Aerodynamics Fundamentals

Aerodynamics is the study of how air moves around objects and the resulting forces created by such interaction. It plays a crucial role in the performance, stability, control, and safety of aircraft.

There are four main that govern an aircraft’s motion: lift, weight, thrust, and drag. These forces determine how an aircraft accelerates, climbs, descends, and maintains flight.

Lift is the aerodynamic force acting perpendicular to the aircraft’s flight direction and results from the difference in pressure and shear stresses produced by air flowing around an aircraft. Most of an aircraft’s lift is generated by its wings.

Drag is the aerodynamic force that resists the motion of the aircraft through the air. It depends on many factors such as the aircraft’s shape, size, speed, and the properties of the surrounding air.

Thrust is the force produced by an aircraft’s propulsion system. It generally acts in the forward direction and allows the aircraft to overcome opposing drag forces. The Boeing 787, for example, is typically powered by the Rolls-Royce Trent 1000 or General Electric GEnx engines.

Weight is the gravitational force pulling the aircraft towards the center of the earth. Although the aircraft’s mass can be unevenly distributed throughout the structure, engineers can represent its weight as acting through a single point called the center of gravity (CG). Another important point of consideration is the aircraft’s weight changing throughout the course of a fligh as fuel is consumed. This changes both the aircraft’s total weight and potentially its center of gravity, which engineers must account for when designing and operating an aircraft.

If you simplify everything down to a flat/straight flight without acceleration, the four forces are approximately balanced, where lift = weight and thrust = drag.

The central challenge in aerodynamics is generating useful aerodynamic forces while minimizing unwanted drag.

Aerodynamics is a branch of fluid dynamics, where air is the ‘fluid’ being studied. Unlike most liquids, certain gases can undergo significant changes in density at different pressures or temperatures. This concept is known as compressibility. At relatively low speeds, air can often be approximated as incompressible, but compressibility becomes an increasingly more important concept as aircraft speed increases. 

Airfoils

When it comes to wing shape, there are a lot of factors that engineers can modify to influence an aircraft’s aerodynamic performance, including camber, thickness, chord length, and aspect ratio. 

Camber describes the curvature of the mean camber line, the curve midway between an airfoil’s upper and lower surfaces, from its leading edge to trailing edge. Low pressure air flows above the airfoil while high pressure air flows below the airfoil, generating lift. Changing the camber changes the pressure distribution around the airfoil, and can therefore affect the lift it generates. In general, airflow around the upper surface of a wing has a lower pressure than the airflow around the lower surface, producing a net upward force.

Bernoulli’s principle describes the relationship between fluid velocity and pressure, stating that higher fluid velocity is associated with a lower static pressure. First published in Daniel Bernoulli’s ‘Hydrodynamica’ in 1738, Bernoulli applied conservation of energy to problems involving fluid motion.

Here is a small design challenge: You need to design a wing for a slow-moving drone. Would you choose a long, narrow wing or a short, wide wing? Why?

The answer will be at the bottom of the newsletter!

How Do Engineers Study Aerodynamics?

There are three main approaches researchers use to study aerodynamics: 

Theory: equations and physical models

Experiment: wind tunnels, pressure sensors, force measurements

Computation: Computational Fluid Dynamics (CFD) simulations

Each approach provides engineers and researchers with different pieces of information. Mathematical models help explain the underlying physics behind fluid motion, experiments help validate and provide measurements from real systems, and computational simulations allow engineers to make predictions or investigate conditions that may be difficult or expensive to test experimentally.

Personal Project: CFD Modeling

Of course, since this month is aerodynamics themed, I had to try out CFD modeling! Computational Fluid Dynamics, or CFD for short, uses numerical methods to simulate fluid flow and predict measurements such as velocity, pressure, and temperature.

I decided to model water flowing around a manta ray. I designed the 3D model in Onshape and then moved to Simscale, which is a popular browser-based engineering platform that runs structural, fluid, thermal, and electromagnetic simulations. For this project, I will be focusing on its fluid simulation capabilities. 

The simulation is currently running, so I will update this page with the results once it has finished.

Author’s Note

As I am writing this letter, I can hear the loud roar of the Blue Angels practicing for the upcoming air show at Miramar Air Station, and I thought that it was super fitting for this month’s aerodynamics theme!

Answers

For a slow-moving drone, a long and narrow wing is generally preferred because it gives the wing a higher aspect ratio, which helps reduce induced drag. Therefore, a high-aspect ratio wing can produce lift more efficiently. 

However, airfoil and wing design always involves trade-offs. A longer, narrower wing can be less structurally robust and less maneuverable compared to a shorter, wider wing, which are important factors to keep in mind when designing an aircraft.

Sources

https://www.sheffield.com/2024/the-importance-of-aircraft-aerodynamics-in-design.html

https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/four-forces-on-an-airplane/

simscale.com

https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/14%3A_The_Behavior_of_Gases/14.01%3A_Compressibility 

https://en.wikipedia.org/wiki/Boeing_787_Dreamliner 

https://en.wikipedia.org/wiki/Bernoulli%27s_principle