Читать книгу Flight Theory and Aerodynamics - Joseph R. Badick - Страница 50

Consider an aircraft departing from an airport located at sea level in Florida on a hot, humid day. Now consider the same aircraft under the same weight, temperature, and pressure conditions on a much drier day from the same airport, the only change is the amount of water vapor in the atmosphere.

Using an online density altitude calculator such as the one provided by the National Oceanic and Atmospheric Administration (NOAA), calculate the impact on density altitude by adjusting the dewpoint values. Could a change in water vapor affect an aircraft’s climb rate?

Density is the mass of the air per unit of volume. The symbol for density is ρ (rho):

Standard sea level density is ρ₀ = 0.002 377 slugs/ft³. Density decreases with an increase in altitude. At 22000 ft, the density is 0.001183 slugs/ft³ (about one‐half of sea level density).

It is desirable in aerodynamics to use density ratios instead of the actual values of density. The symbol for density ratio is σ (sigma):

(2.3)

The universal gas law shows that density is directly proportional to pressure and inversely proportional to absolute temperature:

(2.4)

Forming a ratio gives

R is the gas constant and cancels, so the density ratio, or sigma, is a function of pressure and temperature:

(2.5)