APPROACHING THE SPEED OF SOUND

Aircraft designed to approach the speed of sound look different from low-speed aircraft. To find out why, let's first consider how sound itself travels through air. Slowing down the picture shows how the sound is produced. The vibrating prongs give the air a succession of pushes, and like pressure waves along a spring, these sound waves travel on through the air. The air is alternately compressed and rarefied. And as each wave passes, there is a brief disturbance. Sound waves... are pressure waves. Like ripples on a pond, they spread out from their source in all directions at the same speed. All small disturbances in the air, whether audible or not, travel outwards at this same speed, the speed of sound. What is this speed? Here is a convenient source of sound at sea level. Let's measure the time it takes for the sound of the explosion to travel one mile, the distance between these two forts. The flash is coming now. One, two, three, four. The sound took just under five seconds to travel the mile. That is, at sea level, the speed of sound is about 760 miles per hour. The exact speed depends effectively on only one factor - the temperature of the air. The higher the temperature, the faster the sound travels. On a really hot day, it may reach over 800mph at sea level. But temperature falls with altitude until the stratosphere is reached at about 36,000 feet. Above this height, the temperature remains constant at approximately 60°C below zero. Though the speed of sound is lower, only 660mph. What has the speed of sound to do with high-speed flying? To find out, let's consider first a single point, sending out small pressure waves continuously. Each wave travels outwards at the speed of sound. Now, suppose the point itself is moving. If its speed is less than the speed of sound, the pressure waves still travel out ahead. But if the point is travelling at the speed of sound, the pressure waves cannot travel out ahead of it, for the point is travelling as fast as they are. If the point travels faster than sound, that is at supersonic speed, this happens. But we're not going to deal with this case here. This is the kind of way in which the speed of sound affects high-speed aircraft and therefore, at high speeds, the exact relation between the speed of an aircraft and the speed of sound is important. But remember, the speed of sound varies with temperature, and therefore, with altitude. It's considerably lower in the stratosphere than at sea level. The ratio of an aircraft's true airspeed to the speed of sound where it is flying is called the aircraft's Mach number, after the 19th-century Austrian physicist Ernst Mach. It is usually shortened to "M". At high speeds, it is essential for the pilot to know the Mach number and Mach-metres are fitted to all high-speed aircraft. This is how an aircraft's Mach number is calculated. The aircraft has flown six-tenths of a mile in the time that a sound wave has travelled ten-tenths of a mile. It has flown at six-tenths of the speed of sound in the same atmospheric conditions. So, it's Mach number is 0.6. This aircraft is flying at Mach 0.9. Aircraft flying at the same true airspeed, but at different heights, will have different Mach numbers, for the speed of sound is different in the two cases. The Mach numbers at which an aircraft is intended to operate have a great influence on its design. An aircraft is much more complicated than the point source of pressure waves we saw earlier. So, the behaviour of the air is more complicated, too. To find out about it, let us consider the airflow around a wing. This wing section is symmetrical, like most modern wings. It is in a typical flying attitude. The air is slowed down at the nose to form what is called the stagnation region. It speeds up as it passes round the curvature of the wing. It slows down again towards the trailing edge. These changes of speed cause changes in the air pressure. The yellow regions show reduced, and the green, increased pressure. All these variations in pressure together produce lift and drag. Now, each point on the wing acts like a point source. Here, we're only showing a few such points. Each sends out pressure waves, which travel at the speed of sound and reach the air ahead of the wing. We can use smoke to show how the air flows. The influence of the pressure waves travelling ahead can be seen from the way the streamlines are deflected well ahead of the wing. Lowering a flap changes the entire flow pattern around the wing and affects the airflow ahead. We can see this better with a single streamline. Mark its position well ahead. Even at a distance, the streamline changes direction. The effect of the pressure waves on the air ahead is of great importance. It smooths the flow past an aircraft flying well below the speed of sound. But what happens when approaching the speed of sound? The airflow speeds up as it passes over the wing and reaches its maximum speed at a certain point. Mach number here will always be greater than that of the aircraft as a whole, called the flight Mach number. As the flight Mach number increases, so does the local Mach number at the maximum speed-up point. Eventually, though the aircraft as a whole is flying at less than the speed of sound, just at this point on the wing, the air is moving at the speed of sound. The flight Mach number when this happens is called the critical Mach number of the aircraft, usually written "Mcrit". For any wing section, Mcrit will always be less than one. Aerodynamically, the critical Mach number is very important, for the aircraft has reached a speed at which it meets mixed airflow, part subsonic - that is, less than the speed of sound - part supersonic, greater than the speed of sound. It is the beginning of the transonic speed range. From the behaviour of the aircraft, the pilot has no way of telling that he has reached the critical Mach number, but soon after it has been exceeded, things begin to happen. The Mach-metre on the left has been altered since the actual critical Mach number of this type of aircraft has not yet been released. In this picture, it is 0.9. And soon after Mcrit is exceeded, the aircraft starts to buffet violently. Aircraft vary greatly in their behaviour above the critical Mach number. Some show violent instability, while others, specially designed for transonic flight, may be little disturbed. To find out why problems arise above the critical Mach number, let's use a high-speed wind tunnel. This one's fitted with special optical equipment to show, in colours, regions where the density of the air is changing. These effects can be photographed. Solid objects, like this nozzle, appear silhouetted against a coloured background. When the compressed air jet is turned on, other colours appear. We have chosen red to show regions where density is increasing, and blue, regions where it is decreasing. A symmetrical wing section designed for high speeds is put in the tunnel. At low speed, the air behaves as if it were incompressible. Whatever pressure changes there are are so slight as to cause no colour change. But as speed increases, the air does begin to show signs of compressibility. The colours show regions of increasing and decreasing density. The red area at the leading edge is the stagnation region, where the air is being slowed down and becoming denser. Immediately behind are two blue areas where the rate of speeding-up is greatest, causing a reduction in density. This diagram will remind us of what's happening. As the flight Mach number increases, the flow at the point of maximum speed-up on the wing reaches Mach one, the speed of sound. The wing has reached its critical Mach number. And when this is exceeded... a sudden sharp region of increasing density forms on the wing just behind the point of maximum speed-up. This is a shockwave. It is a sudden jump in the pressure of the air. It grows and moves back as the Mach number increases. Shockwaves can even be seen with the naked eye in certain atmospheric conditions. Watch them streaming back from the nose of this missile. There. This time, we're going to hold the picture still for a few seconds. Let us see a shockwave form again in the wind tunnel. A diagram shows what happens. At the critical Mach number, there is a point on the wing where M equals one. At higher speeds, the point grows into an area in which the flow is supersonic, that is, where M is greater than one. Outside this area, the flow is still subsonic. M is less than one. The rear boundary of the area is the shockwave itself. As the aircraft accelerates, so the area of supersonic flow increases and the shockwave moves back, growing larger and stronger. A shockwave at right angles to the airstream is the means by which the airflow suddenly decelerates from supersonic to subsonic speed. How are shockwaves formed? On the rear part of the wing, every point, such as this one, sends out innumerable tiny pressure waves at the speed of sound. In the forward direction, these waves meet airflow in the opposite direction and make less and less progress until they reach a stage where they can't travel any further forward because the airflow itself is moving backwards supersonically. It's like trying to step off an escalator going the wrong way. The pressure waves constantly pile up here, and this is the shockwave. A shockwave is a very narrow region, about 1/10,000 of an inch thick. Across this region, the supersonic airflow is violently reduced to subsonic speed. Much of the air's energy of movement, or kinetic energy, is dissipated as heat. The temperature of the air rises suddenly as it passes through the shockwave. There is also a sudden rise in pressure. The energy wasted as heat in the shockwave must be continuously supplied by the engines, otherwise, the aircraft would decelerate. So, as the aircraft approaches the speed of sound, it meets an additional kind of drag called wave drag. Wave drag is a large proportion of total drag at transonic speeds. As the speed of airflow is increased still further, the region of supersonic flow goes on growing larger, and a second supersonic region starts to form on the lower surface of the wing with another shockwave. At speeds approaching the speed of sound, the most important result of a shockwave is to cause the airflow to separate from the wing's surface. This is called shock-induced separation. It produces a large turbulent wake, which alters the pressure distribution, lift is produced, and the turbulence creates drag. In this aircraft, the turbulence strikes the tailplane, causes violent buffeting, and so, limits its speed. Other aircraft experienced such serious troubles as sudden loss of stability and reduced effectiveness of the controls. We can't see separation in flight, but this aircraft has wool tufts stuck on the wings. They lie flush when the airflow is smooth and flap when shock-induced separation occurs. But shockwaves don't only appear on wings. Speed-up of the airflow occurs around the canopy and many other parts of an aircraft. Wherever it's great enough, there, shockwaves will form. Shockwaves cause a vast increase in drag and may cause serious control troubles. One way of avoiding these troubles is to put them off to still higher speeds by raising the aircraft's critical Mach number. There are two chief design methods for doing this. The first is to use relatively thin wings. That is, wing sections with maximum thickness small compared with width or chord. Thin wings, in this sense, can of course be very deep. The thinner the wing, the less the air is speeded up, and so the critical Mach number is raised. These two wings are of the same general shape and of the same angle of incidence. The tunnel speed is the same for each. As it increases... a shockwave forms on the thicker wing at a Mach number of 0.8. But not a sign yet on the thinner one. Here it comes, at last, at a Mach number of 0.9. But wings can't be made too thin or the landing speed would be too high. The other important way of raising the critical Mach number is by the use of sweepback. To understand this, let's look from above at the flow over a wing. The contour of the wing section determines the amount the air is speeded up. Now, sweep the wing back. We can represent the velocity of the airflow ahead of the wing by an arrow of a certain length. We can consider this velocity is made up of two smaller components - a yellow component at right angles to the leading edge and a red component parallel to the leading edge. The red component can be considered to flow along the span and is not speeded up by the wing section. But the yellow component flows across the section and is speeded up by it. It is, therefore, only this component which affects the wing's critical Mach number. So, the maximum speed reached over the swept wing will be less than it would be over a similar straight wing. In particular, if the flow over the straight wing has reached the speed of sound, at the same flight speed, the flow over the swept wing will still be subsonic. The swept wing has, therefore, a higher Mcrit. The greater the sweepback, the more the critical Mach number is raised. For instance, suppose the straight wing has a critical Mach number of 0.8. By sweeping the wing back to 35 degrees, the Mcrit is raised to 0.98. But this is only the theoretical result for an infinitely long wing. In practice, the critical Mach number would only be raised to about 0.9. Sweepback, like thin wings, brings its own problems at low speeds, such as tip stalling. So, designers must compromise between high-speed and low-speed requirements. And aircraft designed to fly near the speed of sound use thin wings, together with sweepback, to obtain a high critical Mach number. Various arrangements have been adopted, all very different in appearance. If the swept wing is too thin to contain the engines, they can be mounted externally in pods. Alternatively, the engines can be buried in the wing routes, which are more highly swept than the remainder of the wing. A special kind of sweepback is the crescent wing. Here, the sweepback is reduced in stages to avoid tip stalling. The delta wing combines a high degree of sweepback and great strength. The delta's large wing area makes it very manoeuvrable and gives it a good performance at high altitudes. The simple delta shape can have many variations. Some deltas have a tailplane to improve manoeuvrability at the expense of slight extra drag. Today, aircraft are being designed to fly at speeds far above their critical Mach numbers. Problems of extra drag and loss of control caused by shockwaves are being overcome. But for many years ahead, airliners will cruise below their critical Mach numbers. In this way, they will avoid shockwaves altogether and attain long range and economy of operation. At the same time, designers will aim to make critical Mach numbers as high as possible, thus permitting passenger flight at speeds approaching the speed of sound.