Static Aeroelastic Analysis

The vast majority of lift for a fixed-wing aircraft is from the wings. The horizontal stabilizer and the elevator also produce some lift, part of which is also used to maneuver the aircraft.

Control surfaces are parts of the aircraft structure which can be used to control and maneuver the aircraft. The major flight control surfaces on typical fixed-wing aircraft are the ailerons, elevators, and rudder. Ailerons allow control over the roll of the aircraft, which allows turning the aircraft left or right, elevators control the pitch, and the rudder controls the yaw of the aircraft, which also aids in turning the aircraft left or right.

Trimming an aircraft allows it to maintain a set attitude without any control input. Trimming can be accomplished by adjusting the aerodynamic forces and moments on an aircraft by adjusting the control surfaces. In actual aircraft, trim adjustments control the movement of trim tabs on the major control surfaces (ailerons, elevators, and rudder) which subsequently move the actual control surfaces, due to local aerodynamic effects. Once trimmed, the aircraft can hold the expected attitude, without control input (for example, manual pilot effort to hold the yoke is not required if the pitch trim is set correctly to hold the expected pitch attitude, by trimming the elevator).

A trim analysis determines unknown trim values for the unconstrained trim variables listed on the AESTAT entry and the control surface via AESURF entry. Any trim variables can be defined and constrained on the TRIM Bulk Data Entry. Trim variables not constrained by the TRIM Bulk Data Entry, are considered as variables in the equation of motion solution for static aeroelastic trim analysis and are determined as part of the trim solution. Additionally, aerodynamic forces and pressures on the aerodynamic elements may be obtained via the AEROF and APRES Case Control commands, respectively.

Divergence can occur when deflection of lifting surfaces of an aircraft leads to additional lift, which in turn, leads to further deflection in the same direction. A divergence analysis determines divergence dynamic pressures. The lowest eigenvalue correlates with the critical divergence dynamic pressure.

The structural model can be defined using traditional finite elements, along with their corresponding grid points. For instance, the wing of an aircraft can be modeled with 1D or 2D elements.

The aerodynamic model can be constructed with panels (CAERO1 entry, with corresponding PAERO1 property). The panels are divided into boxes, based on the input defined on the CAERO1 entry.

Control surfaces can be defined using the AESURF entry (along with AELIST), which reference the boxes on the panels already defined via CAERO1 entry. Any CAERO1 panel boxes which are not referenced as control surfaces are part of the non-control entities which can generate aerodynamic lift (for example, the parts of an aircraft wing not associated with control surfaces).

The AEROS Bulk Data Entry can be used to define the reference aerodynamic coordinate system.

Symmetry along the center line can be defined on the AESYMXZ and AESYMXY I/O Entries, and if such entries are not defined, then it is interpreted from the SYMXZ and SYMXY fields, respectively of the AEROS Bulk Data Entry.

Splines are entities which connect the aerodynamic degrees of freedom with the structural degrees of freedom, via interpolation. For definition of splines, the SPLINE1 (surface spline), SPLINE2 (beam spline), and SPLINE4 (curved surface spline) Bulk Data Entries are available. Splines interpolate motion and/or forces between aerodynamic degrees of freedom and structural degrees of freedom. Splines require the input of both aerodynamic model box IDs and corresponding structural grid SETs that are to be connected.

Trim analysis determines the unknown trim variables to trim the aircraft for a particular attitude or flight loading condition. All trim variables of interest for a particular flight loading condition can be listed on the AESTAT Bulk Data Entries, and the control surfaces can be defined on the AESURF Bulk Data Entries. Apart from the unknown trim variables of interest, all other trim variables should be constrained via the TRIM Bulk/Subcase Data Entry pair. Rigid body motions that are constrained by the TRIM Bulk Data should be defined on the SUPORT Bulk Data Entry to balance the rigid body motion in those degrees of freedom. Any rigid body motion that is not defined as a trim variable should be constrained via SPC/SPC1 Bulk Data Entries.

The Mach number and the dynamic pressure should be input on the TRIM Bulk Data Entry. If a rigid trim analysis is required, this can be requested by setting the AEQR field to 0.0 on the TRIM Bulk Data Entry. The rigid trim analysis ignores the effect of structural deformation on aerodynamic loading. Links between AESTAT and AESURF entries can be defined using the AELINK entry.

The parameter PARAM, AUNITS, can be used to switch the interpreted units of acceleration on TRIM Bulk Data from units of gravity (g’s) to physical units of distance per time squared.

Divergence analysis determines the divergence dynamic pressures which are the eigenvalues from a complex eigenvalue analysis. The analysis is activated by a DIVERG Subcase Entry pointing to a corresponding DIVERG Bulk Data Entry. The DIVERG Bulk Data Entry contains information regarding the number of eigenvalues to be extracted and the Mach numbers for which these eigenvalues are to be extracted. A CMETHOD Case Control Entry referencing a EIGC Bulk Data Entry should be specified to activate complex eigenvalue extraction.