777-200LR Flight Test Journal: Archives

19 October 2005

Testing the flutter

Frank Roney, Loads and Dynamics manager, Structures

Keith Wing, lead flight test engineer, Structures

Safety, obviously, is the top consideration in flight testing the 777- 200LR or any other Boeing airplane. The Structures group plays a key role in making sure all flight tests that could affect the airplane platform are conducted within guidelines that guarantee its structural integrity and airworthiness. We don't want to inadvertently damage an airplane in flight test.

The Loads and Dynamics organization determines the airplane's structural requirements based on the configuration definition and basic performance characteristics. The airplane design incorporates these requirements and is tested in two ways: to validate that the requirements have been met, and to ensure that FAA certification criteria have been addressed. One of the first tests we perform is "flight flutter," to make sure the airplane is stable in flight at speeds greater than 270 knots, or Mach 0.70. Flutter is a phenomenon that is related to vibration modes. When an airframe is exposed to high aerodynamic forces, these vibrations could become unstable and grow to a point where the structure may fail. Our goal is to make sure the aerodynamics, weight and stiffness come together to make an aero-elastically stable airplane.

As a result, flight flutter testing is considered "high risk." Consequently, it is the flight test engineer's responsibility to produce a safe test plan. One tool we use to mitigate the risk during flight flutter testing is telemetry. Using telemetry allows us to have a minimum crew (pilot and co-pilot) onboard the aircraft while the balance of the test crew supports the flight from the Telemetry Room where they analyze data and keep the airplane safe.

One of our main safety-related jobs in the flight test program is to prepare a series of structural Temporary Operating Limitations (TOLs). To get an airplane certified, we test all of its performance requirements, including stalls, engine-outs, 2.5 G maneuvers, and so forth. The TOLs set test-limit values so that if the tests are conducted within the prescribed constraints, we can be confident that the maneuvers will be completed safely. If the tests exceed the guidelines, we have to conduct data analysis and/or inspections to make certain that the airplane structure will not be, or has not been compromised.

The initial TOL values are less than or equal to limit load. Limit load is established by a mixture of FAA certification criteria and Boeing design requirements. Coupled with a safety factor, this establishes the strength at which we design and build the airplane. And as much as we aim to get our structure-related testing done inside the TOL values, we know that many tests just can't be done that way. For example, speed and over-weight landing tests need to push the envelope. Or we may be required to go to the edge of 2.5 Gs and stall the airplane. We design the airplanes to take the stress and loads, but only under closely controlled conditions. This is where the Flight Test Structures group gets involved. We monitor the day-to-day testing performed by other flight test disciplines, such as Stability and Control, and make sure what they're doing is safe.

Overall, we believe a lot of the Structures group's talents come to the forefront when we're able to develop a test that works for everybody. We feel the pilots are very comfortable with stability and control and how the airplane feels and how it's controllable. But they depend on us to make sure it's structurally capable. We all use our processes to make sure the airplane is safe. That's one thing I think we can hang our hat on. We have the best processes in place to make sure safety happens, from start to finish.