As an engineer, I rarely encourage people to ignore the manual. On the other hand, if you never experiment, you will never learn. As such, I would like to pass on my own experience in deviation from the manual.
The manual in question here is the construction manual for the Sonerai II that I am (very slowly) building from scratch.
The Sonerai and Sonerai II are small VW conversion powered aircraft. They were designed by John Monnett—best known as the founder of Sonex Aircraft company.
The Sonerais were designed back in the 1970s and use a rag on steel-tube fuselage with aluminum wings. They are a very versatile design which can be built as a one-seater or a two-seater. They can be built with a low wing or a mid-wing. They had the option to build a tricycle or a tailwheel airplane in one set of plans (though it is now common, at the time they were designed this gear option was novel). The wings also detach easily making the aircraft readily trailerable.
The normal procedure for building a Sonerai fuselage calls for first building a flat, sturdy table. The table is then marked with lines to indicate the tubes that make up the sides of the fuselage. Next, little blocks are nailed to the table to hold the tubes captive. The tubes are carefully cut out and pieced together in a close-fitting jigsaw puzzle.
This works well because the left and right sides of the fuselage are identical. The sides can be built flat, then propped up vertically, and all of the horizontal tubes that turn flat pieces into a three-dimensional fuselage are then added.
The problem with this flat layout is that the sides are not straight when fully assembled. The sides have a bend toward the center of the aircraft just aft of the cockpit. The longerons are parallel to each other when viewed from the top through the cockpit and then come together at a point for the tail.
This creates two issues. The first problem is that if the sides are built flat, both top and bottom longerons must be carefully bent inward after the side is lifted out of the assembly jig. The longerons are three-quarter-inch 0.035-inch 4130 steel. This size of tube requires either heating to bend, packing with sand, or careful bending inside of dies. Once assembled into a complete fuselage side, the only real practical way to bend is by heating.
The second problem is that the layout is incorrect if using the dimensions called out in the plans! The plans give the offsets for vertical bars (and what passes for bulkheads in a tube-and-rag aircraft) for the assembled aircraft. This requires the builder to use some careful geometry to work out what the station measurements are while the side is laid out flat. Most builders just end up with an aircraft that is about an inch shorter than plan, but to be fair, this slightly shorter fuselage does not affect flying qualities.
I elected to go with an alternative procedure after reading the book Construction of Tubular Steel Fuselages by Dave Russo. In this case, instead of laying out the sides on a table, the top and bottom are laid out on a table and the sides are constructed on a 3D jig.
One of the reasons this alternative method of construction works well for the Sonerai is that the fuselage’s top is flat. The top longerons have an inward bend, but with no up or down component. This allows the builder to take measurements right from the plans, lay the top out on table, and bend the longerons to the intended shape in a bending jig or using the appropriate tools to prevent the tube from collapsing. This method of making one bend at a time makes the bend much easier.
The bottom layout is more complicated. The problem is that the bottom is three-dimensional. It bends upward at each end with the flat section in the middle. This still allows the longeron bends to be made one at a time, but it does require modifying the table. “Ramps” are made on the table to prop up the ends and provide the three-dimensional aspect of the fuselage bottom.
To make the sides, I elected to assemble the fuselage upside down because the fuselage top was tacked together on the table second. This way, the top of the fuselage did not have to move at all. Instead, some additional jigs were added to the table to hold the bottom in place.
Some movable jigs were created to hold the sides’ vertical tubes in their correct place for tacking. These were designed to fit between the top and bottom so they could move from side to side. This makes sure the vertical tubes and the braces on each side ended up in the same spot.
Once the side tubes are tacked in place, the fuselage can be popped out of the various jigs and the weld-out can start for real. At this point, the tubing has gone from a box in my driveway to something that will, with a little luck and effort, fly someday!
