Refer to figure.
When designing the main rotor blades of helicopters, especially those without powered steering (which means the pilot has to manually control the direction and stability), there’s an important requirement to consider: changes in the angle of attack (AoA) should cause minimal movement of the center of pressure.

• Angle of Attack (AoA): This is the angle between the rotor blade's chord line (the imaginary line running from the leading edge to the trailing edge) and the air flowing over it. Adjusting the AoA impacts how much lift the rotor blade generates. Increasing the AoA generally increases lift, while decreasing it reduces lift.
• Center of Pressure: This is the point along the rotor blade where the lift force is considered to act. If this point shifts significantly when you change the angle of attack, it can lead to instability and make the helicopter harder to control.

Why Is This Important?

• Stability: If the center of pressure moves a lot when the rotor blades change their angle of attack, it can create unexpected changes in how the helicopter behaves in the air. For pilots, this can feel like the helicopter is “fighting back” against their inputs, making it tricky to maintain stable flight.
• Control: Helicopters without powered steering rely on the pilot’s skill to control the aircraft. If the center of pressure shifts too much, it means the pilot has to constantly adjust their inputs to keep the helicopter stable, which can be challenging.
• Predictable Behavior: Keeping the center of pressure movement to a minimum leads to more predictable performance of the rotor blades. This means that pilots can anticipate how the helicopter will respond to their controls, leading to safer and more efficient flight.

In summary, ensuring that changes in the angle of attack produce minimal movement of the center of pressure is a key design feature for rotor blades on helicopters without powered steering. This helps make the helicopter more stable and easier to fly, allowing pilots to focus more on flying rather than fighting against the aircraft's natural tendencies.