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Lindberg 1/48 Republic XF-91 Thunderceptor


Shorty84

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Hi guys,

 

this is my entry for this groupbuild, the Lindberg Republic XF-91 Thunderceptor, a mixed propulsion interceptor prototype.

The kit, first released in 1955 is quite typical for its time: Low parts count, no wheel wells, no cockpit (it has a pilot and a rudimentary seat though), raised panel lines and rivets but a generally sound shape. That means some scratchbuilding and maybe the odd bit of aftermarket (decals, cockpit from an F-84) will be necessary.

xf-91_1skjcg.jpg

 

There is not a lot in the way of references, so the 'Air Force Legends Nr.210' will be consulted in the course of the build.

xf-91_2i3jwt.jpg

 

I guess not everybody is familiar with this type so a few words about the original:

 

The XF-91 started life as USAAF Project MX-809, calling for a high performance interceptor to protect U.S airspace from enemy bombers. An Invitation to Bid (ITB) was sent out to various aircraft manufacturers in December 1945, calling for some pretty challenging performance specs: A total of 25,5 min of combat operation, divided into 2,5 min time-to-climb to an altitude of 47.500ft (14.500m), 15 min of cruise time at 486kts (900kph), 3 min of combat at an average speed of 688kts (1274kph) and 5 min of descent time from 47.500ft to landing.

This basically called for supersonic performance, requiring a dual propulsion (jet/rocket) design. Remember, this was some 22 months before the X-1 broke the sound barrier.

 

Work started in January 1946 and design work was headed by Alexander Kartveli who already designed the sucessful P-47 Thunderbolt and P(F)-84 Thunderjet. The aircraft was to be powered by an General Electric J-47 turbojet and a four-chambered Curtiss-Wright XLR-27 rocket engine. Furthermore, the XF-91 featured some unusual design design features like inverse tapered wings (to prevent the so called "Sabre dance", an aerodynamic phenomenon plaguing most early swept-wing aircraft, resulting in violent pitch-up motions of the aircraft, leading to loss of control and its destruction) and the possibility to adjust the main wings angle of incidence to improve visibility during landing.

 

First flight happened in February 1949, at first with the Turbojet engine only as the rocket motor was way behind schedule. Despite it's unusual design features the flight tests revealed no severe problems. While test flights continued (a second prototype joined the program in the meantime) high performance test were still not possible as the rocket engines were still not available. In the end, the XLR-27 was cancelled and in 1952, Republic decided to install an XLR-11 (used in the X-1) as an interim measure. With both engines, the XF-91 reached Mach 1.07 in December 1952, making it the first U.S combat aircraft breaking the sound barrier in level flight.

 

Yet, the days of the XF-91 were counted. With the delays of the rocket motor the aircraft became already obsolete. The USAF already eyed on much more capable designs which were developed under the "1954 interceptor" concept (leading to the F-102, F-103 and F-106), featuring supersonic performance without complicated rocket engines, radar, fire-control systems and guided missiles needed for night and all-weather operation.

 

Like many designs of this era, the engine proved to be its ultimate downfall, not the airframe. While Republic tried to improve the design by adding radar and missile armament (unguided 2.75in 'Might Mouse' FFARs as well as HVARs and even AIM-4 Falcons were planned) aeronautical development went at a rapid pace, making more advanced designs available within a short time.

 

Cheers

Markus

 

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Welcome to the GB Markus and with an interesting as well an unusual subject choice. Although the kit may be basic you certainly seem to have a plan in place to make the most of the opportunity it provides :thumbsup:

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20 hours ago, Corsairfoxfouruncle said:

Well as the Thunderceptor is my all time favorite never was I will follow your build

Hi Dennis, glad to have you on board. This kit will be a good opportunity for some "proper modelling" so I hope I can do it justice.

 

1 hour ago, exdraken said:

 

You are back to modelling I assume :)

Hi Werner, yes definitely. I thought a groupbuild is a good opportunity to get the juices flowing and build one of those lesser known, yet interesting, designs.

 

Cheers

Markus

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The Lindberg XF-91 is an oldie but a goodie from the "Golden Age" of plastic models. Being kinda basic, you have all kinds of room slight of hand improvements. Can't wait to see how it finishes up.

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Hi Loren,

 

thanks for your feedback. I've developed a taste for these "Golden Oldies" as they depict subjects not done by any other manufacturer today and have now a sizeable collection of vintage kits with the XF-91 being the latest purchase.

 

Have started with surface preperation in the meantime (read: much sanding to get rid of all rivets) so pictures will follow soon.

 

Cheers

Markus

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10 hours ago, LorenSharp said:

Sand, sand polish, polish.

You forgot: rescribe, fill again, rescribe, rerivet.

 

9 hours ago, quangster said:

At least you won’t need Aires for the wheel wells 😂

Oh yeah, I'm really thankful for that... 🤨

Edited by Shorty84
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  • 3 months later...

Hi all,

 

I think you've all guessed it by now based on my absence from this thread for months, but this build will not be finished within this GB. To be completely honest, I really wasn't in the mood to do much modelling during the last months. Let's call it "Covid-fratigue" or "Covid blues" which caused me to lose motivation for almost everything since Christmas. We are in various forms of Lockdown here since November and the cold weather, no sun, coupled with the fact that everydays routine is the same for months now (sleeping, working, eating) has really drawn a lot of energy. I'm really sorry that I did not participate here and commented on the great builds which happened here in the meantime 😪

 

Anyway, this build is not dead yet, and I will continue it as a regular WIP once the GB closes. Now as the weather gets better and the sun is greeting us again, motivation to build something is coming back. So even though I've not done a lot of modelling I want to share with you the progress I've made in the meantime.

 

This is what the kit looks like. A typical product of its time, with raised panel lines and boiler sized rivets.

xf-91_1_3fikfj.jpg

 

xf-91_1_4uhknx.jpg

 

xf-91_1_5uhjo1.jpg

 

Before doing anything else I had to decide which time of it's career I wanted to represent. As with most prototypes, they were modified constantly, and you really have to settle for a specific timeframe and add your details accordingly. I wanted to hang the big external fuel tanks under the wings and a study of various pics suggested that they were only carried early in the aircraft's career before the XLR-11 rocket engine was installed. Later, F-84 tanks were used, so I settled for the first prototype as it appeared in 1950 during the tests at Edwards AFB.

 

The first course of action was to sand down all the "details" and create a smooth surface. The styrene proved to be surprisingly hard, so I had to use coarse grit sandpaper to remove all raised details. Then something happened I've never seen before. Even though the surface was absolutely smooth, the rivets and panel lines left some "shadows", means you could still see them even if completely sanded down. So it looks as if they are still there.

Next step was to trace new panel lines using a pencil. Here I mostly relied on photos as there are no reliable plans out there showing the correct paneling. Especially the belly was hard to do as most photos don't show this area at all or the quality of the existing pictures is not good enough to determine any details. Still, I think I've managed to add most of the main lines correctly.

 

On the wings the MLG cutout and the ailerons have to be enlarged, and I've filled the holes for the rockets with stretched sprue.

xf-91_1_6htj6q.jpg

 

xf-91_1_7mtkoi.jpg

 

On the fuselage I've started to draw various vents, again based on staring for hours to a lot of photos.

xf-91_1_98skox.jpg

 

xf-91_1_10ljj5w.jpg

 

xf-91_1_11dzjdd.jpg

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One of the first modifications I had to do was on the fuselage. From various photos and looking at the surviving second prototype it became clear that the fuselage sides were flat and not rounded as on the kit. So out came again the coarse sandpaper and a lot of elbow grease later both fuselage sides were flattened. Due to the hard plastic this was not an easy operation and my arm really hurt afterwards.

xf-91_1_8ogkck.jpg

 

The Air Force Legends book contains a set of scale plans and as I was interested how the kit compared to them, I scanned and printed them in 1/48. Considering its vintage, the kit compares quite well to the plans. The fuselage is a good match, still there are some discrepancies:

  • The fuselage is too slim. It should be fatter around the belly and behind the cockpit
  • The cockpit fairing (and as a consequence the clear parts) are a bit too big
  • The nose intake is too big in diameter

xf-91_1_13fukgr.jpg

 

The wings and the horizontal stabilizers on the other hand are quite off. While the horizontal stabilizer will be modified anyway (it should be slotted through a cutout in the vertical stabilizer, leaving a big gap and no positive attachment) I'm not sure if I will leave the wings "as is" or not.

xf-91_1_14x3jjh.jpg

 

I decided that I could not live with the intake, so I prepared a plan to decrease the diameter. In the kit, the intake has a diameter of 15.5mm while according to the scale plans it should be just 13.5mm. This may not sound much but makes quite a difference in appearance.

xf-91_1_1528kda.jpg

 

xf-91_1_16imk00.jpg

 

In order to create a smaller intake diameter I removed a "<"-shaped wedge in the horizontal plane and made some vertical cuts in order to force the parts together again. The new joint line was soaked in liquid cement and held in place with tape, curing for a whole day. Here I noticed another oddity with the styrene. At first, it seems it does not react to the cement at all but after a while it fuses together nicely.

xf-91_1_170mjwh.jpg

 

xf-91_1_18g9jbc.jpg

 

A comparison with the plans...looks good.

xf-91_1_19uqkhx.jpg

 

A final check against a photo of the real thing...yes, wonderful.

xf-91_1_20yfkzp.jpg

 

Another area to be modified was the lower rocket engine fairing. I removed a bulge which shouldn't be there and thinned down the fin keel considerably. Again, due to the hard styrene this was quite a bit of work and I had to sand away so much material that some areas became translucent.

xf-91_1_21i1kw5.jpg

 

 

That's it for the moment. I hope you will follow my build here and later in the WIP-section. Comments and critiques are welcome as always.

 

Cheers

Markus

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  • 7 months later...

Hi @exdrakenWerner,

 

well, yes and no. I did some further work on the kit, finally modifying the wings to match the sweep angle of the scale plans. I've also purchased an F-84F resin cockpit, which will be fitted in the future.

 

Unfortunately, due to some family related stuff, I've not really done much modelling lately. But if accepted, this build is my entry for the upcoming 'Prototypes, Racers, Research, Record breakers, Special schemes mega GB' in 2022.

 

So it will get built!

 

Cheers

Markus

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  • 1 month later...
On 11/1/2021 at 3:38 PM, Shorty84 said:

..if accepted, this build is my entry for the upcoming 'Prototypes, Racers, Research, Record breakers, Special schemes mega GB' in 2022.

Of course! :thumbsup:

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  • 4 months later...

Just checked in on your project today. This is one that had slipped passed me prec viously and I am very interested to see how it goes as   , for my sins,  two of these to tackle one day. You are off to a great start with that nose and  the fuselage clean up.  Wishing you   well in your struggle.

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