An extract from ‘Introduction to Flight Test Engineering‘, AGARDograph 300 Volume 14, The Research and Technology Organisation (RTO) of NATO, September 1995 by F.N. Stoliker
(Contributed by Group Captain Malteesh Prabhu (Retd), Group Director (Flt Testing), NFTC, ADA, Bangalore and Vice President, SFTE India Chapter, Bangalore)
In the beginning of flight testing the major issues were: How long can I stay in the air? How high? What speed can I reach? There was no such specialist as the FTE we know today. One man often performed many functions, sometimes he was designer, manufacturer, mechanic, pilot, tester, FTE, etc., all at the same time. In the first three decades of aviation, flight testing was almost entirely aimed at performance, stability and control. There were hardly any “aircraft subsystems” to be tested besides those for propulsion and control. The exceptions were the machine-guns and the cameras in the military aircraft. The forward firing machine-guns were firing through the propeller plane and, therefore, had to be properly synchronized to the propeller position, or else ….
Aeronautical development up to World War I (WWI) proceeded initially through the efforts of a few individuals. They could identify and “understand” (more or less) the deficiencies of their aircraft, which they rectified by empirical modification. However, when (comparatively) vast numbers of aircraft were deployed during WWI, flown by pilots who had little or no aeronautical training, the deficiencies in contemporary designs became embarrassing. As a result, there was considerable pressure to find solutions.
This prompted a variety of investigations into the problems encountered, conducted by men with more formal scientific (but not necessarily aeronautical) background. From that period and through the twenties and thirties there appears to have been a huge increase in scientific interest in aeronautical matters and in attempts to describe aerodynamic phenomena (e.g., flow fields, lift/ drag characteristics, boundary layers, etc.) in mathematical terms. In particular, the theory of aircraft stability and control was developed by many contributors. It is hard to tell when the FTE function emerged. Was it somewhere in the late twenties? The driving forces behind the emergence of flight test engineering as a discipline, were probably the development of the theory of flight, particularly that of stability and control, and the development of a scientific/mathematical basis for predicting the flight behaviour of a given aircraft design.
While many of the theories of aerodynamic phenomena could be validated via wind tunnel testing, validation of overall aircraft behaviour required full scale flight testing. The development of underlying theories and, later, specific requirements, demanded testing that could not be undertaken, unaided, by the pilot. A new breed of engineer – the FTE – arose who understood the theories, and could undertake the necessary measurements in flight (aided by primitive “flight test instrumentation”). Thus, the FTE acted as the interface between theory and practice, working in very close cooperation with the designer(s). He took over many of the technical and scientific functions but worked closely with the pilot who was responsible for the actual flying and the flight safety. The pilot was also responsible for providing a subjective evaluation of the aircraft flying qualities, system operation, and operational utility. The function of FTE was probably further re-enforced by the introduction of quantitative general requirements in respect of flying qualities (MIL-SPECs in the USA, AvP 970 in the UK), and the introduction of Federal Aviation Regulations (FAR) and Joint Airworthiness Requirements (JAR, Europe) and the need to demonstrate that they were satisfied. Moreover, by identifying specific criteria to be met, those requirements must have strongly influenced the nature of the flight test programs conducted.
The many developments in aircraft and aircraft equipment design and the increasing scope and sophistication of predictive techniques modified the scope of flight testing and the role of the FTE. For testing the performance of new subsystems, new methods had to be developed which required close cooperation with the system experts involved. It was the input from these specialists that forced the FTE to develop new flight test methods and techniques.
This was the beginning of a period in which the testing of subsystems would gradually start to take a great percentage of the total testing time. Especially when electronic systems for various purposes made their appearance the scope of flight testing started changing drastically again. In-flight assessment of structural and electronic system performance introduced new disciplines, and theoretical treatment of the traditional disciplines of stability and control and performance became more and more complicated. As a result, the FTE could no longer remain an expert in all matters being tested and he started to depend on expertise from specialist departments or organizations. The FTE became more of a technical manager of the flight test program with responsibility for detailed specification of test points and analysis/interpretation of the results being disseminated to specialists in each discipline. As new technologies were incorporated, and the technical bases of design became ever more diverse and complicated, the FTE became less of an airborne expert and more of a person responsible for coordinating and managing all the various activities involved so that the objectives of a particular series of flight tests are met.
One of the early books regarding certain aspects of flight test engineering was “Notes on Practical Airplane Performance Testing” by G.B. Patterson, published in 1919. It was an attempt to define standardized procedures for performance flight testing. Another book in the same year was NACA Report No. 70, “Preliminary Report on Free Flight Tests” by E.P. Warner and F.H. Norton. Other early books were “Flight Testing of Aircraft”, by E.H. Barksdale, published in 1926, and “A Manual of Flight Test Procedure”, by W.F. Gerhardt and L.V. Kerber, in 1927. The latter book defined standardized procedures for performance flight testing. E.L. Pratt wrote “Flight Test Manual” in 1928. Through such books, and the introduction of courses in aeronautical engineering in universities a cadre of engineers was educated to understand the theories underlying flight. Many of those recruited by industry and government organizations were assigned to flight test work, leading to the establishment and development of flight test engineering as a specific discipline.
The development of flight test engineering received its major impetus during and immediately after WWII. A significant breakthrough was the publication of the NACA treatise by William Phillips, “Application and Prediction of Flying Qualities”, published in the late forties. This was the first recognized attempt to express aircraft flying qualities in quantitative terms. Prior to this date, aircraft designers and developers had to rely on the test pilot’s qualitative comments to relate actual flying qualities to those predicted by design and wind tunnel data. Following the publication of Phillips’ work, several aerodynamic text books were published which presented aerodynamics in terms usable by the FTE (i.e., related to the real aircraft). These include “Airplane Performance Stability and Control” by Perkins and Hage and “Aerodynamics of the Helicopter” by Gessow and Myers. The first text book devoted entirely to the field of flight test engineering was “Flight Testing” by Benson Hamlin, published in 1946.
During WWII the need arose to give the pilots and engineers who were closely engaged in the execution of flight test programs, a formal, specialized training in flight test techniques and procedures. In the UK the “Test Pilot’s School” was founded at Boscombe Down in 1943. It was to “provide suitably trained pilots for test flying duties in Aeronautical R&D Establishments within the Service and the Industry”. It was later renamed “Empire Test Pilot School” (ETPS). The ETPS was soon followed by the US Army Air Force’s Flight Test Training Unit in September,1944, and then by the US Navy. The US Air Force school at the re-named Wright-Patterson Air Force Base (AFB), Ohio, was moved to Edwards AFB in 1951. In France the “Ecole du Personnel Navigant d’Essais et de Réception” (E.P.N.E.R.) was opened in Istres. Although these schools were called Test Pilot’s Schools, they all adapted courses for FTEs. The first “fixed wing” course for FTEs started in February 1973 at Edwards AFB and was soon followed by the ETPS in 1974. The first class for rotary aircraft was offered by the ETPS in 1975. Flight test engineering became recognized as a distinct academic discipline. By 1970 it was included in the aeronautical engineering departments in several universities.
An important part of the FTE’s job is to reduce flight test data and get meaningful results out of it. In the beginning of flight testing the main source of flight test information was the flight test pilot’s subjective judgement. At best the pilot had some basic instruments the readings of which he could jot down on his kneepad if the manoeuvre permitted that. NACA, in 1930, was probably the first to use special flight instruments to record measurands of interest during flight tests for the determination of aircraft handling qualities. At a later stage, cameras were used to photograph or film the pilot’s instrument panel or other panels specially installed in the test aircraft for the purpose of the flight test. After WWII special flight test instruments became available called photo trace recorders. Soon, development of transducers or sensors also began which could convert a physical phenomenon into an electrical output and then record them on magnetic tape (later on computer hard disks). The number of parameters recorded and analysed during flight tests increased sharply from a few tens just after WWII to some tens of thousands for the flight testing of present-day aircraft. The tools used for flight test data analysis advanced from the initial hand-cranked mechanical calculator and the slide rule to highly sophisticated digital computers today.
The engineering simulator has also become an indispensable tool for the FTE. Its most important use is the prediction of safe flight envelopes during the flight test program. Furthermore, it supports flight test preparation and crew training, development of safe and efficient test methods and manoeuvres, and final data analysis. Simulator technology and CFD have improved the ability of the FTE to predict potential problem areas in the flight envelope. They also allow pilots and engineers to familiarize themselves with the predicted aircraft characteristics and allow evaluation of some of the man-machine interfaces.
As seen in the previous paragraphs, changes in technology, requirements and test philosophy have brought about drastic changes in flight test methods and the scope of the work of the FTE during the past few decades. The FTE, as we know him today, has become the central figure in any flight test program. He is responsible for the preparation, coordination, organization, execution of the program and for reporting the results. Looking back, it can be said that the evolution of aviation, since that memorable event of the first powered flight in 1903, has been enormous. So has been the evolution of the profession of FTE. Being an FTE is an interesting, challenging and rewarding job. The somewhat older FTEs have witnessed a big evolution in flight test tools, techniques and capabilities. And maybe the younger FTEs are in for an even more interesting period to come. For those younger engineers a word of caution: Never become so enamoured with the sophisticated, fully automated and computerized data acquisition, processing and analysis systems that you allow yourself to be relegated to the roles of observer and computer operator. Intelligent engineering analysis, as can be provided only by a knowledgeable FTE, will always be essential in the test and evaluation of aircraft systems.
Safe Flight Testing!
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