Saturday, November 22, 2008

Metal , Wood, and Cloth

Drag wires are stripped, all the paint brushed out of the threads, nuts loosened and checked, repainted, and on the right wing.

Aileron control rod replacement is in from England. (The internet is so cool - you can get parts anywhere, as long as they exist and your owner group is strong!)

Yesterday, learned how to use a shear, a brake, and rollers for cutting, bending, and curving sheet metal. Learning how to use and mastering are completely seperate states. I have a lot of work ahead if I'm forming new aileron cove. I wish I could re-use the old cove, but whoever took it off the plane was, ah, less than careful.

The new leading edge will be interesting. In 1941, they used 0.012 inch aluminum - yes, twelve-thousandths of an inch thick. This is roughly as thick as a pop can's side walls. See, back before World War II, Aluminum refining had not yet been perfected, and it was not that common a material - so it was expensive! Therefore, very thin sheets were common - you wanted to use the absolute least amount of this rare, expensive material necessary to get the job done. These days, aluminum is extremely common and cheap - but you can't find anything thinner than 0.016 - that's, what, 25% thicker? Heavier, which is bad, but far sturdier, which is good.

If you're wondering how the leading edge avoided being dented when it was pop-can wall thick, the answer is that when covered, the cloth and dope went over the metal, providing a second layer of reinforcement for strengthening, stiffening, and protection against corrosion and collision with bugs. As for accidental knocks when clearing snow off with a broom, or dents from being leaned on or bumped when fueled? Well, those were just a part of life. Be careful or you get 'em.

The aluminum was also SO, or as jewelry-makers call it, full-soft. It can be bent and shaped by hand, and stays bent. This is also called, on the T Hardness scale, T0. (T0-T6) T1 & T2 are darned rare and hard to find, but T3 & T6 are common - or "half-hard and full-hard" to jewelers. So, I'm planning on replacing the first 4 feet of the leading edge with 0.016 T3 Aluminum, or much harder, dent-resistant, thicker aluminum than was originally on it. This way, should someone lean an elbow on the wing while putting gas in (like yours truly, who has dislocated her right shoulder before, and it'll never be as strong again for hefting jugs of gas), it won't be thrashed and dented for the next 8 years until recovered.

The rest of the leading edge I'll likely get in S0 and form by hand on the leading edge. It's easier, and more correct to original form.

The old aileron cove looked like swiss cheese - because in 1941, airplanes were covered with linen or cotton, and a highly flammable dope (glue.) How flammable? Google Hindenburg. The dope didn't stick to metal, so they laid a second piece of linen on the back of the aileron cove's convex curve, inside the wing, and glued it down. Then, when the doped cloth was laid on the outside of the wing, over the aileron cove, it stuck to the doped cloth on the inside through the holes.

These days, we have options like Poly-Stits and Ceconite. Not flammable, or not nearly so, and it sticks to metal. Interestingly, this means that unless I duplicate the linen-on-the-back trick, all the swiss-cheese holes will now detract from the ability of the fabric to stick to the metal while the metal is held in shape. Hmmm.

On grinding - my right rear spar was 1/4 inch too long. Clearly, the person who cut them was better at woodworking than me, but not as perfectionist. So I got a miter saw, discovered that the amound to be cut was half the blade width, got two files, and carefully took off the offending 1/4 inch by hand. It is now varnished, and when the varnish is dry, I'll put the butt attach fittings on.

...It's so close I can taste it, though the completed wing is still months away. Tonight, priming the wingtip bow that defines the rounded edge of the wing, and seeing if I can figure out how to put it on.

3 comments:

Jenny said...

Awesome!

Sorry I missed you today.

Did the bow work out?

Tony B. said...

I built radio controlled airplanes and used nitrate dope in the covering of some models. Nasty stuff it was. Wear a respirator or get loopy.

Sound like you've a pretty big project in front of you.

On a Wing and a Whim said...

I actually didn't get to the bow; I reached a point where I spent two hours cleaning everything, double-checking all the nuts, tensioning drag wires, putting on other hardware... there's always something to do!

You'll love seeing the hangar next - it's cleaned up, and littered with sharpie-on-masking-tape notes-to-self.

Tony,
Dope is definitely best done with a respirator and a well-ventilated area, that's for sure! I'm not so worried about it - it's the MEK (methyl-ethyl-ketone) that makes me paranoid. It is a big project, but one piece at time, two steps forward, one step back...

The journey of a thousand miles doesn't begin with a single step, see, it begins with a 7/16 wrench and some sandpaper, and a deep breath of "I can do this!"