Application Summary
Military aircraft, rotorcraft, and selected fixed-wing aircraft are often adapted to accommodate a wide variety of armament and other accessory gear that they were not originally designed to handle. The nature of continually changing peacetime and battlefield requirements puts a severe demand on engineers to make these components fit and operate efficiently and safely within a limited development time. Although the aircraft are generally designed to handle the additional payload, placing new components where they will not interfere with vehicle performance is paramount. For example, the airframe and wing structures are critically supported and reinforced, and any extra weight or a vibrating load in the wrong place could cause early failures.

Potential Solution
Structural engineers perform special tests to ensure that the retrofitted components are properly mounted and secure and the aircraft are safe. The engineers typically use multi-channel data acquisition systems and accelerometers for modal analysis to measure vibrations before and after the components are mounted.

IOtech’s Solution
Structural dynamics engineers in the aircraft industry have preferred IOtech ZonicBooks for several years to conduct modal analyses for several reasons: small size, portability, accuracy, sampling speed, and quick and easy set up. They also like the eZ-Analyst software because it is easy to use and lets engineers manipulate the data in many ways.

Rotorcraft industry engineers explain that helicopters are especially susceptible to vibrations generated by the aerodynamics of rotating main and tail rotors at discrete excitation frequencies that relate to the number of blades and rotational speed. When new components are added to a helicopter, especially at an extremity such as the tailboom or wings, the engineers conduct modal tests to ensure that resonances are not present and that the vibrations are not high enough to adversely affect the fatigue life of structures and components, which in turn, can cause early failures.

“For example, we currently use the ZonicBook to look for resonances using modal tests,” says one engineer. The different payloads, which include sophisticated electronic components, put a stress on the wings and struts. The combination of fixed with rotating components such as the main rotor can set up some severe vibrations. For example, on one particular aircraft, a new component had to be installed on the tip of the wings. “We conducted some impact tests to see if we had any resonance problems with wing frequency and rotor frequency,” says the engineer. “Sometimes a resonance does appear, but it might be of such low amplitude that it presents no problem. Textbooks usually warn that resonances must be avoided; however, on some craft, a few resonances can be tolerated when they appear during a transient condition such as during rotor speed ramp up. But no resonances are allowed to exist within the rotors operational speed envelope. Otherwise, components need to be redesigned or vibration isolation must be added. It is not unusual for a helicopter to carry several hundred pounds of vibration isolation equipment.”

The modal testing usually takes two to three hours; but modal analysis takes longer, and the time depends on the specific problem. An 8-channel system uses the first channel as reference, which may be an impact force or random or sinusoidal excitation. For relatively small and stiff structures, impact testing yields satisfactory results. The other 7 channels (eZ-Analyst software can support up to 56 channels) measure accelerometers strategically placed around the wings, struts, and other structures. When more than 7 channels are needed in one test, the accelerometers are relocated and the modal testing continues in the same manner.

“After the ground vibration tests are performed and flight safety is met, up to 56 channels of dynamic signal measurements can be taken with the ZonicBook on board. Its compact size and design for portability are perfect qualities for the job, and its ability to run from a 12V battery makes it ideal for flight tests,” claims the engineer.

Acceleration and vibration are often recorded during flight. The wiring and sensor installation for up to 8 channels can take as long as two hours. The ZonicBook can be simply strapped down in the passenger seat or fastened to the floor with Velcro. It can be operated by the flight test engineer or from a remote location, such as the co-pilot using an on-off remote switch. Flight measurements from all accelerometers at a given flight maneuver can made by running the analyzer continuously for about 60 seconds, or by partitioning it into several bursts to obtain the desired resolution. The amplitudes typically fall in the range of 0.02g pk-pk to 0.7g pk-pk in the cabin, but they can be as high as 5g pk-pk at extremities. Typically, the frequency range of interest is one to 100 Hz, which encompasses several harmonics of the main and tail rotor. For rotating shafts the frequency range is much higher, such as 200 Hz and beyond. These measurements are needed for an Airworthiness Release of the new component or a redesign if necessary.

The eZ-Analyst software is also used for flight vibration measurement. “The eZ-Analyst software is easy to use and provides all the data I need to conduct a successful analysis,” says the engineer.” I particularly like its ability to let me manipulate the data in any manner I like. We use the ZonicBook and eZ-Analyst software for all the helicopters and a number of fixed-wing aircraft as well.”

Conclusion
Structural dynamics engineers frequently retrofit military helicopters and fixed-wing aircraft with a variety of weapons and electronic systems that could potentially alter the dynamic behavior of the platform structure and generate unacceptable vibrations. To keep the aircraft safe, however, engineers use an IOtech ZonicBook and accelerometers to perform ground modal and flight tests that locate any trouble spots so they can be fixed, immediately and effectively.

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