Subsonic Aerodynamics Basics

Subsonic Aerodynamics - Basics

• Mass Conservation: Mass remains constant over time; it cannot be created or destroyed, only rearranged in space.
• Bernoulli's Theorem: In streamlined fluid flow, the sum of all energies (static and dynamic pressure) is constant. Static pressure + dynamic pressure = constant.
• Total Pressure Constant: Dynamic pressure (1/2ρV²) + Static pressure = constant.
• Bernoulli's Equation: PT = PS + 1/2ρV² (Total pressure = Static pressure + Dynamic pressure)
• Dynamic Pressure: Is equal to 1/2 ρV². It's zero when velocity is zero. It increases proportionally with the square of velocity.
• Static Pressure and Density: If temperature is constant and pressure increases, density increases. If density is constant, dynamic pressure increases with velocity squared.
• Venturi Effect: Static pressure in a venturi throat is lower than in undisturbed airflow, while dynamic pressure is higher. Total pressure remains the same.
• Incompressible Flow: Density does not change.
• Temperature and Humidity: Density of the atmosphere decreases with an increase in humidity

Aerodynamic Center and Centre of Pressure

• Aerodynamic center: The point where the pitching moment coefficient doesn't change with varying angles of attack. Located at approximately 25% chord.
• Centre of Pressure (CP): The point where the single resultant aerodynamic force on the airfoil acts. Its position varies with the angle of attack.

Wing Loading and Forces

• Wing Loading: Ratio of aircraft weight to wing area (W/S).
• Drag: Acts in the direction of relative wind (airflow).
• Lift: Perpendicular to relative wind (airflow).
• Angle of Attack: Angle between wing chord line and direction of relative airflow.
• Angle of Incidence: Angle between wing chord line and relative undisturbed airflow.

Lift and Drag Coefficients (CL and CD)

• CL vs AoA (Angle of Attack): CL is max at critical AoA and decreases after that point (stalling angle)
• CD vs AoA: CD is lower at lower AoA, generally decreasing gradually up to stall.
• Lift related to velocity: Lift is proportional to the square of velocity (V^2) and air density (1/2ρV²)
• Aerodynamic Center location: Is at 25% chord location.

Aerodynamic Concepts:

• Aspect Ratio: wingspan / mean chord
• Taper Ratio: ratio of tip chord to root chord
• Sweep Angle: Angle between the quarter-chord line and the lateral axis.
• Thickness to Chord Ratio: Ratio of maximum thickness to chord length of an airfoil section.
• Camber: The difference between the upper and lower surfaces' curves of an airfoil.
• Mean Camber Line: Line midway between upper and lower surfaces.

Aerodynamic Coefficients

• CL: Lift Coefficient (lift/qS)
• CD: Drag Coefficient (drag/qS)
• CL/CD Max: Optimum lift-to-drag ratio important for aircraft performance, efficiency and stability.

Subsonic and Supersonic Flow

• Mach number (M): Ratio of the speed of an object to the local speed of sound (TAS / local speed of sound)
• Transonic Range: Speeds just above and below the speed of sound.
• Critical Mach Number (M_crit): The Mach number at which the airflow over a part of the aircraft reaches sonic speed.
• Drag Divergence Mach Number (M_drag): The Mach number where the drag starts to increase rapidly.

Three-Dimensional Flow

• Induced Drag: Drag caused by the downwash of air from the wings. It is proportional to (CL^2 / AR).
• Aspect ration: Increasing aspect ratio decreases induced drag, because of the effect of tip vortices. Higher aspect ratios = high lift and low drag, as in an eliptical shaped wing compared to a rectangular one. Generally increasing aspect ration is beneficial for the aero plane's efficiency.

Stall Angle of Attack

• Stall: The speed and/or angle of attack at which an aeroplane is no longer able to generate enough lift to maintain its altitude
• Critical AoA: The minimum angle needed to cause a stall.
• Effect of Wing Shape: A wing shape can be more effective to increase the critical/stall angle of attack by using flaps, slats, spoiler and winglets.

Control Surface Deflections (+ or -)

• Aerodynamic Forces: Vary with respect to the speed, and angle of attack, of the moving air.
• Adverse Yaw: A yawing moment that acts in opposite direction to the turn, it requires differential ailerons to counteract the moment.

Engine and Propeller Considerations

• Static Stability: The tendency of an aircraft to return to its original trim condition after a disturbance. (e.g., a gust).
• Dynamic Stability: The aeroplanes response to a disturbance over time. (e.g. a gust)
• Inertia: An important effect of engine location regarding propeller gyroscopic effect, and sensitivity to Dutch roll and yaw.
• Propeller efficiency effects propeller twisting from the root to the tips, blade pitch, and blade radius.

Summary of High and Low Speed Flight

• High Speed: Primarily concerned with supersonic airflow and the formation of shock waves, leading to drag increase and potentially stability loss (exceeding critical conditions).
• Low Speed: Primarily concerned with angle of attack and stalling, control surfaces, adverse yaw, and potential stability issues when using flaps or at stall speed.

This quiz covers fundamental concepts in subsonic aerodynamics, including mass conservation, Bernoulli's theorem, and the Venturi effect. It explains how dynamic and static pressures interact in fluid flow. Test your understanding of these essential principles in aerodynamics.