Subject: VIBRATION AND FATIGUE
EVALUATION OF AIRPLANE PROPELLERS Date: 9/17/01 Initiated By: ANE-110 AC No: 20-66A Change:
1. PURPOSE. This advisory circular (AC) provides guidance and describes one method, but not the only method, for demonstrating compliance with §§23.907 and 25.907 of Title 14 of the Code of Federal Regulations, for the evaluation of vibration stresses on propellers installed on airplanes. Like all AC material, this AC is not, in itself, mandatory and does not constitute a regulation. While these guidelines are not mandatory, they are derived from extensive Federal Aviation Administration (FAA) and industry experience in determining compliance with the pertinent regulations.
2. CANCELLATION. AC 20-66, Vibration Evaluation of Airplane Propellers, dated 1/29/70, is canceled.
3. RELATED DOCUMENTS.
a. Regulations. Sections 35.4, 35.37, 23.907, and 25.907.
b. Advisory Circulars.
(1) AC 35.37-1A, Guidance Material for Fatigue Limit Tests and Composite Blade Fatigue Substantiation, dated 9/17/01.
(2) AC 20-107A, Composite Aircraft Structure, dated 4/25/84.
(3) AC 21-26, Quality Control for the Manufacture of Composite Structures, dated 6/26/89.
(4) AC 25.571-1C, Damage Tolerance and Fatigue Evaluation of Structure, dated 4/29/98.
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(5) AC 20-95, Fatigue Evaluation of Rotorcraft Structure, dated 5/18/76.
c. Mechanical Vibrations, Den Hartog, J. P.; McGraw-Hill Book Company, 1956.
4. DEFINITIONS. For the purposes of this AC, the following definitions apply:
a. Damage tolerance. Damage tolerance is the attribute of the structure that permits it to retain its required residual strength for a period of use after the structure has sustained a given level of damage from fatigue, corrosion, accident, or a discrete source.
b. End of life condition. The end of life condition is the physical condition of the component when it has sustained the maximum extent of damage but maintains sufficient residual strength to meet all airworthiness loading requirements. This condition is defined during certification.
c. Fail-safe. Fail-safe is the attribute of the structure that permits it to retain its required residual strength for a period of use without repair after the failure or partial failure of a principal structural element.
d. Fixed pitch wood propellers of conventional design.
(1) A fixed pitch wood propeller of conventional design is a propeller that has the following physical properties:
(a) One piece laminated wood construction;
(b) Two or four blades;
(c) Surface coatings that do not contribute significantly to the propeller strength; and
(d) Surface coatings that only provide environmental protection.
(2) A fixed pitch propeller that has a composite shell over a wood core does not qualify as conventional design if the composite shell contributes significantly to the strength and frequency response of the propeller.
(3) A fixed pitch wood propeller with a fabric or composite covering for environmental protection that does not significantly alter the structure qualifies as conventional design.
e. Limit load. This term refers to the maximum load expected in service.
f. Minimum flying weight. This term refers to the airplane empty weight with minimum crew and fuel.
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g. Principal structural element. This term refers to the element that contributes significantly to the carrying of propeller loads and whose integrity is essential in maintaining the overall structural integrity of the propeller.
h. Safe-life. The safe-life of a structure is that number of events, such as flights, landings, or flight hours, during which there is a low probability that the strength will degrade below its design value due to fatigue damage.
i. Scatter factor. The scatter factor is a life reduction factor used in the interpretation of fatigue analysis and test results.
5. DISCUSSION. The vibration evaluation of a propeller on an airplane involves identifying propeller vibratory loads or stresses on the airplane and evaluating fatigue. The purpose of the evaluation is to show that the propeller can be operated safely within the structural limitations of the propeller. In this AC the term “loads” may, in context, be used interchangeably with stresses, strains, moments and any other appropriate engineering term. The applicable requirements are §§23.907 and 25.907. These regulations are addressed during airplane type certificate (TC) projects and are frequently addressed during airplane amended TC or supplemental type certificate (STC) projects.
a. Personnel and Organizations. The following personnel and organizations should participate in the vibration evaluation of a propeller on an airplane:
(1) Airplane applicant (airplane company/modifier).
(2) Propulsion system installation engineer/Project manager for the Aircraft Certification Office (ACO) working the airplane certification project (PACO - Project ACO).
(3) Propeller TC holder (propeller manufacturer).
(4) Propeller certification engineer/Project manager for the ACO that manages the original propeller TC project (CMACO - Certificate management ACO).
(5) Project officer for the Small Airplane or Transport Airplane Directorate (Airplane directorate).
(6) Project officer of the Engine and Propeller Directorate (Propeller directorate).
b. Applicable Projects.
(1) Airplane projects that should have an evaluation, re-evaluation, or review of propeller vibration and fatigue include, but are not limited to, the following:
(a) Installing or changing a propeller in TC, amended TC, or STC programs.
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(b) Increasing propeller RPM or power, or both, including an increase in engine power at higher altitudes.
(c) Installing a new engine or engine model.
(d) Installing floats.
(e) Increasing multi-engine airplane maximum gross weight or decreasing minimum flying weight.
(f) Modifying airplane nacelle tilt or toe.
(g) Modifying wing lift on multi-engine aircraft.
(h) Increasing VMO, VNE, and VD.
(i) Adding a new TC category to an airplane, such as acrobatic or commuter.
(2) The listed projects should include a review, when applicable, of the Engine Installation Manual limitations to assure that the installation is within the engine manufacturer’s approved data.
(3) The evaluation should consider changes that affect propeller shaft bending moments and, on reciprocating engines, engine crankshaft vibration. These changes include, but are not limited to, the following:
(a) Number of blades;
(b) Diameter;
(c) Moment of inertia;
(d) Overhung moment (distance from the propeller center of gravity (CG) to the engine); and
(e) Weight.
c. Propeller Structural Data. Propeller structural data used to conduct the fatigue evaluation is typically generated during propeller certification to meet the requirements of part 35. This data, generated under part 35, is used to support the vibration tests and fatigue evaluation on the airplane. The propeller manufacturer usually retains the structural data for the propeller fatigue evaluation on the airplane. In some cases, the propeller manufacturer may supply the installer with the appropriate strain gage locations and the ‘pass-fail’ measured stress criteria for the airplane propeller vibration test. If the propeller structural data is unavailable through the propeller manufacturer, it may be generated using AC 35.37-1A.
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d. Background. Appendix 1 provides basic information on propeller excitation sources and airplane and propeller interactions. This information may be useful to guide the process of planning and implementing the propeller vibration and fatigue evaluation program. Appendix 2 provides considerations for planning a vibration test for propellers on reciprocating and turbine engine installations.
6. VIBRATION TESTS.
a. Vibration Test Conditions. The conditions the propeller vibration test should evaluate vary significantly with each installation. A primary factor in identifying test conditions is the type of engine (reciprocating or turbine). Other significant factors are the type of airplane and the maneuvers included in the airplane operating envelope. Appendix 2 provides an example of test conditions from propeller vibration testing on part 23 category reciprocating and turbine engine installations.
(1) Reciprocating Engine Installations. The propeller vibration testing on reciprocating engine installations should focus on evaluating the possible combinations of engine power and RPM, during ground operations and in flight. Propeller vibration for single reciprocating engine tractor-type installations in the normal category should depend on these combinations. The test should verify that the aerodynamic effects on propeller loading are not a significant factor in the propeller vibratory load amplitudes.
(a) Variables. A number of other factors can influence the propeller vibration characteristics and load amplitudes during testing:
1. Engines with fuel injection tend to run smoother than engines without fuel injection, due to more uniform fuel distribution and cylinder pressures. Propellers installed on engines with fuel injection tend to exhibit slightly lower vibratory load amplitudes than those installed on identical engines without fuel injection. Therefore, when the only change to an installation is the addition of a fuel injection system, a vibration test generally does not have to be repeated.
2. Engines with the higher compression ratios tend to produce higher propeller vibratory load amplitudes, due to the increase in cylinder pressures.
3. Engine crankshaft torsional damper configurations and crankshafts have a significant effect on propeller vibratory load. Any change to the damper configuration or crankshaft changes the propeller vibration characteristics and load amplitudes.
4. Propeller indexing relative to the crankshaft can affect propeller vibration characteristics and stress amplitudes. Some installations provide the option of multiple indexing positions for the propeller; some propellers exhibit unequal blade to blade vibratory load amplitudes.
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5. Turbocharged and supercharged engines produce rated power at higher altitudes and operate at higher manifold pressures than normally aspirated engines. Evaluation of propellers on these engines should include testing at the maximum altitude at which maximum power can be maintained and in incremental combinations of manifold pressure and RPM.
(b) Other installations. Propellers installed on other reciprocating engine installations should have additional testing to evaluate the contribution of aerodynamic loading to the propeller vibratory load amplitudes. Other reciprocating engine installations include, but are not limited to, the following:
1. Twin engine airplanes;
2. Pusher airplanes; and
3. Aerobatics airplanes.
(2) Turbine Engine Installations. The propeller vibration testing on turbine engine installations should focus on evaluating the installation characteristics and flight conditions that affect the aerodynamic loading of the propeller.
(a) Variables. A number of other factors can influence the propeller vibration characteristics and load amplitudes during testing.
1. Changes in airplane weight can have a significant influence on the propeller load amplitudes. The evaluation should consider both the maximum gross weight and the minimum flying weight. If the airplane cannot be tested at these weights, the measured propeller loads should be analytically extrapolated to these airplane weights for the evaluation.
2. Take-off rotation on many turbine engine installations produces propeller vibratory load amplitudes that can be at or near the applicable limits. The use of flaps during take-off tends to reduce the propeller vibratory loading by reducing the angle of attack. Many airplane flight manuals specify a specific flap position for take-off but do not prohibit take-off with the flaps retracted. The evaluation should consider all possible flap positions including, if applicable, the fully retracted position.
3. The evaluation should include turbine engine installations at more than one altitude, to identify any adverse effects on propeller loading due to altitude. This should include the maximum altitude at which maximum power can be maintained. This is particularly significant for engines that are capable of producing rated power at high altitudes.
(b) Thrust alignment. The thrust alignment of the engine as it is installed in the airplane will significantly affect the magnitude of the once-per-revolution (1P) aerodynamic loading. The airplane manufacturer or modifier should work with the propeller manufacturer to determine the engine thrust alignment that best satisfies the requirements of both airplane handling qualities and propeller loading.
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(3) Additional Considerations. Paragraphs 6.a.(1) and (2) of this AC discuss considerations for conventional spark ignition aviation gas reciprocating and turbine engine configurations. Other engine configurations, such as diesel engines and rotary engines, have been certified or are in development; these configurations may have unique vibratory characteristics that may require additional evaluation.
b. Vibration Test Methods.
(1) Propeller Vibration Survey. The propeller vibration survey measures propeller loads on the airplane. The propeller vibration survey should use an electric resistance strain gage, coupled with an electronic signal processing system and a recording device. The recording device records strain gage signals during the test in flight and ground operating conditions suitable to evaluate the propeller loads throughout the airplane operational envelope. The processing system processes strain gage signals and other applicable information after the flight test.
(2) Strain Gages. Strain gages vary in size, resistance, material, and construction. When selecting the type of strain gage, consider the required result, operating environment, and system instrumentation requirements. The manufacturers of the strain gage provide complete application guides and installation instructions. The location of blade strain gages is generally determined from static shake tests, analytic predictions, and previous rotating test results.
(a) Strain gages should be placed on the propeller to measure surface strain that can be converted to stress or load with proper calibration. When placing strain gages on the propeller for the flight stress survey, there are many elements to consider. Consider the fatigue tests conducted for §35.37, since the measured flight test data will be used in conjunction with the fatigue test data. The blades should also have strain gages at reference locations to relate the data to applicable fatigue limits. The location and quantity of strain gages during the vibration survey are governed by the requirements of the evaluation. If the strain gages do not provide an accurate picture of the propeller loading, the subsequent evaluations will not provide useful results. Strain gages are usually installed on the propeller blades along the camber side at the point of maximum thickness; this is typically the point farthest from the blade neutral axis. Propeller blades should have a sufficient number of strain gages to define the load distribution along the blade and to identify the location of peak. Additionally, strain gages should be placed on:
1. Other areas dictated by the specific blade design;
2. Hub unit;
3. Pitch change components;
4. Engine shaft; or
5. Other components requiring load information.
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(b) It is not always possible to place a strain gage where the maximum strain or stress occurs. In these cases, use reference strain gages. Strain gage readings from one location on the propeller may be related to other locations based on previous analysis or test data.
(c) For the evaluation of flutter conditions, 45° gages (shear stress gages) should be installed on the blade surface at a location that is sensitive to blade torsional vibration.
(d) In some cases, strain gages should be installed on the engine shaft. Data from these gages can determine the torsional forcing frequencies produced by a reciprocating engine and the bending moment on the engine shaft due to propeller aerodynamic and gyroscopic loads.
Sometimes this data is required to evaluate the bending moment that must be withstood to conduct a propeller hub flange evaluation. In other cases, the engine shaft data and the propeller data are acquired concurrently, and the engine data is passed along to the engine manufacturers for their own fatigue evaluation.
(3) Instrumentation Systems. Many instrumentation options are available to record the stress survey data. The fundamental system design may be analog or digital, use frequency modulation or amplitude modulation, etc.
(a) The instrumentation should have the following characteristics:
1. Data calibration to assure the accuracy of the recorded signals.
2. Verification of adequate system frequency response, including sampling rates for digital systems.
3. Verification that the data amplitudes are not overloading the system, clipping or otherwise distorting the data.
4. On-line monitoring of the data for flight safety and signal quality verification.
5. Adequate sensitivity for the recorded data to be above the system noise level.
6. Time correlation to relate the strain gage data to the airplane data.
(b) Common problems with data recording systems result from a lack of understanding of the capabilities of the components within the data recording system. Each electronic component has a range of frequency and amplitude capability. If the strain gage signals of importance are higher in frequency or amplitude than the capability of the equipment, the strain gage results will not be accurate. A system designed for a large diameter propeller on a turbine engine transport airplane may not be adequate for a small diameter propeller on a general aviation airplane. The system requirements may also be different for a metallic blade and a composite blade. Each installation should be reviewed to determine if the instrumentation system is adequate to record propeller load.
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(c) Instrumentation systems may not have the capability to measure steady load as well as vibratory load. If the system does not measure the steady load, conduct the appropriate analysis to determine the steady load levels.
(4) Test Data. The strain gage signals and certain airplane operating conditions are recorded during the stress survey. Data is also logged during the stress survey. The combination of recorded and logged data fits the needs of the fatigue evaluation. The following list of test parameters is more extensive than necessary for many flight and ground test programs. Evaluate the overall test program requirements to determine which parameters should be recorded or logged. Consider the following parameters:
(a) Voice record;
(b) Pitch angle;
(c) Strain gage signals;
(d) Altitude;
(e) 1P speed phase pulse;
(f) Blade angle;
(g) Propeller RPM;
(h) Airplane gross weight;
(i) Propeller torque;
(j) Flap setting;
(k) Airspeed;
(l) Ground cross-wind speed;
(m) Airplane CG acceleration;
(n) Ground cross-wind direction;
(o) Yaw angle; or
(p) Weather conditions.
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(5) Data Reduction. Data reduction involves determining the magnitude of vibratory stresses at each test condition and, when applicable, evaluating the propeller response frequency content to assess the propeller response.
(a) The flight and ground test data being evaluated has continuously varying steady and vibratory amplitudes and frequency content. The evaluation of a test condition may be in the form of a mean stress and a peak vibratory stress or a statistically sampled vibratory stress.
When values other than the peak vibratory stress are used, an evaluation should be conducted to show the significance of higher stresses that are excluded from the fatigue evaluation.
(b) Other techniques may be used to process the cyclic content of the measured data to assess the cumulative exposure to the loads. A “rainflow” load cycle accumulation methodology is suitable to describe load cycle content of a load history for fatigue evaluation.
7. STRESS PEAKS AND RESONANT CONDITIONS. Stress peaks are generally due to resonant conditions (critical speeds). For background information on resonant conditions and associated stress peaks, refer to Appendix 1 and Mechanical Vibration. When the natural frequency of a propeller blade vibratory mode is near or at the frequency of a vibratory force acting on the blade, the vibratory response of the blade is magnified and a stress peak results.
Resonant conditions should be examined thoroughly to determine if operating restrictions are required and to determine their extent. Not all resonant conditions result in stress peaks. A stress peak only occurs when the vibratory forces are significant enough to excite the blade. A significant resonant condition is one that may have an effect on the fatigue life of the propeller.
a. Pre-Stress Survey Evaluation. Before the vibratory stress survey, the propeller blade natural frequencies should be identified to determine where to expect resonant conditions. The propeller blade natural frequencies and mode shapes can be identified by analysis, static shake tests, previous vibratory stress measurements and, when available, the results of earlier vibration stress surveys on similar propellers. If a resonant condition occurs where it is not expected or if unexpected resonant conditions are found, further testing should be conducted to determine the force acting on the blade and the blade vibratory mode involved. A resonant condition found previously on a similar installation should be found on the installation being tested.
b. Stress Survey Tests. When testing indicates that a stress peak is within the propeller operating range, further testing may be necessary to determine if RPM restrictions, such as placards, will be needed to avoid the resonant conditions. The test program should be modified to obtain further detail regarding the extent of the stress peak. When testing indicates that a stress peak is just beyond the propeller operating range, testing should be conducted sufficiently above the operating range to determine the maximum stresses that could occur from potential overspeed conditions. Overspeed conditions may result from conditions such as overspeed governor checks, transients, or tachometers that are not properly calibrated.
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c. Identification of the Forces. When a significant resonant condition is found during the vibratory stress survey, sufficient testing should be conducted to permit the identification of the vibratory force acting on the blades. Typical forces are the firing impulses from a reciprocating engine, aerodynamic forces from operation in ground cross-winds, and aerodynamic forces from operation in the wake of the airplane on pusher installations. The aerodynamic forces act at integer multiples of the propeller rotational speed, known as “P” orders. The “P” stands for perpropeller revolution. The term “2P” means two force cycles per-propeller revolution. The reciprocating engine forces act at multiples of the engine speed and may be integer or fractional multiples of the rotational speed. The engine forces are known as “E” orders. The “E” stands for per-engine revolution. The significant vibratory forces are identified to assess the potential variability in the magnitude of the force to determine the extent of any operating restriction. For example, the stress peak caused by ground cross-winds will increase significantly as the crosswinds increase. Operation in a 15 knot rear-quartering cross-wind may be acceptable; operation in winds above 15 knots may be unacceptable.
d. Identification of the Blade Modes. When a significant resonant condition is found during the stress survey, sufficient testing should be conducted to permit the identification of the blade natural frequency and blade vibratory mode shape. This may be accomplished by comparing the measured resonance frequency with analytical predictions, test results, and previous test experience. In addition, the measured strain distribution on the blade may also be used, either directly or filtered at the resonant frequency, to provide further information to identify the blade vibratory mode shape. The significant blade natural frequencies are identified to assess the potential variability in the natural frequency to determine the extent of any operating restriction.
For example, the natural frequencies of an aluminum blade at the maximum thickness permitted within the manufacturing tolerances will be different than the natural frequencies of the same aluminum blade after it has worn to the minimum thickness permitted in service. The shift in natural frequency will shift the location of the resonant condition and resulting stress peak. Also, composite blade natural frequencies may be lower for blades that have been repaired, have multiple coats of paint, or both.
e. Operating Restrictions. When it is determined that an operating restriction, such as a placard, is required to avoid a stress peak due to a resonant condition, the maximum and minimum permissible RPM values should take into account such items as manufacturing variability, service wear, repairs and the excitation force variability. These factors may be assessed by test, analysis, service experience or any other suitable method. For example, Figure 1 shows the measured vibratory stress for a blade during an RPM traverse on the ground in a cross-wind. A stress peak is found at 1000 RPM. A review of calculated blade frequencies in the critical speed diagram in Figure 2 shows that this stress peak is due to a 2P aerodynamic force and a first bending mode natural frequency critical speed. If the RPM restriction were set based on only the measured data, Figure 1, the upper and lower bounds would be set to 1100 and 900 RPM respectively. This restricted operating band is not sufficient when blade manufacturing and wear limits are taken into account. Figure 2 shows how calculated blade frequencies shift the stress peak up and down to account for frequency variability. The critical speed shift is then used to shift the stress peak in Figure 1 using the assumption that the shape of the stress peak remains the same. A blade with the maximum manufacturing tolerances and paint will lower the
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In addition, the magnitude of the stress amplitude should be assessed to determine if further restrictions are needed. The operating restriction may prohibit operation above a maximum rotational speed or may prohibit continuous operation in a restricted rotational speed range to enable the pilot to pass through that range.
f. Continued Airworthiness Monitoring. The following procedures are recommended to maintain RPM restrictions and vibratory stress levels.
(1) Periodically check and calibrate the airplane tachometer.
(2) Check and replace the engine vibration dampers at engine overhaul, if needed.
(3) Assess the natural frequency of propeller blades at intervals over the life of the fleet.
This assessment is to determine if there are service use dependent changes in the blade natural frequency that were not taken into account during certification of the propeller. This may be accomplished by static frequency tests or similarity to existing designs when service use dependent changes are known.
(4) Evaluate the natural frequency of propeller blades to assess the effect of repairs on frequency and RPM restrictions.
8. PROPELLER FLUTTER. The propeller should be shown to be free from the harmful effects of flutter. This is done by evaluating the measured vibratory response and showing that flutter does not exist within the operational envelope of the propeller. The harmful effects of flutter are high blade loads that result in unacceptable fatigue life. Propeller flutter is typically associated with torsional vibration of a blade. Propeller flutter typically occurs during high power operation on the ground or during reverse operation on landing. If flutter is found within the operational envelope of the propeller, further testing and fatigue evaluation may be necessary to show that the flutter conditions will not cause harmful effects within the operational life of the propeller. Limitations may be needed to avoid flutter or to limit the exposure to flutter.
9. SIMILARITY. The propeller vibration stresses may be determined by similarity. To determine similarity, the operating environment is shown to be similar from one airplane to another. When shown to be sufficiently similar, the vibratory load measurement and the fatigue evaluation of the baseline airplane apply to the target airplane. Similarity is generally not used to determine propeller vibration loads on large multi-engine airplanes or acrobatic airplanes, due to the inability to accurately evaluate the aerodynamic environment. Similarity approvals are only valid when compared directly to actual measured data. If airplane “B” is approved based on similarity to airplane “A,” airplane “C” cannot be approved based on similarity to airplane “B;”
only comparisons to airplane “A,” the baseline airplane, are valid.