A work in progress...
An unexpected F-111 issue was the significant time and effort required to develop the engine inlet, an unintended consequence of combining a little-used quarter-round configuration bounded on one side by the fuselage and its top by the underside of the wing with the first attempt at adding an afterburner to a turbofan jet engine, the Pratt & Whitney TF30. It was a non-afterburning low-bypass turbofan engine being developed for the Navy’s subsonic Douglas F6D Missileer. The turbofan configuration—the first few stages of the compressor were significantly bigger in diameter so air accelerated at its periphery bypassed the combustion process and flowed aft past the turbine section into the tailpipe—was developed to power civil airliners, reducing fuel consumption in cruise. Fuel efficiency was essential for both the USAF and USN applications, long range for the former and time on station for the latter. The Air Force’s Mach 2 requirement necessitated an afterburner and a variable-geometry inlet that managed the air volume and velocity over a wide speed range to the engine requirements and limitations. provided a quarter-round, variable-geometry inlet spike was incorporated. The TF30 would be the first turbofan engine to be fitted with an afterburner; the Spey was the second. Both the TF30 and the Spey proved to be finicky about the quality of the air provided to them relative to an axial-flow engine, prone to compressor stalls. It also didn’t help that the bypass air channel increased the susceptibility of the compressor to stall due to the afterburner light-offs. Modifications to both the inlet and the engine became necessary.
One feature of the original inlet was the cowl position for low versus high-speed flight. For takeoff and landing (in the event of the need to wave off), the pilot was to position them forward (extended/open) so there was a large vertical opening into the engine inlet duct to provide additional air for maximum thrust. The control philosophy evolved during early flight test. Tragically, the cowl-position switches were mis-set before the takeoff of Ship 4 (BuNo 151973) on 21 April 1967. This F-111B had a three-position switch (Store/Open/Close) cowl switch for each engine. Store was for ground use only (it overrode the landing gear handle position) to close the cowl if desired. Open extended the cowl if the gear was down. Close retracted an extended cowl when the landing gear was up. Unfortunately, the takeoff was made with both switches in the Close rather than Open position, so seven seconds after the landing gear had retracted (it took that long for the jackscrew actuator to close them), there were compression stalls on both engines, significantly reducing thrust. The attempt to eject the crew capsule just before the airplane crashed beside the runway failed when the ejection handle malfunctioned. Both Grumman test pilots were killed.
The first three F-111Bs were built with and retained the original F-111A inlet. Four and Five received the Triple Plow I inlet. Six initially flew with an early configuration Triple Plow II inlet, which was replaced with one almost identical to the production Triple Plow II inlet (the difference was two suck-in doors instead of three). Seven received that inlet as well.
The most notable difference between the original inlet and the Triple Plow I was a kink at the top of the splitter plate that moved most of it farther outboard from the fuselage. However, vortex generators were also added inside the inlet to provide a less disturbed flow of air to the engine. Triple Plow II was a greater offset of the inlet from the fuselage due to the thicker boundary layer at Mach 2.5. As a result, no splitter plate was required. The spike, however, had to be lengthened accordingly. Another change was the replacement of the powered auxiliary air opening in favor of spring-loaded doors that positioned themselves aerodynamically depending on aircraft speed and engine thrust.
Except for the length, the spike configuration and function was never changed. It provided management of the air to the engine at high transonic and supersonic speeds with its variable geometry.
