777-200LR Flight Test Journal: Archives

14 September 2005

Shhh...quiet please

Boeing Commercial Airplanes: Belur Shivashankara, Eric Nesbitt, Bill Herkes, Stefan Uellenberg and Ron Olsen

Charlotte Whitfield and Mehdi Khorrami, NASA

Steve Petersen, General Electric

Hwa Kwan and Jeff Moe, Goodrich Corp.

Rob Stoker, Boeing Phantom Works

August was an extraordinary month for Flight Test, even by Boeing standards. Several significant flight test programs were running at the same time, including the two 777-200LRs - WD001 and WD002 - which have been chronicled in this journal.

Flight Test also is sometimes asked to support long-term technology research that won't find its way onto a production airplane for 3-5 years, or longer. That's what happened this summer. With the 777-200LRs in the home stretch to certification, let's look at Flight Test's involvement with long-term technology research that one day will make a big difference to our airline customers.

Using a 777-300ER made available to us by All Nippon Airways, we worked with several key partners to test future technology that can substantially reduce airplane noise on landings and take-offs, a big issue the public and airline passengers have with air travel. The 777 is a good platform because the airplane operates with extremely efficient and quiet engines and uses the most advanced noise reduction technology available.

The noise reduction test program is called Quiet Technology Demonstrator (QTD) 2. General Electric, Goodrich and NASA came to Boeing's remote airfield in Glasgow, Mont., to test technology that targets engines and landing gear, the most prominent airplane noise producers. It will find its first commercial use several years down the road on the 787 and the 747 Advanced programs.

Reducing landing and take-off noise is important because there are places that our airplanes fly - especially in Europe and Japan - where noise regulations get tougher all the time. Already many local community noise regulations are more stringent than current FAA noise certification requirements. Most of our airplanes already meet even tougher international noise standards that take effect next year. However, we need to keep finding new and better ways to make our airplanes quieter and more efficient.

In this journal entry we'll look at the testing itself. In our next entry we'll look closer at specific technology that can substantially reduce airplane noise inside and outside the cabin. We conduct two broad categories of noise measurements: interior or cabin noise, which is what passengers hear; and community noise, which refers to people on the ground who hear the airplane.

Typically we do interior noise measurements at climb-out and cruise conditions. There are two basic sets of instrumentations; side-body microphones that measure the noise field on the fuselage's exterior, paired with interior microphones. Inside the cabin, microphones are mounted in the seats roughly at passenger head locations. By comparing inside and outside data, we can determine the attenuation that's experienced as the sound enters the cabin. This enables more effective cabin insulation.

For community noise tests, we have more than 600 microphones on the ground that measure the noise the airplane generates when it does simulated approaches and takeoffs. The airplane actually never touches ground in the flybys, but it simulates approach and take-off conditions.

We have single microphones that measure the level of noise hitting the ground, plus microphone arrays that process hundreds of microphones at the same time and generate maps of noise-source locations. Boeing has fine-tuned that technology to where we can determine if the noise source is coming out of the engine's front or back, the landing gear or maybe off of a flap.

We use the microphones in a technique that's most commonly called "acoustic camera." An analogy will help explain: If you throw a rock in a pond, the waves go out and hit the sides of the pond, each at a slightly different time. Even if the pond has angles, you can move backward from where the wave hits to its point of origin, which is where the rock hit the water. If you have multiple rocks it gets more difficult.

But that's essentially what we do. We have multiple sources of sound on the airplane and computers allow us to back out where each noise comes from. We can make the calculations in just about real-time. Within six minutes after the airplane flies by the microphones, we have a map of the noise sources.

The Montana testing will give us an enormous amount of data in isolating noise origins on the airplane. We're planning static engine testing next summer that will be even more precise in identifying individual noise contributors, such as with the inlets and fan exhaust.