Wing design constraints and adopted solutions

The wing of the Hornet is a one-piece cantilever structure consisting of two spars with compressed plywood webs, extruded light-alloy bottom booms and spruce top booms.

A stressed plywood double top skin is reinforced by wooden stringers placed spanwise, and the bottom skin is of Alclad reinforced by extruded duralumin stringers extending from the outer engine-rib to the tip.

Structurally it is based upon the Mosquito and embodies much of the same wood construction. In itself, however, it is a unique blend of wood and metal, this combination having been necessitated by the higher safety factor required.

From a production viewpoint the most interesting point is probably the use for the first time in an aircraft primary structure of a synthetic-resin bonding process to join wood and metal.

This process is used in the main structural members, the mainplane spars which are of special interest in themselves and typify the extreme structural efficiency achieved in the design of the aircraft gained in the manufacture of the Mosquito

The aircraft is a single-seater, smaller than the Mosquito but requiring a comparable fuel supply (in a smaller wing) in order to give it approximately the same range.

Wing span was kept to the minimum, commensurate with take-off, climb and altitude performance needs so as to keep the weight down, to reduce the bending moment on the wing, and to ensure, with spring-tab ailerons, a quick and easy roll, comparable with contemporary interceptors.

The wing taper was kept as sharp as possible, again to minimize bending moment and to keep a good rolling performance, but without risking wing-dropping near the stall. The wingtip was squared to permit the aileron to extend outboard as far as possible, again in order to obtain a good rolling quality.

The aerofoil section expresses the compromise between low drag at cruising speeds and postponement of compressibility troubles to as high a Mach number as possible.

Another compromise, desire for a small wing on the one hand and the avoidance of too large a turning circle and too high a landing speed on the other, led to the adoption of a wing load of 42 lb/ft² (on combat weight).

The difficulty was increased by the fact that a safety factor of 10 was needed, a requirement that could not be met by the construction used in the Mosquito (which had a factor of 8) owing to the bulk of the additional material that would be needed to give the greater strength. At the same time, in order to avoid a complete changeover of production technique with consequent reorganization and inevitable delays in the shops, it was desired to make use of the Mosquito type of construction to as great an extent as possible.

The outcome of these conflicting requirements is a wing structure based upon the general principle of using metal to carry tensile loads and wood for compression and shear stresses.

The wing structure is based upon the customary two spars which are, however, far from conventional in construction.

A single-web spar became possible because de Havilland developed a method of forming a birch multi-ply web turned over at right-angles along its upper edge so as to pick up the shear load directly on to the upper skin of the wing, eliminating the top boom of the spar.

This makes for a narrow spar with a saving in weight (and a lessening of shrinkage) on the length of the pick-up bolts. In addition, it gives more room for internal fuel tanks.

In short, the main tension stresses are carried in the tapered, extruded T-section aluminium-alloy lower boom of the spar, and the main compression stresses are carried by the unbroken top surface of the wing, comprising two plywood skins with span wise spruce stringers placed between them at close intervals.

The employment of extruded tension booms enabled a complete under skin of stiffened light-alloy sheet to be used, greatly increasing the torsional stiffness of the wing box.

The basis of both is an inverted L-section moulding of laminated birch of which the vertical corresponds to the web and the horizontal to the top boom. The top spar boom may be described as a sandwich made up of longitudinal spruce members between the two birch plywood, top-surface interspar skins of which the lower one is glued to the horizontal flange of the spar moulding.

This portion of the wing closely resembles that of the Mosquito and the two skins, which carry the main compression stresses, are established as in the earlier aircraft by solid, square section, spruce stringers of the same thickness as the boom members. Plywood sheet glued to the outer web face of the spar moulding projects above the horizontal flange and is glued to the outer edge of the spruce boom. Tensile loads are taken by light alloy (D.T.D. 363A) extruded bottom booms glued along each side of the laminated birch web. In this radical departure from conventional engineering practice, the Redux bonding process has been adopted.

Veneer is first attached to the boom by Redux and normal cold-setting urea glue is then used as a joint between the veneer and the birch web.

In its combination of laminated wood moulding and light-alloy extrusions the Hornet spar is undoubtedly unique and from a design point of view it is extremely efficient. It has a high strength-weight ratio and in this respect compares more than favorably with the all-wood box-type spar of the Mosquito. It is also extremely small in bulk.

Leading and trailing-edge units are of conventional construction in light alloy.