Range Performance
The ability of an airplane to convert fuel energy into flying distance is one of the most important items of airplane performance. In flying operations, the problem of efficient range operation of an airplane appears in two general forms: (1) to extract the maximum flying distance from a given fuel load or (2) to fly a specified distance with a minimum expenditure of fuel. A common denominator for each of these operating problems is the "specific range"; that is, nautical miles of flying distance per pound of fuel. Cruise flight operations for maximum range should be conducted so that the airplane obtains maximum specific range throughout the flight. The specific range can be defined by the following relationship:
nautical miles or
nautical miles/hr. or
knots
I
flight hours or
flight hours/hr. or
1 {p309} If maximum endurance is desired, the flight condition must provide a minimum of fuel flow. While the peak value of specific range would provide maximum
range operation, long range cruise operation is generally recommended at some
slightly higher airspeed. Most long range cruise operations are conducted at the
flight condition which provides 99 percent of the absolute maximum specific
range. The advantage of such operation is that 1 percent of range is traded for
3 to 5 percent higher cruise speed. Since the higher cruise speed has a great
number of advantages, the small sacrifice of range is a fair bargain. The values
of specific range versus speed are affected by three principal variables: (1)
airplane gross weight, (2) altitude, and (3) the external aerodynamic
configuration of the airplane. These are the source of range and endurance
operating data included in the performance section of the airplane's flight
handbook.
Total range is dependent on both fuel available and specific range. When range and economy of operation are the principal goals, the pilot must ensure that the airplane will be operated at the recommended long range cruise condition. By this procedure, the airplane will be capable of its maximum design operating radius, or can achieve flight distances less than the maximum with a maximum of fuel reserve at the destination. The propeller driven airplane combines the propeller with the reciprocating engine for propulsive power. In the case of the reciprocating engine, fuel flow is determined mainly by the shaft power put into the propeller rather than thrust. Thus, the fuel flow can be related directly to the power required to maintain the airplane in steady, level flight. This fact allows for the determination of range through analysis of power required versus speed - variation of fuel flow versus speed. The maximum endurance condition would be obtained at the point of minimum power required since this would require the lowest fuel flow to keep the airplane in steady, level flight. Maximum range condition would occur where the proportion between speed and power required is greatest (Fig. 17-58). The maximum range condition is obtained at maximum lift/drag ratio (L/D max) and it is important to note that for a given airplane configuration, the maximum lift/drag ratio occurs at a particular angle of attack and lift coefficient, and is unaffected by weight or altitude. The flight condition of maximum lift/drag ratio is achieved at one particular value of lift coefficient for a given airplane configuration. Hence, a variation of gross weight will alter the values of airspeed, power required, and specific range obtained at the maximum lift/drag ratio. The variations of speed and power required must be monitored by
the pilot as part of the cruise control procedure to maintain the maximum
lift/drag ratio. When the airplane's fuel weight is a small part of the gross
weight and the airplane's range is small, the cruise control procedure can be
simplified to essentially maintaining a constant speed and power setting
throughout the time of cruise flight. On the other hand, the long range airplane
has a fuel weight which is a considerable part of the gross weight, and cruise
control procedures must employ scheduled airspeed and power changes to maintain
optimum range conditions.
The airplane equipped with the reciprocating engine will
experience very little, if any, variation of specific range with altitude at low
altitudes. There is negligible variation of brake specific fuel consumption for
values of brake horsepower below the maximum cruise power rating of the engine
which is the lean range of engine operation. Thus, an increase in altitude will
produce a decrease in specific range only when the increased power requirement
exceeds the maximum cruise power rating of the engine. One advantage of
supercharging is that the cruise power may be maintained at high altitude and
the airplane may achieve the range at high altitude with the corresponding
increase in true airspeed. The principal differences in the high altitude cruise
and low altitude cruise are the true airspeeds and climb fuel requirements.
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