Project News
Projects which inspire others to build their own aircraft Compiled by Mike Slaughter
Project News A slightly different tack for Project News this month, we have a submission detailing a specific construction technique. As space is tight, let’s get straight into it. Two months ago, we had an update from Andy Best on his long running Blue Two original design, he was reporting difficulty forming a bubble canopy for the aircraft and wondered if any readers could assist with some advice on the subject. I’m delighted to say that a couple of fellow builders were put in touch, and it turned out that Tony Razzell was somewhat further down a parallel path. We saw Tony’s own build in this column back in early 2018,
when he gave us an update on his Nicollier Menestrel II, a rather nice wooden scratch-built type with very ‘Spitfiresque’ wings. Andy and Tony have been in touch and, fingers crossed, this will get Andy past his difficulty. One thing led to another, and Tony has kindly given Project News a detailed account of his exploration into the process of forming a complex three-dimensional canopy. So, I’m going to cut the waffle and hand you straight over to Tony To get in touch with Project News, and tell your story, report a milestone or just to send a picture, email: projectnews@laaarchive.org.uk. Please share your story!
Experiments in forming acrylic sheet – A lockdown project By Tony Razzell
F
or my Nicollier Menestrel II project, I had reached the point where I needed to start thinking about a canopy. Whilst canopies are available from France, by the time VAT, transport (and now with Brexit, import duties) are taken into account, it starts getting very expensive. Being someone who likes to get back to first principles and make things from scratch, I wondered how hard it would be to make my own. In this article I describe my journey so far. Thermoplastic polymers soften when heated and can then be formed to shape, and a wide variety have been developed for different applications. Polymethyl Methacrylate, or PMMA, is a thermoplastic polymer which was developed in the UK in the early 1930s by ICI, and in Germany by Rohm & Haas AG. It is a transparent material that can be either cast or extruded to form thin sheets, the latter being more amenable to vacuum forming, and is widely used for light aircraft windshields and canopies. There are a number of trade names, including the well-known Perspex, Plexiglas, Lucite etc. It is sold widely as ‘acrylic’ sheet, which is the term I will use for this article. I started by reading up as much as I could about
10 | LIGHT AVIATION | August 2021
thermoforming acrylic sheet at home, and there is a good range of information available on the internet (bibliography at the end of the article). There are a number of forming methods applicable to light aircraft canopies, summarised in Table 1 as follows: Table 1 – Acrylic Sheet Forming Methods Method
Tooling
Comments
Free blown bubble
Lower edge of canopy formed by oval hole in 12-18mm ply sheet, pressure boundary formed by another 12-18mm ply panel, seal between rear ply panel and acrylic
• Good optical quality, no ‘mark off’ on surface • Limited to simple cross sections (part of a circle) • Sheet thins significantly at top of canopy • Large sheet needed, significant wastage likely, large oven
Drape forming
Male form tool covered in felt and coated with grease to aid slip during forming
• Complex cross sections possible • Tooling time & cost • Manual assistance needed for forming (four big blokes) • Mark-off risk on internal surface
