Figure 11-5.  Drag versus speed.  Figure 11-6.  Power versus speed.  aircraft is operated in steady, level flight at twice as great a  speed, the induced drag is one-fourth the original value, and  the power required to overcome that drag is only one-half  the original value.  When an aircraft is in steady, level flight, the condition of  equilibrium must prevail. The unaccelerated condition of  flight is achieved with the aircraft trimmed for lift equal  to weight and the powerplant set for a thrust to equal the  aircraft drag.  The maximum level flight speed for the aircraft is obtained  when the power or thrust required equals the maximum power  or thrust available from the powerplant.  [Figure 11-6]  The  minimum level flight airspeed is not usually defined by thrust  or power requirement since conditions of stall or stability and  control problems generally predominate.  If an aircraft is to move, fly, and perform, work must act  upon it. Work involves force moving the aircraft. The aircraft  acquires mechanical energy when it moves. Mechanical  energy comes in two forms: (1) Kinetic Energy (KE), the  energy of speed; (2) Potential Energy (PE), the stored energy  of position.  Aircraft motion (KE) is described by its velocity (airspeed).  Aircraft position (PE) is described by its height (altitude).  Both KE and PE are directly proportional to the object’s  mass. KE is directly proportional to the square of the object’s  velocity (airspeed). PE is directly proportional to the object’s  height (altitude). The formulas below summarize these  energy relationships:  We sometimes use the terms “power” and “thrust”  interchangeably when discussing climb performance. This  erroneously implies the terms are synonymous. It is important  to distinguish between these terms. Thrust is a force or  pressure exerted on an object. Thrust is measured in pounds  (lb) or newtons (N). Power, however, is a measurement of  the rate of performing work or transferring energy (KE and  PE). Power is typically measured in horsepower (hp) or  kilowatts (kw). We can think of power as the motion (KE  and PE) a force (thrust) creates when exerted on an object  over a period of time.  Positive climb performance occurs when an aircraft gains PE  by increasing altitude. Two basic factors, or a combination  of the two factors, contribute to positive climb performance  in most aircraft:  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)  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).  11-6  Climb Performance  Drag  Speed  Total drag Parasite drag Stall Induced drag L/D$_{MAX}$ Power required  Speed  Low cruise speed  Min. speed  High cruise speed  M a x i m u m  a va il a b l e p o w e r  Maximum level flight speed  m = object mass KE = ½ × m × v 2  v = object velocity  m = object mass  PE = m × g × h  g = gravity field strength  h = object height 