Subsonic aircraft

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Czech subsonic multi-role combat aircraft Aero L-159 Alca. Aero L-159 ALCA (Czech Air Force) (6437228061).jpg
Czech subsonic multi-role combat aircraft Aero L-159 Alca.

A subsonic aircraft is an aircraft with a maximum speed less than the speed of sound (Mach 1). The term technically describes an aircraft that flies below its critical Mach number, typically around Mach 0.8. All current civil aircraft, including airliners, helicopters and airships, as well as many military types, are subsonic.

Aircraft machine that is able to fly by gaining support from the air other than the reactions of the air against the earth’s surface

An aircraft is a machine that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines. Common examples of aircraft include airplanes, helicopters, airships, gliders, and hot air balloons.

The speed of sound is the distance travelled per unit time by a sound wave as it propagates through an elastic medium. At 20 °C (68 °F), the speed of sound in air is about 343 meters per second, or a kilometre in 2.9 s or a mile in 4.7 s. It depends strongly on temperature, but also varies by several meters per second, depending on which gases exist in the medium through which a soundwave is propagating.

Mach number Ratio of speed of object moving through fluid and local speed of sound

In fluid dynamics, the Mach number is a dimensionless quantity representing the ratio of flow velocity past a boundary to the local speed of sound.



Although high speeds are usually desirable in an aircraft, supersonic flight requires much bigger engines, higher fuel consumption and more advanced materials than subsonic flight. A subsonic type therefore costs far less than the equivalent supersonic design, has greater range and causes less harm to the environment.

The less harsh subsonic environment also allows a much wider range of aircraft types, such as balloons, airships and rotorcraft, allowing them to fill a much wider range of roles.

Balloon (aeronautics) type of aerostat that remains aloft due to its buoyancy

In aeronautics, a balloon is an unpowered aerostat, which remains aloft or floats due to its buoyancy. A balloon may be free, moving with the wind, or tethered to a fixed point. It is distinct from an airship, which is a powered aerostat that can propel itself through the air in a controlled manner.

Airship type of aerostat or lighter-than-air aircraft

An airship or dirigible balloon is a type of aerostat or lighter-than-air aircraft that can navigate through the air under its own power. Aerostats gain their lift from large gasbags filled with a lifting gas that is less dense than the surrounding air.

Rotorcraft Heavier-than-air aircraft which generates lift over rotating wings

A rotorcraft or rotary-wing aircraft is a heavier-than-air flying machine that uses lift generated by wings, called rotary wings or rotor blades, that revolve around a mast. Several rotor blades mounted on a single mast are referred to as a rotor. The International Civil Aviation Organization (ICAO) defines a rotorcraft as "supported in flight by the reactions of the air on one or more rotors". Rotorcraft generally include those aircraft where one or more rotors are required to provide lift throughout the entire flight, such as helicopters, autogyros, and gyrodynes. Compound rotorcraft may also include additional thrust engines or propellers and static lifting surfaces.

Subsonic aerodynamics

Subsonic flight is characterised aerodynamically by incompressible flow, where dynamic pressure changes due to motion through the air cause the air to flow away from areas of high dynamic pressure to areas of lower dynamic pressure, leaving the static pressure and density of the surrounding air constant. At high subsonic speeds, compressibility effects begin to appear. [1]

Dynamic pressure is the increase in a moving fluid's pressure over its static value due to motion. In incompressible fluid dynamics, it is indicated as , or Q, defined by:

In fluid mechanics the term static pressure has several uses:

The density, or more precisely, the volumetric mass density, of a substance is its mass per unit volume. The symbol most often used for density is ρ, although the Latin letter D can also be used. Mathematically, density is defined as mass divided by volume:


The propeller is one of the most efficient sources of thrust available and is common on subsonic aeroplanes and airships. Sometimes it is enclosed in the form of a ducted fan. At higher subsonic speeds and at high altitudes, such as attained by most airliners, the high-bypass turbofan becomes necessary. Pure jets such as the turbojet and ramjet are inefficient at subsonic speeds and not often used.

Propeller fan that transmits rotational motion into thrust

A propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid is accelerated behind the blade. Propeller dynamics, like those of aircraft wings, can be modelled by Bernoulli's principle and Newton's third law. Most marine propellers are screw propellers with fixed helical blades rotating around a horizontal axis or propeller shaft.

Ducted fan

A ducted fan is air moving arrangement whereby a mechanical fan, which is a type of propeller, is mounted within a cylindrical shroud or duct. The duct reduces losses in thrust from the tips of the propeller blades, and varying the cross-section of the duct allows the designer to advantageously affect the velocity and pressure of the airflow according to Bernoulli's principle. Ducted fan propulsion is used in aircraft, airships, airboats, hovercraft and fan packs.

Altitude or height is defined based on the context in which it is used. As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context. Although the term altitude is commonly used to mean the height above sea level of a location, in geography the term elevation is often preferred for this usage.

Wing design

A Spitfire built in 1945 shows off its straight elliptical wing Spitfire mark19 ps853 planform arp.jpg
A Spitfire built in 1945 shows off its straight elliptical wing

The span and area of a wing are both important to the lift characteristics. They are related by the aspect ratio, which is the ratio of the span, measured from tip to tip, to the average chord, measured from leading edge to trailing edge.

Wingspan distance from one wingtip to the other wingtip of an airplane or an animal (insect, bird, bat)

The wingspan of a bird or an airplane is the distance from one wingtip to the other wingtip. For example, the Boeing 777-200 has a wingspan of 60.93 metres, and a wandering albatross caught in 1965 had a wingspan of 3.63 metres, 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 fixed-wing aircraft such as ornithopters. In humans, the term wingspan also refers to the arm span, which is distance between the length from one end of an individual's arms to the other when raised parallel to the ground at shoulder height at a 90º angle. Former professional basketball player Manute Bol stands at 7 ft 7 in (2.31 m) and owns one of the largest wingspans at 8 ft 6 in (2.59 m).

Lift (force) force; aerodynamics term

A fluid flowing past the surface of a body exerts a force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the force parallel to the flow direction. Lift conventionally acts in an upward direction in order to counter the force of gravity, but it can act in any direction at right angles to the flow.

Chord (aeronautics) imaginary straight line joining the leading and trailing edges of an aerofoil

In aeronautics, a chord is the imaginary straight line joining the leading and trailing edges of an aerofoil. The chord length is the distance between the trailing edge and the point on the leading edge where the chord intersects the leading edge.

The aerodynamic efficiency of a wing is described by its lift/drag ratio, with a wing giving high lift for little drag being the most efficient. A higher aspect ratio gives a higher lift/drag ratio and so is more efficient. [2]

In fluid dynamics, drag is a force acting opposite to the relative motion of any object moving with respect to a surrounding fluid. This can exist between two fluid layers or a fluid and a solid surface. Unlike other resistive forces, such as dry friction, which are nearly independent of velocity, drag forces depend on velocity. Drag force is proportional to the velocity for a laminar flow and the squared velocity for a turbulent flow. Even though the ultimate cause of a drag is viscous friction, the turbulent drag is independent of viscosity.

The drag of a wing consists of two components, the induced drag, which is related to the production of lift, and the profile drag, largely due to skin friction which is contributed to by the whole wing area. [3] It is therefore desirable for a wing to have the least area compatible with the desired lift characteristics. This is best achieved with a high aspect ratio, and high-performance types often have this kind of wing.

But other considerations such as light weight, structural stiffness, manoeuvrability, ground handling and so on often benefit from a shorter span and, consequently a less efficient wing. Small, low-altitude general aviation planes typically have aspect ratios of six or seven; airliners of 12 or more; and high-performance sailplanes of 30 or more.

At speeds above the critical Mach number, the airflow begins to become transonic, with local airflow in some places causing small sonic shock waves to form. This soon leads to the shock stall, causing a rapid increase in drag. [4] The wings of fast subsonic craft such as jet airliners tend to be swept in order to delay the onset of these shock waves.

In theory, induced drag is at its minimum when the span wise distribution of lift is elliptical. A number of factors influence induced drag, however, and as a practical matter a wing of elliptical planform, like that of the Supermarine Spitfire fighter of World War II, is not necessarily the most efficient. The wings of jet airliners, which are highly optimized for efficiency, are far from elliptical in shape.

The ratio of tip chord to root chord is called the taper ratio. Taper has the desirable effect of reducing the root bending stress by shifting the lift inboard, but it has been argued by some noted designers, including John Thorp and Karl Bergey, that an untapered rectangular planform is best for aeroplanes of less than 6,000 pounds gross weight.

See also

Related Research Articles

Supersonic speed rate of travel that exceeds the speed of sound

Supersonic travel is a rate of travel of an object that exceeds the speed of sound (Mach 1). For objects traveling in dry air of a temperature of 20 °C (68 °F) at sea level, this speed is approximately 344 m/s, 1,125 ft/s, 768 mph, 667 knots, or 1,235 km/h. Speeds greater than five times the speed of sound (Mach 5) are often referred to as hypersonic. Flights during which only some parts of the air surrounding an object, such as the ends of rotor blades, reach supersonic speeds are called transonic. This occurs typically somewhere between Mach 0.8 and Mach 1.2.

Delta wing wing shaped in the form of a triangle

The delta wing is a wing shaped in the form of a triangle. It is named for its similarity in shape to the Greek uppercase letter delta (Δ).

Flying wing tailless fixed-wing aircraft with no definite fuselage

A flying wing is a tailless fixed-wing aircraft that has no definite fuselage. The crew, payload, fuel, and equipment are typically housed inside the main wing structure, although a flying wing may have various small protuberances such as pods, nacelles, blisters, booms, or vertical stabilizers.

Supersonic transport commercial airliner able to fly faster than the speed of sound

A supersonic transport (SST) is a civilian supersonic aircraft designed to transport passengers at speeds greater than the speed of sound. To date, the only SSTs to see regular service have been Concorde and the Tupolev Tu-144. The last passenger flight of the Tu-144 was in June 1978 and it was last flown in 1999 by NASA. Concorde's last commercial flight was in October 2003, with a November 26, 2003 ferry flight being its last airborne operation. Following the permanent cessation of flying by Concorde, there are no remaining SSTs in commercial service. Several companies have each proposed a supersonic business jet, which may bring supersonic transport back again.

Swept wing

A swept wing is a wing that angles either backward or occasionally forward from its root rather than in a straight sideways direction. Wing sweep has the effect of delaying the shock waves and accompanying aerodynamic drag rise caused by fluid compressibility near the speed of sound, improving performance. Swept wings are therefore often used on jet aircraft designed to fly at these speeds. Swept wings are also sometimes used for other reasons, such as structural convenience or visibility.

In aeronautics, wave drag is a component of the aerodynamic drag on aircraft wings and fuselage, propeller blade tips and projectiles moving at transonic and supersonic speeds, due to the presence of shock waves. Wave drag is independent of viscous effects, and tends to present itself as a sudden and dramatic increase in drag as the vehicle increases speed to the Critical Mach number. It is the sudden and dramatic rise of wave drag that leads to the concept of a sound barrier.

Aspect ratio (aeronautics) ratio of an aircrafts wing span to its mean chord

In aeronautics, the aspect ratio of a wing is the ratio of its span to its mean chord. It is equal to the square of the wingspan divided by the wing area. Thus, a long, narrow wing has a high aspect ratio, whereas a short, wide wing has a low aspect ratio.

In aerodynamics, lift-induced drag, induced drag, vortex drag, or sometimes drag due to lift, is an aerodynamic 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.

T-tail aircraft empennage configuration in which the horizontal plane is at or near the top of the vertical stabilizer

A T-tail is an empennage configuration in which the tailplane is mounted to the top of the fin. The arrangement looks like the capital letter T, hence the name. The T-tail differs from the standard configuration in which the tailplane is mounted to the fuselage at the base of the fin.

Critical Mach number The lowest Mach number at which the airflow over some point of the aircraft reaches the speed of sound

In aerodynamics, the critical Mach number of an aircraft is the lowest Mach number at which the airflow over some point of the aircraft reaches the speed of sound, but does not exceed it. At the lower critical Mach number, airflow around the entire aircraft is subsonic. At the upper critical Mach number, airflow around the entire aircraft is supersonic.

Supercritical airfoil

A supercritical airfoil is an airfoil designed primarily to delay the onset of wave drag in the transonic speed range. Supercritical airfoils are characterized by their flattened upper surface, highly cambered ("downward-curved") aft section, and larger leading-edge radius compared with NACA 6-series laminar airfoil shapes. Standard wing shapes are designed to create lower pressure over the top of the wing. The camber of the wing determines how much the air accelerates around the wing. As the speed of the aircraft approaches the speed of sound, the air accelerating around the wing reaches Mach 1 and shockwaves begin to form. The formation of these shockwaves causes wave drag. Supercritical airfoils are designed to minimize this effect by flattening the upper surface of the wing.


In aerodynamics, pitch-up is a severe form of stall in an aircraft. It is directly related to inherent properties of all swept wings, and seen primarily on those platforms. Unlike conventional low-speed stalls, pitch-up can occur at any speed, and are especially dangerous when they take place in the transonic; at these speeds the aerodynamic loads can become so high as to break up the aircraft, as occurred in 1964 when a F-105 Thunderchief of the USAF Thunderbirds broke up in mid-air. It can also occur at low speeds, in which case it has been called a Sabre dance, a particularly dangerous behaviour of swept wings that became apparent during the development of the USAF F-100 Super Sabre.

Wing configuration Describes the general shape and layout of an aircraft wing

The wing configuration of a fixed-wing aircraft is its arrangement of lifting and related surfaces.

Trapezoidal wing

A trapezoidal wing is a straight-edged and tapered wing planform. It may have any aspect ratio and may or may not be swept.

Supersonic airfoils

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.

Boeing Truss-Braced Wing

Boeing Truss-Braced Wing are airliner designs studied by Boeing with braced, high aspect ratio wings.

Crescent wing

The crescent wing is a fixed-wing aircraft configuration in which a swept wing has a greater sweep angle on the inboard section than the outboard, giving the wing a crescent shape.

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Thickness-to-chord ratio

In aeronautics, the thickness-to-chord ratio, sometimes simply chord ratio or thickness ratio, compares the maximum vertical thickness of a wing to its chord. It is a key measure of the performance of a wing planform when it is operating at transonic speeds.



  1. Clancy 2005 Page 232.
  2. Clancy 2005 Pages 84.
  3. Clancy 2005 Pages 65, 82.
  4. Clancy 2005 Page 279.