F-111B Conversion Part Five - Odds and Ends

 A Work In Progress

Instrumentation Probe: Some of the F-111Bs were flown with instrumentation probes at some point during development. These provide more accurate air data (static and dynamic pressure, angles of attack and yaw) than sources adjacent to the fuselage. This can be added by CAT 4 R72020 “Shock-sensing probe”.

 

Inflight Refueling Probe: The Air Force preferred the boom and port concept for inflight refueling because of the much higher flow rate required to minimize the time needed to fill the tanks of its big bombers. The Navy adopted the lighter, simpler probe and drogue concept instead since the flow rates were adequate for smaller aircraft. See https://tailspintopics.blogspot.com/2012/01/f-111b-inflight-refueling-probe.html

 
Self Boarding: Except for a brief period, self-boarding was required of U.S. Navy carrier aircraft (the F4D and A4D were exempt). Separate ladders were a nuisance aboard crowded flight decks, particularly given the wind-over-deck velocities sometimes required for launches and recoveries. The F-111’s self-boarding capability was provided by steps integral to a hinged door on the lower side of the fuselage below the cockpit. See https://tailspintopics.blogspot.com/2024/04/f-111b-self-boarding.html

Engine Shroud/Ejector/Tail Feathers: Almost all afterburning engines had a variable-area nozzle located at the back end of the engine. The F-111's TF30 engine included a unusual extension aft of that. My guess is that it was intended to increase mass flow and therefore fuel efficiency when afterburning was not being used. It also may have served as thermal protection for the aircraft structure on either side of the aft end of the engine. For some reason, the shroud/ejector was initially shorter than the production one. Early F-111Bs flew with one or the other before the long one became standard. CAT 4 provides both lengths (and the Revell kit was produced with the short version). Note that the ejector did not have a variable diameter although the later version does appear to be be one, probably because it had "feathers", presumably to accommodate the hoop stress resulting from thermal expansion. Note that the F-111F had a unique shroud configuration.

Boat Tail: Boat tail is a term of art for the aft end of an airplane’s fuselage. Considerable wind tunnel testing initially resulted in a knife-edge end to the fuselage under the vertical fin between the engines. The stabilators were mounted on so-called speed bumps (probably dictated by area ruling) outboard of the engine nacelles. Nevertheless, F-111 drag predictions proved to be optimistic. Most of the problem was diagnosed to be excessive drag created by the aft end of the fuselage. Modifications resulted which were passed on to Grumman for incorporation in the F-111Bs.

The original boat tail tapered to a sharp edge on 970/971/972/973. Note the fuel dump nozzle. 

Drag reduction resulted in the addition of a blunt boat tail beginning with 974. Note the small fairing added aft of the arresting hook, presumably to reduce drag. At this point the fuel dump is now flush with the end of the fuselage. 

(The shroud/ejector appears to be short but it is actually the long version)

The production boat tail was introduced on 714 ("A" and "B" are not the F-111 designation suffixes but denote the different configurations. Note that the blunt end on 714 is not as deep as 974's. 

The fairing aft of the arresting hook on 714/715 was integrated with the fuel dump nozzle:

 

Arresting Hook: While the USAF F-111s had arresting hooks for emergency landings at fields with an arresting cable, the F-111Bs had a far more robust installation. This is the configuration of the installation on 970/971/972:

973 had a unique boat tail/hook configuration, the knife edge combined with a small fairing after the hook point.

974 had a similar fairing in the same location relative to the hook point.

 

714/715 had a more robust arresting hook. Also note the wheel added to the tail bumper.

Vertical Fin Tip and Horizontal Tail: The F-111B vertical fin tip was initially identical to the F-111A's. Beginning with 973, it was no longer a fairing for the aft-facing IR detector retained on the F-111A. The modification can be represented by cutting the fairing off longitudinally, fairing it into the fin surface about 2/3ds of the way aft, and raising the white tail light to the top of the modified fin tip.

 

 There were detail exceptions: 971/972 used for Phoenix testing had a camera mounted on the top of the tail and for some reason 973 had a small fairing on top of the fin.

 The camera fairing had a rectangular view port:

Fot its at-sea trials, 974 had something mounted on the tip of the fin, probably a camera:

The F-111B stabilators were never fitted with the USAF F-111 threat warning antennas and the speed bump fairing was never removed (it's provided in CAT 4 R72016).

F-111B Conversion Part Four - Cockpit

 

 Currently a work in progress.

 

The seven F-111B cockpits differed in two major ways: ejection seats or crew escape module; and the Phoenix Airborne Missile Control System (AMCS) was installed or not.

The first three F-111Bs (BuNos 151970, 971, and 972) had ejection seats because qualification of the crew module did not support the aircraft manufacturing schedule. See https://tailspintopics.blogspot.com/2016/07/f-111ab-ejection-seats.html.

As far as I know, only the F-111Bs used by Hughes for its Phoenix system development program had the Phoenix system displays and controls installed. These were BuNos 151971, 972, and 152715 (152714 was also operated by Hughes for a time, reportedly for pilot checkout and proficiency flying). This included the Vertical Display Indicator Group (VDIG), which consisted of a large TV-like presentation (Direct View Indicator, similar to the one in the A-6 Intruder) and a head-up display (Precision Indicator).

This is the instrument panel of BuNo 151972. The head-up display is just visible above the glare shield.

 The left console forward:

 The left console aft:

 Center console:

 

 

 The right hand console from the F-111B flight manual (the top of the control stick for the AMCS cursor is just barely visible in the first photo: note that only the F-111Bs with the Hughes' system installed would have the AMCS panels installed):

The lighting system mock up was similar.
 

The Vertical Display Indicator Group (the PI extended through a slot in the glare shield):

The VDIG was essential to the Phoenix system control, one of its several modes:

All the other F-111Bs had Grumman-designed substitutes: at least two variations of a large, mostly blank, sheet metal panel in the right hand crew position, one the same shape and dimensions as the Phoenix system installation; and a gyro-driven, three-axis attitude indicator and separate flight instruments in place of the VDIG for the pilot.

This is the cockpit of BuNo 152715 prior to the installation of the Phoenix system. Note the sheet metal equivalent of the system in the right hand crew position and the large artificial horizon in place of the VDIG:

This is the cockpit of BuNo 151973 that was used by Grumman for various flight-test requirements. The right side panel in this case is flush with the pilot's instrument panel. The red-orange color designated flight-test-specific hardware and calibrated instruments. The yellow handles with black stripes on either side of the center console activated the ejection of the crew module.

 

This is the cockpit of BuNo 151974 that was eventually used for at-sea evaluation aboard Coral Sea. Note that the right hand panel is the same as the one initially installed in BuNo 152715. The big aeronautical map on the lower half of it is centered on the Long Island, New York area.

 The instrument panel of BuNo 151974 with another variation of a substitute for the VDIG:

This is the stripped cockpit of BuNo 152714 after it was stricken with yet another variation of the right hand instrument panel that included provisions for several flight instruments, possibly including an altimeter (it almost certainly never had the Phoenix systeme installed even though it was assigned to Hughes for a time).

 

Note that the 1/72 Hasegawa/Hobby 2000 kit has a representation of the fixed component on the hatches of the crew thermal-protection system that should be removed for the F-111B.

 

The Cat 4 Conversion Set R72016 provides parts for the first three F-111Bs including the Phoenix instrument panel and the ejection seats.

F-111B Conversion Part Three - Inlets

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.