Here is an interesting concept in model wing construction that probably would have become very popular in the prefabricated model market if the foam wing had not been invented. Honeycomb has a very high strength-to-weight ratio when a compression force is applied parallel to the cell height. When sandwiched between balsa (or maybe even cardboard) upper and lower wing 'skins,' author Shepherd has found the scheme to be very satisfactory for all kinds of models. BTW, words such as 'aluminium,' 'aerofoil,' 'centre,' etc., are not misspellings to the folks on the other side of the pond. Oh, and a 'stone' as used here is a unit of weight (1 stone = 14 pounds), not a physical rock ;-)
Fantastic Strength in this New Type of Wing Structure
- By P. A. Shepherd
Are you seeking a method of constructing wings that is cheap, very strong and quick? These last three things are not usually closely associated, but it is hoped to show that this is possible in the following method. Wings described are especially suitable for radio models, large gliders, and control-line types.
In the full-size aircraft industry the same principle is known as double skin honeycomb construction and is used for floors, doors and other components. With a core in aluminium foil, honeycomb structures form much of the Vickers Vanguard control surfaces. Basis of the model version is 009 in. Kraft paper honeycomb manufactured by Dufaylite Developments Ltd., of Boreham Wood, Herts. It is available in thicknesses ranging from a quarter of an inch to six inches and with hexagons from three-eighths to 1 1/2 in. wide in sheets measuring 36 x 12 in. (expanded). These compress to 1 in. x 1 in. x 18 in. rather like a paper garland, it also expands like a paper garland and whilst it is compressed the required aerofoil profile may be drawn on the end and the "block" sanded to shape (Fig. 1).
Heading shows rigid "Dufaylite Honeycomb" in impregnated materials" and inset. Mr. Shepherd's test sample which we can assure readers, will stand up to a car running over it, or a 16-stone heavyweight tramping on it! Below, a block of "Dufaylite Honeycomb" centre; with symmetrical rib marked on face for shaping, and at right, shaped for covering with sheet.
Next stage in construction is to expand the honeycomb so that the hexagonal cells are approximately equal in size. It should be noted that during this operation the material contracts by one-third in a chordwise direction and due allowance should be made for this. The brown paper core should then be pinned to the building board in the stretched state.
Next job is to apply the upper and lower skins. 1/32 in. thick balsa should suffice for most wings up to about 48 in. span or in certain cases up to as much as six feet. Since balsa is normally obtainable in 4 in. wide sheets, as the maximum, wings of greater chord than 4 in. should have a stringer of about 1/8 in. x 1/32 in. balsa let in flush with the surface to enable two balsa sheets or more as required to be joined to give maximum strength (Fig. 2). The skins can then be glued in place and pinned to the board until dry. Best adhesive has been found to be Aerolite 306 with the resin applied to the wood skin and the hardener to the honeycomb. A very close second to Aerolite is LePages P.V.A., this is certainly easier to apply, particularly if run on the edges while honeycomb is compressed to the block stage, before pinning down between surfaces. When the wing skins and honeycomb are dry and hard, the leading edge strip should be glued in place and after the whole assembly is well and truly dry the wing may be subjected to a "standing upon" examination to test it! One test specimen has had a weight of seventeen stone on it (the heaviest man we could find!).
Optimum size to give maximum strength with lightest weight for our purposes seems to be a honeycomb size of 3/4 in. or 1 in. across the flats of the hexagons. Anything smaller than this pushes the weight up.
Bee keepers who keep extra large bees may welcome any left overs!
Price varies with the size of the hexagons and the thickness, the smaller the honeycomb the higher the price and, of course, the thicker material carries the higher price. As a general guide the size required for model building would cost as little as 2 1/2 d. to 5 d. per sq. ft. (expanded) dependent on thickness ranging from 1/4 in. to 1 in. depth.
Test pieces indicate that wings under about 36 in. span may be slightly heavier than conventional methods of construction; but the strength is incomparable. A honeycomb with leading and trailing edge of balsa but tissue-covered and doped would give good results with excellent airfoil maintenance, superior to many standard surface spar methods.
Undercambered or symmetrical sections are no more difficult to make than the popular Clark Y with its flat undersurface.
Try a double skin construction and amaze yourself.
Think of a C/L model which will last a couple of seasons with ease and remember that this form of construction lends itself to components other than wings - why not fuselages, cowlings - even model boxes?
Posted March 19, 2016