Subject: Propeller Vibration and Fatigue Date: 3/24/11 AC No: 20-66B Initiated by: ANE-111
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 (14 CFR) for the evaluation of vibratory stresses on propellers installed on airplanes. This evaluation uses fa tigue and structural data obtained in accordance with 14 CFR part 35.
2. Applicability .
a. The guidance provided in this document is directed to propeller manufacturers, modifiers, and foreign regulatory authorities.
b. This material is neither mandatory nor regulatory in nature and does not constitute a regulation. It describes acceptable means, but not the only means, for demonstrating compliance with the applicable regulations. The FAA will consider other methods of demonstrating compliance that an applicant may elect to present. Terms such as “should,” “shall,” “may,” and “must” are used only in the sense of ensuring applicability of this particular method of compliance when the acceptable method of compliance in this document is used. While these guidelines are not mandatory, they are derived from extensive FAA and industry experience in determining compliance with the relevant regulations. On the other hand, if the FAA becomes aware of circumstances that convince us that following this AC would not result in compliance with the applicable regulations, we will not be bound by the terms of this AC, and we may require additional substantiation as the basis for finding compliance.
c. This material does not change, create any additional, authorize changes in, or permit deviations from existing regulatory requirements.
3. Cancellation . This AC cancels AC 20-66A, Vibrati on Evaluation of Airplane Propellers, September 17, 2001.
4. Related Documents .
a. Regulations. §§ 35.4, 35.37, 23.907, and 25.907.
b. Advisory Circulars. Please check the FAA’s website at http://www.faa.gov/regulations_policies/ for the latest revision of the following documents:
(1) AC 35-1, Certification of Propellers, December 19, 2008.
(2) AC 35.37-1B, Propeller Fatigue Limits and Evaluation, March 4, 2011.
(3) AC 20-107B, Composite Aircraft Structure, September 8, 2009.
(4) AC 21-26, Quality Control for the Manufacture of Composite Structures, June 26, 1989.
(5) AC 25.571-1C, Damage Tolerance and Fatigue Evaluation of Structure, April 29, 1998.
c. Mechanical Vibrations, Den Hartog, J. P.; McGraw-Hill Book Company, 1956.
5. Definitions. For the purposes of this AC, the following definitions apply:
a. Damage tolerance. 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 physical condition of the component when it has sustained maximum damage but maintains sufficient residual strength to meet all airworthiness loading requirements. This condition is defined during certification.
c. Fail-safe. 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. Limit load. The maximum load expected in service.
e. Minimum flying weight. The airplane empty weight with minimum crew and fuel.
f. Principal structural element. 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.
g. Probability of Detection (POD). A quantitative statistical measure for detecting a particular type of anomaly (flaw) over a range of sizes using a specific nondestructive inspection technique under specific conditions. Typically, the mean POD curve is used.
h. Safe-life. That number of events, such as stress cycles, flights, landings, or flight hours, during which there is a low probability that the strength of a structure will degrade below its design value due to fatigue damage.
i. Scatter factor. A life reduction factor used in the interpretation of fatigue analysis and test results.
6. 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.
(6) Project officer of the Engine and 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:
(a) Installing or changing a propeller in TC, amended TC, or STC programs.
(b) Increasing propeller rpm or power, or both, including an increase in engine power at higher altitudes.
(c) Modifications to the engine or airframe that alter the power of the engine such as, but not limited to, the addition of a turbocharger or turbonormalizer, increased boost pressure, 3
increased compression ratio, increased rpm, altered ignition timing, electronic ignition, full authority digital engine controls (FADEC), or tuned induction or exhaust.
(d) Change to the mass or stiffness of the engine crankshaft/counterweight assembly including dampers.
(e) Installing a new engine or engine model.
(f) Installing floats.
(g) Increasing multi-engine airplane maximum gross weight or decreasing minimum flying weight.
(h) Modifying airplane nacelle tilt or toe.
(i) Modifying wing lift on multi-engine aircraft.
(j) Increasing VMO, VNE, and VD.
(k) Adding a new TC or use category to an airplane, such as acrobatic, commuter, utility, restricted, or other special use category.
(2) The listed projects should include a review, when applicable, of the engine installation manual limitations to ensure 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:
(a) Number of blades;
(b) Diameter;
(c) Moment of inertia;
(d) Propeller extensions;
(e) Overhung moment (distance from the propeller center of gravity to the engine);
and
(f) Weight.
c. Propeller Structural Data. Propeller structural data used to conduct the fatigue evaluation, is typically generated during propeller type certification to meet the requirements of part 35. This data supports the vibration tests and fatigue evaluation on the airplane. The 4
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 gauge 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, applicants may generate it using AC 35.37-1B.
d. Background. Appendix 2 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 3 provides considerations for planning a vibration test for propellers on reciprocating and turbine engine installations.
7. 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). Othe r significant factors are the type of airplane and the maneuvers included in the airplane operating envelope. Appendix 3 provides an example of test conditions from propeller vibration testing on part 23 category reciprocating and turbine engine installations.
(1) Reciprocating Engine Installations. Propeller vibration testing on reciprocating engine installations should focus on evaluating possible combinations of engine power and rpm during ground and flight operations. Propeller vibration for single reciprocating engine tractortype installations in the normal category depends 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 propeller vibration characteristics and load amplitudes during testing:
1 Engines with higher compression ratios tend to produce higher propeller vibratory load amplitudes due to the increase in cylinder pressures.
2 Engine crankshaft torsional dampers 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.
3 Propeller indexing relative to the crankshaft can affect propeller vibration characteristics and stress amplitudes.
4 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:
1 Twin-engine airplanes;
2 Pusher airplanes; and 3 Aerobatic airplanes.
(c) Ground operations should be conducted in calm and cross winds. Applicants should consider brakes locked and takeoff power conditions, single engine taxi, functional checks such as feather checks, and reverse thrust during taxi and landing operations.
(2) Turbine Engine Installations. The propeller vibration testing on turbine engine installations should focus on evaluating the installation characteristics and flight and ground operation 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. These factors may include changes such as airplane weight; position of flaps; and cross wind, as explained below.
1 Changes in airplane weight can have a significant influence on the propeller load amplitudes. An applicant’s evaluation should consider both the airplane maximum gross weight and its 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.
4. Engine water injection systems or autofeather and automatic power increase systems should also be considered since these systems increase the engine power.
5. Ground operations should be conducted in calm and cross winds.
Considerations should include brakes locked and takeoff power conditions, single engine taxi, functional checks such as feather checks, and reverse thrust during taxi and landing operations.
(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.
(3) Additional Considerations. Paragraphs 7a.(1) and (2) of this AC provide considerations for conventional spark ignition aviation gas reciprocating and turbine engine configurations. Other engine configurations, such as diesel engines and rotary engines, 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. This survey should use electric resistance strain gauges, coupled with an electronic signal processing system and a recording device. The recording device records strain gauge signals suitable to evaluate the propeller loads throughout the airplane's operational envelope. The processing system processes strain gauge signals and other applicable information after the flight test.
(2) Strain gauges. Strain gauges vary in size, resistance, material, and construction.
When selecting the type of strain gauge, consider the required result, operating environment, and system instrumentation requirements. The location of blade strain gauges is generally determined from static shake tests, analytic predictions, and previous rotating test results.
(a) Strain gauges should be placed on the propeller to measure surface strain that can be converted to stress or load with proper calibration. When placing strain gauges on the propeller for the flight stress survey, many elements should be considered. First, consider the fatigue tests conducted for § 35.37, since the measured flight test data will be used in conjunction with the fatigue test data. Also, place the strain gauges at reference locations that relate the data to applicable fatigue limits. The location and quantity of strain gauges during the vibration survey are governed by the requirements of the evaluation. If the strain gauges do not provide an accurate picture of the propeller loading, the subsequent evaluations will not provide useful results.
(b) Strain gauges 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. Use a sufficient number of strain gauges to define the load distribution along the blade and to identify the locations of peak stress.
(c) Applicants should consider placing strain gauges on the following components when analysis cannot adequately demonstrate stresses suitable for fatigue evaluation:
1 Other areas of the blade dictated by the specific blade design;
2 Hub unit;
3 Pitch change components;
4 Blade counterweights;
5 Engine shaft; and 6 Other components requiring load information.
(d) Placing a strain gauge where the maximum strain or stress occurs is not always possible. In these cases, use reference strain gauges. Strain gauge readings from one location on the propeller may be related to other locations based on previous analysis or test data.
(e) For the evaluation of flutter conditions, install 45 gauges (shear stress gauges) on the blade surface at a location that is sensitive to blade torsional vibration.
(f) In some cases, strain gauges should be installed on the engine shaft. Data from gauges on the shaft provides information on the torsional forcing frequencies produced by a reciprocating engine and on the propeller shaft bending moment due to propeller aerodynamic and gyroscopic loads. This data may be needed to evaluate whether the engine shaft and propeller mounting flange have sufficient structural capacity. The engine shaft data is often provided 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 or use frequency or amplitude modulation.
(a) The instrumentation should have the following characteristics:
1 Data calibration to ensure 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; and 8
6 Time correlation to relate the strain gauge 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 important strain gauge signals are higher in frequency or amplitude than the capability of the equipment, they will be incorrectly recorded, and consequently, the results will not be accurate. Similar problems arise when a system designed for a large diameter propeller on a turbine engine transport airplane is used for a small diameter propeller on a general aviation airplane. The system requirements may also be different between metallic and composite blades. Review each installation to determine if the instrumentation system is adequate to record propeller load.
(c) Instrumentation systems may not have the capability to measure steady load as well as vibratory load. If the system does not measure steady load, conduct the appropriate analysis to determine the steady load levels.
(4) Test data. The strain gauge signals and certain airplane operating conditions are recorded during the stress survey. Test information is also logged during the stress survey to supplement the recording. The combination of recorded and logged data fits the needs of the fatigue evaluation. The following are some of test parameters to consider for flight and ground test programs. Evaluate the overall test program requirements to determine which parameters should be recorded or logged.
(a) Voice record;
(b) Pitch angle;
(c) Strain gauge signals;
(d) Altitude;
(e) Once-per-revolution (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 center of gravity acceleration;
(n) Ground cross-wind direction;
(o) Yaw angle; and
(p) Weather conditions.
(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, conduct an evaluation 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-counting algorithm (also known as the “rain-flow counting method”) is a methodology suitable for describing the load cycle content of a load history for fatigue evaluation.
8. 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 2 and the reference identified in paragraph 4c of this AC titled “Mechanical Vibrations.” 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. Examine resonant conditions 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 affect the fatigue life of the propeller.
a. Pre-Stress Survey Evaluation. Before the vibratory stress survey, identify the propeller blade natural frequencies 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 unexpectedly, conduct further testing to determine the force acting on the blade and the blade vibratory mode involved. A resonant condition found previously on a similar installation should also be found on the installation being tested.
b. Stress Survey Tests. When testing indicates 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 a stress peak is just beyond the propeller operating range, conduct testing 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.
c. Identification of the Forces. When a significant resonant condition is found during the vibratory stress survey, conduct sufficient testing 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 ground cross-winds; and aerodynamic forces from the wake of the airplane on pusher installations. The aerodynamic forces act at integer multiples of the propeller rotational speed known as “P” (per-propeller revolution) orders. 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” (per-engine revolution) orders. Th e 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 cross-winds increase. Th erefore, operating in a 15 knot rear-quartering cross-wind may be acceptable, while operating in winds above 15 knots may not.
d. Identification of the Blade Modes. When a significant resonant condition is found during the stress survey, conduct sufficient testing to permit the identification of the blade natural frequency and vibratory mode shape. Do this 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.
Use the significant blade natural frequencies 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 from the natural frequencies of this 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.
(1) When 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 consider 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. Placards are typically set 100 rpm higher and lower than the peak stress rpm or as shown by testing.
(2) For example, figure 1-1 in Appendix 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 1-2 in Appendix 1 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 was set based on only the measured data, figure 1-1's 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 1-2 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 1-1 assuming that the shape of the stress peak remains the same. A blade with the maximum manufacturing tolerances and paint will lower the critical speed to 950 rpm, and blade wear will increase the critical speed to 1050 rpm. This increases the upper and lower bounds of the rpm restriction to 1150 and 850 rpm, respectively.
(3) In addition, assess the magnitude of the stress amplitude 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. We recommend the following procedures 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 should determine if any service use dependent changes in the blade's natural frequency exist that were not considered during propeller certification . This may be done 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.
9. 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. It 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 exposure to flutter.