The study of aerodynamic phenomena encompasses a diverse range of acoustic and pressure testing requirements within inherently extreme environments, including aircraft altitude, airspeed and laminar flow measurements. The ultimate goal of such testing is the development of more lightweight, higher performance aircraft with greater fuel efficiency and reduced atmospheric pollution. Testing parameters must closely mimic actual in-flight conditions, factoring in such variables as external temperature, pressure, wind speed and meteorological factors, while closely adhering to budgetary constraints and internal production schedules.

Jet in a wind tunnel test .Wind tunnel testing is therefore a common means of achieving these objectives. Manufacturing time and cost limitations also create emphasis on the use of sensors and instrumentation with proven pedigree to effectively support these applications.

For more than 60 years, Meggitt Sensing Systems has been at the forefront of the design and manufacture of piezoresistive pressure transducers for aerodynamic measurements. For flight test airflow dynamic pressure measurements, the Endevco model 8510C series, a rugged, miniature, high-sensitivity, high resonance gage piezoresistive pressure transducer with 225 mV full scale output, is often specified at the 15 psi range. Testing advantages associated with use of the series include a highly compact and lightweight package, with a face diameter of just 3.86 mm and a weight of 2.3 grams; excellent non-linearity of < 1% to three times over-range with 4x minimum burst pressure; 20,000 g shock resistance; high stability in thermal transients; and wideband frequency response. Its proprietary four-arm strain gage bridge and sculpted silicon diaphragm design produces twice the sensitivity of traditional flat diaphragms, resulting in improved resolution. Integral hybrid temperature compensation provides stable performance over a range of -18?C to +93?C.

The model 8510C series also features a 10-32 UNF-2A mounting thread and comes standard with a 30 in integral cable. Models include a vent tube that can be connected to any standard reference manifold used for differential pressure measurements or referenced to ambient atmosphere.

For small-scale wind tunnel testing, Meggitt offers the Endevco model 8507C series of rugged, miniature high-intensity acoustic microphones and piezoresistive pressure transducers. Available in ranges from 1-15 psig with 300 mV full-scale output and minimum 20x burst pressure, the Endevco model 8507C features the same active four-arm strain gage bridge design as the 8510C, with maximum sensitivity, high resonance frequency and wideband frequency response. Self-contained hybrid temperature compensation provides stable performance over a range of -18?C to +93?C, with excellent measurement stability during thermal transients. The sensors also feature linear performance to 3x range and high shock resistance.

The 2.34 mm cylindrical housing of the model 8507C is designed for space constrained installations, or those not requiring threaded mounting. Its small size even permits flush mounting on curved surfaces for use in supersonic jet airflow fields; hypersonic, transonic and 'quiet flow' wind tunnel testing; and turbulent airflow measurements.

Wind Tunnel Computer Simulation

For wind tunnel testing in higher-temperature environments, the Endevco model 8540 series is an alternative solution. These rugged, miniature high-sensitivity piezoresistive absolute pressure transducers offer internal sensitivity compensation and zero trim for high-temperature accuracy to +260?C and diminished lifetime operation to +316?C with excellent linearity and high resonance frequencies for static and dynamic pressure measurements. In addition, the series exhibits low photo-flash sensitivity and high stability during temperature transients.

Available in ranges from 15-500 psia, Meggitt's Endevco 8540 series transducers also employ silicon strain gages, bonded to a micro-machined silicon diaphragm and housed in a rugged stainless steel case, for maximum sensitivity and wide frequency response, and with full scale output to 300 mV. The 8540 series features a 3.8 mm face diameter, facilitating flush mounting for aircraft skin pressure measurements. Its high-frequency response also allows it to be used within other aerospace testing applications such as rocket, jet engine and auxiliary power unit monitoring.

In addition to pressure transducers, Meggitt also offers a full range of acoustic sensors and instrumentation in support of aerodynamic measurements. Microphone and preamplifier combinations, available in 0.25 in and 0.5 in versions with TEDS onboard memory storage capabilities, are often specified to measure larger channel airflow conditions. Low-profile surface microphones, such as the Endevco model EM40PS, are also used to support wind tunnel noise testing, turbulent flow measurements, acoustic radiation surface measurements and wind-induced airborne vehicle noise monitoring applications.

(Article courtesy: Meggitt Sensing Systems, USA. The company is represented in India by Aimil Ltd, New Delhi. Email: marcom@aimil.com)