bulb root (turbine blade attachment method). A rounded end on a turbine blade that mates with a rounded hole in the periphery of the turbine disk, or wheel. Bulb-root blades are loose when the wheel is cold to allow for different rates of expansion between the blades and the wheel, while holding the blade against the centrifugal loads. Other types of blade roots are the dovetail and the fir-tree.
bulk cargo. Cargo capable of being stacked on the floor of the aircraft carrying it.
bulkhead. A structural partition that divides the fuselage of an aircraft into compartments or bays. A bulkhead strengthens the structure and acts as a wall.
bumping (sheet metal forming method). A method of shrinking or stretching sheet metal into compound curves by hand-hammering it into a sandbag or into or around wood or metal forming blocks.
bundled cable. An electrical cable made up of individually insulated wires. A bundled cable is tied together with lacing cord or with special plastic wire-wrapping straps.
bungee cord. An elastic cord made of a series of small strips of rubber or rubber bands. These strips or bands are encased in a braided cloth cover that holds and protects the rubber, yet allows the rubber to stretch. Bungee cords are used in some of the simpler aircraft landing gears to absorb shock, and the energy in a stretched bungee cord may be used to crank a large aircraft reciprocating engine.
buoyancy. The uplifting force produced on an object when it is placed in a fluid. If a block of wood is placed in a container of water, the wood will displace some of the water, and the water on the outside of the block will push against it. The buoyancy, or the force with which the water pushes on the block, is the same as the weight of the water displaced by the block.