We've spent time over the last two weeks finalizing the design of the drive spool and building a prototype of the spool itself and the shaft upon which it is mounted. The drive spools are four large pulleys (one at each rotor) wound with high-strength line that is pulled off to spin the rotor. Each spool must be able to withstand the torque transmitted through it as well as the immense crushing force of the line wrapped around it. However, the part must be as light as possible; the target weight for the entire spool is less than 300 g.
Based on several factors (the power transmission to each rotor, the strength of the drive line, etc.) it was determined that the spool should be 141 cm in diameter. No solid structure could be thought of that would satisfy the weight constraint. Given that the team is composed entirely of avid cyclists, a viable solution was readily apparent: make a giant bike wheel out of carbon fiber with Kevlar spokes. Using a multi-segment plywood form, a carbon-fiber rim of the desired diameter and cross-section was created which weighed less than 180 g. The bulk of the fiber was oriented circumferentially, but a small portion were set perpendicular to the rest to ensure that the structure could not split.
The rim initially seemed quite flimsy due to its large diameter and small cross-section, but the spokes were going to be arranged to keep it stiff. After several breaking-strength tests on the Kevlar line, we calculated that the spool would need a total of 384 spokes to transmit the torque load (and even that number was below the ideal factor of safety). The rim could be stabilized by spacing the spokes widely on the drive shaft, but it was suggested that this might cause the Kevlar to peel off of the rim due to the angle of those joints. The solution proposed was to bond the spokes to one side of the rim, and have them cross to the opposite side before bonding them to the drive shaft (the crossover is visible several centimeters in from the rim in the attached photos). We spent an afternoon winding the spokes, using over 270 m of Kevlar line.
Upon removing the finished assembly from the jig used for winding spokes, we found that it was still very flexible; obviously we had misunderstood some critical aspect of the structure of a bicycle wheel. By bending it slightly and observing which spoke went slack and which took up tension, it was determined that the crossover point should be glued in place so that the strands could not slide past each other. We put the entire assemble back on its jig and made this improvement.
The rim was much stiffer, but still not nearly stiff enough. However, it deformed differently at this stage than previously. Instead of entire spokes becoming slack, the lines all remained tense from the hub to the glued crossover point. Outside of that, the rim could still bend and buckle. To solve this problem, we added an additional set of spokes that run directly from the rim to the hub without crossing over in between. These spokes risk the peeling effect mentioned earlier, but care was taken to reinforce the bonds in an attempt to avoid that mode of failure. This also gives the added benefit of reducing the tension that each line must bear, as there are now 512 spokes in total. This modification had exactly the desired effect, making the entire structure highly inflexible.
The final phase of prototyping is to test the new part. As shown in the photo, we have made a lashed T-joint similar to the mount for the rotor blades. By fixing the T-joint in place and hanging successively heavier weights on a line wrapped around the spool, we will load the system to failure. The question is: what will fail first? Will the rim be crushed by the spooled line? Will the spokes begin to snap? Will the drive shaft itself buckle in torsion? Will the T-joint fail? These questions will only be answered by careful testing. If all goes as planned, we will determine tomorrow if our current design is strong enough, or if the drive spool needs to be stronger still.
Spool Diameter: 141 cm
Rim Width: 18 mm
Rim Depth: 14 mm
Number of Kevlar Spokes: 512
Total Length of Kevlar: 376 m
Total Mass of Spool: <300 g
Designed Torque Load: 846 N m (624 lb-ft)
We made a giant bike wheel that the pilot draws line off of to spin the rotor blades. It has 512 Kevlar spokes. Tomorrow we're going to hang weights on it until it breaks.