1/72 Link Celestial Navigation Trainer

by Fred Harris


The Celestial Navigation Trainer resulted from a request to Link Aviation by the RAF to develop a simulator to aid the training of navigators involved with ferry-flights across the Atlantic in WW2. The RAAF ordered three CNTs in 1944,suitably modified for use in the Southern Hemisphere. A63-1 was installed at East Sale with Tocumwaal and Rathmines considered as locations for the other two. It is possible that the Rathmines one was installed but servicing difficulties and lack of spares backing resulted in the E. Sale installation being the most successful unit. A63-1 operated intermittently until 1957 when it was superseded by the D.R. Nav. Trainer and was finally dismantled and sold for scrap at auction in August, 1962.

THE TRAINER - The simulator was housed in a 45 foot high octagonal shaped silo. A fully equipped fuselage pod capable of housing a pilot, navigator, bomb-aimer and wireless operator was mounted on a universal joint atop a rotatable tower located directly under the celestial dome.  The fuselage was capable of limited pitch, bank and turn, actuated by vacuum operated bellows. Attached to the tower was a screen on which terrain images were projected from a camera below. The image travelled along the screen according to the direction and speed of the simulated aircraft in flight.  Wind direction and velocity could also be introduced into the terrain movement. Various scale images were projected onto the screen to simulate changing altitudes.  The dome had a system of lights to represent stars and constellations and was mounted on a rail to allow it to move to simulate the passage of time.  The trainer could also be used to train bomb-aimers.

The model

Despite the recent finding of considerably more reference material for the CNT some problems remained. Three sets of artist's impressions of the interior of the silo each showed significant detail differences. Also, only two dimensions were given in the references and even here a contradiction occurred. The height of the silo was quoted as 45 feet  but the width either 22 feet or 26 feet.  By proportional scaling of a photo of the erected silo the 26ft dimension was settled on. Another photo indicated that the walls were approximately six inches (150mm) thickand the I-beam columns of the frame approximately 12 inches (300mm) wide. From this information and the interior perspectives, a representative set of drawings was prepared. One critical item however remained, soucing a tea strainer of suitable size for the dome. If none was available, what would be the alternative? A search through several kitchenware shops finally located a 70mm diameter strainer which. after separation from the frame, gave a slightly distorted hemispherical mesh 64mm diameter.  The shape was improved by stretching and burnishing the mesh over a spherical glass 'snow dome' ornament which was exactly the right size.

Silo Walls and Roof - The walls and roof were the first items made. A wall panel was cut from Evergreen metal siding No 4527 and backed with 1mm plasticard to increase the thickness to the required 2mm (1/72 scale 150mm). Evergreen 4526 would have been a more accurate scale choice but was not available. A roof panel was cut from 1mm plasticard and covered with .125mm (.005”) aluminium litho sheet lapped clincker fashion. Five wall panels and eight roof panels were cast in resin in RTV moulds. Unusually, several of the castings cured with an oily film on the surface away from the mould but when wiped off the casting seemed to have cured satisfactorily- but trouble emerged later. The 'concrete' floor and base is .75mm plasticard.

Frame - The frame was constructed from Evergreen 3/16 inch I-beam, 1/16 inch and 0.080 inch angle and 3/16 inch I-beam slit lengthwise to give the T sections. Gusset plates were cut from litho sheet.

Aircraft Support Tower - The tower was made from .5mm brass rod soldered together using a perspex jig. Perspex is an ideal material for many things in modelling and in this case, being an insulator, does not conduct heat away from the soldered joint, solder does not stick to it and it does not scorch like wood.

Dome Rail - The dome rail is a curved I section beam with vertical stiffeners. The web of the rail is a part annulus scibed and cut from 1mm plasticard with .5mm by 2mm Evergreen strip flanges glued in place with liquid glue.

- The curved catwalk is two strips of .5mm plasticard wrapped around a former and glued together. This is a more reliable method of making curved items than wrapping heavier gauge strips and heating to set the shape.

Steps and Handrails - The steps are resin castings from a length found in the spares box. The handrails are .5mm brass rod soldered in a simple jig milled from wood.

The rest of the model was straight forward modelling except for the girder cage for the dome.

The Dome
- A jig was made by machining a 4mm wide by 1mm deep groove in a piece of perspex. The girder is .5mm brass rod which required annealing to enable it to be bent into a semicircular shape without springback. After numerous modifications of increasing complexity to the jig a set of four girders was finally achieved. One girder was left intact, one cut in half and two bent to an included angle of 90 degrees.  The straight girder ran across the dome, the bent ones forming the 45/135 degree sections and the half pieces the 90 degree section. The pieces were attached to the mesh by lacing with fine wire and solder. The bottom rings were fomed from soft floral arrangement wire assembled in the girder jig.

Assembly - Again, simple jigs were cut from 2”x1” pine to assemble the walls. It was when painting that the earlier problems with the resin re-emerged. The paint would not adhere to the inside of two of the wall panels and most of the roof panels. After a number of attempts to seal the surfaces, the inside of the roof and the two recalcitrant wall panels were lined with litho plate attached with double-sided tape. To avoid possible failure of the tape at a later date, all joints were superglued together and around the edges.
The internals were assembled slowly and with regular checking to avoid foul-ups then the silo added after which  the front section of the roof was removed to allow the upper section of the framework to be visible. The assembly was mounted on a commercial base and the surrounding area covered with ground flock.

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