Pitch-compensated lift feedforward: hold altitude under forward thrust

Reported symptom: the ship sinks as soon as it moves forward. Confirmed in
code -- altitude.lua had no pitch input at all. Pitch only fed the front/aft
differential; nothing compensated the common-mode lift. So when the propeller
pushes the nose up, the lift vector tilts with the hull and its vertical
component drops ~cos(pitch), and only the slow altitude integrator (~5-10 s)
fought it -> a multi-block sag every time you accelerate.

- altitude.lua takes measured pitch and boosts common-mode lift by 1/cos(pitch),
  bounded by cfg.pitchLiftMax (default 1.5 = full comp to ~48 deg, 1.0 = off).
  Sign-agnostic (cos is even); feedforward adds no loop gain so it can't
  destabilize -- altitude feedback still owns steady-state.
- FLY HUD shows the live boost (liftFF xN.NN); u_sum in fly.log reflects it.

Physically motivated but unverified against the mod's exact lift model: if lift
were world-vertical the ship would climb slightly on accel instead (feedback
corrects it) -- still better than sinking. Tune pitchLiftMax from flight.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

files/autopilot/autopilot/altitude.lua

@@ -22,8 +22,9 @@ function M.new(cfg, calib)
 end
 
 -- vMeas: MAALT vertikal fart (b/s). dt: faktisk steg (s). saturated: mixeren klemte en utgang
--- (anti-windup -> frys integralet). Returnerer u_front_common, u_rear_common + diag.
-function Alt:compute(altitude, target, vMeas, dt, saturated, lowAltTighten)
+-- (anti-windup -> frys integralet). pitchDeg: MAALT pitch (grader) for loft-feedforward.
+-- Returnerer u_front_common, u_rear_common + diag.
+function Alt:compute(altitude, target, vMeas, dt, saturated, lowAltTighten, pitchDeg)
   local cfg, calib = self.cfg, self.calib
   local uHoverSum = calib.uHoverFront + calib.uHoverRear   -- noeytral baseline; uTrim laerer resten
 
@@ -46,10 +47,24 @@ function Alt:compute(altitude, target, vMeas, dt, saturated, lowAltTighten)
   end
   local u_sum = utils.clamp(uHoverSum + self.uTrim + cfg.Kp_climb * vErr, 2 * cfg.absFloor, 2 * cfg.maxSignal)
 
+  -- PITCH-KOMPENSERT LOFT (feedforward). Naar skroget pitcher (typisk fordi propellen dytter og
+  -- nesa loefter seg), tilter loftvektoren med skroget og den VERTIKALE komponenten faller ~cos(pitch)
+  -- -> skipet synker "saa fort du gir framdrift". Vi gjenoppretter den vertikale komponenten
+  -- UMIDDELBART (1/cos), bundet, i stedet for aa vente paa den trege loft-integratoren. Fortegns-
+  -- uavhengig (cos er jamn): nese opp ELLER ned taper vertikalt loft likt. Feedforward tilfoerer
+  -- ingen sloyfe-gain -> destabiliserer ikke; hoyde-feedbacken eier fortsatt steady-state.
+  local ffBoost = 1.0
+  if pitchDeg then
+    local c = math.cos(math.rad(pitchDeg))
+    if c > 0.2 then ffBoost = utils.clamp(1 / c, 1.0, cfg.pitchLiftMax or 1.0) end
+  end
+  u_sum = utils.clamp(u_sum * ffBoost, 2 * cfg.absFloor, 2 * cfg.maxSignal)
+
   -- fordel paa segmentene etter trim-forholdet (pitch-loopen legger differansen oppaa)
   local u_front = u_sum * calib.uHoverFront / uHoverSum
   local u_rear  = u_sum * calib.uHoverRear  / uHoverSum
-  return u_front, u_rear, { v_des = v_des, vErr = vErr, uTrim = self.uTrim, u_sum = u_sum, hold = self.hold }
+  return u_front, u_rear,
+    { v_des = v_des, vErr = vErr, uTrim = self.uTrim, u_sum = u_sum, hold = self.hold, pitchFF = ffBoost }
 end
 
 return M

files/autopilot/autopilot/config.lua

@@ -46,6 +46,11 @@ return {
   vMaxDown    = 0.8,      -- b/s maks synkerate
   altDeadband = 3.0,      -- blokker -- innenfor baandet: HOLD, ikke chase eksakt hoyde
   altBandHyst = 2.0,      -- ekstra margin foer vi forlater baandet (hysterese mot grensesyklus)
+  -- Pitch-kompensert loft (feedforward, §altitude): naar skroget pitcher taper loftet sin vertikale
+  -- komponent (~cos(pitch)) og skipet synker naar du gir framdrift. Vi booster felles-loftet med
+  -- 1/cos(pitch), bundet av dette taket. 1.0 = AV. 1.5 = full komp opp til ~48 grader pitch.
+  -- Hev om skipet fremdeles synker under framdrift; senk om det STIGER naar du akselererer.
+  pitchLiftMax = 1.5,
 
   -- ============ HOYDE: G-FRI FART-PI (Lag 6, self-learning) ============
   -- Erstatter den g-avhengige modell-inversjonen (svingte fordi g/hover var ukalibrert). uTrim er

files/autopilot/autopilot/main.lua

@@ -785,7 +785,7 @@ local function runFly()
     local frontHeat, rearHeat, sat, diag, u_diff = nil, nil, true, {}, 0
     if state == "FLYING" and valid then
       local lowAlt = (est.altitude - launchAlt) < cfg.lowAltCaution
-      local uFc, uRc, ad = actl:compute(est.altitude, target, est.verticalVelocity, dt, prevSat, lowAlt)
+      local uFc, uRc, ad = actl:compute(est.altitude, target, est.verticalVelocity, dt, prevSat, lowAlt, est.pitch)
       u_diff = pctl:compute(est.pitch, est.pitchRate, dt, prevSat)
       if not cfg.aftSide then u_diff = 0 end   -- ett-ballong-skip: ingen differensiell autoritet
       local floor
@@ -832,7 +832,7 @@ local function runFly()
     print(string.format("Pitch  %7.1f  pRate %+.1f  trim %+.2f", est.pitch, est.pitchRate, pctl.iTrim))
     print(string.format("v_meas %6.2f  v_hat %6.2f  (e_v %+.2f)", est.verticalVelocity, obs.v_hat, e_v))
     print(string.format("u_sum %.1f  hTrim %+.2f  vDes %+.2f", diag.u_sum or 0, actl.uTrim, diag.v_des or 0))
-    print(string.format("fore %2d   aft %2d", of, oa))
+    print(string.format("fore %2d   aft %2d   liftFF x%.2f", of, oa, diag.pitchFF or 1.0))
 
     -- kringkast status til pilot-konsollen
     link.publishStatus({