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ing aircraft stability and control derivatives.7 It is a comprehensrve compilation
of all the available engineering methods for obtai:rung the stability and control
derivatives of airplane- type configurations.
    The modern CFD methods are capable ofproviding more accurate estimates of
static and some dynamic derivatives. But, at present, the level of effort imtolved
precludes their application at preliminary design stages. In view of this, the CFD
methods are usually used at a sufficiently later stage in the velucle design process
when the configuration takes its final form.
        The experimental methods for obtaining dynamic stability derivatives in ground-
based test facilities8-10 mainly consist ofeither forced oscillation or free oscillation
technique wherein the test model undergoes an oscillatorjr motion in pitch, roll, or
yaw. Another approach that is often used to obtain dynamic stability derivatives
in ground-based test faciMes is the so-called free fiight or semifree fiight test
technique.ll  Here, the test model is allowed to fly either free or semifree [loosely
constrained in some degree(s) of freedom] in the viewing area of the test facility.
The model motion is recorded in the form ofvarious accelerometer outputs or higX-
speed motion picture records. The dynamic stability derivatives are then obtained
implicitly by matching the predicted time history of the model with the observed
 time history. Because the time and effort involved in fabricating and testing aircraft
models in the ground-based facilities are considerable, these approaches are also
usually used in the final stages of the vehicle design.
      On the other hand, the semiempirical or engineering methods provide quick and
cost-effective estimates of the static and dynamic stability derivatives, which can
be readily used for assessing the vehicle fiyability, stability, and control. Also, the
 predictions based on these engineering methods can be used in the design of fiight
control and guidance systems. The only disadvantage is that such predictions are
 less accurate compared to the CFD or the wind-tunnel test data. However, this type
 ofinformation is very useful in the early stages of the design to quickly evaluate
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PERFORMANCE, STABILITY, DYNAMICS, AND CONTROL
    Flight Path
_/_h _
Fig.4.18    Forces acting on an airplane during a pull-out maneuver.
For the disturbed flight,
                 Fx -. Fxo + AFx -. -(Do + AD) - W sin(0o + AO) + T
                                                                                                                                                                      \ U)?-
   Fz -- Fzo + AFz = -(Lo + AL) + W cos(0o + AO) - ( v~   [(Uo +RAf/
so that
                                                 AFx -. -AD - W cos 0o AO
                        AFz = -AL - W sinOoAO - (V;~) [2URAU ]
 Proceedingin a similar way as we did for the small disturbance equations ofmotion
for a steady flight in a vertical plane, we obtain
                                    Cxu - -2CD - CDu
　
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