Aeroplane Aerodynamics
What are the three primary flight controls and the axis each controls?
Ailerons — longitudinal axis (roll). Elevators — lateral axis (pitch). Rudder — vertical/normal axis (yaw).
Define MACH NUMBER and state the speed of sound at standard sea level.
Mach Number = True Airspeed ÷ Local Speed of Sound. Speed of sound at standard sea level (15°C) = 661.7 knots.
Distinguish between COLLECTIVE and CYCLIC pitch control.
Collective: changes ALL blade angles equally and simultaneously — changes total rotor thrust MAGNITUDE (climb/descent). Cyclic: changes individual blade angles progressively through each revolution — tilts the tip path plane to change thrust DIRECTION (forward/backward/lateral movement).
What are the THREE types of flight control system, and which is most widely used?
1. Cable — MOST WIDELY USED (structural deflections do not affect it). 2. Push-pull rod. 3. Torque tube. Most aircraft use combinations of all three.
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Check answers
What is ADVERSE YAW, what causes it, and name TWO design solutions?
The nose yaws AWAY from the intended turn due to greater induced drag on the down-going aileron. Solutions (any two): differential ailerons · Frise ailerons · rudder coordination · roll spoilers.
Define the transonic speed range and describe the two significant aerodynamic events that occur in it.
M 0.75 to M 1.20. Events: 1. Shock waves form — boundary layer separation causes buffet. 2. Aerodynamic centre shifts from 25% to 50% chord, causing large trim/stability changes (Mach tuck).
What is GYROSCOPIC PRECESSION and how does it affect helicopter control input timing?
A rotating body reacts to an applied force 90° LATER in the direction of rotation. To tilt the rotor disc in a desired direction, cyclic input must be applied 90° BEFORE the desired reaction. Control linkages are designed to compensate for this automatically.
What is ARTIFICIAL FEEL, why is it needed in powered control systems, and name TWO types?
Needed because powered systems isolate the pilot from aerodynamic forces — without feel, pilot cannot sense proximity to limits or control neutral. Types (any two): Spring feel (tulip cam and spring) · Q-feel (varies with dynamic pressure/airspeed) · Bobweight (varies with load factor/g).
Describe how a CANARD configuration prevents inadvertent stall/spin.
The canard stalls BEFORE the main wing, dropping the nose automatically to recover speed. The main wing never stalls, aileron control is maintained at all times — virtually eliminating inadvertent stall/spin.
Describe the AREA RULE and explain the waisted fuselage shape it produces.
To minimise transonic drag, cross-sectional area must vary smoothly along the aircraft length. Where wings add area, the fuselage must be NARROWED to keep total cross-section consistent. Produces the characteristic 'Coke-bottle' or waisted fuselage, reducing shock wave formation and wave drag.
What is TORQUE REACTION, how does the standard tail rotor counter it, and name THREE alternative anti-torque systems?
Main rotor torque causes the fuselage to rotate in the OPPOSITE direction. Tail rotor: produces sideways thrust opposing the fuselage rotation. Alternatives: 1. Tandem rotor (counter-rotating fore and aft). 2. Fenestron (shrouded fan in tail fin). 3. NOTAR (air through tail boom slots — Coanda effect).
Explain MACH TUCK and describe how the MACH TRIM SYSTEM corrects it.
As the aircraft enters the transonic range, the aerodynamic centre shifts aft (~50% chord), moving the centre of pressure rearward. This produces a nose-DOWN pitching tendency. Mach trim: an air data computer senses Mach number, drives the Mach trim actuator to reposition the elevator nose-UP automatically.
State the maximum CL achievable with (a) a plain flap, (b) a slotted flap, and explain how the slotted flap achieves the higher value.
(a) Plain flap: CL max = 2.0. (b) Slotted flap: CL max = 2.6. The slot channels high-pressure air from below the wing over the flap upper surface, re-energising the boundary layer and preventing flap stall — allowing greater deflection and higher CL.
Distinguish between a NORMAL SHOCK WAVE and an OBLIQUE SHOCK WAVE — where each forms and what happens to airflow velocity.
Normal: perpendicular to airflow, forms on wing upper surface (transonic) or ahead of blunt leading edge (supersonic). Airflow slows to SUBSONIC. Oblique: angled, forms at wedge leading edge in supersonic flight. Airflow slows but REMAINS SUPERSONIC. Both increase static pressure and density.
Describe DISSYMMETRY OF LIFT in forward flight and explain how the helicopter automatically corrects it.
Advancing blade (higher airspeed) produces more lift than retreating blade. Without correction, the helicopter would roll toward the retreating side. Corrected by blade FLAPPING: advancing blade flaps UP (reducing AoA and lift); retreating blade flaps DOWN (increasing AoA and lift) — 'flapping to equality.'
Describe the FOUR A320 control law levels in order of decreasing protection.
1. Normal Law: full three-axis control + all flight envelope protections + load alleviation. 2. Alternate Law (two sub-levels: with/without some protections). 3. Direct Law: no protection — surface proportional to stick. 4. Mechanical: rudder and horizontal stabiliser only — mechanical control.
Distinguish between SERVO TAB, BALANCE TAB, ANTI-BALANCE TAB and SPRING TAB — direction of movement relative to primary surface and purpose of each.
Servo tab: moves OPPOSITE to surface — it IS the primary pilot input; surface follows. Balance tab: moves OPPOSITE — ASSISTS pilot in moving surface. Anti-balance tab: moves SAME direction — OPPOSES movement, increases stick force. Spring tab: inactive at low speed (rigid horn); activates at high speed to assist pilot.
Describe AERODYNAMIC HEATING — where it is greatest, what it depends on, and THREE consequences for aircraft design.
Greatest at stagnation points where velocity reduction converts kinetic energy to heat. Depends on true airspeed (not altitude). Consequences: 1. Aluminium alloy strength reduced — titanium/steel required above Mach ~2. 2. Fuel may reach spontaneous ignition temperature. 3. High inlet temperature degrades turbojet thrust (turbine temperature limit reached earlier, less fuel can be added).
State the FOUR hovering conditions and explain TRANSLATING TENDENCY — its cause and two design corrections.
Four conditions: constant position, height, rotor RPM, and heading. Translating tendency: helicopter drifts sideways in direction of tail rotor thrust (resultant of main rotor torque couple and tail rotor thrust). Corrections: 1. Offset the rotor mast against the drift. 2. Build a cyclic 'bias' into the control linkage, holding the stick slightly offset.
List FIVE advantages of Fly-By-Wire and explain how RELAXED STATIC STABILITY enables fuel saving.
Advantages: weight saving · reduced maintenance · gust load alleviation · manoeuvre envelope protection · improved handling · fuel saving. Relaxed stability: CG moved near neutral point reduces tailplane lift required for trim, so a SMALLER tailplane can be designed — less weight, less trim drag, less fuel burn. Only safe with FBW computers providing continuous stabilisation.