Professional Pilot, October 2016

22 ft 67 m PROFESSIONAL PILOT October 2016 129 decided to abandon this project and instead focus on their very successful range of intercontinental large cabin corporate jets High ops costs and sonic booms Expensive operating costs due to high fuel burn are one reason why supersonic aircraft have always had a difficult economic case Concorde was a European prestige project subsidized by the French and British governments But note that the German government and Lufthansa never took part in this Germany has a more central European location and overland the Concorde could not operate supersonic even over France and Britain because the boom was prohibitively noisy for residents on the ground At the time it was thought that the subsonic overland part of the flight to the US east coast became too large to still allow for a real time advantage for passengers As mentioned civil supersonic flight has been banned over the landmass of the US since the 1970s and many other countries have banned it since The reason is the perceived impact of the sonic boom In times of heightened tension during the Cold War military jets accelerated to supersonic speeds and flew towards the Iron Curtain sending supersonic booms to the other side as a political signal of resolve This caused windows to break and other discomfort on the ground Sound travels as a pressure wave through the air with a speed of roughly 340 m s depending mostly on temperature and atmospheric conditions This equals a speed of about 660 kts close to the surface at a standard temperature of 15 C 56 F The speed of sound is very familiar to pilots as our cruise speed at high altitude is expressed as a percentage of the speed of sound called the Mach number after Austrian physicist and professor Ernst Mach Mach 100 is the speed of sound under current conditions If an aircraft is slower than Mach 1 we hear it coming as the sound travels ahead of it At transsonic speeds in the area of Mach 1 the sound travels with the aircraft And at supersonic speeds above M 1 we see the aircraft first we hear nothing at first but then we receive the compressed wake of the aircaft and we hear it as 170 ft 518 m a sudden boom which sounds very similar to a far away explosion Supersonic aerodynamics Supersonic aerodynamics are very different from normal subsonic aerodynamics Airflow behaves differently because of formation of shockwaves and an accompanying change of lift and center of lift For this reason we talk of subsonic flight until Mach 08 transsonic flight between Mach 08 and Mach 12 and supersonic or hypersonic flight above that Todays common civil airliners and corporate jets very often operate in the low transsonic range between Mach 08 and Mach 09 With a swept wing design the onset of the main shockwave formation is delayed Small local shockwaves may however form in certain phases of flight and sometimes you can even see them on the wing as a blur While the problems of transsonic and supersonic aerodynamics and stability have largely been solved the sonic boom has remained as the biggest obstacle to overcome As the aircraft travels at supersonic speed the pressure waves created by the hull moving through the air are compressed into a single shockwave that forms a cone with the aircraft at the front peak As this cone reaches the ground the shockwave is heard and felt by the observer At the aircraft there is a rise in pressure at the nose decreasing to negative pressure at the tail with a sudden return to normal pressure This pressure distribution is called the N curve sometimes causing even a double boom as the pressure changes twice rapidly The boom follows the path of the aircraft on the ground with a lateral extension depending on altitude and speed This footprint is called the boom carpet From military aircraft we know that a boom carpet is felt and heard on the ground even at cruising altitudes of 70000 ft Much research has gone into how to minimize or even eliminate the sonic boom It was found that larger wingspans create stronger effects on the ground which is one reason why the somewhat smaller corporate supersonic jets seem to be a feasible next step Also a longer slimmer aircraft shape seems to distribute the shockwave in a more acceptable way The so called Quiet Spike a telescopic spike designed jointly by NASA and Gulfstream that extends in front of the aircraft and aerodynamically extends the length of the aircraft was tested successfully in 2006 From all this it can be seen that the proposal to build a supersonic corporate jet requires not only a strong will but also dedication and deep pockets to support a lot of research and engineering Therefore Aerions plan to certify the AS2 as the 1st corporate supersonic jet warrants a closer look especially since Flexjet placed a firm order for 20 of these 100 million aircraft without even knowing the engine manufacturer A closer look at the Aerion AS2 While Concorde travelled at Mach 2 Aerions AS2 is designed for a maximum speed of Mach 15 This allows for a wing that performs well 61 ft 186 m