R2-b/R3: embedded3_added_mass (the cut-cell added-mass pin, the overset instrument's parameters: n 64/128 C_m 1.087/1.127 vs the confined potential-flow 1.071 + Stokes 0.064 = 1.135, the overset's 1.097/1.127); flag tests: the capsule's tip inset (RTX_E3_FLAG_TIP_INSET, default FLAG_HALF — the apex ON the benchmark's tip A; every earlier record had it 10 mm beyond, =0 restores), root fillet knob RTX_E3_FLAG_FILLET (5 mm: no effect), the tip CSV column = the record's tip in recorded mode

Co-Authored-By: Claude Fable 5.1 <[email protected]>

crates/specialized/rtx-cfd/tests/embedded3_flag_reference_2d.rs

@@ -50,9 +50,34 @@ fn deflection(s: f64, t: f64) -> (f64, f64) {
     )
 }
 
+/// R3 (2026-09-21): the capsule's tip inset (`RTX_E3_FLAG_TIP_INSET`,
+/// metres, default `FLAG_HALF`): the last centreline point is pulled back
+/// along the last segment so the capsule's apex sits ON the benchmark's
+/// tip A. Every record before this date had the apex 10 mm beyond A
+/// (`=0` restores them).
+fn tip_inset() -> f64 {
+    std::env::var("RTX_E3_FLAG_TIP_INSET")
+        .ok()
+        .and_then(|v| v.parse().ok())
+        .unwrap_or(FLAG_HALF)
+}
+
 /// The centreline point `m` of `n` at `t`: (x, y, vx, vy) — the first
-/// mode, or (S2-9) the recorded FSI2 kinematics along its stations.
+/// mode, or (S2-9) the recorded FSI2 kinematics along its stations; the
+/// last point carries the tip inset.
 fn centreline(m: usize, n: usize, t: f64) -> (f64, f64, f64, f64) {
+    let inset = tip_inset();
+    if m < n || inset <= 0.0 {
+        return centreline_raw(m, n, t);
+    }
+    let (ax, ay, _, _) = centreline_raw(n - 1, n, t);
+    let (bx, by, bvx, bvy) = centreline_raw(n, n, t);
+    let len = ((bx - ax).powi(2) + (by - ay).powi(2)).sqrt();
+    let f = (1.0 - inset / len).max(0.0);
+    (ax + f * (bx - ax), ay + f * (by - ay), bvx, bvy)
+}
+
+fn centreline_raw(m: usize, n: usize, t: f64) -> (f64, f64, f64, f64) {
     let s = m as f64 / n as f64;
     if let Some(rec) = recorded() {
         thread_local! {