[ SAILS • Structure ]

THE FLYING CUBES : STRUCTURE

[general design] [structure] [films] [brains] [motors] [sensors] [communication] [development]

MATERIALS

Like for all rigid flying objects, the structure of the [ SAILS ] aerobots has to fulfill lightness, rigidity and stability criteria. It must also be dismountable, in order to allow easy transportation of the aerobots for experiments, demonstrations or shows/performances. Many materials and configurations have been tried, from extruded styrofoam to carbon fiber. Extruded styrofoam (Fig. 1) shows a surprisingly good behavior to shear constraints, and has strong load bearing capacities for such a light material ; its cost and availability could have make it a primary choice for the project. Unfortunately, its stability in time is rather bad. Carbon fiber has unmatchable resistance and rigidity properties, but it was not suitable for this phase of the project because of its density (around 1.6) and price ; it is also more difficult to work with. We are still planning to develop optimal fiber carbon structures for later aerobots. Considerations of cost, availability and workability finally led us to concentrate on light woods for the first flying cubes. Trusses in the Fig. 2 model are made with polycarbonate tubes, connected at the corners with small cubes made from balsa plywood. The edges are supported by supports of different shapes ; the ducted fans are installed in the middle of the tubes, whose radius increases to make room for them. This model did not pass structural resistance tests (more photogtaphs can be seen on Photo gallery 1). Balsa structures were then tried. They were incredibly light, and showed a more satisfying rigidity. But their fragility made them almost impossible to handle : some of the wood pieces were so thin that they could be unvolontarily broken by someone who would not even notice touching them. Next models were balsa-basswood composites ; they were abandoned for the same reason. The final trusses are completely made from basswood. On the M170 model (fig. 3), each truss weigh barely 80g each, which amounts to slightly less than 1 kg for the complete170cm-edge structure.

Needless to say, we could not find (even in hobby shops) pieces of wood with the proper dimensions and shapes : no shop would hold pieces with a 3mm-side equilateral triangular cross-section. We had to buy 50 x 200 mm beams that were cut on saw benches, with a 75% loss, since most of our pieces are thinner than the thickness of the saw blades.

FIG. 1 - Extruded styrofoam structure prototype.

FIG. 2 - Computer model. Structure with polycarbonate tubes and balsa edges and corners.

FIG. 3 - Two final structures for a M170, entirely made from basswood.

TRUSS DESIGN

The resultant of the forces acting on the trusses is rather complex. The main forces are the buoyancy force of the helium, which is evenly distributed along the trusses, and the tension of the stretched membranes, which is much stronger and more difficult to analyze. The air resistance on the front face can become important when the aerobot moves.

The tension exerced by the stretched membranes creates torsion constraints on the trusses, whose center section tends to rotate towards the center of the cube. This deformation is very hard to control : it would require a simultaneous tensioning on all faces, which is impossible, since this operation must be made face by face. This means that the trusses surrounding one face are first bended towards the center of the face, and then bended towards the center of the cube when the membranes on the surrounding faces are tensioned. These two strengthes act very differently on the trusses, and could lead to very different designs if considered separately. To solve this problem, we had to develop a truss that is rigid enough to resist strengthes from various directions, even if it becomes partly overdesigned when all the membranes are tensioned.

All our attempts were trusses with a triangular cross-section. Our first designs were trusses with a rectangle-triangular cross-section which increased from ends to center, in order to limit the arrow at their midpoint. Tests with stretched membranes (fig. 5) quickly led us to realize that the weak point of the trusses were their articulation at the corner : controlling the rotation at the corners would have the effect of limiting the arrow at the center of the trusses, even with a constant section. Diagonal pieces were added on all faces to act as braces. We reached an arrow of about 2 mm at the center of each edge which we are currently trying to reduce to less than 1 mm, so as to make any gap invisible when two flying cubes assemble.

FIG. 4 - Bench for assembling composite balsa/basswood trusses

FIG. 5 - A composite balsa/basswood truss ready for a tension resistance test. The transparent thermoshrinkable film will be heat-stretched, and the arrow at the midpoint of the truss will be measured.

FIG. 6 - A building bench for basswood trusses, developed by architect and designer Guillaume Credoz. It allows to assemble one truss in about 90 minutes.

GLUING AND ASSEMBLING

The trusses are made of wood pieces glued together ; they are assembled with a truss-building bench (fig. 6), developed after the final design, which was developed thanks to clever insights by architect and team member Guillaume Credoz, was adopted. Gluing is not even an easy process : the small size of all the pieces limit the contact surfaces between the different parts. A first gluing is made with medium cyanoacrylate glue ; the truss is then removed from the bench, and small drops of epoxy resin are added to almost every gluing points in order to have "3D glued joints". Needless to say, even with the best possible gluing, the trusses remain relatively fragile because of the thinness of their elements. Trusses are then assembled with a metal tripod, L-shaped nylon squares, and nylon bolts and nuts (see fig. 8 and 9).

FIG. 7 - A basswood truss. The viewpoint reveals the equilateral triangular, homogeneous cross-section, and the 15 degres inclination from the vertical. L-shaped basswood piece reconstitute the edges, and provides the surfaces required for attaching the thermoshrinkable films.

FIG. 8 - A corner, seen from the exterior of the cube. The L-squares connecting the inside edges of the trusses are clearly visible, as well as the 45 degrees cuttings at the ends of the trusses, and the recesses heads of the nylon bolts that connect the outside edges.

FIG. 9 - A corner, seen from the interior of the cube. The steel tripod connecting the L-shaped edges can be seen, with the nylon nuts used to fix it on the edges. The black threads that appear on this picture are used to limit the deformation of the helium bladder ; they are made from an inextensible material and are stretched diagonally across the faces.

FUTURE DEVELOPEMENTS

Research work implying finite element analysis and carbon fiber is currently going on to see if better weight, rigidity and solidity can be achieved with this material, while keeping the weight similar to that of a basswood structure. This also implies a completely new strategy for truss assembling. Deadline for this work module is May 2006. Follow-up on this work will be displayed on this page.