Part of the Boeing Company of Philadelphia, PA, is responsible for the design and development of military rotorcraft. The company is most famous for the CH47 Chinook tandem rotor-bladed helicopter, the latest evolution in a long line heavy lifting helicopters that began with the innovative Boeing Vertol over 50 years ago.
These days Boeing also partners with Sikorsky for the tail section of the RAH-66 Comanche, and with Bell on the distinctive V-22 Osprey tilt-rotor aircraft. Boeing makes active use of LMS software and systems in the Structural, Wind Tunnel, and Acoustics Departments to make major savings in the time it takes to complete their development tasks.
Structural/Mechanical Test Lab
Lead Test Engineer, Mark Bounds, gave an insight into his world. “While completely new designs are rare, there are many upgrade programs and investigations to extend the working life of in-service aircraft, some of which involve major structural changes. I am responsible for the structural testing of the airframe, rotor blades, and major components to ensure that the dynamic performance of each major aircraft variant matches the design predictions. In particular, it is important that any airframe resonances do not coincide with rotor-induced forces.” Mark uses a 128 channel LMS CADA-X system based upon the HP VXI front-end for structural testing and analysis. Fourier Monitor is the base module for data acquisition, with MIMO random, impact and stepped sine being used for modal surveys. Advanced Modal Analysis, Acoustic Intensity, and Time Data Processing (TMON) modules are used for analysis.
According to Mark, the main issues with a Chinook structural survey are problems caused by non-linearities (due to slippage in the rivets, for example) and the massive downwash forces injected into the airframe by the three-bladed contra-rotating rotors. The Chinook rotors turn at a constant 225rpm (rotor pitch being used to change speed) so that the dominant forces are sinusoidal in nature at 3.75Hz and ‘multiples of three’ harmonics. There are also lower level forces at the natural frequencies of the rotors themselves. While random and impact testing methods have their uses for a quick overview, sine testing is the most appropriate technique for system characterization. Only sine testing can reliably describe what is really happening at the operating point where such large forces interact with a highly non-linear structure.
Correspondingly, a so-called “Ground Vibration Test” is more than the traditional low-level test used for fixed wing aircraft. The airframe is suspended at the two rotor hubs by banks of steel springs. Six 2000lbf electro-dynamic shakers then simulate the rotor-induced vibrations: 2 in-phase for vertical shake, 2 out-of-phase to simulate pitch, and the others to simulate lateral and longitudinal forces. Multisine inputs at varying force levels up to 50Hz are used, while typically 240 accelerometers pick-up the responses. Immediately after each test run, a quick modal analysis of the data is made to ensure data integrity, the data is then correlated with the MSC/NASTRAN models by the analysts in the Dynamics group.
“The LMS CADA-X system has the flexibility we need to run the rather special tests we do”, said Mark. “But the real gains for us are in productivity: a test program can now take only five days instead of the six weeks it used to, and analysis is completed in just a tenth of the time.”
Mark also studies manufacturing variances in rotor blades, and dynamic strain effects in transmission units. In the latter, hundreds of combinations of torque and misalignments are tested over six weeks.
Data are collected using the throughput mode in the LMS CADA-X system, and merged with other measurements taken by a Metrum recorder. TMON is then used to analyze the 8GBytes or so of time samples. “The flexibility and speed of analysis in TMON is incredible”, concluded Mark, “I am a great fan of the LMS system”.
Wind Tunnel
The largest wind tunnel within the Boeing Company, the Philadelphia Wind Tunnel Facility is truly impressive. 42 tonnes of air are driven around the 275m circuit by a 12m diameter 10MW fan at speeds up to 400kph at the 6m x 6m throat. It only takes a few minutes to get the air up to the maximum speed. Unlike fixed wing testing, where time invariant conditions are the norm, a rotary wing test is very much more complicated. The 3m helicopter model has a rotor turning at nearly 2000 rpm which means a complex interaction of aerodynamic flow: as the rotor turns each data sample is different to the next. The tunnel is used for routine tests of whole models and components, as well as in a research mode to develop ideas or to troubleshoot problems. Uniquely constructed to be a versatile tool, the tunnel tests fixed wing, rotary wing, and ground effects vehicles. Occasionally, the facility is available for contract work to outside organizations.
The LMS system is used for high-speed data acquisition. Because the very nature of the time dependant problem, huge volumes of test data need to be acquired. Typically, 128 channels of pressure data are acquired at 25kHz bandwidths for a one hour throughput run. There are also many runs in a four-month project (for different angles of attack and model configurations) so the final data volumes are truly awesome. “We plan to add more channels to meet the ever-increasing demand for more data”, said Bill Grauer, Wind Tunnel Manager. “In our department, time is money. The results could only be analyzed by the next day using our homegrown data system - with LMS we can rapidly digest the data during the run and take decisions to re-design the test on-the-fly.” Chief of Testing, Bob Wozniak added, “The LMS system is a popular piece of equipment, and we are getting great support from the local LMS people”.
Acoustics
Rotorcraft can produce high noise levels, both externally as well as within the aircraft. Both helicopters and tiltrotors exhibit high levels of noise inside the cabin, although the sound characteristics are fundamentally different. The interior sound field of helicopters is often dominated by high-frequency (> 2kHz) noise being generated by the transmissions while the sound field of tiltrotors is dominated by low-frequency (< 200Hz) rotor noise.
This low-frequency noise is a result of the close proximity of the blade tip to the fuselage when the rotors are tilted in airplane mode for cruise flight. Tom Zientek, Technical Specialist in the Acoustics department, is working to improve the sound quality for tiltrotors, specifically the V22 Osprey. Tom uses LMS SYSNOISE to model the interior noise and works with an LMS CADA-X Sound Quality system to find the best way to improve the aircraft interior noise. “Reducing troop and pilot fatigue induced by high noise levels is1 becoming increasingly important to our customers”, Tom commented. “I’m using the LMS Sound Quality system to study the importance of the level of each rotor harmonic to improve the sound quality inside a tiltrotors. I’m especially pleased to hear that SYSNOISE will run significantly faster in the next release, as saving time is particularly important when running so many what-if scenarios.”
