Basically the design consists of taking Pete Bowers' basic Flybaby 1A, stretching the fuselage one foot forward of the wing, adding a bigger engine, placing the passenger on the center of gravity, and moving the pilot aft to balance the heavier engine placed farther forward.
I had a rough sketch of Harold Matteson's fuselage as well as copies of some construction photos, but I didn't have any of the measurements or the dimensions of the structural members. I also decided I wanted to avoid having any diagonals at the top of the cockpit sides so I ended up starting from scratch on the truss design.
Neither of the existing tandems were strong climbers. I decided that 65 Hp really wasn't enough, and since I was starting from scratch I could chose any engine weight within reason without making the plane nose heavy. I measured and weighed a friend's 125 Hp Lycoming O-290 and used that as my design assumption. This left open the possibility of using an O-235, O-290, or a derated O-320. Bowers lists 125 Hp as the highest horsepower allowed for the unmodified plane. I assume that a 90-100% increase in horsepower over the 65 Hp used on the other tandems ought to cure any anemic climb performance, and should allow for spectacular short field take-offs.
While the boost in power should allow for short field take-offs, the increase in weight from an added passenger, a heavier engine, and added structure should increase stall speed and the landing runway requirement. Taking the stock Flybaby gross weight at about 1000 lbs, and making a deeply pessimistic estimate of 200 lbs for second passenger and 100 lbs of additional empty weight coming to a pessimistic gross of 1300 lbs. This should yield an increase in stall speed from 45 mph to 54 mph. If I were to add plain flaps inboard of the ailerons that should bring the stall speed back down to 47 mph. Slotted flaps might just get the stall speed back all the way down to 45 mph and the landing roll back down to 400 ft. I plan to build the wings without flaps at least at first. Flight test should reveal if addition of flaps is necessary.
The addition of extra passenger weight, extra engine weight, longer moment arms for both, and the additional thrust loads imposed by the larger engine all suggest that the fuselage should be strengthened somewhat. As a general rule the structural members of the fuselage truss were increased on the order of 33%. Most of the 3/4" x 3/4" members were increased to 1" x 3/4". 3/4" x 1/2" to 3/4" x 3/4" and some other members were increased even more for reasons of simple construction rather than for structural strength. The double bottom longeron section has been lengthened to extend aft of the new pilot's position. I plan to use the 1" spars and increased wire size that Bowers recommends as an aerobatic beef up, and I may build the landing gear out of Douglas Fir rather than Sitka Spruce for added strength.
Once I had determined all the positions of the fuselage members, I built a jig table and built a full scale mockup out of cheap wood, corrugated cardboard and carpenter's glue. The mockup was remarkably strong and I built in rudimentary seats and flight controls. I hauled the mockup up to the local airport for an EAA meeting. After all the cracks about my cardboard airplane, and the pizza box composite construction, and speculation about how many g's it would take dry and how many wet, we started trying the thing on for size. I was able to get in and out easily enough by holding my weight on my hands and sliding my feet down behind the instrument panel. I was happy with that and with my seated position until one of our tech councilors came in and saw it. He made me put on a parachute and sit in it and consider the case where the plane is on fire and you would really rather be able to exit quickly.
I hauled the cardboard flyer back home, borrowed the parachute and
set about cutting and duct taping the crossmembers around the cockpit
until my knees cleared the instrument panel in all conditions. I
ended up both reclining the seatbacks a few degrees and moving both
instrument panels forward. I was then able to sit in the plane with
a 'chute on and with my feet on the rudder pedals and then draw up my
knees to my chest without any interference. I transferred the
changes to the drawings and the jig table and re-ran the numbers.