Part of Advanced Wind Turbine
Carving is shaping a plank into an airfoil with hand tools; wood is the most accessible blade material and can be repaired in the village.
Wood Wants Straight Grain And No Knots
The Blade Materials page weighs the options. For carving, the carrier chooses straight-grained, knot-free wood: spruce, pine, cedar or even well-seasoned fir. It calls these “hardwood”, but all four are conifers, which botanists class as softwoods, so the working rule is straight grain, no knots, and seasoned dry, not hardness. Run the grain along the blade’s length so the fibres, not the glue lines, carry the bending, and keep every knot out of the root, where the bending load is highest.
Method: Carve The Blade
You need a drawknife, a spokeshave, sandpaper, a pencil, and a template for each station, as in Blade Length, Chord, & Twist.
Step 1: Cut the plank. Blade length × maximum chord × 8 to 10 cm thick (the carrier’s thickness), from the seasoned plank.
Step 2: Draw the plan. Mark the tapered outline on the plank face, widest at the root and narrowest at the tip. Mark stations every 10 cm.
Step 3: Rough out the plan. Saw away the waste outside the outline, leaving a margin of a few millimeters.
Step 4: Carve the underside. Per the carrier, the flat underside is the easy one. It is flat across the chord, but twisted along the blade so each station sits at its pitch angle. Check each station with the pitch gauge.
Step 5: Carve the top. The curved upper surface needs careful work, taking wood off a little at a time, to the profile template at each station. Fair the surface between stations with the spokeshave until the transitions are smooth to the hand.
Step 6: Round the leading edge and thin the trailing edge, which cuts drag (Airfoil Profile (Lift vs Drag)).
Step 7: Seal. Several coats of oil, varnish or paint, including the end grain at the root and the tip, as the carrier says.
Step 8: Balance. All three blades must weigh the same within 1 to 2%. On a 1.5 kg blade that is 15 to 30 g. Shave the heavy blade to match. Then balance the finished rotor on a pivot (Blade Balancing, which says this removes 80 to 90% of vibration).
Worked Example: The Load On The Root Bolts
The numbers below are illustrative; weigh your own blade.
- Assume a blade mass of 1.5 kg with its center of mass 0.6 m from the axis, on the 381 RPM rotor from the blade-length page.
- Angular speed ω = 381 × 2π ÷ 60 = 39.9 rad/s.
- Centrifugal force = m × ω² × r = 1.5 × 39.9² × 0.6 = 1,430 N, roughly the weight of 146 kg.
- At twice the speed, 4 × 1,430 = 5,720 N.
Force goes with the square of speed, so the root bolts, the hub plate and the wood around the bolt holes must be sized for the runaway case, and not for the design speed (Dump Load & Overspeed Protection).
Cracks Start At The Root And The Bolt Holes
Look along each blade for cracks in the grain, especially near the root and around the bolt holes, and for water getting under the finish. A rotor turning at 250 RPM all year bends each blade 131 million times (Terrain Effects & Turbulence); cracks grow. Re-seal before the wood is bare.
Common Mistakes
- Knots near the root — the crack starts at the knot.
- Balancing the weight but not the shape — one blade carved fatter at the tip pulls differently from a lighter one.
- Unsealed end grain — it takes up water, swells and splits.
Safety
A cracked blade is thrown, not dropped
A blade that fails at 381 RPM leaves with the energy of its tip speed. Do not run a rotor with a crack, loose bolt or an unbalanced blade, and keep people out of the plane of the rotor. Wear eye protection and a dust mask when carving; fine wood dust irritates the lungs.
What’s Next
- Learn next: Sheet Metal Blades for a quicker, less efficient alternative
- Combine with: Rotor Disc Construction for the hub the blades bolt to