Stretching the Fuselage
I will be converting Mach 2’s 1/72 DC-8 into a -73 freighter.
Comparing the fuselage station diagram of short body DC-8 to that of a -61/63 stretch shows that the forward plug is 240 inches long (6 windows) and the aft plug is 200 inches long (5 windows).
Fuselage plugs have to be very straight. A long fuselage such as this will exaggerate any misalignment in the plugs, and make the airplane appear crooked. The procedure I followed came from the May 2015 issue of Fine Scale Modeler (The brilliantly written article on page 44). To keep things as straight as possible, I created one plug at a time, letting the glue dry thoroughly before moving onto the next.
I started with the left forward plug, as shown in the next image. I sawed the left forward fuselage, and spread it apart to make a gap the same sized as the plug. Next, using the uncut right fuselage as a splint, I solidly taped everything together to keep things correctly aligned. Now, I glued in as many Evergreen styrene strips into the gap as my fat fingers would allow. Since the fuselage halves are taped together, you have to work from the outside. Let this dry thoroughly. When the glue is dry, untape the fuselage halves, and work from the inside to fill in the plug area with as many Evergreen strips as possible to create a really strong joint. I also added a couple of vertical stiffeners to keep the crown and belly from sagging inwards. Tape the fuselage halves back together to ensure this second batch of strips dries straight.
The next image shows the sequence in which I completed the remaining three plugs. The left rear plug is next. Again, use the uncut right fuselage as a splint to keep everything straight. Once both plugs on the left side are completed and the glue is dry, use the left side as a splint and repeat the procedure to create the plugs on the right side. Staggering the plugs on the left and right sides improves straightness and strength. St this point, the fuselage is stretched, aligned correctly, and very strong.
Now, the Evergreen strips need to be skinned over smoothly with some Evergreen sheet. It will be easier to apply these sheets if they are pre-curved to the fuselage diameter. This is done by gently warming the sheets while they are wrapped around a circular form, as shown in the next image. Find something around the house which has the same diameter as the fuselage, around which you can gently warm and bend the plastic sheet. Use elastic bands to press the plastic sheet against the curved surface. Caution – If you place the elastic bands directly against the plastic sheet, they will dig into the heat-softened plastic sheet, damaging its nice smooth surface. I found that placing a thin metal sheet (ie. Soda pop can) between the plastic sheet and the elastic bands will keep the elastics from digging in. What thickness of plastic sheet do you use for the skin? Unfortunately, the fuselage wall thickness of the Mach 2 kit varies between 0.050 and 0.090 inches. I opted for a skin that will be laminated using multiple layers of thin and easy-to-work-with plastic sheet, a combination of 0.020 and 0.030 inch thick sheets. Accordingly, the number of layers of skin panels varies from place to place around the plugs depending on the local fuselage wall thickness. Putty and sanding eventually smooths everything out.
The double bubble fuselage crease must also be reproduced in the plugs. The easiest way to do this is shown in the following image. Simply arrange the Evergreen skin sheets so that they form a joint at the crease line.
I changed a few of the fuselage details, both to improve on the basic kit and also to do the -73F conversion. In order to position details correctly on the kit, I used the fuselage station diagram shown at the beginning of this article. Although the Mach 2 kit did not line up exactly with the drawings, it was nevertheless “close enough for my aged eye”. Note that DC-8 door and window configurations varied significantly, especially on the Super 8’s. There is no such thing as a standard configuration. Make sure you research the specific airframe you are modeling. So, on with the fuselage changes I made.
The cabin windows on the right side of this kit are a bit too low as shown in the image below. This means that any fuselage stripe running along the window line meets the cockpit windows at too low a position. The 1/72 Heller 707 had a similar problem, but on the Mach 2 DC-8 the problem is only on the right side.
On my previous build, I noticed the problem too late, so the only way to fix it was to tweak things a bit when I painted on the right window stripe. This time, I raised the cabin windows on the right side. Luckily, this build was a freighter so I puttied over most of the windows, and there were very few cabin windows left to raise. Skipping ahead, the following image shows the finished model the raised right side windows. The tops of the cabin windows correctly line up with mid-way up the cockpit window.
The nose landing gear on the kit was too far forward, so I moved it aft. The following drawing shows its correct position.
I needed to position the main deck cargo door on the -73F, but the only fuselage station diagram I had for a Stretch 8 was for a passenger version, with no cargo door. Therefore, to correctly position the cargo door on my -73F, I adapted the following dimensions from a short body freighter.
On the nose, I faired over some of the “nostrils” on the air intake scoops. I puttied over all three of the kit’s exhaust vents on each side of the nose, and I created two new rectangular shaped heat exchanger exhausts on each side of the nose.
The under-floor baggage doors on a stretch are different than those on a short body. I used the following drawing to position them. Note that the baggage door immediately aft of the wing varies in position depending on the airframe. That is why mine varies from the position shown in the sketch. Check your references.
The left front cabin door on the kit was a bit too far forward, so I moved it aft to match the position shown in the fuselage station diagram. Previous builds have shown that the right stabilizer is too high up on the fuselage. I will correct this later when I glue on the stabilizers. Finally, the horizontal stabilizer sealing plates moulded into the fuselage were too long, so I sanded them off and will replace them later with plastic card. I etched in the extra cabin exits just forward and just aft of the wing. Note that these extra exits vary in size and location on different airframes, so check your references. I re-etched the left rear cabin door, which on a stretch is 20 inches further aft compared to its position on a short body. On the fuselage right side, I re-etched the right rear cabin door, which on a stretch, needs to be moved aft, so it is immediately opposite the left rear cabin door. Finally, I added extra holes in the wing root area, which I will use later for wing root reinforcement.
In my previous build, I sawed off the rudder and constructed new hinges to make it appear more realistic. In retrospect, that was a lot of work, so this time I tried to get the same look with less effort by simply adding small pieces of Evergreen strip to the rudder hinge areas. The result is shown in the image below, and to my eye it looks just as good. Note that Mach 2 has incorrectly moulded the uppermost rudder hinge. It should actually be parallel to the ground, not inclined like the others.
I similarly added Evergreen strips to the elevator hinges, and constructed some elevator trim tab fairings. These fairings appear on both the left and right sides, but on the bottom surfaces only.
Here are wing station diagrams. Unfortunately, the kit dimensions did not match the drawings very well, so I ended up scaling things only roughly.
Mach 2’s wing-to-fuselage joint is weak. I strengthened it by adding a large piece of Evergreen plastic strip to the inboard end of the wing, and cutting matching holes in the sides to the fuselage. Ensure your design allows for a generous wing dihedral. The wing will be solidly anchored into the fuselage a bit later in the build process.
Here is a summary of everything I did to the wings.
I cut the flaps away by following the engraved lines on the kit wing, added some Evergreen plastic tube to simulate the flap nose, and blended it in using putty. Evergreen strips were used to simulate the flap vanes and tracks. I made the flap attachments from thick Evergreen plastic strips, which I buried deep inside the flap and wing for maximum strength.
Sawing away the flaps from the wing compromised its rigidity, so I added a stiffening bulkhead along the wing trailing edge.
Leading Edge Slots
The slots are difficult to model since they are buried in the wing behind the leading edge. I found that temporarily cutting away the leading edges in front of the slots gave me better access and greatly eased the process of modeling the slots. The slot positions engraved in the wing of the Mach 2 kit are incorrect. They should actually be more outboard, as shown above in the photos and wing station diagrams. Note that on the -62/63 models, the outboard ends of the slots were perpendicular to the leading edge, rather than parallel to the engine pylon as they were in -61 and earlier aircraft.
The Mach 2 kit does not have sufficient wing and tailplane dihedral, the main landing gear legs are too long, the nose landing gear leg is too short, and the engine pylons are too high.
All these problems must be corrected or the engine nacelles will be too close to the ground.
The easiest and most precise way to solve the problem is to build a support frame out of popsicle sticks, that holds the fuselage and wings at their correct distances off the ground. Once these bits are solidly supported in their correct positions, you simply glue them together.
Use the dimensions from the following ground clearance chart.
First, build a popsicle stick support to prop up the fuselage so that the passenger doors are the correct distances off the ground (Distances “A” and “B”).
Next, build a popsicle stick support to prop up each wingtip so that the wingtips are the correct distance off the ground (Distance “R”).
Finally, build a popsicle stick support to prop up each stabilizer tip so that they are the correct distance off the ground (Distance “Q”).
Later, when we install the engine nacelles, they will have to be distances “M” and “N” off the ground.
This support will also be used to adjust the landing gear legs to their correct lengths.
At this point, the fuselage halves are only held together with elastic bands. DO NOT GLUE THE FUSELAGE HALVES TOGETHER JUST YET.
Now, glue the wings and stabilizers onto the fuselage.
Since all the bits are supported at their correct distances off the ground, the dihedrals will automatically be set to the correct values. You can double check that the dihedrals are 6.5 degrees (wing) and 10 degrees (stabilizer).
Once the glue is hard, take the elastics off the fuselage halves, and reinforce the interior of the wing-to-fuselage joint.
Increasing the wing dihedral to the correct value leaves a gap on the bottom of the wing root which needs to be filled.
Now, you can glue the fuselage halves together.
I used the Contrails Resin CFM-56 engines and pylons. Since these were designed for the KC-135R kit, I had to alter the pylon contours a bit. I also strengthened the pylon-to-wing joint by adding a tab to the top of each pylon.
I also made small changes on the bottom of the wing.
Finally, with the fuselage and wings propped up on the popsicle stick supports, I adjusted the pylons to attain the nacelle ground clearance distances “M” and “N” specified in the table.
The landing gear provided in this kit was not the right length. The mains were too long and and the nose was too short. To correct the problem, I simply propped the model up on the popsicle stick supports so that the front and rear cabin doors were the correct distance off the ground. Then I shaved or added as required to the tops of the landing gear legs.
I accidentally shaved a bit too much off the top of one of the main landing gear legs, and ended up with wingtips at slightly different distances off the ground. To fix the problem, I just shaved a bit off the tire bottoms on one side.
I used little circular jewels available from the craft store to simulate the landing and taxy lights. The wingtip lights consisted of clear colourless plastic covers with a small red or green bulb buried within. To simulate these, I cut away the tip of the wing where the lights go. I then trimmed a bit of clear sprue to fit exactly into this cut-out and act as a clear colourless lens. Next, I drilled a tiny hole in the lens and filled it with red or green paint.
Thanks to earlier builders, I was warned that the right rear stabilizer is mounted too high and must be lowered. I placed the aircraft in my popsicle stick supports, and lowered the stabilizer attachment point until it matched the opposite side.