When we think about aviation, we often think about the roar of engines, the thrill of takeoff, and the gentle hum as we cruise in the sky. However, for those living near airports or for those conscious about noise pollution, this noise can be a significant issue. This is where “hush kits” come in – the unsung heroes of aviation noise reduction. Hush kits, fitted to older aircraft, help keep the skies quieter and more environmentally friendly.
Hush kits are aerodynamic devices designed to reduce the noise produced by older jet engines. Primarily, these kits are installed on older turbojet and low-bypass turbofan engines, which are typically louder than the high-bypass turbofan engines of today. Notable aircraft like the Boeing 727, Douglas DC-9, BAC One-Eleven, and Boeing 737-200 have been equipped with hush kits to tame their engine noise.
The most common form of a hush kit is a multi-lobe exhaust mixer. This ingenious device is fitted to the engine’s rear, where it mixes the jet core’s exhaust gases with the surrounding air and some bypass air. This principle, in essence, mimics the noise reduction techniques employed in modern high-bypass turbofan engines.
How Do Hush Kits Work?
Hush kits work by making several modifications to the exhaust, which include acoustically treated tailpipes, revised inlet nacelles, and guide vanes. These alterations help reduce the high-pitched noise produced by the small, high-speed fan of older engine designs. This type of noise is less of an issue with modern engines, thanks to larger front fans that spin at slower speeds.
However, there’s a trade-off. Hush kits can adversely affect the aircraft’s range and performance due to the extra weight they add. For example, the hush kit fitted to the Gulfstream II adds 106 kilograms (234 lb) to the total airplane weight, causing around a 1.6% reduction in aircraft range. Similarly, on larger aircraft like FedEx’s Boeing 727s, the hush kits add 410 kg (900 lb) of extra weight, resulting in a slight increase in fuel burn for short trips.
Regulation and Hush Kits
Although hush kits effectively reduce noise emissions from older aircraft, they can’t always reduce noise to the level of modern planes at a reasonable cost. This led to regulatory disputes in the late ’90s and early 2000s between the US and the EU, culminating in strict noise compliance laws. Today, many of these regulations still stand, further emphasizing the relevance and importance of hush kits in aviation.
The Inventors of Modern Hush Kits
While the idea of a hush kit-like device has been around since the 1960s, the first modern hush kit was patented by Thomas W. Carr, Alan Marsh, and Philip Rose in the mid-1980s. After their patent expired in 2006, the technology entered the public domain, leading to widespread use of hush kits across various aircraft models.
In the grand scheme of aviation, hush kits play a critical role. They enable older aircraft to continue operating in noise-regulated areas, reducing noise pollution around airports, and indirectly contributing to a more serene cabin environment. While they may affect an aircraft’s performance and range slightly, the trade-off is well worth the benefits they bring.
First love never dies. I fell in love with airplanes and aviation when I was a kid. My dream was to become a pilot, but destiny led me to another path: to be an aviation digital media content creator and a small business owner. My passion for aviation inspires me to bring you quality content through my website and social accounts. Aviation is indeed in my blood and blog!
