a Koch Chart or a flight computer with a density altitude  function.  [Figure 4-7] If a chart is not available, the density altitude can be estimated  by adding 120 feet for every degree Celsius above the ISA. For  example, at 3,000 feet PA, the ISA prediction is 9 °C (15 °C –  [lapse rate of 2 °C per 1,000 feet  x 3 = 6 °C]). However, if the  actual temperature is 20 °C (11 °C more than that predicted  by ISA) then the difference of 11 °C is multiplied by 120 feet  equaling 1,320. Adding this figure to the original 3,000 feet  provides a density altitude of 4,320 feet (3,000 feet + 1,320 feet). Lift always acts in a direction perpendicular to the relative  wind and to the lateral axis of the aircraft. The fact that lift is  referenced to the wing, not to the Earth’s surface, is the source  of many errors in learning flight control. Lift is not always  “up.” Its direction relative to the Earth’s surface changes as  the pilot maneuvers the aircraft. The magnitude of the force of lift is directly proportional to  the density of the air, the area of the wings, and the airspeed.  It also depends upon the type of wing and the AOA. Lift  increases with an increase in AOA up to the stalling angle,  at which point it decreases with any further increase in AOA.  In conventional aircraft, lift is therefore controlled by varying  the AOA and speed. An examination of  Figure 4-8 illustrates the relationship  between pitch and power while controlling flightpath and  airspeed. In order to maintain a constant lift, as airspeed is  When induced drag and parasite drag are plotted on a graph,  the total drag on the aircraft appears in the form of a “drag  curve.” Graph A of  Figure 4-9  shows a curve based on thrust  versus drag, which is primarily used for jet aircraft. Graph B  If the pilot wants the aircraft to accelerate while maintaining  altitude, thrust must be increased to overcome drag. As  the aircraft speeds up, lift is increased. To prevent gaining  altitude, the pitch angle must be lowered to reduce the AOA  and maintain altitude. To decelerate while maintaining  altitude, thrust must be decreased to less than the value of  drag. As the aircraft slows down, lift is reduced. To prevent  losing altitude, the pitch angle must be increased in order to  increase the AOA and maintain altitude. Thrust is controlled by using the throttle to establish or  maintain desired airspeeds. The most precise method  of controlling flightpath is to use pitch control while  simultaneously using power (thrust) to control airspeed. In  order to maintain a constant lift, a change in pitch requires a  change in power, and vice versa. reduced, pitch must be increased. The pilot controls pitch  through the elevators, which control the AOA. When back  pressure is applied on the elevator control, the tail lowers  and the nose rises, thus increasing the wing’s AOA and lift.  Under most conditions the elevator is placing downward  pressure on the tail. This pressure requires energy that is  taken from aircraft performance (speed). Therefore, when  the CG is closer to the aft portion of the aircraft the elevator  downward forces are less. This results in less energy used for  downward forces, in turn resulting in more energy applied  to aircraft performance. Drag Curves Pitch/Power Relationship Lift Altitude and Temperature Effects TO FIND  the effect of altitude temperature CONNECT  the temperature and airport altitude by straight line READ  the increase in take-off distance and the decrease in rate of  climb from standard sea level values here  120 110 100 90 80 70 60 50 40 30 20 10 0 −10 −20 −30 −40 18 14 12 10 8 6 4 2 0 −2 Airport pressure altitude—Thousand  of feet (read your altmeter set to 29.92  "Hg) 90 80 280 200 140 100 60 40 20 0 70 60 40 20 0 Percent decrease in rate of climb Add this percent to your normal take off distance Figure 4-7.  Koch chart sample. Figure 4-8.  Relationship of lift to AOA. 1.8  1.6  1.4  1.2  1.0  0.8  0.6  0.4  0.2  0  2  4  6  8  10  12  14  16  18  20  22  24  C L-MAX 2  11  20  Angle of Attack (degrees) Lift Coefficient - C L  Stall 1.5  4-6