The test loads will be applied and reacted using mating aircraft parts or special fixtures which simulate the mating parts. The methods and apparatus to be used for the static tests of the castings are discussed in the apparatus and method section of this report.
Testing will be conducted in…(location).
TEST SPECIMEN The castings which will be tested are listed in numerical order in figure AC 29.621-2.
Those castings which, after structural analysis, show less than a 1.5 casting factor will be tested. All directions are given with reference to a forward facing position in the rotorcraft.
On the basis of a radiographic examination, the three castings which are of the poorest acceptable quality in the first production lot of castings will be selected as test specimens. The poorest of the three castings will be selected as the initial test casting and its radiograph or ASTM standard will be used as the standard for accepting future castings of the particular part unless later standards are approved. Three castings must be tested for each critical condition for each part.
Conformity Inspection Each machined casting will be subjected to an FAA/AUTHORITY conformity inspection prior to testing to determine compliance with the type design drawings. A conformity report for each casting may be incorporated in Part II, Test Results, of this report.
The test specimen will be permanently marked or defaced after testing to preclude its use on a rotorcraft.
See figure AC 29.621-2 for an example of a convenient means of listing castings.
TEST LOAD The test load(s) to be applied to each casting represents the critical loading condition(s) for that casting. The critical conditions on each of the castings were determined by the design criteria and substantiating data approved by the FAA/AUTHORITY.
The design criteria for all of the castings to be static tested may fall into one of two categories. The load factors and structural acceptability requirements for each category are discussed below. Casting factors that are included on the load sheets of each part do not apply in the discussion below. (See paragraph AC 29.621b(2) for casting factors.)
Page D - 21
Castings Designed to Limit Load Conditions A structural analysis of each test casting showing the critical design limit load conditions is given in the data, (reference report number here). The load factors for the static test of the castings are as follows:
1.15 x design limit load = design yield load
1.50 x design limit load = design ultimate load Castings Designed Only to Crash Landing Conditions The castings in this category were designed using a crash landing load factor for the design ultimate load. The design yield load criteria of 1.15 x limit load need not apply to these castings. The test loads for these castings may be given in terms of design ultimate load on the individual casting load sheets shown in Part I of this report.
Test Procedures Depending on the results of the initial static test of each casting, the following procedure will be used.
a. If in the initial test of critical castings the casting is found to have a casting factor of 1.5 (1.5 x design ultimate load), the casting will be considered acceptable and no further tests will be conducted.
b. If in the initial test(s) the critical casting is found to have a casting factor less than 1.5 but equal to or greater than 1.25, two additional castings will be tested for each critical load condition. Each must also show a minimum casting factor of 1.25.
c. If in the initial test, or in one of two additional tests, a casting shows a casting factor less than 1.25 times design ultimate or yields prior to reaching 1.15 times design limit load, the casting will be redesigned and retested. The yield criteria are also applicable to the first two procedures with the exception of critical castings designed to crash landing conditions.
TEST APPARATUS AND METHOD The Model XYZ casting static tests will be conducted using fixtures designed to simulate the installation of the castings in the aircraft. Where practical, mating aircraft parts will be used to apply and react test loads. When practical, the static tests will be conducted with mating castings assembled when the critical loads for the mating castings are compatible; otherwise, fixtures simulating the mating parts may be designed and fabricated for the tests. Assembly hardware used to mount test castings will be the same as hardware used on the rotorcraft. All bolt torques and other assembly notes will conform to the type design assembly instructions.
Page D - 22
The tests will be conducted using calibrated load measuring devices such as hydraulic cylinders and pressure gages, load cells, strain gage bridges, or dead weights.
Deflections of the casting may be measured using graduated dial indicators or scales in all tests. The deflection indicators will be based or mounted on the casting and will measure casting deflection only, when possible, otherwise the indicators will be based on the fixture and measure deflection of the casting relative to the fixture. Deflection readings will be made at 20 percent increments of limit load through 100 percent of limit load and at 115 percent of limit load. These increments may be changed if necessary.
Permanent deformation readings will be made after relieving 115 percent and 150 percent of limit load.
See figure AC 29.621-1 as an example of a load sheet.
Page D - 23
+5,000 # TENS.
ULTIMATE LOADS -1,000 # COMP Tension Comp.
Tension Reaction Compression Reaction FIGURE AC 29.621-1
EXAMPLE OF CASTING LOAD SHEET
RETRACT ACTUATOR SUPPORT - LANDING GEAR Page D - 24
Include spherical bearing with clamped-up bolt and a link in the test setup to confirm the stability. Loads are based on a jam condition with actuator operating at 1,700 PSI pressure maximum.
A 1.25 casting factor is included in these loads.
These loads were derived from data in approved structural loads and analysis report.
END OF SAMPLE REPORT
(4) The format of the previous guidance material may be changed to accommodate the applicant’s method of data presentation.
(5) Nonstructural castings may be tested and included in the test report.
(6) Cast fluid containers, including hydraulic fluid containers, may be tested as prescribed in other rules of FAR Part 29 and a test proposal and test results report may be included in the casting test report or an appropriate report may be referenced for convenience. We recommend use of one report to contain test data or reference to test data for all castings used on the rotorcraft.
Page D - 25
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.
Page D - 28
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.
Page D – 28.2
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).
Page D - 29
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.
Page D - 30