For the obvious reasons, I invariably get too busy to blog exactly when things get interesting...
 Linear Bob, single-stranded |
 The apex with breakaway link |
 Single-Strand Bob, full view |
Implementing Bob turned out to be very easy, an exercise in “junkyard engineering”. After looking at the weight and strength requirements, we chose to forgo the thick cable in favor of steel chain, and place it only at the bottom end of the pyramid. We thus connected three 3/16” steel cables (same as the climb cable) directly to the breakaway link, and added 100 feet of 3/8” drag chain at the end of each. The total weight of the chain us 400 lbs.
The dimensions of the chain were chosen based on the rate of mass-accumulation we want to achieve – for example, if the chain weights 2 pound per foot, than as the helicopter rises one foot, it lifts 3 lengths of chains, 1.4 feet each, plus a bit of sag – a total of about 4.5 feet, and so is accumulating mass at a rate of 9 pounds per foot.
Seattle is a good town for finding cheap chain. A few phone calls to used marine equipment stores, and there it was – a barrel of 100 m and 400 pounds of 3/8” chain, weighing about 1.2 pounds per foot. Perfect – we can use it as is, or double it up. This chain has shorter links than a standard trade chain, which means it weighs more per foot. Maybe an old anchor chain. Perfecter.
The point masses for step Bob should weigh about 500 pounds total – 167 pounds each, and should be sturdy enough to be beaten around a little bit, and cheap. Truck tires did the trick, and Tires Inc were happy to donate a few used ones to our cause. We ended up taking only three 75 lbs tires, so were on the light side. (this will show in the video of the flight)
Since the forces at the end of the chains are now very low, we used soft line to tie the ends of the chains to our cars. We deployed it on this beautiful field not far from Northwest’s HQ, hooked up the helicopter, and in no time were ready for the first test flight.
The sequence of images clearly shows how Bob works - The helicopter picks up the cable from a figure-8 coil we set up, and after the coil is exhausted it picks up the apex of the pyramid. The pyramid “stands up” until the chain begin to rise, at which point the rate of pickup decreases and equilibrium is reached. The pilot doesn’t have to stop the helicopter – it does it all by itself.
 Double Stranded Bob |
 Double Stranded, Step Bob |