Aerodynamic elements are boxes or segments of bodies that are combined to idealize the vehicle for the computation of aerodynamic forces. These elements, like structural elements, are defined by their geometry and their motions are defined by degrees of freedom at aerodynamic grid points. Requirements of the aerodynamic theory dictate the geometry of the boxes. The Doublet‑Lattice (DLM) and ZONA51 methods assume trapezoidal boxes with their edges parallel to the free‑stream velocity. By the use of aerodynamic input data, aerodynamic elements and grid points are automatically generated to help ensure that many of the theoretical requirements are met.

Aerodynamic calculations are performed using a Cartesian coordinate system. By the usual convention, the flow is in the positive X‑direction, and the X‑axis of every aerodynamic element must be parallel to the flow in its undeformed position. (This is an assumption of aerodynamic small disturbance theory.) The structural coordinate systems may be defined independently, since the use of the same system for both may place an undesirable restriction upon the description of the structural model. Any MSC.Nastran Cartesian system may be specified for the aerodynamic coordinates, with the resulting flow defined in the direction of the X‑axis. All aerodynamic element and grid point data are transformed to the aerodynamic coordinate system. All the global (displacement) coordinate systems of the aerodynamic grid points will have their T1‑directions in the flow direction. Their T3‑directions will be normal to the element in the case of boxes, and parallel to the aerodynamic T2‑ and/or T3‑directions in the case of bodies.

The aerodynamic grid points are physically located at the centers of the boxes for the lifting surface theories and at the centers of body elements for the DLM. A second set of grid points, used only for display, is located at the element corners. Grid point numbers are generated based upon the element identification number.

Both sets of grid points are numbered beginning with the user provided ID of the lifting surface. The centroidal grids are numbered from the inboard leading edge box and then incremented by one, first in the chordwise direction and then in the spanwise direction. The corner grid numbering begins at the leading edge inboard corner and again proceeds first chordwise and then spanwise. In terms of the graphical display, the centroidal grids can be thought of as element ID’s and corner points as node ID’s.

The aerodynamic theories in MSC.Nastran have additional downwash locations that need to be defined here. These points are designated as comprising the j‑set of aerodynamic control points. The j‑set is not a user set; it is a notational set to identify aerodynamic matrices used in the solution processing. Physically, these are points on the element where the downwash vectors are computed. The location of these points is a function of the aerodynamic method employed: