Aeronautical Engineering key terms

A dimensionless ratio of inertial forces to viscous forces in a flow, equal to density times velocity times length divided by viscosity. It indicates whether flow is laminar or turbulent and governs dynamic similarity in wind-tunnel testing.

The ratio of an object's speed to the local speed of sound in the surrounding medium. Values above roughly 0.3 make compressibility effects significant, and crossing Mach 1 produces shock waves and wave drag.

An energy relation for steady, incompressible, inviscid flow along a streamline: static pressure plus dynamic pressure plus potential energy per unit volume stays constant. It explains why faster-moving air has lower static pressure.

A dimensional-analysis result stating that a physically meaningful relationship among n variables involving m fundamental dimensions can be rewritten as a function of n minus m independent dimensionless groups.

The slope of the linear elastic portion of a stress-strain curve, expressing how much stress a material carries per unit of elastic strain. High values mean a stiff material that deflects little under load.

The axial compressive load at which a slender, perfectly straight column becomes unstable and deflects laterally. It varies inversely with the square of effective length and directly with flexural stiffness.

A plot of cyclic stress amplitude against the number of cycles to failure for a material or structural detail. It supports fatigue life estimates, and some steels show an endurance limit below which life is treated as infinite.

A numerical technique that divides a continuous structure into many small elements with assumed shape functions, then solves assembled equilibrium equations to approximate stresses, strains, and displacements under load.

When free vibration of a structure is assumed harmonic, the resulting eigenvalue problem yields natural frequencies as eigenvalues and corresponding deformation patterns as eigenvectors, called mode shapes.

An integral transform converting a function of time into a function of complex frequency, turning linear differential equations into algebraic ones. Engineers use it for control-system and transient-response analysis.

A symmetric bell-shaped probability distribution defined by its mean and standard deviation. In engineering it models scatter in material properties, measurement error, and many tolerance stack-ups.

The principle that energy is conserved: heat added to a system equals the change in its internal energy plus the work it performs. It underlies cycle analysis of engines and efficiency calculations.

The ratio of peak local stress at a geometric discontinuity, such as a hole or fillet, to the nominal stress in the surrounding section. It drives fatigue and fracture initiation at such details.

The angle between a wing's chord line and the oncoming relative airflow. Increasing it raises lift up to the critical angle, beyond which the wing stalls.

A dimensionless number relating lift to dynamic pressure and wing area. It varies mainly with angle of attack and Reynolds number and lets data from different speeds and sizes be compared.

Drag produced as a by-product of lift, caused by the spanwise pressure distribution and trailing vortices. It falls with higher aspect ratio and rises steeply at high lift coefficients.

A curve plotting drag coefficient against lift coefficient for a given aircraft configuration. It summarises parasite and induced drag and is used to find best lift-to-drag ratio and performance speeds.

Wing span squared divided by wing planform area. High aspect ratio wings, typical of gliders, reduce induced drag; low aspect ratio suits fast, manoeuvrable aircraft.

The thin layer of air next to a surface where viscous effects dominate and velocity rises from zero at the wall to the free-stream value. Its state, laminar or turbulent, controls skin friction and separation.

The free-stream Mach number at which airflow somewhere on the aircraft first reaches Mach 1, initiating shock waves and rapid drag rise. Swept wings and thin airfoils raise this value.

The distance between the centre of gravity and the neutral point, expressed as a fraction of mean aerodynamic chord. Positive margin makes an aircraft pitch-stable; too much reduces manoeuvrability.

The ratio of total aerodynamic lift to aircraft weight, usually quoted in g. It defines structural design loads, and a V-n diagram bounds the combinations of speed and load factor permitted.

A self-excited aeroelastic instability in which structural vibration couples with unsteady aerodynamic forces and grows rapidly above a critical speed. It can destroy a wing in seconds and must be excluded from the flight envelope.

A structural design and inspection philosophy that assumes undetected cracks exist and requires the structure to withstand design loads until they are found. It sets inspection intervals and allowable flaw sizes.

The main spanwise structural member of a wing that carries bending and shear loads from root to tip. Spars are usually joined to the fuselage through reinforced frames.

For a turbofan, the mass flow of air through the fan duct divided by the mass flow through the engine core. Higher ratios improve propulsive efficiency and reduce fuel burn and noise.

Fuel mass or weight burned per unit of thrust per unit time. A lower value means a more efficient engine, and it is a key input to range and endurance calculations.

A breakdown of stable compressor flow in which pressure and flow oscillate violently, sometimes reversing direction. It can cause flameout and mechanical damage and is avoided by bleed valves and careful scheduling.

Time-dependent plastic deformation of a material held under sustained stress at elevated temperature. It limits turbine blade and disc life and is assessed by stress-rupture testing and design margins.

Aircraft weight divided by reference wing area, usually expressed in newtons per square metre. Higher values give faster cruise and a rougher ride but raise stall, approach, and field-length requirements.

An approval issued by an airworthiness authority confirming that an aircraft design complies with the applicable certification requirements. It precedes production and must be amended for major design changes.

An approval for a significant modification to an already certificated product, such as a new avionics suite or cargo conversion, issued after showing continued compliance with airworthiness requirements.

The set of processes, inspections, and maintenance actions that keep a certificated aircraft compliant with its approved design and safe throughout its operational life.

A documented organisational framework for identifying hazards, assessing and mitigating risk, and assuring safety performance. Regulators increasingly require it from operators, maintenance organisations, and design organisations.

A workplace philosophy that encourages reporting of errors without fear of punishment while still holding people accountable for deliberate recklessness or intentional violations, supporting safety learning and fair treatment.

A situation in which a professional's personal financial or other interest could improperly influence, or appear to influence, engineering judgements made on behalf of an employer or the public.

Reporting, internally or to a regulator, a practice that a professional reasonably believes endangers public safety or breaks the law. Ethical codes recognise a duty to act on such concerns.

Failure to exercise the degree of care a reasonable, competent engineer would provide in similar circumstances, resulting in harm. It requires a duty of care, a breach, causation, and damage.

Regulations restricting transfer of controlled designs, software, and technical data to foreign persons or countries. Engineers must check classification before sharing drawings, even with colleagues of other nationalities.