Aircraft Performance

deriuqer rewoP Maximum available power High cruise speed Low cruise speed Min. speed Speed deeps thgilf level mumixaM Figure 11-6. Power versus speed. garD energy comes in two forms: (1) Kinetic Energy (KE), the energy of speed; (2) Potential Energy (PE), the stored energy Total drag of position. Aircraft motion (KE) is described by its velocity (airspeed). L/D MAX Parasite drag Aircraft position (PE) is described by its height (altitude). Both KE and PE are directly proportional to the object’s Stall mass. KE is directly proportional to the square of the object’s Induced drag velocity (airspeed). PE is directly proportional to the object’s height (altitude). The formulas below summarize these energy relationships: Speed m = object mass Figure 11-5. Drag versus speed. KE = ½ × m × v2 v = object velocity aircraft is operated in steady, level flight at twice as great a m = object mass speed, the induced drag is one-fourth the original value, and PE = m × g × h g = gravity field strength the power required to overcome that drag is only one-half h = object height the original value. We sometimes use the terms “power” and “thrust” When an aircraft is in steady, level flight, the condition of interchangeably when discussing climb performance. This equilibrium must prevail. The unaccelerated condition of erroneously implies the terms are synonymous. It is important flight is achieved with the aircraft trimmed for lift equal to distinguish between these terms. Thrust is a force or to weight and the powerplant set for a thrust to equal the pressure exerted on an object. Thrust is measured in pounds aircraft drag. (lb) or newtons (N). Power, however, is a measurement of the rate of performing work or transferring energy (KE and The maximum level flight speed for the aircraft is obtained PE). Power is typically measured in horsepower (hp) or when the power or thrust required equals the maximum power kilowatts (kw). We can think of power as the motion (KE or thrust available from the powerplant. [Figure 11-6] The and PE) a force (thrust) creates when exerted on an object minimum level flight airspeed is not usually defined by thrust over a period of time. or power requirement since conditions of stall or stability and control problems generally predominate. Positive climb performance occurs when an aircraft gains PE by increasing altitude. Two basic factors, or a combination Climb Performance of the two factors, contribute to positive climb performance If an aircraft is to move, fly, and perform, work must act in most aircraft: upon it. Work involves force moving the aircraft. The aircraft acquires mechanical energy when it moves. Mechanical 1. The aircraft climbs (gains PE) using excess power above that required to maintain level flight, or 2. The aircraft climbs by converting airspeed (KE) to altitude (PE). As an example of factor 1 above, an aircraft with an engine capable of producing 200 horsepower (at a given altitude) is using only 130 horsepower to maintain level flight at that altitude. This leaves 70 horsepower available to climb. The pilot holds airspeed constant and increases power to perform the climb. As an example of factor 2, an aircraft is flying level at 120 knots. The pilot leaves the engine power setting constant but applies other control inputs to perform a climb. The climb, sometimes called a zoom climb, converts the airspeed (KE)