getting to grips with aircraft performance 如何掌握飞机性能 2(10)

F. CRUISE
1. GENERAL
1.1. Introduction
The main objective of the previous chapters is to comply with the airworthiness requirements of JAR/FAR 25 and JAR-OPS 1/FAR 121. This section deals with another objective. That of decreasing Direct Operating Costs (DOC).
Direct Operating Costs include:
.
Fixed costs (taxes, insurance, etc…),

.
Flight-time related costs (crew, hourly maintenance costs, depreciation),

.
Fuel-consumption related costs.

The right choice of flight level and speed allows these DOCs to be minimized. In other words, as time and fuel consumption are closely related, cruise planning is established by making the right speed and flight level choices. In the following chapters, we will review some speed and altitude optimization criteria.
1.2. Specific Range
The specific range (SR) is the distance covered per fuel unit.
Basically speaking, the specific range is equal to:

ground speed (GS)
SR (Ground) =
fuel consumption per hour (FF)
Considering air distance, the specific range is equal to:
true air speed (TAS)
SR (Air) =
fuel consumption per hour (FF)
As TAS is expressed in nautical miles per hour (NM/h), and Fuel Flow (FF) in kilograms per hour (kg/h), the SR is expressed in NM/kg or NM/ton.
Moreover, SR depends on aerodynamic characteristics (Mach and L/D), engine performance (Specific Fuel Consumption)1, aircraft weight (mg) and sound velocity at sea level (a0).
1 The Specific Fuel Consumption (SFC) is equal to the fuel flow (FF) divided by the available thrust. It is expressed in kg/h.N (kilogram per hour per Newton) and represents the fuel consumption per thrust unit.

M . L/D  ê. SR . m ê. SR . SFC ê. SR .
2. SPEED OPTIMIZATION
2.1. All Engine Operating Cruise Speeds
2.1.1. Maximum Range Mach Number (MMR)
Figure F1 illustrates the specific range as a function of Mach number for a given weight at a constant altitude.
As a result, for a given weight, a maximum specific range value exists and the corresponding Mach number is called Maximum Range Mach number (MMR).

MMR

The advantage of the Maximum Range Mach number is that the fuel consumption for a given distance is at its minimum. It also corresponds to the maximum distance an aircraft can fly with a given fuel quantity.
During cruise, the aircraft’s weight decreases due to fuel burn. At the same time, the specific range increases, but MMR decreases (Figure F2). The Mach number must therefore be adjusted to correspond to weight changes during the entire flight at constant altitude.

. Pressure Altitude Influence

Variations of the maximum range Mach number are summarized as follows:
PA = constant weight . . MMR . weight = constant PA . . MMR .

2.1.2. Long-Range Cruise Mach Number (MLRC)
An alternative to MMR is to increase cruise speed with only a slight increase in fuel consumption. Typically, the long-range cruise Mach number (MLRC) provides this possibility.
　
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