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The Boeing 720B…..What’s not to love?
Anyways, let’s get started. Starting with the fuselage, i did the customary thing of mixing epoxy with copper shot and i poured it into the right fuselage half.
I filled the cabin windows (this time) with styrene bar stock. This was glued in place with black Starbond CA, and the excess was cut away and the “stumps” filed and sanded flush.
The fuselage was glued together using Tamiya Extra Thin on the nose, tail, and vertical fin and the remainder was glued and filled with Black Starbond CA. Aside from the rough texture (which sanded out okay), construction of the fuselage was routine.
Next came the Number 1 engine, which served as a test case for the re-engineering of the wing-pylon interface.
Roden broke each engine down into 7 parts: Front fan, Left/Right fan cowl/shroud, Left/Right “Hot Section” of the engine, the translating aft cowl/tail cone, and a turbine wheel bullet that i did not use.
The Black Starbond CA comes in handy for putting these together. My recommended method of construction is to block sand the back of the fan front, glue the left and right shroud halves together, block sand THAT flat, and then assemble the fan section to the shrouds, and fill/ sand this assembly separately. While this is drying, glue the left and right halves of the hot section together, but do not glue on the fan or the translating cowl just yet. Hold off on that until the pylon-wing gaps are all fixed.
After the left and right halves of the hot section and pylon are glued together, it’s then time to lay a piece of bare metal foil on the wing underside, and glue the partially assembled nacelle on top of the foil, in position on the wing.
If all goes to plan, you should have something approximating this…
The assembly photos are a little out of sequence, but the point to drive home is to get the pylon parts together and then start tweaking the pylon saddle and the “foot” on the wing first, then glue/fill/sand the wing pylon and then only after that glue the remaining bits of the engine nacelle together.
And if you are like me, who didn’t set the wing dihedral/pylon angle correctly the first time, you can always go back and repeat the procedure, like i did here to fix the problem on Number 2.
With the mistakes out of the way, the engine nacelles assembled and cleaned up….and with some additional carving of the upper surfaces of the wing fairing/pylon intersections, we get this:
And with this, we endeth Episode 1. Next post will be devoted to fixing the wing-fuselage joint. Until next time, keep those letters and cards coming.
-d-
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Beautiful work so far. Does this model include the classic double lobe crease on the fuselage?
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Great thread Convair990A.
I bought this kit last year and I’ll use this as a reference when I do my own build.
I took the advice of other members and bought the AA engines and am hoping they don’t require as much work, being a less expert / lazy modeller.
Look forward to your follow-up posts!
Tom.
Don’t it always seem to go that you don’t know what you’ve got till it’s gone …
Well, if you’re building a straight 720, you don’t have much choice. The Roden JT-3s are more like JT-4s…..sort of.
AA does a great job with their engines and especially if you’re using JT-3s its undoubtedly the best way to go.
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Good work so far, excited to see how this goes!
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Well, the next task is to optimize the fit of the wing to the wing root fairing and so far, the story has been a little weird. Stay tuned.
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Just an update on the 720B:
I glued up the right wing halves and started roughing in Number 3 and 4 engines yesterday. I have this project running in parallel with an A3D Skywarrior which is serving as a test case for my theories on mounting engines in pods.
Also i have the Tamiya F-4B in work as well as a Buccaneer.
Anyway, there’s something funny going on (funny strange, not funny, ha ha) with the wing root on the left side and i’m still analyzing whats going on. I should know a bit more when i finalize the wing installation for the right wing so i can take some measurements and compare.
-d-
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Greetings!
When you talk about the engines you could probably avoid confusing people by not just saying JT-3… Both fan and turbojet engines are JT3 engines, the turbojet one is a JT3C and the fan engine is a JT3D engine, or more specifically, a JT3C-7 (or -12 on Eastern’s 720) and the fan engine is a JT3D-1.
I know what you mean, but it could easily be confusing to some.
Generally speaking, how do you find the fit of the other parts? I remember when the kit first appeared it was trashed badly by some individuals on this forum.
Best regards,
Jeff Jarvis
God's "Curse" to aviation!
Regards the fit, it’s not horrible, though there may be some cross sectional thickness differences at the left wing root though i’ve been able to straighten that out, i think.
Like other Roden kits, the fuselage halves have that rough sandpapery texture, but it sands out okay.
The locating pads and “feet” mounted to the lower wing planks hurt engine alignment a lot more than they help. As i documented above, i carved the pads off and sanded the wing surface smooth. The feet which constitute the upper surface of the pylon fairing have to be pared down at the front and sides and filed considerably to line up better with the pylons. The locating pins in the engine pylons themselves are wimpy and ideally should be replaced, though there is not much cross sectional area to play with.
Up to this point, really no worse than the Super VC10 build.
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Well believe it or not, some more progress has been made on the 720B.
I notched out the wingtips and added some lenses from colored red and blue-green sprue…
The lens as you see it here is roughly 60% oversize but it will be masked off and painted over. Left wing? Same story.
The mating surfaces of the right wing and wing root were sanded (hopefully) flat. I installed a new rear spar, and bare metal foil was sandwiched between wing and fuselage as i glued it in place and filled the gaps around it with Starbond Black CA….
This was then hit with accelerator applied with a throwaway micro brush.
When dry, the wing was carefully pulled apart from the fuselage, the excess foil was peeled away with a tweezers, and i proceeded with squaring up the starboard wing root with sanding sticks.
The locating pins on the engine nacelles are pretty lame. After some reflection, I decided to install a copper locating pin at the location of the front nacelle mounting pin. Doing this had knock-on effects that forced me to re-align, re-fill, and re sand the gaps between the pylons and the engine nacelles. Not hard, just tedious. If i’d known this ahead of time i would have installed the pins from the get go. Having said that, this integration isn’t the trainwreck that the pylons in the Minicraft 707 kits are.
So, since i don’t know what the best sequence of assembly would be for a 4-engined jet airliner like this, i’ve decided to experiment with gluing number 1 engine in place on the wing. We will then discover how hard it is to blend, mask, paint, work around etc. I’d like to get access to the engine nacelles from as many angles as possible and right now, that means doing the dirty work before the wings get glued to the fuselage. Besides, the fuselage is due for its share of cleanup work anyway.
More to come…
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Good work, thanks for the update! Best of luck with your #1 engine experiment, interested to see where it goes. I generally try to fit and fill the pylons, then paint the engines separately and glue them on at the very end.
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Yeah, i’ve done what i think is the best job of assembling, cleaning up, and tailoring the fit of the pylons to the wings (gap mitigation). However, the fit i can attain without additional sanding and filling on the pylon is about as good as i can get it. The fit of the pylons to the wing is indifferent, at best. Not the unmitigated disaster of the Minicraft 707, but definitely not the hassle-free installation of an AA kit, either.
Complicating matters further is setting the proper angle of the pylon, relative to the wing. My assessment is Roden didn’t take this into account.
This is kind of an applied experiment in how hard it will be to prime, troubleshoot, mask, paint, and mask off an engine nacelle, in situ…..
d-bateable.
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Greetings!
I’m not sure if you mean the angle of the pod canted up or down front to back, or if you mean how the engine hangs relative to the dihedral, but on the 707/720, the engine cants outward with the pylon at right angles to the wing dihedral. From broadside it is hardly noticeable, but from head-on it is easily seen. This is true regardless of the engine type, turbojet or turbofan. Check photos. The walkaround right here on the 707-153B will give you a good view.
Best regards,
Jeff Jarvis
God's "Curse" to aviation!
I am speaking of the angle of the pylon, relative to the wing dihedral. I’ve tried to incorporate this into my build.
Film at 11.
-d-
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Doubling down on the crazy, 720B powerplant integration begins…
As always, keep those letters and cards coming.
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Greetings!
It looks pretty good to me. Those pylons are difficult to get “Just right” with such a small width of pylon sailboat fairing area to work with anyway, but it looks like you managed to get it looking about right. It is so subtle that many folks do not realize that the pylons cant outward. Boeing did it that way to ensure that they would not have any localized flow issues creating drag at the pylon/wing leading edge juncture. That turned out to be a drag problem area on the early DC-8. Suspending the engines further forward by 40 inches and undercutting the pylon on the DC-8-62 and -63 completely eliminated the drag problem.
Do you have access to any photos of the flap area on the bottom of the wing? It’s a relatively minor thing since most folks do not look at the underside of the wing, but Roden did screw up that area on the kit. It should have cove doors ahead of the flaps on both inboard and outboard sections, and it should be a relatively simple fix with a little putty and rescribing, but it is perhaps better done before attaching the engines. The cove doors open upwards when flaps are extended and their purpose is to cover up all of the junk in the flap well so the air flows smoothly over and under the flap. All 707 and 720 airplanes with the various different wings have them.
I’m looking forward to seeing the finished model. Good job with all of your patience!
Best regards,
Jeff Jarvis
God's "Curse" to aviation!
Hi Jeff,
First off, thanks for providing some background on why Boeing set the pylon angles that way. I was always wondering why they went to the trouble to do that.
I knew the early DC-8s had unanticipated drag problems (what plane DOESN’T?) but i didn’t know it was a flow/interference issue between engine, pylon and wing. Moving the engine forward by 40 inches is a pretty significant re-engineering effort! Now i wonder if that issue was also one that affected the Convair 990 which isn’t talked about.
Regards the 720 wing: Roden made an effort to depict the flap cove doors, but it was either half-hearted or based on incorrect information. They look just like the upper wing spoilers. The issue with the Roden wing is, it is missing just. so. much. detail on the lower surface. It becomes obvious when you compare the wing to that in the Minicraft KC-135 (a nice kit once you look past the Bizarro World side cockpit windows).
Plus, i still need to extend the horizontal tailplanes…
Adding all those lower wing details, without silver plastic (for example the Revell 727 kit) to work with will be time consuming and stressful. This may be my only Roden Boeing 720, and since my next airliner will be a Trident or a Convair, i’m inclined to simply sit on the sidelines and wait for the family of 707s to come from Ukraine.
Plus there’s that DC-8-32 that’s not too far off as well.
-d-
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Greetings!
You are doubtless aware that the Convair 990 had numerous shortfalls which required fixes all over, somewhat like the MD-11 did.
It did not take long for the test people to determine that drag was higher than expected on the 990, and there was also a problem at speed and with full tanks in the outer Whitcomb fairings which caused oscillations in the outboard engines, especially in a banked turn. You would say it was similar to flutter in that it could ultimately cause severe damage if not dampened, and it had to be eliminated. The solution was to move those engines rearward 28 inches, thus stiffening the pylons in effect, and that worked.
The rearward movement of the outer pods also increased interference drag, and the tailpipe arrangement on all four engines was also a source of drag. The tailpipes were revised in shape and length on all four, and the wing to body fairing near the rear of the wing was also revised in shape and a slight bulge was added. In addition, American Airlines who wanted SPEED above all else added the spats on the inner side of the tailpipe.
All the other buyers were after range more than speed and their airplanes had no spats and a smaller outboard pylon that angled up to the rear of the wing, whereas the spatted engines had a pylon that curved up to the trailing edge of the wing. There was also a change to the pylon/leading edge junction and the leading edge slats were deleted and changed to Kruger type flaps for the entire leading edge.
After all was done, the airplane met the guarantees that Convair had made on the second set of guarantees to American. The contract had been changed when it became obvious that the 635 MPH cruise and 4,000NM range was not achievable. The second contract specified 620MPH, but at that speed, the 990 could not do BOS-LAX or IDL-LAX, but American accepted them anyway mainly because they needed the capacity. The fleet started being leased out and sold as early as 1967. When operated a little slower by SWISSAIR, VARIG and others, the 990 would do close to a 3,500 NM segment.
Ironically, the DC-8, which was tagged by many as being s-l-o-o-o-w compared to the 707 and the 880/990 airplanes, cruised along quite well at M.82, and once the price of fuel started skyrocketing in the 1970’s, everybody discovered that the best economy speed to operate the DC-8 and 707 was M.80, and SPANTAX, who had bragged about how fast the 990 was started slowing down, and kept going slower and slower until they were only doing……M.80 as well.
The Revell kit (1/135) does not have the mods on it of the 990A, but the Eastern Express 1/144 kit has all of the right features to be a 990A right out of the box. What a little gem!
It is a beautiful airplane, but commercially, it was a failure with only 37 built, and I never had a ride on one.
Best regards,
Jeff Jarvis
God's "Curse" to aviation!
Yeah, i’ve cut my EE Convair 990A off the sprues and test fit a lot of it.
So far, i like it. So much so, that i bought 3 of them.
It’s a super little kit, though you need to put up with people bitching about the price tag.
Last time i checked, it was a DC-8 that went over Mach 1, and not a 707…..
-d-
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Gorgeous work on my least favorite part of building 4-engine jetliner models, Dave. Can’t wait to see this one finished!!
Jeff
Greetings!
Right you are about the DC-8 being deliberately taken over M1.0, and to be specific, it was M1.012 and Chuck Yeager was flying the F-104 chase plane that was observing. On that flight the DC-8 also went up to just over 51,000 feet, far higher than you would go with passengers. The airplane was a DC-8-43 which was delivered to Canadian Pacific a few days later. I was on the airplane in Opa Locka, Florida a few months before it was scrapped and it had a plaque noting the achievement.
My reference to the DC-8 being accused of being s-l-o-o-o-w was mainly because none of the DC-8 operators chose to try for speed records like American did with their little 707 and 720 airplanes. Once they set the records, they slowed down to normal speeds. Although some people have claimed the DC-8 to not be capable of going fast, when I was requested to try to make up time so as to make the sort in SDF by our dispatchers, I occasionally did M.86 in the DC-8. Although it is not cost effective (and is not in a 707 or 990 either), the airplane was capable of doing it. On all of the narrow body jets I flew (707, DC-8, 727) we operated at M.80 for economy, and we did M.84 in the 747.
When you do the EE 990A, what airline markings are you going to do? I hope you do a VARIG! Their colors looked great on every airplane they ever operated IMHO.
Best regards,
Jeff Jarvis
God's "Curse" to aviation!
Well Jeff, we got three engines on the wings now; number 3 is last to go on.
I cant emphasize enough the importance of incorporating a metal locating pin in the installation. Using a combination of CA for the pin and Tamiya extra thin for the pylon will result in a solid joint that can withstand side “wobble” loads without cracking halfway.
Number 2 engine did not have a pin, and i cracked the glue joint twice.
Number 3 engine is in cleanup right now. A combination of missing/obliterated panel lines and sanding scratches need to be dealt with.
There really is no “right” way to do this. It’s a quest for the least terrible option…..
-d-
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Jeff, i haven’t committed to a carrier just yet, though right now the front runners are Modern Air, Garuda and Maybe Alaska. Alaska is the front runner for my -880.
I do agree the Varig scheme looks lovely though i was thinking about applying it to a DC-10 or an Electra.
Forgot to mention that i’ve been using my Atlantis 990 as something of a testbed for tools and techniques. For example, i filled the recessed cabin windows and cockpit windows with a mix of CA and dental resin. Having the EE Coronado in hand, i can use it as a guide to incorporate some improvements to bring the Revell kit somewhere closer to production standard. The toughest part would probably be modifying the trailing edge of the wing at the root, and re-crafting the wing-fuselage fairing.
Might be good enough for an article by itself. Also wondering if its worth it to try to rehabilitate the Minicraft 707 wings and engines.
-d-
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And now, a part of the build that is just as fun* as installing the engines- The cockpit windscreen or as Boeing calls it, the “Cab”.
I used Mig Acrylic white glue since its a very strong joint when fully set. Excess can be rinsed away with a wet paint brush, and it gives me some working time in order to align the canopy.
The gaps at front and back were the worst. Starbond black CA fills the gap but also makes for a much stronger joint. There was no fogging, though i accelerated the glue promptly. I then installed the Montex masks, which actually fit pretty good.
So, time to bomb it with silver primer to see how it looks….
Well….. isn’t that lovely.
Next step was to go back again with more Starbond and fill the longitudinal seams…
We then went “Full Winchester” on the canopy, and this is the end result….
With the windscreen integrated to my satisfaction, i started from nose to tail and began re-scribing panel lines lost during the sanding process. Much of this was lost during the filling and sanding process with the cabin windows. Mostly straight lines. I used Tamiya masking tape for curves for all the lateral bulkhead/frame joints, and 3M Scotch transparent tape for longitudinal lines, like the window belt line, etc…
The odd appearance of that Evergreen bar stock used to fill the windows somewhat undercuts the serious nature of this narrative, but nothing paint and alcohol can’t fix.
Well, i know you want all this excitement to continue but that’s all i have for now. The Number 3 engine is drying on the wing, and i guess its back to the powerplants for the next installment.
I know- the excitement is contagious.
-d-
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Greetings!
It’s all looking very nice! The nose looks quite Boeing to me.
Do you have any idea how much time you have put in for any of your models? Obviously, you need to have patience and determined discipline to see these projects through to the end.
Best regards,
Jeff Jarvis
God's "Curse" to aviation!
Jeff, that’s hard for me to answer. I don’t keep any kind of working logs. Normally, i have 2 models in the build pipeline and i work on one until i feel like i’ve had enough, or i’m starting to make msitakes.
However right now i have 5 that i rotate through which i spend a couple of hours on each. They are:
F-4B Phantom (Tamiya)
A3D Skywarrior (Hasegawa)
Blackburn Buccaneer (Airfix)
Boeing 720B (Roden)
F4U-1 Corsair (Tamiya).
The Buccaneer and Skywarrior are in what i call the “End Game”. The Phantom is in decals, the 720B is starting to look recognizable, and the Corsair is still having its cockpit worked on.
Spent last night looking at Northwest Orient 720B pics on the mighty Google…..
-d-
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Looking good! Couple questions:
- Any particular reason for the silver primer? Does it make it easier to see high and low spots?
- Do you ever run into difficulties sanding the CA? Whenever I use it as a gap filler, it cures harder than the surrounding material and gives hard-to-sand bumps. That’s why I’ve moved toward Mr Surfacer 500, which is softer than both plastic and resin, and thus easier to work with IMHO.
- Have you tried Tamiya Extra Thin on clear parts? I like using that because it doesn’t fog up, and creates a strong bond between the clear part and the rest of the styrene.
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Hi there,
1) I am using a combination of Mr Color H8 Silver and Tamiya LP-11 silver for two purposes: first, silver is an excellent primer and it highlights pinholes, sanding scratches, seams, etc. The LP-11 is also serving as a base coat for my engine nacelles, over which i’m applying progressively darker shades for things like the rear translating cowl, the heated inlet lips, and the hi temp area in the vicinity of the turbocompressors.
2) I used to use zap-a-gap, but i got disenchanted with it for the very reasons you mentioned. I also tried (am still trying) a mix of medium viscosity CA and dental resin powder. Still learning when and where exactly to use it, so it’s an evolving study.
I switched to using Starbond Black CA woordworking glue and wood knot filler. It’s an interesting animal. Its impregnated with some sort of rubber particles, giving it a distinctly dark black.dark brown colour. It has the same viscosity as medium viscosity CA, but being able to actually see the stuff helps.
It seems to sand more easily than conventional CA glues do but it can still be sanded and polished to glass-like smoothness.
I do use Mr Surfacer 500 as sort of a “special teams” filler, once the bulk of the gap has been filled with CA, CA+Resin powder, etc. The 90-degree joint between engine pylon and lower wing plank, would be an example. The CA is used largely for “gap mitigation”, and the Mr Surfacer does the remaining work.
3) I use Tamiya extra thin extensively, but since my cockpit windows are clear, i prefer to use MiG AMMO acrylic glue to install the windows since it doesn’t fog and the excess can be cleaned up with water. Between that, and using CA filler on the outside, the bond between canopy and fuselage is strong enough.
This is only my second airliner after the VC10, so it too is definitely a testbed for all manner of things….. like gluing the engine nacelles to the wing, first….
-d-
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Thanks for the explanations. I’ll have to give the silver a try sometime.
Have you ever gotten fogging with Tamiya Extra Thin? I’ve not, so I’m curious if I got lucky or if it actually works well anyway. My hesitancy with using glues that don’t weld the plastic is knocking something loose.
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Where are you getting the Starbond black CA?
Gene
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Some people like to use the newer Testor’s liquid cement, but my fear is, like thin CA, liquid cement sometimes has a habit of going where you don’t want or expect it to go.
AMMO glue is thick and gloopy, but it won’t run on me, the excess cleans up with water, and when its fully set up, between the glue bond, any CA filler and successive paint and clear coats, the joint is more than strong enough.
david
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I am starting to road-test the Tamiya LP- series metallic lacquers…
There are probably one or two more shades to be added. Lots of variation in colours, depending on the time frame of photo you go from…
Some photos- not all- show a darker metallic area above the fan cowl just behind the leading edge of the pylon, but there’s no turbocompressor there…
More on this slow-motion disaster movie, as it develops….
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Jeff, unfortunately i have no experience with Alclad so i can’t provide data there. I stay away from the stuff based on what i’ve heard in the field: It needs a gloss black undercoat, the Gloss Black can go bad and when it goes bad it goes REALLY bad, It’s delicate to handle, etc.
Having said that, i have yet to find a bare metal finish that wont rub off the leading edge of the wings.
All i can say for sure right now, is Tamiya Silver holds up well to masking with Tamiya tape, and it seems inter-mixable with Mr Color Lacquers. Just thin it to the consistency of Testors Metalizer and it seems to behave fine. Right now i’m experimenting with creating a Corogard, and the Tamiya LP-11 seems to take light aircraft grey (H338) and Orange (H59) from Mr Color just fine.
It *may* have better scratch covering characteristics than Mr Color. I usually finish up with a grit somewhere in the neighbourhood of 1500, depending on how much grit has been removed from a worn sanding sponge. My rule of thumb is, if you’re putting your base coat of LP-11 silver on an engine nacelle, and you see scratches…. its become a primer coat, LOL.
Onward and….um, well, onward.
-d-
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Progress report on No. 2 engine…
You can see i wrapped up No. 1 engine in Tamiya tape, to protect it from collateral damage…
Several iterations are STILL required to get the right shade of that heated intake lip…
Wondering if the Turbocompressor hot section isn’t dark enough.
Still a little bit of work to go. There is a strip of either Corogard, or Titanium that runs along the upper part of the engine pylon up to where pylon meets wing. Jet engines are not clones; they’re more like siblings. No two are exactly the same, and so it will be with this model.
Stay tuned for the next less-than-exciting episode…..
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For those of you in the audience that are wondering….
Nothing has gone wrong with the 720B; i’m in the middle of relocating from the Vancouver area to the Seattle-Bremerton area, and all of my stuff is in a state of storage.
I’ll resume work on the 720B once i get my model room up and running, in the new house.
-d-
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This is coming along great. The different metals on the engine pylon, and other areas, look different in different lighting conditions. So I would say from the pictures your work looks excellent. Clearly you’ve reviewed a great number of reference photos.
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Thanks! When i resume this build, we’re probably looking at modifying the tailplanes, much the way i did with the Revell 727, the difference being that the gaps aren’t nearly as bad.
The Roden provided attachment point is pretty lame. In addition, i will need to extend the stabilizers out at the tips to account for the 720B mod. They just phoned it in.
I know the pivot idea will work, because i already gamed it out on the 72 scale AMT KC-135.
-d-
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Well, the double lobe crease is there….but you have to look for it. Its not as sharply defined as the one on the old Airfix 707-400 series. It is….. well, vague.
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