Aircraft Aerodynamics: The science explains how things fly

10 min readMay 22, 2020

Aircraft has been first invented by Wright Brothers in December, 1903 where the flight can only last up to 59 seconds over a distance of 852 feet. And look how much aircraft industry has been developed until now, it is totally fantastic and amazing!

However, I have been curious about some questions like,

How do the plane actually fly ?
Why engineers do not make aeroplanes to have flappy wings imitating bird flying wings ?
How fast can modern aeroplane can literally travel ? And what does those fancy words like, Aerodynamics, and Supersonic means ?

Based on these questions, I am therefore inspired to write a new article about the “aircraft aerodynamics” in order to answer my curiosity that I had in mind.

What is Aerodynamics ?

By definition, aerodynamics is a study of how gases interact with moving bodies. Because the gas that we encounter most is air, aerodynamics is primarily concerned with the forces of drag and lift, which are caused by air passing over and around solid bodies. Engineers apply the principles of aerodynamics to the designs of many different things, including buildings, bridges and even soccer balls; however, of primary concern is the aerodynamics of aircraft and automobiles.

How do aircraft stay up in the air ?

Four force of the flight

Figure 1. An illustration of 4 forces acting to object in different direction.

To understand the motion of the objects, we need to understand the definition of the force.

A force is any interaction that, when unopposed, will change the motion of an object. A force can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a push or a pull. A force has both magnitude and direction, making it a vector quantity.

In aircraft aerodynamics, we only consider 4 main direction of forces which covers in both horizontal direction and vertical direction.

Vertical direction force

Lift: A force that acts in opposite direction to the weight force. It is the force which make objects stays on a surface, floating on water, or even fly.
Weight: A force which comes from gravity pulling down on objects

Horizontal direction force

Thrust: A force in the same direction of the movement. It is the pushing force that moves objects forward.
Drag: A force in an opposite direction of the movement. It slows some moving objects down or making objects more difficult to move in particular direction.

As illustrated in Figure 1, any objects can fly in the air if and only if the lift is greater than the weight force, and will move forward when thrust is greater than drag force. In aeronautical engineering aspects, engineers has to design devices or components which generates the lift force and thrust force in order to fly.

Why not a flappy wings ?

We know that human can achieve their goals to fly across the globe because they are inspired by birds. But we cannot just make the aeroplane with a flappy wing because of many reasons. e.g. higher cost, requirements of many heavy weight components (more weight force, of course), etc. But how aeroplane can fly without the flappy wings ?

The answer is the shape of an aeroplanes’ wings. Aeronautical engineers has come up with the knowledge of designing the shape of the wing such that the air that pass through the wing can generate sufficient amount of lift and thrust force to make airplane fly.

Most of aeroplane wings are designed to have the suitable curvature on the top surface and flatter surface on the bottom of the wing. Such shape allows air flow over the top faster than under the bottom. As a result, the less air pressure makes the wing and the aeroplane it is attached to to move up.

By using the curves to change air pressure is a trick used on many aircrafts. For example, helicopter rotor blades also rely on this techniques. Even kites and sailboats also utilise this technique to make it fly and float on the water surface, respectively.

Airfoil

As mentioned in the previous section, the shape of aeroplanes’ wings plays a crucial roles in providing lift and thrust to aeroplanes. To design about the shape of airplane wings, the term “airfoil” are extensively mentioned in aircraft design.

Airfoils is the cross sectional shape of the wing blade (of a propeller, rotor, or turbine), or sail. The lift on an airfoil is primarily the result of its angle of attack. When oriented at a suitable angle, the airfoil deflects the oncoming air (for fixed-wing aircraft, a downward force), resulting in a force on the airfoil in the direction opposite to the deflection. Upper surface is generally associated with higher velocity and lower static pressure. Lower surface has a comparatively higher static pressure than the suction surface. The pressure gradient between these two surfaces contributes to the lift force generated for a given airfoil.

Figure 3. An illustration of airfoils structure

As illustrated in Figure 3, there are about 8 factors which has influences to the shape of the airfoil. These factors must be carefully designed in airfoil design in order to allow the airfoil shape to be optimised suitable for many scenarios as well as providing optimal angle of attack during the flight to increase the coefficient of the lift of aeroplanes.

Leading edge: A point of minimum radius of the airfoil.
Trailing edge: A point at the tail of the airfoil.
Chord line: A straight line going from the leading edge to the trailing edge.
Camber line: A curved line which is constituted by the midpoints of all airfoil cross-section segments perpendicular to the Chord.
Maximum thickness: The longest vertical distance between upper surface and lower surface.
Maximum camber thickness: The longest vertical distance between camber line and chord line.
Upper surface: The top surface of the airfoil at which the higher air pressure is produced.
Lower surface: The top surface of the airfoil at which the lower air pressure is produced.

Figure 4. An illustration of different shapes of airfoils can result in different in aerodynamics.

Angle of attack

Figure 5. An illustration of many types of angles involved during the flight.

Angle of attack is an angle between the oncoming air or relative wind and a reference line on the airplane or wing. Most foil shapes require a positive angle of attack to generate lift, but cambered airfoils can generate lift at zero angle of attack. An increase in angle of attack results in an increase in both 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. However, the trend of the angle of attack with respect to the coefficient of lift is illustrated in the Figure 6.

Note: Coefficient of Lift is a dimensionless coefficient that relates the lift generated by a lifting body to the fluid density around the body, the fluid velocity and an associated reference area.

Figure 6. Line graph representing the relationship between angle of attack and coefficient of lift.

Four Speed of the flight

How do we categorise the speed of flight ?

As show in Figure 8, the speed of the flight can be classified by a numerical unit called Mach. Mach unit is a dimensionless quantity representing the ratio of flow velocity past a boundary to the local speed of the sound.

With simple mathematics, the aircraft will fly at the speed of sound if the aircraft is travelling at Mach 1 (322 m/s or 1195 km/hr or 717 miles/hr). In the same way, an aircraft is said to fly at twice speed of the sound in the air when such aircraft is flying at Mach 2.

However, the speed of the flight is categorised into 4 categories based on the maximum Mach unit which an aeroplane can travel in the air. This includes, subsonic, transonic, supersonic, and hypersonic.

Subsonic

At subsonic speed, the aircraft is traveling slower than the speed of the sound — less than about Mach 0.8.

The types of flying objects which falls to this category include everything that flies slowly (all general aircraft, parachute, etc.) Examples of commercial aircraft are Boeing 777 and Airbus 330, and smaller regional jets which have less than 100 seats. Moreover, most older military jets also fall into the subsonic category. For instances, F-100 Super Sabre (developed in 1950) which is flown by US Air Force for 25 years.

Transonic

At transonic speed, an aircraft is approaching the speed of sound but hasn’t yet reached and surpassed Mach 1.

At some places on the aircraft the speed will exceed Mach 1, while at others it will be less than Mach 1. There are a handful of aircraft that fly deep in the transonic regime.

The line between subsonic and transonic is blurry. There are even transonic flows on both of the subsonic commercial airliner. In some cases, you can even see the shadow of the shocks on the upper wing.

Supersonic

At supersonic speed, the aircraft is traveling in speed range from Mach 1.2 to Mach 5.

This includes rockets, such as the Space Shuttle, fly at supersonic speeds immediately after liftoff and for about 45 seconds until about two minutes after launch. During this time, the shuttle accelerates from Mach 1 to Mach 5.

Also many types of military aircrafts are also capable of supersonic flight. For example, F-4 Phantom II model aircraft (image available in Figure 11)

Hypersonic

At hypersonic speed, the aircraft is traveling faster than Mach 5.

At this fast speed, only military aircraft are permitted to complete the flight at this speed. Examples are hypersonic X-15 (shown in Figure 12) can fly at Mach 6.7 (7247 km/hr), and Starry Sky-2 hypersonic aircraft which can fly at the speed of Mach 5.5.

As an aircraft travels at this speed, there exist the distinction between supersonic aircrafts and hypersonic aircrafts in the aspect of temperature changes.

At speeds above Mach 5, most metals will be melt or become extremely soft such that these aircrafts cannot be used for any types of structure. As a result, hypersonic aircraft must be tested with extreme measures for heat protection (such as tiles and blankets protecting the space shuttle). Therefore the material used for constructing hypersonic aircrafts must be carefully selected and often extremely expensive. Considering the facts that the melting points of metals we can find in the market are relatively low compared to those that are used for hypersonic aircraft construction. For instances, Aluminium will be melt at 648 degree celsius and steel will be melt at 1371 degree celsius.

How do we test the flight ?

Aerodynamics Simulation Software

Aerodynamics simulation software relies on the combination of technologies between Computational Fluid Dynamics (CFD) and Computer Aided Engineering (CAD).

Computational Fluid Dynamics (CFD): is a branch of fluid mechanics that uses numerical analysis and data structures to analyse and solve problems that involve fluid flows. Computers are used to perform the calculations required to simulate the free-stream flow of the fluid, and the interaction of the fluid (liquids and gases) with surfaces defined by boundary conditions.
Computer Aided Engineering (CAD): is the use of computer software to simulate performance in order to improve product designs or assist in the resolution of engineering problems for a wide range of industries.

Examples of aerodynamics simulation software are AirShaper, AutoCAD, etc.

Wind Tunnel

Wind tunnel is a large tubes with multiple air-releasing channels to generate moving air inside the chamber. The goal of the wind tunnel is to copy the actions of the objects during the flight.

In the wind tunnel, powerful fans move air through the tube. then, the object under test is fastened in the tunnel so that the object will not move before powering on the air-releasing channels. The air moving around the still object illustrates what would happen if the object are moving through the air. The observation of air movement can be observed by putting smoke or dye in the chamber.

However, the wind tunnel is very useful in spacecraft industries. This is because wind tunnel is widely used to only demonstrate the behaviours of spacecraft during launching and landing. We cannot observe the behaviours of spacecraft while moving in the vacuum environment because there will be no air flowing in the vacuum.

Figure 14. An illustration of wind tunnel.

Contributor

This article is written for an assignment of the course Science Technology and Modern World at King Mongkut’s Institute of Technology (KMITL), Thailand.

The article is used for educational purpose only.

Warakorn Jetlohasiri