A flare, often magnesium, that is dropped from an airplane to illuminate a ground area; a small parachute decreases the rate of descent.

A flare, often magnesium, that is dropped from an airplane to illuminate a ground area; a small parachute decreases the rate of descent.

Related Terms 
POINCARE SURFACE OF SECTION
A method of displaying the character of a particular trajectory without examining its complete time development, in which the trajectory is sampled periodically, and the rate of change of a quantity under study is plotted against the value of that quantity at the beginning of each period. Also known as surface of section.

PLUMMET
A loosefitting metal plug in a tapered rotameter tube which moves upward (or downward) with an increase (or decrease) in fluid flow rate upward through the tube. Also known as float.

PLASTICOVISCOSITY
Plasticity in which the rate of deformation of a body subjected to stresses greater than the yield stress is a linear function of the stress.

PERFORMANCE NUMBER
One of a series of numbers (constituting the PN, or performancenumber, scale) used to convert fuel antiknock values in terms of a reference fuel into an index which is an indication of relative engine performance; used mostly to rate aviation gasolines with octane values greater than 100.

PERMEAMETER
1. A laboratory device for measurement of permeability of materials, for example, soil or rocks; consists of a powder bed of known dimension and degree of packing through which the particles are forced; pressure drop and rate of flow are related to particle size, and pressure drop is related to surface area. 2. A device for measuring the coefficient of permeability by measuring the flow of fluid through a sample across which there is a pressure drop produced by gravity. 3. An instrument for measuring the magnetic flux or flux density produced standard output is being exceeded.

PERFORMANCE CHART
Graphical representations showing the abilities of rotating equipment at various operating conditions; for example, the performance curve for a compressor would include rotor speed for various intake and outlet pressures versus gas flow rate adjusted for temperature, density, viscosity, head, and other factors.

OVERSPIN
In a spinstabilized projectile, the overstability that results when the rate of spin is too great for the particular design of projectile, so that its nose does not turn downward as it passes the summit of the trajectory and follows the descending branch. Also known as overstabilization.

OCTANE SCALE
Series of arbitrary numbers from 0 to 120.3 used to rate the octane number of a gasoline; nheptane is 0 octane, isooctane is 100, and isooctane 6 milliliters TEL (tetraethyllead) is 120.3.

NEWTON'S LAW OF COOLING
The law that the rate of heat flow out of an object by both natural convection and radiation is proportional to the temperature difference between the object and its environment, and to the surface area of the object.

NERNST HEAT THEOREM
The theorem expressing that the rate of change of free energy of a homogeneous system with temperature, and also the rate of change of enthalpy with temperature, approaches zero as the temperature approaches absolute zero.

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