# What Are the Fundamentals of Aerodynamics?

### Key Takeaways

• Lift, drag, thrust, and weight are the major forces acting in an aerodynamic system.

• The aerodynamic forces and moment act at the center of pressure and are influenced by the fluid state as well as the body’s speed and direction.

• The understanding of boundary layer and flow separation concepts for turbulent modeling can facilitate aerodynamic design to achieve equilibrium.

A model of the aerodynamic behavior of an airplane

We have all heard about the Wright brothers and their successful invention of the first aircraft. However, their success was not achieved overnight; it took numerous failed attempts to crack the perfect aerodynamic design code. Their work formed the base reference for aerodynamic engineers and scientists to understand the aerodynamic principles of commercial aircraft design.

Understanding the fundamentals of aerodynamics means discerning fundamental aerodynamic variables, including pressure, density, velocity, and temperature as well as factors like acting forces, flow types, airfoil design, boundary layers, and more. In this article, we will discuss these fundamental concepts and their importance in designing a safe aircraft.

## What Are Aerodynamic Forces and Moments?

Aerodynamic forces acting on a body

### Aerodynamic Forces

The first step to understanding aerodynamic properties is to have knowledge about the forces acting on a body. The main aerodynamic forces of flight include:

• Lift: Lift is generated when the fluid and airfoil interact with each other, with either of them being in motion. A force is generated that acts against the weight of the aircraft, counterbalancing to hold the plane in the air.

• Drag: Drag is the resisting force that works against the aerodynamic motion or thrust. For the airplane in motion, the air resists the forward motion due to the opposing drag force, reducing the velocity.

• Thrust: Thrust is the forward propelling force in the aircraft, which acts against the resistance of drag. Constant thrust maintains constant airplane speed while its increase or decrease may be required during lift-off or landing. Jet engines and propellers in aircraft can create thrust.

• Weight: Weight can be expressed as the force of gravity pulling the craft towards the Earth. Thus, it is always directed downwards from the center of mass of the airplane. Lift, the opposing force to weight, is required to enable flight. During flight, weight can constantly change, affecting the balance of the aircraft. Thus, constant control is always required.

### Moments

In the flow field, the aerodynamic forces produce a moment, which acts at the center of pressure. The center of pressure is the point where the total pressure force acts on the body that is moving through the fluid. The forces and moment acting on the body are relative to factors such as the state and attributes of the fluid medium, shape, and dimension of the moving body as well as its speed and orientation.

# Types of Flow

Aerodynamic behavior is affected by the nature of the flow. Depending on the basic flow parameters—pressure, density, velocity, and temperature, and their relationship—flow can be classified into the following types.

 Incompressible flow The density of the fluid remains constant. In aerodynamics, the flow is considered incompressible for Mach number (M) less than 0.3. Compressible flow The fluid density significantly changes with distance. Most flows in nature are compressible. Steady flow The fluid flows uniformly without a change in velocity at any point in time. Unsteady flow The velocity of flow can change with time. Uniform flow The flow parameters remain the same at different points in a flow system. Non-uniform flow The flow parameters vary at different points in a flow system. Viscous flow Viscosity of the fluid affects the fluid flow. Inviscid flow Viscosity has no effect on fluid flow. Laminar flow Fluid flows in a streamlined, parallel layer. Turbulent flow Fluid layers intermix and velocity fluctuates during the flow.

The flows mentioned are mostly relative to the pressure-density factor. When velocity is explored, the flow can be classified as transonic, subsonic, or supersonic.

• Subsonic: Velocity of flow < velocity of sound, (M<1)
• Supersonic: Velocity of flow > velocity of sound, (M>1)
• Transonic: Velocity of flow between subsonic and supersonic, (0.8<M<1.2)

### Turbulent Flow

The study of the turbulent flow regime is an important part of aerodynamic design, as it deals with the variation in pressure, velocity, flow separation, unsteady vortex creation, and other chaotic factors that are generally considered unfavorable for aerodynamic lift. The turbulent flow can be distinguished with Reynolds number (Re):

Note that:

ρ : density of the fluid

V : fluid velocity

D: hydraulic diameter (of the pipe, tube, or duct)

μ : fluid viscosity

For a Reynolds number greater than 3500, the flow is considered turbulent. This can easily be identified in a less viscous fluid with higher velocities.

Vortices of various sizes and frequencies are also created with turbulence.  When these vortices interact and exchange energies with each other, the result is an increment in drag which acts negatively against the aerodynamic motion.

Flow separation is caused when the boundary layer is detached due to a change in the pressure gradient. This flow separation in aerodynamics means an increase in drag and a reduction in lift.  Capturing all these adverse and variable factors of flow precisely in turbulence modeling has been an ongoing challenge for designers and engineers.

## Boundary Layers

As mentioned above, flow passing through the boundary layer respective to its viscosity, flow velocity, and pressure is an important fundamental of aerodynamics. When the air flows through the airfoil, the thin layer of fluid near its immediate vicinity is called the boundary layer. This boundary layer can be laminar or turbulent. For a higher Reynold number, when the velocity becomes high and the flow unsteady, the turbulent boundary layer becomes prominent. At a higher angle of attack, this turbulent layer can break away to cause flow separation. The skin friction contributing to the turbulent boundary layer can cause drag and affect lift in the aerodynamic system.

## Analyzing the Fundamentals of Aerodynamics With CFD

When solving aerodynamic problems, the key is to understand the acting flow regime and calculate the associated forces and boundary layers. This also involves the analysis of different aerodynamic variables in different flow conditions to analyze the ideal lift and drag permissible in the design.

Determining these influencing factors is easier with the proper CFD tools. Along with Reynolds number calculations for flow regime investigation, CFD simulation also enables the numerical analysis of acting forces and aerodynamic variables. The simulation ability of platforms such as Fidelity and the meshing ability of Fidelity Pointwise can help with the comprehensive visualization and advanced analysis of the fundamentals of aerodynamics for efficient design solutions.