Aerodynamics

OpenConcept’s aerodynamics components provide an estimate of drag as a function of lift coefficient, flight conditions, and other parameters.

Simple drag polar: PolarDrag

PolarDrag is the most basic aerodynamic model and uses a parabolic drag polar. This component computes the drag force given the flight condition (dynamic pressure and lift coefficient) at each node along the mission profile. In run script, users should set the values for the following aircraft design parameters, or declare them as design variables.

Aircraft design variables for drag polar model

Variable name	Property
ac|geom|wing|S_ref	Reference wing area
ac|geom|wing|AR	Wing aspect ratio
e	Wing Oswald efficiency
CD0	Zero-lift drag coefficient

Using OpenAeroStruct

Instead of the simple drag polar model, you can use OpenAeroStruct to compute the drag. This allows you to parametrize the aircraft wing design in more details and explore the effects of wing geometry, including taper, sweep, and twist. OpenAeroStruct implements the vortex-lattice method (VLM) for aerodynamics and beam-based finite element method (FEM) for structures (in the case of the aerostructural drag polar). For more detail, please check the documentation.

The wing is currently limited to simple planform geometries. Additionally, the wing does not include a tail to trim it because there is no OpenConcept weight position model with which to trim.

OpenConcept uses a surrogate model trained by OpenAeroStruct analyses to reduce the computational cost. The data generation and surrogate training is automated; specifying the training grid manually may improve accuracy or decrease computational cost.

VLM-based aerodynamic model: VLMDragPolar

This model uses the vortex-lattice method (VLM) to compute the drag. The inputs to this model are the flight conditions (Mach number, altitude, dynamic pressure, lift coefficient) and aircraft design parameters.

Users should set the following design parameters and options in the run script.

Aircraft design variables for VLM-based model

Variable name	Property	Type
ac|geom|wing|S_ref	Reference wing area	float
ac|geom|wing|AR	Wing aspect ratio	float
ac|geom|wing|taper	Taper ratio	float
ac|geom|wing|c4sweep	Sweep angle at quarter chord	float
ac|geom|wing|twist	Spanwise distribution of twist, from wint tip to root.	1D ndarray, lendth num_twist
ac|aero|CD_nonwing	Drag coefficient of components other than the wing; e.g. fuselage, tail, interference drag, etc.	float
fltcond|TempIncrement	Temperature increment for non-standard day	float

Options for VLM-based model

Variable name	Property	Type
num_x	VLM mesh size (number of vertices) in chordwise direction.	int
num_y	VLM mesh size (number of vertices) in spanwise direction.	int
num_twist	Number of spanwise control points for twist distribution.	int

There are other advanced options, e.g., the surrogate training points in Mach-alpha-altitude space. The default settings should work fine for these advanced options, but if you want to make changes, please refer to the source docs.

Aerostructural model: AeroStructDragPolar

This model is similar to the VLM-based aerodynamic model, but it performs aerostructural analysis (that couples VLM and structural FEM) instead of aerodynamic analysis (just FEM). This means that we now consider the wing deformation due to aerodynamic loads, which is important for high aspect ratio wings. The structural model does not include point loads (e.g., for the engine) or distributed fuel loads.

The additional input variables users need to set in the run script are listed below. Like the num_twist option, you may need to set num_toverc, num_skin, and num_spar.

Additional design variables for aerostructural model

Variable name	Property	Type
ac|geom|wing|toverc	Spanwise distribution of thickness to chord ratio.	1D ndarray, lendth num_toverc
ac|geom|wing|skin_thickness	Spanwise distribution of skin thickness.	1D ndarray, lendth num_skin
ac|geom|wing|spar_thickness	Spanwise distribution of spar thickness.	1D ndarray, lendth num_spar

In addition to the drag, the aerostructural model also outputs the structural failure indicator (failure) and wing weight (ac|weights|W_wing). The failure variable must be negative (failure <= 0) to constrain wingbox stresses to be less than the yield stress.

Understanding the surrogate modeling

OpenConcept uses surrogate models based on OpenAeroStruct analyses to reduce the computational cost for mission analysis. The surrogate models are trained in the 3D input space of Mach number, angle of attack, and altitude. The outputs of the surrogate models are CL and CD (and failure for AeroStructDragPolar).

For more details about the surrogate models, see our paper.

Other models

The aerodynamics module also includes a couple components that may be useful to be aware of:

• StallSpeed, which uses \(C_{L, \text{max}}\), aircraft weight, and wing area to compute the stall speed

• Lift, which computes lift force using lift coefficient, wing area, and dynamic pressure