Digital DATCOM Forces And Moments — Robotics/Aerodynamics

Digital DATCOM Forces and Moments

Robotics / Aerodynamics

Reads the stability and control coefficients a USAF Digital DATCOM run printed, looks them up at the flight condition, and turns them into forces and moments about the moment reference centre:

C_s = [C_D, C_Yββ, C_L, C_lββ, C_m, C_nββ](α, M, h)
C_d = [0, C_Ypp_s, C_Lα̇α̇ + C_Lqq_s, C_lpp_s + C_lrr_s, C_mqq_s + C_mα̇α̇, C_npp_s + C_nrr_s] · [1, b, c̄, b, c̄, b] / 2|V|
F = R(α_V) · q̄S[−C₀, C₁, −C₂], M = R(α_V) · q̄S[bC₃, c̄C₄, bC₅], C = C_s + C_d

The coefficients are per degree, so α, β and the rates enter in degrees; p_s = p cosα_V + r sinα_V, q_s = q − α̇ and r_s = r cosα_V − p sinα_V are the stability-axis rates, α_V the incidence of V and R(α_V) the stability-to-body rotation. S, c̄ and b are the file's SREF, CBARR and BLREF. The dynamic part C_d exists only when the case carries the DAMP card.

Ports

• alpha – the angle of attack α, radians, [1,1].
• beta – the sideslip angle β, radians, [1,1].
• Mach – the Mach number M, [1,1].
• h – the altitude, metres (feet in the English units), [1,1].
• qbar – the dynamic pressure q̄, pascals (psi in the English units), [1,1].
• V – the velocity, [3,1], m/s (ft/s or knots): body axes, or wind axes when Force Axes is Wind, when it is first turned into body axes through the α and β inputs.
• alpha_dot – α̇, rad/s, [1,1]. DAMP cases only.
• pqr – the body rates [p; q; r], rad/s, [3,1]. DAMP cases only.
• F – the forces, [3,1], newtons (pounds): body axes, or [F_x; F_y; F_z] in wind axes.
• M – the moments, [3,1], N·m (ft·lb), always body axes.

The block has eight inputs on a fresh block. A case without the DAMP card takes six: remove the last two input ports, and the block says so if the count and the file disagree.

Parameters

• DATCOM File – the path of the output file DATCOM wrote (for006.dat), 1976 version. Empty (the default) selects the built-in example: a made-up trainer, three Mach numbers, three altitudes and seven angles of attack, with the DAMP card, in DATCOM's own layout. The file is read the way MATLAB's datcomimport reads it: the breakpoints are the $FLTCON card values (MACH, ALT, ALSCHD) and S, c̄, b the $OPTINS ones; a page counts only under the card Mach and altitude that equal the ones it prints; and every cell no page filled, every blank and every NA reads 99999. Not read, and the block says which: a metric (DIM M) case, a case without $OPTINS, the BUILD, PART, TRIM and SAVE cards, and $GRNDEF.
• Case – which case of the file, counting from 1 (default 1).
• Units – the units of the ports:
  • Metric (MKS) (default) – newtons, N·m, metres, m/s, pascals.
  • English (Velocity in ft/s) – pounds, ft·lb, feet, ft/s, psi.
  • English (Velocity in kts) – the same with the velocity in knots.
• Force Axes – Body (default) or Wind: the axes of F, and of the V input.
• Interpolation Method – None - flat (default): the table entry at the breakpoint at or below the input (the last one once the input reaches it); Linear: linear along α, then M, then h.
• Extrapolation Method – None - clip (default) or Linear: whether a Linear lookup continues the end segments past the table.
• Process Out-of-Range Input – Clip to Range (default) or Linear Extrapolation: whether an input outside the breakpoints is clipped before the lookup.
• Sampling Time (s) – zero or less inherits the solver's rate; a positive value runs the block at that period.

Code export

All ten targets: Python, MATLAB, Java, Rust, C, C++, VHDL, Verilog, SystemVerilog and PLC Structured Text. The case's tables and every setting are written into the exported code, so an export needs no file. The three HDL targets carry the arithmetic in real: simulation-only, and the forces must stay inside a Q16.16 port's ±32768 to be read from one.

Simulink bridge

Import and export, mapped to Aerospace Blockset's aerolibadyn/Digital DATCOM Forces and Moments (drawn on two lines). DATCOM File and Case cross together as the block's dcase, written as subsref(datcomimport('file',true,0),substruct('{}',{case})) and read back from that form; a dcase naming a workspace variable is reported on import, and the built-in example, having no file, is reported on export. Units → units, Force Axes → fmode, Interpolation Method → imethod, Extrapolation Method → emethod, Process Out-of-Range Input → rmethod, each one for one. action is always None: the Simulink block's warning or error on an out-of-range input has no counterpart here. The Simulink block defines no SampleTime parameter, measured, so the rate stays on the ICore side. The global Sampling Time (s) → SampleTime pair therefore does not cross.

Notes

• Algebraic and stateless, and not linear, so there is no state space.
• It reproduces, it does not repair. DATCOM prints a derivative that does not change with α once, on the first row, so C_Yβ, C_nβ, C_Lq and C_mq are 99999 at every other angle of attack – and the Simulink block multiplies that in. So does this one. Fill the column in the file to use a constant derivative.
• Verified against R2026a: the reader against datcomimport, cell for cell, on MathWorks' two example files and nine synthetic ones (twelve cases); the arithmetic against the Simulink block within 1e-11 relative over 21 configurations – every unit system, both axes, both interpolations, every extrapolation pairing, per-degree and per-radian derivatives, one-point axes, inputs exactly on the breakpoints.