Worth repeating

1. Veeduber Jul 28 2002, 9:52 pm show options
Newsgroups: rec.aviation.homebuilt
From: (Veeduber) - Find messages by this author


(Originally posted message to the Fly5k mailing list in October, 2001.)
- - - - - - - - -

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.

Eh?

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.

-R.S.Hoover

wrote:


1. Veeduber Jul 28 2002, 9:52 pm show options


Thank you VeeDuber for an excellent article!
John