LDA (localizer-type directional aid). A navaid used for nonprecision instrument approaches, with utility and accuracy comparable to a localizer. An LDA is not a part of a complete ILS and is not aligned with the runway.
L/D or lift/drag ratio (aerodynamic parameter). A measure of efficiency of an airfoil. L/D is the ratio of the lift to the total drag, or the coefficient of lift to the coefficient of drag at a specified angle of attack. The lift/drag ratio is commonly called the L-over-D ratio.
lead. A soft, malleable, ductile, bluish-white, dense, metallic chemical element. Lead’s symbol is Pb, its atomic number is 82, and its atomic weight is 207.19. Lead is used in the manufacture of soft solder, type metal, plates for lead-acid batteries, and radiation shielding.
lead-acid battery. A popular form of storage battery used in automobiles and airplanes. The positive plate of a fully charged lead-acid battery is made of lead peroxide, and the negative plate is pure lead. The electrolyte is a solution of sulfuric acid and water. When a lead-acid battery discharges, both plates change into lead sulfate, and some of the acid in the electrolyte changes into water.
Passing electrical current through a lead-acid battery from its negative terminal to its positive terminal reverses the chemical action which took place as the battery discharged, and the battery becomes charged again. In the charging process, sulfate ions are driven from the plates back into the electrolyte.
leading current (electrical current). Current flowing in an AC circuit that has more capacitive reactance than inductive reactance. Current must flow into a capacitive circuit before voltage can build up across it; therefore, in a capacitive circuit, the current leads the voltage.
leading edge. The edge of a moving object that reaches a point in space or time ahead of the rest of the object. In an airplane wing or a helicopter rotor, the leading edge is the part of the wing or rotor the moving air touches first. In a pulse of electrical energy, the leading edge is the first part of the pulse that moves away from the quiescent, or at-rest, state.
leading-edge flap (aerodynamic control). A type of aerodynamic control surface, used on some airplanes to increase the lift produced by the wing. Part of the leading edge bends downward to increase the camber (the curvature) of the airfoil. Increasing the camber increases the pressure difference across the wing, which in turn increases the lift. Leading-edge flaps are used for both takeoff and landing, to increase the lift at low airspeed.
lead-lag hinge (helicopter rotor system). The hinge in the root of a helicopter rotor blade that allows the blade tip to move back and forth in its plane of rotation. Movement about the lead-lag hinge is called drag, and this movement is opposed by drag dampers. A lead-lag hinge is also called an alpha hinge.