AEROFORUM

                                   PITCHING MOMENTS PITCHING MOMENTS HAPPEN WHEN TWO EQUAL PRESSURE FORCES ACT IN DIFFERENT DIRECTION FROM THE SAME CENTER. PRESSURE DISTRIBUTION AROUND AN ASYMMETRICAL AIRFOIL IN NEGATIVE ANGLE OF ATTACK GIVES ZERO LIFT AND PRODUCES A PITCHING MOMENT WHICH IS NOSE DOWN. IN NEGATIVE  ANGLE OF ATTACKS THERE ARE TWO PRESSURE VECTORS WHICH CONSTITUTE A COUPLE, HENCE NO LIFT IS CREATED AND THE AIRFOIL IS PITCHED NOSE DOWN. WHEN THE SAME AIRFOIL IS PLACED AT ZERO DEGREES OR POSITIVE ANGLE OF ATTACKS THE PRESSURE DISTRIBUTION VARIES IN WHICH UPPER SURFACE OF THE AIRFOIL  CREATES MORE LIFT THAN THE LOWER SURFACE . THUS THE AIRFOIL CREATES LIFT EVEN AT ZERO DEGREES OF ANGLE OF ATTACK AND DUE TO THIS AN INCREASED NOSE DOWN PITCHING MOMENT ALSO OCCURS.  PITCHING MOMENT TAKES PLACE BECAUSE THE PRESSURE VECTORS ACTS AT A DISTANCE FROM THE CENTER OF PRESSURE WHICH RESEMBLES A COUPLE. BUT IN CASE OF SYMMETRICAL AIRFOILS THERE ARE NO PITCHING MOMENTS