Flying tests

Like many manufacturers, aircraft manufacturer Hawker Beechcraft (Wichita, KS, www.hawkerbeechcraft.com) must make small changes to its products to improve quality or replace obsolete parts. Senior electrical engineer Rex Pawlak and other instrumentation engineers have developed flexible data-acquisition systems that flight-test engineers use in flight to evaluate such design changes.

Flight-test engineers use the systems to measure parameters such as air speed, altitude, fuel pressure, and oil pressure using either the aircraft’s built-in sensors or independent sensors. Each system includes a notebook computer and, when needed, a signal-conditioning chassis for the independent sensors (figure). “We prefer to use sensors with digital outputs over those with analog outputs,” said Pawlak. “Digital outputs require fewer wires.” A test may include measurements using strain gages mounted on a wing spar or other structural component.

A flexible data-acquisition system makes in-flight measurements on aircraft.

If test requirements go beyond the capabilities of the built-in sensors, then additional sensors will be fitted to the aircraft. The engineers will connect thermocouple and RTD probes for temperature measurements and strain gages for mechanical measurements to a chassis containing signal-conditioning modules. In addition, they may also connect pressure sensors to the computer through an RS-232 link configured in a daisy-chain topology. A test may require 20 to 30 pressure transducers, although some need only two to five. After engineers specify the sensors, technicians install them in a test aircraft. Installing additional sensors often requires a technician to install tubes and connectors to systems such as fuel lines. Engineers will also customize the PC software to capture, display, and record the specified data. “We will customize the software to best match the needs of a flight-test engineer,” said Pawlak. “For example, some prefer analog [dial] displays, while others prefer digital [numeric] displays.”

During a flight, a test engineer monitors the measurements on the screen and records data when the aircraft is “on condition.” For example, if a test plan calls for pressure, temperature, or strain measurements at 35,000 ft, the test engineer won’t record data during ascent and descent. Often, a test plan calls for test engineers to record data during multiple conditions.

“When designing a data-acquisition system, always look for the simplest solution that meets the test requirements,” said Pawlak. “RS-232 is the least invasive and most convenient solution, followed by monitoring the ship’s internal bus. Use external sensors and signal conditioning only when needed.”