5.9 ROLLED LEADING EDGES
Before the empennage control surfaces can be installed on the stabilizers, the leading edges must be
formed. The object here is to achieve a smoothly curved surface that fits neatly between the skin overhang
of the stabilizer.
Simply pulling the overhanging skins together results in an angle or crease where they cross the edge of the spar. To avoid
creasing the skins the curve is started by rolling the edge of the skin. A piece of 3/4 or 1 inch diameter steel water pipe, a
broomstick, or something of similar diameter about four inches longer than the skin will be needed.
Tape the edge of the skin to the pipe along its entire length. Use vise grips or a small pipe wrench clamped to the pipe as a handle
and roll the skin around the pipe. Maintain pressure down toward the work surface and away from the spar to prevent the skin from
bending right at the spar. This will not produce the final shape, but it will produce a curve in the skin that allows the skin to be closed
with a minimum of spring-back. If working solo, it will be easier to use a shorter pipe and do each skin section individually.
NOTE: Not fully forming the skin, but instead just pulling the two halves together and riveting, causes a lot of pre-load on
the skin and is a common cause of skin cracking at the forward end of the skin stiffeners.
Finish the bend by hand, squeezing the skin
until the holes match. Drill the holes full size
then clean up the holes (it is hard to get
to the inside of the curved skin with a
deburring tool, but in this case a quick
rub along the holes with a scotchbrite
pad is good enough) and rivet. Blind
rivets are used here. They are simple
to set with a hand pop-riveting tool, but
difficult to drill out. Make sure that the
heads of the rivets are firmly against the
skin before squeezing. See the
Leading Edge Detail on the
appropriate drawing.
MAINTAIN PRESSURE
DOWN & AWAY FROM
SPAR WHILE ROLLING
FIGURE 1: ROLLED LEADING EDGES
TAPE
SKIN
PIPE
TABLE
RUDDER
5.10 LAP JOINTS
When riveting a lap joint, the expansion of the aluminum caused by the setting pressure and the expansion
of the rivet causes the overhanging edge of the sheet to bend upwards slightly. This creates a number of
problems, like making the lap joint appear wider than it is, making painting more difficult, etc. One easy method of minimizing this
effect is by pre-bending the last 1/4 inch of the skin downward just a small amount before it is clecoed and riveted. When the rivets
are driven, the skin will be flattened and the pre-set in the skin edge will tend to hold it flush. Except for very rare circumstances,
the amount of bend (break) is very small and when done properly is almost undetectable with your eye. The goal is to do just
enough to keep the skin lying flat but not too much or it will be obvious that it was done.
There are several methods that may be used for making this slight edge bend. Using a hand seamer and moving progressively
down the sheet, making a very light bend to avoid bend marks between succeeding grips with the seamer. On long thin pieces put
the sheet on an even-edged table with about a 1/4 inch overhang and draw a block of hardwood or plastic along the edge with just
enough downward pressure to cause the slight bend as the block moves. UHMW blocks with different depth slots to set the bend
depth may also be used. Slip the slot over the skin, apply a bending force and pull the block towards you, sliding it along the edge
of the skin. Do not try to form the bend all in one pass.
Some tool suppliers sell tools for this purpose, usually two small rollers mounted on a variety of different tools. The edge of the
aluminum sheet is placed between the rollers and a bending pressure is held as the tool is drawn down the edge. Use these with
caution as they may tend to stretch a long edge and make it wavy. Avoid over bending the edge and causing a worse visual effect
than before. Experiment with scrap material first.
5.11 FORMING ALUMINUM PARTS FROM SHEET
Some of Van's kits require forming several ribs and bulkheads from flat aluminum sheet. This can be a useful skill in the Standard
kits as well. Sheet metal press brakes are only capable of producing straight bends, so parts with flanges along curved edges must
be bent over “formblocks.”
formblocks are made of hardwood or dense particle board. They are cut to the contour of the finished part with allowances for the
thickness of the material. Edges are radiused so as the part is bent around the block, it does not bend too sharply and crack. A
blank part, with the appropriate material for flanges, notches at the corners, etc., is cut from sheet stock and sandwiched between
the formblock and a “tool cap”. This tool cap looks very much like a formblock, but does not have to be exact.
The tool cap functions to keep the part from bowing or distorting when the flanges are bent. The formblock, blank and tool cap are
all aligned with tooling holes and clamped together with bolts. The protruding edge of the blank part is bent around the formblock
with a mallet or lead bar. Final adjustments to the flange are made with a hand seamer and fluting pliers.
5.12 VINYL COATING
Many of the alclad parts are supplied with a thin (usually blue) vinyl coating to prevent scratching during the manufacturing of the
parts. The vinyl may be left on during drilling but should be removed for dimpling, priming and final installation. It is possible to
remove strips of vinyl along rivet lines with a soldering iron. Carefully round and smooth the tip of the iron so it will not scratch the
aluminum. The time in labor required for this added protection during construction should only be considered of value if you intend
to leave the airplane polished bare aluminum. If you intend to paint, the preparation process will include scuffing/deglossing all of
the skin surfaces anyway to provide good paint adhesion.
The adhesive on the vinyl strengthens with age, so if the coating is left on for more than a few weeks, it may become very difficult to
remove. Corrosion has been found under the vinyl in some instances. If vinyl covered parts must be stored for long periods remove
the vinyl first.
PAGE
REVISION:
DATE:
VAN'S AIRCRAFT, INC.
05-13
RV-ALL
2
04/15/13
