Monday, March 24, 2008

Other Ribs

A two-place Primay Glider may appear to be a contradiction of terms but having such a beast on the flight-line can be the difference between a Group devoted to pancake breakfasts and one having a successful flight-training program for youngsters.

Since cost is always a factor in real grass-roots aviation, the two-place was built almost entirely from materials that were locally available. The 68" ribs were routed from doorskins and fitted with a grooved cap-strip. Airfoil is an early Eiffel, I think one of the '400' series.


Tuesday, March 18, 2008

Bending Extruded Angle

Extrusions of every type are common stuff in airplanes but many homebuilders may not be aware of the fact the extruded shape usually receives additional forming. Extruded angle, for example, may be opened or closed by as much as twenty degrees (depending on the alloy). It is also common practice to alter the width of the flanges by milling or sawing, cutting them to a taper or machining them to accept stringers or other fittings. The thickness of an extrusion may also be altered to produce joggles or indentations needed to accommodate other components. Extrusions may be formed into circles, ovals or round-cornered rectangles and used as the frames for ports, hatches and so forth. When the flange is on the inside of the curve the extrusion is usually drilled & slotted to accommodate the bend, which is then reenforced with a doubler or scab plate. But when the flange is on the outside of the curve it may be bent to the required shape by hammering alone.

Re-forming an extrusion differs only slightly from every other type of forming operation. For serial production the manufacturer will usually design tooling using rollers or dies to do the re-forming so the required number of parts can be produced accurately and quickly without calling on a skilled metalsmith. But for one-offs, such as a homebuilt, the work is usually done by hand using mallets of wood, rawhide or lead, as appropriate, bapping the angle over a maple vee-block to form curves, over a polished cylindrical anvil to open the angle or simply tapping directly on the flanges to close them (or to form an angle that tapers along the length of the member). For homebuilts, tapering an extrusion is usually done with a table saw and most milling operations are accomplished with files.

The fact an extrusion arrives at the plant (or in your driveway) a quarter of an inch thick with two inch flanges and a perfect ninety degree angle between them does not mean it has to stay that way. The extrusion is simply a convenient starting point for whatever shape the design happens to require. This sort of thing is a normal part of building metal airplanes and because of it, is seldom mentioned specifically in manuals devoted to aviation metalsmithing although most include illustrations of the various procedures used to reform extrusions.

I've taken the trouble to post this message after a confused and rather funny exchange with a fellow building a Teenie Two, which uses extrusions for a number of components. On the wing spars the extrusions are filed, milled or routed so as to nest with the shear-web in a manner similar to the method used on the Spitfire and which the builder had no trouble understanding. But at the firewall the extrusions must be reformed to match angles dictated by the shape of the fuselage and the builder was not aware that it's perfectly legal to reform the extrusion by bending, so long as the resulting angle does not violate the malleability nor the temper of the stock being used. He also knew about slotting the inner flange to facilitate the formation of an inward bend. The funny part of the exchange was my assumption the fellow knew about bapping extrusions into whatever shape is required for an outward bend, which does not require slotting. With that assumption as the foundation, his question about making the required angles caused me to think he was using an anvil of the wrong radius or simply wasn't hitting the thing hard enough, so I'm telling him to use a bar of smaller diameter, a heavier hammer or to hit it harder, convincing him I was a total loon when it came to tin-bending :-)

Calvin Parker is an excellent metalsmith, as shown by the simple sophistication of his design. Unfortunately the techniques needed to produce those elegantly simple junctions of extruded angle are left as an exercise for the student.

If you've never re-formed an extrusion you might want to try making ring of the stuff. Basic tool is a hammer and a sturdy block of wood, slotted to accept the down-angle, with a generous vee under the area where the bending will occur. Put the work-piece on the block, tap (gently) near the junction of the flanges with a heavy mallet as if you were trying to drive the piece into the vee of the block and you will see the piece begin to curve. The required stretching of the material in the down-angle is then adjusted and made more regular by light hammering near the edge of the flange whilst supported on a steel block. The result is an artful curve which if done for the full length of the piece will produce a neat ring of extruded aluminum, as for the base of a nacelle or the frame of a window or hatch.


Tuesday, March 11, 2008

Ancient History

Found in a drawer. A box of 35mm slides, mis-labeled. That's me on the right, visiting bureaucrat on the left. Da Nang, RVN, 1967.

Departing Japan, 1965 --->

Thanksgiving, 2008.

Yeah, I know. But if it still works, why throw it away?

Wednesday, March 5, 2008

Sparks: Color, Length & Myths

A lot of experts offer all sorts of helpful hints for checking out your ignition system based on the color or length of the spark you'll see when you hold the coil's center-lead near a ground while cranking the engine.

Virtually all of that 'expert' advice is bullshit.

Faulty color perception is a sex-linked trait that effects mostly males and is surprisingly common. (There is a form of monochromatic visual abnormality that effects both men and women but in those cases, which are extremely rare, they see no color at all, everything is in shades of gray.)

Show a classroom of would-be mechanics a color slide of a spark... or even a picture of a car(!) and their descriptions of the color will vary wildly. Bluish white, sez one. Except it was sorta reddish white to the guy next to him and purplish white to the guy over there.

The point here is that before you can use color perception as a diagnostic tool you will have to calibrate your 'instrument' :-)

Second point: What gives the spark its color to begin with?

If the spark took place in a vacuum tube, it will always emit the same
spectrum. But if the spark occurs in air, the color will be effected by the
gases... and contaminants... that make up the ambient atmosphere. Damp day?
Different color than a dry day. Inside a shop with lots of exhaust gases floating
around? Different color. Spray booth next door? Different color.

The atmosphere (or lack of it) in which the spark occurs also effects the gap
the spark can jump, as does the surface smoothness of the two surfaces. If you
have a rough surface you can usually induce a relatively low-voltage spark to
jump the gap. Two polished surfaces, you'll need a higher voltage to initiate
the spark. (Why do you think it's unwise to stand under a tree during a
thunder storm? :-)

The bottom line is that visually checking to see if you've got a spark tells you only
that the induction coil and points are working. Any effort to evaluate the
spark voltage based on color or the length of the spark will produce a wide
variation of conclusions even among skilled mechanics.

Here's a good basic rule for mechanics: Unless someone is shooting at you,
don't guess. Measure.

Once you've learned to do something the right way and have done it for a while,
your store of experience will become a valid guide. But don't assume your
perception of a given phenomenon is universally shared. Perceptions are based
on your senses. Color perception in males varies widely and a surprising
number of youngsters today are functionally deaf to certain frequencies, unable
to hear some mechanical problems that are clearly evident to others. In the
same vein, smokers and guys doing nose-candy often have no sense of smell at
all, unable to smell a burned clutch or overheated engine that may be evident
to you from a block away.

-Bob Hoover