Supersonic flight
Demonstrator (Fig. 4) , 28 in which an elongated, blunt ‘Nose Glove’ was fitted to the supersonic aircraft. The long nose prevented shock waves from the engine inlets and wings from merging with the bow pressure wave, reducing the ‘N’ pressure spike observed on the ground to a more rounded, smaller curve. 29 The result was a boom 82% the size of the normal F-5 shock wave 30 – a resounding success. Furthermore, the blunter nose, although creating much drag, created a pressure spike at the front of the aircraft, which raised the surrounding air temperature, increasing the speed of sound and thus spreading the shock wave and dampening it ever more. 31 In the next few years, aircraft such as the NASA/Lockheed X-59 QSTA, 32 will be produced as technology demonstrators to further study and test existing body-shaping theories, 33 including powerplant locations and lift distribution, which could muffle the sonic boom further. 34 However, these aircraft will also highlight the many drawbacks of a streamlined, shaped design: not only does shaping the fuselage lead to increased dynamic drag and landing distance requirements for SSTs, but it also increases manufacturing costs whilst reducing the overall useful volume of the fuselage. 35 Hence, whilst this technology seems very promising and not too distant, a large, inexpensive, shaped SST airliner may still be off the drawing board for some time to come. While some proposals to mitigate sonic booms are already well into the testing stages, engineers continue to come up with ingenious design solutions which, if proved correct, could further revolutionize SST design and production very soon. My favourite solution relies on simple physics and trigonometry applied to the surface of an SST leading edge. 36 Let us imagine the leading edge of a supersonic wing as a pointed wedge (Fig. 5). 37 For low supersonic Mach numbers, up to around Mach 1.8, a small change in the angle of this wedge (angle α) produces a very large variation 38 in the angle of shock wave propagation from the wing (angle β ), according to the equation: 39 Fig. 4: F-5E Demonstrator
Fig. 5
(𝑘 + 1)𝑀 2 2(𝑀 2 𝑠𝑖𝑛 2 𝛽 − 1)
cot 𝛼 = tan 𝛽 [
− 1]
28 Taken from ‘ Shaped Sonic BoomDemonstration ’ (2020) Wikipedia. https://en.wikipedia.org/wiki/Shaped_Sonic_Boom_Demonstration [Accessed 28/07/2020] 29 Scott 2003: 36 30 Gonzalez, C. (2015) Supersonic Flight: Overcoming the Sonic Boom. Page 1. Available at: https://cdn.baseplatform.io/files/base/ebm/machinedesign/document/2019/03/machinedesign_3246_supers onicflightovercomingthesonicboom.pdf [Accessed 20/07/2020] 31 Scott 2003: 36 32 Quiet Supersonic Technology Airliner 33 Cariosca, Locke, Boyd, Lewis and Hallion 2019: 19 34 NASA Chat: Taking the ‘ Boom ’ Out of Booms (2011). Available at: https://www.nasa.gov/connect/chat/sonic_boom_chat.html [Accessed 20/07/2020] 35 Sandu. C, Sandu. R and Olariu 2019: 1 36 The foremost edge of an aerofoil, especially a wing – however this technology could also effectively be fitted to the nose of a Supersonic Transport for further noise mitigation. 37 Taken from Sandu. C, Sandu. R and Olariu 2019: 8 38 A 1 ˚ alteration in angle α leads to a 3.5 ˚ alteration of angle β at Mach 1.3 39 Sandu. C, Sandu. R and Olariu 2019: 7
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