Basic Aerodynamics for Private Pilots

Fundamental Forces of Flight and Aerodynamic Equilibrium

In the study of basic aerodynamics, four primary forces act upon an aircraft in flight: Lift, Weight, Thrust, and Drag. These forces must be in a state of equilibrium for specific flight conditions. Specifically, when an aircraft is in straight and level flight, lift is equal to weight and thrust is equal to drag. This equilibrium of forces occurs during non-accelerated flight. If an aircraft is at rest on the ground or accelerating, these forces are not in the same state of balance. The rudder, one of the primary control surfaces, is specifically designed to control the yaw of the aircraft, which is the movement around its vertical axis.

Principles of Airflow, Lift, and Angle of Attack

Aerodynamic lift is fundamentally explained by Bernoulli's Principle, which states that as air travels faster over the curved upper surface of an airfoil, the pressure on that upper surface is reduced, creating a pressure differential that generates lift. A critical concept in understanding this lift is the "Angle of Attack." This is defined as the acute angle between the chord line of the wing and the relative wind. The chord line is the imaginary straight line joining the leading and trailing edges of the airfoil. It is important to distinguish this from the angle of incidence or the angle between the longitudinal axis and the horizon. The angle of attack at which a wing stalls remains constant regardless of changes in the aircraft's gross weight, though factors like center of gravity ($CG$) positioning can influence other flight characteristics.

Aircraft Stability and Center of Gravity Dynamics

An aircraft that is described as inherently stable is one that requires less effort for the pilot to control. Longitudinal stability is primarily determined by the relative location of the center of gravity ($CG$) with respect to the center of lift. The positioning of the $CG$ has drastic effects on flight safety and handling. If an aircraft is loaded such that the $CG$ is located behind the aft (posterior) limit, the pilot may experience extreme difficulty in recovering from a stall. Furthermore, a $CG$ located further toward the rear limit makes the aircraft less stable at all airspeeds. Conversely, in aircraft other than those with T-tails, reducing power causes the nose to pitch down because the downwash on the elevators from the propeller slipstream is reduced, which in turn reduces the effectiveness of the elevator. Additionally, changes in the center of pressure on a wing affect the aerodynamic balance and controlability of the aircraft.

Wingtip Vortices and Wake Turbulence Avoidance

Wingtip vortices are a byproduct of lift; therefore, they are created only when an aircraft is generating lift. The intensity of these vortices—often referred to as wake turbulence—is at its maximum when the generating aircraft is heavy, clean (flaps and gear retracted), and slow. These vortices circulate outward, upward, and around each wingtip and have a tendency to sink below the flight path of the generating aircraft. To avoid these hazards when landing behind a large aircraft, a pilot should stay above the large aircraft's final approach flight path and land beyond its touchdown point. When departing behind a large or heavy aircraft, the pilot should maneuver the aircraft above and upwind of the heavy aircraft's path. Particular caution is required during light tailwind conditions, as this can move the vortices into the landing or takeoff paths. Pilots must be alert to these vortices as they sink into the flight paths of aircraft operating below the aircraft generating the turbulence.

Load Factors and Structural Limitations

The load factor of an aircraft is the ratio of the total load supported by the airplane's wings to the actual weight of the airplane and its contents. Certain maneuvers increase this factor; for example, turns increase the load factor compared to straight and level flight. The amount of excess load that can be imposed on a wing is dependent on the speed of the aircraft. When an aircraft is subjected to an increased load factor, particularly during an approach to a stall, it will enter the stall at a higher indicated airspeed. At a constant altitude, the weight supported by the structure increases with the bank angle. For instance, an aircraft weighing $2,300\,lb$ in a $60^{\circ}$ banked turn must support approximately $4,600\,lb$. An aircraft weighing $3,300\,lb$ in a $30^{\circ}$ banked turn supports approximately $3,960\,lb$. An aircraft weighing $4,500\,lb$ in a $45^{\circ}$ banked turn supports approximately $6,750\,lb$.

Ground Effect and Flap Operations

Ground effect is the result of the earth's surface interfering with the airflow patterns around an aircraft, specifically when the aircraft is less than one wingspan length above the surface. This phenomenon causes a decrease in induced drag, which can result in