FOR THE AMATEUR CONSTRUCTOR

The Covering of Wing Leading Edges

A task which faces the amateur constructor is the plywood covering of wing leading edges. This is a job which must be carried out properly and the performance of the completed aeroplane depends to a large extent on the result. A smooth, evencontoured forward edge is essential to preserve airflow over the wing at low speeds. If the leading edge is uneven or badly finished, the airflow will break up into small eddies thereby building up the profile drag and destroying lift. This means a higher stalling speed and a lower cruising speed. One can therefore appreciate the importance of taking pains over the construction and finish of this all-important portion of the wing.

Leading edge covering is quite an easy operation provided that it is approached in the correct manner. The secret of a good smooth leading edge lies in the accurate shaping of the ribs before the plywood is attached. If there is any unevenness whatsoever in the nose ribs, the plywood will inevitably be a mass of bulges and Iumps.

Even with careful rib construction there are bound to be slight discrepancies in the rib profiles. The major part of this error should be corrected before the ribs are assembled to the spars by trgping them all up together. The ribs are placed in a pile and clamped together with short lengths of timber corresponding to the spar dimensions placed in the spar slots to locate them correctly. The stack of ribs is then secured in a woodwork vice with the capstrips uppermost. Using a straightedge held spanwise across the ribs, check at close intervals along the chord for any irregularities between them. With a finely set smoothing plane, clean the ribs 'to uniformity paying particular attention to the portion forward of the main spar. If, as is the case with the Druine Turbi and Turbulent, there are nose ribs between each main rib, these should also be clamped into place with the main ribs using suitable packing aft of the mainspar.

Check with a square to see that the ribs are true and that the prepared chbrd profiles are parallel to the spar. Turn the stack of ribs over and repeat the operation for the lower camber.

When the ribs have been built onto the wing and the leading edge ready for plywood covering, the wing must first be checked for alignment as, once the nose ribs are boxed in, the wing will be stiff and any twist will be set in it. To true the wing and box in the nose ribs, mount the main-plane vertically-that is to say with the trailing edge lowermost. Make up three supports using 2" x 2" timber as shown in the accompanying sketch. One is used to support the wing at the centre-section and should be fixed securely to the floor to prevent movement. Attach the wing to the support by means of 'G' clamps. The wing may now be brought up level by means of packing under the rear spar 4ileron gap. At all times when clamping or packing anything onto a structure, ensure adequate protection for it against local crushing by placing large blocks of wood under the area of pressure to spread the load. This is particularly the case when olamping as the pressure applied may be far greater than is either intended or necessary and crushing will invariably take place'if no protection is afforded for the aircraft woodwork. The remaining two supports are now fixed in place near the wing tips and are clamped to the main spar.

With the aid of a spirit level, check that the wing is level by lining up the uppermost face of the main spar. Any errors may be corrected by packing under the supports. Again remember to pack between the supporting timbers and not directly against the structure for the weight of the wing alone is sufficient to permit crushing against a small packing wedge or block.

Select two stations on each wing. They should be as far apart as is practicable but must be positioned where the spar is of uniform thickness. It is useless to try to take any rigging dimensions from the tip where the spar tapers in section. As a rule, it is advisable to choose a point two ribs outboard from the centre section and two ribs inboard from the tip. At these stations on each wing, place a plumb line over the main spar so that it hangs over the under surface .of the wing to within about an inch of the floor. Tack a short strip of wood about l" x $" and about eight inches long across the rear spar adjacent to and protruding several inches beyond the plumb line. These are rigging reference pieces against which is read the distance from the spar centre line to the plumb line.

Carefully slide the plumb line along the main spar until it is within about *t" of the reference

Popular Flying, Octoberf November, 1957

/s_E!-ts_-_A,Gtu t1.'o*Sm,-q r*q r- Inrx th ao y'r,rrhi{,r:a!t[!!sA, piece. Mark a line with a pencil at the point where the plumb line comes up to the edge of the reference piece. Repeat this for the others' Measure from the spar centre line to this point of reference. Both root end or centre section readings should be similar. If the outboard readings differ from the inboard ones, the wing must be twisted-or rather an existing twist rectifiedby adjusting the amount of packing between the rear spar and the support at the tip. Using this method, it is possible to true the wing exactly, but an error of approximately #" is permissible.

The structure is now ready for the final preparation before the attachment of the leading edge plywood skin.

Take a straightedge of at least sufficient length to touch three ribs. Place it across the ribs and, working on three at a time, check spanwise €very inch to detect any high spots upon which the straightedge will rock. Remove these high spots with a finely set plane, spokeshave or sandpaper. If the straightedge shows a hollow or depression, glue onto that rib a thin slip of wood and plane it to shape when the glue has dried.

This period of careful checking and double checking cannot be hurried over. The degree of thoroughness with which it is executed will be shown by the standard of the skinning' Pay particular attention to the leading edge slot ribs -these are just as vital.

It is advisable to skin the wing in sections, one piece at a time, rather than to scarf up the entire length of plywood at once. Each scarf must fall on a rib so that the joint is supported for gluing.

Check with a tape measure or a scrap strip of ply the width of the sheet required to pass from the base over to the top and cut the plywood sheet accordingly, leaving at least an inch or two extra. As a general rule, the grain of nose rib skinning ply runs spanwise (parallel to the spar) and not chordwise. Remember this when ,cutting the panels. Mark a line on the ply sheet corresponding to the leading edge of the wing, and fold the sheet along this line, taking gteat care not to split the sheet. If the ends of the sheet have been scarfed already, avoid damaging the edges.

With the help of an assistant, pour boiling water over the outer edge of the bend. This will enable the sheet to be folded to a tight radius. When the ply has become supple enough by repeated applications of boiling water to the outer surface, the sheet may be clamped between two boards and left to dry out overnight. During this process of pre-forming plywood, the sheet must not be forced round-but allowed to take up the curve gradually as it softens.

When the piece is dry and set, the skinning may be begun. Always start from the bottom or lower surface of the wing and work round to the upper side.

Mix up srlfficient glue for the job and apply liberally to the ribs. If Aerolite 306 is being used, paint the inner surface of the plywood with the hardener, using a broad, soft brush. lf Aerodux 185 is to be used, the ply must be offered into position flrst and the position of the ribs and the spar marked onto it with a pencil so that the resin may be applied to both surfaces in contact.

Put the sheet in place over the leading edge ancl tack it to the under surface of the main spar having flrst made sure that the sheet is squarely aligned to the wing. it should be pbinted out that each section of the ply should be affixed completely within a reasonable time to prevent the glue hardening before the task is finished. Do not apply one half of a section and then leave it; Each piece must be tackled without interruption. Using $" long brass gimp pins and one inch wide plywood tacking strips, nail the sheet to the spar and ribs at intervals of half an inch. Work from the lower or aft edge of the sheet and move progressively upwards, thereby eliminating any wrinkles in the ply and also making doubly sure that there is no air trapped under the ply on large contact glue surfaces.

By attaching a four foot (approximately) length at a time, it is possible to complete the tacking of each length within the shuffiing time of the glue. In the bradding operation, stop about one inch short of the leading edge. This is to permit the lifting up of the sheet from the other (top) side to apply glue for the top surface if this has not already been done. Make sure that there is no gap between the two applications of glue if they are done separately

Before attempting further bradding, it is necessary to hold the sheet in firm, intimate contact with the top camber of the ribs, eliminating all bulges, to permit proper adhesion. The best method to achieve this is to strap or tie the panel firmly down and then wedge it. By means of protective padding around the rear spar, tie ropes or long webbing straps round the leading edge and under the rear spar. Endeavour to place each strap adjacent to a rib. For a four foot wide section, three straps are sufficient. Obtain about half a dozen four foot long lengths of 1" x !" or thereabouts. Insert one of these aft of the main spar under the'straps. Work it up over the plywood and under the straps thereby tensioning them. Slide it right up to a point as near the leading edge as possible. Check to see that the ply on the actual leading edge is in firm contact with the ribs and the leading edge member without sponginess. Drive more wooden strips in under the ropes until the ply is held firmly down all over.

Tack through tacking strips all along the leading edge member. Using short lengths of tacking strip, brad through to the ribs between the wedging strips. It is not necessary to tack the ply under the wedges. Work from the nose downwards (aft) finishing with the spar. Leave the section to dry overnight. Remove the wedges, straps, tacking strips and brads and clean up the scarf joint to take the next section of plywood.

During the attachment of the leading edge, it is advisable to verify at frequent intervals that the wing has not moved or been knocked out of alignment by examining the plumb lines and reference pieces.

Pay particular attention to scarf joints in the nose plywood. See that they are clean and a good tit before gluing. To do this, it may be found

Popular Flying, Octoberf November, 1957 desirable to put the sheet into place and strap it down without glue. When it is flrmly in Slace, the correct position of the scarf may be checked. When bradding through a scarfjoint, always use a strip of short grained ply as a tacking strip separated from the joint by a piece of waxed paper.

Should it be necessary, as for example on a wing tip, to join two panels of plywood at the leading edge, this operation may be tackled as follows. Cut one piece of plywood to form the lower or under side panel. Leave at least three or four inches overlap at the leading edge. If it is necessary to preform the ply, either pour boiling water onto it to plasticise it, or boil it in a large basin of water. Whilst it is still wet and hot, place it in position and clamp it upAllow it to dry thoroughly before attempting to glue it. When finally gluing into place, strap or clamp the overlap at the leading edge firmly round to the other side. Ensure that it is in contact with the leading edge or tip member.

After the glue has dried, unstrap and carefully pare down the leading edge with a plane or spokeshave until a scarf has been cut along the member. Now cut the top panel, also leaving a good surplus at the leading edge. Pre-form if required as above. Affix this in the same manner> pulling the overlap well round to the other side in firm contact with the other ply panel. Make sure that there is plenty of glue in the join. When dry, unstrap and plane away the surplus woodThe result is a 'back to face' scarf flrmly sup* ported by the leading edge or tip member.

To attempt to cut the scarf before attachment would produce a feather edge too weak and fragile to bend round. Furthermore, to apply even pressure to so soft an edge would present great difficulty.

When the whole nose portion of the wing has been plywood covered, sandpaper it thoroughly and remove all brads and lumps of dried glue. As a final check on all sanding, there is no better method than to run the hand over the surfaceIf it draws blood, you have more to do !

The next operation is the application of fabric and finish to the wing, to be dealt with shortly-