Breaking the Sound Barrier

Supermarine Spitfire (Artists Impression) (1)

The Battle of Hastings in 1066 marked the last successful conquer of the British Isles. In the summer of 1940 the Spitfire prevented the most recent attempt of invasion and marked one of the turning points in WWII. Between July and October 1940 an astonishing 747 Spitfires were delivered of which 361 were destroyed and 352 were damaged. Combining aerodynamic beauty of elliptical wings with a slender yet aggressive fuselage, the Spitfire evolved to be more than an airplane; it is symbolic for British defiance and the war-effort in the 1940’s.

Like other incredible aeronautical achievements the Spitfire was a product of a design competition that fostered great feats of engineering innovation. RJ Mitchell (1895 – 1937) was blessed with a short, yet brilliant life as an aerodynamicist and is posthumously famous for the development of the Spitfire. As Head Engineer at the Supermarine aircraft company Mitchell was instrumental in designing a series of sea-planes which won the Schneider Trophy three consecutive times between 1927 and 1931. At the time, the Schneider trophy was a prize competition for seaplanes organised by the financier and aircraft enthusiast Jacques Schneider to encourage technical advances in civil aviation. Since the race was based on navigating a 280 km (later 350 km) triangular course as quickly as possible the competition quickly evolved into a contest for pure speed that facilitated rapid technological advancements. Between 1911 and 1931 the winning average speed to navigate the course increased from 73.6 km/h (45.7 mph), flown by the French Deperdussin aircraft, to 547.3 km/h (340.1 mph), achieved by Mitchell’s Supermarine S.6B. Furthermore, the S.6B also set a new maximum speed record of 655.8 km/h (407.5 mph) or about Mach 0.61 at SI standard temperature and pressure.

Supermarine S6.B (2)

In March 1936 Supermarine tested a new aircraft, which had evolved directly from the winning S6.B sea-plane, featuring the radical novel elliptical wing and powered by the supercharged V-12 Rolls-Royce Merlin engine. The biggest advantage of the elliptical wing planform is that it minimises induced drag i.e. the drag that occurs due to redirection of airflow by the wings to create lift, by facilitating equivalent downwash over all parts of the wing. Although the Spitfire was a great step forwards in aircraft design the limited performance of piston engines at high altitudes (thinner air = less drag) prevented this aircraft from reaching supersonic speeds. The supersonic era required the invention of an entirely new system of propulsion; the Jet Engine proposed by Frank Whittle and Hans von Ohain during WWII.

Bell X-1 (3)

The dawn of supersonic flight after WWII was also the birth of the experimental X-programme that is still active today to test and evaluate novel aerodynamic concepts and aviation technologies. Captain Chuck Yeager, an American WWII fighter ace, became the first supersonic pilot in 1947 when the chief test pilot for the Bell Corporation refused to fly the rocket-powered Bell X-1 experimental aircraft without any additional danger pay. The X-1 closely resembled a large bullet with short stubby wings for higher structural efficiency and less drag at higher speeds. The X-1 was strapped to the belly of a B-29 bomber and then dropped at 20,000 feet, at which point Yeager fired the rocket motors producing 6000 pounds of thrust and propelling the aircraft to Mach 0.85 as it climbed to higher altitudes. The X-1 leveled off at 40,000 feet where Yeager fully opened the throttle, pushing the aircraft into a flow regime for which there was no available wind tunnel data. At this point shock waves started to form over the wings and the Mach meter in the cockpit registered 0.98 … 0.99 … 1.02 and then jumped to Mach 1.06 at which point the fuel in the tanks ran out. This first supersonic flight lasted just 14 seconds but will be recorded firmly in aviation history for the rest of time. The name of the Bell chief test pilot? Can’t remember!

Gallery

References

Article largely based on [6]

[1] http://www.warbirdalley.com/spitpic.htm

[2] http://blog.incipeindustries.com/supermarine-s6b

[3] http://upload.wikimedia.org/wikipedia/commons/thumb/3/33/Bell_X-1_color.jpg/300px-Bell_X-1_color.jpg

[4] http://en.wikipedia.org/wiki/File:Bell_X-1A_in_the_belly_of_a_B-29.jpg

[5] http://www.aviationexplorer.com/bell_x1_facts.htm

[6] Lock, G.D. (01. 2009). “Fluid Mechanics with a Historical Perspective”. University of Bath. Bath, UK