F-111B Conversion Part Three - Inlets

 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. 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 most, if not all, of the inlet development work. The F-111B inlets varied over time. These consisted of the original inlet and two subsequent ones that were approved for USAF production: Triple Plow I and Triple Plow II. "Triple Plow" reflects the fact that the inlet now removed low-energy boundary layer air from entering it slightly differently from the lower wing surface and inlet ramp, directly diverting it rather than internally redirecting it, i.e. plowing it away like some of the fuselage boundary layer air.

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.

Note that the following illustration will be revised to more accurately depict the various iterations flown on the F-111Bs. 

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/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.

F-111B Conversion Part Two: Landing Gear

 A work in progress

The nose gear of the F-111B was very different but so was the main landing gear to a degree. As it happened, with two exceptions, the landing gear of the first three F-111Bs (BuNos 151970/71/72) was essentially identical to the landing gear on production F-111As, making the landing gear in the Hasegawa/Hobby 2000 kits usable as-is for them The two exceptions were that the nose gear on BuNo 151972  was damaged in the first of its two emergency landings and replaced with an F-111B nose-tow gear (at some point prior to its retirement, an F-111A nose landing gear was reinstalled) and the different aft main landing gear door on all seven F-111Bs.

BuNo 151972 with a nose-tow nose landing gear:

The aft main landing gear door was originally identical on the prototype F-111As and Bs. During extension and retraction it was articulated on two pairs of links so that on extension it swung down and back, remaining parallel to the bottom of the fuselage.

One pair of links was attached to pivot points on the main landing gear trunnion to which all its other parts were attached, one on either side of it, just below its pivot point on the back of the wheel well. The other pair were attached on the door just outboard of the first pair and to pivot points on each side of the wheel well. When the landing gear was extended or retracted, the door was driven by the first pair of links and remained parallel to the bottom of the fuselage when extended and retracted by the second pair of links (idlers).

Note that the trunnion on the 172 Hasegawa/Hobby 2000 kits is an open frame that should be covered with a thin piece of plastic.

A montage to illustrate the shape of the links and the location of the door: 

Apparently the rigging of the original aft main landing gear door was both finicky and subject to going out of adjustment, requiring a gear swing to remediate. As a result, early in F-111A production/operation, the door was simply mounted on the back side of the trunnion. This required that about six inches be removed from the back end of the door to clear the bottom of the fuselage when the landing gear was extended, so when it was retracted, there was a six inch wide opening on the bottom of the fuselage aft of the door.

This is the arrangement on all the F-111 kits except for the Revell, which has a simplified version of its original articulation. All seven of the F-111Bs flew with the original door, although in production it seems likely that it would have been mounted on the trunnion as well. While no aftermarket F-111B main landing gear tires are available, the F-111 kit tires can be narrowed and reduced in diameter accordingly.

All of the F-111Bs after the first three were delivered with the Navy landing gear. The F-111B main landing gear was the same as the F-111A’s and subsequent shore-based F-111s with the exception of the tires and the aft main landing gear door. At some point, to minimize a tipback problem in service, at least BuNo 152715 was modified to move the main landing gear wheels eight inches aft.

The Navy nose landing gear was completely different, however, except for the tire size. It was redesigned to add the nose-landing-gear tow and hold back for catapult launch as well as the Automatic Carrier Landing System radar corner-reflector. Both the strut and its actuation reflect the requirement that the nose landing gear react both the hold back and launch loads. Although no aftermarket F-111B nose landing gear is available, scratch building one is relatively easy.  Except for the wheels, the USAF nose gear isn't much use in that regard, though.

 Note that its shock strut angles forward (this is 151970 with the USAF nose gear) and ends well above the tire; the red markings on the nose wheel door were there to verify that the gear was fully extended:

 The Navy nose gear had a somewhat longer (the shock strut extended down to the axle) and beefier shock strut; the retraction actuator was also much beefier and attached above the shock strut piston:

 The Navy nose gear strut was mounted vertically (note the location of the wheel relative to the nose landing gear door):

 Ready to launch (the trail bar would hold the airplane back at full thrust and then release at the fitting to the strut when the catapult fired, remaining on the ship for reuse):

 The radar reflector was attached to the scissors so when the strut was extended in flight, it was raised to be vertical and above the right tire. Note the amber approach light, indicating to the LSO that the pilot is at the proper angle of attack.

 

F-111B Conversion Part One

A matrix of the seven different F-111Bs by Bureau Number is provided HERE along with a illustrations of the features and/or links that describe them.

There have been a few 1/72 and 1/48 conversions released to convert an USAF F-111 kit to the F-111B configuration. Most were only partial representations and there were only a couple of decal sheets available. Nevertheless, Bill Gillman (Navy Bird) was able to make do with them: Click HERE and HERE

 HERE is another 1/72 build by DrPlastic in 2012 using the Pete's Hangar conversion set (not recommended even if you could find one).

All of these conversion options have been eclipsed by the recent CAT 4 release of mostly 3D printed parts and a comprehensive decal sheet and instructions for any of the first three F-111Bs, which diverge the least from the production USAF F-111s. Click HERE for a description of the sets and a source.

These are the 3D-printed ejection seats provided in the CAT 4 1/72 conversion set R72016 that provides the majority of changes for the first three F-111Bs. The protective cover in front of the upper and lower ejection handles has been removed on the left seat.

 
There are various F-111 kits that provide a basis for that but their quality and build-ability varies significantly. I’m only familiar first hand with the 1/72 alternatives; however, Googling “1/48th F-111 kits” will result in the many detailed reviews that will provide a basis for selection.

Four 1/72 kits were originally issued, reissued as variants, and also distributed by other kit manufacturers:  

- Hasegawa/Hobby 2000: Unless your standards for accuracy and build-ability are low, these are the only acceptable starting point (also, the CAT 4 conversion sets are designed for them).

- Esci/Ertl/AMT/Gunze/Italeri: Disappointing in accuracy and fit but buildable (full disclosure, Paul Boyer binned his attempt at it)

- Revell/Monogram: The first release provided parts for both the development F-111A and F-111B airplanes. It was quite accurate in shape for those aircraft except for, ironically, the F-111B nose. The landing gear was retractable but necessarily simplistic as was the interior of the separable crew module. Later releases deleted the F-111B option.

- Airfix/MPC/Heller: Minimal detail (e.g. cockpit, no wheel wells), and the crew module is a separate, ill-fitting assembly

Note that one significant difference in some kits is the wing span but CAT 4 has created a conversion (R72025) for the low-aspect-ratio wing that was present on the F-111A/D/E/F to the higher aspect-ratio F-111B wing (the F-111C/G and FB-111A had the same wing span as the F-111B).

 Other options are DIY and using the wing extensions from the original Revell F-111A/B kit.

 Do It Yourself (degree of difficulty reduced if you glue the flaps and slats up and scribe the ones on the extension):

 Revell Kit Wing Tip Extensions:

 However, the CAT4 modification includes the extension of the trailing edge flaps and leading edge slats, which would be difficult to accurately represent deployed otherwise.

Another consideration is the two different lengths of the engine shroud aft of the afterburner. Both of these were present at one time or another on all of the first five F-111Bs and the ones in the Hasegawa kit are all long and configured with “feathers” that resemble those on variable-area afterburner nozzles, unlike the smooth exterior of the shrouds on the F-111Bs (my guess is that the feather configuration was added to reduce the hoop stress from thermal expansion). CAT4 provides a shroud length option or the kit shroud feathers can be filled and smoothed to the F-111B configuration.

All of the F-111 kits will have the USAF landing gear. As it happens, the first three F-111Bs flew with them but the remainder were delivered with the very different Navy nose landing gear and main landing gear wheels. Also, except for the Revell, the F-111 kits do not have the original retractable aft main landing gear door flown on the first seven F-111Bs.

The engine inlets also varied among the kits with some overlap of those on the first seven F-111Bs (General Dynamics was responsible for inlet development; Grumman was roughly one inlet modification behind in production). As noted on the Conversion Set Application matrix, which specific F-111B you model will determine which Hasegawa/Hobby 2000 kit to buy.

The F-111B cockpits were notably different. The first three F-111Bs had ejection seats. All had flight controls only on the left hand side. The F-111Bs delivered to Hughes for Phoenix system installation (151971/2 and 152715) had the right side instrument panel provided in the CAT 4 set R72016 but the remainder had a flight-test-only substitute with few instruments and switches.

To be continued in Part 2

An F-111B Configuration Update

 6 April 2026: I should have prefaced this post as being a preliminary draft, i.e. corrections and additions to come.

The Configuration Difference matrix has been updated 

CAT4 is in the process of releasing decals and conversion sets for the F-111B. As a result, I'm reviewing and updating previous F-111B posts providing modeler-oriented configuration details of the seven that were built and flown. This was the original post from 2009 that summarized them:

 https://tailspintopics.blogspot.com/2009/10/grumman-f-111b.html

A comparison summary by BuNo:

For the difference in the inflight refueling installation for the short vs. long nose, see https://tailspintopics.blogspot.com/2012/01/f-111b-inflight-refueling-probe.html

For the landing gear and inlet differences, scroll down here: https://tailspintopics.blogspot.com/2009/10/grumman-f-111b.html

For more on note 2) above, https://tailspintopics.blogspot.com/2011/09/f-111b-aft-main-landing-gear-door.html. Also, the F-111B aft main landing gear door extends a bit farther aft than production USAF F-111s because it was attached to the landing gear and therefore had to be trimmed to clear the bottom of the fuselage when the landing gear was extended.

For more on changes to the main landing gear for production, see https://tailspintopics.blogspot.com/2017/02/the-f-111b-production-main-landing-gear.html 

The first three F-111Bs had ejection seats. See https://tailspintopics.blogspot.com/2016/07/f-111ab-ejection-seats.html

 The rotating glove was added to the fuselage adjacent to the leading edge of the wing to increase lift for takeoff and landing, offsetting the increase in gross weight. The leading edge slat was also extended inboard.

 The auxiliary flap was a late addition to further increase the lift on takeoff and landing. See https://tailspintopics.blogspot.com/2014/09/f-111-auxiliary-flaps.html

The original venting of some of the boundary layer air kept out of the inlet was on the side of the fuselage under the wing. Ducts were subsequently added to increase the suction.

 

The venting was subsequently moved to the top of fuselage.

 

 

 A comparison of the "A" and "B" boat tails (in this instance, A and B represent boat-tail configurations, not the F-111A vs the F-111B).

Not included in the comparison above are the two different engine exhaust shrouds that were present on the F-111 prototypes. These shrouds were mounted on the engine behind the afterburner, probably to provide sonic and thermal protection to the adjacent aircraft structure. Free-floating "blow-in" doors were located forward of the shrouds. My understanding is that these were either open or closed in flight depending on the difference in external and internal (engine bay) pressure. The shrouds were originally just slightly tapered metal cylinders. In production, at least the aft portion of the shrouds consisted of linked longitudinal panels resembling an afterburner nozzle but were not controlled and had only limited movement, probably to relieve stress from thermal expansion of the shroud; the nomenclature changed from "shroud" to "feathers". The blow-in doors were also then fixed in the open position.

There were two different shroud lengths. The original ones were longer and the shorter ones possibly introduced as a weight reduction change that was short-lived.

 Note that 151970 originally flew with long (green arrow) shrouds but was photographed at Edwards AFB with short ones (red arrow). 151973 was also flown with both short and long shrouds.