Plane: fixed climb rate for quadplanes in CRUISE

the high loop rate of quadplanes led to less than 1cm/loop change in
height, which got truncated to zero. Adjusting height at 10Hz fixes
that.

Thanks to Marco for reporting this!

ArduPlane/Plane.h

@@ -712,6 +712,12 @@ private:
         // lookahead value for height error reporting
         float lookahead;
 #endif
+
+        // last input for FBWB/CRUISE height control
+        float last_elevator_input;
+
+        // last time we checked for pilot control of height
+        uint32_t last_elev_check_us;
     } target_altitude {};
 
     float relative_altitude = 0.0f;

ArduPlane/navigation.cpp

@@ -248,29 +248,38 @@ void Plane::update_cruise()
  */
 void Plane::update_fbwb_speed_height(void)
 {
-    static float last_elevator_input;
-    float elevator_input;
-    elevator_input = channel_pitch->get_control_in() / 4500.0f;
+    uint32_t now = micros();
+    if (now - target_altitude.last_elev_check_us >= 100000) {
+        // we don't run this on every loop as it would give too small granularity on quadplanes at 300Hz, and
+        // give below 1cm altitude change, which would result in no climb or descent
+        float dt = (now - target_altitude.last_elev_check_us) * 1.0e-6;
+        dt = constrain_float(dt, 0.1, 0.15);
+
+        target_altitude.last_elev_check_us = now;
+        
+        float elevator_input = channel_pitch->get_control_in() / 4500.0f;
     
         if (g.flybywire_elev_reverse) {
             elevator_input = -elevator_input;
         }
 
-    change_target_altitude(g.flybywire_climb_rate * elevator_input * perf.delta_us_fast_loop * 0.0001f);
+        int32_t alt_change_cm = g.flybywire_climb_rate * elevator_input * dt * 100;
+        change_target_altitude(alt_change_cm);
         
-    if (is_zero(elevator_input) && !is_zero(last_elevator_input)) {
+        if (is_zero(elevator_input) && !is_zero(target_altitude.last_elevator_input)) {
             // the user has just released the elevator, lock in
             // the current altitude
             set_target_altitude_current();
         }
         
+        target_altitude.last_elevator_input = elevator_input;
+    }
+    
     // check for FBWB altitude limit
     check_minimum_altitude();
 
     altitude_error_cm = calc_altitude_error_cm();
     
-    last_elevator_input = elevator_input;
-    
     calc_throttle();
     calc_nav_pitch();
 }