pub fn side_sum(count: u32, base_angle_rad: f64, flow_roll_rad: f64) -> f64Expand description
Σ sin(φ − θ_k) cos(φ − θ_k) over count evenly spaced fins at θ_k in a lateral airflow at
φ: the side-force share, perpendicular to the flow’s plane. Each fin sees the local angle
α sin Λ_k (Niskanen 2009 eq. 3.50) and pushes along its own normal; eq. 3.51 keeps the part of
that push in the flow’s plane, sin² Λ_k, and this is the part across it. The sum vanishes for
three or more fins; for one or two it doesn’t: two fins at 45° to the flow push along their
common normal, √2 times their in-plane share. Derived here from eq. 3.50; Niskanen drops it.