POF CHAPTER 3- AIRFLOW

"What is the main focus of Chapter 3 in Principles of Flight?"

"The study of airflow and the two key principles that describe its behaviour; the equation of continuity and Bernoulli's theorem."

"What happens when a flow of air encounters an object?"

"It usually attaches to the surface of the object and flows smoothly around it."

"What two conditions are necessary for airflow attachment?"

"The object must be relatively thin and smooth with gradual surface changes; and it must be presented at a small angle to the airflow."

"What phenomenon explains airflow attachment?"

"The Coanda effect."

"What is the Coanda effect?"

"A phenomenon where a high-speed jet attaches itself to a nearby surface and follows its curvature; even as the surface bends away from the initial flow."

"What causes the Coanda effect to occur?"

"The entrainment of air by the jet; when entrainment is blocked on one side; the jet turns toward the blockage."

"What is entrainment in airflow?"

"The pulling-in of surrounding air by a high-speed jet; which can cause low-pressure regions near surfaces."

"What results from entrainment between a jet and a thin surface?"

"An area of relatively low pressure forms; creating a pressure gradient that causes the flow to bend toward the surface."

"What does the Coanda effect explain in aerodynamics?"

"It explains why airflow bends towards the upper surface of a wing."

"What is the steady state flow field?"

"The condition where airflow around a body has stabilised; with constant direction and speed at each point."

"What do lines drawn in a flow field represent?"

"They show the direction and relative speed of the airflow around an object."

"What is a streamline?"

"A line showing the instantaneous direction of airflow at every point in a steady flow."

"Can two streamlines cross each other?"

"No; because two different flow directions cannot exist at the same point in space."

"What is a stream tube?"

"A volume of air bounded by streamlines; in which all air flows along the tube and none enters or leaves through the sides."

"What does the stream tube concept demonstrate?"

"That whatever mass of air enters the tube must also leave it; preserving constant mass flow."

"What is a pathline?"

"The trajectory that an individual particle of air follows through the flow."

"How do streamlines and pathlines relate in steady flow?"

"They coincide; showing identical patterns of motion."

"When are pathlines used instead of streamlines?"

"In turbulent or unsteady flow; where the direction and speed of air vary unpredictably."

"Why can pathlines cross but streamlines cannot?"

"Because pathlines show different particles moving through the same region at different times."

"What type of flow is described as smooth and predictable?"

"Streamline or laminar flow."

"Why is laminar flow also called streamlined flow?"

"Because air moves in layers or laminations with little mixing between them."

"What causes turbulent flow?"

"Irregular surfaces or abrupt shape changes; which break down laminar flow into chaotic motion."

"How is turbulent flow shown in diagrams?"

"As short; jumbled; irregular pathlines indicating random motion."

"Why does turbulent flow create more drag?"

"Because converting laminar flow into turbulence requires energy; increasing resistance."

"What is vortex flow?"

"A rotating mass of air that has both steady and turbulent characteristics; forming a helical motion."

"Why does vortex flow require energy?"

"Because creating rotational motion in the air consumes kinetic energy; increasing drag."

"How does vortex flow behave over time?"

"It gradually loses energy; becomes larger and slower; and eventually dissipates."

"What type of flow does the air over an aircraft contain?"

"A combination of laminar; turbulent; and vortex flow."

"What is the difference between two-dimensional and three-dimensional flow?"

"Two-dimensional flow occurs in a single plane such as over an aerofoil section; while three-dimensional flow includes motion in all directions around a full wing."

"Why is two-dimensional flow useful for study?"

"Because it simplifies analysis and helps explain basic aerodynamic principles."

"Why is three-dimensional flow necessary to understand a real wing?"

"Because the airflow interacts across the span and includes complex vortices."

"What assumptions are made about airflow for basic aerodynamic analysis?"

"That air behaves as an ideal fluid."

"What are the two main properties of an ideal fluid?"

"It is incompressible and has no viscosity."

"What does incompressible mean in this context?"

"That the air's density remains constant as it moves or encounters an object."

"When is incompressible flow a valid assumption?"

"At low speeds; typically below about 300 knots true airspeed."

"What is the second assumption about air in an ideal fluid?"

"That it has no viscosity; so there is no internal friction."

"What is the effect of viscosity in real airflow?"

"It creates drag and causes the boundary layer; effects studied in later chapters."

"What frame of reference is usually assumed when analysing airflow?"

"A stationary aircraft with moving air."

"What does this frame of reference imply?"

"That the air possesses kinetic energy; not the aircraft."

"What physical law underlies the equation of continuity?"

"The principle of conservation of mass."

"What does the equation of continuity state?"

"That the mass flow of air entering a stream tube equals the mass flow leaving it."

"What variables determine mass flow in a stream tube?"

"Cross-sectional area; velocity; and air density."

"What is the mathematical form of the continuity equation?"

"A × V × ρ = constant."

"What happens to velocity if the cross-sectional area decreases while density remains constant?"

"The velocity increases to maintain constant mass flow."

"What device illustrates the equation of continuity?"

"A venturi tube; which has a converging section; a narrow throat; and a diverging section."

"What happens to airflow speed through the venturi throat?"

"It increases to its maximum value where the area is smallest."

"What happens to airflow speed as the venturi expands after the throat?"

"It decreases again as cross-sectional area increases."

"What does streamline spacing indicate about velocity?"

"Closer streamlines indicate higher velocity; wider spacing indicates lower velocity."

"Up to what speed can airflow be treated as incompressible?"

"Up to about 300 knots true airspeed."

"What changes when airflow approaches high subsonic speeds?"

"Compressibility effects become significant and density can no longer be assumed constant."

"What fundamental law is Bernoulli's theorem based on?"

"The law of conservation of energy."

"What does Bernoulli's theorem state?"

"That in an ideal fluid; the sum of pressure energy and kinetic energy per unit volume remains constant."

"What is the equation form of Bernoulli's theorem?"

"Static pressure plus dynamic pressure equals a constant value; Ps + ½ρV² = constant."

"What does total pressure represent in Bernoulli's equation?"

"The sum of static pressure and dynamic pressure."

"How is Bernoulli's theorem expressed using conventional symbols?"

"Ps + q = Pt; where q is dynamic pressure and Pt is total pressure."

"What does Bernoulli's theorem explain in a venturi?"

"As velocity increases through the throat; dynamic pressure rises and static pressure falls to maintain constant total pressure."

"Where is static pressure highest in a venturi?"

"At the entrance and exit; where flow velocity is lowest."

"Where is dynamic pressure highest in a venturi?"

"At the throat; where velocity is greatest."

"How do static and dynamic pressure vary relative to each other?"

"They vary inversely; as one increases the other decreases."

"What remains constant in Bernoulli's system?"

"The total pressure; which equals static pressure plus dynamic pressure."

"What happens to total pressure if flow energy changes?"

"It changes proportionally to the total energy of the airflow."

"How does Bernoulli's theorem relate to airflow over an aerofoil?"

"It explains that faster-moving air over the upper surface has lower static pressure; contributing to lift."

"What combination of principles describes most aerodynamic flow behaviour?"

"The equation of continuity and Bernoulli's theorem together describe the relationships between area; velocity; and pressure."

"What are the three main types of airflow encountered around aircraft surfaces?"

"Laminar; turbulent; and vortex flow."

"What does the equation of continuity ensure about airflow through a stream tube?"

"That the mass flow rate remains constant at all points along the tube."

"In low-speed flight what relationship does the equation of continuity simplify to?"

"A₁V₁ = A₂V₂ = A₃V₃ for constant density."

"What does a reduction in cross-sectional area cause in airflow?"

"An increase in velocity and a decrease in static pressure."

"What does an increase in streamline spacing indicate?"

"That the airflow is slowing down."

"According to Bernoulli's theorem what happens at the narrowest point in a venturi?"

"The airflow reaches its maximum velocity; dynamic pressure is greatest; static pressure is least; but total pressure remains unchanged."

"What causes the flow to slow again after the throat of a venturi?"

"The increasing area reduces velocity; restoring static pressure."

"Why are continuity and Bernoulli's principles considered complementary?"

"Because one describes conservation of mass and the other conservation of energy; both essential to understanding lift and drag."

"What does the airflow summary of the chapter conclude?"

"That airflow around an aircraft includes laminar; turbulent; and vortex components governed by the laws of continuity and Bernoulli."

"How does Bernoulli's theorem help explain lift?"

"It shows that increased flow speed over the wing reduces static pressure; creating an upward pressure differential."

"What is the key relationship between pressure and velocity in steady airflow?"

"As velocity increases; static pressure decreases; maintaining constant total pressure."

"Why is two-dimensional analysis limited?"

"Because it only represents flow across one section and ignores spanwise or three-dimensional effects."

"Why is understanding airflow essential for pilots?"

"Because it underpins all aerodynamic principles including lift; drag; stability; and control."

"What are the two central physical laws governing steady airflow?"

"The conservation of mass and the conservation of energy."