AC_PID: removed AFF from heli PID

this is not used any more. Discussed with Rob on dev call

libraries/AC_PID/AC_HELI_PID.cpp

@@ -44,10 +44,7 @@ const AP_Param::GroupInfo AC_HELI_PID::var_info[] = {
     // @User: Advanced
     AP_GROUPINFO("ILMI", 7, AC_HELI_PID, _leak_min, AC_PID_LEAK_MIN),
 
-    // @Param: AFF
-    // @DisplayName: Acceleration FF FeedForward Gain
-    // @Description: Acceleration FF Gain which produces an output value that is proportional to the change in demanded input
-    AP_GROUPINFO("AFF",    8, AC_HELI_PID, _aff, 0),
+    // index 8 was for AFF, now removed
 
     AP_GROUPEND
 };
@@ -57,7 +54,6 @@ AC_HELI_PID::AC_HELI_PID(float initial_p, float initial_i, float initial_d, floa
     AC_PID(initial_p, initial_i, initial_d, initial_imax, initial_filt_hz, dt)
 {
     _vff = initial_vff;
-    _aff = 0;
     _last_requested_rate = 0;
 }
 
@@ -67,22 +63,6 @@ float AC_HELI_PID::get_vff(float requested_rate)
     return _pid_info.FF;
 }
 
-float AC_HELI_PID::get_aff(float requested_rate)
-{
-    float derivative;
-
-    // calculate derivative
-    if (_dt > 0.0f) {
-        derivative = (requested_rate - _last_requested_rate) / _dt;
-    } else {
-        derivative = 0;
-    }
-
-    _pid_info.AFF = derivative * _aff;
-    _last_requested_rate = requested_rate;
-    return _pid_info.AFF;
-}
-
 // This is an integrator which tends to decay to zero naturally
 // if the error is zero.
 

libraries/AC_PID/AC_HELI_PID.h

@@ -23,9 +23,6 @@ public:
     /// get_vff - return Velocity FeedForward Term 
     float       get_vff(float requested_rate);
 
-    /// get_avff - return Acceleration FeedForward Term 
-    float       get_aff(float requested_rate);
-
     /// get_leaky_i - replacement for get_i but output is leaded at leak_rate
     float       get_leaky_i(float leak_rate);
 
@@ -38,7 +35,6 @@ public:
 private:
     AP_Float        _vff;
     AP_Float        _leak_min;
-    AP_Float        _aff;
 
     float           _last_requested_rate;       // Requested rate from last iteration, used to calculate rate change of requested rate
 };