Thanks to the generous assistance of Jim Rotramel (an F-111 Weapons System Officer and mission planner, he is the author of Operation Eldorado Canyon: The 1986 US Bombing Raid on Libya), I'm declaring victory on this post although it is still subject to change from time to time as a result of new information or realization of errors.
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. resulting in a quarter-round, variable-geometry inlet spike and cone. 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 handle was down. Close retracted an extended cowl when the landing gear handle was up. As a result, the cowl would be open when the landing gear handle was down. Unfortunately, the takeoff was made with both switches in the Close rather than Open position, possibly because it had been left in the Close position after the previous flight. As a result, seven seconds after the landing gear handle had been raised after takeoff (it took that long for the jack-screw 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.
General Dynamics did all of the inlet development work. The F-111B inlet configuration varied as GD made progress in reducing compressor stalls). These consisted of the original inlet, a subsequent short-lived improvement, and two that were approved for USAF production: Triple Plow I and Triple Plow II. The first three F-111Bs (BuNos 151970, 971, and 972) were built with and retained the original F-111A inlet. Four and Five (BuNos 151973 and 974) first flew with an early modification of the original inlet. Five subsequently received the Triple Plow I inlet. Six (BuNo 152714) 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 (BuNo 152715) received that inlet as well.
This updated and corrected illustration summarizes the various iterations flown on the F-111Bs.
The original F-111B inlet was the same as the first F-111A's.
It was the inlet on BuNo 151970, 971, and 972. However there were subsequent modifications. The first were vents on the bottom of the cowl aft of the inlet to discharge the splitter plate bleed and then underwing ducts rather than simple vents to discharge the glove bleed.
The inlets on 151973 and 974 had a different splitter plate (the bottom was level, not angled upward) and ducts under the cowl to discharge the splitter-plate bleed. There are now vortex generators in the inlet (note the ones on the spike) to provide a more uniform and consistent flow of air to the engine.
The ducts under the wing have been eliminated. My guess is that the glove bleed for these particular inlets was discharged out of enlarged vents in the top of the wing glove.
The next set of F-111B inlets resulted from GD's "Triple Plow" development program. The name reflects the fact that the inlet now removed low-energy
boundary layer air from entering it slightly differently from the lower glove surface and inlet splitter plate, directly diverting it rather than
internally redirecting it, i.e. plowing it away like some of the
fuselage boundary layer air. This is Triple Plow I (note the array of vortex generators inside the inlet):
BuNo 151974 was modified with the Triple Plow I inlet at some point before its at-sea evaluation aboard Coral Sea. 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 it
farther outboard from the fuselage.
The new splitter plate also extended slightly aft of the inlet compared to the original one (note that 151974's Triple Plow splitter plate was not painted gray like its first one):
Triple Plow II had 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. 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/cone 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.
At low transonic and below speeds, the spike was positioned all the way forward and the cone collapsed. At supersonic speeds, the spike moved aft and the cone expanded to control the speed and volume of the air being provided to the engine.
BuNo 152714 first flew with a variant of the Triple Plow II inlet that retained the splitter plate, which no longer has the kink as the spike has been moved outboard. The cowl no longer translates fore and aft, the required opening being replaced by two aerodynamically actuated doors rather than the three on the USAF production Triple Plow II inlet.
Note that spike extends farther forward of the Mach sensor and has been moved outboard closer to it. The splitter plate was subsequently removed but the fuselage not repainted accordingly.
The doors appear to have been added to existing cowls.
Craig Kaston Photos
BuNo 152715 first flew with the splitter plate removed but again, with only two doors on the modified cowl.
One problem with the Hasegawa/Hobby 2000 kits with the Triple Plow II inlets is that the spikes appear to have not been lengthened compared to the Triple Plow I's.
Hobartville Hobbies in Australia sells seamless Triple Plow I and II inlets/ducts in 1/72, 1/48, and 1/32 scales. Their website: https://www.hobartvillehobbies.com.au. I can't vouch for their accuracy, quality, or fit but the illustrations look good and depict the vortex generators in the ducts that would be difficult to add to the parts in the plastic kits.
