Flutter and Aeroelastic Stability

5/20/2018 Flutter and Aeroelastic Stability

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.-

I

Flutter and

Aeroelastic Stability

A Self-Study Video Course

Developed and Presented by

Gerald C. Lakin and James R. Haynes

Transport Airplane Directorate

Aircraft Certification Division

Federal Aviation Administration

Adapted from the September 16 & 17,1997, IVT Satellite Broadcast

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Table

of

Contents

GETTING STARTED

How Do I Use This Self-Study Package?

. . . . . . . .

I. AIRFRAME ENGINEERING CURRICULUM

What Does the Curriculum Cover7

. . . . . . . . . . .

Two-Week Job Function Course

. . . . . . . . . .

Overviews of Technical Subjects

. . . . . . . . . .

Core Technical Subjects Courses

. . . . . . . . .

II. SELF-STUDY VIDEO COURSE ORIENTATION

About This Self-Study Video Course

. . . . . . . . . . .

Who Is the Target Audience?

. . . . . . . . . . . . . .

Who Are the Instructors? . . . . . . . . . . . . . . . . .

What Will You Learn?

. . . . . . . . . . . . . . . . .

How WillThis Course Help You On-the-Job?

. . . . .

What Topics Does the Course Cover?

. . . . . . . . . .

What Are Some Good References?

. . . . . . . . . . .

III. SELF-ASSESSMENT

Pre- & Post-Course Self-Assessment Questions

. . . .

APPENDICES

A.

Flutter/Aeroelastic Stability Video

Presentation Visuals

B. FAR Parts for Flutter/Aeroelastic Stability

C.

Aeroelastic Instability Definitions

D. Flutter Analysis

E. Self-Study Video Course Evaluation Form

1

2

3

4

5

6

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Self-Study Video Course

Federal Aviation Administration September, 1997

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Getting Started

How Do I Use

This self-study package consists of a VHS videotape

This Self-Study (approximately 6 hours) and the accompanying self-study guide.

Package? This self-study guide provides you with the position of this

course in the Airframe Engineering Curriculum, an orientation to

the self-study video course, support materials for use with the

videotape, a self-assessment, and the course evaluation.

Follow these steps to complete your study.

1. Read Section I, Airframe Engineering Curriculum, to

familiarize yourself with the the overall scope and format of

the curriculum.

2. Read Section II, Self-Study Video Course Orientation, before

viewing the videotape to get an overview of the purpose of

the course, the target audience, the instructor(s), what you

will learn, how this course will help you on-the-job, the

topics covered in the course, and some good references.

3. Answer the pre-course self-assessment questions in Section III,

Self-Assessment.

4. Turn to Appendix A, FlutterIAeroelastic Stability Video

Presentation Visuals, and start the videotape.

5. Refer to Appendix B, FAR Parts for Flutter/Aeroelastic

Stability, as needed during the broadcast and as a reference

tool.

6. Appendices C and D are reference materials compiled by the

instructors. Appendix C provides the definitions of

instability terms. Appendix D presents information on

developing the flutter stability equation and on solving and

interpreting equation results.

7. Complete the post-course self-assessment in Section III,

Self-Assessment.

8. Complete the Self-Study Video Course Evaluation Form in

Appendix E and send it to your Directorate/Division Training

Manager (ATM).




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Airframe Engineering Curriculum

I.


What Does the

Curriculum

Cover?

The Airframe Engineering Curriculum fits into the broader AIR

Training Program that is summarized in the following figure.

An Overview

1 Job Functon I

ASI

1

ASE Systems

/ Job Functon

I

; ASE

I

I Propulsion

/ Jab Functon

Flght Tea

Job Functon

I

First Year with Aircraft Certification

Technical Tranng

DACT. OAT

1-

Continuing Development

Within the context of the AIR Training Program, the Airframe

Engineering Curriculum is designed to effectively meet the

critical safety mission of the FAA by addressing the following

Service goals:

Standardization

l Promote standardization throughout the organization in task

accomplishment and application of airworthiness

regulations in order to achieve uniform compliance.




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Job Performance Proficiencv

l Reduce significantly the time required for newly hired

engineers to attain full job performance proficiency.

Customer Service

l Establish and maintain appropriate, effective, and

responsive communication, collaboration, leadership, and

teamwork with both internal and external customers.

In addition to the Service goals, the Airframe Engineering

Curriculum is designed to provide ASEs with job function

training in three domains:

Tasks and procedures governing the work of engineers in

design approval, technical project management, certificate

management, and designee management.

FAR airworthiness requirements that are the purview of

airframe engineers. Generally they are subparts C and D of

FAR Parts 23,25,27, and 29.

Technical subjects essential for all new engineers to meet

both introductory requirements and, later, minimum

technical proficiency level requirements.

The resulting Airframe Engineering Curriculum structure

consists of three main types of training opportunities:

1. Two-Week Job Function Course

2. Overviews of Technical Subjects

3. Follow-on Core Technical Subjects Courses.

Two-Week Job The Two-Week Job Function Course uses an instructor-led,

Function

classroom-based format with lecture, discussion, and individual

Course

and group activities. Supporting materials used in the course

include print, overhead transparencies, videotapes, job aids,

and documents and sample reports.




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The course is divided into the following two major sections:

Week 1

l

Certification Tasks - includes design approval, technical

project management, certification management, and DER

management.

2eek

l FAR Requirements and Key FAR Sections - includes

training in the subparts of the FAR that apply to airframe

engineers (subparts C and D) at two levels: an overview of

those subparts across FARs 23,25,27, and 29; and in-depth

discussion of significant sections of the FAR that are

important to the Service. The importance of these sections

may stem from problems in interpretation and application of

requirements, technical complexity of a design, high

visibility projects, or safety considerations that are

paramount.

Overviews of

Technical

Subjects

High-level overviews of ten technical subjects are presented by

NRSs or other senior engineers. These overviews will be

available in two modes:

l An initial live three- to four-hour IVT satellite broadcast

with accompanying course material will be received at each

Directorate and other downlink sites.

l A Video/Self-Study Training Package adapted from the

initial IVT presentation and accompanying course material

,

will be available through the Directorate Training Manager.

Basic concepts and FAA-specific applications and examples

are provided for each of the following ten technical subjects:

l

Aircraft Loads

l Fatigue/Fracture Mechanics/Damage Tolerance

l Composite Materials




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Aif$iiame Enginizering Curriculum

l

Crashworthiness/Occupant Protection

l Material Properties/Manufacturing Processes of Metal

l Stress Analysis

l Flutter/Aeroelastic Stability

l Structural Test Methods

l Design and Construction

l

Repairs and Modifications.

Each technical subject overview is designed to not only

provide ASEs with the FAA perspective on the topic, but also

serve as an indicator of what further training may be needed.

Core Technical

As a follow-on to the Overviews of Technical Subjects, the

Subjects

curriculum will provide more in-depth training on the

Courses

following three subject areas:

l Basic Loads

l

Stress Analysis and Structural Test Methods

l Repairs and Modifications.

These core technical subjects are essential to the technical work

of the airframe engineer in a regulatory environment regardless

of product or technology. Training in each of the core subjects

will be designed to bring airframe engineers to a minimum

level of technical proficiency and to help promote proficiency

in the application of the technical knowledge in an office work

environment.

Additional technical training for engineers beyond these core

subjects will depend largely on AC0 organizational needs

stemming from customer requirements, products certified,

emerging technology, and the number of staff requiring more

specialized training. In short, the more advanced the technical

training required, the more individualized it becomes.



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Self-Study Video Course Orientation

II.


About This

Self-Study

Video Course

Flutter and Aeroelastic Stability is one in a series of ten

Overviews of Technical Topics in the Airframe Engineering

Curriculum designed to prepare you to meet the critical safety

mission of the FAA. [For more information on the Airframe

Curriculum, refer back to Section I of this guide.]

This self-study video course is based on the original eight-hour

Interactive Video Teletraining (IVT) broadcast on September

16 and 17, 1997. The broadest was televised from the IVT

studio at the FAA Academy in Oklahoma City to various sites

around the country. James R. Haynes and Gerald C. Lakin,

senior engineers in the Transport Airplane Directorate

Standards Staff, are the instructors. They will provide you with

basic aircraft flutter concepts, criteria, regulations, and

examples. During the broadcast, participants were able to

communicate with the instructor either through a microphone

and/or simple-to-use Viewer Response System keypads.

Appendix A contains the actual visual support material used

by the instructors during the broadcast. You can use these

visuals to follow along with the videotape and record notes

directly on the pages.

Appendix B is a listing of relevant FAR Parts.

Appendices C (Aeroelasiic Instability Definitions) and D

(Flutter Analysis) is information pulled together by the

instructors as additional reference materials.

Appendix E contains the Self-Study Video Course

Evaluation Form. Please fill out this form after completing

this course and send the form to your Directorate/Division

Training Manager (ATM).




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Who Is the

Target

Audience?

This course is designed for:

l New and experienced FAA airframe engineers who are not

proficient or expert in flutter/aeroelastic stability, related

FAA regulations, and/or the application of this knowledge

to ensuring aircraft safety.

l Others with interest, such as multidisciplinary engineers or

flight test pilots.

Who Are the

Instructors?

James (Jim) R. Haynes received a BS in Aeronautical

Engineering( 1967) and an MS in Systems Engineering( 1973),

and has performed additional post-graduate studies in

aeroelasticity and flutter at Renssalaer Polytechnic and the

University of Oklahoma. He is a Registered Professional

(Mechanical) Engineer. He has 28 years of experience as an

aeronautical engineer and has worked as an FAA airframe

engineer for the last 26 years.

Jim Haynes

Jim was an FAA Academy instructor from 1974-77 where he

taught certification and engineering courses, including the

Airplane Flutter and Aircraft Vibration courses. He has been

the principle technical specialist for flutter for the FAA

Transport Airplane Directorate since 1980. He was responsible

for the development of Advisory Circular 25.629- 1, Flutter

Substantiation of Transport Category Airplanes and the

principle author of the current version of FAR section 25.629,

Aeroelastic Stability Requirements. Jim is the FAA

representative of the Aviation Rulemaking Advisory

Committee (ARAC) working group for the development of

regulations and advisory material concerning structures, loads,

and flutter.




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S;e& hdy Video Course Orientation

Gerald (Gerry) C. Lakin received a BS (1961) and MS (1965)

in Engineering Mechanics and did post-graduate work in

Aeronautics and Astronautics (1966 and 1970). He is a

Registered Professional Engineer and Senior Member of

AIAA. He worked as an engineer and manager in structural

dynamics (primarily flutter and vibration) for 36 years,

including 30 years at the Boeing Company where he was

involved in many commercial, military and space programs.

He was an FAA DER at Boeing and consulted as an

independent DER in the general aviation industry. He has

published papers in the AIAA and ASME technical journals

and taught the introductory course in Flutter and Vibration at

Boeing.

Gerry joined the FAA in 1995 as a Project Officer in the

Transport Airplane Directorate Standards Staff where he shares

Part 25 oversight responsibilities for the Los Angeles and

Atlanta ACOs. Some of the projects he is associated with are

the civil certification of the McDonnell Douglas C- 17 and

Blended Wing Body airplanes, and the China Bilateral

Airworthiness Assessment. He has provided training on

certification projects and consulted on continued airworthiness

issues involving flutter and vibration.



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Se f-Study Video Course Orientation

What Will You

Learn?

After completing this course, you will have a basic

understanding of the concepts and principles of Flutter and

Aeroelastic Suitability, including:

What aeroelastic instabilities are and how they differ from

forced vibration response. In addition you will be able to

visualize common aircraft flutter modes.

Regulatory requirements, advisory material and policy for

small and transport category airplanes.

Analytical and experimental means of demonstrating

compliance.

Factors that influence aeroelastic stability and detail design

considerations.

Further reading material and studies, including analytical

tools and additional training.

How Will This

Course Help

You On-the- .

Job?

After completing this course, you should be able to:

l

0

Understand and recognize the characteristics of the different

types of aeroelastic instabilities.

List the FAR requirements, guidance, and policy that deal

with aeroelastic stability.

Review applicants flutter certification plan and understand

the general approach to demonstrate compliance.

Describe parameters that can effect aeroelastic stability and

relate this to detail design features and design changes.

Understand that a great deal of education, training, and

experience are necessary to make journeyman findings of

compliance.

0

0

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Se&-Sttidjl Video Course Orientation

What Topics

The following topic outline is intended to give you an overview

Does the

of the course content.

Course Cover?

I

II.

III.

IV

tntroduction

Background and Review

A. Basic definitions

1. What is and isnt instability

2. Instability vs. forced vibration

3. Definition of flutter

B. Vibration modes

1. Single degree of freedom

2. Two degrees of freedom

3. Multi-degrees (continuous) of freedom

4. Free-free vibration modes

5. Coupled modes

6. Full airplane structural dynamic model

C. The mechanism of classical flutter

The Aeroelastic Stability Requirements

A. Transport airplanes

B. Small airplanes

Demonstration of Compliance

A. General

1. Methods of compliance

2. Compliance philosophy

3. Flutter certification plan



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B. Flutter analyses

1. Flutter stability equation

2. Solving

3. Interpreting results

4. Analytical investigations

5. Reporting the results

C. Ground vibration tests

1. Objectives

2. Test plan and conformity

3. Test article and support system

4. Test results

5. Other ground tests

D. Wind tunnel tests

1. Purposes

2. Dynamic similarity

3. Instrumentation and testing techniques

4. Tunnel investigations and parametric variations

5. Flutter model video

E. Flight Flutter Tests

1. Purpose

2. Test article (configurations and conformity)

3. Instrumentation and testing techniques

4. Flight investigations and test conditions

5. How results are used in the compliance program



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V.

What Effects Aeroelastic Stability

A. Changes, modifications and repairs

B. Assessing changes

VI. Flutter Prevention

A. Detail design and service information

VII. Further Study, Analytical Tools, and Training

A. Main referenced texts

B. Standard analytical tools

C. Recommended training courses

D. Recommended training activities

VIII. Summary

What Are

Some Good

References?

The following are good published references for

flutter/aeroelastic stability information:

Garrick, I.E. and Reed, W.H. III,,Historical Development of

Flutter, ALU-8 l-059 1 CP, 198 1.

Theodorsen, T.; General Theory of Aerodynamic Instability

and the Mechanism of Flutter, NACA Report 496, 193 5.

Theodorsen, T. and Garrick, I.E.; Mechanism ofFlutter -- A

Theoretical and Experimental Investigation of the

Flutter Problem, NACA Report 685, 1940.

Theodorsen, T. and Garrick, I.E.; Nonstationary Flow about a

Wing-Aileron-Tab Combination Including Aerodynamic

Balance, NACA Report 736, 1942.



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S&f-Sttidy Video Course Orientation

Theodorsen, T. and Garrick, I.E.; Flutter Calculations in Three

Degrees of Freedom, NACA Report 74 1, 1942.

Fung, Y.C.; Fung, Y.C.; An Introduction to the Theory of

Aeroelasticity, Dover Publications, Inc. 1969.

Scanlan, R.H. and Rosenbaum, R; Introduction to the Study of

Aircraft Vibration and Flutter, The MacMillan Co.,

1962.

Bisplinhoff, R.L. and Ashley, H.; Principles of Aeroelasticity,

Dover Publications, Inc., 1962.

Langley Research Center, Flutter Testing Techniques, NASA

SP-415, October 9-10, 1976.

Pope, Alan and Rae, William, Low Speed Wind Tunnel Testing,

Wiley-Interscience, Inc., 1984.

Advisory Group For Aerospace Research and Development

(AGARD); Manual on Aeroelasticity, October 1968.




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Self-Assessment

III. Self-Assessment

Pre- & Post-

Course Self-

Assessment

Questions

During the IVT broadcast, the instructor asked participants to

respond at the beginning and end of the presentation to the

following questions about flutter certification and analysis.

Rate your level of understanding for each of the following

statements before and after completing the course.

1. I understand what constitutes an aeroelastic instability.

Very

Moderately Not

Cmfid t

Confident Confidenl

BEFORE THE COURSE:

clen cl 0

AFTER THE COURSE: 0

0 Cl

2. I understand the regulatory requirements related to aircraft

flutter.

Very

Moderately Not

Confident Confident Confident

BEFORE THE COURSE: cl Cl 0

AFTER THE COURSE: Cl cl Cl

3. I understand the analytical and experimental means of

compliance for flutter requirements.

Very Moderately Not

Confident Confident Confident

BEFORE THE COURSE: 0 Cl cl

AFTER THE COURSE: cl cl cl


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Appendix A


Video Presentation Visuals

Appendix A



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Appendix A

What you will learn

l What aeroelastic instabilities are

l

Regulatory requirements

l Acceptable means of compliance

l

What affects flutter and flutter

prevention

l Avenues of further study and

development

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Appendix A

Why aeroelastic stability

has been (and is) important

l Flutter can be catastrophic and it can

occur without warning.

l

It is a historic as well as current

problein:

- It was responsible for one of the first

catastrophic accidents in aviation history.

- As recently as 1991, a large transport

airplane was lost due to flutter with 50

fatalities (Accident Board finding).

3

Why aeroelastic stability is

becoming more important

l Airplanes are larger, more flexible.

l Airplanes use advanced flight control

systems that couple directly with the

structural modes of the airplane.

l There .are an increasing number of major

modifications being accomplished that

have a significant affect on aeroelastic

stability.

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Appendix A

Affect on design

l Aeroelastic stability .concerns dictate

the design of a significant amount of

structure on a transport airplane:

- Location of nacelles

-Wing box stiffness

- Vertical fin stiffness

- Pylon stiffness and engine mount fail-safe

structure

- Control surface rigidity, geometry, and

inertial properties.

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Appendix A

What is and isnt instability

Instabilities

Not Instabilities

l

Flutter

l Forced Vibration

l

Whirl Flutter

- Oscillatory failures

l Divergence

- Prop wash excitation

l Control Reversal

l

Buffeting

l

Deformation

- Flow separation

instability

- Mach buffet

l

Aileron buzz??

7

Instability vs. forced vibration

l Forced vibration: A system that has

an external forcing function that is

independent of the motion of the

system.

l Stability phenomena are notforced

vibration because the forcing

function is dependent on the motion

of the system.




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Appendix A

Flutter

An unstable, self-excited structural

oscillation at a definite frequency

where energy is extracted from the

airstream by the motion of the

structure.

The deformation and motion of the

structure result in forces on the

structure that tend to

maintain or

augment the motion.




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Appendix A

Session overview

l Single degree of freedom

l

Two degrees of freedom

l Multi-degree (continuous)

l ,Free-free vibration modes

II

l

Coupled modes

l Full airplane structural dynamic

model

Single degree of freedom

l

Fa=Ma

l -Kx=Ma

K

harmonic motion

a

= -(j 4

l but a = -02x

l -to( =

mo2Mx

. [WM-02]x=0

This is the basic form of the

stability equation no matter

how complex the system.

12



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Appendix A

Two degrees of freedom

. h

= M,a, and

F*=M2a2 ')

. IFI = MW

. -wl00=w1w

. -02[M]{x}+[K]{x}=0

. [M-K- w21]{x}={O}

l Two frequencies and two mode

This is a two deareehapes.

of freedom system.

13

Continuous structures

l Inertial forces

proportional to

displacement

l Infinite degrees of

freedom

l Modeled with

lumped masses

and massless

connecting beams

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Appendix A

Free-free modes

l All inertial forces

and moments

balanced

l SpmeMc and

antbsymmettk

modes if there is a

plane of symmetry

l Othenrvise;

asymmettfc mocks

15

Coupled modes

Elastic axis\

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Appendix A

Structural dynamic model

Lumped mass beam model (&tick model)

is commonly used, even for complex

airplanes.

I7

Summary

l Single degree of freedom

l Two degrees of freedom

l Multi-degree (continuous)

l

Free-free vibration modes

l Coupled modes

l

Full airplane structural dynamic

model



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Appendix A

19

Session overview

l A control surface mechanism

l A main surface mechanism

l

Other mechanisms involving

main and control surfaces

20



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Appendix A

Wing aeroelastic

displacements

X

elastic axis

.

inertia

$3XS

b

2--

Y

z

21

Wing bending and control

surface rotation

22



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Appendix A

Flutter


oscillation at 8 deMte frequency

where energy is extracted from the


structure.

Wing bending and

control surface rotation

24

o,,~= K,/ M, bending frequency

op2= K, / I,, control surface frequency




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Appendix A

Aerodynamic stiffness

Structural mode frequency

versus airspeed

frequenc$

Oh bending

mode

mode

I

0 airspeed, U

26



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Appendix A

Flutter


oscillation at a definite frequency

where energy4s extracted from the


structure.

The deformation and motion of the

structure result in fibrces on the

structure that tend to maintain or

augment the motion.

27

Wing tip deflections

along the flight path

Work done = L, h (stabilizing)




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Appendix A

Wing tip deflections

along the flight path

Work done = L, h (destabilizing)

Aeroelastic stability

g = (1/P)~og(X,/X*)

Unstable

30




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Appendix A

Unbalanced control surface

damping

g

0

gs

stable

Partially balanced

control surface

rnz cg towards

hinge line

32



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Appendix A

Partially balanced

control surface

unstable

Mass balanced control surface

34



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Appendix A

Balanced control surface

damping

g 0

gs

unstable

stable

airspeed

35

Control surface frequency

surface

mode

0

airspeed, U

36




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Appendix A

Wing bending and torsion

a;

m,*=K,/M, bending frequency

oa2=Ka/la, torsion frequency

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Appendix A

Aerodynamic stiffness

59

Structural mode frequency

versus airspeed

frequency

bending

speed, U

40



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Appendix A


airspeed

Mass balanced wing

Ma

42




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Appendix A

Mass balanced wing

damping

unstable

go

airspeed

stable

(ls-*m

43

Wing torsion frequency

increased

%x1

%

44

speed, U




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Appendix A

Wing mounted engine

reduced torsion frequency - unfavorable

cg moved forward - favorable

45

Mass matrix

w s,. sp sg

bending

% Ia pap pai5

torsion

% PaP IP pPs

control

surface

% pa6 Pps Is

tab

46




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Appendix A

Session summary

l A control surface mechanism

l A main surface mechanism

l Other mechanisms involving

main and control surfaces

47


Requirements

Part 25

Transport

Airplanes

4ll



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Appendix A

Session overview

l Principle references

l Instability phenom.ena

l Means of compliance

l Flutter clearance envelopes

- Normal envelope and normal conditions

- Failure envelope and failure conditions

l Mass balance criteria

l Flight flutter test requirement

49

Principle references

l FAR 25.629 Aeroelastic stability

requirements

l Advisory Circular AC256294

Flutter Substantiation of Transport

Category Airplanes

l Military Specification, Mil-A-8870

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Appendix A

Instability phenomena

l Flutter

l

Divergence

l Control Reversal

l Loss of stability and control due to

structural deformation

l Whirl flutter

Means of compliance

. Aeroelastic analysis,

l Wind tunnel tests,

l Ground vibration tests,

l Flight flutter tests, or

l

Any other tests found necessary

by the administrator.

52



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Appendix A

Compliance philosophy

GVT

from

53

l

instability

4

Stability envelopes

l Envelope for normal configurations

and conditions.

l Envelope for failures, malfunctions

and adverse conditions.

s4



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Appendix A

Normal envelope

55

Mach Number

Normal conditions

l Fuel and payload capacities and

distributions.

l For airplanes approved for icing:

ice masses under normal flight in

icing conditions.

l Pressurized and unpressurized.

l

Any other configuration or condition

approved for operation. (MMEL)

56



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Appendix A

Failure envelope

Mach Number

Failure envelope (floor)

D

8

E

1.15 v,

.I

a

E

I

9

I

M,+0.05

s

W

Mach Number

58




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Appendix A

Failure envelope (floor)

Maximum Alt

ii

5

intersection

I

V,i

1.15

II

I

vc

Airspeed (EA;)

39

Composite failure envelope

60

Equivalent Airspeed



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Appendix A

Failure conditions

l Damage or failure conditions considered

under 25.671, 25.672 or 25.1309.

- Hinge failures, tab controls, actuator

failures and combinations of control

system failures.

l Single failures of independent flutter

damper systems.

l

Damage tolerance, discrete source, and

bird strikes.

61

Failure conditions

l

Single failures in supports of large mass

items (engines, external fuel tanks, etc.).

l Single failures of supports of large

rotating masses (propellers and

turbofans).

l Mismanagement of fuel (and zero fuel).

l Inadvertent encounters with icing.

l

Any other combination of failures not

shown to be extremely improbable.

62



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Appendix A

Concentrated mass balance

* Rigidity: 2 times the frequency of the

mode likely to flutter (MU-A-8870).

l Limit load factors (per AC 25.629-l):

- 1009 normal to the plane of the surface

- 30g parallel to the hinge line

- 30g in the plane of the surface

perpendicular to the hinge line.

l Adequate for repair, re-painting, and

the accumulation of ice and water.

63

Flight flutter tests

l Flight flutter tests to V,, mandatory for

all new transports (Amendment 25-77).

l

Flight flutter tests to V,, mandatory for

changes that have a significant effect on

aeroelastic modes (Amendment 25-77).

l Prior to Amendment 25-77 flight flutter

tests were mandatory if the Mach

number exceeded 0.8.

I

64




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Appendix A

Session summary


l Instability phenomena



- Normal envelope and normal

conditions

- Failure envelope and failure conditions

l Mass balance criteria

l Flight flutter test requirement

65


Requirements

Part 23

Small Airplanes



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Appendix A

Session overview

l


l What is covered by the rule


l

Simplified criteria

67


l FAR 23.629 Flutter

l FAR 23.659 Mass balance

l Advisory Circular 23.629-IA Means

of compliance with section 23.629,

Flutter

l FAA Report 45 Simplified Flutter

Prevention Criteria

l NACA Technical Report 685

68



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Appendix A

Session overview

l

Main references



l Simplified criteria

69

Categories of airplanes

l General small airplanes

- Less than 260 knots and less than

Mach 0.5.

- Fixed stabilizers and no T or

boom tails.

- No unusual mass distributions.

70

l Special small airplanes

- Airplanes not meeting the General

definition.




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Appendix A

Small airplane instabilities

l

Flutter

l Divergence

l Control Reversal

l

Whirl flutter (turbopropeller

airplanes)

71

Substantiation envelope

l

Must be free from instability within

the design V-n envelope at speeds

up to l.ZV, if shown by analysis.

l If other methods are used, then the

speed depends on the method.

l Adequate tolerances must be

established for speed, damping,

mass balance, and control system

stiffness throughout the envelope.

72



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Appendix A

Fail-safe requirements

l

For General airplanes: single failures

in tab systems.

l

For Special airplanes: single failures

in primary flight control systems, flutter

dampers, or tab systems.

l

For any airplane electing the fail-safe or

damage tolerance approach: Freedom

from flutter after the failure condition.

13

Session overview

l

Main references

l

What is covered by the rule


l

Simplified criteria

74



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Appendix A

Means of compliance

l Rational analysis

l Analysis and flight tests

l Ground vibration tests

l Simplified Criteria (Report 45)

2D analyses

l Two dimensional analysis is

sometimes acceptable for small

conventional airplanes with large

aspect ratio. Ref - AC 23.629-IA,

Appendix 2.

l NACA Report 685 contains a very

useful collection of parametric charts

that are based on 2-D flutter solutions.

76



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Appendix A

3D analyses

l Current methods are full airplane 3D

modal analyses.

l The traditional approach described

in AC 23.629~IA, Appendix 2, will be

described in more detail later.

Whirlmode analysis

l Whirlmode substantiation for

turbopropeller airplanes (both single

and multiple engines).

l Must include stiffness and damping

variations (deteriorated or failed

engine mounts).

l Appendix 2 of AC 23.629~IA contains

a detailed explanation of the analytical

method for whirlmode analysis.

18



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Appendix A

Analysis plus flight tests

l Although 5 23.629 seems to allow

flight tests alone as substantiation,

this is not normally practical.

l Critical configurations and critical

flight conditions must be established

by analysis before flight testing.

l

Special category airplanes require fail

safe substantiation which is difficult to

do by flight.

79

Ground vibration tests

l Part 23.629 requires that the natural

frequencies be determined by tests,

irrespective of the method of

substantiation.

l Necessary for use in simplified criteria.

l Necessary for validation of analyses.

l In some cases, mav help determine

critical configurations for flight testing.




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Appendix A

Session overview

l

Main references



l

Simplified criteria

Report 45 criteria

l

Wing torsional stiffness

l Aileron balance and free play

l Elevator & rudder mass balance

l

Tab balance, free play, and rigidity

l

Balance weight attachments -

stiffness and strength

82



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Appendix A

Report 45 limitations

l Simplified criteria is limited to use

only with airplanes meeting the

General definition.

l The wing and aileron criteria of

Report 45 are more restricted.

- No large mass distributions on the

wing.

83

Wing torsional stiffness

Report45



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Appendix A

Aileron balance criterion

Report 45

.I

E

E

ts

IS

V, = Design dive speed, mph

Parallel axes criterion

+ Report 45

66

Flutter speed factor vf = 3




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Appendix A

Perpendicular axes criterion

x

Report 45

1.5 times the

highest frequency with which the

control surface could couple in

flutter.

l Limit load factors; 24g normal to the

surface and 12g in the other two

directions. (Same as Part 23 and AC

23.629-l A.)

89

Session summary


l

What is covered by the rule


l Simplified criteria

90



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Appendix A

onstration

of

Compliance

91

Means of compliance

l Analyses

l Ground vibration tests (GVT)

l Wind tunnel modeltests

l Flight flutter tests (FFT)

l

Any other tests found necessary

92



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Appendix A

Compliance philosophy

93

Flutter certification plan

l Description of the airplane project.

l Means of showing compliance.

l Schedule of accomplishment.

l Data approval and test witnessing

responsibilities.

94




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Appendix A

95

Flutter analyses overview

l Flutter stability equation

l Solving

l Interpreting results

l Analytical investigations

l

Reporting the results

96



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----. ._

Appendix A

The mathematical model

WI (;a + WI 1x1 = (FI

Assume that the vibration is

constant amplitude and sinusoidal.

{ii) =

- a2 {x}

- a2 VW @I + WI w = m

91

Structural dynamic model

52

Lumped mass beam model (stick model)

is commonly used, even for complex

airplanes.

98



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Appendix A

Selecting structural modes

First solve the structural model

without any aerodynamic forces.

-a2 [Ml {XI + WI 1x1 = uu

Select a set of these frequencies

and mode shapes (usually the first

20-30 modes or up to 3540 Hz).

99

Superposition of modes

The flutter modes in air (at an airspeed)

are considered to consist of a linear

superposition of the selected structural

modes.

LOO



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Appendix A

Generalized coordinates

The new unknown variables, q,

which are the piopottions of each

structural mode contained in the

flutter mode, are called the

g

coordinates.neralized

101

The generalized equation

Multiply through on the left by the

transpose of the matrix [F]

[4dTCMI[~I Ii

neralized mass

CM

[#lTIM] [+] = generalized stiffness = E

-02iiF{q} + K{q) = [Q][F]

102



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Appendix A

103

Structural damping

Structural damping is

proportional to the spring

force but 90 degrees out of

phase.

Mii + igKx + Kx = 0

Mii + (7 + ig)Kx = 0

Mii + K,x = 0 where K, is complex

0

0

0

104

Handling damping

Structural damping can be ignored for

now and a global structural damping

considered when plotting the results.

(AC23.629~IA or Method 1 of AC25.629~1)

or.....

The structural damping measured for

each mode in the ground vibration test

can be included directly in the generalized

stiffness matrix. (Method 2, AC25.629~1).

In the later case, the stiffness matrix is

corn plex.



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Appendix A

Handling damping

l

Structural damping can be ignored for now

and a global structural damping considered

when plotting the results. (AC23.629~IA or

Method 1 of AC25.629~1) or.....

l

The structural damping measured for each

mode in the ground vibration test can be

included directly in the generalized stiffness

matrix. (Method 2, AC25.629~1).

l

In the later case the stiffness matrix is

complex.

105

Unsteady aerodynamics

Oscillation at frequency, a

Aerodynamic force coefficients depend on

the reduced frequency, k

106

k= -

V




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Appendix A

Aerodynamic model

107

Aerodynamic model

Doublet Lattice




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Appendix A

Aerodynamic forces

AK = Aerodynamic Influence Coefficients

F = -02Q[AIC] {x} or -02Q[AIC] [$] {

q}

so:

MTF = -02Q [MT WC1 Ml 191

[91T[AICl @I= A

A = generalized aerodynamic matrix

109

Flutter stability equation

--a2~ljj +jr{& = --a2m{@

[+D2Mm2QA+fl@=0

110



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Appendix A



l Solving



l


III

Solution difficulties

l Equation is inconsistent since it is

only solvable for neutral stability,

yet it has damping (aero and maybe

structural).

l AIC depends on reduced frequency,

k, and k depends on the flutter

frequency which is unknown.



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Appendix A

Artificial damping

Add some extra artificial structural

damping. Artificial damping forces are:

The main idea is that we can solve for

the artificial damping that results in

neutral stability, then the total system

damping must be the negative of this

result.

-0 H{qj +

(1

+ igg{q} = a Qz{q)

113

Final flutter equation

This is the standard eigenvalue form:

P-W~q~=~O~

where:

h=(l +ig)/02

II4


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Appendix A

Solving the flutter equation

l For the desired altitude, choose a value

of k.

l

Solve for hs using an extraction

algorithm.

l

From h (complex number) get g (artificial

damping) and o for each mode.

l

Then from the assumed k=boN, find V

l Plot o and g against V for each mode.

l

Choose another k and repeat.

115


l

Flutter stability equation

l Solving



l


116



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Appendix A

v-g & v-tD plots

],I

I17

Damping criteria

(structural damping included)

I

I

I

I

D

1.15,

(I l

vD)




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Appendix A

v-g & v-cl, plots

(structural damping not included)

I ~~--

gzo.03

I

I

F

E

D 1m5vD

E

(I g2v,)

d

119

,

Damping criteria

(structural damping not included)

Iii,--

I

g=0.03

F

E

D

1115

E (I .2v,q

G

120



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Appendix A



l Solving



l Reporting the results

121

Analytical investigations

Flutter stability can be very

sensitive to small changes is

certain parameters.

Parametric investigations are

necessary to understand the

sensitivity of these parameters and

ensure adequate margin for

stability.

122



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Appendix A


l Normal conditions

- Conditions within the flutter

clearance envelope

- Parametric investigations of

parameters about their nominal

values.

l

Failure conditions

- Specific failure configurations

- Parametric investigations to

envelope failure conditions.

123

Parametric investigations

l Fuselage rigidity (pressurized and

unpressurized).

l Wing & fuselage rigidity near limit

load.

l Nacelle frequency and aerodynamic

force variations.

l Stabilizer pitch stiffness variations.

124



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Appendix A


l Control surface inertial properties

(static balance, dynamic balance) to

establish the service limitations.

l Control surface aerodynamic

derivatives (normal practice is 75% to

125% nominal).

l

Automatic flight controls variations

- Double gains (+6 decibels)

- Phase variation of +/- 60 degrees.

125


l

Control surface rotation stiffness

variations

- For powered systems, hydraulic

failures, actuator deterioration,

disconnects, hinge failures, etc.

- For conventional mechanical

controls, all frequencies from 0 up

to 1.5 times the fixed stick

frequency should be investigated as

normal conditions.

126




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Appendix A



l Solving


l


l


127

Analytical investigations report

l Airplane general arrangement


l Weights, fuel and payload distributions

l

Discussion of aerodynamic (math) model

l Discussion of structural (math) model

l

Validation of integrated model



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Appendix A

Analytical investigations report

129

l Discussion of important modes

and frequencies

l V-g & V-o plots for nominal

configurations

l Parametric plots for important

parameter investigations

l V-g & V-O plots for significant

failure conditions.

Flutter analyses summary


0, Solving

l

Interpreting results

l


l


130



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Appendix A


l Objectives

l Test plan and conformity

l Test article and support system

l

Test results

l

Other ground tests

132




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Appendix A

Test objectives

l Define vibration mode frequencies, mode

shapes and damping of aircraft.

l Validate mathematical model.

l Use data directly in simplified flutter

criteria.

l Evaluate modifications to a certified

design.

l Study characteristics such as nonlinearity.

133


l 0 bjectives

l

Test plan and conformity

l

Test article and support system

l Test results

l

Other ground tests

I34



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Appendix A

Test

plan and conformity

l FAA approved test plan.

l AC0 engineer should witness as much of

test as possible.

l Test conducted under supervision of DER.

l

Test article must be in conformity

- Structurally complete

- Weight and mass distribution, not details.

l Equipment and setup usually not

conformed.

13s


l 0 bjectives


l

Test article and support system

l

Test results

l

Other ground tests

136



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Appendix A

Test support system

l

Rigid body frequencies on support

system l/2 or less of lowest elastic

mode frequency.

l Common methods of support

- Deflated tires with blocked struts

- Suspend from springs, e.g. bungee

cords,

- Soft support such as air bags.

137

Test article

l Airplane configuration according to

FAA-approved test plan.

l Include all equipment of mass.

l Control surfaces

- Faired to primary surface

- Fixed or free

- Power on.

l Investigate flaps down configuration.

138



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Appendix A

Test equipment

l Electromagnetic exciter most common

- Wide frequency range

- Force applied through drive rod

- Harmonic, transient or iandom force input

- Force or amplitude feedback control

l Accelerometers used to measure amplitude

l Data system

- Time history recorder

139

- Frequency domain analysis

Test procedures

l Resonance dwell and damping

decay

- Hand shake

- Sinusoidal exciting force with

shaker

l

Modes identified from frequency sweep

l

Roving and/or distributed accelerometers

map out mode shapes and node lines

140



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Appendix A

Resonance dwell and decay

141

Test procedures

l

Resonance dwell and damping decay

l


- Force away from node line - at extremity

- More than one shaker/position may

,be used

- More than one reference acceleration

may be used

l Roving and/or distributed accelerometers map

out mode shapes and node lines

I42



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Appendix A

Vibration frequency

sweep

mode 1

reference

amplitude

frequency, f, Hz

143

lest procedures

l

Resonance dwell and damping decay

l


l Roving and/or distributed

accelerometers map out mode

shapes and node lines

144




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Appendix A

Wing vibration mode shapes

I

.

n h\ n

u u \

V

bending

node line

Ground vibration test video




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Appendix A


l

0 bjectives



l Test results

l Other ground tests

I41

Modes normally encountered

l

Wing vertical and chordwise bending, torsion

l

Fuselage torsion, stab antisymm bending

l

Fuselage side bending, fin bending, torsion

l Fuselage vet-t bending, stab symm bending

l Stab pitch, yaw and torsion

l

Nacelle, external store modes

l

Control surface and system modes

14a



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.

Appendix A

Use of data

to show compliance

l DER recommend approval

l Validation of vibration modes

l

Directly with Report 45 criteria

l Directly in flutter analyses

l Demonstrate before and after

modification (always accompanied

with analyses)

149


l Objectives



l Test results

l Other ground tests

150



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Appendix A

Other types of ground tests

l

Control surface and tab mass

properties

l

Control surface and tab freeplay

l Control system stiffness &

damping test

l Stiffness test .

l Other required by FAA

\

151

Control surface mass

.

&e-I B_ f

I

CG: I =(R/W)xL

I, = wLQ/4p2fzL)-l]

Details given in Report 45

152



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Appendix A

Control surface and tab

freeplay

ent

Is3

GVT Summary

l Objectives

l

Test plan and conformity

y Test article and support system

l Test results

l

Other ground tests

IS4



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Appendix A

win

dT

unnel ests

155

Wind tunnel tests

l

Purposes

l Dynamic similarity

l Instrumentation and testing

techniques

l Tunnel investigations and

f parametric variations

9 Flutter model video



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Appendix A

Validation of flutter analyses

l Validate (or calibrate) analysis for a few

configurations - compliance made by

computer analysis.

l Confirm flutter speed boundary.

l Visual study of flutter modes - video.

l Compressibility effects.

l Damping trends with airspeed (flight test).

IS7

Support analyses

in showing compliance

l Determine flutter margins

l Full span, asymmetric

configurations such as failures

and adverse conditions

158



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Appendix A

Complex configurations 88

special effects

l Test configurations difficult to

analyze, e.g. control surfaces,

t-tails and whirl flutter

l Investigations, e.g. effect of angle

of attack on flutter speed

159

Wind tunnel, tests

l Purposes

l

Dynamic similarity


techniques


parametric variations

l

Flutter model video

160



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Appendiw A

Model scale factors

l Length (L,IL,)

- Tunnel width / airplane wing span

l Velocity (V,/ V,)

- Tunnel speed / airplane design speed, or

- Mm/Ma

= 1 for high speed testing

l Density (Pm/Pa)

- Wind tunnel / airplane altitude at critical

161

speed and Mach No.

Dynamic similarity

l

For similar aerodynamic loading,

inertia, elastic, viscous and

gravitational forces must act in same

relationship in the tunnel...

0

. ..these quantities would have to be

matched between airplane and model:

Mach number

Reynolds number

Froude number.

162



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Appendix A

163

Wind tunnel tests

l

Purpose

l

Dynamic similarity


techniques



l Flutter model video

Instrumentation and

testing techniques

l Model flown similar to flight flutter test

l Flutter speed approached slowly

l Model excited by various means

l

Damping and frequency of various modes

tracked using model instrumentation

l

Flutter speeds are often reached,

sometimes leading to catastrophic

resu Its

I64



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Appendix A

165

Wind tunnel tests

l Purpose

l Dynamic similarity .


techniques

l

Tunnel investigations and



Wing flutter speed vs fuel

Flutter

Speed

\A

166

I I I I I

0

25 50 75 100

Wing Fuel



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Appendix A

Wing flutter speed vs

nacelle strut stiffness

Flutter

Speed

short body

Jccl

long body

nominal

167

1

I I I

I

0.8 0.9 1.0 1.1 1.2

Vertical Frequency Ratio

T-tail flutter speed.vs

anhedral angle

;. OEW

Flutter

Speed

Full, aft cg

f

I I

I I I

0 0.5 1.0 1.5 ?.O

166

An hedral Angle Ratio




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Appendix A

169

Wind tunnel tests

l Purpose

l Dynamic similarity


techniques

l




Wind tunnel test video

170



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Appendix A

Wind tunnel tests summary

171

l Purposes

l

Dynamic similarity


techniques

l




I72



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Appendix A


173

l Purpose

l Test Article (configurations and

conformity)


techniques

l

Flight investigations and test

conditions

l How results are used for compliance


l

Purpose

174



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Appendix A

Purpose

l Demonstrate airplane is free from

flutter & vibration throughout

V&M,, envelope

l Confirm flutter analyses

l Demonstrate failures/adverse

conditions to V,,/M,,

l Investigate other phenomena

- e.g. control surface buzz

17s


l Purpose

l Test article (configurations and

conformity)

176




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Appendix A

Configuration and conformity

l Airplane must be in conformity per FAA

approved test plan

l Flutter critical payloads and fuel loading

l Automatic flight control systems

- Autopilot, yaw damper, load alleviation

device

l

Other flutter critical parameters

- Control surface mass balance and freeplay

l Minimum crew - hazardous test (No FAA

pilots or engineers)

177


l

Purpose

l

Test article (configurations and

conformity)

l Instrumentation and testing techniques

178



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Appendix A

Airplane instrumentation

l Uninstrumented flight test isunacceptable

l Sufficient quantity & distribution to

identify amplitude of critical flutter modes

l Sufficiently wide frequency band

l Monitor in real time (recording/telemetry)

l Relative displacements double integrated

from. accelerometers

l Control surface positions measured

179

Airplane instrumentdion

0 vertical

0 lateral



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Appendix A

Methods of exciting the airframe

l FAR 23.629(b) Flight flutter tests

must be made......and to show that -

(1) Proper and adequate attempts

to induce flutter have been made

within the speed range up to V,

(V,, for transport airplanes).

181


l

Abrupt control surface input (kick)

l Kicks always required

l Elevator - symmetric

l Ailerons and rudder - antisymmetric

l Suitable for low frequency wing modes

amplitude

182



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Appendix A


l Control sutface sweep

l

Usually electronically with sine wave

generator

l Broad frequency range

l Limited by frequency response of

control system


l

External airfoil vane

l Also driven by sine wave generator

l

Used for high frequencies (to 25Hz)

l Not limited by frequency response

of control system

l Vane must be flutter free

primary

surface



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Appendix A


l Other methods of excitation

- Inertial rotating mass

- Explosive impulse (bonkers)

- Turbulence in the air

Test procedure

l Test at constant altitude beginning at slow

speed, incrementally increasing to V, .

l At each speed, excite airplane and measure

frequency and damping of flutter modes.

l Clearance given to higher speed once

acceptable damping trends established.

l

Configuration held constant throughout

test.

186




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Appendix A

Frequency & damping

measurement

g = (l/N log y1/ Yt

time

T= l/f

Frequency & damping

measurement



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Appendix A

Frequency & damping

measurement

frequency, f

L

0000

OO

x xxxxx

1111II

I

I I

speed

189

0

VD

1.2&,


l

Purpose

l

Test Article (configurations and

conformity)

l Instrumentation and testing techniques

l Flight investigations and test conditions

190




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Appendix A

Test conditions

l Test at high altitude td M,,

(minimum dynamic pressure, q)

l Test at low altitude to V,,

(maximum q)

l Test at middle altitude to V,, / M,,

(maximum q and critical Mach No.)

l Some modes are critical at high

altitude (e.g. control sutface flutter)

191

Test conditions

control surface mode

Altitude. h

192

Airspeed? V



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Appendix A

L

Fligtit test investigations

l Regulatory requirements

l Payload and fuel loading

l Interaction of systems and

structures

l

Flight conditions, e.g. AOA and load

factor

l Flight test of failures and adverse

conditions not imposed by FAA

193


l

Purpose


conformity)


techniques

l

Flight investigations and test

conditions

l How results are used for compliance



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Appendix A

Compliance program

l FAA approved test plan

l Witnessing of test by instrumentation

l DER should supervise conduct of test

l DER recommend approval of data

l

Summary submitted for TIA

l Complete test reportsubmitted before TC

Flight flutter

I%



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Appendix A

Flight flutter tests summary

l Purpose


conformity)


techniques

l Flight investigations and test

conditions

l

How results are used for compliance

In

What affects

aeroelastic stability?

Changes

Modifications .

Repairs .



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Appendix A

Categories of changes

l Mass or inertial changes

l Structural changes

l Aerodynamic changes

l

System changes

199

Inertial changes

l

Engine/Nacelle (mass & cg)

l Fuel tank capacities (usage

schedules, or activation of dry bays)

l Wing mounted external fuel tanks.

l

Significant changes to payload

(cargo conversions)



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Appendix A

Inertial changes

l Painting and repairing of

control surfaces

l Installation of mass items

on main surfaces (logo or

navigation lights, etc)

l Winglets

Aerodynamic changes

l Nacelle configuration

(quite nacelle modifications)

l

Wing tip modifications

l

Winglets



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Appendix A

a

a

a

a

Structural changes

Fuselage cut outs (especially aft

fuselage cut outs for cargo

conversions)

Wing leading edge cut outs

(weather radar, landing lights, etc.)

Extended fuselages (plugs)

Cargo floors

.

Automatic controls

l New.autopilots or yaw dampers.

l Changes to autopilot or yaw-

damper gains and phases.

l Changes to control laws:

- Automatic flight controls

- Wing load alleviation systems

- Active flutter suppression systems.

104



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Appendix A

Asse,ssing changes

l Structural changes - Vibration analysis

plus before-and-after Ground Vibration

tests.

l Flight Flutter tests are necessary if there is

a significant effect on aeroelastic modes.

l For simple airplanes and simple changes

NACA Report 685 might be useful.

l A complete flutter substantiation program

is often necessary.

205

Typical curve from NACA 685



Septem

Documents

Flutter and Aeroelastic Stability