219
3.3.14 Aerodynamic Balancing
The fioating characteristics of a control surface depend on the hinge-moment
characteristics and, as we will see later, the stick force also depends on the hinge-
moment characteristics. Too low values of the hinge-moment would make the
control highly sensitive to small disturbances, whereas too high values would make
the controls sluggish to operate. Therefore, a careful design of the hinge-moment
parameters Cha and ChtS.e iS necessanjt to achieve a proper balance between these
two conflicting requirements. The control of hinge-moment parameters is called
aerodynamic balancing.
When the hingeline is at the control surface leading edge. both Cha and C,,b.e are
negative. If the hingeline is moved further aft, both Chct and Cn8.e become more
positive because the control surface forward of the hingeline produces an opposing
moment to that produced by the surface aft of the hingeline. The net hinge moment,
which is the algebraic sum of these two moments,is greatly reduced. This type of
aerodynamic balance is called set-back hinge balance (Fig. 3.45a). However, one
has to be careful because too much area forward of the hingeline may lead to an
overbalance of the control at some flight conditions that may affect the pilot's feel
of the aircraft.
Horn balance (Fig. 3.45b) is similar to the set-back hinge, except that all the
area ahead of the hingeline is concentrated on one part of the surface. The horn
Section A-A
Hing tlini
a) Set-back hingeLine b) Horn balance
Bevtl
c) Internal balance d) Beveled trailing edge
e) Tab
Fig.3.45 Aerodynamic balancing concepts.
220 PERFORMANCE, STABILfTY, DYNAMICS, AND CONTROL
Estimation of Ct" atsubsonic speeds. We havel
Cha- -A' A )Chab,,+AChcr (3.121)
Ar +2cos Ac/4
STATIC STABILITY AND CONTROL
221
where Ar is the aspect ratio of the main control surface and, in this case, it is the
aspect ratio of the horizontal tail. The estimation of the parameter Chabd is done
as follows:l
Chabal =
[CC:. ]
C;lct
c/a=cc,C. ](C,,).eory
(3.122)
(3.123)
HINGE
LINE
o NACAO
. NAC~OO:; }
o NACA 66009 j
o NACA0009 I
~. NACA 0015 J
4 NACA0009
(chor) balance
HINGE
LINE
ELLIPI'IC NOSE
SHARP NOSE
HINGE
LINE
tc
F --7r-- {- 2
┏━━━━━┳━━━━┓
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┣━━━━━╋━━━━┫
┃ ┃ ┃
┗━━━━━┻━━━━┛
SUBSONIC SPEEDS
fflNGE
LINE
~ ~
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┃ ┃ ┃ ┃ ┃\ ┃\ ┃.l ┃ ┃ ┃ ┃
┣━╋━╋━╋━╋━╋━╋━━━╋━━╋━━╋━┫
┃ ┃ ┃ ┃ ┃~ ┃X ┃\1. ┃~ ┃~b ┃ ┃
┣━╋━╋━╋━╋━╋━╋━━━┻━━┻━━┻━┫
┃ ┃ ┃ ┃ ┃ ┃ ┃ ┃
┣━╋━╋━╋━╋━╋━╋━━━┳━━┳━━━━┫
┃ ┃ ┃ ┃ ┃ ┃ ┃~~E. ┃Q~ ┃S ~ ┃
┃ ┃ ┃ ┃ ┃ ┃\ ┃ ┃ ┃ ┃
┣━╋━╋━╋━╋━╋━╋━━━╋━━╋━━┳━┫
┃ ┃ ┃ ┃ ┃ ┃ ┃ ┃ ┃ ┃ ┃
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