Bill Matthews takes us step by step on coverting the EC-135N to a 720B.
Author: Bill Matthews
Submitted by:   Date: 03-06-2004
Comments: (2)   Ratings:

At the 2002 British Nationals at Telford, an editor I knew approached me with a parcel under his arm. "Do you think that you could put windows into this" he asked, unrolling the parcel to disclose a very large kit. I recognised it as a model of the EC-135N in 1/72 scale. Derived from the AMT KC-135, this new version is made by Amtech. Windowless it certainly was, while its grossly misshapen nose suggested that I was being asked to bite something that would take a good deal of chewing. However, being a confirmed rusher-in to situations where any angel of normal mental capacity would fear to tiptoe, much less tread, I blithely agreed. What I didn't know was that, by the time I had finished the model, his magazine would have ceased publication!

Getting Started
Though superficially similar in appearance, the civil and military derivatives of Boeing's pioneering dash-eighty prototype are very different aircraft, more cousins than brothers. Of all of them, my favourite is the 720, or, if fitted with JT3D turbofans, 720B. This was Boeing's hot-rod, a smaller, lighter, cheaper and marginally faster version of the original 707-120. It was developed in 1957 to capture a large order from United Airlines; TWA and Delta had already ordered Convair 880's, and Boeing realised that, unless they took drastic action, United would do the same. They succeeded: making its first flight in November 1959, the 720 not only won United's order, but also orders from fourteen other airlines for 134 further aircraft over the next eight years. Most had long careers, some remaining in airline service well into the 'eighties. Save for a few used for test or executive purposes, they are all gone now, most being scrapped for spares for the USAF's tanker fleet.

When I got home, I sat down and wondered what on Earth I was going to do about that nose. In 1/144 scale I could probably recapture its original shapely contours, but in 1/72 - ? Then I remembered that a friend in the IPMS Airliner SIG, Hilton Jones of Cardiff, had combined a Heller 707 with an Airfix C-130 to produce, of all things, a Vanguard. (Don't ask me how he did it, but I've seen the model, and it's beautiful). I rang Hilton: had he any left-over sections from the 707? He had, and they included both the nose and all the transparencies. Better still, he offered me a selection of decals by Flightpath - I had no idea that anyone had issued 707 liveries in 1/72 scale. I chose Continental, one of the most beautiful schemes ever carried by an airliner. Without Hilton's help, I doubt whether conversion from the EC-135N would have been possible. If, however, one of the original AMT KC-135's, now out of production, can be found, the problem disappears.

The Kit
The kit is generally of good quality, accurate and finely detailed. The fit of some of the parts could be better, and in some places, notably the fin and starboard wing, there was quite severe distortion. This was, however, a test shot, and production-standard kits probably do not suffer from these faults. To save time, and also to overcome the distortion, Superglue was used for assembly.

The first job was to widen the fuselage. When a civil derivative of the KC-135 was first offered to the airlines, they insisted that the upper lobe of the fuselage be widened by 4", to permit six-abreast seating. Reluctantly, Boeing complied, and this was the key factor in the commercial success of the entire 707 series. Four inches doesn't sound much, but in 1/72 scale it works out to a noticeable .056". A strip of .030" card was therefore glued to the top edge of each fuselage half, tapering off at each end. This widening also produces the so-called "crease" at floor level, actually the line along which the slightly different radii meet. An overlap strip of .020" card was fitted along the joint, as one does on a vacform.

EC-135N Vs. 720B
Comparison of photographs makes it very obvious that the 720B's wing was much further aft than that of the EC-135N. The question was, how much further? One of the problems I had in modelling this aircraft was that I could find no accurate plan of it. There is, however, a superb book on the 720 by Jon Proctor, which includes a large number of true side-view photographs. I also knew that the frame spacing of all 707, 727 and 737 fuselages is 20", so that the distance between window centres is also 20". Another photo in the book showed that an extension of the line along the leading edge, from tip to inner nacelle (ignoring the extension inboard of the inner engines) hit the fuselage under the centre of window no.15 on the port side of an American Airlines 720. I specify American Airlines because 720 window arrangements differ from airline to airline; luckily, Contintental's aircraft had the same arrangement. Measurement and calculation showed that the wing had to be moved back three frames (60" = 0.833" in 1/72 scale). Using a razor saw, the entire wing fairing was cut away from the left fuselage half . Vertical cuts were made at the front and rear ends of the fairing, joined by a horizontal cut ?" above its top line. This cut was then extended aft by exactly 0.833", a third vertical cut meeting it at its rear end. The fairing was then rejoined to the fuselage half along its top and rear edges, and the gap at its forward end made good with a piece from the Heller kit, again glued along its rear and upper edges only. All this went very well, and once everything was in place, little filing or filling was needed. Using the left fuselage half as a jig, the process was repeated on the right half. Inside the halves, the joints were cleaned up and strengthening plates of .020" card glued across them.

The 720B's fuselage is one frame (0.277") longer than that of the KC-135, so the cuts at the forward end of the insert were extended to the top of the fuselage halves, and a section from the Heller kit inserted between the two parts of each. The freight door, crew entry door and various small windows were glued into place and faired over. The refuelling boom housing was cut away, and the gaps filled with scrap plastic. The Heller kit's nose was also fitted. Finally, a score about 0.05" deep, fading out at each end and disappearing under the wing fairing 0.085" below its top line, was cut into the fuselage halves with a P-cutter. This runs aft for 11" from a point 3" forward of the wing fairing. The edges of this were then scraped away with a small triangular blade to blend smoothly into the curvature of the fuselage.

Next came the most tedious part of the whole conversion - drilling out, shaping and glazing the windows. Even in 1/144 scale, I do not think that decal windows are realistic; in 1/72, transparencies are essential. The first job is to check the number of windows, as these vary from airline to airline, and even at different stages in the same aircraft's career. In Continental's case, there were 49 on the port side, with the leading edge of the wing under the centre of no.15. On the starboard side there are 39. These line up with those on the port side, omitting the forward 10, where the galley is. Marking these out is far more difficult than it sounds - a spacing error of only .005" will result in the row of 49 windows being nearly ?" too long. I found it best to make a marker from two pins fixed rigidly into a flat piece of plastic, the distance between their points set exactly on a vernier scale. These are pressed into the plastic along a line running back from the lower rear corner of the pilot's windows, re-checking every five marks. It takes ages!

Once all the marks are correct, each is drilled through with a no 80 drill held in a pin vice (spin this backwards and forwards to prevent slippage), followed by a no. 70, then 1/16" then 7/64". Even though their shape is not exactly correct, I used the windows from the Heller kit. Lines were drawn along the fuselage to their vertical size, and the holes shaped to these. I use a round, rather than a square, watchmaker's file for this, as it will not twist and cut ugly irregularities in the window frames. The Heller windows are only 0.020" thick, so a deep trench has to be cut inside each fuselage half, as in the Heller kit, to allow them to lie flush with the fuselage sides. This was done with the edge of a 5" warding file, finished by scraping. When all the windows fit correctly, spray the fuselage halves white. If the chosen livery features a cheat line through the windows, apply the decal, and when it is thoroughly dry, punch out the window apertures, again using the round file, and glue the windows into place. Cover windows and decal with a strip of non-adhesive paper held into place with masking tape, and at last the job's done.

The Cockpit
The cockpit interior is mostly dark grey, with the seats in lighter grey. Part 5 becomes the flight engineer's station, part 12 the galley. Parts 9 and 15, the seats, and the instrument panel are also fitted, all other interior detail being omitted. The nosewheel well and leg are glued into place, as are the walls of the main gear wells. The floor is now cemented into place; although it no longer lines up with the moulded attachments, it goes in without trouble. After adding 20 gms of nose weight, the fuselage halves were joined. Distortion caused some difficulties, but eventually they went together satisfactorily. Make sure that the white paint blends smoothly into the plastic exposed by rubbing down the joint. All the early 707's had ventral fins, in various sizes. That of the 720B is 3" long and ?" wide, and was made from scrap plastic.

The Wings
The wings are in the five sections more or less standard on kits of this size. The joint between the centre section and the lower outer sections is very poor, and it took a good deal of work to achieve an acceptable fit. It might be best to join these parts before cementing the wing halves together. The front wall of the main gear bay projects into the wing root to form a short spar, but the wings are very flexible, and should have rigid spars extending at least as far as the outer engines. Make these from thick plasticard, and cement them to the stub spar, ensuring that the wings have the correct dihedral of 7 degs on their centre line.

On the 720, Boeing modified the standard 707 wing by adding a fibreglass "glove" to the leading edge, which improved performance by increasing sweepback and reducing t/c ratio. Again, I had no accurate details, but from photographs, it seemed that the extension was about 33" at the root, increasing the sweepback to the inner engines by 4 degs. The revised wing section is almost flat on top, the lower surface sweeping up to meet the leading edge. These extensions, made from the leading edges of old Airfix 1/144 scale 707 wings, override the kit's mounting stubs, and the wing-to-fuselage joint has to be carefully re-shaped to obtain a good fit.

The KC-135 has only one air-conditioning intake below the wing, whereas all civil 707's and 720's have two, which are longer than that of the military aircraft. New intakes have to be made from scrap plastic. The projections immediately outboard of the wheel wells must be removed, and the Concorde-shaped recesses under the wingtips filled. The small blisters above the undercarriage are too large, and their length, width and height all have to be reduced. The last job on the wings is to drill two lines of no. 77 holes to take the vortex generators. One, of 25 holes, begins .085" forward of the rear spar line 2.45" out from the wing root. The other, of 23 holes, is 0.75" forward of this. The holes are .090" apart. The vortex generators themselves are made from copper fuse wire flattened at approximately ?" intervals with a nail punch and cut apart with scissors. They project .05" from the wing, but are not fitted at this stage.

The Engines / Undercarriage
Amtech's kit includes both JT3C turbojets and JT3D turbofans. Both were used on 720's, but Continental only used turbofan-engined 720B's. The engines went together well enough, but it proved impossible to get rid of the seams inside the intakes. I therefore made liners from rolled .010" card to cover them (photo 5). Owing to the change in wing section, the inner engines adopted a markedly "nose high" angle. To correct this, a V-shaped slice .05" wide at the rear and tapering to a point at the front was cut most of the way across their pylons. Rejoining the cut edges restored the engines to the correct incidence, the cut then being filled and re-shaped. The fit of the pylons to the wing was not good, and needed a good deal of awkward filing and filling to achieve a smooth joint. Note that the tailplane of the turbojet-powered 720 spans 44" less than that of the 720B, and the kit's tailplane has to be shortened by 0.61" if making this version.

The undercarriage came straight from the kit. It is detailed, easy to fit, though the doors were troublesome, and realistic. The 720B actually has smaller wheels than the KC-135, but I did not bother to correct them.

The only area that was painted was the fuselage, which in Continental's scheme is white overall. Engine nacelles, the rear third of the wing, and the forward and rear thirds of the tailplane are natural metal, and these areas were covered with foil. The forward two-thirds of the wing and the centre third of the tailplane are finished in grey Corogard. Decal was used for this. The Flightpath decal for the livery was one of the best I have ever used, thin, strong, quick to slide, and beautifully-printed. It included the window outlines, which were applied in groups of five, and also went on easily. When the decals had set, they were pricked through over the line of holes in the wing, and the vortex generators fitted. This proved easier than I had expected.

Final Steps
At last my 720B was complete - at least I thought it was. It was only when I looked at photographs of it that I realised that something was badly wrong; it didn't have the jaunty look of the 720, but seemed somehow sad. It didn't take long to spot the problem - insufficient dihedral. I've no excuse for this mistake; Rob Sherry's article, originally published in the Newsletter of the IPMS-USA Airliner SIG, specifically warned that the kit was inaccurate in this respect. I wondered long and hard what to do. Should I ignore the error, and just assume that most of those who saw the model wouldn't spot it, or should I put it right. In the end I decided to; whatever anyone else thought, I knew it was wrong every time I looked at it. I could put the dihedral right, but what would all the hacking about do to the now-unobtainable decals?

With my heart in my mouth, I sawed straight across the wing undersurfaces from the leading edge into the wheel well. The wings were then bent upwards, and wedges of 2mm plasticard forced in. Even this wasn't enough, and further wedges were inserted until a line between the wingtips ran through the eyebrow windows, as specified by Rob. This process, not surprisingly, cracked part of the upper surface joint, but otherwise was easy enough. Restoring the wing underside was a different matter, and took quite a while. The corogard decal over this area was largely destroyed; luckily I had just enough left over to make it good. The joint on the upper surface also had to re-cemented, filled and rubbed down; in the process, the orange stripe of the cheatline was also damaged. Vince Klimas very kindly offered me a replacement decal, but I decided to see if I could repair mine. A mix of Xtracolor X323 and X332 produced an exact match to the decal, and it now looks as good as new. The red and gold were touched up with Humbrol 19 and 16 respectively, and you have to look very closely to see any evidence of repair.

At last, after five months, my 720B was complete. With a span of nearly 22" it looked mightily imposing and pretty as well - but I think I'll stick to straight-from-the-box modelling for a while!

Bill Matthews

by Bill Matthews

Member Comments :

comment by: skippiebg posted on 10-10-2006 #2705
Bill, you must be excused if you thought "pearls before swine" at the lack of comments. I have always wanted to praise this article, but been short of time, and am sure this applies to others. Now I'm contemplating doing a mix-and-match using a Heller 707-320B and a Heller C-135 (to a 320 Intercontinental and 720B), this article was the first reference I reached for. Superb!
comment by: b2707sst posted on 10-09-2009 #8685
Interesting subject. Would love to see your article photos, but they're showing as a html code.