Aerodinamik mülakat soruları

Four forces: lift, weight, thrust and drag. Lift balances weight and thrust balances drag; any imbalance makes the aircraft accelerate, decelerate, climb or descend.

Flying at the best lift/drag ratio, which means the optimum angle of attack and its corresponding minimum drag speed (VIMD, VMD).

No, still-air glide range does not change with weight, provided the aircraft flies the best-L/D angle of attack for its weight. A heavier aircraft must glide faster and so reaches the ground sooner.

It is the vertical component of the aircraft's velocity, usually given in feet per minute. It tells you how long a given height change will take.

A heavier aircraft descends faster at the same angle of attack, because it has to fly at a higher airspeed to hold that angle of attack.

An aerofoil is a shaped surface that produces a large lift force for relatively little drag when set at a small angle to the airflow. Examples are wings, tailplanes, fins and propeller blades.

The chord line is the straight line joining the leading edge to the trailing edge of the aerofoil section.

The standard mean chord is the wing area divided by the wingspan (SMC = S / b). It is the average chord of a tapered wing.

The mean camber line runs from the leading edge to the trailing edge, at every point halfway between the upper and lower surfaces of the aerofoil.

The angle of incidence is the fixed angle between the wing's chord line and the aircraft's longitudinal axis (datum), set when the wing is built. It is also called rigging incidence.

Angle of attack is the angle between the aerofoil's chord line and the relative airflow (the free-stream flow meeting the wing).

Washout is a twist built into the wing so that the incidence decreases from root to tip. The root then reaches the stalling angle first, keeping the tips and ailerons flying.

Dihedral is the upward angle of the wings from root to tip, seen from the front. It gives lateral (roll) stability.

Anhedral is the downward angle of the wings from root to tip, seen from the front. It reduces lateral stability.

Lift is the aerodynamic force perpendicular to the relative airflow. It comes from the pressure difference between the wing's lower and upper surfaces, created as the wing turns the airflow downward.

L = CL × ½ρV² × S. Lift equals the lift coefficient times dynamic pressure times wing area.

CL is a dimensionless number for how effectively a wing produces lift. It depends on the wing section shape and configuration and on the angle of attack.

The centre of pressure is the point on the chord where the total aerodynamic (lift) force acts. On a conventional cambered aerofoil it moves forward as angle of attack increases and aft as it decreases, until the stall.

If lift and weight do not act through the same point, they form a couple. CG ahead of the centre of pressure gives a nose-down moment, and CG behind it gives a nose-up moment.

Aspect ratio is span divided by mean chord (AR = b² / S). A high AR gives low induced drag and a good lift/drag ratio. A low AR is structurally stiffer and suits high-speed or highly manoeuvrable designs.

In steady flight lift roughly equals weight, so it is greatest when the aircraft is heaviest, around takeoff and initial climb. It also exceeds weight whenever the load factor rises, as in rotation, pull-ups and turns.

Direct lift control (DLC) changes the wing's lift directly, usually with symmetrically deflected spoilers or fast-acting flaps, without first changing pitch attitude. This allows precise flight-path control, for example on approach.

High-lift devices increase CLmax so the aircraft can fly safely at lower speeds. Trailing-edge flaps (for example Fowler flaps) add camber and area, while leading-edge slats, slots and Krueger flaps delay separation to a higher stalling angle.

Drag is the aerodynamic force that resists the aircraft's motion through the air. It acts parallel to and opposite the flight path, and thrust must overcome it.

Total drag = profile (parasite) drag + induced drag. Profile drag rises with the square of speed, while induced drag falls as speed increases. They are equal at minimum drag speed (VIMD), where total drag is lowest and L/D is best.

A straight-winged piston aircraft has a sharply U-shaped drag curve: steep induced-drag rise at low speed, steep profile-drag rise at high speed, and a clear minimum at VIMD. Below VIMD speed is noticeably unstable.

A swept-wing jet has a flatter total-drag curve with a less distinct minimum, and VIMD is a higher speed relative to stall. Speed instability below VIMD develops gradually and is easy to miss.

If thrust and drag act along different lines they form a couple. With thrust below the drag line (underwing engines), more thrust pitches the nose up. With thrust above it (tail or high-mounted engines), more thrust pitches the nose down.

Devices that deliberately add drag: flaps at large deflection, spoilers or speed brakes (in flight) and lift dumpers (on the ground), the landing gear, thrust reversers (ground only), and on some aircraft braking parachutes.

A lifting wing has higher pressure underneath than on top. At the tip the air spills from the high-pressure lower surface around to the low-pressure upper surface, and this rolls up into a trailing vortex behind each tip.

Spanwise flow forms the tip vortices (and so induced drag), reduces aileron effectiveness, and, on swept wings, thickens the boundary layer toward the tips, which encourages tip stall.

Wingtip vortices cause induced drag, create wake turbulence that sinks behind and below the aircraft, and change the downwash over the tailplane, which affects longitudinal stability and trim.

With wing fences and vortex generators, which help keep the upper-surface flow chordwise. Sawtooth (dog-tooth) leading edges, leading-edge notches, washout and winglets also help.

Both control the boundary layer to cut down spanwise flow and delay separation. Vortex generators re-energise the boundary layer by mixing in fast air, and fences physically block spanwise drift toward the tips.

Winglets are small near-vertical aerofoils at the wingtips. They weaken the tip vortex by limiting the flow of air around the tip from the lower to the upper surface, which reduces induced drag and improves fuel efficiency.

Aerodynamically, the wing must be able to make lift equal to the weight, which depends on wing design, achievable speed (engine power) and air density. Structurally, certified maximum weights (MTOW, MLW, MZFW) protect the airframe.

Overweight reduces performance (longer takeoff and landing, poorer climb and ceiling, less range), raises stall speeds, reduces manoeuvrability, wears tyres and brakes faster, and erodes structural safety margins.

The centre of gravity (CG) is the single point through which the total weight of the aircraft can be considered to act.

An arm is the horizontal distance from the reference datum to the point where an item's weight acts (its CG). By convention arms aft of the datum are positive and arms forward of it are negative.

A moment is the turning effect of a weight about the datum: moment = weight × arm.

By the horizontal tailplane. It produces an up or down force (set by elevator or stabiliser trim) whose moment about the CG cancels the lift-weight pitching moment.

The CG range (envelope) is the band between the most forward and most aft permitted CG positions, set out in the flight manual. Within it, the tailplane can still balance the aircraft, keep it longitudinally stable and give adequate pitch control.

The forward limit makes sure the tail can still rotate and flare the aircraft without excessive stick force or trim drag. The aft limit makes sure the aircraft stays longitudinally stable, with adequate stick-force feel.

Too far forward: very stable and heavy in pitch, with poor rotation and flare authority, more trim drag and a higher stall speed. Too far aft: unstable, with light and over-sensitive pitch control, a risk of over-stressing, and weak nose-down recovery authority.

Loading depends on the basic and dry operating weight plus payload (passengers, baggage, cargo) and fuel. All must respect the maximum weights (MTOW, MLW, MZFW and so on) and keep the CG within the envelope for every phase of flight.

Toward an aft CG, within limits. It reduces the tail download and trim drag needed for the cruise attitude, which lowers fuel burn and increases range.

A forward CG raises the stall speed. The tail must produce a larger download to balance the nose-heavy aircraft, which adds to the load the wing must carry, as if the aircraft were heavier.

Because its CG can move a long way during a flight and between different loadings, mostly from burning large amounts of fuel stored along a swept wing. It needs a powerful, usually trimmable, tailplane to stay stable and controllable across that range.

Changes in how weight is distributed. Mainly this is fuel burn or fuel transfer, plus movement of passengers, crew and trolleys, and smaller effects from gear and flap retraction.

An aft CG increases the minimum control speeds. It shortens the rudder's moment arm about the CG, so the rudder needs more speed to counter asymmetric thrust. A forward CG lengthens the arm and lowers them.

Momentum is mass × velocity. It makes an aircraft keep its existing speed and flight path for a time after the controls or thrust change. Heavy, fast jets have much more momentum than light piston aircraft, so they respond more slowly and must be flown with anticipation.

A heavier aircraft needs an earlier top of descent. At a given idle-thrust speed (IAS or Mach) it descends on a shallower gradient, so it covers more distance while losing the same height.

Positive g is a load factor above zero: the wing's lift acts toward the aircraft's roof, so occupants are pressed into their seats. Load factor n = lift / weight, and steady level flight is +1 g.

Negative g is a load factor below zero. The aircraft's lift acts "downward" relative to the airframe, so occupants and loose items are pushed toward the ceiling, against their straps.

Use sweepback, a thin section (low thickness/chord ratio) and low or supercritical camber to raise the critical Mach number and cut wave and profile drag. Then add high-lift devices and a strong structure so airfield performance and strength remain acceptable.

The wing mainly responds to the velocity component perpendicular to its leading edge. Sweep reduces that component to V × cos(sweep angle), so the aircraft can fly faster before the local flow over the wing becomes sonic.

In two stages. First get the most from the clean wing (aerofoil, camber, twist and planform), then add enough high-lift devices (leading-edge slats or Krueger flaps and multi-slotted or Fowler flaps) to give acceptable takeoff and landing performance.

Mainly higher cruise Mach numbers, because sweep delays shock formation and drag rise, and a smoother ride in turbulence, because the swept wing's lower lift-curve slope makes it less sensitive to gusts.

Lower CLmax and lift-curve slope, so higher stall and approach speeds. Speed instability at low speed, because of a flat drag curve with a high VIMD. A tip-stall tendency that causes pitch-up. Also Dutch roll and a heavier, more complex structure.

Without design countermeasures, a swept or tapered wing stalls at the tips first. Lost lift outboard (and therefore aft) moves the centre of pressure forward, giving a pitch-up that drives the aircraft deeper into the stall.

With thrust left unchanged, a speed-stable aircraft drifts back to its trimmed speed after a disturbance, while a speed-unstable one moves further away. Above VIMD speed is stable, and below VIMD it is unstable.

Mach number is the ratio of the aircraft's true airspeed to the local speed of sound: M = TAS / LSS. Mach 0.5 means flying at half the local speed of sound.

Mcrit is the free-stream Mach number at which the accelerated airflow somewhere on the aircraft, usually over the wing upper surface, first reaches Mach 1. Above it shock waves form, drag rises sharply and buffet can begin.

Approaching and passing Mcrit you get Mach buffet from shock waves, a steep drag rise, changing stick forces, a nose-down pitching tendency (Mach tuck) and, in extreme cases, possible loss of control.

It moves aft. The upper-surface shock moves rearward and lift is redistributed chordwise, and on a swept wing the inboard sections lose lift first, so lift shifts outboard, which also means further aft.

Mach tuck is the nose-down pitching tendency that occurs above Mcrit as the centre of pressure moves aft of its normal position, increasing the nose-down moment about the CG.

A Mach trimmer automatically counteracts Mach tuck above Mcrit by adding nose-up trim as Mach number increases. This restores normal stick-force and speed stability.

It is a system that senses Mach number and moves the elevator or trimmable stabilizer nose-up in proportion. It holds pitch attitude and stick forces normal from Mcrit up to MDF. If it fails, a reduced MMO normally applies.

At high speed, air ahead of the aircraft is compressed and its density rises. This makes airspeed indicators over-read and changes the wing pressure pattern, leading to shock waves, drag rise and buffet near Mcrit.

For a given TAS, Mach number depends on the local speed of sound, which depends only on air temperature. Colder air means a lower speed of sound and therefore a higher Mach number for the same TAS.

The speed margin is the gap between the normal maximum operating speed (VNO/VNE or VMO/MMO) and the demonstrated dive speed (VDF/MDF). It is wide on piston props but small on jets, which also accelerate easily, so jets need careful speed management.

The manoeuvre envelope is the band between a lower speed limit near the stall and an upper limit at high speed. It is either Vs to VDF/MDF (absolute) or about 1.2–1.3 Vs to VMO/MMO (normal operating), and it shrinks with altitude.

Coffin corner is the altitude at which low-speed (stall) buffet and high-speed (Mach) buffet speeds meet, leaving almost no usable speed range. Operational ceilings are set well below it.

A wing stalls when it exceeds its critical angle of attack. The boundary layer separates from the upper surface and lift falls sharply while drag rises. This can happen at any airspeed, attitude or weight, and recovery requires reducing angle of attack.

The stall occurs at a fixed critical angle of attack, but the indicated stall speed changes with weight and load factor, CG position, altitude/Mach, wing design, configuration, and power/slipstream.

Stall speed increases with weight and is proportional to the square root of weight: Vs2 = Vs1 × √(W2 / W1).

Leading-edge slots and slats re-energise the boundary layer and allow higher angles of attack and CLmax. Washout and higher-camber tip sections make the tips stall later than the root.

Movement of the centre of pressure. On a typical wing designed to stall at the root first, the remaining lift comes from further aft, so the CP moves back and the nose pitches down, which is a stable reaction.

It rises. At high Mach, CLmax falls because of compressibility, so the EAS stall speed increases. In addition, the compressibility correction between IAS and EAS grows, so the indicated stall speed rises further.

A forward CG raises the stall speed and an aft CG lowers it. With a forward CG the tail must produce more downforce, so the wing has to carry more than the aircraft's weight, which increases effective weight.

A deep stall is a stall from which recovery may be impossible. Typical of T-tail, rear-engine, swept-wing jets, it combines a nose-up pitch-up at the stall with a tailplane blanked by the stalled wing's wake. BEA Flight 548 (Trident, 1972) is the classic example.

Stall warning systems (aural warning and stick shaker) alert the crew before the stall. Stick pushers, fitted mainly to aircraft with poor stall characteristics, automatically push the nose down. Fly-by-wire aircraft add angle-of-attack envelope protection.

Dutch roll is a coupled, oscillating yaw-and-roll motion, typical of swept-wing jets. The nose yaws side to side while the wings roll alternately, and the roll part is the most noticeable.

Mainly wing sweep. When a swept-wing aircraft yaws, the advancing wing becomes effectively less swept and faster, so it produces more lift, while the retreating wing produces less. The resulting roll couples with yaw into an oscillation.

Engage or re-engage the yaw damper or autopilot if possible. If flying manually without a yaw damper, damp it with small, well-timed aileron inputs against the roll, and avoid rudder inputs, which usually make it worse.

Yaw dampers. Swept-wing jets have a fin that is deliberately not large enough to damp Dutch roll naturally, so a yaw damper adds artificial directional damping through the rudder.

Directional stability is the aircraft's natural tendency to return to its original heading after a yaw disturbance. It is mainly provided by the fin, which meets the airflow at an angle in a sideslip and produces a restoring force.

A spirally stable aircraft rolls back towards wings-level when ailerons are released in a turn. A spirally unstable one, typically with a dominant fin compared with lateral stability, gradually steepens its bank, drops its nose and enters a spiral dive.

Lateral stability is the tendency to roll back towards wings-level about the longitudinal axis after a sideslip. It comes mainly from dihedral effect (dihedral, sweep, high wing) and, to a smaller extent, from side loads on the fin.

Longitudinal stability is the aircraft's tendency to return to its trimmed pitch attitude and angle of attack after a disturbance about the lateral axis. It is provided mainly by the tailplane and depends on the CG being ahead of the neutral point.

Dynamic stability in pitch, roll and yaw generally gets worse at high altitude because aerodynamic damping falls in thin air. Dutch roll damping deteriorates most, while spiral stability tends to improve.

A narrow operating speed range (coffin corner), reduced manoeuvrability (small buffet margin), reduced aerodynamic damping, and reduced stability.

Elevator for pitch about the lateral axis, ailerons for roll about the longitudinal axis, and rudder for yaw about the normal (vertical) axis. On many jets roll spoilers help the ailerons, and the trimmable stabilizer or all-moving tail supplements pitch.

The elevator is the hinged rear part of the horizontal tail. Pulling the column raises it, creating a downward tail force that pitches the nose up. Pushing lowers it, creating an upward tail force that pitches the nose down.

Ailerons are hinged surfaces on the outboard trailing edges of the wings, moving in opposite directions. The down aileron increases lift on its wing and the up aileron reduces it, producing roll. Banking tilts the lift vector, which turns the aircraft.

The rudder is the hinged rear part of the fin, controlled by the pedals. Left pedal moves the rudder left, the fin produces a sideways force to the right at the tail, and the nose yaws left about the normal axis.

Effectiveness increases with airspeed because control forces depend on dynamic pressure (q = ½ρV²). Large deflections are needed at low speed and small ones at high speed for the same response.

Elevator reversal is an aeroelastic effect at very high speed. Air loads on the deflected elevator twist the flexible tailplane so much that the elevator's effect is cancelled or reversed, giving a pitch response opposite to the input.

Adverse yaw is a yaw away from the intended turn when rolling. The wing with the down-going aileron produces more lift and therefore more induced drag, pulling the nose the opposite way.

At high speed, the large load from a down-going aileron twists the flexible wing leading-edge-down. This reduces that wing's angle of attack and can cancel or reverse the intended roll. It occurs above the aileron reversal speed, mainly on thin, swept wings.

At high speed, a rudder input can roll the aircraft away from the yaw direction, because the fin's side force acts well above the roll axis. For this reason rudder travel is limited at high speed.

Spoilers are hinged panels on the upper wing surface that rise to disrupt airflow, reducing lift and increasing drag. Used asymmetrically they give roll control. Symmetrically in flight they act as speed brakes, and fully on the ground as lift dumpers.

Roll control (with the ailerons), in-flight speed braking, and ground lift dumping. Roll control is usually regarded as the primary function.

The difference is how they give roll control while already deployed as speed brakes. Differential spoilers rise further on one side and retract on the other. Non-differential spoilers can only rise further on one side, and give no roll control if already fully extended.

Four relate to roll control because ailerons are limited: size limits, wing twist and reversal, reduced effectiveness at high speed on swept wings, and strong roll from yaw. Two relate to braking: in-flight speed brakes and ground lift dumping.

At very high speed the air load on deployed spoilers can exceed what their hydraulic actuators can hold, so the panels are pushed back towards retracted (blowback). The load depends on airspeed and deflection angle.

By design and speed management. Spoilers are built not to blow back within the normal operating envelope, so keeping the aircraft within VMO/MMO and following speed brake limitations prevents it.

Reduce speed, mainly by reducing thrust and using pitch as required, until the spoilers can deploy normally. Then recycle the speed brake lever and follow the overspeed or upset recovery procedure.

Slats move forward and down from the leading edge, opening a slot and increasing camber and slightly the chord. This re-energises the boundary layer and raises the stalling angle and CLmax, lowering the stall speed.

Krueger flaps are leading-edge devices hinged from the lower surface that swing forward and down. They increase leading-edge camber and CLmax and help prevent leading-edge separation at high angles of attack.

Fowler flaps are trailing-edge flaps that slide aft on tracks and then deflect down. They increase both wing area and camber, and are usually slotted. Small settings add mostly lift; large settings add mostly drag.

To increase CLmax by increasing camber and, with Fowler-type flaps, wing area. This allows lower safe speeds for takeoff and landing, and shorter field lengths.

Lift and drag increase and the pitching moment changes. The net pitch change depends on CP movement, tailplane downwash and where flap drag acts relative to the CG. Many aircraft pitch nose-up, but it is type-dependent.

CLmax and lift at a given angle of attack fall, and the stall speed rises. Unless the aircraft has accelerated sufficiently and the pitch attitude is increased, it will sink or approach the stall.

Within the approved takeoff range, more flap gives a higher CLmax, lower VR and V2 and a shorter ground run, at the cost of climb gradient. Flap settings outside the takeoff range lengthen the run: too much adds excessive drag and too little requires a much higher VR.

A yaw damper senses yaw rate and applies small opposing rudder inputs. It prevents Dutch roll and helps turn coordination. A roll damper acts through the ailerons to damp residual Dutch roll and turbulence-induced roll and to improve spiral stability.

An early design that moves the rudder through the same control run as the pilot, so the pedals move with its inputs. This shows it is working but adds to pedal loads, so it was often switched off for takeoff and landing.

A series yaw damper acts on the rudder downstream of the pilot's input, usually near the rudder actuator in the tail. It moves the rudder without moving the pedals, so it does not add pedal forces and can stay engaged for takeoff and landing.

It is a horizontal tail whose whole surface can change incidence. It is called a trimmable stabilizer when it handles pitch trim and has a separate elevator for manoeuvring, and a variable-incidence or all-moving tailplane when the whole surface also provides pitch control.

The trimmable stabilizer provides the longitudinal balancing (trim) force. This leaves the elevator's full range available for pitch control and manoeuvring, and lets it cover both large and small changes in pitching moment.

To balance a wide CG range, to balance a wide speed range, to handle large trim changes from high-lift device extension and retraction, and to keep elevator trim drag to a minimum.

Unless the jam happens to match the required trim, the smaller elevator must provide the balancing force. The result is heavy stick forces, slower pitch response and reduced remaining elevator authority for manoeuvring and flare.

An aft CG is preferred. It reduces the nose-up elevator effort needed at low speed and in the flare, because the stabilizer is usually stuck at a cruise-trim setting that cannot be retrimmed for the approach.

Hold the speed at which it jammed for as long as possible, land at a nearby airport, move the CG aft, slow down late, and fly a long, stable approach with configuration changes made early and one at a time.

A runaway is uncommanded, continuous stabilizer trim movement, which can drive the stabilizer towards its stops. Hold the column firmly, disconnect the autopilot, set the stab trim cutout switches to cutout and, if motion continues, grip and hold the trim wheel.

Pitch control authority is reduced or lost. The aircraft responds less to column inputs, which makes attitude changes, configuration changes and especially the landing flare difficult.

Aft, within limits. An aft CG reduces the nose-up elevator deflection needed to balance the aircraft at low speed and in the landing flare, so you rely less on the weakened elevator.

If the elevator is jammed, control pitch with stabilizer trim (and thrust). If it is degraded, check whether the remaining authority is enough to land. If not, treat it as jammed. Move the CG aft, configure early and use reduced landing flap.

Without proper feel the column becomes light. Small inputs can produce large elevator deflections and overstress the structure, especially at high speed. Use slow, small, smooth inputs and avoid turbulence.

Forward. A forward CG raises natural pitch stability, so each bit of elevator produces a smaller pitch change and the controls feel heavier. That cuts the risk of an accidental large input overstressing the elevator.

A deflected surface's air load acts at its centre of pressure, behind the hinge. Load times arm gives the hinge moment, which pushes the surface back to neutral. Aerodynamic balancing or powered controls deal with it.

A tab is a small hinged surface on the trailing edge of a primary control surface. Its aerodynamic force produces a moment about the main surface's hinge, which is used for trimming, for balancing (reducing stick force) or for servo operation of the surface.

A balance tab is a tab mechanically linked to move opposite to the main control surface. Its force creates a moment that partly cancels the surface's hinge moment, reducing the pilot's stick force.

Both are forms of aerodynamic balance. A set-back hinge moves the hinge line aft, closer to the centre of pressure, so the arm and hinge moment shrink. A horn balance puts surface area ahead of the hinge, which creates an opposing moment.

A mass balance is a weight fixed ahead of a control surface's hinge line to move the surface's CG onto or ahead of the hinge. Its main purpose is to prevent control surface flutter, not to reduce stick force.

On large, fast jets the hinge moments from big surfaces at high dynamic pressure exceed what a pilot can manage, even with aerodynamic balancing. Hydraulic actuators provide enough force to handle the full air load.

Irreversible powered controls give the pilot no feedback of the air loads on the surface. Artificial feel restores a sense of how much control is applied and its effect, so the pilot can avoid overcontrolling and overstressing the aircraft.

The simplest system is a spring feel unit (spring box) in the control run. It gives a force proportional to deflection and a self-centring action, but the force does not change with speed, so it suits only aircraft with a narrow speed and altitude range.

Q-feel is an artificial feel system that makes the control force depend on dynamic pressure (q = ½ρV²) as well as on deflection. It is typically used on powered elevators, and on some aircraft also on the rudder or ailerons.

Dynamic pressure, from pitot (total) and static pressure, and control surface (or stabilizer) position. Together they set the feel force.

Active controls are surfaces that move automatically in response to sensed conditions or other surface movements, without a direct pilot command. Examples are geared balance tabs and auto-slats that extend near the stall.