Aerodynamic center

The torques or moments acting on an airfoil moving through a fluid can be accounted for by the

net lift applied at some point on the foil, and a separate net pitching moment about that point whose magnitude

varies with the choice of where the lift is chosen to be applied. The aerodynamic center is the point at which

the pitching moment coefficient for the airfoil does not vary with lift coefficienti.e. angle of attack, so this choice

makes analysis simpler [1]

.

where CL is the aircraft lift coefficient.

Aerodynamic force

Forces on an aerofoil.

Aerodynamic force is the resultant force exerted on a body by the air (or some other gas) in

which the body is immersed, and is due to the relative motion between the body and the fluid.

An aerodynamic force arises from two causes: [1]

[2]

[3]

the force due to the pressure on the surface of the body

the force due to viscosity, also known as skin friction

When a body is exposed to the wind it experiences a force in the direction in which the wind

is moving. This is an aerodynamic force. When a body is moving in air or some other gas the

aerodynamic force is usually called drag.

When an airfoil or a wing or a glider is moving relative to the air it generates an aerodynamic

force that is partly parallel to the direction of relative motion, and partly perpendicular to the

direction of relative motion. This aerodynamic force is commonly resolved into two

components:[4][5]

Drag is the component parallel to the direction of relative motion.

Lift is the component perpendicular to the direction of relative motion.

The force created by a propeller or a jet engine is called thrust and it is also an aerodynamic

force. The aerodynamic force on a powered airplane is commonly resolved into three

components:[6][7]

thrust, lift and drag

The only other force acting on a glider or powered airplane is its weight. (Weight is a body

force, not an aerodynamic force.)

Aileron

.

An aircraft rolling with its ailerons

Ailerons are hinged flight control surfaces attached to the trailing edge of the wing of a fixed-wing aircraft. The

ailerons are used to control the aircraft in roll, which results in a change in heading due to the tilting of the lift

vector. The two ailerons are typically interconnected so that one goes down when the other goes up: the

downgoing aileron increases the lift on its wing while the upgoing aileron reduces the lift on its wing, producing a

rolling moment about the aircraft's longitudinal axis.[1]

The word aileron is French for "little wing".

Airspeed indicator

The airspeed indicator or airspeed gauge is an instrument used in an aircraft to display the craft's airspeed,

typically in knots, to the pilot.

The airspeed indicator is used by the pilot during all phases of flight, from take-off, climb, cruise, descent and

landing in order to maintain airspeeds specific to the aircraft type and operating conditions as specified in the

Operating Manual.

Altimeter

Diagram showing the face of the "three-pointer" sensitive aircraft altimeter displaying an altitude of 10,180 feet.

An altimeter is an instrument used to measure the altitude of an object above a fixed level. The

measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of

depth underwater.

Angle of attack

In this diagram, the black lines represent the flow of a fluid around a two-dimensional airfoil shape. The angle α is the angle of attack.

Angle of attack (AOA, α, Greek letter alpha) is a term used in fluid dynamics to describe the angle between a

reference line on a lifting body (often the chord line of an airfoil) and the vector representing the relative motion

between the lifting body and the fluid through which it is moving. Angle of attack is the angle between the lifting

body's reference line and the oncoming flow. This article focuses on the most common application, the angle of

attack of a wing or airfoil moving through air.

In Aerodynamics, angle of attack is used to describe the angle between the chord line of the wing of a fixed-wing

aircraft and the vector representing the relative motion between the aircraft and the atmosphere. Since a wing

can have twist, a chord line of the whole wing may not be definable, so an alternate reference line is simply

defined. Often, the chord line of the root of the wing is chosen as the reference line. Another alternative is to use

a horizontal line on the fuselage as the reference line (and also as the longitudinal axis).[1]

Some books[2][3]

adopt

the so called absolute angle of attack: zero angle of attack corresponds to zero coefficient of lift.

Angle of incidence

Angle of incidence

Angle of incidence is a measure of deviation of something from "straight on", for example:

in the approach of a ray to a surface, or

the angle at which the wing or horizontal tail of an airplane is installed on the fuselage, measured relative to

the axis of the fuselage.

Aspect ratio (wing)

In aerodynamics, the aspect ratio of a wing is essentially the ratio of its length to its breadth (chord). A high

aspect ratio indicates long, narrow wings, whereas a low aspect ratio indicates short, stubby wings.[1]

For most wings the length of the chord is not a constant but varies along the wing, so the aspect ratio AR is

defined as the square of the wingspan b divided by the area S of the wing planform[2][3]

—this is equal to the

length-to-breadth ratio for constant breadth.

Boundary layer

Boundary layer visualization, showing transition from laminar to turbulent condition

In physics and fluid mechanics, a boundary layer is that layer of fluid in the immediate

vicinity of a bounding surface where effects of viscosity of the fluid are considered in detail.

In the Earth's atmosphere, the planetary boundary layer is the air layer near the ground

affected by diurnal heat, moisture or momentum transfer to or from the surface. On an

aircraft wing the boundary layer is the part of the flow close to the wing. The boundary

layer effect occurs at the field region in which all changes occur in the flow pattern. The

boundary layer distorts surrounding non-viscous flow. It is a phenomenon of viscous forces.

This effect is related to the Reynolds number.

Laminar boundary layers come in various forms and can be loosely classified according to

their structure and the circumstances under which they are created. The thin shear layer

which develops on an oscillating body is an example of a Stokes boundary layer, whilst the

Blasius boundary layer refers to the well-known similarity solution for the steady boundary

layer attached to a flat plate held in an oncoming unidirectional flow. When a fluid rotates,

viscous forces may be balanced by the Coriolis effect, rather than convective inertia, leading

to the formation of an Ekman layer. Thermal boundary layers also exist in heat transfer.

Multiple types of boundary layers can coexist near a surface simultaneously.

Center of gravity of an aircraft

The center-of-gravity (CG) is the point at which an aircraft would balance if it were possible to suspend it at that

point. It is the mass center of the aircraft, or the theoretical point at which the entire weight of the aircraft is

assumed to be concentrated.[1]

Its distance from the reference datum is determined by dividing the total moment

by the total weight of the aircraft.[2]

The center-of-gravity point affects the stability of the aircraft. To ensure the

aircraft is safe to fly, the center-of-gravity must fall within specified limits established by the manufacturer.

Center of pressure

The center of pressure is the point on a body where the total sum of a pressure field acts, causing a force and

no moment about that point. The total force vector acting at the center of pressure is the value of the integrated

vectorial pressure field. The resultant force and center of pressure location produce equivalent force and moment

on the body as the original pressure field. Pressure fields occur in both static (hydrostatic) and dynamic

(aerodynamic) fluid mechanics. Specification of the center of pressure, the reference point from which the center

of pressure is referenced, and the associated force vector allows the moment generated about any point to be

computed by a translation from the reference point to the desired new point.

Critical Mach number

Transonic flow patterns on an aircraft wing showing the effects at critical mach.

In aerodynamics, the critical Mach number (Mcr) of an aircraft is the lowest Mach

number at which the airflow over any part of the aircraft reaches the speed of sound.[1]

For all aircraft in flight, the airflow around the aircraft is not exactly the same as the airspeed

of the aircraft due to the airflow speeding up and slowing down to travel around the aircraft

structure. At the Critical Mach number, local airflow in some areas near the airframe reaches

the speed of sound, even though the aircraft itself has an airspeed lower than Mach 1.0. This

creates a weak shock wave. At speeds faster than the Critical Mach number:

Drag divergence Mach number

The drag divergence Mach number (not to be confused with critical Mach number) is the Mach number at

which the aerodynamic drag on an airfoil or airframe begins to increase rapidly as the Mach number continues to

increase[1]

. This increase can cause the drag coefficient to rise to more than ten times its low speed value.

The value of the drag divergence Mach number is typically greater than 0.6; therefore it is a transonic effect. The

drag divergence Mach number is usually close to, and always greater than, the critical Mach number. Generally,

the drag coefficient peaks at Mach 1.0 and begins to decrease again after the transition into

the supersonic regime above approximately Mach 1.2.

The large increase in drag is caused by the formation of a shock wave on the upper surface of the airfoil, which

can induce flow separation and adverse pressure gradients on the aft portion of the wing. This effect requires

that aircraft intended to fly at supersonic speeds have a large amount of thrust. In early development

of transonic and supersonic aircraft, a steep dive was often used to provide extra acceleration through the high

drag region around Mach 1.0. This steep increase in drag gave rise to the popular false notion of an

unbreakable sound barrier, because it seemed that no aircraft technology in the foreseeable future would have

enough propulsive force or control authority to overcome it. Indeed, one of the popular analytical methods for

calculating drag at high speeds, the Prandtl-Glauert rule, predicts an infinite amount of drag at Mach 1.0

Endurance (aircraft)

In aviation, Endurance is the maximum length of time that an aircraft can spend in cruising flight. Endurance is

sometimes erroneously equated with range. The two concepts are distinctly different: range is a measure

of distance flown while endurance is a measure of time spent in the air. For example, a typical sailplane exhibits

high endurance characteristics but poor range characteristics.

Endurance can be written as:

Engine

An engine or motor is a machine designed to convert energy into useful mechanical motion.[1][2]

Devices converting heat energy into motion are referred to as engines,[3]

which come in many types. A common

type is a heat engine such as an internal combustion engine which typically burns a fuel with air and uses the hot

gases for generating power. External combustion engines such as steam engines use heat to generate motion

via a separate working fluid.

A common type of motor is the electric motor. This takes electrical energy and generates mechanical motion via

varying electromagnetic fields.

Flap (aircraft)

Flaps are hinged surfaces on the trailing edge of the wings of a fixed-wing aircraft. As flaps are extended,

the stalling speed of the aircraft is reduced, which means that the aircraft can fly safely at lower speeds

(especially during take off and landing). Flaps are also used on the leading edge of the wings of some high-speed

jet aircraft, where they may be called Krueger flaps

Extending flaps increases the camber of the wing airfoil, thus raising the maximum lift coefficient. This increase in

maximum lift coefficient allows the aircraft to generate a given amount of lift with a lower speed. Therefore,

extending the flaps reduces the stalling speed of the aircraft.

Flow separation

Airflow separating from a wing at a high angle of attack

All solid objects travelling through a fluid (or alternatively a stationary object exposed to a moving fluid) acquire

a boundary layer of fluid around them where viscous forces occur in the layer of fluid close to the solid surface.

Boundary layers can be either laminar or turbulent. A reasonable assessment of whether the boundary layer will

be laminar or turbulent can be made by calculating the Reynolds number of the local flow conditions.

Flow separation occurs when the boundary layer travels far enough against an adverse pressure gradient that

the speed of the boundary layer relative to the object falls almost to zero.[1][2]

The fluid flow becomes detached

from the surface of the object, and instead takes the forms of eddies and vortices. In aerodynamics, flow

separation can often result in increased drag, particularly pressure drag which is caused by

the pressure differential between the front and rear surfaces of the object as it travels through the fluid. For this

reason much effort and research has gone into the design of aerodynamic and hydrodynamic surfaces which

delay flow separation and keep the local flow attached for as long as possible. Examples of this include the fur on

a tennis ball, dimples on a golf ball, turbulators on a glider, which induce an early transition to turbulent flow

regime; vortex generators on light aircraft, for controlling the separation pattern; and leading edge extensions for

high angles of attack on the wings of aircraft such as the F/A-18 Hornet.

Boundary layer separation occurs when the portion of the boundary layer closest to the wall or leading edge

reverses in flow direction. As a result, the overall boundary layer initially thickens suddenly and is then forced off

the surface by the reversed flow at its bottom.[3]

High-lift device

In aircraft design, high-lift devices are moving surfaces or stationary components intended to increase lift during

certain flight conditions. They include common devices such as flaps and slats, as well as less common features

such as leading edge extensions and blown flaps.

Horseshoe vortex

The horseshoe vortex model is a simplified representation of the vortex system of a wing. In this model the

wing vorticity is modelled by a bound vortex of constant circulation, travelling with the wing, and two trailing

vortices, therefore having a shape vaguely reminiscent of a horseshoe.[1][2]

(The starting vortex created as the

wing begins to move through the fluid is considered to have been dissipated by the action of viscosity, as are

the trailing vortices well behind the aircraft.)

The trailing vortices are responsible for the component of the downwash which creates induced drag.[3]

The horseshoe vortex model is unrealistic in implying a constant circulation (and hence by the Kutta–Joukowski

theorem constant lift) at all sections on the wingspan. In a more realistic model (due toLudwig Prandtl) the vortex

strength reduces along the wingspan, and the loss in vortex strength is shed as a vortex-sheet from the trailing

edge, rather than just at the wing-tips.[4]

However, by using the horseshoe vortex model with a reduced effective

wingspan but same midplane circulation, the flows induced far from the aircraft can be adequately modelled.

Hypersonic speed

In aerodynamics, a hypersonic speed is one that is highly supersonic. Since the 1970s, the term has

generally been assumed to refer to speeds of Mach 5 (5 times the speed of sound) and above. The hypersonic

regime is a subset of the supersonic regime.

The precise Mach number at which a craft can be said to be flying at hypersonic speed is elusive, especially

since physical changes in the airflow (moleculardissociation, ionization) occur at quite different speeds.

Generally, a combination of effects become important "as a whole" around Mach 5. The hypersonic regime is

often defined as speeds where ramjets do not produce net thrust. This is a nebulous definition in itself, as there

exists a proposed change to allow them to operate in the hypersonic regime

Internal combustion engine The internal combustion engine is an engine in which the combustion of a fuel (normally a fossil fuel)

occurs with an oxidizer (usually air) in a combustion chamber. In an internal combustion engine, the expansion of

the high-temperature and -pressure gases produced by combustion applies direct force to some component of

the engine, such as pistons, turbine blades, or a nozzle. This force moves the component over a distance,

generating useful mechanical energy.

The term internal combustion engine usually refers to an engine in which combustion is intermittent, such as the

more familiar four-stroke and two-stroke piston engines, along with variants, such as the six-stroke piston engine

and the Wankel rotary engine. A second class of internal combustion engines use continuous combustion: gas

turbines, jet engines and most rocket engines, each of which are internal combustion engines on the same

principle

Kutta condition

The Kutta condition is a principle in steady flow fluid dynamics, especially aerodynamics, that is

applicable to solid bodies which have sharp corners such as the trailing edges of airfoils. It is named

for German mathematician and aerodynamicist Martin Wilhelm Kutta.

Kuethe and Schetzer state the Kutta condition as follows:[1]

A body with a sharp trailing edge which is moving through a fluid will create about itself a circulation of

sufficient strength to hold the rear stagnation point at the trailing edge.

In fluid flow around a body with a sharp corner the Kutta condition refers to the flow pattern in which fluid

approaches the corner from both directions, meets at the corner, and then flows away from the body. None

of the fluid flows around the corner and remains attached to the body.

The Kutta condition is significant when using the Kutta–Joukowski theorem to calculate the lift generated by

an airfoil. The value of circulation of the flow around the airfoil must be that value which would cause the

Kutta condition to exist.

Laminar flow

Laminar flow, sometimes known as streamline flow, occurs when a fluid flows in parallel

layers, with no disruption between the layers.[1]

At low velocities the fluid tends to flow without

lateral mixing, and adjacent layers slide past one another like playing cards. There are no cross

currents perpendicular to the direction of flow, nor eddies or swirls of fluids.[2]

In laminar flow the

motion of the particles of fluid is very orderly with all particles moving in straight lines parallel to the

pipe walls.[3]

In fluid dynamics, laminar flow is a flow regime characterized by high momentum

diffusion and low momentum convection.

Leading edge slot A leading edge slot is an aerodynamic feature of the wing of some aircraft to reduce the stall

speed and promote good low-speed handling qualities. A leading edge slot is a span-wise gap in each wing,

allowing air to flow from below the wing to its upper surface. In this manner they allow flight at higher angles of

attack and thus reduce the stall speed.[1]

Lift coefficient The lift coefficient ( or ) is a dimensionless coefficient that relates the lift generated by an

aerodynamic body such as a wing or complete aircraft, the dynamic pressure of the fluid flow around the

body, and a reference area associated with the body. It is also used to refer to the aerodynamic lift

characteristics of a 2D airfoil section, whereby the reference "area" is taken as the airfoilchord.[1][2]

It may

also be described as the ratio of lift pressure to dynamic pressure.

Lift-induced drag

In aerodynamics, lift-induced drag, induced drag, vortex drag, or sometimes drag due to

lift, is a drag force that occurs whenever a moving object redirects the airflow coming at it.

This drag force occurs in airplanes due to wings or a lifting body redirecting air to cause lift

and also in cars with airfoil wings that redirect air to cause a downforce. With other

parameters remaining the same, induced drag increases as the angle of attack increases

Lift-to-drag ratio

In aerodynamics, the lift-to-drag ratio, or L/D ratio ("ell-over-dee"), is the amount of lift generated by a wing or vehicle, divided by the drag it

creates by moving through the air. A higher or more favorable L/D ratio is typically one of the major goals in aircraft design; since a particular

aircraft's required lift is set by its weight, delivering that lift with lower drag leads directly to better fuel economy, climb performance, and glide

ratio.

The term is calculated for any particular airspeed by measuring the lift generated, then dividing by the drag at that speed. These vary with speed,

so the results are typically plotted on a 2D graph. In almost all cases the graph forms a U-shape, due to the two main components of drag.

Mach tuck

Mach tuck is an aerodynamic effect, whereby the nose of an aircraft tends to pitch downwards as the airflow

around the wing reaches supersonic speeds. The aircraft will be subsonic, and traveling significantly

below Mach 1.0, when it first experiences this effect.[1]

Shock wave on upper surface of wing moves rearwards as aircraft mach increases

Oblique shock

A small scale X-15 placed in a NASA supersonic wind tunnel produces an oblique shock wave at the nose of the model (along with

other shocks).

An oblique shock wave, unlike a normal shock, is inclined with respect to the incident upstream flow direction. It

will occur when a supersonic flow encounters a corner that effectively turns the flow into itself and compresses.

The upstream streamlines are uniformly deflected after the shock wave. The most common way to produce an

oblique shock wave is to place a wedge into supersonic, compressible flow. Similar to a normal shock wave, the

oblique shock wave consists of a very thin region across which nearly discontinuous changes in the

thermodynamic properties of a gas occur. While the upstream and downstream flow directions are unchanged

across a normal shock, they are different for flow across an oblique shock wave.

Parafoil

Illustrations from Jalbert's 1966 patent, showing the keels and the airfoil shape.

The NASA X-38 prototype makes a gentle lakebed landing at the end of a July 1999 test flight at the Dryden Flight Research Center.

A parafoil is a nonrigid (textile) airfoil with an aerodynamic cell structure which is inflated by the wind. Ram-air

inflation forces the parafoil into a classic wingcross-section. Parafoils are most commonly constructed out

of ripstop nylon.

Parasitic drag

Parasitic drag (also called skin friction drag) is drag caused by moving a solid object

through a fluid medium (in the case of aerodynamics, more specifically, a gaseous medium).

Parasitic drag is made up of many components, the most prominent being form drag. Skin

friction and interference drag are also major components of parasitic drag.

In aviation, induced drag tends to be greater at lower speeds because a high angle of attack is

required to maintain lift, creating more drag. However, as speed increases the induced drag

becomes much less, but parasitic drag increases because the fluid is flowing faster around

protruding objects increasing friction or drag. At even higher transonic and supersonic

speeds, wave drag enters the picture. Each of these forms of drag changes in proportion to the

others based on speed. The combined overall drag curve therefore shows a minimum at some

airspeed - an aircraft flying at this speed will be at or close to its optimal efficiency. Pilots

will use this speed to maximize endurance (minimum fuel consumption). However, to

maximize gliding range in the event of an engine failure, the aircraft's speed would have to be

at the point of minimum power, which occurs at lower speeds than minimum drag.

Range (aircraft)

The maximal total range is the distance an aircraft can fly between takeoff and landing, as limited by fuel

capacity in powered aircraft, or cross-country speed and environmental conditions in unpowered aircraft.

Ferry range means the maximum range the aircraft can fly. This usually means maximum fuel load, optionally

with extra fuel tanks and minimum equipment. It refers to transport of aircraft for use on remote location.

Combat range is the maximum range the aircraft can fly when carrying ordnance.

Combat radius is a related measure based on the maximum distance a warplane can travel from its base of

operations, accomplish some objective, and return to its original airfield with minimal reserves.

The fuel time limit for powered aircraft is fixed by the fuel load and rate of consumption. When all fuel is

consumed, the engines stop and the aircraft will lose its propulsion. For unpowered aircraft, the maximum flight

time is variable, limited by available daylight hours, weather conditions, and pilot endurance.

The range can be seen as the cross-country ground speed multiplied by the maximum time in the air. The range

equation will be derived in this article for propeller and jet aircraft.

Propeller (aircraft)

Propeller

The feathered propellers of an RAF Hercules C.4

Aircraft propellers or airscrews[1]

convert rotary motion from piston engines or turboprops to provide

propulsive force. They may be fixed or variable pitch. Early aircraft propellers were carved by hand from

solid or laminated wood with later propellers being constructed from metal. The most modern propeller

designs use high-technology composite materials.

The propeller is usually attached to the crankshaft of a piston engine, either directly or through a reduction

unit. Light aircraft engines often do not require the complexity of gearing but on larger engines

and turboprop aircraft it is essential.

Rib (aircraft)

Rib

Wing ribs of a de Havilland DH.60 Moth

In an aircraft, ribs are forming elements of the structure of a wing, especially in traditional construction.

By analogy with the anatomical definition of "rib", the ribs attach to the main spar, and by being repeated at

frequent intervals, form a skeletal shape for the wing. Usually ribs incorporate the airfoil shape of the wing, and

the skin adopts this shape when stretched over the ribs.

Rudder

A rudder is a device used to steer a ship, boat, submarine, hovercraft, aircraft or other conveyance that

moves through a medium (generally air or water). On an aircraft the rudder is used primarily to counter adverse

yaw and p-factor and is not the primary control used to turn the airplane. A rudder operates by redirecting the

fluid past the hull or fuselage, thus imparting a turning or yawing motion to the craft. In basic form, a rudder is a

flat plane or sheet of material attached with hinges to the craft's stern, tail or after end. Often rudders are shaped

so as to minimize hydrodynamic or aerodynamic drag. On simple watercraft, a tiller -- essentially, a stick or pole

acting as a lever arm—may be attached to the top of the rudder to allow it to be turned by a helmsman. In larger

vessels, cables, pushrods, or hydraulics may be used to link rudders to steering wheels. In typical aircraft, the

rudder is operated by pedals via mechanical linkages or hydraulics.

Shock wave

Schlieren photograph of an attached shock on a sharp-nosed supersonic body.

A shock wave (also called shock front or simply "shock") is a type of propagating disturbance. Like an

ordinary wave, it carries energy and can propagate through a medium (solid, liquid, gas or plasma) or in

some cases in the absence of a material medium, through a field such as the electromagnetic field. Shock

waves are characterized by an abrupt, nearly discontinuous change in the characteristics of the

medium.[1]

Across a shock there is always an extremely rapid rise in pressure, temperature and density of

the flow. In supersonic flows, expansion is achieved through an expansion fan. A shock wave travels

through most media at a higher speed than an ordinary wave.

Sonic boom

A sonic boom is the sound associated with the shock waves created by the supersonic flight of

an aircraft. Sonic booms generate enormous amounts of sound energy, sounding much like an explosion.

The crack of a supersonic bullet passing overhead is an example of a sonic boom in miniature.

Spar (aviation)

In a fixed-wing aircraft, the spar is often the main structural member of the wing, running spanwise

at right angles (or thereabouts depending on wing sweep) to the fuselage. The spar carries flight

loads and the weight of the wings whilst on the ground. Other structural and forming members such

as ribs may be attached to the spar or spars, with stressed skin construction also sharing the loads

where it is used. There may be more than one spar in a wing or none at all. However, where a single

spar carries the majority of the forces on it, it is known as the main spar.[1]

Spars are also used in other aircraft aerofoil surfaces such as the tailplane and fin and serve a

similar function, although the loads transmitted may be different to those of a wing spar.

Spoiler (aeronautics)

In aeronautics a spoiler (sometimes called a lift dumper) is a device intended to reduce lift in an aircraft.

Spoilers are plates on the top surface of a wing which can be extended upward into the airflow and spoil it. By

doing so, the spoiler creates a carefully controlled stall over the portion of the wing behind it, greatly reducing the

lift of that wing section. Spoilers differ from airbrakes in that airbrakes are designed to increase drag making little

change to lift, while spoilers greatly reduce lift making only a moderate increase in drag.

Spoilers are used by gliders to control their rate of descent and thus achieve a controlled landing at a desired

spot. An increased rate of descent could also be achieved by lowering the nose of an aircraft, but this would

result in an excessive landing speed. However spoilers enable the approach to be made at a safe speed for

landing.

Supersonic airfoils

A United States Navy F/A-18E/F Super Hornet in transonic flight

A supersonic airfoil is a cross-section geometry designed to generate lift efficiently at supersonic speeds.

The need for such a design arises when an aircraft is required to operate consistently in the supersonic

flight regime.

Supersonic airfoils generally have a thin section formed of either angled planes or opposed arcs (called

"double wedge airfoils" and "biconvex airfoils" respectively), with very sharp leading and trailing edges. The

sharp edges prevent the formation of a detached bow shock in front of the airfoil as it moves through the

air.[1]

. This shape is in contrast to subsonic airfoils, which often have rounded leading edges to reduce flow

separation over a wide range of angle of attack[2]

. A rounded edge would behave as a blunt body in

supersonic flight and thus would form a bow shock, which greatly increases wave drag. The airfoils'

thickness, camber, and angle of attack are varied to achieve a design that will cause a slight deviation in

the direction of the surrounding airflow[3]

.

Swept wing

A B-52 Stratofortress showing wing with a large sweepback angle.

A swept wing is a wing planform with a wing root to wingtip direction angled beyond (usually aftward) the

spanwise axis, generally used to delay the drag rise caused by fluid compressibility.

Unusual variants of this design feature are forward sweep, variable sweep wings and pivoting wings. Swept

wings as a means of reducing wave drag were first used on jet fighter aircraft. Today they have become almost

universal on all but the slowest jets (such as the A-10). The four-engine propeller-driven Tu-95aircraft also has

swept wings.

The angle of sweep which characterizes a swept wing is conventionally measured along the 25% chord line. If

the 25% chord line varies in sweep angle, the leading edge is used; if that varies, the sweep is expressed in

sections (e.g., 25 degrees from 0 to 50% span, 15 degrees from 50% to wingtip).

Delta wing

The delta wing is a wing planform in the form of a triangle. It is named for its similarity in shape to

the Greek uppercase letter delta (Δ).

Tailplane

Tailplane or horizontal stabilizer of aBoeing 737

A tailplane, also known as horizontal stabilizer (or horizontal stabiliser), is a small lifting surface located on

the tail (empennage) behind the main lifting surfaces of a fixed-wing aircraft as well as other non-fixed wing

aircraft such as helicopters and gyroplanes. However, not all fixed-wing aircraft have tailplanes, such as those

configured with canards (where the "tail-plane" is located in front), flying-wing aircraft, where there is no tail,

and v-tail aircraft where the fin/rudder and tail-plane are combined to form two diagonal surfaces in a V layout.

The tailplane serves three purposes: equilibrium, stability and control.

Thrust-to-weight ratio

Thrust-to-weight ratio is a ratio of thrust to weight of a rocket, jet engine, propeller engine, or a vehicle

propelled by such an engine. It is a dimensionless quantity and is an indicator of the performance of the engine or

vehicle.

The instantaneous thrust-to-weight ratio of a vehicle varies continually during operation due to progressive

consumption of fuel or propellant, and in some cases due to a gravity gradient. The thrust-to-weight ratio based

on initial thrust and weight is often published and used as a figure of merit for quantitative comparison of the

initial performance of vehicles.

Transonic speed

Transonic speed is an aeronautics term referring to the condition of flight in which a range of velocities of

airflow exist surrounding and flowing past an air vehicle or an airfoil that are concurrently below, at, and above

the speed of sound in the range of Mach 0.8 to 1.2, i.e. 600-900 mph. This condition depends not only on the

travel speed of the craft, but also on the pressure and temperature of the airflow of the vehicle's local

environment. It is formally defined as the range of speeds between the critical Mach number, when some parts of

the airflow over an air vehicle or air foil are supersonic, and a higher speed, typically near Mach 1.2, when all of

the airflow is supersonic. Between these speeds some of the airflow is supersonic, and some is not.

Most modern jet powered aircraft are engineered to operate at transonic air speeds. Transonic airspeeds see a

rapid increase of drag from about Mach 0.8, and it is the fuel costs of the drag that typically limits the airspeed.

Attempts to reduce wave drag can be seen on all high-speed aircraft; most notable is the use of swept wings, but

another common form is a wasp-waist fuselage as a side effect of the Whitcomb area rule.

Turbulence

In fluid dynamics, turbulence or turbulent flow is a fluid regime characterized by

chaotic, stochastic property changes. This includes low momentum diffusion, high

momentum convection, and rapid variation of pressure and velocity in space and time. Nobel

Laureate Richard Feynman describes turbulence as "the most important unsolved problem of

classical physics."[1]

Flow that is not turbulent is called laminar flow. While there is no

theorem relating Reynolds number to turbulence, flows with high Reynolds numbers usually

become turbulent, while those with low Reynolds numbers usually remain laminar. For pipe

flow, a Reynolds number above about 4000 will most likely correspond to turbulent flow,

while a Reynold's number below 2100 indicates laminar flow. The region in between (2100 <

Re < 4000) is called the transition region. In turbulent flow, unsteady vortices appear on

many scales and interact with each other. Drag due to boundary layer skin friction increases.

The structure and location of boundary layer separation often changes, sometimes resulting in

a reduction of overall drag. Although laminar-turbulent transition is not governed by

Reynolds number, the same transition occurs if the size of the object is gradually increased,

or the viscosity of the fluid is decreased, or if the density of the fluid is increased.

Vortex

Vortex created by the passage of an aircraft wing, revealed by colored smoke

A vortex (plural: vortices) is a spinning, often turbulent, flow of fluid. Any spiral motion with

closed streamlines is vortex flow. The motion of the fluid swirling rapidly around a center is called a vortex.

The speed and rate of rotation of the fluid in a free (irrotational) vortex are greatest at the center, and

decrease progressively with distance from the center, whereas the speed of a forced (rotational) vortex is

zero at the center and increases proportional to the distance from the center. Both types of vortices exhibit

a pressure minimum at the center, though the pressure minimum in a free vortex is much lower.

Washout (aviation)

Washout refers to a feature of wing design to deliberately reduce the lift distribution across the span of

the wing of an aircraft. The wing is designed so that angle of incidence (angle to the fuselage) is higher at

the wing roots and decreases across the span, becoming lowest at the wing tip. This is usually to ensure

that, at the stall, the wing root stalls before the wing tips, providing the aircraft with continued aileron control

and some resistance to spinning. Washout may also be used to modify the spanwise lift distribution to

reduce lift-induced drag.

Washout is commonly achieved by designing the wing with a slight twist, reducing the angle of

incidence from root to tip, and therefore causing a lower angle of attack at the tips than at the roots. This

feature, pioneered in fighter aircraft like the Spitfire[1]

, is sometimes referred to as structural washout, to

distinguish it from aerodynamic washout.

Wing loading

In aerodynamics, wing loading is the loaded weight of the aircraft

divided by the area of the wing. The faster an aircraft flies, the more

lift is produced by each unit area of wing, so a smaller wing can carry

the same weight in level flight, operating at a higher wing loading.

Correspondingly, the landing and take-off speeds will be higher. The

high wing loading also decreases maneuverability. The same

constraints apply to birds and bats.

Wing twist

Wing twist is an aerodynamic feature added to aircraft wings to adjust lift distribution along the wing.

Often, the purpose of lift redistribution is to ensure that the wing tip is the last part of the wing surface to stall, for

example when executing a roll or steep climb; it involves twisting the wingtip a small amount downwards in

relation to the rest of the wing. This ensures that the effective angle of attack is always lower at the wingtip than

at the root, meaning the root will stall before the tip. This is desirable is because the aircraft's flight control

surfaces are often located at the wingtip, and the variable stall characteristics of a twisted wing alert the pilot to

the advancing stall while still allowing the control surfaces to remain effective, meaning the pilot can usually

prevent the aircraft from stalling fully before control is completely lost.

Vorticity Vorticity is a concept used in fluid dynamics. In the simplest sense, vorticity is the tendency for elements of the

fluid to "spin."

More formally, vorticity can be related to the amount of "circulation" or "rotation" (or more strictly, the local

angular rate of rotation) in a fluid.[1]

The average vorticity ωav in a small region of fluid flow is equal to

the circulation Γ around the boundary of the small region, divided by the area A of the small region.[1]

Notionally, the vorticity at a point in a fluid is the limit as the area of the small region of fluid approaches zero at the point:

[1]

Mathematically, vorticity is a vector field and is defined as the curl of the velocity field:

Wingspan

Wingspan

The distance A to B is the wingspan of this Aer Lingus

Airbus A320.

The wingspan (or just span) of an airplane or a bird, is the distance from one wingtip to the

other wingtip. For example, the Boeing 777 has a wingspan of about 60 metres (197 ft); and a

Wandering Albatross (Diomedea exulans) caught in 1965 had a wingspan of 3.63 metres

(11 ft 11 in), the official record for a living bird.

The term wingspan, more technically extent, is also used for other winged animals such as

pterosaurs, bats, insects, etc, and other winged aircraft such as ornithopters.

Degrees of freedom

1. Moving up and down (heaving); 2. Moving left and right (swaying); 3. Moving forward and backward (surging); 4. Tilting forward and backward (pitching); 5. Turning left and right (yawing); 6. Tilting side to side (rolling).

Area rule

The Whitcomb area rule, also called the transonic area rule, is a design technique

used to reduce an aircraft's drag at transonic and supersonic speeds, particularly

between Mach 0.75 and 1.2.

This is one of the most important operating speed ranges for commercial and military fixed-wing aircraft today, with transonic acceleration being considered an important performance metric for combat aircraft, necessarily dependent upon transonic drag

The area rule also holds true at speeds higher than the speed of sound, but in this

case the body arrangement is in respect to the Mach line for the design speed. For

instance, at Mach 1.3 the angle of the Mach cone formed off the body of the aircraft

will be at about μ = arcsin (1/M) = 50.3 deg (μ is the angle of the Mach cone, or

simply Mach angle). In this case the "perfect shape" is biased rearward, which is why

aircraft designed for high speed cruise tend to be arranged with the wings at the

rear.[1] A classic example of such a design is Concorde. When applying the

supersonic area rule, the condition that the plane defining the cross-section meet the

longitudinal axis at the Mach angle μ no longer prescribes a unique plane for μ other

than the 90 degrees given by M=1. The correct procedure is to average over all

possible orientations of the intersecting plane.