Aerospace Flight Test
Introduction Part I
Flight testing provides a significant challenge to the instrumentation engineer. On rocket and missile systems, data transmission occurs via radio frequency (RF) telemetry, which is often encrypted. On air and rotor craft, a lesser amount of data transmission occurs by RF transmission, while the majority is stored onboard in high capacity digital recorders. RF transmission can become constrained by factors such as flight vehicle orientation, ionization products of rocket and missile system plumes, and rotor craft blade rotation. Thus, unless real time analysis is required, onboard recording is preferred.
Hundreds to more than 1000 data channels of instrumentation may be required on a given flight vehicle. An individual data channel’s frequency response allocation is typically less than 2k Hz. The routing of all of these channels to the RF transmitter(s) or recorder can require tens of miles of cables. Not every data channel is recorded on every flight. Separate measurements may be required for Vehicle State of Health monitoring (e.g., rotor craft Health Usage Monitoring (HUMS).
Strain gages usually constitute a high percentage of data channels with accelerometers, pressure transducers, and temperature transducers all a close second. Microphones are also frequently used for measurements such as cockpit noise. Other miscellaneous sensors include angular (e.g.,synchros) and linear (e.g., LVDTs, potentiometers) displacement transducers, flow meters, heat flux gages, torque transducers, force transducers, and more. Video is also a useful diagnostic. Instrumentation locations for aircraft can encompass the entire fuselage, wing(s), engine(s), landing gear, and empennage.
Vibratory flutter measurements enable the study of the aeroelastic stability of an aircraft so that a safe flight envelope can be defined. This study typically uses accelerometers with DC response (e.g., MEMS – 37XX Series). In some instances, dependent on aircraft size and resonant frequencies, low impedance, ICP® piezoelectric accelerometers can suffice.
Aircraft buffeting measurements result in the definition of vibration induced load inputs to the structure and components. These provide a basis for the generation of structural testing requirements. Buffeting measurements require higher frequency response than flutter and are typically made by ICP® piezoelectric accelerometers. Acceleration rigid body motion is recorded by a triaxial array of high accuracy, DC response accelerometers at the flight vehicle’s center of gravity.
Introduction Part II
Flight vehicle structural design margins are assessed based on a combination of material properties (e.g., yield, ultimate, rupture) as well as fatigue considerations. Strain gages acquire these measurements. In some instances, particularly on rotor craft, structural members have strain gages configured to separate various force and moment components.
Other measurements of interest include load inputs to an air or rotor craft structure associated with landings, ordnance release, rapid application of engine thrust, turbulence, and more. DC response load cells and pressure transducers can be used for these measurements. If the measurement is sufficiently dynamic in nature, ICP® piezoelectric force and pressure transducers may be used. Last, the effect of onboard ordnance on the air or rotor craft in terms of associated aerodynamic and inertial loads and stability must also be measured.
PCB Piezotronics, Inc. manufactures transducers and signal conditioning to handle the majority of the aforementioned requirements. Specifically, PCB® offers a wide array of DC response, charge, and ICP® piezoelectric accelerometers, force transducers, and pressure transducers. In addition, an assortment of microphones, torque transducers, and high-sensitivity strain gages are available. Many of these transducers operate off of MIL-STD- 28 ± 4 VDC. The majority of manufacturers of airborne signal conditioners provide ICP®-compatible constant current supplies. All PCB transducers operate over the normal range of aerospace temperatures with some capable of operating over much wider extremes.
Flight Testing – A Varied and Complex Test Set
These web pages documents many of the sensors and signal conditioners offered by PCB Piezotronics to the flight test community. It is complemented by PCB’s other aerospace and defense sensors for other applications such as aerospace vehicle ground testing, environmental testing, Health and Usage Monitoring (HUMS), fuze/safe and arm, and blast testing.
Because of the complexity of the flight test application and breadth of PCB’s product line, this site offers the most commonly used subset of PCB’s flight test sensors and signal conditioners. For a complete exploration of other options, we invite inquiries to PCB's application engineering team (see contact information on the bottom of this page). The variety of flight test measurement requirements creates a proliferation of sensor/signal conditioning types. For example, this brief catalog contains, among others, the following sensor types, each of which is targeted at specific flight test procedures:
- Accelerometers for specialized dynamic tests such as flutter and stability/controllability characterization
- Accelerometers for load factor measurement
- High-temperature accelerometers and pressure sensors for measuring engine-excited vibration, as well as combustion and compressor instabilities
- Accelerometers and dynamic force sensors for the measurement of vehicle responses to loads for fatigue, strength, and stiffness/ compliance characterization – many of these sensors may also be used for such specialized testing as aircraft carrier qualification
- Pressure sensors and microphones for characterization of cockpit/cabin, payload and external acoustic environments. These sensors span a variety of pressure dynamic ranges from low-level cabin sound pressure to launch acoustics environments to cyclic pressures capable of inducing high-cycle fatigue
- Accelerometers and dynamic pressure sensors for characterizing the interaction between engines, airborne subsystems and the vehicle structure
- Accelerometers for the measurement of the aircraft and related systems’ responses to mission, such as ordnance firing/release
- Accelerometers for the characterization of ordnance performance, related to the above
Compounding the complexities of meeting such a variety of measurement types, the flight test environment is particularly challenging.
Flight Testing – A Demanding Application
Flight testing presents some of the test community’s greatest challenges. It is extremely expensive, test article availability is inevitably limited, timeframes are often compressed and unpredictable, and the sensors with associated instrumentation have to perform properly on the first attempt, even in rigorous environments. This testing can be a single event or it may encompass multiple tests over months or even years.
Through decades of collaboration with flight test engineers, PCB® has developed a set of sensors and signal conditioners tailored to flight test’s demanding environments. These include:
- Internally amplified (ICP®) triaxial accelerometers, as small as a 0.25 inch cube, that add minimal weight and occupy very little volume n Conveniently packaged signal conditioners that accept poorly regulated on-board power and condition signals from piezoelectric sensors
- DC accelerometers (those that measure down to zero Hz) that include internal power regulation to accept a broad range of power voltages
- Thermally insensitive piezoelectric accelerometers for rapidly changing temperature environments
- In appropriate sensors, integral temperature compensation
- Low profile accelerometers and piezoelectric pressure sensors, for minimal aerodynamic disruption
- Hermetic sensors and contamination-resistant connectors, for wet or dusty environments
- Accelerometers that meet strict outgassing limits, for space applications
- Robust connectors and cables manufactured by PCB®
- Radiation tolerant accelerometers, for space applications
- Accelerometers that tolerate shock load orders of magnitude larger than their maximum dynamic ranges
- Low impedance ICP® sensors and instrumentation that maximize electromagnetic interference rejection
Sensors that include built-in filtering to identify customer defined performance or to protect integral amplifiers from saturation, for specific applications
With all these challenges, PCB® recognizes the importance of working closely with flight test instrumentation engineers and sharing lessons learned over the years. In fact, many of our “flight test” sensors started their lives as “specials” designed and built for specific flight test programs.
PCB's Offering to the Flight Test Community
Building upon a foundation of one of the world’s largest and most diverse sensor and related electronics product lines.
- Acceleration from DC (e.g., due to load factors and gravity) to very high frequencies across dynamic ranges measured in micro-g’s up to a maximum of 120,000 g’s
- Pressure from acoustic levels to 100,000 psi or more, and from DC to hundreds of kHz
- Force and torque sensors for both static and dynamic measurement applications
- TEDS (Transducer Electronic Data Sheet) on selected sensors and signal conditioners for large number of channel test systems minimize human record keeping errors
Complementing PCB’s sensor line is signal conditioning, specifically designed for aerospace vehicle power availability, severe vibration environments, challenging EMI conditions, constrained space requirements, and the temperature extremes encountered in flight testing.
PCB’s instrumentation and electronic engineering staff is experienced in design for flight test applications. We can quickly and efficiently modify sensors and electronics for specific or unique flight requirements, when necessary. PCB’s long standing commitment to Total Customer Satisfaction extends to the willingness to devote engineering and manufacturing resources to such unique and challenging requirements.