Make a sheet-metal blade by cutting the tapered plan from 1 to 2 mm steel or aluminum, curving it over a pipe former to an approximate airfoil, twisting it to the station angles, and bolting it to a hub plate. It is faster than carving and adequate for battery charging, but less efficient than a carved blade, so design it for the low end of the tip speed ratio range; a 1.5 mm aluminum blade of 1 m length weighs about 0.55 kg against 1.6 kg in steel.
Part of Advanced Wind Turbine
A sheet-metal blade is a cut, curved and twisted plate; it trades aerodynamic quality for speed and simplicity of making.
Fast To Make, Less Efficient To Run
The carrier’s description is brief: cut the blade shape from 1 to 2 mm steel or aluminum, bend it to an approximate airfoil profile with a form or by hand, and rivet or bolt it to a hub plate. It calls the result less efficient than carved wood, a flat plate against a true airfoil, but adequate for battery charging. Choose sheet when the village has more metal than seasoned timber, or when the first turbine must be running soon.
The loss shows up at the tip. As worked out in Airfoil Profile (Lift vs Drag), a section keeps only the fraction 1 − λ_r × D ÷ L of its lift as torque, where λ_r is the local speed ratio and L/D the lift-to-drag ratio. A cruder section has a lower L/D, so it loses more at a high tip speed ratio. My inference from that formula is to design a sheet-metal rotor near the low end of the carrier’s 5 to 7 range.
Method: Cut, Form, Twist And Fix
Step 1: Cut the plan. Mark the tapered outline from the blade-length drawing (Blade Length, Chord, & Twist) on the sheet and cut it with shears or a cold chisel, leaving a margin. File the edges smooth; a sharp burr is a cut waiting to happen.
Step 2: Curve it. Lay the sheet over a pipe or a half-round wooden former and form a gentle curve across the chord by pressing and rolling by hand. A flat strip curved in this way has the cambered shape the carrier asks for, with the hollow underneath.
Step 3: Stiffen the leading edge. A thin sheet flexes and flutters in the wind. Fold the leading edge over a rod to make a rolled hem, and for a long blade rivet a flat strip along the blade as a spine. These are common practice, recalled and not from the corpus.
Step 4: Twist to the stations. Clamp the root flat, then twist the blade by hand, checking the angle at each station with the pitch gauge until it matches the table. Check again after a day, since a sheet springs back a little.
Step 5: Fix to the hub plate with at least two bolts, and large washers to spread the load. Holes near the edge or too close together are where thin sheet cracks first.
Step 6: Balance the three blades on a pivot (Blade Balancing), and paint steel against rust.
Worked Example: Weight And Root Load
A 1.0 m blade with an average chord of 13.5 cm (the mid-value from the blade-length page) and a thickness of 1.5 mm has a volume of 1.0 × 0.135 × 0.0015 = 0.000203 m³.
- Steel at about 7,850 kg/m³ (a standard handbook density, recalled): 0.000203 × 7,850 = 1.6 kg.
- Aluminum at about 2,700 kg/m³ (also recalled): 0.000203 × 2,700 = 0.55 kg, a third of the steel.
- Centrifugal force at 381 RPM, with the center of mass 0.6 m out: for 0.55 kg, 0.55 × 39.9² × 0.6 = 525 N; for 1.6 kg, 1.6 × 39.9² × 0.6 = 1,528 N. Doubling the speed multiplies each by four.
Aluminum loads its hub bolts far less, though thin aluminum fatigues sooner than steel at a hole, which is why washers and generous edge distance matter.
Common Mistakes
- No stiffening — the blade flutters and the cracks start at the root.
- Sharp-edged holes and rough cuts — they start fatigue cracks.
- Different thickness sheets for different blades — the set will not balance.
Safety
Thin metal edges cut, and a thrown blade is lethal
Cut edges are razor sharp: file them, and wear gloves when handling. A blade that cracks at the root leaves the rotor at its tip speed (30 m/s at a tip speed ratio of 6 in a 5 m/s wind). Inspect root bolts and holes each month, and never stand in the rotor plane while the turbine is turning.
What’s Next
- Learn next: Rotor Disc Construction for the hub and magnet rotor
- Combine with: Carving Wooden Blades to compare the wood option