Application Technology

By Ian Parker, freelancer, ianparkerwriter@aol.com

Aircraft were hugely significant in the Allies’ victory in the Second World War and several types remain close to the hearts of Britons. These include the Spitfire, Hurricane and Lancaster, but less well known is the De Havilland Mosquito, which outperformed them all. Why? Because it was made mainly of wood. 

Initially decried as Freeman’s Folly, after the Air Chief Marshall who defended the design, it later became known as the Wooden Wonder as it flew past every other type in the sky. There are just a few flying examples around the world, but The People’s Mosquito project in the UK is building a new one that is expected to fly as early as 2027 – with the fuselage moulds already complete. 

Far from being outdated, its structure and manufacture have caught the attention of Airbus, which is contributing to the build, because the techniques are applicable to current aircraft and spacecraft. “Airbus is a strong believer in preserving wider aviation heritage and we are proud to be supporting The People’s Mosquito in this endeavour. For us, the Mosquito is more than a beautiful and iconic aircraft; Airbus has a direct relationship to it – as more than 90 were built at our Broughton factory in Flintshire, UK. That passion and dedication is reflected today in our skilled workforce – whose talents are focused on building wings for the Airbus commercial aircraft fleet,” explained Jeremy Greaves, vice-president, corporate affairs and strategy, Airbus UK. 

As the editor of Aerospace Materials magazine in the 1990s, I often looked into how old and new could come together, in terms of the materials and their processing and application. There was a fashion to use the new stuff, but it didn’t always make economic sense. If a new material is 50% better than its predecessor, it’s no good if it costs ten times as much. 

Engineers often pride themselves on advanced developments. They think they’ve got something completely new, but then a wider view reveals that nature got there millions or sometimes billions of years ago. Composite materials (fibres in a resin) have become increasingly popular in aircraft production, in some cases for the whole airframe of carbon, aramid or glass fibre. Wood has been described as nature’s own composite and if different types are put together, they can produce an amazing material combination, particularly for airframes. 

Since the Wright brothers first flew in 1903, wood has been an important material for aerospace and it continues to be excellent for many applications, with the younger Wooden Wonder proof of that. The aircraft is effectively glued and screwed together, so its fasteners and fixings are part of the reason that it was – and is – a brilliant aircraft.

Building a Mosquito also involves many skills outside the aircraft industry. Furniture makers, cabinet makers, coachbuilders and even coffin makers are being used, with the car industry also playing a vital part¹. Hence, production spread from the pressed aviation industry to workers who had the availability of time, with skills and abilities to add to the war effort.

Structure 

The oval-section fuselage was a frameless monocoque shell built in two vertically separate halves formed over a mahogany or concrete mould – with pressure applied with band clamps. Some of the ½ inch – ¾ inch shell sandwich skins comprised ³/₃₂ inch birch three-ply outers, with ⁷/₁₆ inch cores of Ecuadorean balsa. In many smaller but vital areas, such as around apertures and attachment zones, stronger timbers, including aircraft quality spruce, replaced the balsa core. 

The main areas of the sandwich skin were only 0.55 inch (14mm) thick. Together with various forms of wood reinforcement, often of laminated construction, the sandwich skin gave great stiffness and torsional resistance. The separate fuselage halves speeded construction, permitting internal access by personnel working in parallel with others.

Work on the separate half-fuselages included installation of control mechanisms and cabling. Screwed inserts in the inner skins that would be under stress in service were reinforced using round shear plates made from a Fabric Bakelite composite.

Transverse bulkheads were also compositely built-up with several species of timber, plywood and balsa. Seven vertically halved bulkheads were installed within each moulded fuselage shell before the main ‘boxing up’ operation. Bulkhead number seven was especially strongly built, since it carried the fitments and transmitted the aerodynamic loadings for the tailplane and rudder.

The fuselage had a large ventral section cut-out, strongly reinforced, that allowed the fuselage to be lowered onto the wing centre-section at a later stage of assembly. For early production aircraft, the structural assembly adhesive was Casein-based. At a later stage, this was replaced by Aerolite, a synthetic urea-formaldehyde type, which was more durable.

To provide for the edge joints for the fuselage halves, zones near the outer edges of the shells had their balsa sandwich cores replaced by much stronger inner laminations of birch plywood. For the bonding together of the two halves (boxing up), a longitudinal cut was machined into these edges. The profile of this cut was a form of V-groove. 

Part of the edge bonding process also included adding further longitudinal plywood lap strips on the outside of the shells. The half bulkheads of each shell were bonded to each other in a similar way. Two laminated wooden clamps were used in the after portion of the fuselage to provide supports during this complex gluing work. The resulting large structural components had to be kept completely still and held correctly until the glue cured.

For finishing, a covering of doped madapollam (a fine, plain woven cotton) fabric was stretched tightly over the shell and several coats of red, followed by silver dope, were added – followed by the final camouflage paint.

The all-wood wing pairs comprised a single structural unit throughout the wingspan, with no central longitudinal joint. Instead, the spars ran from wingtip to wingtip. There was a single continuous main spar and another continuous rear spar. Because of the combination of dihedral with the forward sweep of the trailing edges of the wings, this rear spar was one of the most complex units to laminate and to finish, by machining after the bonding and curing. 

It tapered from the wing roots towards the wingtips. Both principal spars were of ply box construction, using in general 0.25 inch plywood webs with laminated spruce flanges, plus a number of additional reinforcements and special details.

Spruce and plywood ribs were connected with gusset joints. Some heavy-duty ribs contained pieces of ash and walnut, as well as the special five ply that included veneers laid up at 45 degrees. The upper skin construction was in two layers of 0.25 inch five-ply birch, separated by Douglas fir stringers running in the span-wise direction.

The wings were covered with madapollam fabric and doped in a similar manner to the fuselage. The wing was installed into the roots by means of four large attachment points. The engine radiators were fitted in the inner wing, just outboard of the fuselage on either side – giving less drag. The radiators themselves were split into three sections – an oil cooler section outboard, the middle section forming the coolant radiator and the inboard section serving the cabin heater.

The wing contained metal-framed and skinned ailerons, but the flaps were made of wood and were hydraulically controlled. The nacelles were mostly wood, although for strength, the engine mounts were all metal, as were the undercarriage parts. Engine mounts of welded steel tube were added, along with simple landing gear oleos filled with rubber blocks. 

Wood was used to carry only in-plane loads, with metal fittings used for all triaxially loaded components, such as landing gear, engine mounts, control-surface mounting brackets, and the wing-to-fuselage junction. The outer leading edge had to be brought 22 inches (56cm) further forward to accommodate this design. The main tail unit was all wood. The rudder and elevator were aluminium alloy framed and fabric-covered. The total weight of metal castings and forgings used in the aircraft was only 280lb (130kg).  

Power to the people

The People’s Mosquito is a UK-based charity, which plans to put a Mosquito back in the air by working with appointed contractors Aero Vintage Limited, through their Retrotec division. It’s building the UK’s first new Mosquito for over 70 years, designated RL249.  In doing so, it’s recreating parts from over 22,000 original De Havilland drawings to ensure the aircraft meets strict CAA airworthiness standards. 

Under the expert guidance of Guy Black, Retrotec has established a reputation for delivering some of the most authentic aircraft restorations in the world over the past 30+ years. The business, which is fully accredited by the UK Civil Aviation Authority, offers services including design, parts manufacture and reconstruction of complete aircraft – all delivered by highly experienced engineers.

The Retrotec workshops are situated a few miles north of Hastings, East Sussex, where the company employs a small team of dedicated and highly skilled staff. Late last year, The Princess Royal, Princess Anne, visited Retrotec and spoke to former ‘Mossie’ pilot 101-year-old Flt Lt George Dunn DFC, LdH. She asked him if it was difficult to fly. He said: “No. The only slight snag with the Mosquito, it would tend to swing on take off.” He said the aircraft was “beautiful, versatile, you had everything – speed, height, you name it, it was there. It was my favourite aircraft. I’ve flown Spitfires and that, but the Mosquito was absolutely out of this world. I can’t believe that the aircraft is being renovated, resurrected and is going to fly again. Unfortunately, I don’t think I’ll still be here when it is completed”.

The Princess Royal said of the rebuild: “The skill sets you had to find to do this are pretty impressive. There’s a lot of boatbuilding techniques here, old skills… and very sensible use of materials.”

John Lilley, chairman and managing director of The People’s Mosquito, explains: “Our charity has three pillars – to fly, to educate, to remember. We’re doing it for joy; we’re also doing it for future generations, not only from a heritage point of view, but a British engineering point of view as well.”

The project is being funded by public and corporate donations and anyone wishing to help can find out more via the project website: www.peoplesmosquito.org.uk. The charity needs to raise a great deal of money to complete the project and the website details how individuals and companies can assist, so that future generations can wonder at The Wooden Wonder.  

¹ https://www.standardmotorclub.org.uk/copy-of-canley-assembly-line-2