Gust load prediction and alleviation on a fighter aircraft

28 Pages·1986·1.943 MB·English

by  J Becker

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AGARD REPORT No.728
Gust Load Prediction and Alleviation
on a Fighter Aircraft
NORTH ATLANTIC TREATY ORGANIZATION
DISTRIBUTION AND AVAILABILITY
ON BACK COVER

AGARD-R-728
NORTH ATLANTIC TREATY ORGANIZATION
ADVISORY GROUP FOR AEROSPACE RESEARCH AND DEVELOPMENT
(ORGANISATION DU TRAITE DE L'ATLANTIQUE NORD)
AGARD Report No.728
GUST LOAD PREDICTION AND ALLEVIATION
ON A FIGHTER AIRCRAFT
by
J.Becker
Papers presented at the 61st Meeting of the Structures and Materials Panel of AGARD in
Oberammergau, Germany on 8—13 September 1985.
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Published June 1986
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ISBN 92-835-1532-3
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CONTENTS
Page
INTRODUCTION 1
CONSISTENCY WITH MILITARY REQUIREMENTS 1
METHODS FOR CALCULATING THE DYNAMIC RESPONSE OF FLEXIBLE AIRCRAFT IN TURBULENCE 1
EXAMPLES OF FLEXIBLE AIRCRAFT TURBULENCE RESPONSE PREDICTIONS 6
RIDE COMFORT EVALUATION 10
STRUCTURAL COUPLING PROBLEMS 12
ACTIVE GUST CONTROL ON FLEXIBLE AIRCRAFT 12
CONCLUSIONS 16
REFERENCES 16

GUST LOAD PREDICTION AND ALLEVIATION ON A FIGHTER AIRCRAFT
by
J. Becker
MESSERSCHMITT-BOLKOW-BLOHM GmbH.
Airplane and Helicopter Division
P.O. Box 801160, 8 Munich 80
W.-Germany
Contribution to the AGARD-Activity
"Flight of Flexible Aircraft in Turbulence"
INTRODUCTION
Turbulence induced dynamic loads and vibrations on a fighter aircraft may play a
considerable role for certain configurations with respect to the design of the structure
and the performance of special missions or they may cause on the other hand limitations
of the flight envelope.
As far as aircombat aircraft are concerned typical restrictions may be found for
instance with respect to design gust loads if wing tip mounted store configurations are
investigated or with respect to ride comfort if low level high speed missions are evalua-
ted. Here both aspects namely design gust critical cases and ride comfort are therefore
reported in more detail in order to demonstrate the problem areas. In this context also
some difficulties of turbulence response prediction methods will be described, especially
with respect to the dynamics of the aircraft with FCS and the interaction of flight '
and structural dynamics, though these .methods are well established. In addition problem
areas of the aircraft with FCS and gust alleviation system are mentioned for the case
of ride improvement. The design of such systems would strongly necessitate the full dyna-
mic description of the flexible fighter aircraft in order to evaluate maximum loads, fati-
gue loads and structural coupling problems with the control system.
CONSISTENCY WITH MILITARY REQUIREMENTS
The following problem areas may be of importance for the aircraft with PCS and for
dynamic load alleviation and ride improvement systems. During the development of a system
for the proper achievement of ride smooting, as for instance based on the requirements as
specified in MIL-F-9490 D (USAF) chapter 3.1.2.12, or for the purpose of gust load alle-
viation (MIL-A-8861 requirements) of course the controlled aircraft must be investigated
with respect to
degradation of flying qualities and handling
adverse effects on airplane strength and rigidity:
flight loads, fatigue loads, vibration, flutter and aeroservoelastic stability, be-
cause conflicts could exist between the "different specifications. In detail the be-
low mentioned aspects should be considered.
stability
destabilisation of aircraft modes or structural modes. Structural coupling problems
in flight followed by degrading effects on stiffness and on elastic mode damping or
aeroservoelastic flutter.
- Static and dynamic loads:
Change of maximum design loads and fatigue loads caused by control surface activi-
ties.
In addition all effects caused by system limitations (actuator power, control sur-
face rates and deflection) should be investigated and a system failure strategy develop-
ped.
METHODS FOR CALCULATING THE DYNAMIC RESPONSE OF FLEXIBLE AIRCRAFT IN TURBULENCE
Several methods are well established for the calculation of the flexible aircraft
in turbulence. Of course these methods have there own advantages but are not necessarily
in a stage for the solution of specific problems especially occuring if static unstable
aircraft configurations are considered or gust alleviation systems are investigated.
Two approaches will in general be used in parallel:
a) The solution of the nonlinear flight dynamic equations of motion of the aircraft
with flight control system dynamic equations together with structural dynamic
equations in time domain.
b) The solution of linearised flight dynamic equations of motion of the aircraft with
coupled structural dynamic equations in frequency and in time domain.
The calculation of the dynamic response in time domain using a) includes the simu-
lation of nonlinear effects at severe disturbance conditions, as for example aero-
dynamic nonlinearities (which are included in the static aerodynamic data set),
rate and deflection limitations of the control surfaces due to marginal actuator
power and inertia coupling at high rates.
Several disadvantages may arise in this approach if effects of the flexible air-
craft on the total aircraft response are only treated in a pure quasisteady approach
as normally used in flight dynamic simulation by the introduction of a flexibilised
steady aerodynamic data set.
Consequently this approach could lead to a wrong prediction of local dynamic res-
ponse and maximum dynamic loads.
In addition this quasisteady approach will not account for time correlated sfruc-
tural dynamic and flight dynamic responses since no aerodynamic coupling exists
between structural dynamic and flight dynamic equations. A superposition of rigid
and elastic aircraft vibrations in the right phase will not be possible. The dis-
advantages therefore could be found in not accurate predictions of maximum or
stochastic vibrations and of dynamic loads. The analytical investigation of struc-
tural coupling effects, which are of interest especially for static unstable air-
craft cannot be performed adequatly by this approach.
The dynamic response approach using linearized equations of motion of the aircraft
around trimmed condition coupled with structural dynamic equations and flight control
equations will give the possibility to introduce the unsteady and coupling effects in
a proper sense. Therefore flight dynamic and structural dynamic responses are described
'
in the right phase and the superposition of vibration and dynamic loads from both con-
tributions can be performed. In addition of course the stability of the total aircraft
can be calculated for the flight control system closed. Open loop phase and gain mar-
gins for the total system can be investigated which would give more insight into the
coupling effects of the flexible aircraft modes on the stability of the flight dynamic
modes and the structural dynamic modes by evaluation of the Bode diagram in the frequency
range 0 - fmax.(fmax is the frequency of the highest flexible mode used in the analyti-
cal model). The linearised model in advance gives information about the transfer fun-
ctions of all state variables, of local accelerations and of dynamic loads due to a gust
input. These evaluations are also of interest especially if gust alleviation systems are
studied. Besides the calculation of power spectral densities of the above mentioned vdri-
ables also the incremental responses at trimmed condition can be calculated with the
linearised model in time domain and nonlinear effects like rate and deflection limita-
tions can be evaluated as well in a first step. Disadvantages can arise through the deri-
vation of unsteady airloads at transsonic speeds or at higher incidences where some limi-
tations of the theoretical procedures could be existent. Correction methods using experi-
mental static or dynamic experimental results should be applied.
APPROACH 1:
Nonlinear flight dynamic equations in body fixed coordinates and structural dynamic
equations without coupling.
Flight dynamic equations:
X = m(~-VR+WQ)
Y = m(v-WP+UR)
Z = m(~-U+QV P)
Pf Q, R roll, pitch yaw rate
Ix' Iy' Iy moment of inertia
xy' xz' yz
? P
Normal force Z = - U2 Fc Rollmoment L = •* U2
Axial force X = — U2 Fc Pitehmoment M - — U2 c Fc
m
<p A
Side force Y = - U2 Fc Yawmoment N = ^ U2 2 sFc
n
c , c , c , c c , c = c (OC , fj, H, Ma)
x y z lf m n
The flexible aerodynamic coefficients are functions of incidence , control sur-
face deflection »•, altitude H and Machnumber Ma.
Additional structural dynamic equations:
M q (t) + g D^q/t) + M co^q (t) + g A^Ma,t) q/t) = Q (a ,Ma,t)
r r r r r o
r generalised mass of natural mode r
M 2 generalised stiffness of natural mode r
A , generalised aerodynamic forces
Q generalised gust force of mode r
q generalised coordinate of mode r
r
The deflection z at a point of the aircraft is assumed to be
B
z (x, y) S 0 <x, y) q
r
0 total aircraft natural mode r
APPROACH 2:
Linearised flight dynamic equations, control and structural dynamic equations are
here formulated in frequency domain.
The structure of the coupled equations of the aircraft dynamics shall be described
in matrix notation as shown in Fig. 1.
MRR PR' 0 0 *R 0 0 XR
+
0 D * 0 K
* XE
E
'PR'
LRR RE XR
*
LER LEE XE PE
FIG, 1 FLEXIBLE AIRCRAFT DYNAMICS COUPLED FLIGHT AND STRUCTURE DYNAMIC EQUATIONS
The following notation are used:
Degree of Freedom Rigid Aircraft
Degree of Freedom Flexible Aircraft
Matrix of Rigid Aircraft Aerodynamic Coeff.
-RR
Matrix of Generalised Aerodynamic Coeff.
-EE
-RE Coupling Terms (Lift and Moment due to Defl.)
-ER Coupling Terms of Rigid Motion to the Generalised Aerodynamics
Rigid Excitation Forces
Generalised Excitation Forces
Matrix of Generalised Stiffness
The aerodynamic terms and the degrees of freedom are complex functions in frequency
consisting of real- and imaginary parts.
The rigid aircraft aerodynamic terms may be introduced using a experimental data
set (rigid) or calculated derivatives LRR (except the drag terms) using computer pro-
grams for the calculation of unsteady aerodynamic forces (Ref. 1, 2, 3), which can be
applied also for the derivation of the LEE, LRE, L , PR and P matrices.
ER E
A more detailed structure of the longitudinal symmetric dynamics of the total air-
craft is shown in matrix notation in Fig. 2, where the first four equations describe
the flight dynamics excited by gust induced forces and moments. They include the aero-
dynamic coupling terms Zj, Mj the normal forces and pitch moments due to total aircraft
elastic modes and the control surface drag, normal force and pitch moment X , Z ,
M . The structure dynamic equations, excited by generalised gust forces Lj<*g, con-
tain also the aerodynamic coupling terms Lj^, , Lj» .