International Standard Atmosphere (ISA)

7/29/2019 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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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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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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?

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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.

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International Standard Atmosphere (ISA)

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