Aircraft Performance

deriuqer rewoP NM recommended long-range cruise condition. By this procedure, specific range = pounds of fuel the aircraft is capable of its maximum design-operating radius or can achieve flight distances less than the maximum with or a maximum of fuel reserve at the destination. NM/hour A propeller-driven aircraft combines the propeller with the specific range = pounds of fuel/hour reciprocating engine for propulsive power. Fuel flow is determined mainly by the shaft power put into the propeller or rather than thrust. Thus, the fuel flow can be related directly to the power required to maintain the aircraft in steady, level knots flight, and on performance charts power can be substituted specific range = fuel flow for fuel flow. This fact allows for the determination of range through analysis of power required versus speed. If maximum specific range is desired, the flight condition must provide a maximum of speed per fuel flow. While The maximum endurance condition would be obtained at the the peak value of specific range would provide maximum point of minimum power required since this would require the range operation, long-range cruise operation is generally lowest fuel flow to keep the airplane in steady, level flight. recommended at a slightly higher airspeed. Most long-range Maximum range condition would occur where the ratio of cruise operations are conducted at the flight condition that speed to power required is greatest. [Figure 11-11] provides 99 percent of the absolute maximum specific range. The advantage of such operation is that one percent of range The maximum range condition is obtained at maximum lift/ is traded for three to five percent higher cruise speed. Since drag ratio (L/D ), and it is important to note that for a given MAX the higher cruise speed has a great number of advantages, the aircraft configuration, the L/D occurs at a particular AOA MAX small sacrifice of range is a fair bargain. The values of specific and lift coefficient and is unaffected by weight or altitude. A range versus speed are affected by three principal variables: variation in weight alters the values of airspeed and power required to obtain the L/D . [Figure 11-12] Different 1. Aircraft gross weight MAX theories exist on how to achieve max range when there is a 2. Altitude headwind or tailwind present. Many say that speeding up in 3. The external aerodynamic configuration of the aircraft. a headwind or slowing down in a tail wind helps to achieve max range. While this theory may be true in a lot of cases, These are the source of range and endurance operating data it is not always true as there are different variables to every included in the performance section of the AFM/POH. situation. Each aircraft configuration is different, and there is not a rule of thumb that encompasses all of them as to how Cruise control of an aircraft implies that the aircraft is to achieve the max range. operated to maintain the recommended long-range cruise condition throughout the flight. Since fuel is consumed during cruise, the gross weight of the aircraft varies and optimum ht g a c i o r n sp tr e o e l d m , e a a lt n it s u t d h e e , c a o n n d t r p o o l w of e t r h s e e o tt p in ti g m c u a m n a a i l r s s o p e v e a d r , y a . l C tit r u u d is e e , L/DMAX Higher w e e i g i ht a sp n e d c i p fi o c w ra e n r g s e e c tt o i n n d g i t t i o o n m . A ai t n th ta e i n b e t g h i e n n 9 in 9 g p o e f r c c r e u n i t s e m fl a ig x h im t, u th m e B w as e i r c w w ei g ht relatively high initial weight of the aircraft requires specific Lo values of airspeed, altitude, and power setting to produce the recommended cruise condition. As fuel is consumed and the aircraft’s gross weight decreases, the optimum airspeed and Constant altitude power setting may decrease, or the optimum altitude may increase. In addition, the optimum specific range increases. Therefore, the pilot must provide the proper cruise control procedure to ensure that optimum conditions are maintained. Speed Total range is dependent on both fuel available and specific Figure 11-12. Effect of weight. range. When range and economy of operation are the principal goals, the pilot must ensure that the aircraft is operated at the