Function ellipsoid_subsonic_pressure_drag 

pub fn ellipsoid_subsonic_pressure_drag(
    fineness_ratio: f64,
    mach: f64,
) -> Result<f64, AeroError>

An elliptical nose’s (or shoulder’s) pressure drag below Mach 0.8, on its increase in area, interpolated in fineness between the forebody pressure drags Hoerner measured at low speed (1965 p. 3-12, Fig. 20) (ADR-173, a blunt ellipsoid’s measured drag):

• from the hemisphere (f = ½) on, the straight line through the hemisphere’s 0.01 and the round head’s −0.05 at f = 1, held at 0 once it gets there (at f = 7/12): a head at least that long is charged no pressure drag, as eq. 3.86 charges a tangent joint none;
• blunter, hpr’s interpolation (Fig. 20 measures no ellipsoid between): eq. B.9’s form between the flat face at f = 0 (the blunt cylinder’s 0.85 q_stag/q, eq. B.2, at the same Mach number) and the hemisphere’s 0.01, C₀ (0.01/C₀)^(ln(f + 1)/ln 1.5): a step at f = 0, the hemisphere at f = ½.

From the hemisphere up the value holds unchanged to Mach 0.8, and blunter heads follow the flat face’s rise with Mach number; that is hpr’s assumption: the measurement is at low speed, and the drag rise starts near the critical Mach number (the surface’s first sonic point, about 0.65 to 0.7 for a hemisphere by the Prandtl–Glauert rule). A 0.577-calibre ellipsoid gets 0.0008, a ¼-calibre one 0.074 at Mach 0.

Errors

AeroError::Domain for a non-positive or non-finite fineness ratio (a zero-length nose is a flat face, PressureDragCurve::step), or a Mach number outside [0, 0.8).