pub fn axial_drag_alpha_factor(alpha_rad: f64) -> Result<f64, AeroError>Expand description
The scaling of axial drag with angle of attack, C_A(α) = C_D0 f(α) (Niskanen 2009 §3.4.7),
with C_A positive along −z_B (toward the tail).
Niskanen describes, without coefficients, a two-part polynomial from f = 1 at α = 0 up to
1.3 at 17° and down to 0 at 90°, with zero slope at all three. hpr uses the lowest-degree
polynomials that meet those conditions, a cubic on each part:
f = 1 + 0.3 (3t² − 2t³), t = α/17°, 0 ≤ α ≤ 17°
f = 1.3 (1 − 3u² + 2u³), u = (α − 17°)/73°, 17° ≤ α ≤ 90°Past 90° the flow meets the tail first and drag pushes toward the nose; hpr mirrors with the
sign reversed, f(α) = −f(180° − α) (an assumption; the source stops at 90°), so f is
continuous through 0 at 90°. The coefficients are derived, not published (the decision record
on subsonic drag, ADR-009).
§Errors
AeroError::Domain for an angle outside [0, π].