Gust Loads on Aircraft: Concepts and Applications

320 Pages·2001·16.612 MB·English

by  Frederic M. Hoblit

Gust Loads on Aircraft:
Concepts and Applications
Frederic M. Hoblit
Consultant
Formerly,
Senior Research and Development Engineer
Lockheed-California Company
AIAA EDUCATION SERIES
J. S. Przemieniecki
Series Editor-in-Chief
Air Force Institute of Technology
Wright-Patterson Air Force Base, Ohio
This volume was sponsored by the Lockheed Aeronautical
Systems Company and was originally published in whole as a
Lockheed Report
Published by
American Institute of Aeronautics and Astronautics, Inc.
370 L'Enfant Promenade, S.W., Washington, D.C. 20024
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American Institute of Aeronautics and Astronautics, Inc., Washington, DC
Library of Congress Cataloging-in-Publication Data
Hoblit, Frederic M.
Gust loads on aircraft: concepts and applications
p. cm.—(AIAA education series)
Bibliography: p.
Includes index.
1. Airplanes—Design and construction. 2. Gust loads. I. Title. II. Series.
TL671.2.H58 1988 629.132'3—dc!9 88-8146 CIP
ISBN 0-930403-45-2
Copyright © 1988 by the American Institute of Aeronautics and Astronautics, Inc. All rights
reserved. Printed in the United States of America. No part of this publication may be
reproduced, distributed, or transmitted, in any form or by any means, or stored in a data base
or retrieval system, without prior written permission of the publisher.
To My Lockheed Mentors,
Melcon A. Melcon, Jack Wignot, and Warren Stauffer
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FOREWORD
One of the objectives of the AIAA Education Series is to provide archival
documentation of the established practices in aircraft design. Gust Loads on
Aircraft: Concepts and Applications by Frederic M. Hoblit is in this special
category; it provides an authoritative exposition of the current engineering
practice of determining gust loads on airplanes, especially the continuous-
turbulence gust loads.
The author begins with the older concept of discrete-gust idealization of
the gust structure, including the subject of static and dynamic discrete-gust
loads, and then moves on to the more modern power-spectral, or continu-
ous-turbulence, concepts and methods. This newer approach has gained
wide acceptance and is being used today as the primary method of
determining the dynamic loads due to gust encounters that are subse-
quently used to verify structural integrity of the airplane.
Because of the relative unfamiliarity of an average reader with the
power-spectral methods, all the necessary mathematical concepts and meth-
ods are provided in the text. The continuous-turbulence gust loads criteria
are discussed with reference to Federal Aviation Authority requirements
and military specifications. This is followed by a detailed discussion of
loads combination and design criteria, gust-response equations of motion,
various short-cut methods, spanwise variation of vertical gust velocity,
treatment of nonlinear systems, and analysis of gust-response flight-test
data. The final chapter addresses the overall philosophy of discrete-gust
concepts and their present applications.
This latest text in the AIAA Education Series provides a unique source
of the design criteria and analysis of aircraft in turbulent air. It should be
useful as a reference material for structural-loads engineers and as a
supplementary text for courses in aeroelasticity, flight mechanics, and flight
controls.
J. S. PRZEMIENIECKI
Editor-in-Chief
AIAA Education Series
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TABLE
OF
CONTENTS
xi Preface
1 Chapter 1. Gust Loads Fundamentals
1.1 Mechanism of Gust Loads Generation
1.2 Characteristics of Turbulence
7 Chapter 2. Discrete-Gust Static Loads
2.1 One-Minus-Cosine Idealization
2.2 FAR Static Gust Requirement
2.3 FAR Gust Loads Formula
2.4 History of FAR Static Gust Requirements
2.5 Effect of Static Aeroelastic Deformation on C ^
L
2.6 Corresponding Military Requirements
15 Chapter 3. Discrete-Gust Dynamic Loads
3.1 Definition
3.2 When, Where, and Why Considered
3.3 Ingredients of the Discrete-Gust Dynamic Loads
Differential Equations
3.4 Sample Results
3.5 Criteria Considerations
21 Chapter 4. Basic PSD Concepts and Application to Gust Loads
4.1 Gust Profile as a Stationary Gaussian Random Process
4.2 Measure of Intensity: RMS Value of y
4.3 Probability Distribution
4.4 Frequency Content and Power-Spectral Density (PSD)
4.5 Gust PSD for Use in Design
4.6 Frequency of Exceedance
4.7 Application of Frequency of Exceedance to Gust Loads
4.8 Determination of P's and b's from Flight Data
4.9 Relation Between Frequency of Exceedance and
Probability of Exceedance
Problems
69 Chapter 5. Continuous Turbulence Gust Loads Criteria
5.1 FAR Requirements
5.2 Military Requirements
5.3 Basic Forms of Criterion
5.4 Design Levels
5.5 Flight Profile Generation
5.6 Modification of Design Envelope Criterion to Account
for NQ
5.7 Combined Vertical and Lateral Gust Loads
5.8 Stability Augmentation or Active Control System (ACS)
97 Chapter 6. Load Combinations and Design Conditions
6.1 The Phasing Problem
6.2 Loads for Stress Analysis
115 Chapter 7. Gust Response Equations of Motion: Formulation
and Solution
7.1 Generalized Coordinates
7.2 Grid Systems
7.3 Basic Data
7.4 Use of Matrices and Matrix Algebra
7.5 Loads as Functions of Generalized Coordinate Response
7.6 Solution for Frequency-Response Functions
7.7 Inclusion of an Active Control System
7.8 Computation of A, N , and p
G
7.9 Checks for Error
123 Chapter 8. Short-Cut Methods
8.1 Plunge-Only Curves
8.2 Plunge-Pitch Curves: Hoblit Parameters
8.3 Plunge-Pitch Curves: Peele Parameters
8.4 Qualitative Relations Between Peele and 1/(1 +2nk) Lift
Growth Assumptions and Definition of an Effective c for
Use in Sees. 8.1.1 and 8.2
8.5 Use of Plunge-Pitch Curves to Give Plunge-Only Data
8.6 Use of Simplified Equations of Motion
8.7 Empirical Adjustments
Problems
165 Chapter 9. Spanwise Variation of Vertical Gust Velocity
9.1 Introduction
9.2 Applications of Three-Dimensional Gust Analysis
9.3 Effect of Three-Dimensional Gust Analysis on Loads
9.4 Effect of L on Three-Dimensional Gust Response
169 Chapter 10. Treatment of Nonlinear Systems
10.1 Sources of Nonlinearities
10.2 Use of Time-History Analysis
10.3 Simplifications Made
10.4 Gust Velocity Time History
10.5 Interpretation of Results
173 Chapter 11. Analysis of Gust-Response Flight-Test Data
11.1 Purpose of Gust-Response Flight Testing
11.2 Basic Approach
11.3 Selection of Samples for Analysis
11.4 Gust Velocity Determination
11.5 UseofPSD's
11.6 PSD Determination: Parameter Selection
11.7 Other Considerations in PSD Determination
11.8 Effects of Computing Options on PSD's and
Frequency-Response Functions
187 Chapter 12. Adequacy of the Stationary-Gaussian Idealization of
the Gust Structure
12.1 Evidence of Possible Inadequacies in the
Stationary-Gaussian Model
12.2 Non-Gaussian Turbulence vs Nonstationary Turbulence
12.3 Evaluation of the Adequacy of the Stationary-Gaussian
Model
12.4 Other Models
197 Chapter 13. Present Place of Discrete-Gust Load Requirements
13.1 Nature of Atmospheric Turbulence
13.2 Tuned Discrete-Gust Analysis
13.3 Static Discrete-Gust Loads
13.4 Computational Considerations
203 Appendix A: Plunge-Only Equation of Motion for an Airplane
Encountering a One-Minus-Cosine Vertical Gust
209 Appendix B: Elementary Introduction to Some Concepts in
Probability Theory and Statistics
B.I Probability of a Single Event
B.2 Some Basic Rules of Probability
B.3 Probability Distribution
B.4 Frequency of Exceedance