Relative Ratio

Calculate relative atmospheric ratios

Library

Flight Parameters

Description

The Relative Ratio block computes the relative atmospheric ratios, including relative temperature ratio (θ), $$\sqrt{\theta }$$, relative pressure ratio (δ), and relative density ratio (σ).

θ represents the ratio of the air stream temperature at a chosen reference station relative to sea level standard atmospheric conditions.

$$\theta =\frac{T}{T_0}$$

δ represents the ratio of the air stream pressure at a chosen reference station relative to sea level standard atmospheric conditions.

$$\delta =\frac{P}{P_0}$$

σ represents the ratio of the air stream density at a chosen reference station relative to sea level standard atmospheric conditions.

$$\sigma =\frac{\rho }{{\rho }_0}$$

The Relative Ratio block icon displays the input units selected from the Units list.

Parameters

Units

Specifies the input units:

Units	Tstatic	Pstatic	rho_static
Metric (MKS)	Kelvin	Pascal	Kilograms per cubic meter
English	Degrees Rankine	Pound force per square inch	Slug per cubic foot

Theta

When selected, the θ is calculated and static temperature is a required input.

Square root of theta

When selected, the $$\sqrt{\theta }$$ is calculated and static temperature is a required input.

Delta

When selected, the δ is calculated and static pressure is a required input.

Sigma

When selected, the σ is calculated and static density is a required input.

Inputs and Outputs

Input	Dimension Type	Description
First		Contains the Mach number.
Second		Contains the ratio between the specific heat at constant pressure (Cp) and the specific heat at constant volume (Cv). For example, (γ = Cp/Cv).
Third		Contains the static temperature.
Fourth		Contains the static pressure.
Fifth		Contains the static density.

Output	Dimension Type	Description
First		Contains the θ.
Second		Contains the $$\sqrt{\theta }$$.
Third		Contains the δ.
Fourth		Contains the σ.

Assumptions

For cases in which total temperature, total pressure, or total density ratio is desired (Mach number is nonzero), the total temperature, total pressure, and total densities are calculated assuming perfect gas (with constant molecular weight, constant pressure specific heat, and constant specific heat ratio) and dry air.

Reference

Aeronautical Vestpocket Handbook, United Technologies Pratt & Whitney, August, 1986.

Introduced before R2006a