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How to convert 707 engines to the Comtran Hushkit version.
Author: Merlin Woodman
Submitted by: LH707   Date: 11-26-2014
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707 Comtran Hush Kit Conversion Guide

In the 1980s, the JT3D-powered 707s were getting pushed out by stricter noise regulations, so a number of companies created hush kits to make them ICAO Stage 2 and Stage 3 compliant. Comtran International created two designs, the first used on the 707-138B and the second for the 707-300B/C. Both of these types had a lengthened bypass duct, and the version for the 300B/C had a lengthened fan case as well. This guide will show you how to create the lengthened bypass duct using a 1:72 Heller 707 engine as a template. The process can be done with other kit engine architectures and in 1:144, but may need adaptations (or more skill) to complete.

First up, glue the halves of the core together, and then saw a slot in them across the YZ-plane using the NACA inlet on the port side as a cut guide, and cutting from the bottom of the engine towards the top. Cut up until where the core will join the pylon, but no further. If you wrap the core with tape, it makes the cut easier to align.

Next, cut out a rectangle of .020” (.010” for 1:144) sheet styrene that’s a bit wider and deeper than the core’s diameter, and chop a notch out of the top of it that’s the exact width of the pylon. Insert the piece of styrene into the slot in the core, and without gluing it, carve it down so there’s a uniform width off of the core that leaves the same gap as the fan bypass duct does further forward. Once you’re happy with the shape on the first disk, pull it out of the slot and use it as a template to make the other three. When they’re complete, glue them all into place in their slots.

When the disks are firmly glued, cut small strips of 0.10” styrene about 5 mm thick and long enough to go around the circumference of the cores. Mark the center of the strip where it goes around the bulge, and mark where the kink occurs where the bulge meets the rest of the core. Next, score the inside of the strip so it makes it easier to bend. Then, use tube glue to attach the middle of the strip to the bulge. Once this dries, glue each side down one at a time, taking care to jam the strip all the way into the kink.

If you’re also doing the lengthened fan duct for a 707-300B/C (not pictured), make a similar slot cut all the way through the fan cowl between the circumferential panel line on the bypass duct and the forward edge of the turbocompressor exhaust panel and add 5 mm of length in this gap you just created, while keeping the fan mount point back in its original position relative to the core (the engine needs to stay the same length, after all). Finally, glue the core into the fan section.

Lastly, add putty into the circumferential gap between the core and the original bypass duct and sand it to shape. It’s helpful to apply the putty up until the panel line under the middle of the turbocompressor exhaust, as this helps create a uniform start to the puttied region. Once you’re happy with the shape, paint and install your hush-kitted engine to your 707.


by Merlin Woodman

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