On October 14, 1947, test pilot Chuck Yeager flew the Bell X-1 faster than the speed of sound over the Mojave Desert. The rocket plane, named Glamorous Glennis, was carried high into the sky by a B-29 Superfortress before being released to fire its own engine. This flight mattered because many engineers feared that aircraft might become uncontrollable near Mach 1.
Yeager's success showed that the sound barrier was not a solid wall, but a challenging region of high-speed aerodynamics that could be understood and overcome.
As an aircraft approaches the speed of sound, pressure waves pile up because they cannot move ahead of the aircraft fast enough. These compressed waves form shock waves that can greatly increase drag, change lift, and shake the aircraft. The Bell X-1 used a bullet-shaped fuselage, thin wings, rocket propulsion, and careful control design to survive this transonic and supersonic region.
Its flight helped launch the age of supersonic aviation and shaped later aircraft such as jet fighters, research planes, and spacecraft reentry vehicles.
Understanding Aviation: Breaking the Sound Barrier
The speed of sound is not a fixed number. Sound travels through air by passing tiny pressure changes from one group of molecules to the next. In warmer air, molecules move faster, so sound moves faster.
At high altitude, the air is usually colder, and the local speed of sound is lower. This is why pilots use Mach number rather than one fixed speed.
Mach number compares aircraft speed with the speed of sound in the air around it. A plane can have the same true speed at two altitudes but a different Mach number.
The hardest region is called transonic flight. It occurs around Mach one, when airflow over parts of an aircraft may be faster than sound even if the aircraft itself is slightly below Mach one. Air speeds up as it moves over curved surfaces, especially the top of a wing.
Small supersonic patches can form there first. When that airflow slows down again, it may cross a shock wave. Across the shock wave, pressure rises sharply and airflow loses energy.
This can cause a sudden drag rise called wave drag. It can reduce lift or move the center of pressure on a wing.
Control was a major engineering problem in this region. Early aircraft often had a tailplane with a hinged elevator. At high speed, shock waves could make the elevator weak or make it difficult for the pilot to move.
The X-1 used an all-moving horizontal tail. Instead of bending only a small flap, the whole tail surface changed angle. This gave the pilot stronger pitch control when the airflow became difficult.
Engineers learned that a stable shape was not enough. The controls, structure, instruments, and pilot training all had to work together. A small change in airflow could produce a large change in the forces on an aircraft.
Students meet these ideas in several physics topics. The drag equation shows why drag rises strongly with speed. In that equation, drag force depends on air density, speed squared, drag coefficient, and area.
Near Mach one, the drag coefficient can rise because of shock waves, so speed squared is not the only concern. The topic connects to pressure, forces, energy, motion, and fluid flow. It is useful to separate airspeed from ground speed, since wind changes ground speed but does not directly determine Mach number.
It is also important to remember that supersonic aircraft create sonic booms. A boom is not a single sound made at one instant. It is the continuing pressure pattern from shock waves reaching an observer as the aircraft passes overhead.
Key Facts
- Speed of sound near sea level: about 343 m/s or 1235 km/h at 20°C.
- Mach number: M = v / c, where v is aircraft speed and c is the local speed of sound.
- Supersonic flight begins when M > 1.
- At Mach 1, pressure waves crowd together and can form strong shock waves.
- Drag force increases with speed according to Fd = 1/2 rho v^2 Cd A.
- The Bell X-1 reached about Mach 1.06 on October 14, 1947, with Chuck Yeager as pilot.
Vocabulary
- Mach number
- Mach number is the ratio of an object's speed to the speed of sound in the surrounding air.
- Shock wave
- A shock wave is a thin region of sudden pressure, temperature, and density change caused by motion at or above the speed of sound.
- Sound barrier
- The sound barrier is the set of aerodynamic problems that occur as an aircraft approaches and passes Mach 1.
- Transonic
- Transonic describes flight near the speed of sound, where some airflow around the aircraft may be subsonic and some may be supersonic.
- Rocket plane
- A rocket plane is an aircraft powered by a rocket engine that carries both fuel and oxidizer instead of using oxygen from the air.
Common Mistakes to Avoid
- Calling the sound barrier a physical wall is wrong because it is not a solid object, but a region where shock waves and drag make flight difficult.
- Using one fixed value for the speed of sound is wrong because the speed of sound depends mainly on air temperature and changes with altitude.
- Thinking a sonic boom happens only at the instant an aircraft crosses Mach 1 is wrong because a supersonic aircraft continuously creates shock waves as it flies.
- Ignoring Mach number and comparing only speed in km/h or mph is wrong because the same aircraft speed can be subsonic or supersonic depending on the local speed of sound.
Practice Questions
- 1 An aircraft flies at 360 m/s where the local speed of sound is 300 m/s. What is its Mach number, and is it subsonic or supersonic?
- 2 The Bell X-1 reaches Mach 1.06 where the local speed of sound is 320 m/s. What is its speed in m/s?
- 3 Explain why the Bell X-1 was carried by a B-29 before release instead of taking off from the runway under rocket power alone.