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Thesis - Leigh Moody.pdf - Bad Request - Cranfield University

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Appendix E / Earth Geometry<br />

_ _<br />

19.3 Air Density<br />

P<br />

ZG<br />

p<br />

P<br />

∈<br />

ZG<br />

p<br />

ZG<br />

[ 11000<br />

, 20 000 ] ⇒ ∂ T ∂ P : = 0<br />

><br />

20 000<br />

⇒<br />

19-6<br />

∂ T<br />

S<br />

∂ P<br />

S<br />

ZG<br />

p<br />

: =<br />

p<br />

0.<br />

001<br />

Equation 19.2-5<br />

Equation 19.2-6<br />

The air density is given by the gas law (sea-level air density is 1.225 kg/m 3 ),<br />

ρ<br />

A<br />

: =<br />

PS<br />

R ⋅ T<br />

g<br />

S<br />

Equation 19.3-1<br />

The cubic polynomial approximation with errors in the range ± 0.01kg/m 3 ,<br />

1.<br />

2231<br />

+<br />

ρ<br />

A<br />

: =<br />

−3<br />

−5<br />

ZG ZG ZG<br />

( − 0.<br />

11477 + ( 3.<br />

7188 x10<br />

− 4.<br />

1034 x10<br />

⋅ P ) ⋅ P ) ⋅ P<br />

AIR DENSITY (kg/m^3)<br />

1.2<br />

1<br />

0.8<br />

0.6<br />

0.4<br />

0.2<br />

0<br />

-5 0 5 10 15 20 25 30 35<br />

GEODETIC HEIGHT (km)<br />

Figure 19-3 : Air Density vs Height<br />

p<br />

p<br />

Equation 19.3-2<br />

Deriving a polynomial approximation for the air density variation with<br />

height using curve fitting to the original equations results in a discontinuity<br />

due to the static temperature since,<br />

∂ ρ<br />

∂ P<br />

A<br />

ZG<br />

p<br />

: =<br />

1<br />

R ⋅ T<br />

g<br />

S<br />

⎪⎧<br />

∂ P<br />

⋅ ⎨<br />

⎪⎩ ∂ Pp<br />

S<br />

ZG<br />

P<br />

−<br />

T<br />

S<br />

S<br />

∂ T<br />

⋅<br />

∂ P<br />

S<br />

ZG<br />

p<br />

⎪⎫<br />

⎬<br />

⎪⎭<br />

Equation 19.3-3<br />

p

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