FIGURE AC
29.621-2 EXAMPLE
CASTINGS TO BE STATIC TESTED FOR MODEL XYZ REF. LOAD
MACHINE OR SHEET CASTING NO. ASSY. NO. NAME AND LOCA TION MATERIAL FIG. NO.
Base Assembly , Pilot’s Collective Column Page D - 26
AC 29.623. § 29.623 BEARING FACTORS.
a. Explanation.
(1) The rule requires use of a minimum bearing factor in free fit joints to account for effects of typical relative motion. A minimum value is not specified in the rule. The factor, appropriate for the applic ation, is applied to the ultimate bearing strength of the softest material used as a bearing. A definition of free fit (clearance fit) is noted in Subparagraph b(7) below.
(2) Specific bearing factors are specified by § 29.685(e) for control system joints subject to angular rotation. These factors are applied to the ultimate bearing strength of the softest material used as a bearing in the control system. Control systems ball, roller, or needle bearings are covered by § 29.685(f).
(3) MIL-HDBK-5C, paragraph 8.3, refers to design standards for plain or journal bearings or bushings. These standards ar e found in Air Force Systems Command Design Handbook 2-1, Airframe, Chapters 2 and 6.
b. Procedures.
(1) Control system joint bearings are discussed under paragraph AC 29.685, § 29.685 of this document but the bearing factors are noted here for convenience.
Section 29.685(e) requires a 2.0 bearing factor for cable systems and a 3.33 bearing factor for push-pull systems other than ball and roller bearing systems. The manufacturer’s static, non-Brinell rating of ball and roller bearings may not be exceeded as stated in § 29.685(f).
(2) A landing gear pivot, grease lubricated, plain bearing is one example of a free fit subject to pounding or vibration. A bearing factor of 2.0 may be used or another factor may be proven for grease lubricated plain bearing or bushing to account for the anticipated higher loads caused by pounding or vibration. See subparagraph AC 29.623b(6) for ball or roller bearings.
(3) A typical engine mount bolt installation with a plain bearing having a free or loose fit (not interference fit), is another example of a sleeve bearing application subject to a design bearing factor. As an example, a bearing factor of 1.85 may be applied to the design loads on the softest material reacting the bearing loads. A different factor will be acceptable if proven. For example, t he design limit load may be calculated for a .312-inch-diameter bolt in a 2-inch-long bearing. The bearing projected area is .312 x 2 = .624-inch-square. The design limit load is 3,000 pounds. The design limit bearing stress is 3,000 pounds/.624-inch-square x 1.85 = 8,894 PSI. If a free or loose fit is not used; i.e., tighter than free fit, a bearing factor is not required.
(4) Military standard part specification, MS 21240, “Bearing, Sleeve Plain, TFE Lined” and MS 21241, “Flanged Bearing, Sleeve Plain, TFE Lined contain allowable Page D - 27
AC 29-2C, Chg 4 5/1/2014 load ratings, static, and dynamic that apply to the particular use of the bearing. An appropriate bearing factor should be applied to the static rating. Military Specification MIL-B-8943A, Amendment 3, “Bearing, Sleeve, Plain, and Flanged, TFE Lined” (temperature range -65° F to 250° F) shows that MS 21240 and MS 21241 sleeve bearings have been superseded by MS 1934/1 and MS 81934/2 sleeve bearings, respectively. Military Specification MIL-B-81934, Amendment 2, “Bearings, Sleeve, Plain and Flanged, Self-Lubricating,” uses TFE liners. These bearings are intended for use in a temperature range from -65° F to +325° F. Whenever a sleeve bearing is used an appropriate bearing factor should be applied to the static rating that is contained in the specification or standard. Other sleeve bearings are contained in standards NAS 72 through NAS 77, NAS 537, and NAS 538. The installation design information is only contained in standards NAS 72 through NAS 74. These types of plain sleeve bearings are designed for clamping to the shaft or bolt with relative motion occurring on the bearing outside diameter. An appropriate bearing factor is required for the application.
(5) The minimum fitting factor 1.15, specified by § 29.625, must be applied as specified to account for load distribution at the fitting. This fitting factor need not apply to plain or journal “bearings” whose “bearing factor” exceeds 1.15.
(6) For airframe and landing gear structural joints, the manufacturer’s static, non-Brinell rating of ball and roller bearings may not be exceeded. ABEC Class 1 bearings or better quality bearings may be used in airframe structural joints and landing gear; ABEC Class 3, 5, or 7 bearings should be used in rotor pivot joints. The non-Brinell rating includes consideration of the bearing factor and no other bearing factor is required.
(7) A free fit was described in American Standards Association (ASA) Standard B4a-1925. The “free fit” clearances and tolerances of this old standard are now called Class RC6, Medium Running Fit, in ASA Standard B4.1, 1955. As an illustration using these standards, a 1-inch diameter shaft and a plain sleeve bearing would have a clearance ranging from .0014 to .0040 inch.
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AC 29.625. § 29.625 FITTING FACTORS.
a. Explanation. A 1.15 factor is specified to assure that the calculated load and stress distribution within any fitting is conservative. Application of the factor is excluded or excepted as stated in the rule.
b. Procedures.
(1) The factor may be applied to the calculated load or stress for the fitting.
(2) The structural substantiating data for the rotorcraft, including the rotor system, must include the prescribed fitting factor. The rotor system includes the flight control system rotor head and hubs and rotor blade attachments, rotor head and hubs, and boosted control system elements. Other typical areas that may be considered are tail rotor gearbox attachment, tailboom to fuselage fittings, transmission pylon attachments, and landing gear attachment to the rotorcraft.
(3) The fitting factor is not required in the following applications:
(i) Joints such as continuous joints in metal plating, welded joints and scarf joints in wood.
(ii) Elements proven by limit or ultimate load tests such as nonboosted control system parts.
(iii) Elements for which a larger load factor is used such as a casting factor, a 1.33 retention factor when required for seats and safety belts, a fatigue factor, bearing factor or special factor greater than 1.15, crash load factors that are the only design case, and crash load factors that exceed limit load factors x 1.5 x 1.15.
(iv) Elements for which the failure mode does not affect safety of flight or occupant safety.
AC 29.625A § 29.625 (Amendment 29-42) FITTING FACTORS.
a. Explanation. Amendment 29-42 added § 29.625(d) that requires a 1.33 factor applied to the emergency landing loads of § 29.561(b)(3) for the substantiation of attachments of each seat, berth, and litter to the structure and each safety belt or harness to the seat, litter, or structure.
b. Procedures. All of the advisory material pertaining to this section remains in effect with the following additions.
(1) A fitting factor of 1.33 must be applied to the emergency landing loads of § 29.561(b)(3) when evaluating the attachments of the seat, berth, and litter to the Page D – 28.1
AC 29-2C, Chg 4 5/1/2014 structure, and each safety belt and harness attachment to the seat, berth, litter, or structure.
(2) The 1.33 factor is required whether analysis or test is used.
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AC 29.629. § 29.629 FLUTTER.
a. Explanation.
(1) The rotorcraft must be free from flutter.
(2) Section 29.251 vibration is an associated flight requirement concerning flight demonstrations. See paragraph AC 29.251 for this standard.
(3) Section 29.571(a)(3) concerns in-flight measurement of loads or stresses.
b. Procedures.
(1) Freedom from flutter may be shown by analysis or appropriately instrumented flight flutter tests.
(2) The flight loads survey proposal submitted for compliance with § 29.571 may also contain tests to fulfill compliance with § 29.629. The flight loads survey program encompasses the envelope of design airspeed and rotor RPM, and sufficient aerodynamic excitation is generally present to excite any latent flutter modes.
(3) Flight loads survey data or flight flutter test data submitted should be reviewed to assure that excessive oscillatory loads of rotors or surfaces will not be encountered.
AC 29.629A. § 29.629 (Amendment 29-40) FLUTTER AND DIVERGENCE.
a. Explanation. Amendment 29-40 adds the requirement that each aerodynamic surface of the rotorcraft must be free from divergence in addition to the requirement of freedom from flutter. The aeroelastic stability evaluations required by this regulation include flutter and divergence. Compliance with this regulatory requirement should be shown by analysis and/or flight test, supported by any other means found necessary by the Administrator. The aeroelastic eval uation of the rotorcraft should include an investigation of the significant elastic, inertia and aerodynamic forces on all aerodynamic surfaces (including rotor blades) and their supporting structure. The forces associated with the rotations and displacements of the plane of the rotors should be considered.
b. Procedures.
(1) It should be shown by analysis that the rotorcraft is free from flutter and divergence (unstable structural distortion due to aerodynamic loading) under any condition of operation including:
(i) Airspeeds up to 1.11 VNE (power on and power off).
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AC 29-2C, Chg 4 5/1/2014
(ii) Main rotor speeds from 0.95 x the minimum permitted speed up to
1.05 x the maximum permitted speed (power on and power off).
(iii) The critical combinations of weight, CG position, load factor and altitude.
(2) Adequate tolerances should be established on those physical quantities which could affect flutter, divergence, or structural distortion to a degree sufficient to cause a significant deterioration in the characteristics of the rotorcraft, such that likely variations in these quantities will not result in flutter or divergence within this envelope.
(3) All physical properties which could contribute to a reduction in the predicted flutter or divergence margins are to be investigated, including stiffness, damping, mass balance, and aerodynamic coefficients. Parame tric variations should be sufficient to cover any possible variation due to manufacturing and maintenance tolerances and environmental factors, and to provide conservatism where estimated values are used.
Linear approximations to non-linear variations may be used.
(4) Where approval for flight in icing conditions is being sought, the effects of ice accretion on unprotected surfaces, including that which might occur as a result of a single system malfunction, should be considered.
(5) Rotorcraft should be demonstrated by suitably instrumented flight tests to be free from flutter and divergence at all combinations of forward speed and rotor RPM (power off and power on), up to 1.11 VNE and 1.05 times the maximum permitted RPM (except that combinations of speed in excess of VNE and rotor speed in excess of the maximum permitted are not required to be tested). Flight tests to demonstrate compliance with flutter and divergence requirements may normally be addressed simultaneously with testing in compliance with §§ 29.251, 29.571, 29.1505 and similar regulations. Special flight tests for fl utter and divergence would not normally be required.
(6) Stabilizing surfaces may be addressed by analysis alone if flutter and divergence margins can be shown to provide adequate conservatism. Flight testing at
1.05 times the maximum power-on rotor speed may also be waived if it is considered impractical, and can be adequately addressed by analysis.
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AC 29.631. § 29.631 (Amendment 29-40) BIRD STRIKE.
a. Explanation. Amendment 40 adds requi rements for continued safe flight and landing after a bird strike. Compliance with § 29.631 must be shown for a 2.2 lb (1.0 kg) bird at a relative velocity equal to the lesser of VNE or VH at altitudes up to 8,000 feet.
For Category A certification, the rotorcraft must be capable of continued safe flight and landing after the described bird strike. For Ca tegory B certification, the rotorcraft must be capable of a safe landing after the bird strike.
b. Procedures. For compliance with FAR 29.631, it should be demonstrated by test or analysis supported by test evidence that,
(1) The windshields will withstand the bird strike, without penetration, and,
(2) The rotorcraft is capable of continued safe flight and landing following impact with a 2.2-lb (1.0 kg) bird at VNE or VH (whichever is the lesser) at altitudes up to 8,000 feet. Areas of impact that are of particu lar interest include flight control surfaces (which includes main and tail rotors) and exposed flight control system components.
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SUBPART D - DESIGN AND CONSTRUCTION ROTORS AC 29.653. § 29.653 (Amendment 29- 3) PRESSURE VENTING AND DRAINAGE OF ROTOR BLADES.
a. Explanation. The rule requires each rotor blade to be provided with venting and drainage means (i.e., holes, etc.) or the blade must be sealed and designed to withstand internal pressure.
b. Procedures. Although the rule prov ides for venting and drainage features, recently certificated blades have been designed to be sealed and to sustain the “maximum pressure differentials expected in service.” For modern blade designs, the internal pressure buildup due to environmental effects and centrifugal acceleration effects (near the tip) can be readily sustained with moisture sealing accomplished. The use of sealed blades is highly advantageous and recommended because of the possibility for severe corrosion damage resulting from trapped moisture and because of the difficulty in finding internal corrosion damage by use of field level inspections.
AC 29.659. § 29.659 (Amendment 29-3) MASS BALANCE.
a. Explanation. The rule requires that mass balancing of rotors and blades be provided, as necessary, to prevent excessive vibration and flutter. Further, the rule requires structural substantiation of the mass balance installation.
b. Procedures.
(1) The weight, geometry, and location of rotor and blade mass balance devices are determined as the requirements of §§ 29.571 and 29.629 are met.
(2) The structural substantiation should show static strength to meet the maneuver and gust loads of §§ 29.337, 29.339, and 29.341. In addition, the main rotor loads of § 29.547(c) should be substantiated. The fatigue strength of the mass balance devices (including structural supports) should meet the requirements of § 29.571.
(3) In addition to the appropriate strength requirements, some recent designs have included features which trap the balance weight inside a limited area even if the primary attachment means (adhesive, bolts, etc.) fail. This type of design feature is recommended because of the severe loading environment to which balance devices are subjected.
AC 29.661. § 29.661 (through Amendment 29-3) ROTOR BLADE CLEARANCE.
a. Explanation.
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SUBPART D - DESIGN AND CONSTRUCTION ROTORS AC 29.653. § 29.653 (Amendment 29-3) PRESSURE VENTING AND DRAINAGE OF ROTOR BLADES.
a. Explanation. T he rule requires each rotor blade to be provided with venting and drainage means (i.e., holes, etc.) or the blade must be sealed and designed to withstand internal pressure.
b. Procedures. Although the rule provides for venting and drainage features, recently certificated blades have been designed to be sealed and to sustain the “maximum pressure differentials expected in service.” For modern blade designs, the internal pressure buildup due to environmental effects and centrifugal acceleration effects (near the tip) can be readily sustained with moisture sealing accomplished. The use of sealed blades is highly advantageous and recommended because of the possibility for severe corrosion damage resulting from trapped moisture and because of the difficulty in finding internal corrosion damage by use of field level inspections.
AC 29.659. § 29.659 (Amendment 29-3) MASS BALANCE.
a. Explanation. The rule requires that mass balancing of rotors and blades be provided, as necessary, to prevent excessive vibration and flutter. Further, the rule requires structural substantiation of the mass balance installation.
b. Procedures.
(1) The weight, geometry, and location of rotor and blade mass balance devices are determined as the requirements of §§ 29.571 and 29.629 are met.
(2) The structural substantiation should show static strength to meet the maneuver and gust loads of §§ 29.337, 29.339, and 29.341. In addition, the main rotor loads of § 29.547(c) should be substantiated. The fatigue strength of the mass balance devices (including structural supports) should meet the requirements of § 29.571.
(3) In addition to the appropriate strength requirements, some recent designs have included features which trap the balance weight inside a limited area even if the primary attachment means (adhesive, bolts, etc.) fail. This type of design feature is recommended because of the severe loading environment to which balance devices are subjected.
AC 29.661 § 29.661 (through Amendment 29-3) ROTOR BLADE CLEARANCE.
a. Explanation.
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(1) This paragraph discusses the regulatory requirement contained in § 29.661.
That requirement is that there must be enough clearance between the rotor blades (main and tail rotor blades) and other parts of the structure to prevent the blades from striking any part of the structure during any operating condition.
(2) In the past, some rotorcraft that have been shown to comply with § 29.661 during the certification process have experienced subsequent accidents involving in-flight contact between the main rotor and airframe (rotor/airframe contact).
Completion of developmental and TIA flight testing without a rotor/airframe contact incident has proven not to be adequate demonstration of compliance with § 29.661 in all cases.
(3) Historically, in-flight rotor/airframe contact accidents have occurred as a result of mast bumping, rotor stall, or excessive rotor flapping due to control manipulation. For some rotorcraft, a more thorough examination may be required to ensure adequate clearances.
b. Procedures. Testing should be conducted by the applicant, prior to FAA/AUTHORITY participation, to ensure that the rotorcraft is in compliance with § 29.661 in all areas of the envelope during all operational maneuvers expected throughout the life of the aircraft. The tests should be performed concurrently with performance, flight characteristics, and flight loads testing. Tests should include:
(1) A blade flapping survey to determine flapping angles/margins, blade bending, and blade clearance from the entire airframe. Data may be gathered from instrumented flapping hinges, instrumented blades, high-speed video from airframe mounted cameras, a chase aircraft, or other acceptable means.
(2) Determine that margin exists between the minimum rotor RPM encountered during testing for compliance with § 29.143(d) and the RPM (power off) at which analysis shows that the rotor will experience a significant stall. A significant stall condition may be defined by the rotor reaching an RPM from which normal operating RPM is unrecoverable due to drag on the main rotor blades or, a stall that results in excessive main rotor flapping. The rotor RPM decay rate under the critical conditions of weight, density altitude, minimum approved power-on rotor RPM must provide a margin between the minimum rotor speed achieved during demonstration of compliance with §§ 29.87 and 29.143(d) and the analytically derived rotor stall RPM for the same conditions. For example, the minimum rotor RPM resulting during H-V tests must allow for a margin above the rotor stall value to allow for variations that may occur during operational flying.
(3) During parts of the certification flight test program, frangible devices (wood dowels) or other means of measuring clearance, may be requested to confirm that the clearances shown in the drawings and verified during company flight tests are adequate in all operating conditions. Balsa wood dowels or styrofoam pads may be clamped to Page D - 33
the aft part of the fuselage and cabin roof within the rotor arc. Such devices may be especially helpful in determining clearance during autorotation and controllability testing under FAR 29.143. If such measuring devices are used, the type inspection report should contain a record of clearance found during the tests. During TIA flight testing, it is not necessary to precisely determine the clearance but only necessary to determine "enough clearance" as stated in the rule.
AC 29.663. § 29.663 GROUND RESONANCE PREVENTION MEANS.
a. Explanation.
(1) This section, adopted in Amendment 29-3, and amended by Amendment 29-30, requires reliability and damping action investigation for the ground resonance prevention means which typically includes the shock struts. Section 29.1529 requires associated maintenance information in the maintenance manual. The probable range of variations in service, not just the allowable range, should be established and investigated as prescribed. This probable range includes operation on the ground, or other appropriate landing surface applicable to the rotorcraft design. Quantitative test data are generally obtained in compliance with this rule although analysis or tests may be employed. The preamble to Amendment 29- 3 contains additional information.
(2) Note that the maintenance information is not contained in the approved mandatory section of the maintenance manual.
(3) Paragraph AC 29.241 concerns demonstrating freedom from ground resonance during certain applicant and TIA verification evaluations or tests of the rotorcraft. Section 29.241 complements the requirements of § 29.663. As noted in paragraph AC 29.241, a specific requirement for a ground vibration survey was removed from CAR Part 7. However, § 29.663 was adopted by Amendment 29- 3 to investigate possible sources of ground resonance and to assure that the reliability of the ground resonance prevention means; i.e., dampers, shock struts, etc., would preclude the occurrence of ground resonance. The total rotorcraft system, including landing gear, struts, tires, etc., is evaluated under this standard.
(4) Viscous dampers in the rotor head have been used for many years to prevent ground resonance. Modern rotorcraft designs may also use elastomeric dampers and may use elastomeric bearings in the rotor head and rotor pylon attachment to the airframe. The standard applies to viscous and elastomeric dampers.
The “probable” range in damping shall be investigated. The standard also requires investigation of the probable range of variations of these dampers, whether viscous or elastomeric, and elastomeric bearings to preclude ground resonance.
(5) Ground resonance can occur due to flexibility in the rotor pylon restraint system as well as with landing gear flexibilities and/or shock struts. See paragraph AC 29.663b(2) for an explanation. An analysis may be done to show the effect of the rotor pylon mount stiffness on ground resonance stability. If the analysis Page D - 34