AEROFORUM

          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.

                                   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

       CENTRE OF PRESSURE When an aircraft is in flight it experiences several loads across its structure which is defined as pressure distribution in technical terms. The pressure varies across the fuselage and the wing structure and it is impossible to analyse it. Hence the overall pressure distribution is represented by a single aerodynamic force called  center  of pressure or total reaction force which lies on the chord line of the aerofoil. The  center  of pressure moves along the chord line when the angle of attack is altered. At normal cruising speeds and low positive angles of attack the  center  of pressure is positioned on the chord line near the  center  of an aerofoil. With increasing angles of attack the  center  of pressure moves forward towards the leading edge and the magnitude of the total reaction force increases. When the aircraft reaches the stalling angle of attack the magnitude of the total reaction force reduces and the angle of attack moves towards the

                       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

                   Wing  components Wing is the main source of lift and stability of an aircraft in flight. Our doubt is how this structure controls the motion of the aircraft? These are the scientific facts behind that; first of all an aircraft wing resembles an air foil which creates lift when it is in moving air. Normally the wing is constructed with many components and they have their own characteristics. They are Spars Ribs Stringers or Longerons Skin Spars are the main structural members that bear the loads acting on the wing; they are extended from fuselage and are in right angles to the fuselage. Ribs provide shape and rigidity to wing, which resemble the shape of an air foil. They are arranges in chord wise manner from wing root to wing tip. Stringers or longerons are thin long materials usually fastened with the skin of the wing. The primary function of the stringers is to transfer the aerodynamic bending loads to the ribs and spars. Ski