Thursday, November 16, 2006

Flying on the Cheap

I wasn't born knowing how to weld.

Or rivet.

Or even knowing how to drive a nail into a hunka wood (without bending, please :-)

My genetic failings were brought to mind by a recent exchange of messages with a wannabee airplane builder who had his own set of genetic failings, which came to light as we discussed inexpensive airplanes. And I mean dirt cheap, as in less than a thousand bucks.

The key to any flying machine that doesn't have a lot of corners is power-to-weight-to-strength, whereas the secret to flying on the cheap appears to be the intelligent use of materials that are commonly available. If you really want to fly, the materials to build a safe, durable, inexpensive airplane are probably available in your own home town.

To achieve this seeming miracle you allow industries other than aviation to subsidize your airplane, such as using the engine from a snowmobile or car, steel tubing used in bicycle frames for your fuselage and so forth. The Hat Trick is how someone without a heavy engineering background can get the maximum strength for the least weight from inexpensive, commonly available materials. Oddly enough, those who have gone before have told us how to do exactly that.

When the question has to do with strength versus weight steel tubing will always stand near the head of the list. And since America is an industrialized nation you have access to structural steel tubing no matter where you live. Structural steel tubing is that welded stuff, formed by rolling a strip of sheet stock into a tube and roller-welding the edges. The modern day stuff comes in several alloys superior to the SAE 1020 mild steel tubing used in tens of thousands of early airframes and is legal for use in repair work on rag & tube airframes. (The details are covered in a CAA bulletin posted in the mid-1950's.) The nice part is that structural steel tubing costs only pennies per foot.

So how much is that, in airplane terms? A husky, over-built fuselage like a Pietenpol uses about ten twenty-foot pieces of tubing. Not all two hundred feet of it; probably about 170' or so, mostly in 5/8 and half inch. But with structural tubing it's usually cheaper to buy a full length than to pay for the cutting, so you end up with a lot left over. A 20' piece of 3/4" x .042, for example, is about $7.50. (I just called Carol over at Escondido Metals to check on that. If I bought $200 worth, they'll deliver it free. Otherwise, it's an extra twenty bucks. [NOTE: Price as of October, 2001]) Other sizes run about the same or a little less, since with non-TSO'd structural steel you're paying for the weight of the metal rather than its size and certification.

That means you're looking at something like $75 for the steel to build a fuselage. A big, two-place fuselage. Okay, yeah... there's lots of tabs and fittings that you gotta cut out of flat stock. And you need a few feet of bushing stock and some 4130 for the axles. And I haven't mentioned the struts (add 40' of 1" tubing). But the point is, the stuff is available and it's dirt cheap. And that's for a fairly large two-place airplane with a proven track record. Build something smaller - a one-place job you can power with a VW engine - and the cost will be even less.

So I mentioned all that to the fellow wanting to fly on the cheap. And got back about three pages of misconceptions, preconceptions and Conventional Wisdom.

First off, he doesn't know how to weld. And made it pretty clear he isn't interested in learning. Which I thought was kind of sad because the only things we truly own are the things we know - the stuff between our ears. All else is transient. It can be taken from us by the stroke of a politician's pen, by disaster or simple fate. But knowledge is yours forever. No one is born knowing how to weld. Or to fly, ride a bicycle, program in Pascal... But no matter our age or background we can learn to do those things. And among the manual arts, welding is one of the most useful as well as being one of the easiest skills to learn. You can take a course on welding at most community colleges (we've got two of them, locally, both eager for your business). You may even find a weldor through your local EAA chapter who will be willing to give you some pointers.

Truth is, welding is mostly practice and you don't need anything for that other than determination and a welding torch. You can buy a torch from Harbor Freight and unless you're living on the moon, there's someone in your neighborhood ready to provide you with flasks of oxygen and acetylene. From that point you're about six hours away from running an acceptable bead. After that, it's simply a matter of practice. As for practice tubing, your local hardware store can provide all you want at about ten cents a foot. It's called Electrical Metallic Conduit - EMT. It comes galvanized but there's this stuff called ‘acid' that does a number on zinc... and galvanizing is zinc. So you cut your practice-welding tubing coupons out of EMT, soak them in dilute acid for about twenty minutes and there's your material. If you don't like the thought of diluting a bucket of hydrochloric acid, go down to the paint store and buy a jug of JASCO (brand name) ‘Prep & Prime'. It's dilute phosphoric acid, used for preparing aluminum and galvanized metal to accept paint. Very benign stuff as acids go -- you eat a lot of it (read the label on a can of soda pop). But besides putting that special tang in Coca-Cola it will also dissolve rust and zinc.

Once you've learned to tack-weld and can run a reasonable bead on tubing, you can build yourself a fuselage. Assuming you aren't too old. Building a fuselage is as much gymnastics as welding :-)

Secondly, the fellow didn't want to consider steel tubing because, according to him, a steel tube fuselage has hundreds of pieces of tubing, each of which has to be cut & fitted to make a perfect joint before you can start to weld.


Even counting the longerons it's rare to find a steel tube fuselage that contains more than sixty pieces and many of those are dupes - you make them in pairs. Smaller the airframe, the fewer the pieces of tubing in the fuselage, down to a minimum of about forty pieces. Not ‘hundreds.' So how many joints is that? On a complicated fuselage, maybe fifty. That's because you don't weld each individual tube, you weld the cluster where those tubes come together. How long does it take? That's up to you but with MIG it seldom takes more than five minutes to do a cluster. Gas takes maybe three times that while TIG takes... as long as it takes. Even so, your set-up time for each cluster will be at least as long as the time it takes to weld the thing, less if you've got the fuselage on a rotisserie. On average, working without a lot of tooling, laying out the sides on the floor and assembling it on a pair of sawhorses, it takes about five man-days to put a fuselage on its gear.

His fear of fitting was equally fey. Fitting steel tubing isn't difficult at all if you have the right tools and approach the task in the proper fashion. Some of the ‘right tools' are a vise at proper working height, hardwood blocks to hold the tubing and some round files of the proper diameter. But the most important tool is the center-line you draw on the tubing in order to keep the notches on the ends aligned. (A nice advantage of welded tubing vs seamless is that the seam provides the center line.)

The truth is, a properly assembled fuselage requires a lot less fitting than you may realize.

If the fuselage uses a Warren truss as in the Pietenpol or Piper Cub, all of the side frame tubes intersect the longerons at an obtuse angle, whereas designs using a Pratt truss, as with the Georgias Special or any of the Heath designs, a majority of the tubes form right angles with the longerons. And fitting the tube for a square corner is dead simple to make using a hole saw or step drill or simply nipping a 22.5 degree angle on each side of the tube and thinning down the inside of the resulting points with a grinder. For angles other than 90, most guys who do any tube work at all have a couple of wheels for their bench grinder that they've rounded to match the most common sizes of tubing, but the truth is, a grinder wheel with a half-inch radius can be used on anything larger, you just have to swing the tubing in an arc.

(A Warren truss looks like the letter ‘W,' which makes it easy to remember. A Pratt truss looks like the letter ‘N.' )

Fuselage sides may be either Warren or Pratt but the cross members tend to follow the Pratt truss pattern. Warren truss, you really should make up a jig to hold things in alignment. Pratt truss, you can lay-out fairly well on any flat surface using nothing more than a framing square and some firebricks to position the tubing.

(An interesting point about steel tube fuselages is that you only build one side. Then you make the other to match :-)

Putting the sides together is dead simple if the upper longerons are straight –– you just lay the thing on its back. Once you have the forward cross members tacked in place you clamp it down, belly up, strike a centerline and bring the aft end of each side to the line. That insures you have an equal angle on either side. From that point the remaining work is straight forward since the cross pieces are virtually perpendicular to the longerons and the diagonals fit in the corners formed by the cross members. Once it's all tacked, to keep the structure straight, you start at the nose and spiral your way aft, welding as you go. If it sneaks out of alignment you heat the guilty cluster and pull things back into alignment using a bar-clamp or turnbuckle diagonally across that frame. With any fuselage smaller than a Fairchild or Waco, as you weld you just keep rolling it over on the saw horses to bring the next cluster to a convenient welding height. With larger airframes, you have to climb right inside the thing. Little planes, like the Heath, just hold under one arm :-)

Light airplanes use lotsa small tubing for the diagonals. Which is good. As the tubing gets smaller (or the wall thicker) the fitting gets easier, since you don't need to make a perfectly scalloped end. For example, when doing the diagonals on a Pratt truss using half inch tubing, you simply chamfer the ‘corners' off the tube with a file or angle-head grinder because it's impossible to form a perfect fishmouth on a diagonal tube, except for earning a welding certificate :-) Why? Because when fabricating a homebuilt fuselage it's unlikely you'll have the benefit of a full fuselage jig. In that case the standard practice is to tack your verticals so as to fix the longerons in place before you start doing your diagonals. And with the verticals (or horizontals, if you're working on the top or bottom) already tacked, a properly notched diagonal CAN'T fit. (Try it :-) So you nick the top (or bottom) off the corner of one end of each diagonal, allowing the tube to fit into place. If that doesn't make sense, draw a half inch circle intersecting a five-eighths inch circle at a 45 degree angle. And don't worry about the gap. When the arc of intersection is less than the diameter of your welding rod (or the thickness of the tubing wall) the scallop will ‘vanish' during the welding.

(The clusters for a welding certificate are ‘open', in that the other end of the tubes are simply hanging in space. Such tests show the examiner you can fit tubing and run a pretty row-of-dimes but you'll never find such a cluster in the real world. There are hundreds of welding ‘certs,' by the way. For basic repair of a certified airframe you need about a half dozen certificates, mostly involved with carbon steel and chrome-moly, welded with gas, mig and tig. Additional certifications cover Monel and stainless, used mostly for exhaust systems, and aluminum for doing tanks. )

Some folks go a little bit crazy over that last point, insisting all of the tubes must be perfect joins. That shows they haven't built many steel tube fuselages. If you drop by an aircraft factory you'll see them doing exactly as I've described above, and using a wire-feed MIG'er for the welding. That isn't to say the work is sloppy, it simply recognizes the fact that a gap smaller than your filler rod or wall thickness (which ever is smaller) isn't significant.

Buy yourself a MIG'er, you don't need to learn how to weld. (Well, maybe not quite :-) Using fine gauge wire, medium feed and low amperage, you can weld .028 wall and do so with the same ease & convenience as quarter inch plate. There's some tricks to it - the usual learning curve associated with using any tool - but it's a doable thing. On the other hand, about the thinnest structural tubing you'll run into is .035, which is pretty easy to weld. Using a MIG'er your welds won't be as pretty as with gas or TIG. They'll come out kinda lumpy until you get the hang of weaving your bead and gain some experience in how the shape of the part concentrates the heat - you can't weld every part of a cluster at the same rate or with the same type of bead - but you CAN weld yourself a fuselage. Kinda lumpy here & there but more than strong enough. (No, you can't grind it down to make it pretty. Structural welding, what you see is what you get.)

Ohmygosh! There I was talking dirt cheap and all of a sudden I'm talking about TIG and MIG and gas welding rigs and bottles of gas and bench grinders and all that expensive stuff! Um, well, actually, it's not that expensive. Those are tools, not skills. Build yourself an airplane then sell all that stuff. In fact, most of those Chinese tools from Harbor Freight are little better than a pre-assembled KIT of parts. If you want it to work you'll probably have to take it apart, clean it, lubricate it and put it back together properly adjusted. And folks are always willing to pay a fair price for a good tool, which yours will be, after spending a bit of time with them.

A final point the fellow made about steel tube fuselages had to do with the added weight of using structural steel tubing instead of two-dollar-a-foot 4130.

In theory, he's correct. (Using TIG you keep the joints nice & tight, rarely need to add any filler rod.) But in actual practice, he's not. TIG welded 4130, using a dozen different diameters and wall thicknesses, will give you a fuselage having the optimum strength-to-weight ratio. A bare Pietenpol-type fuselage for example (ie, no landing gear or center section struts) would come out weighing about 43 pounds. If you make the same fuselage using just three diameters of structural steel tubing and a MIG welder, it'll come out 10% to 12% heavier. (You can calculate this for yourself [All steel weighs about the same.] In fact, you don't even have to calculate the weight of the tubing - it's listed in the various catalogs and machinist's references.)

If you read "...12% heavier..." in a book you'd know right off that was a terrible thing. (Without even building a welded steel tube fuselage you have become an instant Expert :-) But in practical terms that 12% means a fuselage similar to a Pietenpol, fabricated from structural steel instead of several sizes of 4130, weighs 48 pounds instead of 43. Since a wooden fuselage would run close to seventy pounds you'll still come out ahead on the deal. (A lot of older designs used only one size of tubing and were never fabricated in 4130. If built from structural steel tubing they will weigh exactly the same, although they may gain a fraction of a percent in weight if welded with MIG due to the thicker bead.)

Having expounded upon the reasons why a welded steel tube fuselage was a bad, bad idea, the fellow went on to point out the difficulties of finding, and the expense of overhauling, a Model A engine, having taken my mention of the Pietenpol as a suggestion to build one. (I wasn't suggesting anything, merely providing him with information about options of which he seemed unaware... which is also the reason I'm posting this.)

This may come as a surprise but there are modern industrial engines that weigh less than the Model A and produce more torque at an even lower rpm. GM makes a nice one. $1600 brand new in the crate from the factory. It cranks out an honest 65 hp @ 1800 rpm, giving you more than twice the thrust of the Model A. The engine, which has been in production since about 1965, is also available used and overhauled, in both long and short block versions. Just be sure you get it with the Industrial Engine cam instead of the Marine Engine cam. The marine version runs at a much higher rpm. Ford makes a similar engine although I'm not familiar with its specs.

If you stick with the VW engine you'll find at least a dozen early rag & tube designs that will be quite happy to fly behind the thing and generally perform better than the original. Several of Leslie Long's designs fall into this category, as do most of the designs by Church, Corben, Henderson and Heath. Most of these were designed for relatively heavy engines putting out as little as 20 hp. One of the most popular was the Henderson four cylinder motorcycle engine that produced 23hp for a weight of 120 pounds. Plans for most of these early planes are in the public domain, available for a few dollars each in reprints of early aviation journals, such as Fawcett Publications annual ‘Flying and Gliding Manual' reprinted by the EAA.

None of those early designs are what you would call fast. With a heavy, low powered engine they needed a lot of wing. With a big strut-braced wing, adding more power may let you climb like an eagle but doesn't do much for the top end. On the plus side, that big wing usually gave you a very low stall and a short take-off run. There's worse ways to fly :-)

The fellow closed his message with some remarks about the high cost of aircraft certified spruce, which I agree have become a bit ridiculous. But here again we are dealing with an optimized strength-to-weight TSO'd material. You can build a perfectly good wing using Douglas Fir for your spars - or even marine plywood for the shear web with hemlock capstrips. It will be heavier for the same strength but the cost will be a scant fraction of price you'd pay for certified spruce. There is no question as to the strength or utility of such spars, which are described in a 1920's Technical Report from the NACA. Indeed, the major flaw reported in that TR had to do with failure of the casein glue-line, an unheard of occurrence since the introduction of resorcinol and epoxy adhesives.

If you live in an area which has any aerospace industries you may also have the option of making yourself a set of ALUMINUM wings. Here in southern California it's possible to find new-surplus aluminum at give-away prices. Not easy, just possible :-) (I purchase most of my rivets and hardware from the surplus outlets of the several aerospace firms in the San Diego area.)

The whole point of this message is to get across the idea that flying on the cheap doesn't mean an unsafe airframe. At the most it means an airframe that weighs a bit more than one built of certified materials having an optimized strength-to-weight ratio. That added weight was virtually inherent in early designs and shows up as a reduced rate of climb and an increase in your stalling and landing speed. But that addition weight only appears if you religiously use the materials of yesteryear such as motorcycle wheels and Grade A cotton fabric and no aluminum for your leading edge on a scalloped wing that produces left than half the lift of a modern airfoil. And with an antique powerplant on the nose.

The other side of the coin is to take what we've learned over the last 75 years and apply it to those early designs, such as using a modern industrial engine in a Pietenpol airframe, or a VW engine in any of the smaller designs (which has already been done for the Heath, by the way). The use of industrial or go-cart wheels is a no-brainer. (Paz used industrial wheels and non-TSO'd tires on his PL-4, as did hundreds of Varieze builders.) Such wheels are typically stronger and lighter than a motorcycle wheel and offer less drag. Curtis Pitts and Vernon Payne have taught us how to build wood-spar'd wings of impressive strength without going broke buying turnbuckles or making special compression strut fittings. The use of polyester fabric, typically four times as strong for the same weight of Grade A cotton is another no-brainer. 100% polyester (that's ‘Dacron' if you own stock in DuPont) is universally available in widths up to 66" for only a few bucks per yard. (And it really doesn't HAVE to be white :-) Brushed-on urethane varnish mixed with aluminum powder provides a good UV barrier and there's all sorts of urethane enamels that will give you a glossy, easily maintained top coat. The can doesn't have to say ‘aircraft certified to contain good quality paint. Indeed, HOUSE PAINT has been successfully used on the fabric of many lightplanes.

Despite all of the above, this message isn't about rag and tube airplanes. The real message is that the limiting factor in our desire to rise above the ground appears to be not the size of our wallets but the limits we impose upon ourselves. If you want to fly on the cheap, you can. The proof of that is an historical fact. The factors that made such flight possible in the 1930's remain in place today.

-Bob Hoover

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