What is the approximate value of the specific gas constant for dry air, R, in J/(kg·K)?

What is the approximate value of the specific gas constant for dry air, R, in J/(kg·K)?

• A. 410
• B. 183
• C. 1000
• D. 287

Answer: (D)

Air behaves like an ideal gas in many flight-physics equations, and the specific gas constant for dry air connects pressure, density, and temperature in the relation p = ρRT. This constant comes from the universal gas constant divided by the molar mass of dry air. Using the universal constant Ru ≈ 8.314 J/(mol·K) and a dry air molar mass of about 0.02897 kg/mol, you get R ≈ 8.314 / 0.02897 ≈ 287 J/(kg·K). This value is the standard figure used in aviation to relate thermodynamic properties of air. The other numbers don’t align with Ru divided by the typical dry-air molar mass, so 287 is the appropriate approximate value.

Air behaves like an ideal gas in many flight-physics equations, and the specific gas constant for dry air connects pressure, density, and temperature in the relation p = ρRT. This constant comes from the universal gas constant divided by the molar mass of dry air. Using the universal constant Ru ≈ 8.314 J/(mol·K) and a dry air molar mass of about 0.02897 kg/mol, you get R ≈ 8.314 / 0.02897 ≈ 287 J/(kg·K). This value is the standard figure used in aviation to relate thermodynamic properties of air. The other numbers don’t align with Ru divided by the typical dry-air molar mass, so 287 is the appropriate approximate value.