The Grantville Gazette Volumn VI by Eric Flint
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 woode
Spars are the main beams of the wings and transmit air loads to the fuselage (the body of the plane). They may go from wing tip to wing tip or from wing tip to fuselage and support the ribs, compression struts and various attachment fittings. They can be solid wood, solid laminated wood, or built up into 'I' shapes or boxes.
Ribs give the wing and other airfoils their shape. What that shape should be is an engineering problem. Modern engineers go to a NASA database and look up the airfoil contour that will give them the flight characteristics they desire. Jesse Wood is fortunate that Hal Smith had a book with the standard NACA airfoils in it. Without that, in the seventeenth century, you would have to fall back on the Wright brothers' technique of trial and error, using models in a homemade wind tunnel to test their ideas. However, even without the NACA airfoil book, the up-timers have brought back an understanding of the principles of how airfoils work well beyond the Wrights'.
A properly designed and constructed wooden rib can support thousands of times its own weight when in place on the aircraft.
Leading and trailing edge strips give the ribs sideways support and shape the respective edges of the airfoils. Wing-tip bows shape the outer ends of the wings.
Bulkheads and formers shape the fuselage, or body, of the aircraft. They are held in place by longerons and stringers—wooden strips that run the length of the fuselage.
Carpentry
Most of the actual woodworking is well within the capabilities of down-time craftsmen, given the necessary materials. The ribs are the most complex individual part and they're not all that bad, even though there are a lot of them. The usual technique is to take a copy of the design blueprint and attach it to a flat board, then nail small pieces of wood to it to form a jig for assembling the pieces that form the rib. The ribs themselves are formed from thin strips of wood held together by glue and thin wood gussets. The gussets are usually nailed in place to hold them while the glue dries, but the strength of the rib comes from the glue, not the nails. Just about all the other wooden parts are made using techniques that haven't changed much in centuries.
Metal Work
This gets trickier. There are a lot of metal parts in a wooden airplane. Just making the screws, nuts and bolts is going to be a problem in the down-time world, let alone to the standards of strength and consistency flight safety demands. (Remember, you can't just coast over to the side of the road, or even to a stop, if a major part breaks in flight.) In the up-time world, that's done with intense quality control and inspection procedures that use X-rays, Magnaflux™, and fluorescent penetrating dyes, among other things. X-rays probably won't be an option, but, given electricity and wire, Magnafluxing is straightforward. Penetrating dyes may also be possible. Short of this kind of intensive testing, down-time aircraft will be restricted to over-engineering, using bigger stronger and therefore heavier parts than are really required. This results, of course, in a heavier aircraft with poorer performance.
Braided wire cable is also beyond down-time technology for now, so controls and control surfaces are going to be operated by metal pushrods (which are usually tubes) and bell cranks. Bell cranks are simple, pivoting pieces of metal shaped to transfer and redirect the motion of cables and pushrods. They get their name from the crank that converts a straight rope pull to the swinging motion of a church bell. In airplanes, they move the control surfaces, and may also be used for engine controls.
Safety wire is another tricky essential. It's flexible wire, usually made of soft iron or stainless steel, used to secure nuts, bolts and turnbuckles so the vibrations of flight can't loosen them. That means nuts and bolts have to have holes drilled through them for the safety wire to pass through. General purpose safety wire is 0.032 inches in diameter. The holes are barely larger than the diameter of the wire, so they don't compromise the strength of the nut or bolt. Drilling those holes is going to be a challenge to down-time technology. You need safety wire and the drilled nuts and bolts that go with it, unless you can you live with pieces falling off your airplane in flight.
In Conclusion
Even if, by a miracle, I was inside the Ring of Fire with all my tools and books and class notes and training and experience still fresh in my head, Grantville still wouldn't know how to build an airplane from scratch, let alone have—or even know of—all the necessary materials. Even ignoring the engineering and design gap, every substitution you make of down-time materials downgrades the safety and reliability of the aircraft and probably its payload as well. For the first 20 years or so after Kittyhawk, it was considered normal for an airplane to crash every fifty miles or so. Pilots were daredevils. And they had aluminum, steel, cotton, rubber, etc. They just didn't have all the safety regulations and standards in place. (As one of my instructors pointed out, until well after the advent of passenger planes every FAA regulation on the books was put there for the safety of the people on the ground. Think about that.) Down-time, it's iffy whether anyone can make safety wire.
During my mechanic's training, we practiced on an old Stearman biplane. As I recall, its maintenance manual came in two volumes, each about three inches thick. Just the procedure for tensioning the wires that held the wings up (and down) took up several pages and there was only one right way to do it. Some engineer figured out the procedure. Try any other sequence of installing and tensioning those wires and they would be out of balance, incorrect and unsafe, depriving the aircraft of much of its structural strength. Did I mention the special tool you need to measure the tension of the wires? Also, even then, there was only one company left in the entire country that still made the wires, in case you needed to replace one. I have no idea if they're still in business today. Again, dealing with this sort of issue might be overcome by overengineering, using heavier stronger parts than you really need to so that the aircraft can handle the stresses of down-time construction, but that would result, of course, in a heavier aircraft with lower performance.
By the way, once aluminum production comes up to speed, the next thing you need, if you're going to make airplanes with it, is zinc chromate. That's the green primer you may have seen on interior airplane parts, if you've ever seen them at all. Without it, aluminum corrodes and weakens under the vibration and stress of flight and, eventually, pieces start falling off your airplane. How do you make zinc chromate? I have no idea. It comes in cans.
Of course, you're also going to need rivets. Time was I could give you a half hour lecture on rivets alone, covering all the different types, shapes, alloys, purposes, installation procedures, etc. Get back to me about that when Grantville has aluminum.
Note that I haven't even mentioned things like flight instrumentation. How many people even know how an altimeter or air speed indicator works, let alone how to build one? Where are you going to get the down-time expertise to make them to the necessary precision? Do the Grantville machine shops have room in their overbooked schedules to make precision gyroscopes and the associated mechanisms that let them do what a pilot needs them to do? Certainly, no one else down-time has the tools or the skills to do that.
Airplanes—useful, reasonably safe airplanes, anyway—are complicated and difficult to make and they require a lot of industrial infrastructure if they're going to be more than rare curiosities. The time will come when Grantville has that infrastructure in place and airplanes will begin to be common again, but, realistically, that's not going to happen overnight, or even over a year or two.
So, now you have some facts in hand. Go forth and write your tales of down-time aeronautical adventure, ignoring them as you choose. Just don't come to me when pilot and mechanic fans start writing in to tell you what you got wrong. I'll only say, "I told you so."
Mike Spehar, author of the flying scenes in 1633, and creator of Colonel Jesse Woods, head of the USE Air Force in 1633 replies
In general, I agree and sympathize with the author. It would be an engineering feat of unusual skill and luck to design a successful aircraft from scratch—certainly to the precision that Jerry is familiar with. In focusing on the airframe, Jerry hasn't discussed the problems related to power plant design, especially gearing to the propeller, or how to build a suitable landing gear. The problem of power transfer alone cost me many days of research. I likewise could go on and on about air intake vents and the proper design of exhaust systems. And his comments about suitable wood and fabric brought to mind hours of research of my own on the subject. (Parenthetically, I note that he hasn't mentioned anything about internally braced wooden wing construction. Nor did he discuss the possibilities of incorporating light wood veneer and parquet techniques for wing design, though such things are probably not usually taught in the normal A&P courses.)
But, of course, I cheated. I did include an aeronautical engineer in the Ring of Fire, along with a professional Air Force pilot with an aero background, as well as thousands of flying hours (as I have). Most of my personal research led me to believe that there are reasonable workarounds to many of the problems Jerry's mentioned, if one can live with a "belt and suspenders" approach to design, generally by making things stronger than they need to be. Naturally, all those workarounds would add weight to the aircraft, but, as the man says, safety first.
There are a couple of technical points that can be addressed from the above:
Re props: Anyone who's looked closely at a WWI prop could probably duplicate one. Our aero engineer would know the basics and the mathematical formulae for wing and prop design aren't rocket science.
Re the construction of instruments: We've mentioned this problem in a number of places, and even discussed some workarounds. I agree it would be difficult, even impossible, to give any aircraft a full suite of instruments. But needle, ball, and airspeed worked for decades. It's not for nothing that we've stressed the dangers of anything but VFR flight. I've several books describing how to build basic instruments, and I suspect Hal Smith would have them as well. And the scientific principles upon which they are based are even better known, but gyroscopic instruments just won't be happening for the foreseeable future.