The Transonic Range: Aviation’s Undiscovered Country?

With the Cessna Citation X as the lone example in current production specifically to fly at this speed, is the transonic range still the aviation world’s largely undiscovered country?

By: Ringo Bones 

Specifically designed to operate at around Mach 0.935, a velocity only 80 kilometers per hour or 50 miles per hour slower than the speed of sound, it seems that Cessna Aircraft Company’s Citation X is currently the only aircraft of its kind in current production – military or civilian - specifically designed to operate within the “tricky” transonic range (though Boeing’s Sonic Cruiser proposed back in March 2001 was designed to fly at Mach 0.98 or 15-percent faster than existing subsonic passenger planes at the time but only half as fast as the supersonic Concorde, was cancelled in December 2002 due to post 9/11 aviation slump and was since repurposed into the Boeings rather “conventional” Mach 0.85-capable 787 Dreamliner program). As a high-end private jet / corporate jet, the Citation X can also fly nonstop for 6,000 kilometers or the distance between Moscow to Beijing. But given that some modern high-performance military aircraft can even fly at Mach 1.5 without using their afterburners unlike their fuel guzzling Cold War era predecessors, why is it that most aircraft – military or civilian – tend to fly below or bypass into the supersonic region, instead of cruising continuously into the transonic range?  

Any aircraft in flight displaces the air through which it flies, and produces countless small disturbances. Called pressure waves, these radiate from various points on the aircraft’s surface like ripples from a boat. All of them travel at the speed of sound. At subsonic speeds, these waves are able to move harmlessly out ahead of the aircraft. At sonic speeds – i.e. at the speed of sound 760 mph (1,223 km / hr) at sea level which falls off to around 660 mph (1,062 km / hr) at 25,000 feet and above – these pressure waves can no longer move ahead of the aircraft because it is flying as fast as they are, and so they pile up, reinforcing one another to create a high-pressure shock wave. 

The shock wave buildup starts at about Mach 0.8 for most aircraft. Even though the plane is not then moving as fast as sound, the accelerated air moving over the top of the wing reaches supersonic speed and a small localized shock wave is formed. The region from about Mach 0.8 (525 mph or 845 km/hr) to Mach 1.2 (913.45 mph or 1,470 km/hr) is called the transonic range or transonic region because some of the airflow across the aircraft is subsonic and some have already reached supersonic. 

The swept-back wing design that has since become de rigueur in fast airplanes is a result of minimizing the problem of flying in and beyond this transonic range / transonic region. The fact that even a subsonic plane like the venerable Boeing 707 and its related variants could never operate at the speeds it does unless its wings were swept back. This is because the Boeing 707 and its related variants do in fact cruise at speeds of around 600 miles per hour (966 km/hr) at altitudes of 25,000 feet or over. This is more than 90-percent of the speed of sound. At such speeds, a straight-winged 707 would have airspeed over its wings – due to the acceleration needed for lift – already at supersonic speeds. Assuming such a straight-winged 707 variant had sufficient power to overcome the drag created at these speeds, the shock waves set up could cause severe buffeting and lack of control.

However, by sweeping back the wings, the formation of shock waves is delayed. In flight, the swept-back wing meets the air at an angle. The effect of this is that now the velocity of the wind relative to the wing acts in two directions – one at a right angle to the leading edge of the wing, and the other along the span of the wing. Neither of these components is equal to the original velocity of the wind striking the forward edge of the wing – which, in fact, is the speed of the airplane. It is only that part of the wind passing at a right angle to the leading edge of the wing which is accelerated in its passage in order to obtain lift. Since the speed of this wind is less than the forward speed of the airplane, it becomes possible for the airplane to fly much closer to the speed of sound before shock waves begin to form on the wings.  

 

Because shock waves so severely affect an airplane’s stability, the greatest problem for a pilot at the “tricky” transonic range is the change in control characteristics. A wing has a slowly moving layer of air called the “boundary layer” that clings to the surface. Near Mach 1, shock waves can interact with the boundary layer to distort the airflow so that lift may be impaired and control surfaces made ineffectual. This disturbance also adds to the turbulent wake which is created by any wing, whatever its speed. 

Wing shape is obviously important in controlling airflow, but other design solutions have been found. Some are ingeniously simple – i.e. Boeing’s cancelled Sonic Cruiser which flies deep into the transonic range by having a cruise speed 15-percent faster that existing subsonic passenger jets, resorted to having a “supersonic ready” flight control surfaces. Tail surfaces, for example, may be moved up or down to get these out of the wings’ troubled wake. But providing supersonic-ready flight control surfaces to transonic planes may prove to be a not-so-brilliant engineering solution because since the advent of the aviation world’s mastery of flying faster than the speed of sound around the middle of the 20^th Century, supersonic ready flight control mechanisms / flight control surfaces weigh two and a half times than their subsonic counterparts and cost on average four times as much.