Supersonic flight
conditions constantly and adjust the aircraft’s speed accordingly to mainta in a quiet boom, and the environmental measuring capabilities we have today are not up to the standard required as of yet. 16 Therefore, although this idea is certainly promising, and is being vigorously pursued, its implementation could be a long way away.
However, while research into theoretical proposals continue today, one boom- silencing technique has been excessively tested since the 1970s. 17 Aircraft body- shaping is a relatively less ‘out -of-the- box’ method to mitigate sonic booms. The first discovery NASA encountered in researching body-shaping was that the smaller the aircraft, the quieter the boom it produces: 18 a bigger, heavier aircraft displaces more air to produce more lift to sustain its flight. Therefore, it will create sonic booms stronger and louder than those of smaller, lighter SSTs. 19 This concept will be a critical factor in the designs of future supersonic aircraft, which will likely be ‘ b usiness jets’ at first, 20 rather than full-blown airliners, since sonic boom mitigation is far more straightforward with smaller aircraft designs. There are, however, many other shaping techniques that have been proven to reduce the volume of sonic booms. Most popular of these is the introduction of a long, thin, sleek body to future
Fig. 3: As Sonic Booms propagate off sharp corners on the aircraft, they coalesce to form an ‘N’ pressure spike and two booms are heard
concepts, 21 increasing their fineness ratio . 22 When an aircraft goes supersonic, shock waves actually propagate from many areas on the aircraft fuselage, where a sudden change in shape causes a bend in airflow and a change in air pressure. As these many waves travel towards earth, they coalesce and become stronger, until two distinct booms can be heard by observers on the ground (Fig. 3). 23 By streamlining the aircraft fuselage, and eliminating sharp bends and protrusions such as wing-joints and engine inlets, fewer shock waves will propagate off the SST andmerge together, leading to a quieter boom! 24 A long nose and thin design adds to this effect: not only does it produce less parasitic and wave drag at Mach 1+, 25 it also spaces out the shock waves emerging off the fuselage, making them less likely to converge, 26 quietening the sound observed to what is described by as ‘ like a car door shutting ’ . 27 In fact, this principle has been tested on many flying testbeds, including an F-5E Shaped Sonic Boom
16 Cariosca, Locke, Boyd, Lewis and Hallion 2019: 10. 17 Scott 2003: 36. 18 Grose 2016: 32. 19 NASA Facts – Sonic Booms (2003). Available at: https://www.nasa.gov/centers/dryden/pdf/120274main_FS-
016-DFRC.pdf [Accessed 28/07/2020]. 20 Sandu. C, Sandu. R and Olariu 2019: 2. 21 Colarusso 2002: 7-8.
22 The fineness ratio is the ratio of the length of a body to its maximumwidth. Shapes that are short and wide have a low fineness ratio, those that are long and narrow have high fineness ratios. [ ‘Fineness ratio’ (2020) Wikipedia. https://en.wikipedia.org/wiki/Fineness_ratio Accessed 28/07/2020 ]. 23 Taken from Review and prospect of supersonic business jet design (2016). Available at: https://www.researchgate.net/publication/311987911_Review_and_prospect_of_supersonic_business_jet_desi gn [Accessed 28/07/2020]. 24 Colarusso 2002: 8. 25 Anderson and Eberhardt 2010: 146. 26 Scott 2003: 36. 27 Cariosca, Locke, Boyd, Lewis and Hallion 2019: 19.
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