Introduction to Flight Testing. James W. Gregory

Introduction to Flight Testing - James W. Gregory


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      Another hallmark of successful flight testing is the careful probing of the edges of the flight envelope. Notice how the team approached the uncertain conditions associated with loss of control and buffeting. They gingerly pushed the Mach limits higher and higher, with the hope that any loss‐of‐control situation could be quickly recovered from. Despite the accelerated nature of the test program, the team took the time to carefully analyze the data and debrief after each flight. This was essential for gleaning insight from each test condition and informing the next step in the flight test program. They took an incremental buildup approach – starting from low‐risk flights with known characteristics and carefully advancing to higher‐risk flights, where the flight characteristics were unknown and potentially hazardous.

      Also note how the aircraft was instrumented beyond what a normal production aircraft would have been. In fact, the record‐setting XS‐1 (the first airframe built) was only lightly instrumented compared to its sister ship, the second airframe off the production line, which was targeted for a much more detailed exploration of supersonic flight by the NACA team. This instrumentation is critical for understanding exactly what is happening during flight and preserving a record for postflight analysis. The analytical work was done by a large team of engineers, technicians, and, in that day, human “computers” who did many of the detailed computations of the data (see Figure 1.2).

      After some initial renegade flying by Yeager, the flight test team settled into a rhythm of carefully planned and executed flights. Before each flight they carefully planned the objectives and specific maneuvers to fly on the next mission. The injunction was that the flight must proceed exactly as planned, with specific plans for various contingencies and anomalies. This culture of flight testing is absolutely essential for the safety and professionalism of the process. One common phrase captures this mentality of flight testing: “plan the flight, and fly the plan.”

      This initial foray into exploring the flight testing program of the XS‐1 illustrates many of the hallmarks of flight test programs. We'll next discuss some of the different kinds of flight testing being done today. Clearly, not every flight test program is as ambitious or adventurous as the XS‐1 program, but a common objective is to answer the remaining unknown questions that are always present in an aircraft development program, even after rigorous design work backed up by wind tunnel testing and computational studies.

      There are several different kinds of flight testing, driven by the objective of a particular program. These motivations include scientific research, development of new technologies or experimental capabilities, evaluation of operational performance, or airworthiness certification of new aircraft for commercial use. Other kinds of flight tests include production flight test (first flight of a new airframe of an already certified type, to verify compliance with design performance standards), systems flight test (new systems installed, new external stores on a fighter aircraft that must be tested for separation, new avionics systems), and post‐maintenance test flight. Here, we'll focus our attention on flight testing for scientific research, assessment of experimental technologies, developmental test and evaluation, operational test and evaluation, and airworthiness certification programs. Other perspectives on the different kinds of flight testing are provided by Kimberlin (2003), Ward et al. (2006), or Corda (2017).

      1.2.1 Scientific Research

Photo depicts inspection of pressure-sensitive paint on Purdue University's Beechjet 400 following a flight test in 1999 (depicted left to right are Hirotaka Sakaue, Brian Stirm, and Jim Gregory).

      Source: Photo courtesy of Nate Lachendro.

Photo depicts Notre Dame's Dassault Falcon 10.

      Source: U.S. Air Force.

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