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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AIRFOIL TERMINOLOGY

Air foil Terminology What is an airfoil? How an air foil is described in modern science, what are the parameters that determines its shape? An Air foil is primary building block of an aircraft which has two edges namely leading edge and trailing edge, the straight line connecting the leading and trailing edge of the air foil is called as chord line. The line which is drawn equidistant from the upper and lower surfaces of the air foil and connecting the leading and trailing edge is called as mean camber line.The distance between the leading and trailing edge is called chord of an air foil. The maximum distance between the mean camber line and chord line is referred as maximum camber. This is one of the variables that determine the aerodynamic characteristics of the wings. Maximum thickness to chord ratio is expressed in terms of percentage. For subsonic wings the ratio is normally 12-14% Depending on the camber (curvature) of the air foil it is d

Form Drag

Form Drag Form drag is experienced in the surface of the aircraft when the streamlined airflow separates and becomes turbulent. To experiment form drag keep a flat plate on the streamlined airflow, now the pressure before the plate is atmospheric and the pressure after it is below the atmospheric which results in sucking effect behind the plate and vortices are formed. It is mandatory to delay the separation point of airflow and this is achieved by altering the shape or streamline of the given object. When the airflow changes its direction rapidly form drag experienced is higher. A fairing is fitted around the fixed undercarriage leg to reduce form drag to considerable extent. The resultant form drag depends on the length and maximum thickness of the streamlined object. The ratio of length to maximum thickness is called the fitness ratio. Fitness ratio = a/b At subsonic speeds fitness ratio of streamlined shapes is

AERODYNAMIC CENTRE

AERODYNAMIC CENTER IN ASYMMETRICAL AIRFOILS THE CENTER OF PRESSURE MOVES ALONG THE CHORD LINE WITH CHANGES IN ANGLE OF ATTACK. AS THE ANGLE OF ATTACK INCREASES THE CENTER OF PRESSURE MOVES TOWARDS THE LEADING EDGE AND WHEN IT DECREASES IT MOVES TOWARDS THE TRAILING EDGE. A NOSE DOWN PITCHING MOMENT IS ALWAYS PRESENT AND INCREASES IN INTENSITY WITH INCREASING ANGLES OF ATTACK. HOWEVER THERE IS A POINT ON THE CHORD LINE ABOUT WHICH THE PITCHING MOMENT REMAINS CONSTANT,REGARDLESS OF ANY CHANGE IN ANGLE OF ATTACK. THIS POINT IS CALLED AERODYNAMIC CENTER. IN SUBSONIC AIRFLOW IT IS LOCATED AT A DISTANCE 25% FROM LEADING EDGE ON THE CHORD LINE. HOWEVER IN SUPERSONIC AIRFLOW IT IS CONSIDERED TO BE LOCATED ON 50 % FROM LEADING EDGE ON THE CHORD LINE. FOR DESIGN PURPOSES IT IS CONSIDERED TO BE OVERALL LIFT FORCE TO ACT ON THAT POINT COMBINED WITH A PITCHING MOMENT OF CONSTANT STRENGTH.

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