Aerospace Ground Test
Ground testing of a new aircraft design or an aircraft that has undergone significant structural modification, is a prerequisite before any flight-test program can begin. In turn, ground testing follows and complements an extensive structural modeling program, which encompasses flight loads, material fatigue, structural dynamics, airborne and structure borne acoustics and more.
A design life or “life expectancy” goal, in flight cycles (takeoffs and landings) or flight hours, is established early in the development of a new aircraft. Due to their extreme operating environments, military fighter aircraft may have design life expectancies only in the high thousands of flight hours. For civilian transport aircraft, the design life goal is typically in the high tens of thousands of flight cycles. Before first flight, a significant number of these cycles are accumulated during ground testing performed on a full-scale aircraft structure.
Knowledge of the anticipated flight load spectrum enables pressure cycling of the fuselage, as well as hydraulic loading of the wings, empennage and other principal structures. Large data acquisition systems enable monitoring of the applied pressures and loads and resultant structural deflections and strains. Periodic inspections with nondestructive testing equipment often accompany this process to monitor for any resultant crack growth.
As part of the ground test program, ground vibration testing (GVT) is also performed. The purpose of GVT is to obtain experimental vibration data for the entire aircraft structure for validating and improving its structural dynamics model. Assessment of the aircraft structure’s linear or nonlinear behavior is also performed. During GVT testing, the aircraft must be in a configuration as close as possible to flight test. Successful correlation of structural frequencies and mode shapes between GVT and structural modeling enables assignment of accelerometer mounting locations to support the subsequent flight test program. In addition, GVT results provide inputs to predict flutter behavior to ensure the safety of these flight tests.
New engine development to support aircraft design requires its own test program beginning in test cells and culminating with “on-wing” testing in a test bed aircraft. Once the qualified engine(s) become integrated as part of the new aircraft, their noise influence on their surrounding environment and the passenger cabin must be assessed. Therefore, airborne and structure-borne acoustic transmission paths are also characterized as part of the aircraft ground test program.
The preceding has focused on aircraft. Ground testing of rotary wing aircraft and space launch vehicles follows the same pattern. Testing and structural modeling must correlate on the ground to assure structural integrity and safety in subsequent flight tests
Why PCB for Ground Test?
PCB Piezotronics offers various sensors and signal conditioners to the Aerospace Ground Test community. Due to the complexity of Aerospace Ground Test applications and the breadth of PCB’s product line, this catalog offers the most commonly used subset of PCB’s Ground Test sensors and signal conditioners. For a complete exploration of other options, we invite inquiries to PCB's Application Engineering team.
PCB’s service to ground testing applications encompasses sensors and signal conditioning that
- Provide reliable, cost-effective service
- Are specifically tailored to the type of testing involved
- Interface effectively with the data acquisition systems being used by our customer base
For convenience, we categorize our products into four Ground Test application areas:
GVT Ground Vibration Testing
PCB®’s focus on tailoring sensors and signal conditioning to specific applications is illustrated by our line of structural test accelerometers and signal conditioners. These products are designed for convenience, accuracy and reliability, whether the test involves extracting the natural frequencies, vibratory modes shapes and damping of a complete vehicle or an operating subsystem or requires only “quick and dirty” component vibration troubleshooting.
Since this kind of testing typically involves large channel counts, PCB’s GVT specific products include:
- A line of cost effective phase-matched structural accelerometers that feature accessories for simplified mounting. These accelerometers are available in a range of form factors for both single axis and triaxial measurements. High sensitivity piezoelectric force transducers are also available for vibration force control of compliant structures such as satellites.
- Cable management and signal conditioning systems including patch panels, multichannel cable, and bank switch systems. These save substantial installation and data management time and money, while allowing large channel GVT’s to be accomplished with a limited number of data acquisition channels.
- A full line of instrumented modal hammers specifically designed for modal test excitation and force measurement, as an alternative to shaker excitation.
Static and Fatigue Testing
Although PCB® is best known for our dynamic measurement sensors and signal conditioning, we also offer a complete line of fatigue-rated strain gage-based load cells for static and fatigue testing. These are available in either single bridge configurations or dual independent bridge configurations. The latter configuration is used in situations in which measurement redundancy is required. Similarly, DC response pressure transducers are available for aircraft fuselage pressurization cycling testing.
PCB®’s line of piezoelectric force sensors complements our strain gage-based load cells for those applications that require measurement of dynamic (high frequency) forces. These piezoelectric force sensors can be integrated with strain gage-based load cells in a system that measures complex static loads plus high frequency dynamic loads. Again, PCB®’s Application Engineering team is available at your convenience to discuss these measurement system challenges.
Reliability and Functional Testing
A great deal of aerospace ground testing involves exposing systems to realistic loading and operating conditions “on the bench”. PCB®’s line of cost-effective fatigue and non-fatigue rated load cells and torque sensors are widely integrated into bench tests that measure operating loads and torques in aerospace systems. These sensors include reaction and rotating torque sensors with their associated signal conditioners. Among PCB®'s family of torque sensors, the Torkdisc® enables high bandwidth torque measurements in space constrained environments.
As mentioned above, PCB®’s ability to supply both piezoelectric and strain gage-based load cells allows us to uniquely address complex loading situations that involve combinations of high static plus dynamic forces and torques. For instance, integrating dynamic and static force/torque sensors provides our development engineering customers the ability to measure static loads related to an actuation system’s primary function simultaneous with high frequency or fleeting dynamic pressure or force transients that cannot be accurately characterized by strain gage-based sensors.
PCB®’s family of load and torque sensors is complemented by our complete family of AC and DC response accelerometers, as well as our line of dynamic and static pressure sensors. In some cases, even PCB®'s complete line of precision microphones are enlisted, to sense the condition of systems under test or to measure the acoustic environment of the airborne systems.
Acoustic Testing and Certification
Complete line of 1/4", 1/2" and 1" microphones are available in two versions:
- Externally Polarized
The prepolarized type of microphone offers significant savings in signal conditioning costs. While the performance of these microphones is essentially identical to their conventionally powered, externally polarized counterparts, the microphones and their preamplifiers are designed to interface with constant current (e.g. PCB®’s, ICP®) signal conditioners.
Because ICP® signal conditioners are so widely employed for other sensor types, they are an order of magnitude less expensive than conventional microphone power supplies. The cables and connectors used with prepolarized microphones are also substantially less expensive than those employed with externally polarized microphone power supplies. PCB® offers microphones and acoustic pressure sensors that are useful for specialty measurement applications, such as:
- PCB®’s unique line of “HT” microphones and preamplifiers for acoustic characterization in environments as hot as 120 ºC/250 ºF.
- PCB® Model 377A26 probe microphone, high temperature acoustic measurements.
- PCB® Series 106 acoustic pressure sensors are widely used for such applications as launch vehicle acoustic environment characterization.
PCB’s 40+ year history of reliability and responsiveness in providing dynamic measurement sensors now extends to this complete line of acoustic testing products.
PCB uses a range of measurement technologies:
- Piezoelectric for accelerometers, force and pressure sensors
- Capacitive for microphones and DC accelerometers
- Resistive for load cells, pressure sensors and accelerometers
These technologies allow our sensors to measure the performance of mechanical systems across both wide dynamic/amplitude ranges and broad frequency ranges. Whatever your aerospace ground test application, PCB is likely to be a single supplier for all of your mechanical sensing requirements.