A standard Darrieus vertical axis wind turbine (VAWT) has a significant flaw: torque ripples. As its vertical blades rotate, the angle at which they meet the wind changes dramatically. This creates a pulsating, uneven torque on the central shaft. The effect is similar to pedalling a bicycle with jerky, inconsistent pushes instead of smooth, continuous strokes. This pulsation causes vibrations, increases wear on components, and makes the turbine difficult to start in low wind conditions.

The solution is to give the blades a helical twist, like the stripes on a candy cane. This design ensures that some portion of the blade is always at an optimal angle to the wind. By distributing the aerodynamic forces smoothly over the entire rotation, the helical shape cancels out the pulsating torque, leading to much smoother power delivery and enabling the turbine to self-start more reliably.

The geometry of a helical blade is a careful balance of aerodynamic principles. The blade's cross-section, or airfoil, is a critical choice. Symmetrical airfoils, like the NACA 0012, are common because they perform consistently as the angle of attack shifts from positive to negative during each half of the turbine's rotation. An asymmetric airfoil might produce more lift in one half of the rotation, but it would perform poorly in the other, creating its own imbalance.

The magic of the helical design lies in how it manages the angle of attack—the angle between the blade's chord line and the relative wind. For a straight blade, this angle varies wildly. For a helical blade, the twist ensures that the angle of attack stays within a narrow, effective range throughout the entire 360-degree rotation.

The performance of a turbine is often described by its Tip Speed Ratio (TSR), which compares the speed of the blade tips to the speed of the wind.

Helical VAWTs are designed to operate within an optimal TSR range. The amount of twist in the blades is directly related to this target. A higher twist angle (a more tightly wound helix) is generally better for low TSRs, improving start-up torque. A lower twist angle is more efficient at higher TSRs. The exact geometry is a trade-off between self-starting capability and peak power output.

Another key design parameter is solidity, which measures how 'full' the turbine's swept area appears.

A high-solidity turbine (more blades or wider blades) will have excellent starting torque but may have lower peak efficiency due to aerodynamic interference between the blades. A low-solidity design has better high-speed performance but may struggle to self-start. The helical twist, blade solidity, and target TSR must all be calculated together to create an efficient and reliable turbine.