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XFOIL Study Notes (3)

September 29th, 2011 No comments

Workflow

The typical workflow of airfoil analysis using XFOIL is: Run XFOIL -> Load Airfoil Shape (using LOAD command or NACA command) -> Run Analysis (using OPER command) -> Plot results (using PPLO command).

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XFOIL Study Notes (2)

September 29th, 2011 No comments

Units

Most XFOIL operations are performed on the airfoil’s cartesian coordinates x,y, which do not necessarily have a unit chord c. Since the chord is ambiguous for odd shapes, the XFOIL force coefficients C_L, C_D, C_M are obtained by normalizing the forces and moment with only the freestream dynamic pressure (the reference chord is assumed to by unity). Likewise, the XFOIL Reynolds number RE is defined with the freestream velocity and viscosity, and an implied unit chord:

Force Coefficients in XFOIL

The conventional definitions are:

Conventional Force Coefficients

It can be seen that the conventional and XFOIL definitions differ only by the chord factor c or c^2.

Categories: Engineering Tags:

XFOIL Study Notes (1)

September 29th, 2011 No comments

Buffer Airfoil and Current Airfoil

XFOIL uses the concept of a “current airfoil” and “buffer airfoil”. These are the airfoils on which various calculations are performed, and they are distinct from the “polar” x,y coordinates. The “polar” x,y are simply archived data, and do not directly participate in computation. The “polar” x,y must first be transferred into the current airfoil if they are to be used for computation.

Some notes on buffer airfoil and current airfoil:

  • When the buffer airfoil coordinates are read from a file during startup, or read in via the LOAD command, they are by default also copied directly into the current airfoil.
  • The NACA command automatically invokes the paneling routine to create a current airfoil with a suitable paneling.
  • In some cases it is desirable to explicitly re-copy the buffer airfoil into the current airfoil via the PCOP command.
  • If the normalization flag is set (toggled with the NORM command), the buffer airfoil coordinates will be normalized to unit chord and the leading edge will be placed at the origin.
  • The GDES facility allows very extensive manipulation of the buffer airfoil.
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