Why Are F-18s So Loud?

Fighter jets are known for being noisy. They can be heard in the distance even at night. UC engineers are testing a novel design of the nozzle to reduce noise. The new design uses a corrugated seal to prevent hot gases from leaking into other parts of the nozzle. The engine flaps also help reduce noise, especially in cold weather.

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UC engineers test novel nozzle design on cold and heated jets

Engineers from the University of Cincinnati and the Naval Research Laboratory are testing a novel nozzle design on cold and heated jet engines. The idea is to reduce noise without compromising engine performance. They used a scale model of the F-18’s F404 engine, which generates 1,100 degrees Fahrenheit exhaust. While the real thing is likely to be even hotter, the researchers’ tests have already shown that the new design can significantly reduce engine noise.

The researchers have used a lubricated, porous nozzle for their tests. The nozzle is jacketed with a high-viscosity lubricant, which is pulled through the nozzle by the water jet, and is designed to minimize wear and tear on the nozzle.

The nozzle has an average pressure rise that changes with nozzle diameter. The length of the jet tube was chosen to minimize modeling uncertainty.

Corrugated seals prevent hot gases from leaking into other parts of the nozzle

Corrugated seals are often used in jet fuel nozzles to prevent hot gases from leaking into other parts. The rate of leakage depends on the separation at the critical constriction. Plastic deformation reduces this rate by approximately 8 times.

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Although plastic deformation is a complex phenomenon, it can be easily modelled by using simple physics. To achieve this goal, researchers used a simple and convenient model based on the atomic scale. This approach is simple to implement, and can be used to estimate leakage rates in metallic seals.

This leak rate has been measured for 10 minutes. In some cases, the leak rate decreases as the water pressure increases. This is attributed to the fact that water-based impurities clog critical constrictions. As water pressure increases, the squeezing force increases. This reduces interfacial separation at critical junctions and lowers the leak rate.

Engine flaps reduce engine noise on cold jets

Engine flaps have been proven to reduce engine noise in a number of aircraft types, including fighter jets. Developed by John Seiner, associate director of the National Center for Physical Acoustics at the University of Mississippi, the flaps are designed to reduce noise caused by the engine. In a test plane, the flaps reduced noise by four decibels below the wing.

The optimum chevron configuration reduced both high and low-frequency noise. Specifically, the design increased penetration, which reduced the broadband shock noise and peak jet noise. However, decreasing the chevron’s length increased noise at most observation angles. The impact of chevron width was dependent on the frequency and observation angle.

Propulsion-induced engine noise consists of two components: high-frequency fan noise and low-frequency jet rumble. High-frequency fan noise radiates forward through the air inlet, while low-frequency jet noise typically radiates rearward. To meet FAA noise-restrictions, engine noise must be reduced in both components.

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Sound levels of f-18s at night

If you’re worried about the sound of F-18s flying overhead at night, you’re not alone. The noise from these aircraft is extremely loud, at times up to 20 decibels louder than the average commercial jet. The noise is so loud, in fact, that you can’t even hear yourself yelling next to the plane. This noise can be deafening for many people, especially those with hearing impairments. Fortunately, there are ways to limit your exposure to noise and protect your health.

The noise from an F-35 is also significantly louder than that of an F-16 or an A-10. According to the Arizona Star, an F-35 fighter will emit 121 decibels of noise, about 25 decibels more than an F-16. The Air Force says that F-35s are slightly louder at takeoff, but considerably quieter on all approaches.