A stable aerodynamic design requires the balance of forces comprising lift, drag, thrust, and gravity.
Airfoil design accounts for the speed, lift, and performance aspects of flight.
The Coanda effect studies the airfoil shape and its influence over the flow jet in achieving the desired lift.
Understanding fluid flow and the acting aerodynamic forces are important to achieving dynamic equilibrium
With increased safety and quality standards, the complexity of achieving efficiency in aerodynamic design is also increasing. This complexity begins with the study of airfoil shapes and extends to the design of the entire aircraft. The key to cracking the perfect aerodynamic design code is to look at the challenges and opportunities; a high-efficiency aerodynamic design involves a smooth surface, less drag, increased lift, and similar quality that provides flight stability.
For a wide range of flight conditions, achieving precise and accurate aerodynamic designs can be done with computational fluid dynamics (CFD). In this article, we will discuss the factors that influence aerodynamic behavior and how CFD can accommodate them to achieve a quality aerodynamic design.
The Fundamental Aerodynamic Factors
Quality aerodynamic design must begin with a clear understanding of airflow, its interaction with airfoils, and the forces acting on it. There are four basic forces that act on an aircraft—lift, drag, thrust, and gravity. Aerodynamic design teams should have a thorough understanding of these forces to ensure efficiency and stability in design.
Lift: An upward force that is responsible for the flight taking place. A lift is created when there is a difference in air pressure. In aircraft, this pressure difference can be maintained when the air flows at different speeds above and below the wings. The airflow speed is faster above the wing surface, reducing the pressure. The speed is slower at the bottom of the wing, creating relatively high pressure, which balances the flight.
Drag: The resistance that opposes the aerodynamic motion. Drag is generated when the surface comes in contact with fluid in motion. This resistance is the reason for the decreased speed of the aircraft. Excess drag is also the reason for turbulence, which hinders the stability of the aircraft.
Thrust: The force that propels the plane forward. It works against the weight and drag to maintain flight at a constant speed. Propellers or jet engines are responsible for generating thrust, which is generated by passing the fluid (air) through the engine at a relatively high speed.
Gravity: The downward acting force that pulls the airplane towards the Earth. This can be countered by generating enough lift that can defy the gravitational pull. Airplanes are always designed to be lighter so that less thrust and lift are needed to keep the plane in the air.
An ideal airfoil shape enables maximum efficiency in aerodynamic performance. Therefore, its design parameters, such as thickness or angle of attack, are factors of utmost importance for a wide speed range. A thin laminar flow airfoil is usually associated with high-speed aircraft design. These airfoils require significantly less energy to deflect the air while their design also ensures that the flow is uninterrupted and the drag is minimum. The uniform pressure distribution and reduced drag help create a significant lift. Similarly, a low-speed aircraft can be designed to have airfoils with greater thickness.
A number of techniques have been used for airfoil design for aerodynamical applications. The most prominent ones include:
- Direct method: Analyzes the airfoil geometry to calculate the performance
- Inverse method: Supports airfoil design based on the pressure distribution
- Optimization method: Utilizes the existing analysis to maximize the airfoil efficiency
The Coanda Effect
When studying lift for aerodynamic design, we usually find the prominent principles to be:
Bernoulli’s principle: Explains the lift for an airfoil shape due to differential pressure.
The Coanda effect: Analyzes the fluid flowing over an airfoil to optimize its shape so the maximum lift coefficient can be obtained.
The airfoil shape is characterized by its curved top surface and flat bottom surface, the aim of both being the deflection of the air. When the air flows over the curved top surface, the flow jet tends to stay attached to the convex wing surface deflecting the jet of air downwards. This flow attachment is responsible for increasing aerodynamic lift.
The Coanda effect in aircraft can be increased by ensuring that the rear edge of the wing is sharp and is facing diagonally downwards.
Optimal Aerodynamic Design Using CFD Tools
There are many aspects of aerodynamic design where computational fluid dynamics (CFD) is beneficial. For flow analysis, airfoil design, and boundary layer assessment, CFD can take into account the acting forces, shear stress, roughness effect, and the Coanda effect during simulation. For high or low Reynolds numbers, the flow simulation can be done using the Reynolds-Averaged Navier Stokes (RANS) approach.
The features of Fidelity and Fidelity Pointwise from Cadence can help designers build suitable models and provide the computational capabilities needed to assess the associated variables that influence lift in aerodynamic design. Furthermore, with advanced visualization capabilities and high-fidelity simulation, these CFD platforms are ideal tools for achieving an optimal aerodynamic design.