Modeling the Bell XFL-1 Airabonita

31 January 2023: Added a size comparison of the XFL-1 and P-39 empennages.

The Bell XFL-1 Airabonita was a one-off prototype based on Bell's P-39 that competed with the Vought XF4U-1 and the Grumman XF5F-1 following the U.S. Navy's 1938 carrier-based fighter competition.

It looks like it would be a colorful and straightforward conversion of the P-39, kits of which are available in every popular scale from 1/144 to 1/32. In fact, several have been done and documented in articles in modeling magazines. However, most—if not all—fall short of representing the actual XFL-1 configuration. Unfortunately, most of the structure, particularly the canopy, was different in detail from the P-39. Not even the 1/72 XFL-1 kits that are available are accurate, since they have the P-39 wing planform, which was somewhat different in taper and span.

(Note that the XFL-1 wing root is also closer to the fuselage centerline than the P-39's.)

 For example, this is what it takes to convert a P-39 wing to an XFL-1 wing:

The empennage (both initial and final) is also very different from the P-39's (the final horizontal stabilizer is about the same size and shape but the elevator itself has significantly more chord).

Another, more notable difference was the canopy and door, since over-the-nose visibility was critical for a carrier-based airplane.

All this and more is contained in my XFL-1 monograph, available from Steve Ginter:

http://www.ginterbooks.com/NAVAL/NF81.htm

In addition to most of the photographs extant of the XFL-1, there are illustrations of the configuration changes during development like the empennage. Since the Navy specifically included the XFL-1 in the competition to evaluate the performance benefit of the new 1,000-hp Allison liquid-cooled engine,  it includes a summary of the history of aero-engine development, comparing and contrasting the benefits and shortcomings of the liquid-cooled versus air-cooled engine. It also places the XFL-1 in the context of the Navy's rapid transition to monoplanes and new requirements like armor and self-sealing fuel tanks.

Even if you don't need a model of an XFL-1, the monograph provides interesting background on aero engine and Navy airplane development between the World Wars.

Grumman A-6A vs A-6E Intruder

30 January 2016: Corrections from Mick Roth and updated External Differences table

20 September 2015: Added detail provided by Mick Roth.

16 September 2015: Numerous changes have been incorporated over the past few days: Steve Belanger (see http://www.aoadecals.com/) Richard Brumm (also see his comment below) and others have provided additional information. 

A model builder sometimes asks for the differences between the A-6A and the A-6E. There isn’t a simple answer. Initially, the major changes were avionics and other internal items, with the only notable external change being the ECM antennas—and that wasn't true for the first 20 or so A-6Es—and possibly the location of the inboard wing fence. Note also that over 200 As were rebuilt as Es, making Bureau Numbers unreliable as a differentiator. (The first production A-6E was 158041.)

The Bible for the A-6 is Intruder: The Operational History of Grumman’s A-6 by Mark and Rick Morgan, published by Schiffer.  The subtitle doesn't do it justice because it also covers development, configuration changes, and in-service problems.

  Also see http://thanlont.blogspot.com/2012/12/grumman-6-intruder-history.html

Rick generously provided content and illustrations for this post but he only reviewed a first draft of the text so any errors and omissions are all mine.

The external changes can be simplified into the following categories:

Early As

Later As

Early Es

CAINS

TRAM

SWIP/Composite Wing

Note that I have arbitrarily differentiated Early and Late A-6As as circa 1970 after the incorporation of improved defensive systems in this summary.

Also see Brumm comment below.

Due to problems encountered in flight test and Navy evaluation, wingtip-mounted speed brakes were added at BuNo 149940 and the perforated fuselage-mounted speed brakes were deactivated shortly after the first A-6As were delivered to operational squadrons. The fuselage speed brake assembly was deleted entirely in production at BuNo 154170 and replaced with a blank panel. However, earlier A-6s, including those subsequently converted to A-6Es, retained the original perforated structure covering the speed brake well until it had to be replaced for cracking or corrosion. More than 10 were still flying with the original panel in 1991 while deployed for Operation Desert Storm. This is an example of a former A-6A in June 1983.

 Rick Morgan Photo

Early As had fairings attached to the forward portion of the tailhook and early Defensive Electronic Countermeasures (DECM) equipment and the ALE-18 chaff dispenser.

 The ALE-18 dispensed packets of chaff through a small slot on the lower left-hand side of the fuselage between the engine exhaust and the speed brake well. It was the production standard between production number 19 and 433.

 Small ALR-15 Radar Warning Receivers were mounted on the wingtips between the position lights on A-6A production number 19 through 358.

New and improved defensive systems were added as a result of combat experience, on the fly so to speak, which resulted in the Later A configuration. For example, two ALE-29A dispensers were added aft of a truncated Doppler radar antenna fairing beginning in either March 1969 and at production number 434 (Roth) or February 1970, replacing the ALE-18. These contained separate cartridges of either chaff or a flare with 30 tubes per dispenser.

The Later A-6A DECM antennas were boom mounted on the outboard pylon and under the leading edge of the wingtip:

Steve Belanger noted that the boom mounted antenna (AN/ALQ-100) was an airframe change dated 29 February 1968; the wingtip receivers (AN/APR-25), 30 April 1968. Mick Roth reported that the ALQ-100 was added at production number 310 and the APR-25 replaced the ALR-15 at production number 359. However, retrofit might have taken some time although A-6s that were in combat would get them as soon as possible during deployment. It's rare to see an A-6A that has one antenna and not the other but VMA (AW)-533 had only the wingtip receivers circa 1968/1969 on their 1541XX A-6As even though the addition of the ALQ-100 was the earlier change. This is an example:

John Murphy via Steve Belanger

Externally and from a modeler's perspective of the cockpit, the first 72 Es were basically the same as Later As except for ECM antennas (and even then the first A-6Es were delivered with the Later A-6A DECM antennas), the substitution of the GRU-7 seat (also retrofitted to some As) for the GRU-5 and possibly the location of the inboard wing fence.

Early A-6A Cockpit (gunsight protective cover in place):

Later A-6A Cockpit (note the addition of a radar warning display on the instrument panel above the center console):

 Scanned from the VMA(AW)-533 1972-1973 Cruise Book by Steve Belanger

The A-6E cockpit was initially similar to the Later A-6A's except for the GRU-7 ejection seat.

The ejection seats:

Production A-6Es #16 and 18 were delivered to VA-65 with A-6A DECM antennas.

The A-6A ECM antenna on the outboard pylon was subsequently replaced on A-6Es with three in a large fairing on the inboard-wing leading-edge. It had a recess in it for the main landing gear door.

The inboard portion of the A-6A's stall strip on the leading edge, shown here, was retained.

This change to the A-6E inboard leading edge resulted in the inboard wing fence being relocated slightly inboard. On the A-6A (and A-6C contrary to its SAC drawing), it was located slightly but notably outboard of the inboard pylon. On the A-6E, it was moved to be slightly but still clearly inboard of the inboard pylon. Click HERE for the relative locations.

Note that after the first few deployments, the black radomes were replaced with tan ones (the natural color of the fiberglass) to reduce conspicuity. Grey and white paints were subsequently approved that did not affect transitivity.

The inlets and vents on the right shoulder panel were different:

CAINS (Carrier Airborne Inertial Navigation System) was the replacement for the original INS that was one of the avionics carryovers from the A. It resulted in the addition of a large external air scoop on the upper aft fuselage and a small exhaust on the lower left aft fuselage (the original inlet at the base of the vertical fin was blanked off). In production, CAINS preceded the installation of TRAM (Target Recognition Attack Multisensor) turrets but all production A-6s delivered with CAINS had provisions for TRAM, including a removable panel at the bottom of the radome and the relocation of the anticollision light from the nose landing gear door to a light under each engine nacelles (two were doubtless required to meet the minimum viewing angles). The TACAN antenna was relocated to the underside of the left engine nacelle.

Note that the first A-6As had the angle of attack indexer lights evenly spaced along the side of the door and no TACAN antenna on it.

Since CAINS preceded TRAM, all A-6s with the sensor turret under the forward fuselage had the large inlet scoop, but not all A-6s with the large inlet scoop had the turret. However, the latter configuration would be relatively rare although it could occur simply due to a requirement for turret repair with no spare available. This is a former A-6A with the CAINS air scoop but no turret.

TRAM, which provided FLIR (Forward Looking Infra Red) among other things, resulted in a notable change on both sides of the cockpit.

 Note that there is one screen instead of two for the pilot and vice versa on the right side.

As in the earlier A-6s, when in use the bombardier's scopes were covered by a hood to maximize readability.

SWIP (Systems/Weapon Improvement Program) was a package of avionics changes and a new, all-composite wing that would extend the A-6’s service life. Because of delays in the wing’s design and development, some aircraft received some of the SWIP changes before than the wing. The only external SWIP change feature might be the addition of an aft-facing DECM antenna mounted above the fuel dump on the composite wing and possibly on the metal wing. Note that formation ("slime") lights were added to the wingtip (and the fuselage as shown above), possibly in production with BuNo 161092, but beginning in October 1982 in any event.

The composite wing configuration is covered in detail here: http://tailspintopics.blogspot.com/2014/01/grumman-6-wing-fold-differences.html

The later formation lights replaced much smaller rectangular orange lights on the fuselage.

 Note that this A-6E is a former A-6A that has the CAINS air scoop, the perforated panel over the speed brake well, and a panel on the underside of the nose in lieu of the TRAM turret (see http://www.usscoralsea.net/images/cv4319800425FD3RL.jpg). The picture is easy to date as taken in April 1980 since this is one of the airplanes with ID markings added for the Tehran hostage rescue mission.

The SWIP  improvements were supposed to include a head-up display for the pilot but this doesn't seem to have been realized before the A-6s were prematurely put out to pasture.

F7U-3 Nose Landing Gear

Surprise has been expressed about the presence of pre-rotation on the nose landing-gear tires as described HERE. The need is a little more obvious when you realize how long the nose landing gear was with the shock strut fully extended. When the nose wheel touched down, the load of the initial drag of the tires on the nose landing gear down-lock mechanism was magnified by a very long lever. (If you're in a window seat beside or aft of the main landing gear of a de Havilland Dash Eight, you can get a pretty good idea of the loads involved when the wheels meet the runway on landing.) Pre-rotation of the F7U-3 nose-gear tires reduced the kickback load, particularly on the non-skid surface of an aircraft carrier deck.

Note that the bleed air line telescoped to match the movement of the shock strut. Also, this nose gear does not yet have all the claptrap between the wheels required to keep the Davis barrier and barricade straps from being trapped there, possibly resulting in nose gear collapse.

The broad "V" directed the straps inward where they would be cut by the blades below it. My guess is that the small hook in the front of the shock strut was there to collect the activation strap of the Davis barrier (see http://thanlont.blogspot.com/2010/10/barriers-and-barricades-one-more-time.html).

It's Not That Easy To Get It Right

Consider the F7U-3 Cutlass. There are pretty good drawings available, although it turns out that the Vought draftsman was having a bad day when he did this promotional three-view:

 That's not a problem, however, unless this is the only Vought drawing that you rely on.

To get details, you could simply visit one of the survivors that is on display, like the F7U-3M in the National Naval Aviation Museum at Pensacola, Florida. If you can't do that, then there are walk-around photo collections on the interweb, the best of which is Don Hinton's excellent set HERE.

Unfortunately, the NNAM Cutlass has detail shortcomings and differences in configuration that will lead one astray. For starters, it is not an F7U-3M unless F7U-3 outer wing panels have been substituted because it lacks the F7U-3M outboard pylons. (It is also marked with an F7U-3 Bureau Number but that might not be the airframe's in any event.)

(The extended lower inlet was in fact a retrofit on some F7U-3s.)

Another indication that it is an F7U-3 is that it was originally displayed with the inflight refueling probe, which was removed from the F7U-3M to make room for a radar dish antenna.

The major discrepancy is the external fuel tanks it is carrying are not correct for an F7U-3. See http://tailspintopics.blogspot.com/2017/08/relying-on-museum-examples-for-detail.html

The appearance of  the nose gear scissors has varied over the years due to handling damage:

What's more, a unique feature of the F7U-3's nose landing gear has been removed. In order to reduce the kick-back load on the strut on touchdown, the nose wheels were spun up in flight by bleed air impinging on a turbine mounted on the inboard side of the wheel.

 From Doug Reid:

The air came down a pipe on the back side of the strut. It had a telescoping section to accommodate the enormous change in strut length between weight on and off the wheels.

Bill Spidle provided a picture and drawing of the turbine (97 in the illustration above) that spun up the tire.

Also see: http://tailspintopics.blogspot.com/2015/09/f7u-3-nose-landing-gear.html

You can't ask for a better look at the NNAM F7U's cockpit unless you're seated in it. See HERE. Unfortunately, the cockpit is not accurate. One of the throttles is from an F8U Crusader, instruments and switches are missing, the gun sight is similar to the one in the earliest F7U-3s but probably not even correct for one of them, the ejection seat lacks detail, etc. Also see HERE.

While the airplane has an actual Vought F7U-3 ejection seat in it, the seat lacks detail like a cushion/survival pack, a parachute, shoulder harness, the ejection handle and curtain, etc. Also note that the Vought seat was also unconventional in that the seat bucket and head rest could be positioned separately for cruise, landing, air-to-air combat, etc. As a result, the headrests could be in slightly different positions.

I'm not sure about the afterburner. The museum's airplane has exposed afterburner petals.

Don Hinton Photo

In all the pictures I've seen of the F7U-3 afterburner, the petals are surrounded by a shroud that extends from the fuselage.

Another consideration is configuration. The NNAM F7U-3 has the engine inlet with the extended lower lip. This modification increased engine thrust at low speed. However, it was a retrofit to some F7U-3s and most if not all F7U-3Ms so it is not on the first F7U-3s that deployed.

(The vents aft of the gun ports were also a retrofit but most if not all F7U-3s that deployed went out with this mod or got it mid deployment.)

Thus endeth the lesson...

TBM-3 Variants

Rick Morgan (http://rickmorganbooks.com/index.html) and I have been researching the modification of the TBM-3 and -3E for other missions. We've found some errors in the descriptions on the interweb of the various TBM-3 variants. (We're shocked, shocked...)

For a summary and some links, see http://thanlont.blogspot.com/2015/07/post-war-eastern-tbm-variants.html

A-4 Nose Gear Steering

To minimize weight, the Douglas A4D Skyhawk did not have nose gear steering among many other things. Turns on the ground were accomplished by differential braking. It was guided in close quarters and where accuracy was desired, e.g. lining up on the catapult, with a steering bar or "tiller" inserted in the nose wheel hub.

This practice was common for the first Navy carrier-based jets. Nose-gear steering was introduced with the F7U-3 Cutlass because of its size and weight.

When the A-4 was being modified to be a two-seat trainer, one of the features added to improve its marginal crosswind landing capability was a steerable nose gear. This was subsequently adopted on later versions of the single-seat Skyhawk. The changes consisted of a hydraulic actuator mounted in place of the shimmy damper on the right side of the strut just above the fork holding the wheel, hydraulic lines from the nose wheel well to the actuator, and a vertical sleeve to enclose the hydraulic lines. The sleeve was mounted to the bottom of the strut and guided up and down by a collar mounted on the upper back side of the nose landing gear as the shock strut moved in and out. (The slender rod, aka shrink strut, on the left side of the nose landing gear pulled the shock strut into the strut body as the gear retracted. This minimized the volume required for the retracted nose landing gear.)

The illustrations use crops from a set of A-4M walkaround photos by Bill Spidle.

It's Not That Easy to Avoid Error

I did a pretty good post on the Douglas F4D early on that was based on an article I did for the IPMS update: http://tailspintopics.blogspot.com/2010/10/bat-out-of-hell.html

One of the detail configuration issues that I didn't mention was the main landing gear wheel hubs, mostly because I wasn't paying close attention to them.

I was recently asked which of three different wheel hubs installed on F4Ds now on display was the one to use on a fleet airplane. The answer was:

Note that the F9F-8 wheel hub was not used on fleet F4Ds as far as I know. If a model kit manufacturer faithfully copied this particular museum's F4D, it would be incorrect in this regard.

Another detail is the "spat" that was on the earliest F4Ds. This was to accommodate the Davis barrier used on axial-deck carriers (see http://thanlont.blogspot.com/2008/10/davis-barrier-redux.html)

As far as I know, the spats weren't present on any fleet Fords, because they only operated from angle-deck carriers.

Finally, as I was hanging out with some radio-control enthusiasts at the New England Air Museum as they were measuring its F4D for a large-scale flying model, I happened to notice that the wing-tip airfoil had a negative camber. This is one way to provide stability and control to a "tail-less" design. It seems counter-intuitive from an overall lift standpoint but something of the sort is necessary when there is no horizontal tail.