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A primary flight display with the indicated airspeed (IAS) displayed in the form of a vertical "tape" on the left. Indicated airspeed (IAS) is the airspeed of an aircraft as measured by its pitot-static system [1] and displayed by the airspeed indicator (ASI). [2] This is the pilots' primary airspeed reference.
Indicated airspeed is a better measure of power required and lift available than true airspeed. Therefore, IAS is used for controlling the aircraft during taxiing, takeoff, climb, descent, approach or landing. Target speeds for best rate of climb, best range, and best endurance are given in terms of indicated speed.
Since indicated, as opposed to true, airspeed is a better indicator of margin above the stall, true airspeed is not used for controlling the aircraft; for these purposes the indicated airspeed – IAS or KIAS (knots indicated airspeed) – is used. However, since indicated airspeed only shows true speed through the air at standard sea level ...
It is calculated from the pressures developed in the cylinders, measured by a device called an engine indicator – hence indicated horsepower. As the piston advances throughout its stroke, the pressure against the piston generally decreases, and the indicator device usually generates a graph of pressure vs stroke within the working cylinder.
Despite having the dimension of pressure, MEP cannot be measured. [2] When quoted as an indicated mean effective pressure (IMEP), it may be thought of as the average pressure acting on a piston during the different portions of its cycle. When friction losses are subtracted from the IMEP, the result is the brake mean effective pressure (BMEP).
These formulae can then be used to calibrate an airspeed indicator when impact pressure is measured using a water manometer or accurate pressure gauge. If using a water manometer to measure millimeters of water the reference pressure ( P 0 {\displaystyle P_{0}} ) may be entered as 10333 mm H 2 O {\displaystyle H_{2}O} .
The SI unit for pressure is the pascal (Pa), equal to one newton per square metre (N·m −2 or kg·m −1 ·s −2). This special name for the unit was added in 1971; before that, pressure in SI was expressed in units such as N·m −2. When indicated, the zero reference is stated in parentheses following the unit, for example 101 kPa (abs).
The metric units ampere, volt, ohm and coulomb are the only units used. The SI term hertz has replaced the term cycles per second for the same unit of frequency. Energy is often measured in watt hours (SI-based), calories (SI-based), BTUs (customary) or therms (customary) rather than the SI joule.