Flight Basics

         Head Wind A head wind is a wind that flows opposite to the direction of an aircraft, it is favorable in takeoffs and landings as an airfoil moving into a headwind is capable of generating greater lift. During headwind, ground speed of aircraft is equal to the difference of speed of aircraft and speed of wind.  It allows aircraft to reach the lift off speed at a lower ground speed. Air-traffic controllers commonly choose to take off or land in the direction of a runway which will provide head wind. During cruise it will decrease performance by reducing ground speed which in turn increases the fuel requirement for the flight.  Tail Wind A tail wind is a wind that blows in the direction of travel of an aircraft hence, increasing the ground speed of the aircraft, and increases the takeoff distance. During tail wind, ground speed is just equal to the sum of aircraft speed and speed of wind.  It provides a greater ground speed to attain the lift-off speed. During cruise, it will i

  Aerodynamic Center The aerodynamic center is the point on the chord at which the pitching moment coefficient does not vary with lift coefficient, i.e. angle of attack. It is the point where the aerodynamic forces are supposed to act, a fixed location on the chord. Lift and drag forces can be applied at a single point, where they exert zero torque, at center of pressure but it moves with angle of attack as pressure distribution changes thus, makes the aerodynamic analysis complex. For most low speed airfoils aerodynamic center lies at (1/4) of chord back from the leading edge in supersonic airfoils it is nearer to the (1/2) chord location. In case of symmetric airfoils the aerodynamic moment about ac is zero for all angles of attack. With camber, the moment is non-zero and constant for thin airfoils. For a positive camber airfoil the moment is negative and results in the counter-clockwise rotation of the airfoil.

  Streamline Streamlines are the ones that drawn such that their tangents at every point along the streamline are in the same directions as the velocity vectors at that point and forms a powerful tool in understanding flows. This definition leads to the equation of streamline as follows : Streakline A line created by particles in a flow that have previously passed through a common point in the flow field. It is obtained by a simple and efficient integration of particle pathlines within the flow solver as the unsteady flowfield is computed.   Pathline The line traced by a given particle when it flows from one point to another in a flow field. In unsteady flow, the pathlines for different fluid elements passing through same point are not the same.                                          

  Circulation  The Circulation C about a closed curve in a flow field is defined as the negative of the line integral of velocity around that closed curve. It is denoted by the symbol, Γ (Uppercase Gamma). This definition makes it a Scalar Quantity.  It is a kinematic property depending on the velocity of the field and the choice of the curve C. The word 'Circulation' generally means movement in a circle or circuit but in aerodynamics circulation has a very precise technical meaning as per the equation above. It does not necessarily means that fluid elements are moving around in circles in this fluid field rather when circulation exists in a flow it simply means that the line integral taken above is finite. For example, if the airfoil is generating lift, the circulation taken around a closed curve enclosing the airfoil will be finite, although the fluid elements are by no means executing circles around the airfoil. Vorticity Vorticity is mathematically defined as the curl of ve