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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.


Transonic





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).

The Tuck Under Roll Turn

The tuck under roll turn is a fancy turn jet pilots sometimes perform during exhibitions. They love to use this maneuver after a low pass in front of the crowd.
  • Fly straight and level past your imaginary reviewing area. (1)
  • Pull the nose up slightly and apply hard left stick to start an aileron roll. (2)
  • When you have completed three-quarters of the roll (on your side with the cockpit facing to the right), stop the roll and pull back hard on the stick to perform a maximum rate turn to the right. (3)


Figure. The Tuck Under Roll Turn

Fly MAS Using MasterCard And Win Business Class Tickets To London, Melbourne And Shanghai - malaysiakini



SUBANG, From now until February 19, MasterCard cardholders who fly Malaysia Airlines (MAS) stand a chance to win business class tickets to London, Melbourne or Shanghai.

According to MAS Commercial Director Datuk Abdul Rashid Khan, MasterCard International Worldwide cardholders who purchase any products from the on-going Malaysia Airlines Travel Fair (MATF ’09) will automatically be in the running to win business class tickets to the three destinations.

Three winners will be chosen from among cardholders who charge their purchases to their MasterCard. Each winner will receive two MAS Business Class air tickets for travel commencing in Kuala Lumpur.

The prizes will be awarded based on the highest amount spent, with the first prize winner getting a pair of Return Business Class air tickets to London. Subsequently, the second prize comprises Return Business Class tickets to Melbourne while the third prize winner gets a trip for two to Shanghai.

"MasterCard International Worldwide has been a firm supporter of the Malaysia Airlines Travel Fair and they return as ‘Official Card’ for the sixth consecutive year," Abdul Rashid said in a statement here, Monday.

"This year's fair offers customers the choice of 55 destinations and the chance to travel just seven days after securing their booking. So check out the fares, use MasterCard to facilitate the payment and who knows, you could be a winner," he said.

Over one million seats have been up for grabs at discounts of up to 70 percent off the prevailing market fares since February 10.

In conjunction with the travel fair, MASholidays is offering a promotion whereby those who purchase MASholidays packages worth RM1,500 or more in Peninsular Malaysia will be eligible to win a Caltex StarCard worth up to RM400 and a cash rebate of up to RM1,000 towards the purchase of a 42" Samsung LCD TV.

"These are just some of the many ways that Malaysia Airlines aims to reward its customers. The MATF will be running for only a few more days. I hope members of the public will take advantage of this opportunity to enjoy the lowest fares and best deals that we have on offer," Abdul Rashid said.

Customers can visit the offices of over 600 participating travel agents in Malaysia and over 1,500 agents from Singapore, Philippines, Indonesia, Thailand and Vietnam. Malaysia Airlines will also offer these attractive deals through its website http://www.malaysiaairlines.com/ and ticketing offices in Malaysia and in selected ASEAN countries.