Aircraft Performance

contains a smaller mass of air. In other words, the density is consideration. Expect a decrease in overall performance in decreased. In fact, density is directly proportional to pressure. high humidity conditions. If the pressure is doubled, the density is doubled, and if the pressure is lowered, so is the density. This statement is true Performance only at a constant temperature. Performance is a term used to describe the ability of an aircraft to accomplish certain things that make it useful for Effects of Temperature on Density certain purposes. For example, the ability of an aircraft to land Increasing the temperature of a substance decreases its and take off in a very short distance is an important factor density. Conversely, decreasing the temperature increases to the pilot who operates in and out of short, unimproved the density. Thus, the density of air varies inversely with airfields. The ability to carry heavy loads, fly at high altitudes temperature. This statement is true only at a constant pressure. at fast speeds, and/or travel long distances is essential for the performance of airline and executive type aircraft. In the atmosphere, both temperature and pressure decrease with altitude and have conflicting effects upon density. The primary factors most affected by performance are the However, the fairly rapid drop in pressure as altitude is takeoff and landing distance, rate of climb, ceiling, payload, increased usually has the dominant effect. Hence, pilots can range, speed, maneuverability, stability, and fuel economy. expect the density to decrease with altitude. Some of these factors are often directly opposed: for example, high speed versus short landing distance, long range versus Effects of Humidity (Moisture) on Density great payload, and high rate of climb versus fuel economy. The preceding paragraphs are based on the presupposition of It is the preeminence of one or more of these factors that perfectly dry air. In reality, it is never completely dry. The dictates differences between aircraft and explains the high small amount of water vapor suspended in the atmosphere degree of specialization found in modern aircraft. may be negligible under certain conditions, but in other conditions humidity may become an important factor in the The various items of aircraft performance result from the performance of an aircraft. Water vapor is lighter than air; combination of aircraft and powerplant characteristics. The consequently, moist air is lighter than dry air. Therefore, as the aerodynamic characteristics of the aircraft generally define water content of the air increases, the air becomes less dense, the power and thrust requirements at various conditions of increasing density altitude and decreasing performance. It is flight, while powerplant characteristics generally define the lightest or least dense when, in a given set of conditions, it power and thrust available at various conditions of flight. contains the maximum amount of water vapor. The matching of the aerodynamic configuration with the powerplant is accomplished by the manufacturer to provide Humidity, also called relative humidity, refers to the amount maximum performance at the specific design condition (e.g., of water vapor contained in the atmosphere and is expressed range, endurance, and climb). as a percentage of the maximum amount of water vapor the air can hold. This amount varies with the temperature; Straight-and-Level Flight warm air can hold more water vapor, while colder air can All of the principal components of flight performance involve hold less. Perfectly dry air that contains no water vapor has steady-state flight conditions and equilibrium of the aircraft. a relative humidity of zero percent, while saturated air that For the aircraft to remain in steady, level flight, equilibrium cannot hold any more water vapor has a relative humidity must be obtained by a lift equal to the aircraft weight and a of 100 percent. Humidity alone is usually not considered an powerplant thrust equal to the aircraft drag. Thus, the aircraft essential factor in calculating density altitude and aircraft drag defines the thrust required to maintain steady, level performance; however, it does contribute. flight. As presented in Chapter 4, Aerodynamics of Flight, all parts of an aircraft contribute to the drag, either induced The higher the temperature, the greater amount of water (from lifting surfaces) or parasite drag. vapor that the air can hold. When comparing two separate air masses, the first warm and moist (both qualities making air While parasite drag predominates at high speed, induced drag lighter) and the second cold and dry (both qualities making predominates at low speed. [Figure 11-5] For example, if it heavier), the first must be less dense than the second. an aircraft in a steady flight condition at 100 knots is then Pressure, temperature, and humidity have a great influence accelerated to 200 knots, the parasite drag becomes four on aircraft performance because of their effect upon density. times as great, but the power required to overcome that There is no rule-of-thumb or chart used to compute the effects drag is eight times the original value. Conversely, when the of humidity on density altitude, but it must be taken into 11-5