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Mach Number, Mach Tuck, Speed of Sound And Transonic.

Mach number

In the flow of a fluid, the ratio of the flow velocity, V, at a given point in the flow to the local speed of sound, a, at that same point. That is, the Mach number, M, is defined as V/a. In a flowfield where the properties vary in time and/or space, the local value of M will also vary in time and/or space. In aeronautics, Mach number is frequently used to denote the ratio of the airspeed of an aircraft to the speed of sound in the freestream far ahead of the aircraft; this is called the freestream Mach number. The Mach number is a convenient index used to define the following flow regimes: (1) subsonic, where M is less than 1 everywhere throughout the flow; (2) supersonic, where M is greater than 1 everywhere throughout the flow; (3) transonic, where the flow is composed of mixed regions of locally subsonic and supersonic flows, all with local Mach numbers near 1, typically between 0.8 and 1.2; and (4) hypersonic, where (by arbitrary definition) M is 5 or greater.

Perhaps the most important physical aspect of Mach number is in the completely different ways that disturbances propagate in subsonic flow compared to that in a supersonic flow. Shock waves are a ubiquitous aspect of supersonic flows. See also Compressible flow; Shock wave; Sonic boom; Supersonic flight.

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. Note that the aircraft is subsonic, and traveling significantly below Mach 1.0, when it experiences this effect.

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

Initially as airspeed is increased past the critical Mach number, the wing develops an increasing amount of lift, requiring a nose-down force or trim to maintain level flight. With increased speed, and the aft movement of the shock wave, the wing’s center of pressure also moves aft causing the start of a nose-down tendency or “tuck.” If allowed to progress unchecked, in an aircraft not designed for supersonic flight, Mach tuck may occur. Although Mach tuck develops gradually, if it is allowed to progress significantly, the center of pressure can move so far rearward that there is no longer enough elevator authority available to counteract it, and the airplane could enter a steep, sometimes unrecoverable dive. In addition as the shockwave goes towards the rear, it can impinge upon the elevator control surfaces and this can greatly exacerbate the nose down tendency. Partly for this reason, supersonic and subsonic aircraft often have an all-moving tailplane (a stabilator) which lacks separate elevator control surfaces.

Historically, recovery from a mach tuck has not always been possible. In some cases as the aircraft descends the air density increases and the extra drag will slow the aircraft and control will return.

For aircraft such as supersonic bombers or supersonic transports such as Concorde that spend long periods in supersonic, Mach tuck is often compensated for by moving fuel between tanks in the fuselage to change the position of the centre of mass. This minimises the amount of trim required and significantly reduces aerodynamic drag.

Speed Of Sound

The speed at which sound travels in a given medium under specified conditions. The speed of sound at sea level in the International Standard Atmosphere is 1, 108 feet per second (or 658 knots, or 1, 215 kilometers per hour).
Sound is a vibration that travels through an elastic medium as a wave. The speed of sound describes how much distance such a wave travels in a certain amount of time. In dry air at 20 °C (68 °F), the speed of sound is 343 m/s. This also equates to 1235 km/h, 767 mph, 1125 ft/s, or about one mile in five seconds. Although it is commonly used to refer specifically to air, the speed of sound can be measured in virtually any substance. Sound travels faster in liquids and non-porous solids than it does in air.


F/A-18 flying at transonic speed

Transonic is an aeronautics term referring to a range of velocities just below and above the speed of sound (about mach 0.8–1.2). It is defined as the range of speeds between the critical mach number, when some parts of the airflow over an aircraft become 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 spend a considerable amount of time in the transonic state. This is particularly important due to an effect known as wave drag, which is prevalent in these speed ranges. Attempts to combat 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.

Severe instability can occur at transonic speeds. Shock waves move through the air at the speed of sound. When an object such as an aircraft also moves at the speed of sound, these shock waves build up in front of it to form a single, very large shock wave. During transonic flight, the plane must pass through this large shock wave, as well as contending with the instability caused by air moving faster than sound over parts of the wing and slower in other parts. The difference in speed is due to Bernoulli's principle.

Transonic speeds can also occur at the tips of rotor blades of helicopters and aircraft. However, as this puts severe, unequal stresses on the rotor blade, it is avoided and may lead to dangerous accidents if it occurs. It is one of the limiting factors to the size of rotors, and also to the forward speeds of helicopters (as this speed is added to the forward-sweeping (leading) side of the rotor, thus possibly causing localized transonics).


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