An AIRFOIL produces a lifting force that acts at right angles to the airstream and a dragging force that acts in the same direction as the airstream.
What is an Aerofoil in general!?
- An airfoil or aerofoil is the cross-sectional shape of a wing.
- The lift on an airfoil is primarily the result of its angle of attack. This force is known as aerodynamic force and can be resolved into two components: lift and drag.
- An aerofoil is necessary to produce aerodynamic lift. An airfoil-shaped body moving through a fluid produces an aerodynamic force.
Determining an airfoil is not just a peice of cake :
LEADING EDGE:
- The leading edge is the point at the front of the airfoil that has maximum curvature.
TRAILING EDGE:
- The trailing edge is defined similarly as the point of maximum curvature at the rear of the airfoil.
CHORD LINE:
- The chord line is the straight line connecting leading and trailing edges.
CHORD LENGTH:
- The chord length, or simply chord, is the length of the chord line. That is the reference dimension of the airfoil section.
Ever wondered why an aerofoil has specific shape!?
Reason for specific shape :
- The mean camber line or mean line is the locus of points midway between the upper and lower surfaces.
- Its shape depends on the thickness distribution along the chord; The thickness of an airfoil varies along the chord.
Design of Aerofoil:
- Airfoil design is a major facet of aerodynamics. Various airfoils serve different flight regimes.
Symmetrical aerofoil:
- This has identical upper and lower surfaces such that the chord line and mean camber line are the same producing no life at zero ANGLE OF ATTACK. These find applications in most of the light helicopters in their main rotor blades.
Non-symmetrical aerofoil: ( Cambered )
- This has different upper and lower surfaces such that the chord line is placed above with large curvature.
- These have different chord line and chamber line. The advantages of non-symmetrical aerofoil is that the lift to drag ratio and stall characteristics are better and useful lift is produced at zero ANGLE OF ATTACK.
- The disadvantages are that they are not economical and there is a production of undesirable torque.
In the region of the ailerons and near a wingtip a symmetric airfoil can be used to increase the range of angles of attack to avoid spin–stall. Thus a large range of angles can be used without boundary layer separation.
SUBSONIC AIRFOIL:
- Subsonic airfoils have a round or oval like leading edge, which is naturally insensitive to the angle of attack .
- The cross section is not strictly circular, however: the radius of curvature is increased before the wing achieves maximum thickness to minimize the chance of boundary layer separation.
SUPERSONIC AEROFOILS:
- Supersonic airfoils are much more angular in shape and can have a very sharp leading edge, which is very sensitive to angle of attack.
- A supercritical airfoil has its maximum thickness close to the leading edge to have a lot of length to slowly shock the supersonic flow back to subsonic speeds.
- Generally such transonic airfoils and also the supersonic airfoils have a low camber to reduce drag divergence.
ATLAS THE CONCEPT OF LIFT……
Finally, important concepts used to describe the airfoil’s behaviour when moving through a fluid are:
- The aerodynamic center, which is the chord-wise length about which the pitching moment is independent of the lift coefficient and the angle of attack.
- The center of pressure, which is the chord-wise location about which the pitching moment is zero.
Kutta-Joukowski Lift Theorem:
Kutta-Joukowski theorem gives the relation between lift and circulation on a body moving at constant speed in a real fluid with some constant density.
- In fluid flow around a body with a sharp corner, the Kutta condition refers to the flow pattern in which fluid approaches the corner from both directions, meets at the corner, and then flows away from the body.
- None of the fluid flows around the sharp corner.
- The Kutta condition is significant when using the Kutta–Joukowski theorem to calculate the lift created by an airfoil with a sharp trailing edge.
- The value of circulation of the flow around the airfoil must be that value which would cause the Kutta condition to exist.
KUTTA CONDITION:
The Kutta condition is a principle in steady-flow that is applicable to solid bodies with sharp corners, such as the trailing edges of airfoils.
IT STATES THAT :
- THE FLOW SHOULD LEAVE THE TRAILING EDGE SMOOTHLY.
- IF THE
- TRAILING EDGE IS FINITE, THEN THE TRAILING EDGE IS THE STAGNATION POINT.
- TRAILING EDGE IS CUSPED, THE VELOCITIES LEAVING THE TOP AND BOTTOM SURFACE AT TRAILING EDGE ARE FINITE AND EQUAL IN MAGNITUDE AND DIRECTION.
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