# Lecture X: Air Density Factor

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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. This is one reason why high-elevation airports tend to have longer runways than those closer to sea level. Air, like anything else, expands as it heats up, which means that a cubic foot of hot air has fewer molecules, or is less dense, than a cubic foot of cooler air. As we go higher in the atmosphere, the air becomes less dense because there is less air above us squeezing down on the air where we're flying. Heat and height, or altitude, are the two factors that aviators commonly use when calculating air density. This is the reason mountaineers are recommended to take some additional oxygen cylinders with them while climbing mountains. As air density decreases with altitude, the concentration of oxygen also starts dropping at higher altitudes.

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. Describe the effect of increased temperature on aircraft performance, aerodynamically and propulsion? Increased air temperature decreases air density or air pressure. Picture in your mind air molecules in the atmosphere. When it is cold out these molecules are very close together and when it is hot out they are further apart. The effect of this pressure difference caused by temperature is called density altitude. One very hot days when you listen to an airports AWOS/ASOS or ATIS the density altitude is given a form of an altitude in feet. AS an example if you hear density altitude 2000ft, this means that at airport elevation whatever the elevation is, the hot air will act on the airplane as if it is flying at 2000ft. Now, what does this mean to a pilot? Well, as you may know the higher an airplane is in the sky the less dense the air is. Less dense air affects airplanes because there is less mass (molecules) for the airfoils (propeller, wing, horizontal stablizer) to work with (the propeller and wing take 'less of a bite' out of the air). Increased temperature will most affect climb performance and engine output. This is why it is very important a pilot know the density altitude when on approach or landing. In both phases of flight a pilot can expect a higher ground speed at the same airspeed which results in a longer takeoff roll and landing roll-out. To be aware of this phenomenon, while important for all pilots in all areas, is most critical at airports of high elevation. Pilots in the western states (Arizona, New Mexico, Colorado for example) may take off from an airport with a elevation of already 3,000 to 6,000ft. Couple that with a high denisty altitude and you may be pushing a small airplane's limits to operate.

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 humidity effect the way that an airplane flies? Answer Humidity affects the way an airplane flies because of the change in pressure that accompanies changes in humidity. As the humidity goes up, the air pressure for a given volume of air goes down. This means the wings have fewer air molecules to affect as they are pushed through the airmass. Fewer molecules = less lift. The other problem is that jet engines do not like humidity either. Jet engines are built for cold, dry air, and humid air has fewer oxygen molecules to burn per unit volume. Therefore the engine combusts a little bit less and puts out slightly less thrust. There are four factors that decrease the performance of a jet airplane - heavy, hot, high, and humid. Notice that three of those factors all have the net effect of lowering the density of the air. So there you have it. Humidity decreases the performance of most aircraft, not only because of it's effect on the wings, but also the effect on the engines. Answered by: Frank DiBonaventuro, B.S., Air Force officer, Physics Grad, The Citadel Humidity has a major affect on the way planes fly. This is due to the weight of the air when it is humid. When air is humid, it is actually lighter then dry air, contrary to common belief. That is because the water (H2O) weighs less then the N2 or O2 that it replaces. So if you take the fundamentals of lift, which is that the curved part of the wing (the top) will cause air to move by it quicker, causing the bottom of the wing to have a higher pressure (with slower moving air), causing lift. If you take humid air (less dense), then the plane can no longer create the amount of lift it could when the are is dry (more dense). This causes pilots to have longer runways to gain speed before enough air is passing the wings per second to create enough lift, it also forces pilots to fly faster then would be required if the air were dry. Answered by: Steve Smith, None, High School Student

How does the air density affect aircraft performance?
As a memory aid, keep in mind "hot, high, and humid" as the conditions that lower air density.

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