Sagitta (oz3277)

About this Plan

Sagitta. Modello acrobatico sport per motori da .40-.45 (ap. alare cm 162).

Update 8/12/2023: Added article, thanks to Cesare.

Quote (google-translated from the Italian): "Fabio PONTANARI presents: 21-06 'Sagitta'. A petrol model aircraft for Sunday fliers who also love to build.

The birth of this model, although recently built, has very distant origins. In the photo below you can see him with the n. 27 (progressive project number) which is a VVC that comes from an ancient, but unfortunately short history. Built in 1970 with an ST 35 engine it had proven to be excellent in edge figures and looping squares, hourglasses and vertical passes was real fun.

This characteristic was the cause of his premature end, summarized as follows: I come out one and a half meters from the ground, now at one meter, now at... holy cow, I exaggerated! So it was that I left the landing gear on the asphalt, which tore off the wing, which sawed off the tail planes.

The intact fuselage remained in the 'I'll think about it later' corner. In fact he stayed there for 35 years and three moves!

Five years ago it came back into my hands, I liked that line so I decided to rebuild the wings and tail planes. On the fuselage I only had to remove the Modelspan and the paint and cover everything with Oracover maintaining the original colors. Then... with limp legs, I picked up the handle of a “U-Control” again. Another feature of this model was the large undercarriage fairings which always allowed adequate traction on the cables and to remain vertical at very low speed, hence the thought: how will an RC with this configuration behave in aerobatics and in knife flight?

No sooner said than done: drawing board (the kitchen one which is over 2 m), accurate calculation of the CSL both on the horizontal and vertical axis, elliptical wing with study of progressive warping, a friend of the group had had unpleasant problems with a Spitfire and I wanted to experiment the problem thoroughly.

Between calculations, drawings and forays into the Profili2 program (a monument must be made to Duranti!) I spent a pleasant week. On the other hand, to keep your mind awake you either mess around with a few formulas or you only have Puzzle Week left! The result is what you see. If you like retro style airplanes, like me, and aren't afraid of a somewhat demanding construction, perhaps you will also be interested in how to build it.

Fuselage: The two front sides are prepared in lightened 4 mm poplar plywood, the lightening also serves to better adapt the sides to the frame (e) which connects with the trellis area of the tail. The wing recess reinforcements must be glued onto the sides, remembering to stay tight to then adapt them to the wing and the relative fittings.

For the alignment of the frames I did not foresee the joints on the sides, but I first glued the 10 x 10 balsa slats to the plywood sides and then I made the alignment joints directly on the slats.

The servo plate is glued together with the frames and, lastly, the belly (two lightened layers of 12 mm balsa). The lower/rear part is connected to this structure: usual 12 mm balsa, keeping everything well aligned. On this level, the tail truss is built using 5x10 and 5x5 samba or hard balsa strips.

The back, behind the cabin, is made with 5 mm balsa semi-frames and a shaped strip of 12 mm soft balsa while the front part can also be made with a block of balsa or entirely in foam and fibreglass.

The tank/engine area is hinged for quick inspection. The whole thing is covered with Oracover but if, given the style, you use Solartex on the trellis part you won't do any damage, but be careful of the weight or you will be forced to ballast the nose.

Wing: Here things get complicated because with an elliptical wing you have to draw and cut each rib individually, furthermore the warping is not linear but progressive, as shown on the drawing. It is therefore necessary to prepare a simple but precise ladder which, starting from the aileron, where the warping begins, aligns the incidence of the wing at least every two ribs. (I used strips of poplar plywood on which I cut the negative of the belly of the ribs with the right incidence and I simply screwed them, in correspondence with the ribs, onto the work surface).

The ribs that support the trolley are reinforced, from the spar to the leading edge, with two false ribs in 3 mm birch plywood; the classic grooved beech spar is glued between the two ribs. If you can't find it glued, staggered to form the groove, three strips of 5mm birch plywood. Once finished, the wings must be leveled and, leaving space for the fuselage, joined using 5 mm birch plywood reinforcement and 3 mm gussets. Then the central area is covered with 2 mm balsa..."

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