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┃                                              ┃:L  ┃6a >O, c(<o   ┃
┃                                        ,( -  ┃\   ┃    ClZia < 0 ┃
┃ .  (                                         ┃)   ┃              ┃
┃                                              ┣━━┻━━━━━━━┫
┃~ l-_-,                                       ┃fr    L_- J Down    ┃
┣━━━━━━━━━━━━━━━━━━━━━━━╋━━━━━━━━━━┫
┃                                              ┃                    ┃
┗━━━━━━━━━━━━━━━━━━━━━━━┻━━━━━━━━━━┛
Fig.3.103   Sign convention for aileron deflection.
C,8a = a8~
307
(3.377)
where L is the rolling moment.
   On a tailless aircraft (no horizontal tail), the elevator and ailerons are usually
combined into one set of control surfaces called elevons. Differentially deflected,
the elevons produce roll control and, symmetrically deflected, they produce pitch
control.
   Some aircraft also employ spoilers (Fig. 3.104) for enhancing roll control au-
thority. The spoilers are fiat platelike devices mounted on the wing upper surface
and are mainly used for dumping the lift or increasing the drag during the ground
run for landing. Like ailerons, the spoilers have to be deflected differentially for
roll control. Br9cause they are mounted on the top surface of the wings, only up-
ward deflection is possible. Therefore, if the aircraft wants to roll to the right, only
the right spoiler has to be defiected upward. The left spoiler remains in its neutral
position. This causes the right wing to lose lift, whereas the lift on the left wing
remains unchanged.ll~is imbalance in lift creates a rolling moment to the right or
in the direction of the de:flected spoiler.
  The aileron effectrveness is measured by the rolling-moment coefficient per
unit defiection (differential) of the ailerons and is denoted by Cl8a and can be
approximate,ly estimated using the strip theory as follows.
;l
   ':; I
~
STATIC STABILITY AND CONTROL
Fig. 3.105    Strip theory estimation of aileron effectiveness.
309
on the wing planform. As discussed in Chapter 1, for rectangular wings of high
aspect ratio, high taper ratio, and low sweep, the ailerons retain their effectiveness
all the way up to the wing stall because the stall on such wings originates at the
root sections and progresses towards the wing tip.
   Howe'ver, for highly swept-back wings, the stall originates at the wingtip and
spreads towards the wing root. As a result, the ailerons lose their effectiveness
long before the entire wing stalls. In this respect, the swept-forward wing has a
 distinct advantage because the stall progresses from root to tip more or less like a
rectangular wing.
                                         Example 3.11
    For the uulless vehicle configuration ofExamples 3.2 and 3.8, obtain the lateral
stability derivative Ctp for M -- 0.7 and M -. 2.0.
　
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