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NCAA CPL Principles of Flight study materials
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NCAA CPL Principles of Flight practice questions

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Question 1Aerodynamics, airspeed and Mach concepts

In steady, level, unaccelerated flight, which statement correctly describes the relationship among the four forces acting on the airplane?

  1. Thrust exceeds drag while lift equals weight.
  2. Lift equals weight and thrust equals drag. Correct answer.
  3. Lift exceeds weight while thrust equals drag.
  4. Lift equals thrust and weight equals drag.

Explanation

Unaccelerated flight means the net force on the airplane is zero, so opposing forces must balance. Lift offsets weight in the vertical axis and thrust offsets drag along the flight path. Any imbalance would produce acceleration and a change in flight path or speed.

Question 2Aerodynamics, airspeed and Mach concepts

An airplane is in a steady climb at a constant airspeed with the thrust line parallel to the flight path. How do the force components compare along and perpendicular to the flight path?

  1. Thrust equals drag plus the along-path component of weight, and lift equals the perpendicular component of weight. Correct answer.
  2. Thrust equals the component of weight along the path, and lift equals weight.
  3. Thrust equals drag, and lift equals weight.
  4. Thrust equals drag plus the rearward component of weight, and lift equals the rearward component of weight.

Explanation

In a climb the weight vector has a component acting rearward along the flight path, so thrust must balance both drag and that component to hold constant speed. Perpendicular to the path, lift balances only the component of weight normal to the flight path, which is less than total weight. This is why climb performance differs from level flight at the same airspeed.

Question 3Aerodynamics, airspeed and Mach concepts

What is the standard definition of indicated airspeed (IAS)?

  1. Ground speed corrected for wind.
  2. True airspeed corrected for density altitude.
  3. True airspeed corrected for compressibility only.
  4. The speed read directly from the airspeed indicator, uncorrected for installation and position error. Correct answer.

Explanation

Indicated airspeed is simply the value displayed on the airspeed indicator, derived from the difference between pitot (impact) pressure and static pressure. It has not been corrected for instrument, position, or installation error. Calibrated airspeed results after those corrections are applied.

Question 4Aerodynamics, airspeed and Mach concepts

An airplane flies at a constant indicated airspeed while climbing to a higher altitude. Assuming no compressibility effects, what happens to true airspeed?

  1. True airspeed remains constant because IAS is constant.
  2. True airspeed increases because the speed of sound decreases.
  3. True airspeed increases because air density decreases. Correct answer.
  4. True airspeed decreases because static pressure rises.

Explanation

Indicated airspeed reflects dynamic pressure, which depends on density as well as velocity. As altitude increases, density falls, so the same dynamic pressure requires a higher true velocity. That is why true airspeed exceeds indicated airspeed at altitude.

Question 5Aerodynamics, airspeed and Mach concepts

What does calibrated airspeed (CAS) correct indicated airspeed for?

  1. Position and installation error of the pitot-static system. Correct answer.
  2. Wind and ground speed differences.
  3. Compressibility effects at high Mach number.
  4. Density altitude and temperature.

Explanation

Calibrated airspeed is indicated airspeed corrected for errors caused by the location of the static ports and airflow around the airplane, known as position or installation error. These corrections are published as a calibration table for the specific airplane. Density and compressibility corrections are applied later in the chain.

Question 6Aerodynamics, airspeed and Mach concepts

Equivalent airspeed (EAS) is most directly relevant to which aerodynamic consideration?

  1. Estimating the wind correction angle.
  2. Determining structural and dynamic pressure loads at high speed and altitude. Correct answer.
  3. Calculating the takeoff ground roll distance.
  4. Computing fuel flow for a given ground speed.

Explanation

Equivalent airspeed is calibrated airspeed corrected for compressibility and represents the speed at sea-level density that produces the same dynamic pressure. Because aerodynamic loads scale with dynamic pressure, EAS is the appropriate reference for structural limits at high altitude. It is therefore used where dynamic pressure, not true velocity, drives the limit.

Question 7Aerodynamics, airspeed and Mach concepts

Which airspeed is used for navigation flight planning because it reflects the airplane's actual speed through the air mass?

  1. Calibrated airspeed
  2. Equivalent airspeed
  3. True airspeed Correct answer.
  4. Indicated airspeed

Explanation

True airspeed is the airplane's actual velocity relative to the surrounding air mass. Combining TAS with the wind vector yields ground speed, which is what navigation planning requires. IAS and CAS are pressure-based values that do not directly give distance covered over the air mass.

Question 8Aerodynamics, airspeed and Mach concepts

The speed of sound in air depends primarily on which atmospheric property?

  1. Static pressure
  2. Temperature Correct answer.
  3. Air density
  4. Humidity alone

Explanation

The speed of sound in a gas is proportional to the square root of the absolute temperature and the gas constant. In the atmosphere, temperature variation dominates, so sound speed decreases with altitude in the troposphere. Pressure and density changes largely offset each other and do not by themselves change sound speed.

Question 9Aerodynamics, airspeed and Mach concepts

An airplane flies at a true airspeed of 500 knots in air where the local speed of sound is 600 knots. What is its Mach number?

  1. 1.20
  2. 0.83 Correct answer.
  3. 0.60
  4. 1.83

Explanation

Mach number is the ratio of true airspeed to the local speed of sound. Dividing 500 by 600 gives approximately 0.83. The airplane is therefore flying below the critical Mach number region of transonic flow.

Question 10Aerodynamics, airspeed and Mach concepts

What is the critical Mach number of an airplane?

  1. The Mach number at which drag divergence begins at the wing root.
  2. The Mach number at which the airplane's true airspeed equals the speed of sound at sea level.
  3. The Mach number at which airflow somewhere on the airframe first reaches Mach 1. Correct answer.
  4. The Mach number at which the airplane first exceeds the speed of sound.

Explanation

Because air accelerates over curved surfaces, local flow can reach sonic speed while the airplane itself is still subsonic. The free-stream Mach number at which the fastest local flow first reaches Mach 1 is the critical Mach number. It is always lower than the airplane's flight Mach number at the onset of sonic flow.

Question 11Aerodynamics, airspeed and Mach concepts

What causes the rapid rise in drag known as drag divergence as Mach number increases beyond the critical value?

  1. Increased skin friction from colder air at altitude.
  2. Decreased lift coefficient caused by lower density.
  3. Shock waves forming on the wing and the resulting boundary-layer separation. Correct answer.
  4. Reduced thrust available from the engines.

Explanation

Once local flow exceeds Mach 1, shock waves form on the upper surface. The pressure rise across the shock thickens and can separate the boundary layer, producing a large increase in pressure drag called wave drag. This is the mechanism behind drag divergence.

Question 12Aerodynamics, airspeed and Mach concepts

As an airplane approaches its critical Mach number, which stability or control effect is typically observed?

  1. A decrease in the speed of sound over the tail.
  2. A strong increase in static longitudinal stability.
  3. Complete elimination of adverse yaw.
  4. Buffet and possible loss of elevator effectiveness due to shock-induced flow separation. Correct answer.

Explanation

Shock-induced separation can cause buffet and alter the flow over the tail, reducing elevator effectiveness and changing pitch characteristics. These effects are a key reason for Mach limits in high-speed flight. Pilots may also see Mach tuck as the center of pressure shifts aft.

Question 13Aerodynamics, airspeed and Mach concepts

What is Mach tuck?

  1. A nose-down pitching tendency caused by aft movement of the center of pressure in transonic flight. Correct answer.
  2. A rolling tendency caused by aileron reversal.
  3. A nose-up pitching tendency caused by forward movement of the center of pressure.
  4. A yawing oscillation caused by asymmetric shock formation.

Explanation

As shock waves develop, the center of pressure moves aft, producing a nose-down pitching moment that can worsen as speed increases. This tendency is called Mach tuck and can lead to a steep dive if not corrected. It is a significant transonic handling concern.

Question 14Aerodynamics, airspeed and Mach concepts

How does the maximum operating Mach number (MMO) differ from the maximum operating airspeed (VMO) in airplane limitations?

  1. MMO is expressed as a true airspeed and VMO as an indicated airspeed.
  2. MMO limits flight in high-altitude, compressibility-sensitive regimes, while VMO limits dynamic pressure at lower altitudes. Correct answer.
  3. MMO is a structural limit and VMO is an aerodynamic limit.
  4. MMO applies only to supersonic airplanes and VMO only to subsonic airplanes.

Explanation

Both are operating limits, but they protect against different concerns. VMO limits dynamic pressure and structural loads at lower altitudes, while MMO limits compressibility effects and shock-related handling problems at high altitude. Whichever limit is reached first governs the flight envelope.

Question 15Aerodynamics, airspeed and Mach concepts

An airplane has a stall speed of 60 knots in the landing configuration at 1 g. What is the approximate stall speed in a level 60-degree banked turn at the same weight and configuration?

  1. 75 knots
  2. 120 knots
  3. 60 knots
  4. 85 knots Correct answer.

Explanation

In a level 60-degree bank the load factor is 1 divided by cosine 60 degrees, which equals 2. Stall speed scales with the square root of load factor, so it increases by about 1.41, giving roughly 85 knots. This is why accelerated stalls occur at higher speeds in steep turns.

Question 16Aerodynamics, airspeed and Mach concepts

Why does an airplane stall at a higher angle of attack in a coordinated turn than in level flight at the same airspeed?

  1. The wing's critical angle of attack decreases in a turn.
  2. The wing must produce more lift to support the increased load factor. Correct answer.
  3. The horizontal component of lift reduces the stall angle.
  4. The relative wind changes direction because of the turn rate.

Explanation

In a turn the load factor rises because lift must also provide the centripetal force. To generate more lift at a given airspeed, the wing must operate at a higher angle of attack, closer to the critical value. The critical angle itself does not change.

Question 17Aerodynamics, airspeed and Mach concepts

What is the primary reason an airplane's stall speed increases with altitude?

  1. The load factor increases as density decreases.
  2. The critical angle of attack decreases with altitude.
  3. The speed of sound decreases, lowering the critical Mach number.
  4. The wing must fly at a higher true airspeed to produce the same lift in thinner air. Correct answer.

Explanation

Lift depends on density, velocity squared, and lift coefficient. At altitude, lower density requires a higher true airspeed for the same lift, so the true stall speed rises. Indicated stall speed, however, remains nearly constant because it reflects dynamic pressure.

Question 18Aerodynamics, airspeed and Mach concepts

Which statement best explains why induced drag decreases as airspeed increases in level flight?

  1. A higher airspeed increases parasite drag, which cancels induced drag.
  2. A higher airspeed increases the air density around the wing.
  3. A higher airspeed reduces the wing's angle of attack below the zero-lift angle.
  4. A higher airspeed requires a lower lift coefficient, so less downwash and tip vortex strength are produced. Correct answer.

Explanation

At higher speed the wing produces the needed lift with a smaller lift coefficient and lower angle of attack. The pressure difference between the lower and upper surfaces is smaller, so the tip vortices and downwash are weaker and induced drag falls. Parasite drag, by contrast, rises with speed.

Question 19Aerodynamics, airspeed and Mach concepts

At what speed does an airplane in level flight experience its maximum lift-to-drag ratio?

  1. At the speed where induced drag is at its minimum.
  2. At the minimum controllable airspeed.
  3. At the speed where parasite drag equals induced drag. Correct answer.
  4. At the speed where parasite drag is at its minimum.

Explanation

Total drag is the sum of parasite drag, which rises with speed, and induced drag, which falls with speed. The lift-to-drag ratio is greatest when total drag is lowest, which occurs where the two components are equal. This speed gives the best glide ratio and maximum range in still air.

Question 20Aerodynamics, airspeed and Mach concepts

What is the significance of the best-glide speed for a power-off descent?

  1. It produces the longest time aloft for each unit of altitude lost.
  2. It produces the highest rate of descent in still air.
  3. It produces the lowest possible airspeed before stall.
  4. It produces the greatest forward distance for each unit of altitude lost. Correct answer.

Explanation

Best-glide speed corresponds to the maximum lift-to-drag ratio, so the airplane travels the greatest horizontal distance per unit of height lost. It is a distance metric, not a time metric. Minimum-sink speed, which is lower, maximizes time aloft instead.

Question 21Aerodynamics, airspeed and Mach concepts

How does a headwind affect the best-glide speed and the resulting glide range?

  1. Best-glide speed decreases and glide range increases.
  2. Best-glide speed is unchanged and glide range is unchanged.
  3. Best-glide speed increases and glide range decreases. Correct answer.
  4. Best-glide speed increases and glide range increases.

Explanation

A headwind reduces the ground distance covered for a given airspeed, so the airplane should fly faster to reduce exposure time to the wind, increasing the best-glide speed. The overall glide range over the ground is still reduced compared with still air. A tailwind has the opposite effect.

Question 22Aerodynamics, airspeed and Mach concepts

What happens to the airplane's pitch trim requirement when it enters a steady climb at the same airspeed as level flight?

  1. Trim is unaffected because airspeed is constant.
  2. Less nose-up trim is needed because lift is now less than weight. Correct answer.
  3. More nose-down trim is needed because drag is greater.
  4. More nose-up trim is needed because thrust is greater.

Explanation

In a steady climb the lift required is only the component of weight perpendicular to the flight path, which is less than in level flight. With a smaller lift requirement at the same airspeed, the wing needs a lower angle of attack. The resulting pitching moment generally calls for less nose-up trim than level flight.

Question 23Aerodynamics, airspeed and Mach concepts

Which statement correctly describes the relationship between angle of attack and the relative wind?

  1. Angle of attack is the angle between the relative wind and the longitudinal axis.
  2. Angle of attack is the angle between the wing chord line and the horizon.
  3. Angle of attack is the angle between the wing chord line and the relative wind. Correct answer.
  4. Angle of attack is the angle between the flight path and the horizon.

Explanation

Angle of attack is defined as the acute angle between the wing chord line and the direction of the relative wind. It is a local aerodynamic angle and is independent of the airplane's pitch attitude relative to the horizon. This distinction explains why an airplane can stall in a climb or a descent.

Question 24Aerodynamics, airspeed and Mach concepts

During a constant-airspeed, unaccelerated descent, how does the required angle of attack compare with level flight at the same airspeed?

  1. It is lower because thrust must balance a forward weight component. Correct answer.
  2. It is the same because airspeed and weight are unchanged.
  3. It is higher because drag increases in a descent.
  4. It is higher because lift must exceed weight.

Explanation

In a descent the weight vector has a component acting forward along the flight path, which helps balance drag. Perpendicular to the path, the required lift is less than weight, so at the same airspeed the wing needs a lower angle of attack. This is why descents often require nose-down trim.

Question 25Two-dimensional and three-dimensional airflow, lift and drag

In two-dimensional airfoil theory, which statement correctly describes the behavior of the stagnation point as angle of attack increases from zero toward a positive value?

  1. It moves onto the upper surface near the trailing edge.
  2. It moves rearward along the lower surface toward the trailing edge.
  3. It moves forward along the lower surface toward the leading edge. Correct answer.
  4. It remains fixed at the leading edge regardless of angle of attack.

Explanation

At positive angle of attack the forward stagnation point migrates along the lower surface toward the leading edge, because the local flow must stagnate where the oncoming stream meets the inclined surface. This shift changes the pressure distribution and is linked to the leading-edge suction peak.

Question 26Two-dimensional and three-dimensional airflow, lift and drag

Bernoulli's equation applied along a streamline in steady, incompressible, inviscid flow expresses which relationship?

  1. Total pressure decreases as velocity increases along the streamline.
  2. Static pressure plus dynamic pressure remains constant along the streamline. Correct answer.
  3. Static pressure increases as velocity increases along the streamline.
  4. Dynamic pressure remains constant while static pressure varies.

Explanation

For steady incompressible inviscid flow along a streamline, the sum of static pressure and dynamic pressure (plus potential energy) is constant. This means an increase in velocity is accompanied by a decrease in static pressure, which underlies lift generation on an airfoil.

Question 27Two-dimensional and three-dimensional airflow, lift and drag

Which statement best describes the circulation around an airfoil and its role in lift generation according to the Kutta-Joukowski theorem?

  1. Lift depends only on the airfoil thickness and not on circulation.
  2. Lift per unit span equals circulation divided by freestream velocity.
  3. Lift per unit span equals the product of density, freestream velocity, and circulation. Correct answer.
  4. Circulation is zero for any lifting airfoil at positive angle of attack.

Explanation

The Kutta-Joukowski theorem states that lift per unit span is the product of freestream density, freestream velocity, and the bound circulation. Circulation arises because the Kutta condition fixes the rear stagnation point at the trailing edge.

Question 28Two-dimensional and three-dimensional airflow, lift and drag

An airfoil is placed in a wind tunnel at a small positive angle of attack. According to the Kutta condition, what determines the amount of circulation around the airfoil?

  1. The circulation equals the product of chord and angle of attack only.
  2. The circulation is fixed by the tunnel speed alone.
  3. The circulation is set so the rear stagnation point sits on the upper surface.
  4. The circulation adjusts so that flow leaves the trailing edge smoothly. Correct answer.

Explanation

The Kutta condition requires the flow to leave the sharp trailing edge smoothly, which selects a unique value of circulation for a given angle of attack and airfoil shape. Without this condition, potential-flow theory would predict an arbitrary circulation and an unphysical rear stagnation point.

Question 29Two-dimensional and three-dimensional airflow, lift and drag

What is the primary physical cause of induced drag on a finite wing?

  1. Compressibility effects at high subsonic speed.
  2. The rearward inclination of the lift vector due to downwash from wingtip vortices. Correct answer.
  3. Separation of the boundary layer near the trailing edge.
  4. Skin friction along the wing surface.

Explanation

Wingtip vortices create downwash that tilts the local lift vector rearward, producing a drag component called induced drag. Induced drag is therefore a consequence of generating lift with a finite-span wing.

Question 30Two-dimensional and three-dimensional airflow, lift and drag

How does induced drag vary with airspeed for a wing at a constant weight in level flight?

  1. It is zero at low airspeed and maximum at high airspeed.
  2. It decreases as airspeed increases because lift coefficient falls. Correct answer.
  3. It increases as airspeed increases.
  4. It remains constant regardless of airspeed.

Explanation

At constant weight, higher airspeed requires a lower lift coefficient, which reduces the downwash and the induced drag. Induced drag is proportional to the square of the lift coefficient divided by the aspect ratio, so it falls as speed rises.

Question 31Two-dimensional and three-dimensional airflow, lift and drag

Which change increases the aspect ratio of a rectangular wing?

  1. Increasing the span while keeping chord constant. Correct answer.
  2. Increasing the chord while keeping span constant.
  3. Increasing the angle of attack.
  4. Increasing the wing thickness ratio.

Explanation

Aspect ratio is span squared divided by wing area. For a rectangular wing, increasing span while holding chord fixed increases aspect ratio, which reduces induced drag.

Question 32Two-dimensional and three-dimensional airflow, lift and drag

In three-dimensional flow over a finite wing, the effective angle of attack experienced by each section differs from the geometric angle of attack because of which effect?

  1. Compressibility of the freestream.
  2. Skin friction heating of the boundary layer.
  3. Downwash induced by the trailing vortex system. Correct answer.
  4. The thickness of the wing at the root.

Explanation

The trailing vortex system induces a downward component of velocity at the wing, called downwash, which reduces the effective angle of attack seen by each section. This reduction is why a finite wing generates less lift at a given geometric angle than a two-dimensional section.

Question 33Two-dimensional and three-dimensional airflow, lift and drag

For a given lift coefficient, which wing planform produces the lowest induced drag?

  1. A wing with a large amount of sweep.
  2. A high-aspect-ratio wing with an elliptical lift distribution. Correct answer.
  3. A highly tapered delta wing.
  4. A low-aspect-ratio rectangular wing.

Explanation

Induced drag is minimized when the spanwise lift distribution is elliptical, and a high aspect ratio further reduces it. This is why gliders use long, slender wings with carefully shaped planforms.

Question 34Two-dimensional and three-dimensional airflow, lift and drag

What happens to the lift curve slope of a finite wing compared with its two-dimensional airfoil section?

  1. The finite wing has a lower lift curve slope. Correct answer.
  2. The finite wing slope becomes negative.
  3. The finite wing has a higher lift curve slope.
  4. The slopes are identical regardless of aspect ratio.

Explanation

Downwash reduces the effective angle of attack, so the finite wing must be pitched to a higher geometric angle to reach the same lift coefficient. This makes its lift curve slope shallower than the two-dimensional section slope.

Question 35Two-dimensional and three-dimensional airflow, lift and drag

Which quantity is defined as the component of the resultant aerodynamic force perpendicular to the freestream direction?

  1. Thrust
  2. Drag
  3. Lift Correct answer.
  4. Pitching moment

Explanation

Lift is defined as the component of the total aerodynamic force acting perpendicular to the relative wind. Drag is the component parallel to the relative wind.

Question 36Two-dimensional and three-dimensional airflow, lift and drag

At what angle of attack does an airfoil typically reach its maximum lift coefficient in two-dimensional flow?

  1. At a negative angle of attack.
  2. At the critical angle of attack just before stall. Correct answer.
  3. At the zero-lift angle of attack.
  4. At the angle where drag is minimum.

Explanation

Lift coefficient increases with angle of attack until the critical angle is reached, where flow separation begins and lift drops off. Maximum lift coefficient occurs at this critical angle just before stall.

Question 37Two-dimensional and three-dimensional airflow, lift and drag

A wing section stalls when which condition occurs?

  1. The dynamic pressure exceeds the static pressure.
  2. The boundary layer separates extensively from the upper surface. Correct answer.
  3. The Reynolds number falls below the critical value.
  4. The lift coefficient reaches zero.

Explanation

Stall occurs when the angle of attack becomes too large and the adverse pressure gradient causes the boundary layer to separate over a large portion of the upper surface. This separation destroys lift and increases pressure drag.

Question 38Two-dimensional and three-dimensional airflow, lift and drag

Which type of drag is caused by the normal pressure distribution acting on the airfoil and is strongly affected by boundary-layer separation?

  1. Wave drag
  2. Form drag Correct answer.
  3. Skin friction drag
  4. Induced drag

Explanation

Form drag, also called pressure drag, results from the integrated pressure forces over the airfoil surface. When separation occurs, the pressure recovery at the rear is reduced, increasing form drag.

Question 39Two-dimensional and three-dimensional airflow, lift and drag

Which statement correctly describes the relationship between the Reynolds number and boundary-layer behavior on an airfoil?

  1. Higher Reynolds number tends to move transition forward and thin the boundary layer. Correct answer.
  2. Lower Reynolds number always produces turbulent flow over the whole chord.
  3. Reynolds number has no effect on boundary-layer transition.
  4. Higher Reynolds number tends to promote transition to turbulent flow farther aft.

Explanation

Increasing Reynolds number generally moves the transition point forward and produces a thinner, more energetic turbulent boundary layer that resists separation. This can improve maximum lift and reduce separation-related drag.

Question 40Two-dimensional and three-dimensional airflow, lift and drag

What is the main function of a vortex generator on a wing?

  1. To add energy to the boundary layer and delay separation. Correct answer.
  2. To increase the wing's thickness ratio.
  3. To eliminate induced drag entirely.
  4. To reduce the wing's aspect ratio.

Explanation

Vortex generators create small vortices that mix higher-energy freestream air into the boundary layer, helping it resist adverse pressure gradients. This delays separation and can improve control effectiveness at high angles of attack.

Question 41Two-dimensional and three-dimensional airflow, lift and drag

In a three-dimensional wing, what is the effect of increasing aspect ratio on the lift-induced drag coefficient at a fixed lift coefficient?

  1. Induced drag coefficient increases.
  2. Induced drag coefficient becomes negative.
  3. Induced drag coefficient decreases. Correct answer.
  4. Induced drag coefficient stays the same.

Explanation

The induced drag coefficient is proportional to the square of the lift coefficient divided by aspect ratio. At a fixed lift coefficient, increasing aspect ratio reduces the induced drag coefficient.

Question 42Two-dimensional and three-dimensional airflow, lift and drag

Which statement best explains why a swept wing is used on high-speed aircraft?

  1. Sweep increases the wing's aspect ratio.
  2. Sweep lowers the critical angle of attack.
  3. Sweep increases the critical Mach number by reducing the effective normal Mach number. Correct answer.
  4. Sweep eliminates induced drag at all speeds.

Explanation

Sweeping the wing reduces the component of freestream velocity normal to the leading edge, which raises the freestream Mach number at which shock formation begins. This delays the onset of wave drag.

Question 43Two-dimensional and three-dimensional airflow, lift and drag

What is the significance of the zero-lift angle of attack for a cambered airfoil?

  1. It is the angle at which the lift coefficient is zero, typically negative for positive camber. Correct answer.
  2. It is the angle at which drag is minimum.
  3. It is always zero for any cambered airfoil.
  4. It is the angle at which the airfoil stalls.

Explanation

A cambered airfoil produces lift even at zero geometric angle, so its lift coefficient reaches zero at a negative angle called the zero-lift angle. Symmetric airfoils have a zero-lift angle of zero.

Question 44Two-dimensional and three-dimensional airflow, lift and drag

Which factor most directly determines the magnitude of the downwash angle behind a finite wing?

  1. The wing's thickness ratio only.
  2. The ambient humidity.
  3. The span loading and aspect ratio of the wing. Correct answer.
  4. The color of the wing surface.

Explanation

Downwash strength depends on the strength and distribution of the trailing vorticity, which is governed by span loading and aspect ratio. A wing with high lift and low aspect ratio produces stronger downwash.

Question 45Two-dimensional and three-dimensional airflow, lift and drag

What happens to the center of pressure of a cambered airfoil as angle of attack increases toward the stall angle?

  1. It moves rearward toward the trailing edge.
  2. It moves forward toward the leading edge. Correct answer.
  3. It remains exactly at the quarter chord for all angles.
  4. It moves to the wingtip.

Explanation

As angle of attack increases, the lift distribution shifts forward, moving the center of pressure toward the leading edge. This forward movement affects pitching moments and trim requirements.

Question 46Two-dimensional and three-dimensional airflow, lift and drag

Which statement correctly distinguishes the aerodynamic center from the center of pressure?

  1. The aerodynamic center is the point where lift is zero.
  2. The aerodynamic center is the point about which pitching moment is nearly constant with angle of attack. Correct answer.
  3. The aerodynamic center moves significantly with angle of attack.
  4. The center of pressure is fixed at the quarter chord for all airfoils.

Explanation

The aerodynamic center is defined as the chordwise point about which the pitching moment coefficient remains nearly constant as angle of attack changes. The center of pressure, by contrast, is the point where the resultant lift acts and it shifts with angle of attack.

Question 47Two-dimensional and three-dimensional airflow, lift and drag

A wing has an elliptical spanwise lift distribution. What is the consequence for induced drag compared with a wing of the same aspect ratio but a non-elliptical distribution?

  1. Induced drag is maximized for the elliptical distribution.
  2. Induced drag becomes zero for the elliptical distribution.
  3. Induced drag is minimized for the elliptical distribution. Correct answer.
  4. Induced drag is unaffected by the distribution shape.

Explanation

An elliptical lift distribution produces the minimum possible induced drag for a given span and total lift. Non-elliptical distributions have higher induced drag because their downwash is not uniform across the span.

Question 48Two-dimensional and three-dimensional airflow, lift and drag

What is the primary reason that a finite wing stalls at a lower geometric angle of attack at the root than at the tip when washout is incorporated?

  1. Washout reduces the local angle of attack at the tip relative to the root. Correct answer.
  2. Washout increases the wing's aspect ratio.
  3. Washout increases the local angle of attack at the tip.
  4. Washout eliminates downwash at the tip.

Explanation

Washout twists the wing so the tip has a lower incidence than the root. This causes the root to reach its critical angle first, providing stall warning and preserving aileron effectiveness.

Question 49Lift augmentation and drag reduction devices

On a wing fitted with trailing-edge flaps, what happens to the lift curve when the flaps are extended?

  1. The curve shifts downward and the maximum lift coefficient decreases while the stalling angle increases
  2. The curve shifts upward and its maximum lift coefficient increases, but the stalling angle of attack is reduced Correct answer.
  3. The curve keeps the same maximum lift coefficient but moves to a much higher stalling angle of attack
  4. The curve becomes a straight line with no defined stalling angle of attack

Explanation

Flap extension increases camber, so the whole lift curve moves up and the maximum lift coefficient rises. Because the aft loading is already high, the wing reaches its critical angle at a lower angle of attack, so the stall occurs earlier. The zero-lift angle also becomes more negative.

Question 50Lift augmentation and drag reduction devices

What is the primary aerodynamic purpose of a wing leading-edge slat?

  1. To delay the stall by energising the boundary layer over the upper surface at high angles of attack Correct answer.
  2. To act as a speed brake by creating deliberate separation over the wing
  3. To trim the aircraft longitudinally by shifting the centre of pressure aft
  4. To increase the wing's aspect ratio and reduce induced drag at cruise

Explanation

A slat forms a converging slot that accelerates air over the upper surface, adding energy to the boundary layer. This delays separation to a higher angle of attack and raises the maximum lift coefficient. The slat does not change aspect ratio or provide pitch trim.