Caveats

The XFOIL code is not foolproof, and requires some level of aerodynamic expertise and common sense on the part of the user. Although the inviscid analysis (OPERi), geometry design (GDES), and Full-Inverse (MDES) routines are nearly invulnerable to failure, the Mixed-Inverse (QDES) design routines and especially the viscous analysis (OPERv) routines will fail if a “reasonable” problem is not specified.
Typical failure scenarios are:

• Viscous Analysis (OPERv)
  • Massive separation from excessive airfoil thickness, flap deflection, or angle of attack
  • Inherently unsteady flow (von Karman vortex street, etc.)
  • Poor resolution of leading edge pressure spike
  • Poor resolution of small viscous features (e.g. separation bubbles)
  • Reynolds number too low
• Mixed-Inverse Surface Speed Design (QDES)
  • Re-entrant airfoil shape (negative thickness)

A possible consequence of these occurences is an arithmetic fault causing program failure. This is unlikely, but it does happen occasionally. It is therefore a good idea to save any previous work before an ambitious calculation is attempted.

The following situations may give problems strictly due to numerical roundoff:

• Excessively small panel(s) somewhere on the airfoil
• Airfoil located too far from origin
• Airfoil too thin

These situations will rarely result in an arithmatic failure, but will typically result in a “ragged” Cp distribution. Examine the paneling in the GDES menu, making the GSET command if neceesary to set the current paneling. Eliminate excessively small panels my deleting one or more panel nodes with the DELP command.

When performing viscous analysis calculations, it is always a good idea to sequence runs so that alpha does not change too drastically from one case to another. The Newton solution method always uses the last available solution as a starting guess for a new solution, and works best if the change from the old to the new solutions is reasonably small. For this reason, it is best to perform difficult calculations (such as past CLmax) by gradually increasing alpha. The ASEQ command in OPER is convenient for this. If the user insists on a large change from one point to another, it is best to force a re-initialization of the boundary layers with the INIT command from the VPAR menu in OPER before the radical calculation is performed. INIT should always be executed whenever the viscous solution blows up but the program doesn’t crash.

The viscous analysis will execute no more Newton iterations than set by the current iteration limit each time an ALFA, CL, etc. command is issued. If convergence is not achieved within this limit, ALFA or CL can be issued as often as needed (most easily with “!”), with another set of Newton iterations being performed each time. This iteration limit can be changed from its default value of 10 with the ITER command in OPER.

One should always be wary of trusting solutions which show regions of supersonic flow. Such flows can be reliably predicted only with a truly nonlinear field method (such as the MSES code). As a rule of thumb, if the maximum Mach number doesn’t exceed 1.05 anywhere, shock losses will be very small, the Cp distributions will be reasonably accurate, and the drag predicted by XFOIL is likely to be accurate.