PoF- Chapter 9 Three Dimensional Flow

"What is spanwise flow?"

"Flow along the span of the wing; perpendicular to the aircraft's direction of motion."

"Do all wings experience spanwise flow?"

"Yes; all wings experience some degree of spanwise flow depending on design and flight conditions."

"What two main factors influence spanwise flow?"

"The inertia of the air mass and local pressure differences around the aircraft."

"Why does air tend to keep moving in its current direction?"

"Because it possesses momentum and inertia."

"What causes air to change direction despite inertia?"

"Pressure differentials; even small differences cause air to move toward low pressure."

"What determines flow speed when air moves between pressure regions?"

"The steepness of the pressure gradient; the greater the difference and the shorter the distance, the faster the flow."

"What is the main cause of pressure differentials around a wing?"

"Differences in static pressure between the upper and lower surfaces."

"Where are regions of high and low pressure found on a lifting wing?"

"Low pressure exists above the wing and high pressure exists below it."

"What prevents these two pressure regions from mixing along most of the wing?"

"The physical structure of the wing separates the two regions."

"What happens at the wingtip where the wing ends?"

"Air from the high-pressure region below flows around the tip into the low-pressure region above."

"What is this flow around the wingtip called?"

"It initiates a wingtip vortex."

"How does a wingtip vortex form?"

"High-pressure air from beneath the wing flows around the tip into the low-pressure region above, rolling into a rotating mass of air."

"How many tip vortices form on a wing?"

"Two; one at each wingtip."

"In which direction do the wingtip vortices rotate when viewed from behind?"

"Clockwise around the left wingtip and anti-clockwise around the right wingtip."

"What factors primarily affect the intensity of wingtip vortices?"

"The pressure differential between the upper and lower surfaces and the time that differential acts on the air."

"What happens to vortex intensity as lift increases?"

"It increases because the pressure difference is greater."

"When will no vortex form?"

"When no lift is produced and no pressure differential exists between upper and lower surfaces."

"Why does air take time to accelerate into vortical motion?"

"Because it has mass and inertia."

"How does airspeed affect vortex intensity?"

"Slower speeds create stronger vortices because the pressure differential acts on each parcel of air for a longer time."

"What effect does chord length have on vortex strength?"

"Longer chord wings create stronger vortices because each air parcel is influenced longer."

"How does aspect ratio affect vortex intensity?"

"A low aspect ratio wing produces more intense vortices; a high aspect ratio wing produces weaker ones."

"Summarise three factors that reduce tip vortex intensity."

"Increasing speed; increasing aspect ratio; and reducing lift."

"What additional aerodynamic effects result from tip trailing vortices?"

"Downwash; additional drag; and wake turbulence."

"How do tip vortices produce downwash?"

"Their rotation drags surrounding air downward behind the wing."

"Why do tip vortices increase drag?"

"Because the vortices contain rotational energy supplied by the aircraft; energy lost to the air manifests as drag."

"What is wake turbulence?"

"The highly energetic rotating air behind an aircraft produced by its wingtip vortices."

"When is wake turbulence strongest?"

"At low speeds; high angles of attack; and when the aircraft is heavy and clean (flaps retracted)."

"How fast can wake vortex air rotate?"

"Up to about 300 feet per second."

"Why is wake turbulence hazardous?"

"Because it can roll and damage following aircraft; especially lighter ones."

"Summarise the Challenger 604 vs A380 wake turbulence incident."

"A Challenger encountered an A380's wake at FL340; rolled uncontrollably; lost altitude; and sustained severe structural damage."

"At what altitude and distance does wake turbulence typically stabilise?"

"Roughly 900 feet below the generating aircraft and within about 5 NM behind."

"Why does wake turbulence migrate downward?"

"Because the downwash behind the wing drags the vortices downward."

"When are tip vortices weakest?"

"When inboard trailing-edge flaps are extended; reducing the pressure differential at the tips."

"What causes spanwise flow along the wing surfaces?"

"A pressure gradient along the span caused by differences between root and tip pressures."

"In what direction is spanwise flow above the wing?"

"Inwards toward the fuselage."

"In what direction is spanwise flow below the wing?"

"Outwards toward the wingtip."

"Why is spanwise flow strongest near the wingtips?"

"Because the pressure gradient is steepest there and weakest near the roots."

"What results from upper and lower surface spanwise flows meeting at the trailing edge?"

"The formation of trailing-edge vortices."

"How does wing geometry affect trailing-edge vortex strength?"

"On rectangular wings; vortices are stronger near the tips and weaker near the roots."

"What happens to trailing-edge vortices near the tips?"

"They are absorbed into the tip vortex; strengthening it further."

"What overall effect do vortices have on airflow leaving the trailing edge?"

"They impart a downward velocity component to the air; producing downwash."

"In two-dimensional flow; how do upwash and downwash compare?"

"They are equal in magnitude and opposite in direction."

"In three-dimensional flow; how do upwash and downwash compare?"

"Downwash increases while upwash decreases due to the influence of vortices."

"What does the downwash sheet represent?"

"The total downward flow leaving the trailing edge; strongest near the tip vortices."

"What is effective airflow?"

"The resultant airflow direction at the wing after combining forward velocity and downwash velocity."

"How does effective airflow differ from relative airflow?"

"It is inclined slightly downward due to the downwash behind the wing."

"What determines the wing's aerodynamic forces?"

"The direction of the effective airflow; not the free-stream relative airflow."

"What is the induced angle of attack?"

"The angle between the relative airflow and the effective airflow."

"Why does the induced angle of attack exist?"

"Because the effective airflow is deflected downward by downwash."

"What components make up the total aerodynamic reaction in three-dimensional flow?"

"Lift perpendicular to the relative airflow and induced drag parallel to it."

"What is induced drag?"

"The rearward component of lift caused by the backward tilt of the total reaction due to downwash."

"When does induced drag occur?"

"Whenever lift is produced."

"On what two factors does induced drag depend?"

"The induced angle of attack and the intensity of the wingtip vortices."

"What causes induced drag to increase?"

"Stronger vortices and greater downwash; typical at high lift and low airspeed."

"How is induced drag related to lift?"

"Induced drag always accompanies lift and increases with it."

"What new definitions of angle of attack must be refined for 3D flow?"

"The geometric (EASA) angle of attack; the induced angle of attack; and the effective angle of attack."

"What is the EASA definition of angle of attack?"

"The angle between the aircraft's longitudinal axis and the relative airflow."

"How does this differ from the aerofoil definition used in 2D flow?"

"In 2D flow the reference is the chord line; in 3D the reference is the longitudinal axis."

"What is the induced angle of attack?"

"The angle between the effective airflow and the relative airflow."

"When is the induced angle of attack greatest?"

"At low TAS and when vortices are strong; such as at high angles of attack."

"How does induced drag vary with airspeed?"

"It increases at low speeds due to larger angles of attack required to maintain lift."

"What is the effective angle of attack?"

"The angle between the effective airflow and the wing chord line."

"What is the relationship between total; induced; and effective angles of attack?"

"Total angle of attack = induced angle + effective angle."

"What happens to the effective angle of attack as vortices become stronger?"

"It decreases; while the induced angle increases."

"What effect do weaker vortices have on lift and drag proportions?"

"They increase effective angle of attack and improve the lift-to-drag ratio."

"What is desirable in aircraft design regarding vortices?"

"Weaker vortices; because they produce less induced drag and better efficiency."

"How does vortex strength vary across a rectangular wing?"

"Vortices and downwash are strongest near the tips and weakest near the root."

"How does effective angle of attack vary across the span?"

"It is smaller near the tips (where downwash is greater) and larger near the root."

"Where is the lift coefficient largest on a rectangular wing?"

"Near the root; where the effective angle of attack is greater."

"Where is the lift coefficient smallest on a rectangular wing?"

"Near the tip; where downwash is stronger and effective angle of attack is smaller."

"What is the consequence of uneven lift distribution along the span?"

"The root produces more lift per unit area than the tip."

"What are key effects of three-dimensional flow compared to two-dimensional flow?"

"It introduces vortices; downwash; induced drag; and varying lift along the span."

"What are three key causes of spanwise flow?"

"Pressure gradient along the span; wingtip pressure leakage; and inertia of the air mass."

"What flight configuration increases wake turbulence hazard most?"

"Clean configuration; high lift; low speed; heavy weight."

"When is wake turbulence least hazardous?"

"When flaps are extended; speed is high; or lift is reduced."

"Summarise the relationship between induced drag and vortex strength."

"Induced drag increases with vortex intensity and decreases as vortices weaken."