Advisory U.S. Department of Transportation Circular Federal Aviation Administration Subject: Means of Compliance With Date: 10/23/85 AC No:23.629-1A Section 23.629, Flutter Initiated By: ACE-100 Change:
PURPOSE. This advisory circular presents information and guidance to provide one means, but not the only means, of complying with Section 23.629, Flutter (including flutter, airfoil divergence, and control reversal) of Part 23 of the Federal Aviation Regulations. Accordingly, this material is neither mandatory nor regulatory in nature.
2. CANCELLATION. AC 23.629-1, Means of Compliance With FAR 23.629, Flutter, dated January 8, 1979, is cancelled.
3. BACKGROUND.
prompted endeavors to find simplified methods of flutter substantiation.
Although the advent of electronic computers has deemphasized the need to make drastic assumptions previously necessary to enable mathematical treatment of the flutter problem, there remains a need to simplify the flutter problem as much as possible consistent with safety in order to minimize the cost and effort required to show freedom from flutter. Past experiences gained by the necessity to judiciously choose meaningful degrees of freedom, and by the need to make parametric studies to establish practical boundaries of the effectiveness of the various physical quantities, has resulted in a generally recognized set of good practices.
These good practices form the basis for this advisory circular.
Danes m Charteni JEROLD M. CHAVKIN Acting Director, Central Region
CONTENTS Paragraph Page CHAPTER 1. AIRPLANE CATEGORIES
1. General I
2. Special Design I CHAPTER 2.METHODS OF SUBSTANTIATION 3 Simplified Criteria 3
4. Rational Analysis 4
5. Analysis Plus Flight Test 7
6. Ground Testing 7
7. whirl Mode.. 8 CHAPTER 3.MODIFICATIONS TO AIRCRAFT ALREADY CERTIFICATED
8. Reevaluation 9 CHAPTER 4.CONTROL SURFACES AND TABS
9. Response. 11
10. Balance.. 11
11. Vibratory Modes 11
12. Analyses. 11
13. Fail Safe Requirements. 12 CHAPTER 5.DIVERGENCE AND CONTROL REVERSAL
14. General 13
15. Airfoil Divergence 13
16. Control Reversal 13
APPENDIXES APPENDIX I. GROUND TESTING (11 Pages) 1 APPENDIX 2. FLUTTER ANALYSIS (19 Pages) 1 APPENDIX 3. FLIGHT FLUTTER TESTING (7 Pages) 1 APPENDIX 4. ACKNOWLEDGMENTS, REFERENCES, BIBLIOGRAPHY (2 Pgs.) 1 iii(and iv)
CHAPTER 1. AIRPLANE CATEGORIES
1. GENERAL. Airplanes in the general category are those with a typical exterior configuration; i.e., high, mid, or low wing; single fin and single horizontal stabilizer aft-mounted on the fuselage; and tractor powerplant installations.
2. SPECIAL DESIGN. The special design category includes airplanes with certain design features that experience has shown warrant special consideration with regard to flutter. Flutter free operation for these special unconventional configurations may be shown by analyses which include an assessment of the effects of critical parameters. Flight flutter tests to supplement those analyses are recommended. Some of these special unconventional configurations are:
a. Any aircraft with a design dive speed of 260 knots (EAS) or more at altitudes below 14,000 feet and Mach 0.6 or more at altitudes at and above 14,000 feet.
b. Any aircraft approved for flight in icing conditions. (The effect of ice accretions on unprotected surfaces, including those which might occur during system malfunctions, should be considered).
C. Pusher powerplants.
d. Canard geometry.
e. T, V, X, H, or any other unusual tail configuration.
f. Any external pods or stores mounted to wing or other major aerodynamic surface a
g. Fuel tanks outboard of 50% semispan.
h. Tabs which do not meet the irreversibility criteria of chapter 2, paragraph 3.d., and of reference 1, appendix 4.
i. Spring tabs.
j. All-movable tails, i.e., stabilators.
k. Slender boom or twin-boom fuselages.
1. Multiple-articulated control surfaces.
m. Wing spoilers.
n. Hydraulic control Systems with stability augmentation.
0. Full span flaps.
p. Leading edge devices (i.e., slots, etc.).
q. Geared tabs (servo or anti-servo, etc.).
Chap I Par 1 1 (and 2)
CHAPTER 2. METHODS OF SUBSTANTIATION
3. SIMPLIFIED CRITERIA.
a. Guidelines. Airframe and Equipment Engineering Report No. 45 is intended to serve as a guide to the small airplane (VD less than 260 knots EAS at altitudes below 14,000 ft.) designer in the prevention of flutter, aileron reversal, and wing divergence. The material presented relies upon:
(1) A statistical study of the geometric, inertia, and elastic properties of those airplanes which had experienced flutter in flight, and the methods used to eliminate the flutter.
(2) Limited wind-tunnel tests conducted with semi-rigid models.
These were solid models of high rigidity with motion controlled at the root by springs to simulate wing bending and torsion. Springs at the control surface were used to simulate rotation.
(3) Analytic studies based on the two-dimensional study of a representative section of an airfoil.
b. Wing and Aileron. Prevention of wing flutter is attempted through careful attention to three parameters; wing torsional flexibility, aileron balance, and aileron free play.
(1) The aileron balance criteria is obtained from the aileron product of inertia, K, about the wing fundamental bending node line and the aileron hinge line; and the aileron mass moment of inertia, I, about its hinge line. A limit of the parameter, K/I, is set as a function of VD.
(2) A wing torsional flexibility factor, F, is defined and a limit established as a function of VD. In order to apply the criteria, one needs to know wing twist distribution per unit applied torque) wing planform, and limit dive speed.
(3) The total free play of each aileron with the other aileron clamped to the wing must not exceed the specified maximum.
C. Elevator and Rudder. Dynamic balance criteria for the elevator and rudder (similar to the K/I of the aileron) are defined and limits set as a function of limit dive speed. In order to utilize the criteria, the following information is required:
(1)Geometry -horizontal tail semichord at the midspan
- semispan of horizontal tail
- distance from fuselage torsion axis to tip of fin
- semichord of vertical tail measured at 70% span position Chap 2 Par 3 3
(2)Stiffness - Fuselage vertical bending frequency
- Fuselage torsional frequency
- Fuselage lateral bending frequency
(3) Mass - Elevator static balance about hinge line
- Elevator mass moment of inertia about hinge line
- Elevator product of inertia referred to stabilizer centerline and elevator hinge line
- Rudder static balance about hinge line
- Product of inertia of rudder referred to fuselage torsion axis and rudder hinge line
- Rudder mass moment of inertia about hinge line
d. Tabs. It is recommended that all reversible tabs be balanced about the tab hinge line. The degree of static and dynamic balance should be determined by rational analyses (reference chapter 4). In practice, most tabs are irreversible, which means:
(1) For any position of the control surface and tab, no appreciable deflection of the tab can be produced by means of a moment applied directly to the tab when the control surface is held in a fixed position.
(2) The total free play at the tab trailing edge should be less than the following!
(i)If the tab span does not exceed 35 percent of the span of the supporting control surface, the total free play shall not exceed two percent of the distance from the tab hinge line to the trailing edge of the tab perpendicular to the tab hinge line.
(ii)If the tab span equals or exceeds 35 percent of the span of the supporting control surface, the total free play is not to exceed one percent of the distance from the tab hinge line to the trailing edge of the tab perpendicular to the tab hinge line.
(3) The tab natural frequency should be equal to or should exceed the calculated value and expressed as a function of tab and control surface geometry and airplane dive speed. (reference 1).
(4) Spring loaded tabs are free to rotate and thus are not irreversible. Generally, these tabs will require dynamic as well as static balance. Extensive flutter analysis is always needed to define these requirements.
4. RATIONAL ANALYSIS.
a. Review of Past Analysis. Review of previous flutter analyses conducted upon similar aircraft can provide the engineer with useful Chap 2 4 Par 3
1O/23/85 AC 23.629-lA information regarding trends, critical modes, etc. Although in general such a review is not used as a substantiation basis for a new aircraft, it can provide a useful tool in evaluating the effect of modifications to existing certified aircraft. Chapter 3 provides additional comments on this subject.
b. Two-Dimensional Analysis. The flutter characteristics of straight wings (or tails) of large aspect ratio can be predicted reasonably well by considering a "representative section that has two or three degrees of freedom. Translation and pitch are always needed and) for control surfaces, the third freedom would be rotation about the hinge line.
Appendix 2 presents a more thorough discussion of this approach.
c. Three-Dimensional Analysis. Current analysis is based upon consideration of total span, rather than “representative section” discussed in 4.b. above. The behavior is integrated over the whole structure being analyzed. Some idealization is always necessary; the most common being the division of the span into strips. Other types of modeling are also used.
Generalized mathematics are presented in appendix 2.
For Part 23 airplanes, quite often the wing and empennage analyses are conducted separately; however, this is not always adequate for unconventional configurations. Both the symmetric and antisymmetric motions require investigation.
Calculated mass and stiffness distributions are generally used to calculate uncoupled modes and frequencies. These values are then used to conduct a coupled vibration analysis; the resulting coupled modes and frequencies are then usually compared with measured natural modes.
The calculated stiffness-related inputs are generally adjusted until good agreement is obtained with the test data. Once satisfactory agreement is achieved, the coupled vibration analysis is normally used for the flutter calculations.
It is suggested that one perform certain variations in the assumed input conditions to see which parameters are critical. Control surface balance conditions and system frequencies (especially tab frequencies) are often investigated parametrically. The effect of control system tension values at the low and high ends of the tolerance range should be assessed.
It may be advantageous to arbitrarily vary certain main surface frequencies (stiffness), especially torsional frequencies and engine mode frequencies, while leaving other frequencies constant.
Chap 2 Par 4 5
Sometimes it is desirable to evaluate the effect of a slight shift in spanwise node location for a very massive item where the node is located very close to or within the item. (Test data may not be sufficiently accurate for this assessment.)
It is normal practice to run a density-altitude check to include nearsea-level, maximum and any other pertinent altitudes such as the knee of the airspeed-altitude envelope where the design dive speed becomes MACH- limited.
It is desirable to investigate combined wing-empennage modes for high performance (VD of 260 KEAS or above) airplanes, as well as for airplanes with unconventional configurations.
Flutter Analysis Evaluation: For a given set of input parameters, the resulting output generally consists of a number of theoretical damping values (g) with associated airspeeds and frequencies.
Various cross plots of these values among themselves and versus varied input parameters allow a study of trends. Common plots are: damping vs.
equivalent airspeed (g-V plots), control surface balance vs. flutter speed, uncoupled frequency vs. flutter speed, altitude vs. flutter speed, etc.
Normally only the critical items will be extensively compared.
Of particular importance is an evaluation in the neighborhood of the crossing of a damping velocity (g-V) curve toward the unstable damping region, through zero. The typical critical g-V curve will first become increasingly stable and with increasing speed will turn and rise toward or pass through g=O, then at some higher speed may again turn toward the stable region. Typical characteristics are discussed in the following examples:
Examples:
Curves 1 and 2 show slight trends toward instability, but do not approach actual instability.
Chap 2 6 Par 4
Curve 3 crosses the stability axis but, depending on the inherent structural damping, may or may not actually become unstable. Curve 4 is obviously unstable and probably violent, since its slope is steep as it passes through zero. In actual flight it may only be a mile an hour or so between completely stable and extremely unstable explosive flutter. Flight tests are not advisable when this type plot is observed inside or near the flight envelope.
Much can be learned from g-V curves. (Absolute values should be viewed with some reserve as there is no perfect one-to-one correspondence of the analytical parameters and flight parameters.) where the critical curve crosses the axis (with respect to V~ for the airplane) is important. Equally important is the rate of approach to instability (slope of curve).
The general practice is to use a damping value of g=O.03 at 1.2 VD as the flutter limit of the g-V plots. However, this value should be used with caution if the slope of the curve is large (damping decreases very rapidly with an increase in airspeed) between g=O and 0.03. In cases where the slope is steep, it is suggested that the g=O airspeed be at least 1.2 VD.
If flight flutter testing is conducted to verify damping under the above circumstances, extreme caution should be exercised.
For damping curves such as (3), which peak out below 1.2 VD, the predicted damping should be no more unstable than g=O.02 unless justification is provided by other acceptable means.
5. ANALYSIS PLUS FLIGHT TEST. Although paragraph (c) of section 23.629 permits certification based upon flight test only, it is recommended that some analysis precede a flight flutter test. The results of any of the analysis procedures in paragraph 4 would be useful and could be used to provide guidance for formulating a flight flutter test plan. In all cases, as required by paragraph 23.629(a), the natural frequencies of main structural components should be determined by vibration tests or other approved methods prior to conducting any flight testing. A more thorough discussion of flight flutter testing is presented in appendix 3.
6. GROUND TESTING. Comparison of test data may be used in lieu of a totally new analysis in the case of dynamically similar aircraft.
Comparison would usually be based upon geometry, mass and stiffness distributions, speed regime, and more importantly, upon a comparison of the measured coupled vibration modes.
a. Test data would normally include:
(1) Ground Vibration Testing
(2) Control Surfaces and Tab Mass Property Determination
(3) Stiffness Tests
(4) Free Play Measurement of All Tabs
(5) Rotational Frequency for All Tabs
(6) Tab System Rotational Stiffness Chap 2 Par 4 7
b. Appendix 1 presents some guidelines for recommended tests and procedures.
C. The degree of similarity between aircraft that is required for justification can vary greatly. Some of the factors which should be considered are the amount of safety margin available, flutter speed sensitivity to certain parameters, and the thoroughness of the original analysis.
7. WHIRL MODE. Beginning with Amendment 23-7, paragraph 23.629(e) required an investigation of the whirl mode phenomena for multiengine turbopropeller airplanes only. The basis being these airplanes characteristically have wing mounted engines wherein the stability of a flexibly mounted engine/propeller on an elastic wing is of major concern. Amendment 23-31 of paragraph 23.629(e) now requires an investigation of the whirl mode phenomena for both single and multiengine turbopropeller airplanes.
Although airframe influence may be negligible for fuselage mounted single engine tractor configurations, the potential for propeller whirl flutter still exists. For pusher configurations, empennage motion may be significantly affected by engine/propeller forces. Stability of either installation is dictated, in part, by engine mount stiffness, damping, mass properties, motion axes, propeller geometry and propeller advance ratio.
Therefore, to assure freedom from whirl mode flutter, all turbopropeller installation investigations should include, in addition to the appropriate airframe degrees of freedom:
a. Whirl mode degree of freedom which takes into account the stability of the plane of rotation of the propeller and significant elastic, inertial, and aerodynamic forces.
b. Propeller, engine, engine mount, and airplane structure stiffness and damping variations appropriate to the particular configuration; e.g., deterioration of engine isolators, large cantilevered engine installations) etc.
Generalized mathematics are presented in appendix 2. In addition, references 9, 10 and 11 of appendix 4 contain technical information for an acceptable means of demonstrating whirl mode stability.
Chap 2 8 Par 6
CHAPTER 3. MODIFICATIONS TO AIRCRAFT ALREADY CERTIFICATED
8. REEVALUATION. Considerable judgment is often required to determine the degree of reevaluation necessary. If the mass, mass distribution, or the stiffness distribution are affected sufficiently to result in possible significant changes in resonant frequencies of major modes, mode shapes, or mass coupling terms in the flutter equations, then some reevaluation, such as pre-mod and post-mod GVT data comparison, or analysis may be required.
Some examples of significant changes are:
a. Engine (Propeller). A change in mass or mass moment of inertia of the powerplant or in its mounting system (bushings, etc.) or a c.g. shift should be investigated. On single-engine airplanes, such changes will most likely affect fuselage and empennage frequencies and mode shapes. For engines mounted on the wings, the entire airplane may be affected.
For changes in existing designs which entail significant increases in engine power and/or airplane speed, special assessments of the effect on primary and secondary control systems should be made. If tabs are exposed to the propeller slip stream, particularly on airplanes with a dive speed greater than 260 KEAS, it may be necessary to impose the fail-safe criterion discussed in chapter 4.
b. Structural Cutouts. Severing or bridging across major structural members, such as fuselage bulkheads and ribs or stringers of aerodynamic surfaces, may produce discontinuities in stiffness parameters that significantly alter the vibratory response of the structure.
The significance of a change may be ascertained by its effect on the energy terms in the flutter modes being evaluated.
Chap 3 Par 8 9 (and 10)
CHAPTER 4. CONTROL SURFACES AND TABS
9. RESPONSE. The aerodynamic force on an airfoil is very sensitive to control surface displacement, which in turn is responsive to both control motions and aerodynamic forces from tab displacement. Control surface displacement may result from deflection of the control system, deflection of the control surface attachment, or structural deflection of the control surface itself under forces from control application, aerodynamic force due to position or velocity of position change, and inertia force.
10. BALANCE. Control surfaces and tabs are balanced to prevent rotation about their hinges resulting from inertial response to motion in any flutter mode. When the flutter mode consists of motion about some axis perpendicular to the control surface hinge axis, a concentrated ballast is most efficiently used. Caution should be used to assure that its location is in a high response area of the vibratory mode, which is difficult when the mode is complex. Caution should also be used to assure that its attachment is secure. Because the attachment is subjected to oscillatory loads which cause fatigue failures and because a distributed ballast achieves balance against all flutter modes, it is conservative to distribute the ballast in accordance with the spanwise weight distribution of the surfaces. If less than static balance is provided, the effect of variations in the amount of balance should be evaluated. To guard against unintended balance changes in service, sealing and proper drain boles should be provided to minimize the risk of water, ice, or dirt accumulation in a control surface or tab. Excessive accumulation of these substances could alter the static and/or dynamic balance of the control sufficiently to adversely affect flutter characteristics.
11. VIBRATORY MODES. Control surface rotation about its hinge line is affected by various constraints. Control system stiffness and the rigidity of interconnection between control surfaces determine the primary rotational modes. Both symmetric and antisymmetric modes should be considered.
Vibration mode changes resulting from the modifications to the control system such as the addition of a bob weight must be assessed for their effect on flutter. Secondary rotations may result from flexure of the attaching structure or bending of the control surface. This is a major consideration for long short-chord tabs and may affect their effective irreversible characteristics. When it is necessary to raise a tab frequency by redesign, consideration should be given to the contributions of: hinge bending perpendicular to the surface especially near the horn-actuator station), horn length, axial stiffness of the push-pull rod or link, mounting flexibility and lateral stability at push-rod attachment of the tab actuating mechanism.
12. ANALYSES. In most cases involving control surfaces, the flutter speeds are largely governed by the mass balance values and distributions. It is wise for the flutter analyst to cover a range of balance values and distributions to determine the most satisfactory ones. It is common to find that a change which improves one mode degrades another. When conducting a Chap 4 Par 9 11
multi-degree-of-freedom analysis, it is advisable to investigate the effect of control system frequency from zero to about 1-1/2 times the system frequency measured in test. Due to friction, etc., it may be difficult to excite and measure control system frequency accurately. The stiffness can be measured at the surface with the control locked in the cockpit and, using the inertia of the end items, the system frequency can be calculated.
Theoretical values of tab and primary control surface aerodynamic derivatives have, for some configurations, produced higher flutter speeds than flutter model testing. Analytically derived tab and primary control surface aerodynamic coefficients based on strip theory have for some configurations produced higher flutter speeds than wind tunnel tests.
Therefore, flutter speed sensitivity to variations in the theoretical coefficients should be evaluated in all control surface/tab investigations.
13. FAIL SAFE REQUIREMENTS. Amendment 23-23 of paragraph 23.629(f) requires flutter free operation after failure, malfunction, or disconnect of any single tab element. This fail-safe requirement is extended to include a failure, malfunction, or disconnect of any element in the primary flight control system or flutter damper on airplanes with a dive speed in excess of 260 KEAS below 14,000 feet, or MACH 0.6 above 14,000 feet.
Potential failures that require investigation include, but are not limited to, tab or primary control trim actuating system, primary control actuating system (both of which includes bellcranks, pulleys, brackets, and their attachments), and control cables or push rods. Control surface hinges and tab hinges, their attachments, and local portions of structure need not be included as part of the control system in this investigation.
Possible means of compliance to actuating system failures (i.e., actuators, cables, rods) may be achieved by incorporating dual systems9 mass balancing the controls to counter the rotation of a zero stiffness free surface, or by incorporating a combination of the two. Proper mass balancing, particularly for tabs, requires considerable care and knowledge of the flutter mechanism to assure adequacy of the design in suppressing flutter. Dual load path designs should include an assessment of residual strength with a single failure to assure that the remaining path will not fail before the single failure is detected during appropriate specified inspection intervals.
Chap 4 12 Par 12