Acquiring data during flight test
Greg Reed, Contributing Technical Editor -- Test & Measurement World, 4/11/2008 2:27:00 PM
At this year's Aerospace Testing, Design & Manufacturing Expo Europe in Munich, Germany, Dr. Patrick Walter will be presenting a workshop on flight-test instrumentation systems. Walter, a professor of engineering at Texas Christian University and the senior measurement specialist at PCB Piezotronics, recently described to T&MW some of the challenges involved with collecting data during flight test.
Q: What are some primary instrumentation challenges faced when acquiring physical measurements during flight test?
A: For an aircraft, we might encounter temperatures on the tarmac as high as 54°C (130°F). In flight, these external temperatures may drop to –35°C (–30°F). Rocket systems encounter much greater temperature extremes. Aside from influencing the various sensors, these temperature variations can result in internal condensation or ice formation, which can lower electrical impedances in the components of the onboard measurement system.
Strain and acceleration are induced into the flight system’s structure at various times because of turbulence, engine and motor burns, jettisoning of equipment, ordnance release, sonic booms, staging separation, and other events. Acoustic noise is transmitted through the air frame or structure to its interior. Some of the events in the preceding list that induce acceleration can result in high levels of pyrotechnic shock.
At both anticipated and unanticipated times in flight, radars may illuminate the flight vehicle. In addition, RF transmissions can be received by and emitted from the vehicle. Thunder and lightning storms, as well as snow and sleet, can be randomly encountered, producing mechanical, thermal, and electrical inputs to the flight vehicle.
Considering only rocket systems, we may have partial loss of signal (LOS) while transmitting desired measurements to the ground. Plume interference during motor burn can be one of the causes. During atmospheric reentry, the extreme temperatures due to aerodynamic heating produce plasma causing progressive LOS and eventual black-out (all communications are temporarily lost). Finally, flight test vehicles can contain as much as 50 miles of routed instrumentation wiring. A significant number of these wires carry only millivolt-level signals, exacerbating the challenge of maintaining an adequate signal-to-noise ratio.
Q: What are some dynamic and electrical models for transducers?
A: While transducers often appear as simple electromechanical components, they are complex systems with both high-frequency and low-frequency constraints. The combination of these constraints is a function of both the electrical and mechanical characteristics of the transducer.
Q: What are some common measurement errors and their root cause?
A: The various numbers of ways in which to acquire bad data are too extensive to even begin to encompass within a comprehensive listing. Nonlinearities, inadequate system amplitude and phase characteristics, response to other adverse environments, improper sensor coupling, and problems in interface cables are just the beginning of the list.
Q: Can you offer some selection criteria for sensors destined for aerospace applications?
A: It is very important to first define clearly what you are trying to measure. Are you supporting a thermal, structural, or acoustic model? Aerospace systems, as a minimum, can be all three of these. Then it is important to define, as well as you can, what other adverse environments are present. Following this, one must clearly establish the response requirements to support any predictive modeling. Answers to the questions only allow the initiation of a discussion with a sensor manufacturer.
Q: What other types of seminars are available for test engineers to benefit further from your expertise this year?
A: On May 20 to 22, PCB will host its annual three-day corporate seminar at its headquarters in Depew, NY. The seminar, “Successful Measurement of Dynamic Force, Pressure & Acceleration,” also encompasses the aerospace community. In addition, through TCU, annual on-site seminars are presented at Lockheed-Martin, Edwards AFB, Kennedy Space Center, Cold Lake (4 Wing) Canada, and elsewhere.
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