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