Principles of Flight

Effect of Humidity (Moisture) on Density Theories in the Production of Lift The preceding paragraphs refer to air that is perfectly dry. In In order to achieve flight in a machine that is heavier than air, reality, it is never completely dry. The small amount of water there are several obstacles we must overcome. One of those vapor suspended in the atmosphere may be almost negligible obstacles, discussed previously, is the resistance to movement under certain conditions, but in other conditions humidity called drag. The most challenging obstacle to overcome in may become an important factor in the performance of an aviation, however, is the force of gravity. A wing moving aircraft. Water vapor is lighter than air; consequently, moist through air generates the force called lift, also previously air is lighter than dry air. Therefore, as the water content discussed. Lift from the wing that is greater than the force of of the air increases, the air becomes less dense, increasing gravity, directed opposite to the direction of gravity, enables density altitude and decreasing performance. It is lightest or an aircraft to fly. Generating this force called lift is based on least dense when, in a given set of conditions, it contains the some important principles, Newton's basic laws of motion, maximum amount of water vapor. and Bernoulli's principle of differential pressure. Humidity, also called relative humidity, refers to the amount Newton’s Basic Laws of Motion of water vapor contained in the atmosphere and is expressed The formulation of lift has historically been an adaptation as a percentage of the maximum amount of water vapor the over the past few centuries of basic physical laws. These air can hold. This amount varies with temperature. Warm air laws, although seemingly applicable to all aspects of lift, holds more water vapor, while cold air holds less. Perfectly do not explain how lift is formulated. In fact, one must dry air that contains no water vapor has a relative humidity consider the many airfoils that are symmetrical, yet produce of zero percent, while saturated air, which cannot hold any significant lift. more water vapor, has a relative humidity of 100 percent. Humidity alone is usually not considered an important factor The fundamental physical laws governing the forces acting in calculating density altitude and aircraft performance, but upon an aircraft in flight were adopted from postulated it is a contributing factor. theories developed before any human successfully flew an aircraft. The use of these physical laws grew out of the As temperature increases, the air can hold greater amounts Scientific Revolution, which began in Europe in the 1600s. of water vapor. When comparing two separate air masses, Driven by the belief the universe operated in a predictable the first warm and moist (both qualities tending to lighten manner open to human understanding, many philosophers, the air) and the second cold and dry (both qualities making mathematicians, natural scientists, and inventors spent their it heavier), the first must be less dense than the second. lives unlocking the secrets of the universe. One of the most Pressure, temperature, and humidity have a great influence well-known was Sir Isaac Newton, who not only formulated on aircraft performance because of their effect upon density. the law of universal gravitation, but also described the three There are no rules of thumb that can be easily applied, but basic laws of motion. the affect of humidity can be determined using several online formulas. In the first example, the pressure is needed at the Newton’s First Law: “Every object persists in its state of rest altitude for which density altitude is being sought. Using or uniform motion in a straight line unless it is compelled to Figure 4-2, select the barometric pressure closest to the change that state by forces impressed on it.” associated altitude. As an example, the pressure at 8,000 feet is 22.22 "Hg. Using the National Oceanic and Atmospheric This means that nothing starts or stops moving until some Administration (NOAA) website (www.srh.noaa.gov/ outside force causes it to do so. An aircraft at rest on the ramp epz/?n=wxcalc_densityaltitude) for density altitude, enter remains at rest unless a force strong enough to overcome the 22.22 for 8,000 feet in the station pressure window. Enter its inertia is applied. Once it is moving, its inertia keeps a temperature of 80° and a dew point of 75°. The result is a it moving, subject to the various other forces acting on it. density altitude of 11,564 feet. With no humidity, the density These forces may add to its motion, slow it down, or change altitude would be almost 500 feet lower. its direction. Another website (www.wahiduddin.net/calc/density_ Newton’s Second Law: “Force is equal to the change in altitude.htm) provides a more straight forward method of momentum per change in time. For a constant mass, force determining the effects of humidity on density altitude equals mass times acceleration.” without using additional interpretive charts. In any case, the effects of humidity on density altitude include a decrease in overall performance in high humidity conditions. 4-5