Rotorcraft airfoil types

Airfoil

Any object, such as a flat plate, or the deck of a bridge, with an angle of attack in a moving fluid will generate an aerodynamic force perpendicular to the flow. Airfoils are more efficient lifting shapes, able to generate more lift than similarly sized flat plates, and to generate lift with significantly less drag. An airfoil or aerofoil is the cross-sectional shape of a wing, blade. An airfoil-shaped body moved through a fluid produces an aerodynamic force. The component of this force perpendicular to the direction of motion is called lift. The component parallel to the direction of motion is called dragSubsonic flight airfoils have a characteristic shape with a rounded leading edge, followed by a sharp trailing edge, often with a symmetric curvature of upper and lower surfaces.


Cross-sectional shape of a wing, or blade

Main types

  • Symmetrical
  • Non-symmetrical
Airfoil shape types

Symmetrical

The symmetrical airfoil is distinguished by having identical upper and lower surfaces. The mean camber line and chord line are the same on a symmetrical airfoil, and it produces no lift at zero AOA. Symmetrical design can generates more speed and ability to belly fly.

Advantage:
  • Lower cost and.
  • Easier construction compared to asymmetrical (non symmetrical) airfoil.
  • Used in aerobatics helicopter builds.
Disadvantages:
  • Produce less lift than a asymmetrical airfoil. 
  • Have relatively undesirable stall characteristics.

Non-symmetrical

The non-symmetrical airfoil has different upper and lower surfaces, with a greater curvature of the airfoil above the chord line than below. The mean camber line and chord line are different. The non-symmetrical airfoil design can produce useful lift at zero AOA.

Advantage:
  • Non-symmetrical generates more lift at zero AOA than a symmetrical design.
  • Improved lift-to-drag ratio, 
  • Better stall characteristics.
Disadvantages:
  • Center of pressure travel of up to 20 percent of the chord line. This creating undesirable torque on the airfoil structure.
  • Greater production costs.

Blade Twist

Helicopter blade twist with each section A, B, C.

Because of lift differential due to differing rotational relative wind values along the blade, the blade should be designed with a twist to alleviate internal blade stress. And distribute the lifting force more evenly along the blade. Blade twist provides higher pitch angles at the root where velocity is low and lower pitch angles nearer the tip where velocity is higher. This increases the induced air velocity and blade loading near the inboard section of the blade.

Tip shapes
Helicopter three main tips shapes

There are three main types of helicopter tip designs:

  • Parabolic tip,
  • Swept tip,
  • BERP tip.

In addition there are several tip shapes intended to alleviate BVI noise by splitting, or diffusing the tip vortex. Despite a considerable research effort over the last 30 years, no single best tip shape has emerged, perhaps because the tools required for accurate evaluation have not previously been available, or because design requirements and manufacturing constraints have also been evolving. Modern high-resolution CFD methods now offer an opportunity to gain deeper insight into the aerodynamics of blade and tip design, and with continued rapid development, will soon have sufficient maturity to make a major impact on the design process. Based on “A review of helicopter rotor blade tip shapes” in Progress in Aerospace Sciences(PDF)

NACA airfoil

The NACA airfoils for aircraft wings developed by the National Advisory Committee for Aeronautics (NACA). The shape of the NACA airfoils is described using a series of digits following the word “NACA”. The parameters in the numerical code can be entered into equations to precisely generate the cross-section of the airfoil and calculate its properties. For more information see Airfoil Tools.

Conclusion

The above material will allow us to better understand the different types of airfoil and their application. It turns out that despite appearances, this is a very complicated topic. Requiring many mathematical calculations and compromises in developing and manufacturing. Although the OMH v.03 logic does not take into account real time fluid simulation, but for us it is a chance to introduce further visual variants. However, depending on the type used, we should experience the difference in flight characteristics. For our needs at the beginning I prepared three airfoil types based on NACA0012, NACA6409, NACA10. Each has a different characteristic, which will affect the experience of the flight. For example, NACA0012 may allow us to try some aerobatic tricks, like flying upside down.

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