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Stealth Aircraft Engine Noise Reduction

- question from Curtis Yorkey

I think you are asking two separate, if related, questions. The more general question about what makes a stealth plane different from a non-stealth plane covers a wide variety of technologies. The issue of making a plane quieter is more specific and isn't necessarily confined to stealth aircraft alone. It's more of an issue of the kinds of engines used on a plane.

Older military and commercial jets tend to use jet engines that are called turbojets or low-bypass turbofans. These engines suck in a small amount of air and accelerate it out the back at high speed to produce thrust. This process tends to be very noisy. For this reason, stealth planes (as well as newer commercial aircraft) use engines that are called high-bypass turbofans. These engines take in a much larger quantity of air but accelerate it less. However, because the mass of air is so big, you can actually get more thrust than with the older kinds of engines. The added benefit is that these newer engines create a large cushion of slower-moving air around the noisy part of the engine, and this cushion blocks much of the noise so the engine is significantly quieter. Commercial aircraft are also using these new quieter engines to meet government noise regulations.

Another technique used to make a stealth plane quieter is to place the engines on top of the plane so that their noise is blocked from reaching the ground by the body of the aircraft itself. You can see this method used on both the B-2 stealth bomber and F-117 stealth fighter.

Overhead view of a B-2 showing illustrating its buried engines

While I am not aware of any websites that discuss these ideas in detail, here are some pages that might help you out:

How stealth works -- primarily a discussion of radar

Stealth technology -- previous question of the week on several stealth issues, including aural (noise) signature

Jet engines -- previous question of the week on different kinds of jet engines including turbojets and turbofans

We hope that these sources will help in your project. If anyone can recommend further sources of information on this topic, please contact us.
- answer by Jeff Scott, 24 March 2002

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