5.13 FLUTING
There are some conventions when referring to parts of parts.Terms like "flange" and "web" have specific connotations and using
them correctly makes the job of Van's Builders Support personnel much easier. See Figure 1.
Verify that the flanges on parts are perpendicular to the web (unless otherwise specified), so that they will mate correctly with the
skins. Adjust flanges with hand seamers or small wood blocks with slots cut in them as necessary. A pair of duck bill pliers can also
be very handy for adjusting small flanges.
The process used to manufacture parts with curved flanges will leave them slightly bowed and sometimes twisted. Before parts are
installed the flanges must be straightened (any twist in the part can be ignored). This is done by "fluting", that is, putting small
creases or "flutes" along the edge of the flange with special pliers. The flutes effectively shrink the flange material and pull the part
into line.
Fluting diagrams are shown on the plans where required. Otherwise the flutes are simply centered between the pre-punched holes
in the flanges. When making flutes, be sure they are formed towards the inside of the part so that the outer face of the flange
remains flat for the skin to lie on.
Straighten the ribs/bulkheads with fluting pliers and check for straightness by sighting down the web or by laying the part on a flat
table top. On parts with prepunched holes, a straight edge can be used to check that the prepunched holes are aligned. The more
curve that a flange has, the more fluting that will be required. On parts with a varying amount of curve (such as a wing rib), a
different amount of fluting will be required in different areas of the part. Do a little at a time until reaching the desired flatness.
NOTE: Do not confuse the twist in a part with a curved flange. It is normal for light pressure to be required to hold a
twisted part flat on a table. Fluting does not remove twist from a part.
If you have overdone the flutes and curved the rib or
bulkhead the other way, gently squeeze the flutes with
smooth pliers to straighten the ribs.
WEB
FLANGE
FIGURE 1: BASIC TERMINOLOGY
5.14 ALUMINUM TUBING
In RVs, 3003 soft aluminum tubing is used for the fuel lines inside the cockpit and for the brake lines. These lines must be bent with
a tube bender to avoid kinking and to get a professional looking installation. The flared ends of these lines are 37°, not the 45°
found on automotive lines. A good quality flaring tool is a necessary tool to do the proper job. Properly installed, aluminum lines will
last for many years. Here are a few tips on the "properly" part:
Preparing the tube: Soft aluminum tube should be cut with a tubing cutter - not a hacksaw. The resulting end will be square. After
making the cut, deburr the interior edge of the tube end and polish the end of the tube with fine crocus cloth, emery paper, or a
Scotchbrite wheel.
Mounting it in the flaring tool: First, put the AN-818 nut and AN-819 collar on the tube and push them out of the way. There is a
tongue on most flaring tools that serves as a stop. Make sure you have selected the right diameter (Most RV's use 3/8 tubing for
fuel lines and 1/4 tubing for brake lines), insert the tube from the far side of the tool until it hits the stop, then tighten the clamp.
Making the flare: Put a drop of light oil on the cone of the flaring tool. Spin the cone down into the tube and watch it make the flare.
Do not over tighten, which can start thinning the material in the area of the flare. Turn the cone just enough to fully form the flare.
Inspect the flare: Take a good look at the stretched aluminum around the circumference of the flare. You will probably see some
tiny stretch marks, but there should be no cracks or splits.
Install the tube on the flare fitting: Mate the flared end of the tube with the conical end of the AN fitting. Slide the AN-819 collar
down the tube until it rests on the back of the flare. It must be square to the fitting. Any slight angle will make it difficult to start the
nut, and if you do get it started, runs the risk of splitting the aluminum flare.
Slide the AN818 nut over the far end of the tube and engage the threads on the AN fitting. Tighten to the specified torque. You
should have a Standard Aircraft Handbook with the torque tables for these things. (assuming aluminum fittings, for 1/4 tubing it is
40-65 inch-pounds, for 3/8 tubing it is 75-125 inch-pounds.) Later, you can leak test the system.
5.15 STEEL
Most of the steel used in RVs is 4130 normalized, mostly in the form of thin plate and thin wall seamless tubing. This 4130 high
carbon steel is used extensively in the aircraft industry because of its high strength and relatively good workability.
Most steel parts supplied in RV kits are powder coated at the factory, meaning that there is little for the builder to do except install it.
On a few parts, where the sequence of welding or forming operations makes powder coating at the factory impractical, the builder
may paint the part. The best method of cleaning is bead-blasting, but some work with a stiff brush and solvent will do the job.
Steel parts should be primed and painted immediately after cleaning to prevent rust.
Stainless steel will quickly dull cutting tools (drills, deburring tools, reamers, etc.). Use plenty of lubricant (Van's Aircraft uses
Boelube) and keep the cutting speed low. Use a step drill if creating holes over .250 [6.4 mm] in diameter.
Stainless steel edges can be very sharp. Handle with care!
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