# Aeronautical Glossary

Below is a glossary of aeronautical and aerospace related terms used throughout this website. Use the links provided to see additional information on a particular term, or posts and calculators associated with the term.

Advance Ratio The ratio of the freestream fluid (air) velocity to the propeller (rotor) tip speed.
Adverse pressure gradient This occurs when static pressure increases in the direction of flow.
Aerodynamic center The point (fraction of chord) at which the pitching moment coefficient of an airfoil is constant with a changing lift coefficient (angle of attack).
Airfoil The airfoil section refers to the cross-sectional shape of a wing, blade (propeller/rotor) or sail. Airfoil Nomenclature
Introduction to Airfoil Aerodynamics
NACA 4 Series Airfoil Generator
NACA 5 Series Airfoil Generator

Angle of Attack Angle of attack refers to the angle between a reference line on a body and the vector representing the relative motion between the body and the fluid through which it is moving. It is most often used in an aeronautical context to describe the angle between the chordline of an airfoil and the relative wind or resultant direction of airflow. Angle of Attack
Area rule This is a design technique used to reduce the total aircraft drag particularly at transonic and supersonic speeds by a narrowing or ‘waisting’ of the fuselage in the region of the wings.
Aspect ratio This is a measure of the spanwise area distribution of a wing. It is calculated as the ratio of wing span to chord. In cases where the wing is tapered, it is convenient to define the aspect ratio as the square of the wing span divided by the total planform wing area. Aspect Ratio
Introduction to Wing Design Pt. 1
Atmosphere See Standard Atmosphere Standard Atmosphere Calculator
Balanced field length This refers to a runway condition where the aircraft accelerate-stop distance is equal to the take-off distance required for a given aircraft weight, configuration, and thrust setting. It is a term used in both the FAR 25 and CS-25 regulations where an aircraft must be shown to demonstrate that the accelerate-stop distance and take-off distance is less than or equal to a given available runway length both with all engines operating and one engine inoperable.
Bank angle When completing a turn, an aircraft will rotate its wings about the longitudinal axis to an angle relative to the horizontal. The angle between the wing and the horizontal is termed the “bank angle”.
Boundary layer This is a fluid mechanics term that refers to the region of fluid (air) in the immediate vicinity of a body where the viscous effects of the body causes the fluid in this region to attain a velocity less than that of the free-stream.
Camber Camber refers to the asymmetry between the top and bottom surface of an airfoil. A camber line is drawn by a spline connecting a series of points along an airfoil chord equidistant between the upper and lower surface. Airfoils are designed with camber to increase the maximum lift coefficient. Airfoil Nomenclature
Introduction to Airfoil Aerodynamics
Center of gravity This is a geometric property of weight defined as the average location of the weight of an object. It can also be defined as the balance point of an object.
Center of pressure This is a point on a body where the total pressure distribution on that body can be collapsed into a single resultant vector acting at the center of pressure.
Chord On an airfoil, the chord is defined as a straight line joining the leading and trailing edge. Airfoil Nomenclature
Introduction to Airfoil Aerodynamics
Delta wing This refers to a wing planform shaped like a triangle. It is named for its similarity to the Greek uppercase letter Delta (Δ).
Downwash The downward deflection of air caused as a by-product of the aerodynamic process of producing lift by an airfoil, blade or rotor.
Drag An aerodynamic force opposing the direction of motion as a result of a body’s motion through a fluid. An aircraft produces a drag force as it moves through the air, which is counter-acted by the thrust produced by the aircraft engine. Fundamental Forces in Flight
Drag polar A graphical plot showing the relationship between an aircraft’s lift and drag by plotting the dependence of lift (Y-axis) on drag (x-axis). A tangent line drawn from the origin to the polar plot gives the point of maximum aerodynamic efficiency (highest lift-to-drag ratio). Introduction to Airfoil Aerodynamics
Dynamic Pressure Pressure in a fluid as a result of the motion of that fluid. In incompressible flow, the dynamic pressure is defined as $q = \frac{1}{2} \rho V^2$ which forms one of the terms in the Bernoulli Equation.
High-lift device A movable aerodynamic surface present on either the leading edge (slat) or trailing edge (flap) which when deployed increases the effective camber of the airfoil. This in turn increases the maximum lift coefficient and decreases the stall speed. Most commonly used during take-off and landing phases of flight.
Hypersonic A term used to describe flight speeds in excess of five times the speed of sound (Mach 5).
Leading edge The foremost edge of an airfoil section. Airfoil Nomenclature
Introduction to Airfoil Aerodynamics
Lift force A force vector that acts perpendicular to the direction of flight. An aircraft’s wing is the primary generator of the lifting force. This force counteracts the aircraft weight. Fundamental Forces in Flight
Lift coefficient A dimensionless coefficient that relates the lift generated by a lifting surface to the dynamic pressure and an associated reference area (typically wing area). $$C_{L} = \frac{L}{qS} = \frac{L}{\frac{1}{2} \rho V^2 S}$$
Lift slope Defines the change in lift coefficient with angle of attack. The lift slope is constant over a range of angle of attack away from stall. Angle of Attack
Lift-to-drag ratio The ratio of the lifting force to drag force produced by an aircraft. This is an indicator of aerodynamic efficiency.
Limit load This is the maximum load factor authorized during flight. This is the loading that should only occur once (or very seldom) during the lifespan of an aircraft. Aircraft are designed support limit loads without any detrimental permanent deformation.
Mach number The ratio of the speed of a body to the speed of sound in the surrounding medium. The speed of sound is denoted Mach 1 and a speed of Mach 2 is defined as twice the speed of sound.
Mean aerodynamic chord This is a two-dimensional chordwise representation of the entire wing. The pressure distribution over the entire wing can be reduced to a single lift vector and moment around the aerodynamic center of the mean aerodynamic chord (MAC). The calculation of the MAC becomes important where the wing is tapered from root to tip. Wing Plot Tool
Mean camber line See camber Introduction to Airfoil Aerodynamics
NACA National Advisory Committee for Aeronautics. Formed in 1915 and dissolved in 1958, this was the precursor to NASA.
NASA National Aeronautics and Space Administration. NASA website
Neutral point This is a position of the center of gravity that when completing a longitudinal static stability analysis would result in an aircraft with neutral stability.
Oswald efficiency factor This is a measure of span efficiency that represents the change in drag of a three dimensional wing, as compared to the ideal (elliptical) case of equal aspect ratio.
Pitch angle The angle between the horizontal axis and the longitudinal axis of the aircraft. An aircraft will pitch up to climb and pitch down to descend. Pitch angle can also refer to tuning the angle of attack of a propeller blade to control the production of power.
Planform The geometric shape of a wing as viewed from directly above an aircraft. Wing Plot Tool
Quarter chord A point 25 % of a chord length away from the leading edge of an airfoil. This is an important point in the analysis of stability of flight as the quarter chord is used as the assumed aerodynamic center.
Reynolds number An important dimensionless quantity in fluid mechanics which is used to predict the transition from laminar to turbulent flow. It is defined as the ratio of inertial to viscous forces in a fluid. High Reynolds number flows are dominated by inertial forces and exhibit such flow phenomena as eddies, vortices and other flow instabilities. Aircraft generally operate at high Reynolds number states. Reynolds Number Calculator
Scalar A scalar quantity is such a quantity that depends only on magnitude and not direction to fully describe it. Some examples: temperature, volume, mass, density, pressure, speed. Vectors and Scalars
Slat (leading edge) An aerodynamic high-lift device found on the leading edge of a wing, which when deployed, extents forward and downward, increasing the effective camber of the airfoil and thereby increasing the wing’s lift coefficient for a given angle of attack. Slats are generally used during landing to decrease the stall speed which allows the aircraft to land at a lower speed.
Span The lateral length of a wing from wingtip to wingtip. Wing Design Pt. 1
Wing Plot Tool
Spoiler An aerodynamic surface that when extended intentionally reduces the lift component of an airfoil or wing. Spoilers (sometimes called Lift Spoilers or Lift Dumpers) are most often used during the landing phase to dump lift from the wing after touch-down. They can also be used during flight to increase the descent rate or assist with roll control.
Stall A situation reached when an airfoil angle of attack is extended past that which generates the maximum lift coefficient. During stall, the airflow over the upper surface of the airfoil separates resulting in a loss in lift and a subsequent pitch down. Angle of Attack
Standard Atmosphere An atmospheric model relating pressure, temperature, density and viscosity of the Earth’s atmosphere to altitude above the surface. This provides a common reference that may be used by engineers when modelling aircraft performance at a variety of altitudes. Standard Atmosphere Calculator
Static margin The distance between the center of gravity and the neutral point of the aircraft, expressed as a percentage of the mean aerodynamic chord.
Subsonic Any speed below that of the speed of sound (sonic speed).
Supersonic Any speed greater than the speed of sound (sonic speed).
Sweep angle The angle at which a lifting surface is translated backwards (or occasionally forwards) relative to the root chord of the lifting surface. This has the effect of delaying the formation of shock waves and is used on aircraft that fly at or above transonic speeds. Sweep Angle
Taper ratio The ratio of the wing tip to wing root.
Thrust Thrust is the propulsive force that pushes or pulls an aircraft through the air. It is a force vector and is produced by the aircraft’s engines. Fundamental Forces in Flight
Trailing edge The rearmost edge of an airfoil section. Airfoil Nomenclature
Introduction to Airfoil Aerodynamics
Transonic Any speed in the range between the critical Mach number (approximately Mach 0.8) and a higher speed (typically Mach 1.2) where some fraction of the local air velocity is supersonic, but a significant portion is not.
UAV Unmanned Aerial Vehicle (a vehicle piloted by remote control or onboard computers).
Ultimate load factor Limit load multiplied by a prescribed factor of safely (generally 1.5 in aeronautics). An aircraft structure must be able to support an Ultimate loading for a minimum of 3 seconds without structural failure (FAR 25.307).
VTOL An acronym for Vertical Take-off and Landing which refers to any aircraft that can hover, take-off and land vertically.
V-speeds A set of standard terms used to define important airspeeds useful to the safe operation of an aircraft. See the definitions below.
$V_{1}$ Critical Decision Speed – take-off should not be aborted above this speed.
$V_{2}$ Take-off Safety Speed – an aircraft may be safely climbed with one engine inoperative at this speed.
$V_{C}$ Design Cruise Speed.
$V_{D}$ Design Dive Speed – the maximum speed planned to be achieved during flight testing.
$V_{MCA}$ Minimum Control Speed – the minimum speed at which steady flight can be maintained with one engine inoperative and the corresponding opposite engine producing maximum thrust. This airspeed forms a constraint as to the minimum size of the vertical tail and rudder that can be designed.
$V_{NE}$ Never Exceed Speed.
$V_{R}$ Rotation Speed – the speed at which a pilot can add a pitch input to raise the aircraft nose off the ground during take-off.
$V_{REF}$ Landing Reference Speed – the speed at which the aircraft should cross the runway threshold.
$V_{S}$ Stall Speed – the minimum flight speed at which the aircraft is still controllable.
Vector A vector quantity is a quantity that requires both a magnitude and a direction to fully describe it. Some examples: lift, drag, thrust, velocity, displacement. Vectors and Scalars
Wash-in A variation in the angle of incidence along the span of a wing such that the tip is at a higher angle of attack than the root.
Wash-out A variation in the angle of incidence along the span of a wing such that the wing root is at a higher angle of attack than the wing tip.
This is done so as to ensure that stall occurs towards the root before the tip which results in a more benign stall response. Wash-out is the most common example of wing twist.
Weight – Operational Empty (OEW) The basic weight of an aircraft including the crew, all fluids necessary for operation (water, engine oil, coolant), unusable fuel, and any items required for standard operations. OEW excludes usable fuel and payload.
Weight – Usable Fuel The weight of the fuel on board an aircraft that can be consumed by the engines during flight.
Weight – Aircraft Gross (UAW) Also known as All-up Weight (UAW), it is the total weight of an aircraft at any moment during flight or ground operation. The aircraft gross weight will decrease during the course of a flight as fuel and oil is consumed.
Weight – Payload Payload is the carrying capacity of an aircraft. Payload could include cargo, passengers, munitions or extra fuel in the case of an airborne tanker.
Weight – Maximum Takeoff (MTOW) This is the maximum weight at which a pilot is allowed to attempt a take-off. This is the heaviest weight for which an aircraft has been certified to meet all applicable certification requirements.
Wing area The plan surface area of a wing. This is a crucial metric used in the performance analysis and design of an aircraft wing. Wing Area
Wing Plot Tool
Wing Design – Wing Area & Aspect Ratio
Wing loading The mass of an aircraft divided by the wing area. Often taken at MTOW as the aircraft mass varies with fuel burn and payload. Wing Design – Wing Area & Aspect Ratio
Yaw The movement of an aircraft’s nose left or right about an axis running vertically through the aircraft. The aircraft yaw angle is controlled by the application of the rudder on the vertical tail.