Fuselage Pressure Fields caused by the 20-mm, M-61-A1 Rapid Fire Gun

Abstract

Acoustic pressure fields generated by large-caliber rapid-fire weapons are sufficiently intense to constitute sources of concern to airplane designers. The operation of such weapons could result in greatly increased equipment and structure vibration and danger of hearing damage to flight personnel. Acoustic pressures from both fuselage and wing-mounted M-61-A1 guns have been measured on the fuselage and wing of an A-7 airplane. The measurements covered an area extending several feet fore and aft of the gun muzzles. Both static and flight firings were measured. Recordings of acoustic pressure were analyzed for waveform, instantaneous amplitude distributions, spectra, and spacial distribution. The waveform for a burst of fire is approximately a series of decaying sinusoids at the gunfire repetition rate. Peak-to-rms ratios as high as 5 to 6 are not uncommon. On the fuselage near the gun muzzle, peak pressures can reach 2 to 3 psi (177–181 dB SPL). The possible effect of these high-pressure fields on airplane design and operation is discussed in relation to structure and equipment vibration and personnel safety.