Supersonic flight
In normal circumstances, the steady state of a sonic boom means a shock wave cannot be eliminated. However, if this state is converted to a transient one, we can reduce the effect it has on the ground significantly. From the equation, the angle of propagation ( β ) relies on the speed of the aircraft (M), and the wedge angle ( α ). We of course cannot constantly change the speed of an aircraft to produce a transient state, however we could vary the angle of the wing ( α ) constantly for this to occur.
Fig. 6: proposed design for shock wave mitigation through vibrations of aircraft leading edge
Fig. 7: Large distribution of shock wave on ground
This can be achieved through use of an elastic membrane stretched over the leading edge of the wing and vibrated to its resonance frequency. 40 In Fig.
6, 41 an elastic fairing made of carbon fibre composite is stretched over the leading edge of the wing. When the air pressure in the manifold is varied, forces act on the fairing causing it to vibrate at a frequency of around 10Hz. 42 Usually, sonic shock waves are extremely thin when they reach the ground, causing a sudden and abrupt pressure change (Nwave) across a very small region. However, as vibration occurs, angle α varies, and angle β follows suit, distributing the shock waves produced over a much larger area (Fig. 7). 43 As a result, the maximum pressure, and thus impact of the sonic boom is hugely reduced, to a level hardly detectable by residents on the ground. Although the technology is only in its early stages of development and testing today, 44 it is extremely promising, and could well be a feature of SSTs in the very near future. However, while there is a huge engineering effort to develop ‘ b oomless’ supersonic flight today, there is the possibility this may never actually occur. To beginwith, many companies, including Aerion 45 and Boom Technology 46 are currently developing aircraft with no active noise reduction technologies at all. Instead, these enterprises have decided that ‘ b oomless’ flight is not worth the wait or money to develop right now, and instead have decided to get their aircraft in the skies and making money as soon as possible. As a result, it is highly likely that the first generations of SSTs we will see over the next few years will be without boom-limiting capabilities, instead employing the same strategy that failed the Concorde: flying supersonic only over the oceans. With limited range, and very limited routes, it will be interesting to see if this once failed business plan will be economical this time around, and, if it is, perhaps we will see more companies moving away from their current developments to follow suit instead. Indeed, it will certainly be a long while until the FAA even considers a relaxation on trans- continental supersonic flight, 47 withmuch testing and research needed before any actions can be taken, and by that time, who knows what alternative technologies will have been developed, making supersonic flight redundant. For one, the development of low-orbit space flight by companies including
40 This ensures maximum amplitude with minimum power used. 41 Sandu and Brasoveanu 2011: 6. 42 The duration of a natural N shock wave is around 0.1 seconds in length: thus the period of vibration must be under 0.1s for the technology to be effective, leading to a frequency of around 10Hz being desirable. 43 Information and image taken from Sandu. C, Sandu. R and Olariu 2019: 9. 44 Sandu and Brasoveanu 2011: 10-12. 45 Grose 2016: 32. 46 Cariosca, Locke, Boyd, Lewis and Hallion 2019: 20. 47 Grose 2016: 33.
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