曝光台 注意防骗
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Edg*
ng
Edge
Fig. 3.40 Concept of permissible center of gravity travel.
215
The demand on the elevator to trim the CL.max becomes more severe when the
aircraft is operating in close proximity of the ground, which is the case during
landing or ta7ceoff. The most forward limit on the center of gravity location given
by Eq. (3.110) essentially corresponds to an airplane in free fiight where the ground
effects are insignificant.
The proximity of the ground reduces the strength of the wingtip vortices (Fig.
3.41a), As a result, the induced downwash velocities are small ever)rwhere in
comparison with those iri free flight. In particular, the downwash angle 6 at the
tail is much smaller in the presence of the ground compared to that in free flight
(Fig. 3.41b). This leads to an increase in the lift-curve slope of the horizontal
tail (Fig. 3.41c) and a corresponding increase in tail effectiveness. As a result,
the aircraft becomes more stable, leading to a requirement that the most forward
center of gravity location with ground effect lie aft of that in free flight as shown
in Fig. 3.42.
3.3.13 Stick-Free LongitudinaIStabil,7y
Let us assume that the elevator is mounted on a frictionless hinge so that it
can fioat freely under the action of aerodynamic forces when allowed to do so.
The elevator will either float up or down depending on location of the hingeline
relative to its center of pressure. If the hingeline is ahead of the center of pressure,
the elevator tends to float up for positive angles of attack and fioat down at negative
angles of attack as shown in Fig. 3.43.
The horizontal tail angle of attack is given by
so that
ctt - cL'w - iw + it - E + r8e
a's - uw - iw + /r - €
ar - a's + c8e
(3.113)
(3.114)
(3.115)
Thus, when 8e = O, (xt - as. Often, a's is also called the stabilizer angle of attack.
216 PERFORMANCE, STABILITY, DYNAMICS, AND CONTROL
Free Flight
a)
Airplant in Ground
Ettect
with Ground Etttct
Futelage
L
tal Tail
in Free Fl19ht Aerodycnamic Ctntir
b)
c)
Fig.3.41 Schematicillustration ofground effect.
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