Flight Training Blog

The Four Forces of Flight

At the core of aerodynamics are the four forces that act on an aircraft: lift, weight, thrust and drag. These forces are in constant interaction and understanding them is key to flying.

Lift

Generated by the wings, lift is a force that acts perpendicular to the relative wind and opposes weight. Pilots control lift through angle of attack (AoA) and airspeed. The wing shape (airfoil) and the smooth flow of air over it is critical to generate enough lift. Additionally, lift is also affected by the air density, which changes with altitude and weather conditions. Understanding how these factors interplay helps pilots anticipate and adjust to varying flight environments.

Weight

Weight is the force of gravity pulling the aircraft towards the Earth. It acts through the aircraft’s center of gravity (CG). Balancing weight with lift is essential for level flight. Managing the aircraft’s weight distribution, especially fuel and payload, is crucial for maintaining control and stability. Pre-flight checks often include ensuring the CG is within safe limits, which directly impacts the aircraft’s performance and handling characteristics.

The position of the center of gravity (CG) changes with fuel consumption. On short to medium flights, the difference is usually insignificant, but for long-haul flights, it’s a critical factor to consider. Finding the right balance is essential. A more forward CG results in higher fuel consumption and increases the stalling airspeed. Conversely, having the CG too far aft makes the aircraft less stable. For instance, during takeoff, if the aircraft is not properly trimmed, you can notice the difference during rotation. In a 70+ ton aircraft, moving a few hundred kilos from forward to aft can be noticeable. In extreme cases, if the CG is out of limits, the aircraft might even rotate on its own below the designated rotation speed, risking a tail strike.

Thrust

Thrust is produced by the engines and pushes the aircraft forward. Jet engines produce thrust by expelling exhaust gases at high speed, which counteracts drag to maintain the airplane’s speed during flight. Managing thrust is about power settings, fuel efficiency and responding to different phases of flight. Proper use of thrust is critical during takeoff, climb and go-arounds. Moreover, understanding the relationship between thrust and drag is essential for maintaining optimal cruise speeds and ensuring fuel efficiency, especially on long flights.

Jet engines have a delay of a few seconds when you increase thrust. This delay must always be considered during critical phases of flight. To mitigate this, most jet aircraft have different types of idle thrust—one for the ground and one for when in flight. These settings activate automatically in specific situations to reduce the engine’s reaction time.

Drag

Drag opposes thrust and is divided into two main types: parasitic drag and induced drag. Parasitic drag includes form drag, skin friction and interference drag while induced drag is a byproduct of lift. Pilots minimize drag through smooth efficient flight paths and clean aircraft surfaces. Advanced techniques, such as using drag reduction devices like winglets and fairings, also play a significant role in modern aircraft design, enhancing overall aerodynamic efficiency.

Bernoulli’s Principle and Lift

Bernoulli’s principle is key to lift. It states that an increase in the speed of a fluid (air, in our case) results in a decrease in pressure. In the context of Bernoulli’s equation, incompressible flow assumes constant fluid density, while compressible flow accounts for significant changes in fluid density. For pilots that means the faster airflow over the curved top surface of the wing results in lower pressure than the higher pressure underneath the wing and that’s lift. Understanding this helps us understand why smooth airflow over the wings is so important. Pilots can observe Bernoulli’s principle in action during various flight maneuvers, especially when adjusting the flaps and slats to increase the wing’s surface area and enhance lift during takeoff and landing.

The Boundary Layer and Flow Types

The boundary layer is the thin layer of air close to the aircraft’s surface where friction effects are significant. Understanding the flow field, which describes the dynamics and characteristics of fluid motion around objects, is crucial for analyzing how pressure and velocity vary depending on the shape and movement of the aircraft. There are two types of boundary layer flows:

• Laminar flow: Smooth and orderly, less skin friction drag. But can separate from the wing surface more easily which can lead to stall.
• Turbulent flow: Chaotic and mixed, more drag but adheres better to the surface, delays flow separation.

Fluid mechanics plays a vital role in analyzing fluid flows around aerodynamic bodies, forming the basis for understanding air behavior and pressure changes. Pilots often monitor boundary layer behavior using instruments and sensors that detect changes in airflow patterns. Advanced aircraft are equipped with systems that can manipulate the boundary layer, enhancing performance and safety. The assumption of steady flow is important when applying Bernoulli’s equation, as it ensures consistent analysis of pressure and velocity relationships within the fluid. It is in the pilot’s interest to maintain laminar flow where possible to reduce drag but they must be aware of conditions that can transition it to turbulent flow.

Angle of Attack and Stall: The Link

The angle of attack (AoA) is the angle between the wing’s chord line and the relative wind. It’s a fundamental concept that directly affects lift. A higher AoA increases lift – up to a point. Beyond the critical angle of attack the smooth airflow over the wing breaks down and you stall.

In our previous article on angle of attack we discussed in detail what leads to stalls and their recovery procedures. Remember stalls can occur at any airspeed and attitude if the critical AoA is exceeded. Pilots use AoA indicators in the cockpit to keep track of this critical parameter, ensuring they remain within safe limits during different phases of flight, particularly during steep turns, slow flight, and high-angle climbs.

Conclusion

Aerodynamics is more than just a theory; it’s the base of every flight. By understanding and applying these principles we can fly safer and more efficient. Whether you’re climbing to cruise or landing perfectly the forces of lift, weight, thrust and drag are at work. Continuous learning and applying aerodynamic principles in simulated and real-world scenarios help pilots refine their skills and adapt to evolving aviation technologies. Aerodynamics makes you a better pilot and gets you closer to being a master of aviation!