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Control Effectiveness lecture
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AAE 556 Aeroelasticity
Lecture 7-Control effectiveness
1Purdue Aeroelasticity
Reading
i Sections 2.15-2.18– These sections are painfully worked
example problems – read through them to understand principles discussed in class
– Section 2.18.2 has a virtual work example – wait to read this until next week
i Skip 2.19 for now (will do next week)i Read 2.20, 2.20.1 and 2.20.2
Purdue Aeroelasticity2
Purdue Aeroelasticity6-3
i Demonstrate the aeroelastic effect of deflecting aileron surfaces to increase lift or rolling moment
i Examine the ability of an aileron or elevator to produce a change in lift, pitching moment or rolling moment
i Reading – Sections 2.20-2.20.2
Our next goallearn about control effectiveness
i Many of the uncertified minimum ultralights, and perhaps some of the certificated aircraft, have low torsional wing rigidity. This will not only make the ailerons increasingly ineffective with speed (and prone to flutter), but will also place very low limits on g loads.– http://www.auf.asn.a
u/groundschool/flutter.html#flutter
Purdue Aeroelasticity4
Ailerons are required for lateral stabilityThey become increasingly ineffective at high speeds
Purdue Aeroelasticity6-5
The ability of an aileron or elevator to produce a change in lift, pitching moment or rolling moment is changed by
aeroelastic interaction
0
Lift
M AC K
T
0 e
shear center
torsion spring
V
ailerondeflection
L o L
o
AC MAC
L qSC qSC
noM qScC
Purdue Aeroelasticity6-6
Herman Glauert’s estimators for CLd and CMACd
The flap-to-chord ratio is cc
E f
EEEC
C LL 1221cos 1
1 1LMAC
CC E E E
Purdue Aeroelasticity6-7
1 DOF idealized model – no camber Sum moments about the shear center
0 Tsc ACLeM M K
e
0oRemember
Linear problem (what does that mean?)
L
Purdue Aeroelasticity6-8
Solve for the twist angledue only to aileron deflection d
oLT
MACL
qSeCK
CecCqSe
Lift LoL qSCqSCL
Purdue Aeroelasticity6-9
The aeroelastic lift due to deflection
D
L
MAC
DoL
qq1
C
C
ec
qq1
qSCL
oLrigid qSCL
Compare answer to the lift computed ignoring aeroelastic interaction
Purdue Aeroelasticity7-10
The aileron deflection required to generate a fixed increases as q increases
D
RoLo
qqqq
qSCL1
1
R
D
L
oo
qqqq
qSCL
1
1
The required control input is …
Aileron deflection increases as q approaches reversal
Is aileron reversal an instability?
Purdue Aeroelasticity6-11
The most common definition forthe reversal condition
0flexL
We usually use an aileron to produce a rolling moment, not just lift. What is the dynamic pressure to make the lift or rolling moment zero even if we move the aileron?
Is it possible that I deflect and aileron and get no lift?
Purdue Aeroelasticity6-12
How do I make the numerator term in the lift expression equal to zero?
1
01
MAC
D L
L o
D
Cq cq e C
L qSC qq
L=0, reversal
L=infinity, divergence
Purdue Aeroelasticity6-13
Solve for the q at the reversal condition
1 0MACR
D L
Cq cq e C
Rreversal qqq
MAC
LDR C
C
ceqq
numerator=0
MAC
L
L
TR C
C
ScCKqor
Why the minus sign?
Purdue Aeroelasticity6-14
Understanding what the aileron doesTwo different ways to compute pressure
distribution resultants due to aileron deflection
aileron l ift = qSC L o
0 e
aerodynamic cente r M AC = qScC MAC o
(a) aerod ynamic ce nter representatio n
0 e
aileron l ift = qSC L o
d
(b) a ilero n ce nter o f pressure
Purdue Aeroelasticity
15
Force equivalencethe same moment at the AC with 2 different models
MAC oL d qScC
Solve for the distance d to find the CP distance from the AC
0e
d
A lift force at d produces the same result at the AC as a lift force and moment at the AC
L
M
L
Cdc C
=
+
Purdue Aeroelasticity6-16
Aileron center of pressure depends on the aileron chord
1.00.80.60.40.20.00.00
0.10
0.20
Distance from aileron center of pressureto airfoil aerodynamic center(Glauert prediction)
flap to chord ratio
dist
ance
in c
hord
leng
ths
quarter chord location
midchord position
shearcenterlocation
example
Dis
t anc
e af
t of 1
/ 4 c
hord
Aileron flap to chord ratio, E
AC
mid-chord
All-movable surface
Purdue Aeroelasticity6-17
i Control surfaces generate less lift because the control deflection creates a nose-down pitching moment as it generates lift.
i At a special dynamic pressure (a combination of airspeed and altitude) the deflection of an aileron creates more downward lift due to nose-down deflection than upward lift
Summary