ONE of the most interesting projects undertaken by the plastics division of F. G. Miles, Ltd., during the last three years, has been the construction for the British Gliding Association of an experimental wing—moulded in phenolic-impregnated asbestos material—for a high-performance glider. The machine, known as the M.76, is a development of Hugh Kendall's Crabpot I, which gained first place in the 1947 B.G.A. 2-seat design contest.
Financial backing for the project has come from the Miles company, the Kemsley Flying Trust (through the medium of the B.G.A.), and the Ministry of Supply.'
The effect of the chordwise position of boundary laver transition on a sailplane's performance is great, for laminar flow over an increased fraction of the chord gives a much reduced minimum drag coefficient. The achievement of laminar flow over as much as possible of the wing has been hampered in wings of normal construction by surface waviness, and it was felt that one way of obtaining the required improved finish would be to use some form of stabilized plastics construction.
It was decided to use a phenolic/asbestos fibre material stabilized with a paper honeycomb, the manufacturing technique being based on the vacuum moulding process pioneered by the R.A.E. at Farnborough. The 60-ft span wing was to be made in two semi-span sections each consisting of one 30-ft moulding— probably the largest one-piece moulded phenolic/asbestos structures yet manufactured.
The method of construction is, briefly, as follows :—
(1) A model of the wing, with exactly the same shape as the wing itself, is made from concrete, steel and a phenolic/asbestos material.
(2) A layer of soft phenolic/asbestos material is placed upon this model and made rigid by heat cure. This is called the master finer.
(3) A further layer of phenolic/asbestos material is then placed on this and heating mats and thermocouples are put in place and again heat cured. The shell produced is the heart of the main mould.
(4) This mould is then supported in place and covered with concrete.
(5) To make a wing, the soft Durestos felts are cut to shape and placed in position on the loading table, together with the paper honeycomb (Dufaylite) filling. The outer felts are then laid in place, and the table is lowered over a loading trolley.
(6) The loading trolley and soft felts are then moved into the mould. A rubber bag is used to seal the felts and apply suction.
(7) Electrical heaters driven by a 112 kVA generator are used to cure the product, and after about six hours the complete stabilized wing skin is cured.
(8) The wing is then removed from the mould and after trimming is ready for the fitting of ribs and trailing edge closing member.
The steel root fittings are moulded into the wing in situ at the time of cure.
In developing this method of construction, the company has had to face a number of original problems resulting from the use of the new materials. Mould cracking due to temperature variations in the concrete mass and to pinching action of the cured phenolic/asbestos mould surface, and the inconsistency in quality and effect of certain apparently identical mixes of resin, were among the setbacks experienced. A really close liaison between design staff and the operatives has proved invaluable for the successful introduction of the new techniques.
The date when the M.76 will fly is still some time ahead for, up to the present, only starboard wings have been produced, and the glass fibre ribs and trailing edge member, and the ailerons, have not yet been fitted. R.A.E. tests are about to commence, and the first public view of one of the moulded wings will be at the British Plastics Exhibition.
The main value of the Miles project is clearly wider than that of the particular wing; it lies in the "know-how" of the techniques of this method of moulding. For the future, further applications are inevitable—and a number are already being considered.
The possession of a two-seat machine such as the M.76 would obviously be a good thing for Britain in the 1954 world championships.
To guard against delays in the production of the plastic wing, a wooden wing to the same specification is being constructed by Elliotts of Newbury, and is to be fitted to the conventional wooden fuselage built by Miles.
