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 20th 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.
