Sound barrier
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U.S. Navy F/A-18 at transonic speed. The cloud is due to the Prandtl-Glauert singularity.
This is an article about the aviation term. For the 1952 film see The Sound Barrier (film). For the heavy metal band, see Sound Barrier.
In aerodynamics, the sound barrier is the transition at transonic speeds from subsonic to supersonic travel, usually referring to flight. The term came into use during World War II when a number of aircraft started to encounter the effects of compressibility, a grab-bag of unrelated aerodynamic effects, and fell out of use in the 1950s when aircraft started to routinely "break" the sound barrier.
Contents
[hide]
· 1 History
o 1.1 Early problems
o 1.2 Attempts to break the sound barrier
o 1.3 The Sound Barrier fades
· 2 Media
· 3 References
· 4 External links
[edit] History
The bullwhip, or stockwhip, was probably the first human-made object to move faster than sound. The tip of the whip breaks the sound barrier and causes a sharp crack—literally a sonic boom. Many forms of ammunition also achieve supersonic speeds.
[edit] Early problems
The tip of the propeller on many early aircraft may reach supersonic speeds, producing a noticeable buzz that differentiates such aircraft. This is particularly noticeable on the Stearman, and noticeable on the T-6 Texan when it enters a sharp-breaking turn. Note that this is in fact undesirable as the transonic air movement creates disruptive shock waves and turbulence. It is due to these effects that propellers are known to suffer from dramatically decreased performance as they approach the speed of sound. It is easy to demonstrate that the power needs to improve performance are so great that the weight of the required engine grows faster than the power output of the propeller. This problem was one of the issues that led to early research in jet engines, notably by Frank Whittle and Hans von Ohain, who were led to their research specifically in order to avoid these problems in high-speed flight.
Propeller aircraft were nevertheless able to approach the speed of sound in a dive. However this led to numerous crashes for a variety of reasons. These included the rapidly increasing forces on the various control surfaces, which led to the aircraft becoming difficult to control to the point where many suffered from powered flight into terrain when the pilot was unable to overcome the force on the control stick. The Mitsubishi Zero was particularly "well known" for this problem, and several attempts to fix it only made the problem worse. In the case of the Supermarine Spitfire, the wings suffered from low torsional stiffness, and when ailerons were moved the wing tended to flex in a such a way to counteract the control input, leading to a condition known as "roll reversal". This was solved in later models with changes to the wing. The P-38 Lightning suffered from a particularly dangerous interaction of the airflow between the wings and tail surfaces in the dive that made it difficult to "pull out", a problem that was later solved with the addition of a "dive flap" that upset the airflow under these circumstances. Flutter due to the formation of shock waves on curved surfaces was another major problem, which led most famously to the breakup of de Havilland Swallow and death of its pilot, Geoffrey de Havilland Jr.
All of these effects, although unrelated in most ways, led to the concept of a "sound barrier" that would make it difficult, perhaps impossible, for an aircraft to break the speed of sound.
There are, however, several claims that the sound barrier was broken during World War II. Hans Guido Mutke claimed to have broken the sound barrier on April 9, 1945 in a Messerschmitt Me 262. However, this claim is widely disputed by most experts as the Me 262's structure could not support high transonic, let alone supersonic flight and thus this claim lacks a plausible scientific foundation.[1] Similar claims for the Spitfire and other propeller aircraft are even more suspect. It is now known that traditional airspeed gauges using a pitot tube give inaccurately high readings in the transonic, apparently due to shock waves interacting with the tube or the static source. This led to problems then known as "Mach jump".[2]
[edit] Attempts to break the sound barrier
In 1942 the United Kingdom's Ministry of Aviation began a top secret project with Miles Aircraft to develop the world's first aircraft capable of breaking the sound barrier. The project resulted in the development of the prototype Miles M.52 jet aircraft, which was designed to reach 1,000 mph (417 m/s; 1,600 km/h) at 36,000 feet (11 km) in 1 minute 30 seconds.
The aircraft's design introduced many innovations which are still used on today's supersonic aircraft. The single most important development was the all-moving tailplane, giving extra control to counteract the Mach tuck which allowed control to be maintained at supersonic speeds. In the immediate post-war era new data from captured German records suggested that major savings in drag could be had through a variety of means such as swept wings, and Director of Scientific Research, Sir Ben Lockspeiser, decided to cancel the project in light of this new information. Later experimentation on the Miles M.52 design proved that the aircraft would indeed have broken the sound barrier, with an unpiloted 3/10 scale replica of the M.52 achieving Mach 1.5 in October 1948.
US efforts started soon after with the Bell XS-1. Also featuring the all-moving tail, the XS-1, later known as the X-1. In was in the X-1 that Chuck Yeager was the first person to break the sound barrier in level flight on October 14, 1947, flying at an altitude of 45,000 ft (13.7 km).
George Welch made a plausible but officially unverified claim to have broken the sound barrier on October 1, 1947 while flying an XP-86 Sabre. He also claimed to have repeated his supersonic flight on October 14, 1947, 30 minutes before Chuck Yeager broke the sound barrier in the Bell X-1 using the adjustable tail concept. Although evidence from witnesses and instruments strongly imply that Welch achieved supersonic speed, the flights were not properly monitored and cannot be officially recognized. (The XP-86 officially achieved supersonic speed on April 26, 1948.)
The sound barrier was first broken in a vehicle in a sustained way on land in 1948 by a rocket-powered test vehicle at Muroc Air Force Base (now Edwards AFB) in California. It was powered by 6000 pounds of thrust, reaching 1,019 mph.[3]
Jackie Cochran was the first woman to break the sound barrier on May 18, 1953 in a Canadair Sabre, with Yeager as her wingman.
[edit] The Sound Barrier fades
As the science of high-speed flight became more widely understood, a number of changes led to the eventual disappearance of the "sound barrier". Among these were the introduction of swept wings, the area rule, and engines of ever increasing performance. By the 1950s many combat aircraft could routinely break the sound barrier in level flight, although they often suffered from control problems when doing so (Mach tuck). Modern aircraft can transition through the "barrier" without it even being noticeable.
By the late 1950s the issue was so well understood that many companies started investing in the development of supersonic airliners, or SST's, believing that to be the next "natural" step in airliner evolution. History has proven this not to be the case, but both the Concorde and Tupolev Tu-144 both entered service in the 1970s regardless.
Although the Concorde and Tu-144 were the first aircraft to carry commercial passengers at supersonic speeds, they were not the first or only commercial airliners to break the sound barrier. On August 21, 1961 a Douglas DC-8 broke the sound barrier at Mach 1.012 or 660 mph while in a controlled dive through 41,088 feet. The purpose of the flight was to collect data on a new leading-edge design for the wing.[4] Boeing reports that the 747 broke the sound barrier during certification tests. A China Airlines 747 almost certainly broke the sound barrier in an unplanned descent from 41,000 ft to 9500 feet after an in-flight upset on February 19, 1985. It also reached over 5g. [5]
On October 15, 1997, in a vehicle designed and built by a team led by Richard Noble, driver Andy Green became the first person to break the sound barrier in a land vehicle. The vehicle called the ThrustSSC ("Super Sonic Car"), captured the record almost exactly 50 years after Yeager's flight.
[edit] Media
Mach number
From Wikipedia, the free encyclopedia
Jump to: navigation, search
An F/A-18 Hornet at transonic speed and displaying the Prandtl-Glauert singularity just before breaking the sound barrier.
Mach number (Ma) (pronounced: [mæk], [mɑːk]) is a dimensionless measure of relative speed. It is defined as the speed of an object relative to a fluid medium, divided by the speed of sound in that medium:
where
is the Mach number
is the speed of the object relative to the medium and
is the speed of sound in the medium
Mach number is the number of times the speed of sound an object or a duct, or the fluid medium itself, move relative to each other. It is named after Austrian physicist and philosopher Ernst Mach.
Contents
[hide]
· 1 Overview
· 2 High-speed flow around objects
· 3 High-speed flow in a channel
· 4 Calculating Mach Number
· 5 See also
· 6 References
· 7 External links
[edit] Overview
The Mach number is commonly used both with objects travelling at high speed in a fluid, and with high-speed fluid flows inside channels such as nozzles, diffusers or wind tunnels. As it is defined as a ratio of two speeds, it is a dimensionless number. At a temperature of 15 degrees Celsius and at sea level, Mach 1 is 340.3 m/s (1,225 km/h, 761.2 mph, or 661.7 kts) in the Earth's atmosphere. The Mach number is not a constant; it is temperature dependent. Hence in the stratosphere it remains about the same regardless of height, though the air pressure changes with height.
Since the speed of sound increases as the temperature increases, the actual speed of an object travelling at Mach 1 will depend on the fluid temperature around it. Mach number is useful because the fluid behaves in a similar way at the same Mach number. So, an aircraft travelling at Mach 1 at sea level (340.3 m/s, 1,225.08 km/h) will experience shock waves in much the same manner as when it is travelling at Mach 1 at 11,000 m (36,000 ft), even though it is travelling at 295 m/s (654.632 MPH, 1,062 km/h, 86% of its speed at sea level).
It can be shown that the Mach number is also the ratio of inertial forces (also referred to aerodynamic forces) to elastic forces.
[edit] High-speed flow around objects
High speed flight can be classified in five categories:
· sonic: Ma=1
· Subsonic: Ma < 1
· Transonic: 0.8 < Ma < 1.2
· Supersonic: 1.2 < Ma < 5
· Hypersonic: Ma > 5
(For comparison: the required speed for low Earth orbit is ca. 7.5 km·s-1 = Ma 25.4 in air at high altitudes)
At transonic speeds, the flow field around the object includes both sub- and supersonic parts. The transonic regime begins when first zones of Ma>1 flow appear around the object. In case of an airfoil (such as an aircraft's wing), this typically happens above the wing. Supersonic flow can decelerate back to subsonic only in a normal shock; this typically happens before the trailing edge. (Fig.1a)
As the velocity increases, the zone of Ma>1 flow increases towards both leading and trailing edges. As Ma=1 is reached and passed, the normal shock reaches the trailing edge and becomes a weak oblique shock: the flow decelerates over the shock, but remains supersonic. A normal shock is created ahead of the object, and the only subsonic zone in the flow field is a small area around the object's leading edge. (Fig.1b)
(a) (b)
Fig. 1. Mach number in transonic airflow around an airfoil; Ma<1 (a) and Ma>1 (b).
When an aircraft exceeds Mach 1 (i.e. the sound barrier) a large pressure difference is created just in front of the aircraft. This abrupt pressure difference, called a shock wave, spreads backward and outward from the aircraft in a cone shape (a so-called Mach cone). It is this shock wave that causes the sonic boom heard as a fast moving aircraft travels overhead. A person inside the aircraft will not hear this. The higher the speed, the more narrow the cone; at just over Ma=1 it is hardly a cone at all, but closer to a slightly concave plane.
At fully supersonic velocity the shock wave starts to take its cone shape, and flow is either completely supersonic, or (in case of a blunt object), only a very small subsonic flow area remains between the object's nose and the shock wave it creates ahead of itself. (In the case of a sharp object, there is no air between the nose and the shock wave: the shock wave starts from the nose.)
As the Mach number increases, so does the strength of the shock wave and the Mach cone becomes increasingly narrow. As the fluid flow crosses the shock wave, its speed is reduced and temperature, pressure, and density increase. The stronger the shock, the greater the changes. At high enough Mach numbers the temperature increases so much over the shock that ionization and dissociation of gas molecules behind the shock wave begin. Such flows are called hypersonic.
It is clear that any object travelling at hypersonic velocities will likewise be exposed to the same extreme temperatures as the gas behind the nose shock wave, and hence choice of heat-resistant materials becomes important.
[edit] High-speed flow in a channel
As a flow in a channel crosses M=1 becomes supersonic, one significant change takes place. Common sense would lead one to expect that contracting the flow channel would increase the flow speed (i.e. making the channel narrower results in faster air flow) and at subsonic speeds this holds true. However, once the flow becomes supersonic, the relationship of flow area and speed is reversed: expanding the channel actually increases the speed.
The obvious result is that in order to accelerate a flow to supersonic, one needs a convergent-divergent nozzle, where the converging section accelerates the flow to M=1, sonic speeds, and the diverging section continues the acceleration. Such nozzles are called de Laval nozzles and in extreme cases they are able to reach incredible, hypersonic velocities (Mach 13 at sea level).
An aircraft Machmeter or electronic flight information system (EFIS) can display Mach number derived from stagnation pressure (pitot tube) and static pressure.
[edit] Calculating Mach Number
Assuming air to be an ideal gas, the formula to compute Mach number in a subsonic compressible flow is derived from the Bernoulli equation for M<1:[1]
where:
is Mach number
is impact pressure and
is static pressure.
The formula to compute Mach number in a supersonic compressible flow is derived from the Rayleigh Supersonic Pitot equation:
where:
is now impact pressure measured behind a normal shock
As can be seen, M appears on both sides of the equation. The easiest method to solve the supersonic M calculation is to enter both the subsonic and supersonic equations into a computer spreadsheet. First determine if M is indeed greater than 1.0 by calculating M from the subsonic equation. If M is greater than 1.0 at that point, then use the value of M from the subsonic equation as the initial condition in the supersonic equation. Then perform a simple iteration of the supersonic equation, each time using the last computed value of M, until M converges to a value--usually in just a few iterations.[1]