Helicopter Rotor Aerodynamic Design

Let's explore the essential aerodynamic characteristics of helicopter rotors. Like airplane wings, rotor blades are also shaped like airfoils, but there are differences in design considerations compared to airplane design. Here are some key points to consider when designing rotor systems.

---

Components of the Rotor System

The rotor system consists of a mast, hub, and rotor blades. The mast and hub are connected, and the blades are attached to the hub. For a detailed explanation of each component's structure, please refer to the post below.

Components of helicopter

Components of helicopter

When examining the rotor blades closely, you'll notice that the root section, where it attaches to the hub, is thick and wide, while it becomes thinner and narrower towards the tip. The thickness of the airfoil at the tip is typically about 18% of the root thickness to minimize the vortex and drag generated at the tip.

helicopter

Rotor Blade Design

Unlike airplane wings, helicopter rotor blades have varying cross-sectional shapes at different positions, making the design more complex and challenging to interpret. Rotor blade design can be divided into two types: planform geometry design, which focuses on the overall shape when viewed from above, and airfoil design, which focuses on the cross-sectional shape.

helicopter planform, airfoil geometry

The reason for the varying cross-sectional shapes along the rotor blade is the rotation. Rotation implies the presence of linear velocity, which is proportional to the radius. Consequently, the velocity is slower at the root and faster at the tip. If the thickness and angle of the blade were the same at all positions, it would result in varying lift forces at different positions, leading to unstable flight.

Planform Design

Therefore, when designing the planform of the rotor blade, it is essential to achieve uniform lift force distribution. This requires a larger angle of attack at the root where the velocity is slower and a smaller angle of attack at the tip where the velocity is faster.

 

Key Factors in Planform Design

• Definition of characteristics based on radial position
• Chord length
• Twist angle
• Sectional airfoil shape

Airfoil Design

When designing the airfoil shape, it is crucial to understand the airflow around the airfoil. The goal of airfoil design is to maximize lift and increase lift-to-drag ratio while avoiding flow separation.

 

Key Factors in Airfoil Design

• High maximum lift coefficient
• High drag divergence Mach number
• Good lift-to-drag ratio over a wide range of Mach numbers
• Low pitching moment
• Sufficient thickness for structural strength

In summary, rotor blades generate lift through rotation, resulting in varying lift forces at different positions. To achieve a uniform lift distribution across the entire blade, the blade's airfoil shape and angle are designed differently at each position.