A wind turbine blade works by lift, like an aircraft wing, not by drag like a paddle, and a three-blade lift rotor should run with its tips moving 5 to 7 times as fast as the wind. A ceiling-fan-shaped blade captures only 5 to 10% of the wind’s power while a proper airfoil captures 35 to 45%, and no blade can exceed the Betz limit of about 59%.
Part of Advanced Wind Turbine
Blade design is the choice of blade shape, size, twist and number that turns moving air into rotor torque as efficiently as possible.
A Turbine Blade Is A Wing, Not A Paddle
The carrier says modern blades work on lift (an airfoil with low pressure on its curved upper surface), not drag, and that a lift rotor turns at tip speeds 5 to 7 times the wind speed. A drag device such as a paddle or a Savonius rotor can never move faster than the wind that pushes it, which limits how much of the wind it can use. The Blade Design page puts the capture of a ceiling-fan blade at perhaps 5 to 10% against 35 to 45% for a proper airfoil. Those two figures are quoted here, not derived.
The ceiling on any rotor is the Betz limit: a turbine can take at most 16/27, about 59%, of the power in the wind passing through it (Wind Power). Home-built rotors reach 25 to 35% of the available power. The 59% is a limit of physics, not of workmanship.
Tip Speed Ratio Ties The Blade To The Generator
Tip speed ratio, λ, is how many times faster than the wind the blade tip moves. At λ = 6 in a 5 m/s wind, the tip moves 6 × 5 = 30 m/s, or 108 km/h. For a 2 m rotor, whose tip travels a circle of π × 2 = 6.28 m, the rotor turns 30 ÷ 6.28 = 4.78 times a second, which is 286 RPM. The carrier and the Blade Design page both give 5 to 7 for a three-blade rotor; the Wind Power page says 6 to 8. They overlap at 6 to 7, and the figures are quoted without being merged.
The tip speed ratio ties blade angle, rotor RPM and generator together. Too low and the blades stall; too high and drag dominates and the rotor is noisy.
Shape, Size, Material And Number Set The Blade
| Choice | Carrier’s guidance | Detailed in |
|---|---|---|
| Airfoil shape | curved top, flatter underside, lift over drag | Airfoil Profile (Lift vs Drag) |
| Length, chord, twist | twist about 20-25° at the root, 3-5° at the tip; chord widest near the root | Blade Length, Chord, & Twist |
| Material | carved wood, or sheet metal for speed of build | Carving Wooden Blades, Sheet Metal Blades |
| Number | three, for smooth running and good efficiency | Blade Design |
Three blades beat two because a three-blade rotor loads its hub evenly as it yaws; two blades make the load fluctuate. More blades starts the rotor in lighter wind but lowers its peak speed and efficiency.
Worked Example: What A 2 m Rotor Can Make
Power in the wind is P = ½ × ρ × A × v³, with ρ = 1.225 kg/m³ and A = π × 1² = 3.14 m². At 5 m/s: ½ × 1.225 × 3.14 × 125 = 240 W available. At 35% capture: 0.35 × 240 = 84 W, matching the Wind Power page. At 8 m/s the available power is ½ × 1.225 × 3.14 × 512 = 985 W, and 35% of it is 345 W. Wind speed matters far more than blade polish, which is why site and tower come first (Site Selection & Tower Height).
Common Mistakes
- Using a ceiling-fan or flat-paddle blade — it captures a fraction of what an airfoil does.
- Setting the same angle along the whole blade — the tip moves much faster than the root and needs a smaller angle.
- Unequal blades — vibration shakes the bearings and tower.
Safety
A 108 km/h blade tip
At a tip speed ratio of 6 in a 5 m/s wind the tips move at 30 m/s, and faster in stronger wind. A blade that breaks or a hub that loosens throws the blade a long way. Never stand in the rotor’s plane, balance every blade, and fit the overspeed protection described in Dump Load & Overspeed Protection.
What’s Next
- Learn next: Airfoil Profile (Lift vs Drag) for how the shape makes lift
- Combine with: Blade Materials for the choice of material