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small in comparison with wing lift for longitudinal control considerations.
From this equation, we observe that we have the following options for longitu-
drnal control.
Controlof wing camber. The Cmac.w is a function of the wing camber, which
can be altered in a relatively simple manner during fiight by deploying either
leading- or trailing-edge flaps. On taillcss aircraft, this type of control is usually
used. However, a disadvantage of using wing flaps for longitudinal control on
aircraft with horizontal tails is that the flap deflection alters the downwash field
at the horizontal tail, hence the longitudinal stability level of the aircraft. In other
words, the use of flaps for longitudinal control alters the longitudinal stability level
from one trim-lift coefficient to another, This is not desirable. Ideally, the method
PERFORMANCE, STABILITY, DYNAMICS, AND CONTROL
used for control should not affect the stability level of the airplane. As we will see
later, any change in the static stability level affects the pilot's feel of the airplane.
Center of gravity shift. rfhe forward or backward movement of the center of
gravity duringYflight has a strong effect on the equilibrium or trim-lift coefficient as
we have seen in Fig. 3.30. We observe that the equilibrium lift coefficient decreases
and the level of static stability increases 'as the center of gravity moves forward
and vice versa.
The Anglo-French supersonic Concorde airliner uses this method oflongitudi
nal control to partially offset the rearward movement of the aerodynamic center
in the transonic/supersonic region. The center of gravity shift on the Concorde is
effected by transferring the fuel from one wing tank to a~other, which is favorably
located. Although this is a powerful method of longitudinal control, it is mechan-
ically complex and is accompanied by a significant change in the level of static
stability from one trim-lift coefficient to ano~her. As we will discuss later, it affects
the pilot's feel of the airplane.
E/evatorcontro/. Thisis a powerful and perhaps the most widely used method
of longitudinal control. The ele:ator is a small fiap attached at the trailing edge of
the horizontal tail as shown in Fig. 3.34a. A defiection of the elevator alters the
(Jl
Lr.
Tail
ai
T
Ll
\
Hirtge Lir.t
a) Elevator geometry
S*ction A A
b) Pressure distribution
Fig. 3.34 Schematic diagram of elevator geometry and pressure distribution.
STAT.IC STABILITY AND CONTROL
209
pressure djstribuuon as shown in Fig. 3.34b, which in turn changes the tail lift.
Some high-speed aircraft needing significant pitch control authority to counter
the rearward movement of the center of pressure at transonic/supersonic speeds
employ a slab tail or an all-movable honizontal tail.
To understand how the elevator control works, let us return to the pitching
moment equation:
Cm - CL.wXa + Cmac,w + Cm f - CL.t V i r7r
(3.92)
The sign convention for elevator defiection is as follows. We assume that the
elevator deflection is positive ifitis deflected downward or trailing edge down and
negative if it is deflected upward or the trailing edge up.
The effect of elevator deflection on the tail-lift coefficient can be expressed as
where
St
CLJ = arch S rjr
= at(aw -/rn+/r -e+r8e) G 7,
dar
T - -.
d8e
(3.93)
(3.94)
(3.95)
Here, r denotes the elevator effectiveness, which is the changein tail angle ofattack
per unit deflection of the elevator. The parameter r can be estimated as follows.
The lift increment due to a control surface deflection of A8e is given by
Datcom:l
ACL - ACi
(-.,) [((.,)).,.]
(3.96)
where ao'is the sectional (two-dimensional) lift-curve slope and ACt is the section-
lift increment caused by A8e. The parameter (as)cl./(u8)cl is the ratio of the
three-dimensional control effectiveness parameter to the two-dimensional control
effectiveness parameter as given by Fig. 3.35, and Kb is the control surface span
factor. Using Fig. 3.36, Kb can be evaluated as
Kb = (Kb)ri=rro - (Kb)n=m.
Differentiating Eq. (3.96) with respect to 8e and rearranging, we obtain
r = (CI ) [((.,))., ] Kb
(3.97)
(3.98)
where Cl8 can be obtained using the data given in Figs. 3.37a and 3.37b.
It is important to note that the elevator deflection does not affect the stick-
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