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Exploring the Aerodynamics of Symmetrical Airfoil

Key Takeaways

  • In symmetrical airfoil, the upper section is identical to that of the lower section.

  • The symmetrical airfoil cannot produce any lift at a zero angle of attack.

  • System designers can use CFD sol\vers to simulate and visualize symmetrical airfoil performance at different flow conditions. 

 Airfoil condition simulation

When fluid flows around certain structural geometries such as airplanes and automotive or marine components, the knowledge of how the flow parameter interacts is of utmost importance to engineers and system designers. Especially in aeronautics engineering, the study of various aerodynamic parameters over the airfoil provides information on pressure distribution, lift, or drag force. With the help of computational fluid dynamics (CFD), this prediction can be made in a much faster and easier manner. Through characterization of the airfoil, CFD can facilitate analysis of its efficiency when exposed to various speed and force parameters.

However, for the analysis, it is necessary to understand airfoil and identify how its geometry plays an important role in assessing aerodynamic performance. This article aims to explore cambered and symmetrical airfoil in detail. 

Airfoil and Lift

Airfoil, in aerodynamic terms, is the cross-sectional shape of an object, such as wings, rotors, or sails, that is used to generate a lift. The airfoil is streamlined in shape and can be of many designs for different speed requirements. When the airfoil is subjected to flow, its streamlined shape allows the flow to split between the upper and lower surfaces of the airfoil. The pressure in the upper part of the airfoil decreases as the flow stretches over the curved upper surface as compared to the flat lower section where the speed and pressure of the flow remain the same. The resulting pressure difference helps in creating a lift.

Airflow diagram in an airfoil

 

This is the exact mechanism that allows aircraft such as planes or helicopters to stay up in the air. As the rotor blade passes through the air, the downward force on the airfoil helps to develop a significant lift, enabling a successful flight.

However, the lift produced is different depending upon the shape of the airfoil. Usually, the airfoil can be classified based on geometry as either asymmetrical or symmetrical airfoil. 

Symmetrical Airfoil

The airfoil is considered to be symmetrical or uncambered when the upper section of the airfoil from the centerline mirrors that of the lower section, i.e., the upper and lower surfaces are identical. Symmetrical airfoil produces less lift than asymmetrical airfoil.

Airfoil parameters

A typical airfoil is presented in the figure above with:

Chamber line: The line equidistant from the upper and lower surface of the airfoil. 

Chord line:  The imaginary strength line that joins the leading line to the trailing line of the airfoil.

Angle of attack: The angle between the flow direction and the chord line. The larger the angle of attack, the greater the lift.

In the case of the symmetrical airfoil, the chamber line and cord line are the same. At zero angle of attack, the symmetrical airfoil produces no lift. Thus, a positive angle of attack is required if the lift is to be generated. The center of pressure remains relatively constant for a small change in angle of attack. Thus, for a wide range of velocities, symmetric airfoil provides a good lift-drag ratio.

Typically, symmetrical airfoil is incorporated in helicopter rotors or lightweight aerobatic airplanes. The pressure balance provided by symmetric airfoil allows precision performance for such lightweight aircraft.   

Comparing Symmetrical and Cambered Airfoil

Airfoil selection is an important aspect of design in aerospace engineering. Understanding the characteristics of different airfoil sections is important to identify the lift and drag produced during flight. Between the basic airfoil designs: symmetric and cambered or asymmetric airfoil, designers must make a careful selection of design that enhances the flight characteristics for optimal aircraft performance. Below is a comparison between the two:

Symmetrical Airfoil

Cambered Airfoil

  • Identical upper and lower surface design.
     
  • Higher stalling speed.
     
  • Provides a comparatively lower lift-to-drag ratio.
     
  • At zero angle of attack, lift cannot be produced.
     
  • Easy to design.
  • Wide range of upper and lower surface designs.
     
  • Facilitates low stalling speed.
     
  • Provides a higher lift-to-drag ratio for favorable aircraft design.
     
  • At zero angle of attack, a significant lift can be produced. 
     
  • Has considerable design challenges.

CFD Analysis of Symmetric Airfoil

In aerospace analysis, flight characteristics examination is important at various speeds and flow regimes. CFD analysis facilitates the analysis of symmetrical airfoil for aerodynamical efficiency. Not only does it enable visualization of flow over the airfoil geometry, but it also captures the effect of a wide range of velocities and changes in pressures, enabling the calculation of lift and drag for optimal design selection. This is made easier with the simulation and meshing capabilities provided by a CFD solver.

The excellent simulation capabilities of Omnis and mesh generation capabilities of Pointwise facilitate the characterization of a wide range of airfoil designs. Through high-fidelity CFD simulation, airfoil behavior can be analyzed for different flow regimes. 

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