A major design objective of the Naval Aircraft Factory SA-1 of late 1918 was to have as simple a structure as possible. This was achieved, but almost to the point of being crude in some areas. The structure, particularly in the fuselage, has some features well worth considering for today’s ultralights and the lower end of the “licensed” class.
The fuselage of the all wood SA-1 used three longerons, a heritage from the late Santos-Dumont “Demoiselle” designs and the forerunner of the famous Aeronca C-2 and C-3 models. The three longerons allow a rigid rear fuselage to support the tail, eliminating the bunch of wires used to stabilize the tails of the one and two-longeron ultralights in use today. However, as used on the SA-1, the three-longeron arrangement had some notable disadvantages.
For one, the upper longeron was not continuous from the tailpost to the forward A-frame, or pylon. It ended at the upper crosspiece of the rear pylon. Putting the load of the upper longeron into the middle of a crosspiece like this would ordinarily be an engineering no-no, but in this case the load was split by two stout steel tubes and transferred into two separate side beams that connected the two pylons. However, this transfer wasn’t the main function of the tubes. Again, unloading a force into the middle of a beam was not good engineering, but only the aerodynamic tail loads were in that upper longeron. The landing impact loads were taken by the two forward-mounted “tailskids” on the lower longerons.
There was another disadvantage to the three-longeron system as used on the SA-1. For rigidity, every bay had to be braced with crossed wires. This made it nearly impossible for the pilot to get to his seat from the side, which would seem to be the logical way. Instead, he went in though the top, forward of the rear pylon. Thanks to the diagonal steel tubes, the area between the rear pylon, the side beams, and a cross-tube installed farther forward was rigid without the need of crossed wires and provided enough clear space for the pilot’s access.
In the 1924 light plane developed by J.S. Roche, which became the Aeronca C-2 of 1929, the basic three-longeron structure was supplemented by secondary low longerons in the cockpit area that made the structure rigid at the point without the need for crossed wires or diagonal tubes. Entrance could now be made easily from either side. Also, the upper longeron was now extended to the forward structure and both wing panels attached to it rather than to the two parallel side beams as on the SA-1, a good example of design evolution and improvement.
The landing gear of the SA-1 was extremely simple — a pair of non-brake wheels on a steel tube axle bound to the lower longerons with rubber cord that functioned both as fastener and shock absorber. The wheels were too small and too far below the raised pilot’s seat for him to hold on to them for braking. The Roche and the early C-2s also used the straight axle on the longerons but used larger diameter racing sulky wire wheels. The pilot, sitting lower down, could hold them to keep from rolling forward during engine idle or run up. Some pilots wore heavy gloves and grabbed the wheels for actual braking during rollout and taxiing. Later C-2s went to small diameter Goodyear airwheels that eliminated this convenience.
The wheels of the SA-1 were just outboard of the lower longerons, which made for a fairly narrow track gear with associated crosswind taxiing problems, a characteristic shared by many ultralights today, especially those with high-dihedral wings. The Roche and the early C-2s with narrow fuselages had even more of a problem to the point where the Roche added wing tip skids. Later C-2s and the early C-3s reduced the problem significantly by going to tripod landing gear struts that put the wheels well out from fuselage for better crosswind stability.
SIZING IT UP: The pilot of the SA-1, standing on his seat, gives a good idea of the small size of the SA-1 which, except for its relatively heavy 50-60 hp Lawrance engine, is similar in size and structure to many of today’s ultralights.
THE BASICS: The three-longeron fuselage structure of the NAF SA-1. Note the two-piece construction of the lower longeron, with forward projections forming anti-nose-over skids. The three-cylinder Lawrance engine is mounted on two heavy plywood webs bolted to the forward A-frame. Note the curve in the middle of the axle (here without wheels) to clear the torque tube supporting the control stick.
INSIDE SCOOP: A closeup of the cockpit shows all the diagonal brace wires that made it easier for the pilot to get in and out through the top of the fuselage rather than the side. Note that the rear lower longerons are not a continuation of the forward portions, but attach to the rear A-frame above them.
WINGING IT: The wing-spars of the SA-1 were solid spruce, extensively routed for lightness. Note that the pulleys for the wing-warping cables to the rear spar are built into the top and bottom of the rear A-frame, or pylon.
ANOTHER VIEW: Looking into the “cockpit” of the SA-1. Note the splitting of upper longeron loads by diagonal tubes, the open bay for pilot access and the simple way of attaching parallel side beams to the angular A-frame.