A Spitfire very nearly broke the sound barrier in 1944. In the 1930s a small number of aero-engineers recognized that the piston-engine and propeller were providing diminishing returns. Despite reductions in drag and increases in efficiency provided by slippery airframes and variable-pitch propellers, huge increases in engine horsepower were yielding smaller and smaller increases in speed. And as propeller-driven aircraft approached the speed of sound they came up against what the historian of technology Professor Edward Constant calls a presumptive anomaly: progress in a given line of technological development will always be capped by a theoretical upper limit.
A propeller-driven aircraft screws its way through the air somewhat in the way a corkscrew screws through a cork. The tips of the propeller have to travel further and faster than the aircraft, and so when it gets close to sonic speed, the propeller begins to go supersonic, starting at the tips and working inwards. Spectators on the ground underneath Schneider Trophy aircraft would hear a deafening crashing and banging as the propeller tips exceeded the speed of sound.
For the aircraft to go supersonic the whole propeller would have to be travelling through the air much faster than the speed of sound. And there is a problem with that. The drag on the blades increases hugely as they approach Mach 1, the thrust diminishes, and the noise and shockwaves become so destructive that the airframe itself is imperilled. So there was a clear limit to the speed of piston-engined propeller-driven aircraft. This was the presumptive anomaly and everyone in the business of aeroplanes in the 1930’s knew that propeller aircraft were near the limit of what was possible.
The only way you could approach the speed of sound was if your propeller fell off. And this is exactly what happened to Squadron Leader Anthony Martindale in April 1944. Exploring high speeds for the RAF, he put his Merlin-engined Mark XI Supermarine Spitfire into a dive from high altitude. At something approaching 600 mph the reduction gearbox ripped off, taking the propeller with it.
The Spitfire then reached over 620mph (1,000km/h) – Mach 0.92 – as it plunged towards the ground. With the heavy gearbox and propeller missing, Martindale’s Spitfire was now tail-heavy and so this change in the centre of gravity forced it into a steep climb. Martindale lost consciousness due to positive g, and eventually woke up to find his aircraft flying quietly along at 40,000ft (13,000 m) on its own, without a propeller. It could be argued that this Spitfire had just become a jet fighter as it had a Meredith-effect radiator providing jet thrust, and the Merlin engine had backwards-pointing exhaust stubs that did the same.
A consummate pilot, Martindale glided the Spitfire back to base and got out somewhat shaken. His groundcrew pointed out there was even more damage: the speed of the Spitfire’s dive had bent the wings backwards, giving them a swept shape. It was the shape that aircraft wings need to be to break the sound barrier.
After the defeat of the Luftwaffe British aircraft makers turned their energies towards a less visible enemy: the sound barrier. The term is an anomaly; to the fascinated public it suggested an invisible wall somehow to be penetrated by the sharp nose of a jet aircraft. Instead, as we have seen with the propeller-less Spitfire, the barrier is an increase in aerodynamic drag experienced by an aircraft as it approaches the speed of sound. This increased drag makes it difficult to exceed the speed of sound unless the aircraft is specially designed to overcome the drag effects.
The speed of sound on a dry, warm day at ground level is about 767 mph, or 343 metres per second (1234 km/h). If you stand at the end of a football pitch and watch the goalkeeper at the other end kick the ball you will see the kick at the speed of light and then hear the kick at the speed of sound. The noticeable delay is due to the speed of sound being considerably slower. The temperature and the medium that the sound travels in affects the speed: the speed of sound in cold, thin air above 35,000 feet is 660 mph, or 295 metres per second (1063 km/h). That is one reason why supersonic aircraft tend to fly so high. And the speed of sound in diamonds is 27,000 mph! (12,000 metres per second).
Because there is no absolute speed for sound (as there is for light) aeronautical engineers had to come up with a relative measure, so the speed of sound in air is called Mach 1.0 (after the Austrian Ernst Mach). So, an aircraft flying Mach 1.0 at sea level is doing around 767 mph, (666 knots, 1234 km/h), but a plane flying Mach 1.0 at 35,000 ft is flying at around 660 mph (589 knots 1091 km/h) etc. An aeroplane flying at Mach 2.0 is flying twice as fast as the speed of sound. Speeds below Mach 1 are called subsonic, between Mach 0.8-1.2 transonic, and above Mach 1.2 supersonic.
There is nothing particularly unnatural about exceeding the speed of sound. Meteors cracking through the earth’s atmosphere do it. Bullets, shells and rockets do it. Even the crack of a bullwhip is caused by the tip breaking the sound barrier. Dinosaurs with long tails such as Brontosaurus, Apatosaurus, and Diplodocus may have been able to flick their tails at supersonic speeds, with what must have been a terrifying crack.
I remember hearing my first sonic boom on a French beach in the early 1960s, probably made by a Dassault Mirage jet fighter. The sound was a thunderous double bang in the sky which filled all four corners of that long-ago morning. Just as a ship will make bow and stern waves, so too will an aircraft make a series of pressure waves in front of it. These waves travel at the speed of sound and as the speed of the aircraft increases the waves are compressed together because they cannot get out of each other’s way fast enough. When that happens, they merge into a single shock wave which travels at the speed of sound. The double bang is the sound of the shock waves at the front and rear of the aircraft.
To the little boy on the shore that sonic double bang was one event, but in fact the aircraft was laying down a continuous boom all along its course along the coast, like the rolling-out of a red carpet. The supersonic Concorde pilot hears nothing: “You don’t actually hear anything on board. All we see is the pressure wave moving down the aeroplane – it gives an indication on the instruments. And that’s what we see around Mach 1. But we don’t hear the sonic boom or anything like that. That’s rather like the wake of a ship – it’s behind us.”