Latest Work Completed Mantua, New Jersey
Original Site:
September 2004
E-mail: usav8or@yahoo.com
More... The Radical Radial Fuselage.
February 7, 2008 Scarfs... it's winter but were not talking about keeping warm
Yesterday I took an insane amount of time to cut just three of the fuselage tubes. One of those learning curves. I tend not to jump in with both feet when a little mistake, or oversight, will
cost you a nice length of 4130 tubing, which equates into lost time if you don't have enough of it around.
The slowness can also be attributed to the fuselage tables being in the garage and the chopsaw being in the basement. But we're not going to point out any poor planning on my part,
are we? Let me tell ya, this is going to change real quick. Move the saw and the Joint Jigger up next to the fuselage, and with a little bit of cutting I'll be moving along in no time.
OK... so the three tubes I cut were the front vertical tube at station 0.o and the two longeron tubes, upper and lower, that connect to that station. Refering to the plans you'll see that you need to reduce the
size tubing after station 85.o for the upper and station 49.o6 for the lower longeron.
Now that I'm reducing the size of the tubing, I've got to either make a 30 degree scarf joint or a fishmouth joint. The 30 degree scarf is just and angled cut whereas the fishmouth are two angled cuts, at 30 degrees, that look like, well, a fishmouth.
The fishmouth is stronger, but we're not talking about Unlimted Akro g-forces here. So... I made the 30 degree scarf. But the question I had, not sure if you did, but the one I had was which way should the "mouth" of the scraf point, up or down ? Another question thrown
out to the guys on the biplane forum. Erich took a look at a few of his books (Aircraft Maintenance for the Aircraft Mechaninc c. 1941 and Maintenance and Repair of Aerospace Vehicles, his Northrop text book,) the one I have by Tony B, doesn't mention how this should be pointing. Both of Erich's books say it's better to have a
fish mouth, with 30 degree angles. But, both the fish mouth and the scarf are acceptable methods of joining two lengths of tubing. But... to make the splice resistant to bending, and vibration stress, the splice cut should be 30 degrees to the longitude or horizontal axis of the tube.
Dale, one of the guys from the forum that wrote that convincing piece on the Joint Jigger, added... "It doesn't matter (which way the opening faces,) but, fit it where less shows thru the fabric."
Based on those responses, I chose to make a 30 degree scraf joint (it's an easy cut to make with the chopsaw, instead of me hacking away at a fishmouth by hand cutting it and ending up with a less than perfect cut.)
Which way did I face the opening of the cut ? Up for the lower longeron and down for the upper longeron, based on the comment made by Dale. Less weld line would be exposed to the fabric doing it this way... or so I would think.
Now, before I can move on to cutting any additional tubes, I need to drill a 3/8" diameter hole approx 2" forward of the scraf cuts. I will be inserting the reducing tubes in about 3" and will need to make a rosette weld to make them one with the
forward longeron tubes. The 3/8" hole needs to be drilled into both sides of the tube. I may just end up drilling two sets of holes into the scarf joint.
February 10, 2008 How do you attack a rosette ?
Circle just before the strike.
To keep the frame as light as possible... Curtis, and other aircraft designers, reduce the size and thickness of the tubing as it moves towards the rear of the fuselage. Most of the strength is needed up front for
all that weight. As you get to the back for the aircraft you're mainly dealing with a structure to support the tail and the loads imposed upon it. I'm no engineer, but that's the way it is folks.
So how does one decrease the size of the tubing from the front to the back ? You do it in increments using tubing that fits snug into the tubing that it's sliding into. For instance... the R version of the Skybolt, the one I'm crafting... The lower longerons start off as being 1" x .o49. This size tubing goes back to
station 49.o6. From her Curtis reduced it to 7/8" x .05. The 7/8" OD (outside diameter) just fits snug into the ID (inside diameter) of the 1" x .o49 tubing. From here it was again reduced to 3/4" x .o35. It just so happens that the 3/4" tube fits nice and snug in to the 7/8" x .035 tubing. Funny how it works out like that, eh ?
OK, so we got it reduced down to it's smallest form. If we don't somehow hold them into position they'll be sliding forward and back like the neck on a trombone. So wadda we do ? If we were idiots, we could probably glue them together and hope that it held, or, we could do what many thousands of other builders have done in the past and
welded them together using a few different techniques.
The trailing edge of the longeron tubes need to be either cut at a single 30 degree angle or with a fishmouth that has 30 degree cuts. You can read about it in the above post that I made.
The next and final weld that you need to make on the reduction joint, as I'll call it, is a rosette weld, or plug weld. This weld basically consists of drilling a hole (usually the hole's size is 1/4 of the diameter of the tubes OD... but not less than 1/4") through the outside tube.
When the smaller tube is slid into the outter tube you weld the two together. Some builders heat the center of the exposed tube to a liquid form then run the weld line to the outside and into the outer tube. I found that it was easier for me
to heat the center tube to a liquid form then run that puddle to the edge of the outer tube and weld the two tubes together, forming a circular weld line, then fill the "crater" with the weld rod.
I found this method easier. But basically a matter of choice. I think if you're taught, not self-taught, you'd do what the others are doing. Me... self-taught. 'nuff said.
I walked away from an hours worth of fustration tonight. Did I have any learning that I could slip into my pocket of knowledge or was tonight a complete waste of
my time ? I really didn't want to think so, but it was looking more like that with the passage of time.
Out to the garage "workshop" tonight. Earlier today I picked up a new acetylene
tank and was ready to weld up a lot more of this bungee truss.
Who knows... maybe I'd even finish it. I was looking forward
to pushing that glistening puddle around.
Strapped the tank into it's cradle on the wheeled welding cart, hooked up the gauges, checked them for leaks and then fired up the BIG AW207.
Did my usual preheat of the surrounding area then started working on that golden puddle... and working on it... and working on it. Turned up the heat and started working on it...
and working on it... and working... POP ! What the ??? OK... needs more gas flow to keep the gas from going back inside of the tip and igniting inside and popping. Been there done that. Turned up the heat and started working the... POP ! Damn ! Ok... must need a little more heat.
Same result. Ending with a loud POP !
Add a little bit more heat. Long story short. I kept gradually adding more gas flow and kept getting the same result. Got to the point where it was starting to burn away the
surrounding tube without a good puddle forming.
Tried down sizing my tip.
Same gradual results. POP POP POP ! This can't be happening? can it ?
Me, the seasoned puddle pusher, ending up with the un-seasoned puddle pusher's pops. I spent the next hour going up and down the ladder of sizes and gas settings ending up with... yeah, you got it... the same resulting POP ! I finally had to walk away from it all. Took a break. Watched some mindless tv.
It's now 1AM. I came up here with the intentions of filling in the Build Log with an hours worth of fustration and not adding it to the total hours on the log. Nothing learned. So I figured I can't add it to my tally.
But wait, as I began to write down my fustrations, I could now see what was in front of me all the time. Three pieces of 1 1/8" x .o65 tubing is a chunk of metal that's just gonna wanna rob your puddle of just about any amount of heat you throw at it.
I hope I'm right on this... what I need to do is add more heat to the surround tubing. What I thought was enough, was enough for some piddly-ass .o35 or even .o49, but for .o65, enough just wasn't quite enough.
We'll see if more enough is enough for .o65... 'cause enough just wasn't enough.