Cruise performance
Navigation · Chapter 56 · DGCA CPL and ATPL
Far or long? Two different speeds, and a staircase in the sky.
Remember
- FF2 = FF1 × m2 ÷ m1.
- SR = TAS ÷ FF.
- Jet: VMD and 1.32 VMD.
- Wind: not endurance.
- Optimum altitude rises as fuel burns.
Lighter burns less
In level flight, thrust equals drag. A jet’s fuel flow follows the thrust. So less drag, less fuel. And a lighter aeroplane has less drag.
At the same level, fuel flow goes with mass. Ten per cent lighter, ten per cent less.
Check the direction: lighter must mean less.
FF2 = FF1 × mass 2 / mass 1
The score for cruise is specific range: how far on one kilogram.
Specific range = TAS / fuel flow
Far or long
Stay up longest, or go furthest? They are different speeds.
Endurance is about time: least fuel per hour. Range is about distance: least fuel per mile.
For a jet, fuel flow goes with drag. So endurance is at VMD, the bottom of the valley.
Range is a tangent: a straight line from the origin that just touches the curve. It touches at 1.32 VMD: the best deal of speed for drag.
Jet: endurance VMD, range 1.32 VMD. Propeller: endurance VMP, range VMD.
What moves them?
Wind has no effect on endurance: you are counting hours, not miles.
Climbing as you burn
This lesson is part of the Navigation plan. See plans
CAS, TAS and Mach
This lesson is part of the Navigation plan. See plans