Lecture X: Air Density Factor

AIRCRAFT WEIGHT & PERFORMANCE

Understanding Air Density

• In simple terms, density is the mass of anything – including air – divided by the volume it occupies.

• It has great importance in the field of aeronautics and other disciplines of science.

• For example, performance of the aircraft depends on the density of the air in which it flies.

• There are three important factors that affect air density which are• Altitude• Temperature• Humidity

Factors That Affect Air Density• Altitude - higher altitude, lower air density• Air density is less at higher altitudes and more at lower

altitudes. • Low altitude, high pressure draws air molecules close

together, resulting in more mass per unit volume of air. Therefore, with an increase in pressure, air density increases as well.

• Pilots use "density altitude" to relate air density to aircraft performance.

Factors That Affect Air Density

• Temperature - hotter air, lower air density.

Higher temperature makes the molecules of air move at a much faster rate than normal, which results in the separation of molecules from each other. This is why a higher temperature (hot) forces the air density to decrease quickly and a lower temperature (cold) causes air density to increase.

Factors That Affect Air Density

• Humidity – higher humidity(humid), lower air density.• Humidity is referred to level of water vapor.• Although the effect of humidity on air density is not as much as

that of temperature and pressure, it does affect air density. • Regardless of the types of molecules present in the air, the total number

of molecules is always constant; therefore, an increase in humidity causes water molecules (H20) to increase at the expense of oxygen and nitrogen. But the molecular weight of oxygen and nitrogen is greater than the molecular weight of water, and this is the reason why an increase in humidity forces the air density to decrease.

• High humidity always refer as humid air while low humidity normally refer as dry air. *dry air, higher air density

How does the air density affect aircraft

performance?

Effect of Air Density on Aircraft Performance

• Performance of the aircraft depends on the density of the air in which it flies.

• This is because the density of the air has a direct effect on : • LIFT produced by the wings;• POWER/THRUST output of the engine

LIFT Produced By The Wings

• Air density affect the amount of lift that will be generated by wing.

• If we go back to the basic definition of the amount of lift that is generated by a wing then we get to the formula

L=Cl½ρV²S. • Note that one symbol in this formula is the Greek letter . ρ

This symbol “rho” refers to the DENSITY of the air. • Without going too deeply, it is obvious that a decrease in

“rho”, the air density, will result in a decrease in the total lift generated by our wing at a given airspeed.

• Note that, air density decreases on hot days and on humid days

LIFT Produced By The Wings

• Thus, low air density during hot or humid day will cause the amount of lift generated by wing decreases.

• The practical results are that airplane will require a longer take-off run in order to gain enough lift to become airborne.

• This is why airport in the hotter area such in Middle East tend to have longer runways.

POWER/THRUST output of the engine

• Air density also affect the amount of power that will be generated by the engine of your aircraft.

• The engine produces more power on a cold day (higher air density) than it does on a hot.

• This condition is good when aircraft still on the ground to perform take-off.

POWER/THRUST output of the engine

• As aircraft climb to high altitude, air density decreases.• But less dense air at high altitude, is advantage to engine

performance.• That’s because the air is thinner, which means the engines have

less resistance and the plane gets less thrust to move the aircraft forward at a given speed.

• As a result, airplanes can fly more efficiently at higher altitudes with less fuel.

• But, for any aircraft there is maximum altitude that can be reached (service ceiling). Exceed the ceiling is dangerous to aircraft. The reason is air density is too thin or even zero that can cause No Lift & No Thrust condition . Aircraft can stall.

• Most commercial aircraft has service ceiling about 40’000 to 45’000 ft