diff --git a/libraries/AP_Motors/AP_MotorsHeli_Dual.cpp b/libraries/AP_Motors/AP_MotorsHeli_Dual.cpp
index 87bae449b0..3f9adabc39 100644
--- a/libraries/AP_Motors/AP_MotorsHeli_Dual.cpp
+++ b/libraries/AP_Motors/AP_MotorsHeli_Dual.cpp
@@ -124,6 +124,8 @@ const AP_Param::GroupInfo AP_MotorsHeli_Dual::var_info[] = {
     // @Increment: 0.1
     AP_GROUPINFO("YAW_SCALER", 12, AP_MotorsHeli_Dual, _yaw_scaler, 1.0f),
 
+    // Indices 13-15 were used by RSC_PWM_MIN, RSC_PWM_MAX and RSC_PWM_REV and should not be used
+
     // @Param: COL2_MIN
     // @DisplayName: Collective Pitch Minimum for rear swashplate
     // @Description: Lowest possible servo position for the rear swashplate
diff --git a/libraries/AP_Motors/AP_MotorsHeli_Single.cpp b/libraries/AP_Motors/AP_MotorsHeli_Single.cpp
index a4222096b7..b594cadead 100644
--- a/libraries/AP_Motors/AP_MotorsHeli_Single.cpp
+++ b/libraries/AP_Motors/AP_MotorsHeli_Single.cpp
@@ -116,6 +116,7 @@ const AP_Param::GroupInfo AP_MotorsHeli_Single::var_info[] = {
     // @User: Standard
     AP_GROUPINFO("GYR_GAIN_ACRO", 11, AP_MotorsHeli_Single,  _ext_gyro_gain_acro, 0),
 
+    // Indices 16-18 were used by RSC_PWM_MIN, RSC_PWM_MAX and RSC_PWM_REV and should not be used
     // parameters up to and including 29 are reserved for tradheli
 
     AP_GROUPEND