Per-panel pitch rates for flow curvature - #261
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A section rotating about its own spanwise axis sees an incidence that varies linearly along the chord. In thin airfoil theory that is the same boundary condition as parabolic camber, giving A1 = q*c/(2V) and A2 = 0, hence cm_c/4 = (pi/4)(A2 - A1) = -(pi/4) q_hat. The lift half of the same effect was already exact: A0 + A1/2 collapses to the effective incidence at three-quarter chord, which is where the inflow is sampled, so no correction belongs there. Only the moment was missing, and one control point per section cannot represent it because it is an A1 term rather than an A0 shift. Off by default. Enabled through the `flow_curvature` solver setting, applied in both force paths (`calc_forces!` behind `solve!`, and `calculate_results` behind `solve`). Independent of the pivot, so it needs the rate only and not a rotation centre. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
The moment increment responds to the chordwise gradient of normal velocity across a panel. For rigid motion that gradient is omega . y_airf, but a deforming wing has a twist rate that varies along the span and no single body rate expresses it, so flapping and torsion modes could not reach the term. BodyAerodynamics now carries pitch_rate_dist, one rate per panel. The rigid set_va! fills it by projecting omega onto each y_airf, which already differs per panel under dihedral. The distributed set_va! takes it as a keyword, so a structural model can pass rates it computed itself. section_pitch_rate builds one entry from a section's leading and trailing edge velocities. It reduces to omega . y_airf for rigid motion, so one expression covers both cases and the sign convention is pinned in one place rather than rediscovered by every caller. Omitting the keyword zeroes the rates. The distributed set_va! never set omega, so reading it there would have fed the moment a stale rate from whenever the rigid form was last called. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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Folded into #260 — single PR, rebased onto main. |
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Stacked on #260 — review that one first.
Why
#260 reads the flow-curvature rate as
ω ⋅ y_airf. The projection is genuinelyper-panel (panels at different dihedral see different rates from one body rate),
but the source is a single rigid-body
ω, so a deformation field cannot beexpressed at all. Twist and flapping — the modes where this moment is largest,
because a strip's torsional inertia is small while the moment scales with
c²—could not reach the term.
What
What the moment actually responds to is the chordwise gradient of normal velocity
across the panel, not a body rate. So the input becomes that:
pitch_rate_distonBodyAerodynamics, one rate per panel, positive nose-up.set_va!(body_aero, va, omega)fills it by projecting onto eachy_airf— behaviour identical to Optional flow-curvature pitch-rate moment #260.
set_va!(body_aero, va_distribution; pitch_rate_dist)takes it directly, so astructural model passes rates it computed itself.
section_pitch_rate(velocity_leading, velocity_trailing, z_airf, chord),exported, builds one entry from a section's edge velocities.
section_pitch_ratereduces exactly toω ⋅ y_airfunder rigid motion(
v_TE − v_LE = ω × c·x_airf, andy × x = −z), so one expression covers bothcases and the sign convention — where an error flips damping into divergence — is
pinned in one place instead of being rediscovered by each caller. There is a test
asserting that reduction.
Also fixes a trap
The distributed
set_va!never setomega, so #260 would have read whatever therigid form last left there. Omitting the keyword now zeroes the rates rather than
silently reusing a stale value.
Tests
31 pass, including 16 for the distributed path: an antisymmetric twist rate that
no rigid
ωcan produce, per-panelcm_distincrements matchingflow_curvature_cmexactly, opposite signs on the two half-wings, and a lengthmismatch throwing.
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