Part 2 : '[hq: Fabric Couering (Conrinuec{)

TN his previous article Mr. Deverell took the lconstructor through the operation of giving the fabric one coat of dope and continues bv describing the stringing procedure. In a later issue Mr. Deverell will be dealing with the subject of care and maintenance of fabric-covered components.

First, dope on to the fabric al each rib boom, a strip of l" Egyptian tape; this is to reinforce the covering where stringing operations are to be carried out. The "string" used is a waxed linen braided cord, a very strong cord for its size, and rolled flat on the reel in order to avoid unnecessary protrusions in the vicinity of the ribs, when the stringing operation is complete. Those who are not sure of the method of stringing had better have their inspector show them first, as to do it incorrectly will not only spoil the whole job but can be useless and even damage the fabric. The stringing spacings on the aircraft we are concerned with is three inches with a doutble knot on the sixth " stitch "; the knots are usually on the surface for convenience and the joining link between each o'stitch". A stringing needle is required at least nine inches long and about 5164'thick, with an " eye" aL the end to receive tl6 cord. lt is best to have the wing trestled with the chordline horizontal and the wing at a convenient height for working above and below. Next, from the vicinity of the rear surface of the main spar, mark off the three inch spacings on each side of each rib boom, top and bottom, taking care to avoid putting a mark immediately over the aileron cables as it is undesirable to have the cord binding against the aileron cables when passing the cord through the wing. Enough cord is threaded on to the needle to completely string one rib and it should be noted that the cord is not doubled.

We now pass the needle straight down through both surfaces of the wing and on one side of the t2 rib, then pass the needle and cord back up the other side of the rib, to join the end of the cord and be secured by a knot which, if not known, will need to be demonstrated. We next pass the needle straight down through the wing three inches away and continue the process to within three or four inches from the trailing edge. It is a good idea, when passing the needle through

by A. Deuerell

the wing, to feel gently around with it before passing it through the other surface. This ensures that there is no obstacle, such as control cables, in the near vicinity. The stringing is sealed with two inch serrated tape doped on and rubbed down with a pad of fabric. The whole component has now had one coat of dope but before we proceed with the doping operations, there is one more job required to be done.

lt is of vital importance that the component be suitably ventilated not only to remove damp in a natural way but to equalise pressure inside to the surrounding elements. This is achieved by doping on little vent eyelets in each bay on the under surface just in front of the trailing edge. These eyelets are merely little oval shaped pieces of thin acetate with an oval hole of, about f " across the narrow width. When the dope is dry, a hole is cut in the fabric with a sharp pointed knife, through the hole in the eyelet. lncidentaly, boxed-in wood components should also be suitably ventilated for the same reasons.

For the beneflt of future inspection, " Woods Rings " should be doped on in suitable places, such as in the vicinity of control cable pu[[eys, on the under surface. These rings are of the same material as the eyelets only they are approximately four inches diameter. When inI'opttlur b'lying, M urch-April, 1961.

spection in the wing is required, the fabric is cut out from the centre of the ring with a sharp knife and the inside of the wing can be seen. The ring prevents the fabric tearing. After inspection, a rip off patch is then doped over the hole and ring. The second coat of dope is now well scrubbed in to the fabric and allowed to dry after rubbing down with a pad of fabric.

After the third coat of, dope has been brushed on, it is wise at this stage to start preparing a good smooth surface. Great care is needed when gently rubbing the serrated tape edges with a very fine sand paper. These edges can be virtually moulded into the rest of the fabric. A safer way is to very carefully resort to rubbing down with'owet and dry". Be careful not to press on the fabric or saggy patches can occur and do not overdo the rubbing in the vicinity of stringing, ribs and structure or one can rub through the fabric with disastrous results. Thoroughly dry and clean the wing and carefully brush on anoth.er coat of doPe. IncidentaTly, a well ventilated room is required for these doping oper- ations as the fumes can be rather overpowering; also do not attempt to carry out doping operations in extremes of temperature or cold or humid draughts. The effect of the dope relies entirely on the right conditions when applied. For a really good final finish, the final coats, well thinned down, can be sprayed on and allowed to dry for at least 24 hours. Beware of flre risk; do not have open electric or coal fires, etc., anywhere near the building, during these operations. Remember, although dopes appear to be dry within a matter minutes, it takes at least twenty-four hours under the best conditions for the shrinking effect to take place. Before the finisher colours are applied, an aircraft primer sealer is sprayed on. We are always amazed at the remarkable glossy finish that can be achieved on fabric components if things are done the right way and with care. Cellons are always very helpful with advice if asked for. Do not spoil a good job by a hurried finish.