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Assumptions & Limitations

This page documents the physical and numerical boundaries of the lightaero library. As a low-fidelity tool intended for preliminary research, the library makes several standard simplifications that limit its applicability to specific flight regimes.

Major Physical Assumptions

Aerodynamics

lightaero's primary aerodynamic solver is based on the Vortex Lattice Method (VLM).

  • Inviscid Flow: The fluid is assumed to be inviscid (no boundary layers, no separation). While profile drag is estimated using empirical form factors, it does not capture separation-induced drag.
  • Incompressible Flow: The standard VLM implementation assumes the fluid is incompressible. Compressibility effects (Prandtl-Glauert) are currently not implemented.
  • Small Perturbations: The theory assumes small angles of attack and small perturbations to the freestream.
  • Potential Flow: Flow is assumed to be irrotational, except on the vortex filaments themselves.

Validity Regime: Mach Number

Standard VLM is strictly valid for incompressible flow (Mach < 0.3). While calculations may run at higher speeds, results will be physically inaccurate as compressibility effects are ignored [Katz & Plotkin, 2001].

Validity Regime: Angle of Attack

VLM predicts linear lift growth even at very high AoA. In reality, wings stall. Results for \(\alpha > 10^\circ\) should be treated with extreme caution as viscous separation and stall are not modeled.

Structures

  • Linear Elasticity: The structural model assumes materials remain in the linear elastic regime.
  • Euler-Bernoulli Beam Theory: Wings are modeled as 1D beams. Shear deformation and large deflections are not accounted for.
  • Static Analysis: The current implementation is limited to static aeroelastic analysis.

Atmosphere

  • International Standard Atmosphere (ISA): Calculations use the ISA model (ISO 2533 / ICAO) for density and pressure.

Implementation Discrepancies

Altitude Calculation

Geometric vs. Geopotential Altitude

The current atmosphere implementation uses geometric altitude (\(z\)) as the input for density calculations, whereas the ISA standard is defined in terms of geopotential altitude (\(H\)). At 10,000 m, the error is approximately 16m (0.16%), which is generally considered acceptable for low-fidelity research but should be noted for high-precision comparisons.

Structural Dynamics

Static Analysis Only

Although the theoretical documentation provides formulas for mass matrices, the current structural discipline is limited to static analysis. Dynamic effects (flutter, gust response) are not yet implemented.

References

  • Katz, J. and Plotkin, A. Low-Speed Aerodynamics, 2nd ed., Cambridge University Press, 2001.
  • Drela, M. Flight Vehicle Aerodynamics, MIT Press, 2014.
  • ISO 2533:1975, Standard Atmosphere.