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 BECAUSE THE PRESSURE VECTORS ACT AT THE CENTER OF PRESSURE. AT ZERO DEGREES OF ANGLE OF ATTACK THE MAGNITUDE OF PRESSURE VECTORS ARE SAME HENCE THERE IS NO LIFT.

AT POSITIVE ANGLE OF ATTACK THERE IS A POSITIVE LIFT BECAUSE PRESSURE VECTOR IN UPPER SURFACE IS IN A GREATER MAGNITUDE THAN THE LOWER SURFACE. SYMMETRICAL AIRFOILS DO NOT PRODUCE ANY PITCHING MOMENTS AT ANY ANGLE OF ATTACKS.

ASYMMETRICAL AIRFOILS EXPERIENCES PITCHING MOMENTS BECAUSE OF THE MOVEMENT OF CENTER OF PRESSURE ALONG THE CHORD LINE BY CHANGING THE ANGLE OF ATTACKS

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