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we discuss such probler:; of powered flights,let us consider the simple power-off
gliding flight,.
2.3  Gliding Flight
         A glideris an unpowered light airplane. The power to overcome the aerodynamic
drag comes at the expense ofits potential energy or its height above the ground.
Because a glider cannot attain the height on its own, it must be towed by another
powered airplane to the desired height and then launched to fly on its own.
    An interesting example of power-off gliding fiight is the return of the Space
Shuttle orbiter from space. The Space Shuttle orbiter glides back to the Earth and
lands on a runway like a conventional airplane.

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74                PERFORMANCE, STABIU-fY, DYNAMICS, AND CONTROL
Fig. 2-4    Forces acfng on a glider.
      Consider a glider fiying in a vertical plane as shown iri Fig. 2.4. Note that, for
 gliding flight, y  < O. With T : 0, Eqs. (2.7) and (2.8) reduce to the following form:
                                                  D+ W sin y - 0                                     (2.29)
                                                      L - W cos y - 0                                        (2.30)
Kinematic equations as given by Eqs. (2.9) and (2.10) are
                                                  x - V cos y                                      (2.31)
                                                           h = V sin y                                             (2.32)
Generally, the glide angle y is small so that we have
                                           D+ Wy - 0                                 (2.33)
　
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