Bell X-1: The Challenges of Going Supersonic

Because breaking through the speed of sound isn’t easy.

The Bell X-1 kicked off the legendary X-plane series in spectacular fashion when it smashed through the sound barrier in 1947. So, what were the challenges that the designers, engineers and test pilots of Bell Aircraft and the USAF faced when making this leap into the history books over seventy years ago?

Bell X-1 Specification:

General

• Length: 31 ft.
• Wingspan: 22 ft. 10 in.
• Height: 10 ft. 10 in.
• Wing Area: 115 sq. ft.
• Empty Weight: 6,850 lbs.
• Loaded Weight: 14,750 lbs.
• Crew: 1

Performance 

• Power Plant: 1 × Reaction Motors XLR11
• Range: 4 minutes, 45 seconds
• Max Speed: 1,450 mph
• Ceiling: 90,000 ft.

The problem of high-speed flying had long been experienced by pilots and designers alike throughout the 1930s and WW2. As aircraft got faster and faster, issues of structural integrity became more and more apparent. Even when aircraft were developed to withstand these extreme forces, there remained the issue of compressibility.

To use basic terms, as aircraft fly at moderate speeds, air molecules simply move around the surface of the airframe as it passes through the atmosphere. As planes fly close to the speed of sound, however, air molecules simply haven’t the time to get out of the way. This creates a shockwave or compression ahead of aircraft and around their surfaces. This phenomenon was not well understood in 1944 when the X-1 project began.

The Bell X-1 was built to study these dangerous effects of flying at transonic speeds. Transonic refers to the speed range just below and just above the speed of sound. Many challenges had to be overcome. The first challenge was the shape of the aircraft. It was formed like a .50 cal bullet as this shape was known to be stable at transonic speeds. The airframe was built to withstand a maximum of 18 (!) Gs to make sure that it wouldn’t break up under the extreme forces experienced in the realm of transonic travel. 

To propel the aircraft, the Bell X-1 used rocket engines. These were developed during WW2 as JATO (Jet Assisted Take Off) engines for PBY Catalina flying boats amongst other aircraft. The idea was to put four of these engines together. The resulting power unit used ethyl alcohol and liquid oxygen as propellants to generate a max thrust of 6,000 lbf. It was called the Reaction Motors XLR11.

Control, or the loss of it, was one of the biggest issues faced by the X-1 team. On several flights, test pilot Chuck Yeager experienced the same phenomenon that had killed many P-38 and P-47 pilots during the war when they dived to speeds just below the sound barrier: “I was flying at 0.94 Mach at 40,000 feet, experiencing the usual buffeting, when I pulled back on the control wheel and nothing happened!”. Compression shock waves had created an aerodynamic shadow around the tail which completely eliminated the effect of the elevator. 

To solve the elevator problem, a variable-incidence tailplane was developed. This was effectively a stabilator, or all moving tailplane. It mitigated the ‘aerodynamic shadow’ issue. Today, stabilators are almost universal on military combat aircraft that fly supersonically. This was the key to controlling the X-1 at high speeds.

On the morning of October 14th, 1947, Charles E. “Chuck” Yeager piloted the rocket-powered Bell X-1 to a speed of Mach 1.07 taking him faster than the speed of sound. It was a historic moment for aviation and human achievement that saw one 24-year-old test pilot become the fastest person in the world.

More Flite Test Articles on the History of Aviation

P-51 Origins

Hawker Typhoon

Supersonic Airliners

The World’s First Airline

Article by James Whomsley

Editor of FliteTest.com

Contact: james@flitetest.com

YouTube Channel: www.youtube.com/projectairaviation