International Standard Atmosphere (ISA)
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Transcript of International Standard Atmosphere (ISA)
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The most important single factor
influencing learning is what the
learner already knows. Ascertain this
and teach him accordingly.DavidAusubel-Educational psychology: A cognitive approach, 1968.
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Aerodynamics
Physical quantities of a flowing gas
Source of aerodynamic force-flow field
Equation of state for perfect gas
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STREAMLINES
STREAMLINES REPRESENT A FLOW AND IS DEFINED AS
IMAGINARY LINES ACROSS WHICH THERE IS NO FLOW.THE CLOSENESS OF STREAMLINES GIVE AN INDICATION
OF FLOW SPEED
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HARRIER INSTANTANEOUS STREAMLINES
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WATER STREAMLINES ON F-16 MODEL
http://www.aerolab.com/water.html
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TYPES OF FLOWS: FRICTION VS. NO-FRICTION
Viscous: Flows with friction
All real flows are viscous
Inviscid flow is a useful idealization
By neglecting friction analysis of flow is usually much easier!
Inviscid: Flows with no friction
Flow very close to surface of airfoil is
Influenced by friction and is viscous
(boundary layer flow)
Stall (separation) is a viscous phenomena
Flow away from airfoil is not influenced
by friction and is wholly inviscid
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TYPES OF FLOWS:
COMPRESSIBLE VS. INCOMPRESSIBLE
Compressible: Density of fluid elements may change from point to
point All real flows are compressible
Important for gases (rarely important for liquids)
Most important at high speeds
Incompressible: Density of fluid elements is always constant
General Rule of Thumb:
If flow speed is less than about 100 m/s (or less than 225 MPH) flow can be considered incompressible
or
If flow is less than Mach 0.3, flow can be consideredincompressible
Mach number, M: ratio of local velocity to local speed of
sound, V/a
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Flight velocities over the years
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Flight altitudes over the years
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The Standard Atmosphere
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Solar Energy as Radiation
Nearly 150 million kilometers separate the sun and earth, yet solar radiation drives earth's
weather.
E th' At h
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Earth's Atmosphere
99% of atmospheric gases, including water vapor, extend only 30 kilometer (km) above earth's
surface.
Most of our weather, however, occurs within the first 10 to 15 km.
Thin Gaseous envelope
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The Standard Atmosphere
Dynamically changing system
Need of standard atmosphere (P,T and )
Flight test, Wind tunnel results and Aircraftperformance
SAmean values of P,T and as a function of
altitude
Table of common ref
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The Standard Atmosphere
Altitude-Quantitative use Geometric altitude (hG)
Absolute altitude (ha)
Local gravitational acceleration (g)
Sea level gravitational acceleration
(go) Geopotential altitude (h)
Pressure & Density
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Pressure & Density
The amount of force
exerted Over an area
of surface is called
Air pressure!Air Density is
The number of air
Molecules in a given
Space (volume)
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Vertical Pressure Profile
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Vertical Pressure Profile
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Hydrostatic Equation
Force balance on element
of fluid
Variation of Pr and Density
with altitude
dp = - *g*dhGdp = - *go*dh
Geopotential altitude
If the net upward pressure force on the slab is equal to the
downward force of gravity on the slab, the atmosphere is said
to be in hydrostatic balance.
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Geopotential and Geometric Altitude
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?
S
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Summary
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Standard Atmosphere
The values of temperature, pressure and
density are never constant in any given layerof the atmosphere .
Requirement for a standard atmosphere for
the comparison of aircraft performances,calibration of altimeters and other practicaluses.
Standard Atmosphere (ISA) defined by the
International Civil Aviation Organization(ICAO).
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Standard Atmosphere
The ISA assumes a mean sea level Temp of
+15C, a pressure of 1013.25 mb (14.7 psi) and
a density of 1.225 kg /m3.
The temperature lapse rate is assumed to be
uniform at the rate of 6.5K per kilometer up
to the height of 11 km (36,090 ft) above which
it remains constant at 216.66 K
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The Hydrostatic Equation
Air pressure at any height in the atmosphere is due to theforce per unit area exerted by the weight of all of the airlying above that height. Consequently, atmosphericpressure decreases with increasing height above theground.
The net upward force acting on a thin horizontal slab of air,due to the decrease in atmospheric pressure with height, isgenerally very closely in balance with the downward forcedue to gravitational attraction that acts on the slab.
If the net upward pressure force on the slab is equal to thedownward force of gravity on the slab, the atmosphere issaid to be in hydrostatic balance.