Unpowered flight II

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Unpowered flight

Unpowered flight occurs when:
  • A single-engine aircraft has engine failure or runs out of fuel;
  • A multiengine aircraft runs out of fuel;
  • An aircraft has no propulsion system (a glider or a sailplane).
The forces acting on an airplane in a power-off glide are lift, drag, and weight: the thrust is zero because the power is off. The glide flight path makes an angle gamma (γ) below the horizontal. For an equilibrium unaccelerated glide the sum of the forces must be zero.
The weight can be divided into two components:
  • W ⋅ sin(γ) acting in a forward direction;
  • W ⋅ cos (γ) acting downward at right angles to the descent path.
The weight component along the flight path must balance drag and the weight component perpendicular to the flight path must balance lift.
Forces in the glide
The equations of motion for unpowered flight are:


An expression for the glide angle can be recovered from equations of motion by division:


The glide angle is a function only of the L/D ratio. The descent angle will be least when the L/D ratio is the greatest. L/D ratio is a maximum at the optimum angle of attack, and this also corresponds to the minimum drag speed.
There are two unpowered flight programs of special interest: maximum range and maximum endurance. The first is of particular importance to the pilot with engine failure or fuel starvation and in the design of a military glider. The second is the key to the design of sailplanes for soaring.