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On the Design, Construction and Maintenance of Wooden Aircraft

Written by Jerry Hollombe

Introduction

This essay started out to be about what it takes to build an airplane using wood, wire, dope and fabric. It's still about that, but it's also about why there shouldn't be a down-time aerospace industry, nor much of an air force, in the first decade or so post Ring of Fire. I say "shouldn't" because what actually happens is up to the fiction authors and, in my experience, when works of fiction are created, plot and drama trump the details of reality every time. Still, if you're going to break the rules, you should at least know what they are.

I earned my private pilot's license in 1966. At the time, it required a minimum of forty hours flight time. I qualified for my Airframe and Powerplant (A&P) mechanic's license in 1970—one of the very last groups of students to be formally trained in maintaining wooden aircraft. To earn my A&P license I went to school eight hours a day, five days a week, for fourteen months, then passed long and rigorous written and practical exams. Nearly all of what I learned in that time is orthogonal to what a pilot learns. The idea that J. Random Pilot from the twenty-first century would know anything about building and maintaining wooden aircraft is laughable. There were no A&P mechanics in the Ring of Fire—let alone any of my era—so most of what I'm going to talk about below is unknown in Grantville.

Further, as a mechanic I know how to maintain and repair aircraft using mostly off-the-shelf parts and materials. I don't know how to design one. For that you need an aerospace engineer and there is only one in the Ring of Fire, Hal Smith. (Mike Spehar managed to grandfather him in before the Grid became so rigid.) I don't know how to make the precursor chemicals for dope. For that you need a chemist. I don't know how to make the high quality steel to make the wires, nuts, bolts, etc., you need to hold an aircraft together. For that you need a metallurgist. Except in the most general terms, I don't even know how to make a propeller, let alone design one. Trial and error will have to serve.

The following description of the building and maintenance of fabric-covered, wood-framed aircraft is going to include a lot of fiddly details and requirements. Some of them are going to be difficult to implement in the seventeenth century. Whether they are implemented or not is up to the fiction authors, but they should be aware of this: A lot of airplanes crashed and a lot of people died to put those details and standards in place. None of them are entirely frivolous. If you want your airplanes to be credibly able to fly from Peetle to Pootle without crashing six times along the way and want your pilots and passengers to be anything but suicidal daredevils, you'll leave them in place. Also note that even modern private aircraft are inspected annually, commercial aircraft are also inspected every 100 hours of flight and military aircraft are inspected daily, so problems can be detected and repaired early. Finally, when feasible, every pilot does a walk-around inspection of his aircraft before taking it up.

It's been suggested to me that outside of Jesse Wood's air force, down-time pilots will be daredevils. Even if you aren't concerned about their safety, consider the safety of your precious engines, instruments and even rubber tires. You can't afford to build airplanes that crash and burn at every pause in the conversation.

So, let's begin.

Tools

First is a list of the minimum woodworking tools required to maintain a wood framed aircraft. Most of them should be available or makeable in the seventeenth century. Space limits prevent me from describing each one and its use. Mechanics learn about them in the practical shop part of their training.

Backsaw (14 to 18 teeth per inch)
Small bucking bar
Auger bits
Brace
C-clamps
Parallel wood clamps (Jorgenson)
Scribe compass (10 inch, thumbscrew lock)
Hand drill
Twist drills (1/16 to 1/4 inch)
Flashlight
Hammer
Magnetic tack hammer
Pocket knife
Block plane
Jack plane
Diagonal cutting pliers
Coarse wood rasp (half round)
Fine wood rasp (half round)
Dovetail saw
Crosscut hand saw (10 to 14 teeth per inch)
Keyhole saw
Rip saw (5 to 6 teeth per inch)
Screwdrivers
Combination square
Straightedge (36 to 48 inches)

The wooden frame is covered with fabric and the tools for working with that are the same as those used by a tailor or upholsterer. They include assorted needles, scissors, pinking shears, sewing machines and irons. The fabric, in turn, is covered with dope, which I'll talk more about under the materials heading. Dope is applied like paint, with brushes or, if available, a paint sprayer.

Even wooden airplanes have metal parts and fittings and for them you need the usual wrenches and screwdrivers and drills (oh my!). To fabricate the parts from raw stock, you'll need the resources of a machine shop or a blacksmith.

In addition to these mostly generic tools, there are specialty tools needed for doing things that only airplanes need done, like tensioning the wires and cables that hold the wings up (and down). I'll mention them as they come up in context.

Materials

Wood

Aircraft spruce is the wood most commonly used for wooden aircraft structures. Properly cured, it is light in weight and has high tensile strength for loads applied parallel to the grain. "Properly cured" means kiln dried to produce uniform strength and reduce moisture content evenly. To promote even curing, pieces to go in the kiln should be as small as feasible, given the parts they are going to be used to make. (Obviously, beams for wing spars and such are going to be pretty long.) If aircraft quality spruce isn't available, certain other woods may be substituted if they are of sufficient quality: Douglas fir, noble fir, Western hemlock and white or Port Orford cedar. Some of these are not available in seventeenth-century Europe.

In general, the wood should be straight grained and the grain should not deviate more than one inch in fifteen. Wood for spars and other large structural parts should be quarter sawed such that the end grain is nearly perpendicular to the sides of the board. The minimum number of annual rings per inch is six for most woods and eight for Port Orford cedar or Douglas fir. Look for trees growing on the shady side of a hill or in other conditions that lead to slow growth.

Aircraft wood must be free of decay, shakes and checks (splits) and compression failures. Minor defects like small, solid knots and wavy grain are tolerable if they don't appreciably weaken the part, but should be avoided if at all possible.

Glue

Most aircraft construction and repair uses glue to join pieces of wood. A glue joint should be as strong as the surrounding wood. Of the glues available in the seventeenth century, animal and fish glues cannot be used for aircraft work because they are not waterproof. Until synthetic resin glues are reinvented, casein glue will have to do. (The familiar white glue is usually a casein glue. It's made from milk, lime and salt.) It is satisfactory for the purpose as long as it is protected from fungus, usually by chemical additives (zinc borate or formaldehyde may be suitable). All glue left over from a job should be discarded.

Fabric

The most common fabric for modern aircraft is grade A mercerized cotton cloth. (Mercerizing is a chemical treatment that shrinks the material.) Unfortunately, long staple cotton isn't readily available in seventeenth-century Europe, so a substitute must be found.

In the early days of flight, aircraft were covered with Irish linen, which is still acceptable provided it meets quality standards. The main problem with linen is shrinkage. The material must be carefully cut and sewn to allow for that factor or it can tighten up enough to break ribs and damage other aircraft structures.

The minimum tensile strength for the covering fabric is eighty pounds per inch. I.e., a one-inch wide strip of cloth must support at least eighty pounds weight without breaking. It must have a thread count of eighty to eighty-four threads per inch in both length and width and must weigh four ounces or more per square yard. After weaving, the fabric is calendered (pressed wet between hot and cold rollers) to lay the nap.

Fabric may be bias cut (cut diagonally across the weave), which allows a small amount of stretch for fitting purposes.

Surface Tape

Surface tape is used as a reinforcement over stress areas, such as the leading and trailing edges of wings, over rib lacing and seams and around fittings on doped fabric. It is usually cut from the same fabric used to cover the airplane and has identical physical specifications. The tape usually has a pinked (sawtoothed) edge, which improves adhesion and helps inhibit raveling. It should be used to cover all lacing and stitching, but only after the first coat of dope has been applied.

Reinforcing Tape

This is used between the fabric covering a rib and the lacing cord to help distribute load and keep the cord from wearing through the fabric. The material is similar to surface tape, but the warp thread is larger than the fill and it should have a tensile strength of one hundred fifty pounds per half inch. Its width should be matched to the width of the rib it is covering.

Sewing Thread and Cordage

Again, since the customary cotton is not available, linen will have to do.

Machine sewing thread must have a tensile strength of five pounds per strand and weigh about one pound per five thousand yards. It is technically described as white, silk-finish, No. 16 four-cord thread with a left or Z twist.

Hand sewing thread must have a tensile strength of fourteen pounds per strand and weigh one pound per 1650 yards.

Lacing cord is used to attach fabric to the structure of the airplane. It should have a minimum tensile strength of forty pounds single or eighty pounds double. Bee's wax should be used to lightly coat the cord before use by drawing the cord across a piece of wax.

Waxed cord is used to attach leather chafing strips (made of russet strap leather) on parts of the structure that may be subject to rubbing by moving parts such as brace wires and structural tubing. Chafing strips protect against wear and abrasion and the cord holding them in place must be double-twist and waxed.

Leather

Russet strap leather is used for reinforcing where structural parts or controls must pass through the fabric skin. Horsehide, which is thinner, may be substituted in areas of lesser wear.

Miscellaneous

Tacks are used during construction to temporarily hold fabric in place, but only rustproof tacks, made of brass, tinned iron or Monel, should be used for permanently attaching fabric to wood.

Where holes are necessary for drainage, inspection or lacing, grommets are used to reinforce the fabric. Seaplane or marine grommets are shaped to create suction to enhance drainage or ventilation when necessary.

Dope

In order to make aircraft fabric airtight and weatherproof, dope is applied. Dope also causes the fabric to tighten, removing wrinkles. Caution should be exercised here, as too much tightening can damage underlying structures. Clear and pigmented dopes each have their separate purposes. Modern dope is often pigmented with powdered aluminum to provide protection from sunlight. Aluminum is unknown in the seventeenth century. Until powdered aluminum becomes available, you'll have to live without it and plan extra inspection, maintenance and repair to compensate. Final coats of dope are mixed with color pigments to achieve any desired appearance and also provide some protection from sunlight.

Nitrocellulose dope is made by adding glycol sebacate, ethyl acetate, butyl acetate or butyl alcohol to a solution of nitrocellulose. Ethyl alcohol or benzol can be used to thin the dope to desired consistency. The main drawback of nitrocellulose dope is extreme flammability. Once ignited, it burns too fast for fire fighting to be practical, especially in an aircraft aloft. Adding aluminum, when available, only exacerbates the problem. For reference, the crash of the Hindenburg is now attributed to its having been coated with nitrocellulose dope pigmented with aluminum and iron oxide—a combination better known in modern times as "rocket fuel."

Cellulose acetate butyrate (CAB) dope is more resistant to fire than nitrocellulose and penetrates better as well. On the down side, it has a stronger tautening effect, which can damage fabric or structure if care is not taken. It can be applied over nitrocellulose dope.

If you want to know about the chemicals that make dope. I haven't a clue and never did. It's not in my textbooks. As I said at the outset, I'm a mechanic. I know how to maintain and repair airplanes using mostly off-the-shelf materials. Dope is something I ordered from a parts catalog.

Modern aircraft coatings also include fiberglass and assorted other plastics, but down-timers are going to have a hard enough time making the traditional dopes without worrying about up-time synthetics.

Construction and Repair

Vocabulary

As with most technical specialties, there is a broad nomenclature for wooden aircraft. It doesn't exist to keep nonexperts at bay, but rather to precisely specify things that must be so specified and don't exist in any other context. So, we must deal with spars, stringers, bulkheads, ribs, formers, longerons and leading edge strips, all of which have definitions unique to wooden ...