Function terminal_speed_m_s 

pub fn terminal_speed_m_s(
    mass_kg: f64,
    drag_area_m2: f64,
    density_kg_m3: f64,
    gravity_m_s2: f64,
) -> f64

The equilibrium descent speed v_e = √(2 m g/(ρ C_D S)), m/s (Knacke, printed page 5-128).

It is the speed at which the drag area’s drag balances the weight, so it is also the speed a long descent settles at.

The formula is evaluated as written, with no domain checks: a zero drag area or density gives infinity, and a negative mass, density, drag area or gravity gives NaN. Flights validate their devices instead (crate::Simulation::with_recovery).