- The following SKYbrary Articles:
Gain an understanding of:
- Basic aerodynamics
- Aerofoil terminology
- Angle of Attack
- Aerodynamic stall
An Aerofoil is a shape capable of producing lift with relatively high efficiency as it passes through the air.
An aerofoil can have many cross sectional shapes. Different aerofoils are used to construct the aircraft wings. The designers choose the shape that has the best aerodynamic characteristics to suit the purpose, weight and speed of the aircraft. In order to compare and distinguish different aerofoil shapes, an aerofoil’s properties are defined and specific terminology is used:
Chord is a distance between the leading and trailing edges measured along the chord line;
Chord line is a straight line joining the leading and trailing edges of an aerofoil;
Leading edge is a part of an aerofoil (edge) that hits the air particles first;
Lower surface is the surface of an aerofoil between the leading and trailing edges, on the lower surface;
Mean camber line is a line joining the leading and trailing edges of an aerofoil, equidistant from the upper and lower surfaces;
Maximum camber is the maximum distance of the mean camber line from the chord line;
Maximum thickness is the maximum distance of the lower surface from the upper surface.
Trailing edge is a part from an aerofoil (edge) that hits the air particles last;
Upper surface is the surface of an aerofoil between the leading and trailing edges, on the upper surface;
An aerofoil is constructed in such a way that its shape takes advantage of the air’s response to certain physical laws. This develops two actions from the air mass: a positive pressure lifting action from the air mass below the wing, and a negative pressure lifting action from lowered pressure above the wing.
Different aerofoils have different flight characteristics. The weight, speed, and purpose of each aircraft dictate the shape of its aerofoil. Advancements in engineering have made it possible for today’s high-speed jets to take advantage of the concave aerofoil’s high lift characteristics. Leading edge slats, leading edge Krueger Flaps and trailing edge (Fowler) flaps, when extended from the basic wing structure, literally change the aerofoil shape into the classic concave form, thereby generating much greater lift during slow flight conditions.
On the other hand, an aerofoil that is perfectly streamlined and offers little wind resistance sometimes does not have enough lifting power to take the airplane off the ground. Thus, modern airplanes have aerofoils that strike a medium between extremes in design. The shape varies according to the needs of the airplane for which it is designed.
The pressure differential created between the upper and lower surfaces of the wing lifts the wing upward in the direction of the lowered pressure. This lifting force is known as induced lift. Induced lift may be increased, within limits, by:
- increasing the angle of attack of the wing or changing the shape of the airfoil, changing the geometry, e.g., aspect ratio
- increasing the wing area
- increasing the free-stream velocity
- a change in air density.
Aerofoil surfaces of an aircraft include wings, tailplanes, fins, winglets, propeller blades and helicopter rotor blades. Control surfaces (e.g. ailerons, elevators and rudders) are shaped to contribute to the overall aerofoil section of the wing or empennage.
Angle of Attack
The Angle of Attack is the angle at which relative wind meets an aerofoil. It is the angle formed by the Chord line of the aerofoil and the direction of the relative wind or the vector representing the relative motion between the aircraft and the atmosphere.
The angle of attack can be simply described as the difference between where a wing is pointing and where it is going.
An increase in angle of attack results in an increase in lift and induced drag, up to a point. Too high an angle of attack (usually around 17 degrees) and the airflow across the upper surface of the aerofoil becomes detached, resulting in a loss of lift, otherwise known as a Stall.
Since lower density air is less viscous, a wing will stall more easily in it and will do so at higher airspeeds for any given angle of attack.
Since the angle of attack necessary to generate lift increases with decreasing air density, the angle of attack needed to fly safely at higher altitudes must also increase, which raises the stall speed.
Q1: Leading Edge flaps which, when extended from the basic wing structure, change the aerofoil shape into the classic concave form, thereby generating much greater lift are called:
- Serengeti Flaps
- Krueger Flaps
- Yellowstone Flaps
Q2: Stall is caused by high angle of attack, usually around
- 13 degrees
- 17 degrees
- 23 degrees