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generally smooth lenses or almonds. Lenticular clouds most often appear in formations on the lee side of a
mountain peak as a result of lee waves, and they remain nearly stationary with respect to the terrain.
Lenticular clouds are an indicator of extreme turbulence.
Lenz’s law (electrical law). The electrical law explaining induced current. Lenz’s law states that the
current induced in a conductor cut by lines of magnetic flux flows in the direction opposite to that of the
current which produced the lines of flux.
level (horizontal condition of a body). A flat, horizontal surface of a body is said to be level when all
points on the surface are perpendicular to a line which points directly toward the center of the earth.
level (spirit level, or bubble level). An indicating device used to show a level condition. A curved glass
tube is almost, but not completely, filled with a clear liquid, and since the tube is not entirely full, there is a
small air bubble in the liquid. This bubble, being much lighter than the liquid, always rises to the highest
point in the tube.
The curved tube is mounted in a long straight bar, or housing, in such a way that the bubble rises
to the center of the tube when the housing is perpendicular to the pull of gravity (perpendicular to a line
pointing to the center of the earth).
leveling lugs (aircraft component). Brackets or other structural devices installed in an aircraft in such a
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 351
way that when the aircraft is in its level-flight attitude, a straightedge placed across the lugs will be level.
leveling means (aircraft specification). The method specified by the aircraft manufacturer to determine
the level-flight attitude of an aircraft. The leveling means may require the use of a spirit level or a plumb
bob.
level of free convection (meteorology). The level at which a parcel of air lifted dry-adiabatically until it
becomes saturated, and then moist-adiabatically would become warmer than its surroundings in a
conditionally unstable atmosphere.
lever. A rigid bar, free to pivot, or rotate about a point called the fulcrum with an input force applied to one
point, and an output force taken from another point. The lever, one of the simple machines like the wheel,
screw, and inclined plane, can be used to change a force or the speed of movement.
The action of a lever is determined by the placement of the fulcrum and by the distance between
the fulcrum and the points the force is put into and taken from the lever. See first-class lever, second-class
lever, and third-class lever.
levitate. To suspend an object in the air in such a way that it appears to disobey the law of gravity. A
magnetic body can be levitated by suspending it in a strong magnetic field.
Leyden jar. One of the earliest examples of an electrical capacitor. A Leyden jar is a glass container with a
liner of metal foil pressed against its inside surface and a coating of metal foil on the outside of the jar.
When a source of DC electricity is connected across the two pieces of foil, electrostatic energy is stored in
the glass (the dielectric of the capacitor).
liaison aircraft. A type of light military aircraft made popular during World War II because of its ability to
land and takeoff from unimproved terrain. Liaison aircraft have been replaced by helicopters.
life-limited part. An aircraft part whose service life is limited to a specified number of operating hours or
operating cycles.
life-support systems. Systems in an aircraft which make it possible for the occupants to operate in an
environment that does not support life. Oxygen systems and pressurization systems are examples of
life-support systems.
lift (aerodynamic force). An aerodynamic force caused by air flowing over a specially shaped surface
called an airfoil. The airfoil is curved in such a way that the air flowing over the upper surface finds the
surface falling away from it. The air is being pressed onto the surface by the air above it, and in order for it
to remain on the surface, it must speed up. As the air speeds up, its pressure drops, and the air above it
flows down to fill the low-pressure area.
The air below the airfoil finds the surface rising into its flow path, and this slows the air. As the air
slows down, its pressure rises, and it forces the surrounding air away from it. The combination of the low
pressure pulling air down to the airfoil and the high pressure forcing air away from the airfoil causes the air
to be deflected downward. When an airfoil forces air downward, a force equal to the weight of the
deflected air forces the airfoil upward. This force is called lift.
lift coefficient (aerodynamic parameter). A dimensionless number used in the formula for aerodynamic
　
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