Aeroplanes are one of the main means of transportation in the 21st Century. The main part of the Airplane that enables it to fly is the wings (also known as aerofoil). This essay will examine in great detail about the physical characteristics of the aerofoil and the importance of these parts in enabling the airplane to take flight.
In general, an aerofoil should have these 6 characteristics:
1. A streamline appearance, without any abrupt changes of curvature anywhere, no gaps or breaks on upper or lower surface.
2. A nicely rounded front nose (leading edge) and a sharp trailing edge, or a trailing edge coming to a rounded point of a very small curvature.
3. A maximum thickness at about one third of the cord from the leading edge.
4. Neither excessive upper camber, since this may mean excessive drag with no compensating increase in lift.
5. No excessive hollowing out of the under surface, since this may mean excessive drag not compensated for by the increase in lift.
6. No excessive camber below the chord line, since this will decrease the lift, even though minimum drag may be diminished.
These characteristics is shown in the Figure below.
An aerofoil takes advantage of the Bernoulli’s Principle. Bernoulli’s Principle states that the faster the molecules within a fluid move, the less pressure they exert on objects around them. This applies to all fluids, including water, air and gases. Since the top surface has more camber than the bottom surface, the air flows faster over the top of the wing than it does underneath. This results in less air pressure at the top of the wing than the bottom. The difference of air pressure between the top surface and the bottom surface of the wing causes lift.
The amount of lift depends on these things:
1. The wing’s airfoil shape
2. The area and the shape of the wing
3. The angle of attack
4. Density of air
5. Speed of flight
The wing’s airfoil shape:
Airfoil shapes are designed to generate as much lift as possible while incurring as little drag as possible. This section will discuss on how the airfoil shape are formed and how it affects the ability of an airplane’s flight.
A step-by-step construction of an airfoil:
1. The length of an airfoil is determined by placing the leading and trailing edges at their desired distance apart. This length is called the called the chord line.
2. Add curvature with the camber line. The amount of curvature is determine by the camber line. This curvature helps to generate lift.
3. Add thickness above the camber line. The amount of thickness added depends on the amount of strength needed in the wing and speed the airplane usually flies.
4. Add the same amount of thickness below the camber line.
5. The final airfoil shape.
Different airfoil shape creates different amounts of lift and drag. An airplane designed to fly at lower speed will have different airfoil shape than the plane built to fly at supersonic speed. The reason behind this is that the air flows in slightly different ways at different speed and different altitude. In general, an airplane built to fly from low to medium speeds will have a thicker airfoil with a greater camber.
The camber causes air to flow faster at the top of the wing than the bottom, which results in less pressure at the top, thus creating a lift. It is important to notice that greater camber gives greater lift at slower speeds. The airplanes designed to fly at faster speeds (supersonic) and at higher altitudes will have thinner airfoil. This is because when flying close to or at the speed of sound a shock wave forms at the nose of the airplane. NASA researchers discovered that a thin airfoil delays the formation of the shock wave. This reduces drag as the plane moves through the shock wave.
The angle of attack:
Angle of attack is the angle of a wing to the oncoming airflow or the angle between the chord and the oncoming airflow.
When the airfoil is tilted with respect to the airstream, air flows faster at the top of the wing than underneath thus increases the difference in airspeed between the top and bottom of the wing. As the difference in speed increases, the pressure difference will also increase which generates a lift. Normally, the lift will increase when the angle of attack increases. On the contrary, if the angle of attack is too high, the smooth airflow cannot follow the shape of the top of the wing. The airflow will then stop following the shape of the wing and will spread out and away from the wing’s surface. This is called the stall angle. When an airplane reaches stall angle, the wing will stop generating lift. It is important to keep in mind that each airplane has different stall angles.
The area and shape of the wing (High Lift Devices):
The flaps move downwards and backwards and are located at the trailing edge of the wing. It increases the area of the wing and the camber of the airfoil. With this increase in area, the airflow has farther to travel which spreads the pressure difference over a larger area.
An equation for the lift force is
According to this equation, the lift increases as the pressure and area increases.
The slats are located on the leading edge of the airfoil. They slide forward and also have the effect in increasing the area of the wing and the camber.
Both slats and flaps are used during take-off and landing. They enable the airplane to get off the ground more quickly and lands more slowly.
Spoilers are located on top of the wing and has the opposite effect from flaps and slats. It disrupts the airflow over the top of the wing thus reduces the lift. It causes the increase of drag which results the airplane to slow down sooner. It is usually deployed after the airplane has landed and lift is no longer needed.
Design of Wings:
The shape of the wing greatly influences the performance of the airplane. It affects the speed, manoeuvrability and handling of the airplane. There are five basic wing shapes that are used in modern airplanes:
1. Straight wing
2. Sweepback wing
3. Forward-sweep wing
4. Delta wing
1. The straight wing
The straight wing is designed for small, low speed airplanes. This type of wing provides good lift at low speeds and can be found in General Aviation airplanes. This wing provides good and stable flight. It can be made cheaper and lighter. It is not suited for high speed airplanes as is perpendicular to the airstream which has a tendency to create drag.
Rectangular Straight Wing
Tapered Straight Wing
Rounded or Elliptical Straight Wing
2. The sweepback wing
This is the choice of wing for most high speed airplanes. Sweepback wing creates less drag which explains why it is unstable at low speeds. The amount of sweep depends on the purpose of the plane. Commercial airlines use moderate sweepback wing on their airplanes because it causes less drag while maintaining stability at low speed. High speed airplanes (fighters) use greater sweepback wing. These airplanes are not very stable at low speed. They also take-off and descend for landing at a high rate of speed.
Slight Sweepback Wing
Moderate Sweepback Wing
Great Sweepback Wing
3. The Forward-Sweep Wing
This design of the wing has yet to make it into mass production. An airplane (like X-29) is highly manoeuvrable but it is highly unstable too. Because of the instability, a computer based control system must be used in the X-29 to help the pilot fly.
4. Delta Wing
Airplanes with this type of the wing can reach high speeds and their landing speed are very fast because of the high sweep. Many supersonic airplanes such as Concorde has delta wing.
Simple Delta Wing (top) and Complex Delta Wing (bottom)
5. The Swing-Wing
This design attempts to exploit the high lift characteristics of straight wing with the ability of sweep-back wing which enables it to fly at high speed. During landing and take-off, the wings swings to almost straight position. During cruise, the wings will swing to sweep-back position. However, there is a price to pay with this design and that is weight. The hinges that enables the wings to swing are very heavy.
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