# Aerofoil

## Definition of Aerofoil

Aerofoil or also known as Airfoil is a structure with curved surfaces designed to give the most favourable ratio of lift to drag in flight, which is mainly used as the basic form of the fins, wings, and tailplanes of most aircraft. Aerofoil is the cross-section design of the wing, blade, or sail. Lift is the component such that the force is perpendicular to the direction of motion and drag is the component parallel to the direction of motion. A similar idea is being used in the designing of hydrofoils which is used when water is used as the working fluid. A body that is of airfoil-shaped, moving through a fluid produces an aerodynamic force. The design of the aerofoil depends on the weight, speed, and purpose of the aircraft and mainly depends on the aerodynamic characteristics. These are dependent on certain terms that need to be defined to understand the design.

### Aerofoil Terminology

Aerofoil Terminology. An Aerofoil is being designed with a shape that has the capability of producing lift with relatively high efficiency as it passes through the air. An aerofoil can have many cross-sectional shapes. The terms which are related to aerofoils are as follows:

1. Chord: Chord can be defined as the distance between the leading edge, at the front of the aerofoil that is the point, and has maximum curvature and the trailing edge, at the rear of the aerofoil, that is the point with a maximum curvature along the chord line.

2. Chord Line: Chord line is the straight line connecting the leading and trailing edges.

3. Upper Surface: The upper surface is associated with high velocity and low static pressure, which is also known as suction surface.

4. Lower Surface: The lower surface is a higher static pressure surface which is also known as pressure surface.

When the aerofoil is moving through a fluid, the following are the terms used to describe the behavior:

1. Aerodynamic Center: The centre where pitching moment is independent of lift coefficient and angle of attack.

2. Center Of Pressure: The centre where the pitching moment is zero.

3. The Angle Of Attack (AOA): The angle of attack is formed between a reference line on a body and the oncoming flow.

4. Pitching Moment: The moment or torque produced on the aerofoil bye the aerodynamic force is known as Pitching moment.

### Lift Coefficient

The lift coefficient is a relationship between the lift generated by a lifting body to fluid density, fluid velocity, and the associated reference area, this is a dimensionless coefficient. Mathematical representation is as follows:

$C_{L}$ = $\frac{L}{q^{S}}$ = $\frac{L}{(1/2)\rho \mu^{2}S}$ = $\frac{2L}{\rho \mu^{2}S}$

Where,

$C_{L}$  : lift coefficient

L : lift force

S : relevant surface

q : fluid dynamic pressure

$\rho$ : fluid density

$\mu$ : flow speed

### Types of Aerofoil

The types of aerofoils that are used are as follows:

1. Symmetrical Aerofoil: This has identical upper and lower surfaces that produce no life at zero AOA such that the chord line and mean camber line are the same. In most of the light helicopters in their main rotor blades, these applications are fined.

2. Non-symmetrical Aerofoil: Non-symmetrical aerofoil has different upper and lower surfaces such that the chord line is placed above with large curvature, and it is also known as a cambered aerofoil.

3. These have different chord lines and chamber lines. These are the advantages of a non-symmetrical aerofoil, that is the lift to drag ratio and stall characteristics are better and useful lift is produced at zero AOA. The only disadvantages are that they are not economical and there is a production of undesirable torque.

Nonsymmetrical and Symmetrical Airfoil

### Fun Facts

1. In American English it is Airfoil and in British English, it is known as Aerofoil.

2. An Aerofoil or Airfoil is the shape of a wing, or blade of a propeller.

3. Aerofoil was invented by Sir George Cayley.

4. Aerofoil will provide either lift or downforce, when it is moving through a fluid, depending on what it is used for.

5. According to Newton’s third law, the air must exert equal and opposite force on the airfoil, which is known as lift.

6.  Birds fly on the basis of airfoils for wing-lift.

7. The underwater fins of sailboats, such as centerboards, are also lifting foils and operate on the same principles as airfoils. Technically they should be called hydrofoils, but this term has already been taken; generally, they are just referred to as "foils".

8. It is important to note that any thin object such as a flat plate or even the deck of a bridge, at an angle of attack with respect to the airflow, will generate lift; there is nothing "magic" about the shape of an airfoil. However, the lift is generated with the minimum of drag, so it is important for efficiency, the airfoil shape ensures that.