HOW TO BUILD
A light airplane can be made which will fly one or two men with low enough power and small enough
dimensions to be airworthy. The light plane will fly well with one, (and in some instances two passengers) on low power and will have big airplane flight characteristics. As the size increases the
ship is weighted down by design factors until it becomes heavy and powerful enough to fly again. Then it is an airplane.
This lightplane is one that calls for skill in flying nothing superhuman, but just good, careful flying. And
to do that it is important that not only the design factors of the light plane be in proper balance, but it is important that the wing area be right.
Aship loaded 3?
pounds to the square foot will handle a little like a kite. She will be light, and slow, and tender, but very controllable. A ship Loaded seven to ten pounds per square foot will not fly well, if
at all, in the lower powers. About 4 to 5?
pounds per square foot is the best loading.
This loading is distributed over the wing surface and to the ship through the medium of the wing. In its
peculiar construction each square foot of cloth throws the Load it carries ontO a rib or series of ribs. These ribs must be strong enough to throw the load onto the spars. The spars must be strong enough to
absorb this load and transmit it through the struts to the ship proper.
That is the design theory behind good wings. The buiLding of the wings then starts in inverse order to
the distribution of the load. The spars are built first, then the wing ribs, and the whole thing is trussed up
with the drag bracing. Then the cloth slip is put over the wing, sewed on and the whole job is covered.
Spars in a lightplane are usually sized according to the load. The Heath Parasol has four spars (two in each wing) which are roughly 3/4 in. thick by 4 in. deep. This is sufficient on a span of about 25 feet to
give a safety factor of ten to eleven on a load of not over five hundred pounds. The greater the span the deeper and heavier must be the spars.
It all depends upon the design, and the reader is urged to follow the design carefully. If he is designing his own light plane, he will be using some standard wing section, such as a Clark Y, which is pretty
hard to beat, and if he places his spars at 15 percent and 65 percent from the leading edge of the wing
he will distribute the load pretty evenly between the front and back spars. It is impracticable to get an
exactly even loading on both spars. As a general rule the front spar will be carrying most of the load, about 60 percent, because the center of pressures of wings of standard sections are about 28 percent
to 38 percent from the leading edge, going back as the angle of incidence decreases and as the speed increases.
The spars are carefully machined, carefully worked down to avoid splintering and raises which would be the first to show failure in event of undue stress.
The old method of building routed spars is practically obsolete nowadays, and spars of solid unrouted characteristics are usually used. The Waco spar is about 6 in. deep and is 1 in. thick, solid spruce.
Some very good spars have been put into production using plywood, though if built as a web-spar the weight is apt to be greater than with a solid spruce spar. Plywood has been used for wing webs, too,
with the usual light cap strip, and the one advantage they have is that they are stouter in a crash and can be built many at a time, being sawed out on the band saw to a template. Probably for the home
builder the old reliable method of building up a Warren trussed rib using 14 in strips with plywood gusset plates is as good as any. This kind of rib is solid enough for any kind of ship in the lightplane class. As a
matter of fact, theoretically the things could be a lot less in dimension and still be plenty strong enough,
but the trouble for amateur construction then would lie in the fact that breakage would be apt to occur in the fastenings. Here is a case of practical consideration overruling the theoretical.
The accompanying drawing shows the method of building up a wooden template with a sawed-out rib jig box, and how the ribs are made by careful bending and nailing and glueing. This type of rib is easiest
to repair while away on barnstorming tours. The cover can be ripped off and a small stick spliced in, and the cover doped over. Crush a plywood rib and it means rebuilding the wing.
Covering is an important part of the building of a wing, too. The old idea that linen must be used is not
tenable any more. The majority of airplanes aside from factory jobs are flying with a good stiff grade of muslin, doped. The reason, aside from strength, that the majority of builders use linen is that it is
"chemically free" and does not deteriorate in several seasons' use.
The method of covering a wing after the spars and the ribs are on and the drag bracing is in place is to
put a long slip of the cloth over the wing. This is sewed like a bag to the shape of the wing. it is best done on a sewing machine, using a double stitched seam. The lapping at the ends, as shown in the
drawings, is pulled reasonably tight. Then the tape is put on and sewed through around the ribs with a long needle. Then the doping operation commences.
Dope is a chemical that stretches tightly when dry. The wing is put where no direct quick-drying sunlight can hit it, and a liberal thick coating of the dope is brushed in. When it is dry, and not before
that is when it loses its cool, damp feeling?another coat can be put on. Not before!
Then the next coat will see less sag in the cloth. It will begin to be drum tight about the end of the third coat, and one or two more coats can then be put on. Unless the wing is a very heavy one and very
strong it is best not to put on more dope than that, as the dope has a tendency to bleed into the other
coats, and when the whole skin starts its real drying the shrinking force on the wing is more than apt to warp it.
Pigment for any color can be mixed into the dope in the last two coats, and the wing is ready for rigging and ready for use in the greatest of mechanized outdoor sports?Lightplane flying! ...