Choose a blade profile that gives strong lift at a small angle of attack with low drag, and set each section at an angle of attack of 5 to 10 degrees, because beyond about 15 degrees the airflow separates and lift collapses. At a tip speed ratio of 6 the blade tip meets the air at 6.08 times the wind speed, and a section turns lift into useful torque only if its lift-to-drag ratio exceeds 6; with a ratio of 10 it keeps 40% of the tip’s lift as torque.
Part of Advanced Wind Turbine
An airfoil is the cross-section of a blade; its shape decides how much lift it makes per unit of drag when air flows past at a given angle.
The Air The Blade Sees Is Not The Wind
A blade section moving through the rotor plane at speed u = ω × r meets two flows at once: the wind v, straight into the rotor, and its own motion, sideways. The relative wind is their sum:
W = √(v² + u²), at an angle φ to the rotor plane, with tan φ = v ÷ u.
At the tip, with tip speed ratio λ = 6 in a 5 m/s wind, u = 30 m/s, so W = √(25 + 900) = 30.4 m/s, and φ = arctan(5 ÷ 30) = 9.5°. The air hits the tip at 6.08 times the wind speed, almost from the side. The blade’s pitch β (its angle to the rotor plane) is the flow angle φ less the angle of attack α: β = φ − α.
The Blade Design page gives the best angle of attack as 5 to 10° for simple airfoils, and says that beyond about 15° the flow separates from the surface and lift collapses while drag spikes. That is stall.
Why Drag Costs So Much At The Tip
Lift acts at right angles to the relative wind and drag acts along it. Because φ is small near the tip, the lift is tilted forward by φ and the drag backward. The force that actually turns the rotor, along the rotor plane, is:
F_t = L × sin φ − D × cos φ = L × sin φ × (1 − (D ÷ L) × cot φ)
and cot φ equals the local speed ratio λ_r = u ÷ v. So the useful fraction of the lift’s forward push is 1 − λ_r × D ÷ L. At the tip (λ_r = 6) this is:
| Lift-to-drag ratio L/D | Fraction kept at the tip (1 − 6 ÷ (L/D)) | At mid-blade, λ_r = 3 |
|---|---|---|
| 6 | 0 | 0.50 |
| 10 | 0.40 | 0.70 |
| 20 | 0.70 | 0.85 |
The L/D values are examples, not measurements of any particular blade; at L/D of 10 the tip keeps 0.40, which is 40% of its lift. Two things follow. A section with L/D at or below the local speed ratio makes no torque at all. And the outer part of the blade, which moves fastest and sweeps the most area, is the part most spoiled by drag, which is why smooth finish and a good profile matter most near the tip.
How Much Lift Is It?
Lift per meter of blade = ½ × ρ × W² × chord × Cl. With ρ = 1.225 kg/m³, W = 30.4 m/s, an assumed chord of 0.10 m and an assumed lift coefficient Cl of 0.8:
½ × 1.225 × 30.4² = 566 Pa, and 566 × 0.10 × 0.8 = 45 N per meter of blade.
The chord and Cl are illustrative. That is the order of the force a hand-carved tip section must carry, as part of the same load that tries to bend the blade each revolution.
Choosing A Profile You Can Make
- Curved top, flat underside. The carrier describes low pressure on the curved upper surface and high pressure on the flat lower surface. It is the simplest to carve: the flat underside is easy and the curved top needs careful shaping (Carving Wooden Blades).
- Rounded leading edge, thin trailing edge. A blunt or ragged trailing edge adds drag, which is the quantity that matters at the tip.
- Thick at the root, thinner outward. Thickness gives strength where bending is largest (Blade Length, Chord, & Twist). The corpus gives no thickness ratios, so choose from the strength you need, not from a table.
- A flat sheet gives lift but is less efficient than a true airfoil: the carrier calls sheet-metal blades adequate for battery charging (Sheet Metal Blades).
Method: Set The Blade’s Angle At Each Station
Step 1: Choose the design tip speed ratio, 5-7 per the carrier (Blade Design & Aerodynamics).
Step 2: At each station, compute φ = arctan(1 ÷ λ_r) where λ_r = λ × (r ÷ R).
Step 3: Subtract the design angle of attack, 7° if you have no better figure, to get the pitch β. The table is in the blade-length page.
Step 4: Make a template for each station and check the carved section against it with a protractor or an angle gauge.
Step 5: Spin the finished rotor in a steady wind and watch the starting. Poor blades stall at start-up and refuse to turn until the wind is strong.
Common Mistakes
- Using the wind angle as the blade angle — the relative wind is almost sideways at the tip.
- Too steep a pitch near the tip — the section stalls (α above 15°) and drag takes over.
- Rough or wrinkled surfaces — they raise drag, and drag is paid for at the tip.
What’s Next
- Learn next: Blade Length, Chord, & Twist for the dimensions that follow from this angle
- Combine with: Blade Balancing before the blades go on the hub