### Q26: What is the Bernoulli principle as applied to an aerofoil? ^t80q26 - A) Pressure increases where flow velocity increases - B) Where flow velocity increases, pressure decreases - C) Lift is generated solely by the deflection of air downward - D) Drag is independent of velocity **Correct: B)** > **Explanation:** Bernoulli's principle states that in a steady, incompressible flow, an increase in flow velocity is accompanied by a decrease in static pressure, and vice versa. Applied to an aerofoil, the air accelerates over the curved upper surface, creating a region of lower pressure compared to the lower surface. This pressure differential generates lift. While Newton's third law (downwash) also contributes to lift, the Bernoulli pressure distribution is the primary mechanism for conventional subsonic flight. ### Q27: What is adverse yaw? ^t80q27 - A) The tendency to pitch nose-down in a steep turn - B) Unwanted yaw in the direction opposite to the intended turn when ailerons are applied - C) The yaw caused by rudder deflection in crosswind - D) The yaw resulting from asymmetric thrust **Correct: B)** > **Explanation:** Adverse yaw occurs because the down-going aileron (on the wing that rises) increases both lift and induced drag on that wing. The extra drag on the rising wing pulls the nose toward the descending wing — opposite to the intended turn direction. This is why coordinated use of rudder with aileron is essential, and why differential aileron deflection was developed as a design solution. ### Q28: When does ground effect become significant? ^t80q28 - A) At any altitude in calm air - B) Within approximately one wingspan of the ground - C) Only during take-off roll - D) Above 100 m AGL **Correct: B)** > **Explanation:** Ground effect becomes significant when the aircraft is within approximately one wingspan of the surface. The ground physically restricts the development of wingtip vortices and reduces the induced downwash angle, which effectively increases lift and reduces induced drag. Pilots experience this as a floating sensation during the landing flare — the glider wants to keep flying in ground effect, which can cause overshooting the intended touchdown point if not anticipated. ### Q29: What does the term "washout" refer to in wing design? ^t80q29 - A) The reduction of wing chord from root to tip - B) A decrease in the angle of incidence from wing root to tip - C) The cleaning procedure for wing surfaces - D) The loss of lift during a stall **Correct: B)** > **Explanation:** Washout is a deliberate design feature in which the wing's angle of incidence decreases progressively from root to tip (geometric washout) or the aerofoil section changes to produce less lift at the tip (aerodynamic washout). This ensures that the wing root stalls before the tip, preserving aileron effectiveness during a stall and making the stall behaviour more benign and recoverable. Washout is particularly important in gliders with their long, high-aspect-ratio wings. ### Q30: What is the relationship between the angle of attack and the lift coefficient up to the stall? ^t80q30 - A) Lift coefficient decreases as angle of attack increases - B) Lift coefficient increases approximately linearly as angle of attack increases - C) Lift coefficient remains constant regardless of angle of attack - D) Lift coefficient increases exponentially with angle of attack **Correct: B)** > **Explanation:** In the pre-stall regime, the lift coefficient CL increases approximately linearly with angle of attack (AoA). The slope of this line is the lift curve slope (typically about 2π per radian for a thin aerofoil). This linear relationship continues until the critical angle of attack is reached, at which point flow separation causes CL to peak (CL_max) and then drop sharply — the stall. The linearity of the CL vs. AoA relationship is one of the foundational results of aerodynamic theory. ### Q31: How does the flap position affect the stall speed? ^t80q31 - A) Extending flaps raises the stall speed - B) Flap position has no effect on stall speed - C) Extending flaps lowers the stall speed - D) Retracting flaps lowers the stall speed **Correct: C)** > **Explanation:** Extending flaps increases the wing's maximum lift coefficient (CL_max) by adding camber and, in some designs, wing area. From the stall speed formula Vs = sqrt(2W / (ρ × S × CL_max)), a higher CL_max yields a lower stall speed. This allows approach and landing at slower speeds with a shorter ground roll. Retracting flaps removes this benefit and returns stall speed to the higher clean-configuration value. ### Q32: What is the purpose of a laminar-flow aerofoil? ^t80q32 - A) To increase induced drag at low speeds - B) To maximise the region of turbulent boundary layer - C) To reduce skin friction drag by maintaining laminar flow over a larger portion of the wing - D) To improve stall characteristics at high angles of attack **Correct: C)** > **Explanation:** Laminar-flow aerofoils are designed with their maximum thickness further aft than conventional profiles, creating a favourable pressure gradient that keeps the boundary layer laminar over a larger portion of the chord. Since laminar boundary layers produce far less skin friction drag than turbulent ones, the overall profile drag is significantly reduced. Gliders exploit this extensively — clean laminar-flow wings are the reason modern gliders achieve glide ratios exceeding 50:1. ### Q33: How does air density change with increasing altitude? ^t80q33 - A) It increases linearly - B) It remains constant - C) It decreases - D) It increases then decreases **Correct: C)** > **Explanation:** Air density decreases with altitude because atmospheric pressure drops and air expands. In the standard atmosphere, density at 5,500 m is roughly half the sea-level value. Reduced density means reduced dynamic pressure at a given TAS, which is why aircraft performance (lift and drag per unit TAS) degrades at altitude — the aircraft must fly faster in TAS to maintain the same IAS and lift. ### Q34: What is the difference between static stability and dynamic stability? ^t80q34 - A) They are the same concept - B) Static stability is the initial tendency to return to equilibrium; dynamic stability describes whether the subsequent oscillations damp out - C) Dynamic stability is the initial tendency; static stability describes long-term behaviour - D) Static stability only applies to pitch, dynamic stability only to roll **Correct: B)** > **Explanation:** Static stability describes the aircraft's immediate response to a disturbance — whether restoring forces act to push it back toward the original equilibrium. Dynamic stability describes what happens over time: if the resulting oscillations decrease in amplitude and the aircraft eventually returns to its trimmed state, it is dynamically stable. An aircraft can be statically stable but dynamically unstable (oscillations grow), which is a dangerous condition. ### Q35: What is the purpose of vortex generators on a wing? ^t80q35 - A) To increase the laminar boundary layer region - B) To reduce the aircraft's weight - C) To energise the boundary layer and delay flow separation - D) To decrease the stall speed **Correct: C)** > **Explanation:** Vortex generators are small tabs that protrude from the wing surface and create tiny vortices that mix high-energy air from outside the boundary layer into the slower boundary layer flow near the surface. This energised boundary layer can resist adverse pressure gradients more effectively, delaying flow separation and improving control effectiveness at high angles of attack. They trade a small increase in skin friction for a significant delay in stall onset and better aileron authority near the stall. ### Q36: The lift formula L = CL x 0.5 x rho x V² x S contains several variables. Which of these can the pilot directly control in flight? ^t80q36 - A) Air density (rho) - B) Wing area (S) - C) Airspeed (V) and, indirectly, the lift coefficient (CL) through angle of attack - D) All of the above **Correct: C)** > **Explanation:** The pilot can directly change airspeed V (by adjusting pitch attitude) and indirectly change the lift coefficient CL (by changing the angle of attack, or by extending/retracting flaps). Air density ρ changes with altitude and temperature but is not directly controlled. Wing area S is fixed (except in rare variable-geometry designs or Fowler flap configurations). Airspeed and angle of attack are the pilot's primary tools for managing lift. ### Q37: In which direction does the centre of pressure move as the angle of attack increases (pre-stall)? ^t80q37 - A) Rearward along the chord - B) It does not move - C) Forward along the chord - D) Upward, away from the wing surface **Correct: C)** > **Explanation:** As angle of attack increases in the pre-stall range, the pressure distribution shifts such that the centre of pressure moves forward along the chord. This forward CP movement produces a nose-up pitching moment that must be counteracted by the tail — one of the main reasons aircraft require a horizontal stabiliser. At very low (or negative) angles of attack, the CP moves rearward. This CP migration is why the aerodynamic centre concept is useful: the moment about the aerodynamic centre stays constant regardless of AoA. ### Q38: What determines the critical angle of attack at which a wing stalls? ^t80q38 - A) The aircraft's weight - B) The altitude at which the aircraft is flying - C) The airspeed - D) The aerofoil shape (profile geometry) **Correct: D)** > **Explanation:** The critical angle of attack is an inherent property of the aerofoil's geometric shape — it is the angle at which the flow can no longer remain attached to the upper surface and separates, causing the stall. It does not change with weight, altitude, or airspeed. What changes with those factors is the stall speed — the speed at which the wing reaches the critical angle of attack in level flight. The aerofoil geometry (camber, thickness, leading edge radius) determines how well the flow follows the upper surface at high angles. ### Q39: How does induced drag behave with increasing airspeed in level flight? ^t80q39 - A) It decreases continuously - B) It reaches a maximum, then decreases - C) It remains constant - D) It increases with increasing airspeed **Correct: A)** > **Explanation:** Induced drag decreases monotonically with increasing airspeed in level flight: D_induced = 2W^2 / (rho * V^2 * S^2 * pi * AR * e). As V increases, induced drag continuously falls — there is no minimum/maximum within the normal flight envelope. Parasite drag (not induced drag) has the U-shaped curve described in B/C. Total drag has a minimum at the speed where induced drag equals parasite drag; induced drag itself simply decreases with speed. ### Q40: Which types of drag make up total drag? ^t80q40 - A) Induced drag, form drag, and skin-friction drag - B) Interference drag and parasite drag - C) Form drag, skin-friction drag, and interference drag - D) Induced drag and parasite drag **Correct: D)** > **Explanation:** The standard aerodynamic breakdown of total drag is: Total drag = Induced drag + Parasite drag. Induced drag arises from lift generation (wingtip vortices). Parasite drag is the collective term for all non-lift-related drag: form/pressure drag, skin friction drag, and interference drag. Options A and C list sub-components of parasite drag but omit induced drag or incorrectly combine them. Option B omits induced drag, which is a major component especially at low speeds. ### Q41: How do lift and drag change when a stall is approached? ^t80q41 - A) Both lift and drag increase - B) Lift rises while drag falls - C) Lift falls while drag rises - D) Both lift and drag fall **Correct: C)** > **Explanation:** As the critical angle of attack is reached, flow begins to separate from the upper surface, starting at the trailing edge and progressing forward. Once past the critical AoA, the clean attached flow that generated lift breaks down — CL drops sharply. Simultaneously, the separated flow creates a large turbulent wake with very high pressure drag, so CD rises dramatically. The drag polar shows this clearly: the nose of the polar curves sharply as the stall condition is approached, with CL falling and CD rising. ### Q42: To recover from a stall, it is essential to... ^t80q42 - A) Increase the bank angle and reduce the speed - B) Increase the angle of attack and increase the speed - C) Decrease the angle of attack and increase the speed - D) Increase the angle of attack and reduce the speed **Correct: C)** > **Explanation:** Stall recovery requires reducing angle of attack below the critical value so that airflow can re-attach to the upper surface and lift can be restored. The pilot must push forward on the elevator control to lower AoA, which also allows the aircraft to accelerate (or the pilot applies power if available). Increasing AoA (B, D) deepens the stall. Reducing speed (D, A) worsens the condition. Banking (A) increases the load factor, which raises the stall speed — exactly the wrong input. ### Q43: During a stall, how do lift and drag behave? ^t80q43 - A) Lift rises while drag rises - B) Lift rises while drag falls - C) Lift falls while drag falls - D) Lift falls while drag rises **Correct: D)** > **Explanation:** This is the definitive stall characteristic: lift collapses because boundary layer separation destroys the pressure differential that generates it, while drag rises dramatically due to the large turbulent separated wake. The CL vs. AoA curve shows CL_max at the critical angle, then a steep drop — this is the stall. The CD vs. AoA curve rises steeply through and beyond the stall. This combination (less lift, more drag) is why the stall is critical — the aircraft loses lift while simultaneously experiencing high drag that would further reduce speed. ### Q44: The critical angle of attack... ^t80q44 - A) Changes with increasing weight - B) Is independent of the aircraft's weight - C) Increases with a rearward centre of gravity position - D) Decreases with a forward centre of gravity position **Correct: B)** > **Explanation:** The critical (stall) angle of attack is a fixed aerodynamic property of the aerofoil shape — it is the AoA at which flow separation occurs regardless of airspeed, weight, or altitude. What changes with weight is the stall speed (Vs = sqrt(2W / (rho * S * CL_max))), not the stall AoA. A heavier aircraft must fly faster to generate the same lift, but it still stalls at the same critical AoA. C.G. position affects pitch stability and control effectiveness but does not change the aerofoil's critical angle. ### Q45: What leads to a lower stall speed Vs (IAS)? ^t80q45 - A) Higher load factor - B) Lower air density - C) Decreasing weight - D) Lower altitude **Correct: C)** > **Explanation:** From Vs = sqrt(2W / (rho * S * CL_max)): stall speed decreases when weight (W) decreases, since less lift is needed to maintain equilibrium. Lower density (B) increases true airspeed (TAS) stall speed but the IAS stall speed remains approximately constant (since IAS is based on dynamic pressure q = 0.5 * rho * V_TAS^2, which equals 0.5 * rho_0 * V_IAS^2). Higher load factor (A) effectively increases apparent weight (n*W), raising stall speed. Lower altitude means higher density, which slightly lowers TAS stall speed but does not significantly change IAS stall speed. ### Q46: Which statement about a spin is correct? ^t80q46 - A) Speed constantly increases during the spin - B) During recovery, ailerons should be kept neutral - C) During recovery, ailerons should be crossed - D) Only very old aircraft risk spinning **Correct: B)** > **Explanation:** Spin recovery technique (PARE: Power off, Ailerons neutral, Rudder opposite to spin direction, Elevator forward) requires keeping ailerons neutral because using ailerons during a spin can worsen the rotation — applying aileron into the spin raises the inner wing's AoA (which may already be stalled) and can deepen the spin. Rudder opposite to spin direction stops the autorotation; forward elevator then reduces AoA to unstall both wings. Speed does not constantly increase in a spin — the aircraft reaches a stabilised spin with relatively constant speed and rotation rate. ### Q47: The laminar boundary layer on the aerofoil lies between... ^t80q47 - A) The transition point and the separation point - B) The stagnation point and the centre of pressure - C) The transition point and the centre of pressure - D) The stagnation point and the transition point **Correct: D)** > **Explanation:** The boundary layer development follows a specific sequence: flow is divided at the stagnation point, a laminar boundary layer develops from the stagnation point rearward, then at the transition point the laminar layer converts to turbulent, and finally at the separation point the turbulent layer detaches from the surface. The laminar boundary layer therefore occupies the region from the stagnation point to the transition point. Laminar flow aerofoils are designed to push the transition point as far aft as possible to minimise friction drag. ### Q48: What types of boundary layers are found on an aerofoil? ^t80q48 - A) Turbulent layer at the leading edge areas, laminar boundary layer at the trailing areas - B) Laminar boundary layer along the complete upper surface with non-separated airflow - C) Laminar layer at the leading edge areas, turbulent boundary layer at the trailing areas - D) Turbulent boundary layer along the complete upper surface with separated airflow **Correct: C)** > **Explanation:** The natural sequence of boundary layer development on an aerofoil runs from laminar (near the leading edge, where the flow is orderly and Reynolds number is low) to turbulent (further aft, after transition). The reverse sequence (turbulent first, then laminar) does not occur naturally. This forward laminar / aft turbulent arrangement is why designers place the maximum thickness of laminar-flow aerofoils further back — to extend the favourable pressure gradient that maintains laminar flow as far as possible before transition. ### Q49: How does a laminar boundary layer differ from a turbulent one? ^t80q49 - A) The turbulent boundary layer is thicker but produces less skin-friction drag - B) The laminar layer generates lift while the turbulent layer generates drag - C) The laminar layer is thinner and produces more skin-friction drag - D) The turbulent boundary layer can remain attached to the aerofoil at higher angles of attack **Correct: D)** > **Explanation:** The turbulent boundary layer, despite having higher skin friction drag than the laminar layer, has more energetic mixing that allows it to remain attached to the surface against an adverse pressure gradient at higher angles of attack. This is its critical advantage: it resists flow separation better. The laminar boundary layer is indeed thinner (C is partly correct about thickness) and has lower friction drag — but it separates more easily. This is why turbulators are sometimes used on gliders: deliberately triggering transition to turbulent flow to prevent laminar separation bubbles. ### Q50: Which structural element provides lateral (roll) stability? ^t80q50 - A) Elevator - B) Wing dihedral - C) Vertical tail - D) Differential aileron deflection **Correct: B)** > **Explanation:** Lateral (roll) stability — the tendency to return to wings-level after a roll disturbance — is primarily provided by wing dihedral (the upward angle of the wings from horizontal). When a gust rolls the aircraft, the lower wing descends and its angle of attack increases (it meets more airflow), generating more lift and creating a restoring moment back to level. The vertical tail provides directional (yaw) stability; ailerons are roll control surfaces (not stability), and the elevator controls pitch. High-wing aircraft achieve similar lateral stability through the pendulum effect of the fuselage hanging below the wings.