Digital Electronic Engine Control

NASA pioneered in the development and validation of advanced com­puter-controlled electronic systems to optimize engine performance

across the full flight envelope while also improving reliability. One such system was the Digital Electronic Engine Control (DEEC), whose gene­sis can be traced back to NASA Dryden work on the integrated flight and engine control system developed and evaluated in a joint NASA-Air Force program that used two Mach 3+ Lockheed YF-12C aircraft. The YF-12C was a cousin of the SR-71 strategic reconnaissance aircraft, and both air­craft used twin Pratt & Whitney J58 afterburning engines. As the SR-71 neared Mach 3, a significant portion of the engine thrust was produced from the supersonic shock wave that was captured within each engine inlet and exited through the engine nozzle. A serious issue with the oper­ational SR-71 fleet was so-called engine inlet unstarts. These occurred when the airflow into the inlet was not properly matched to that of the engine. This caused the standing shock wave normally located in the inlet to be expelled out the front of the SR-71’s inlet, causing insuffi­cient pressure and airflow for normal engine operations. The result was a sudden loss of thrust on the affected engine. The resulting imbalance in thrust between the two SR-71 engines caused violent yawing, along with pitching and rolling motions. Studies showed that strong vortexes produced by each of the forward fuselage chines passed directly into the

inlets during the yawing motion produced by an unstart. NASA efforts supported development of a computerized automatic inlet sensing and cone control system and helped to optimize the ratio of air passing through the engine to that leaving the inlet through the forward bypass doors. Dryden successfully integrated the engine inlet control, auto-throt­tle, air data, and navigation functions to improve overall performance, with aircraft range being increased 7 percent. Handling qualities were also improved, and the frequency of engine inlet unstarts was greatly reduced. Pratt & Whitney and the Air Force incorporated the improve­ments demonstrated by Dryden into the entire SR-71 fleet in 1983.[1257] The Dryden YF-12C made its last NASA flight on October 31, 1979. On November 7, 1979, it was ferried to the Air Force Museum at Wright – Patterson AFB, OH, where it is now on display.[1258]

The broad objective of the DEEC program, conducted by NASA Dryden between 1981 and 1983, was to demonstrate and evaluate the system on a turbofan engine in a high-performance fighter across its full flight envelope. The program was a joint effort between Dryden, Pratt & Whitney, the Air Force, and NASA Lewis Research Center (now the NASA Glenn Research Center). The DEEC had been commercially devel­oped by Pratt & Whitney based on its experience with the J58 engine during the NASA YF-12 flight research program. It integrated a vari­ety of engine functions to improve performance and extend engine life. The DEEC system was tested on an F100 engine mounted in the left engine bay of a NASA Dryden McDonnell-Douglas F-15 fighter. Engine – mounted and fuel-cooled, the DEEC was a single-channel digital control­ler. Engine inputs to the DEEC included compressor face static pressure and temperature, fan and core rotation speed, burner pressure, turbine inlet temperature, turbine discharge pressure, throttle position, after­burner fuel flow, and fan and compressor speeds. Using these inputs, the DEEC computer set the variable vanes, positioned the compressor air bleed, controlled gas-generator and augmentor fuel flows, adjusted the augmentor segment-sequence valve, and controlled the exhaust noz­zle position. Thirty test missions that accumulated 35.5 flight hours were flown during the 2-year test program, which covered the opera-

tional envelope of the F-15 at speeds up to Mach 2.36 and altitudes up to 60,000 feet. The DEEC evaluation included nearly 1,300 throttle and afterburner transients, more than 150 air starts, maximum accelerations and climbs, and the full spectrum of flight maneuvers. An engine noz­zle instability that caused stalls and blowouts was encountered when operating in afterburner at high altitudes. This instability had not been predicted in previous computer simulations or during ground-testing in NASA high-altitude test facilities. The instability problem was eventually resolved, and stall-free engine operation was demonstrated across the entire F-15 flight envelope. Faster throttle response, improved engine air – start capability, and an increase of more than 10,000 feet in the altitude that could be attained in afterburner without pilot restrictions on throt­tle use were achieved.[1259]

DEEC-equipped engines were then installed on several operational USAF F-15s for service testing, during which they showed major improve­ments in reliability and maintainability. Mean time between failures was doubled, and unscheduled engine removals were reduced by a factor of nine. As a result, DEEC-equipped F100 engines were installed in all USAF F-15 and F-16 aircraft. The DEEC was a major event in the his­tory of jet engine propulsion control and represented a significant tran­sition from hydromechanical to digital-computer-based engine control. Performance improvements made possible by the DEEC included faster throttle responses, improved air-start capability, and an altitude increase of over 10,000 feet in afterburner without pilot restrictions on throttle use. Following the successful NASA test program, the DEEC went into standard use on F100 engines in the Boeing F-15 and the Lockheed F-16. Pratt & Whitney also incorporated digital engine control technology in turbofan engines used on some Boeing commercial jetliners. The lin­eage of similar digital engine control units used on other engines can be traced to the results of NASA’s DEEC test and evaluation program.[1260]