Warbirds in my Workshop

David Glen BSc (Hons) MSc, Model Maker, Journalist and Author

Formerly Spitfire in my Workshop

Retrospective: The chin scoop

Wednesday, 27th February, 2013

The P51’s chin scoop is one of the trickier parts of the airframe to model, but fortunately its subtle curves are captured in NAA cross sectional drawings.  I began by scaling these to exactly 20% of full size and printing off sufficient copies from which I could cut paper templates from the 10 or so sections indicated. Essentially, I planned to create the piece much as I had done for the fuselage and wings, the main difference being that the spacing between the stations would be tiny by comparison. This necessitated cutting my sections, not from ply as previously, but from 0.5mm aluminium sheet.  Using inscribed centre lines and horizontal datum lines as a guide I built up the three dimensional shape ‘bread and butter fashion’, filling the spaces between the alloy with thin plywood cut slightly oversize on my jigsaw. By utilising varying thicknesses of ply in indifferent combinations, including 1mm ply ‘shims’, I was able to fill the interstices between stations quite accurately. For adhesive I used Superglue, holding the job under pressure in the vice as each new layer was added. The end result was a very substantive and stable composite block.

Now all that remained to achieve the required shape to a high degree of accuracy was to file and sand away excess plywood both internally and externally until the cuts just brushed into the aluminium.  The final stage, to render the surface perfectly smooth and unblemished, could not be rushed. This required many cycles of automotive primer, rubbing down after each second or third spray coat so as to remove any trace of the laminated substrate and leave a finish as pristine as the metal skin that would surround it.

However, as belt and braces, my intention was to take a mould from my composite pattern and to cast the final piece using two-part resin. I reasoned that this substantial casting would be totally stable, and that any residual telltale laminations could be removed easily. The photographs show the finished piece, and also the nasty surprise awaiting me… shrinkage in the casting!  The resin supplier’s assurance that this is not supposed to happen was of little comfort.  In the end I reverted to my original pattern, re-examined it for surface integrity and fitted that to the model instead.

First I had to cut away that part of the forward fuselage to accept the refined chin scoop assembly, and as an added embellishment I decided to include at least a short section of the internal ducting, just for those who might peer a little too closely into the Mustang’s ‘smile’.  Again my pictures show the stages in this, in which the method used was very similar to that for the scoop itself. The internal surfaces of the duct were lined with pewter sheet, which conforms relatively easily to tight compound curvatures, and the inspection plate was added for good measure.

Fitting the scoop required care to ensure that it faired perfectly with the rest of the airframe. Given that it was created directly from the manufacturer’s drawings, it was gratifying and a tribute to Arthur Bentley’s draft fuselage profiles that very little dressing around the nose area was needed to achieve this. The trickiest part was to trim downwardly angled segments off the two upper corners of the scoop so that is lateral top-line matched exactly the flow of the cowling panel lines as they curved upwards towards the spinner. Once satisfied the fit was adequate, I installed two sturdy wooden pegs with matching holes in the scoop and fixed the assembly in pace using screws rather then glue, just in case of an unforsen need to dismantle my handiwork!

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My completed composite pattern of the P-51's chin scoop. It is made from thin alloy cross sections sandwiched between plywood.
The corners are cut off at a downward angle to match the cowling panel lines. This image serves to illustrate the significant number of laminations used in achieving the exact shape from the drawings.
A resin casting is taken from the master pattern. Note the drawn cross sections in the foreground.
Now the nose is recessed to accept the finished chin scoop.
Space for the internal ducting is routed out.
A short section of ducting is made. Note the internal lining of pewter and the inspection cover detail.
The dummy ducting is fitted.
But lo and behold, the casting has shrunk...
..while the original pattern fits perfectly!
Nothing for it but to use the original.
Well over a year later the nose section has been alloy skinned, etch primed and is ready for painting. The chin scoop fits perfectly.
Under the paint is impossible to tell that the scoop is not metal like the rest of the airframe.
Mustang in my Workshop book cover

Mustang in my Workshop

A book to inspire, encourage and empower the enthusiastic model maker to scratch build a masterpiece.