7130/97 AC 27-1A and design criteria for the castings are discussed in detail in the test loads section of this report.
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 Table I. 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 Table I 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 250b(2) for casting factors.)
Par 250 313
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 tests) 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.
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7/30/97 АС 27-1A 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 250-1 as an example of a load sheet.
Par 250 315
P +5,000 # TENS.
ULTIMATE LOADS -1,000 # COMP Tension Comp.
Tension Reaction Compression Reaction FIGURE 250-1
EXAMPLE OF CASTING LOAD SHEET
RETRACT ACTUATOR SUPPORT - LANDING GEAR 316 Par 250
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 27 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.
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TABLEI EXAMPLE
CASTINGS TO BE STATIC TESTED FOR MODEL XYZ REF. LOAD
MACHINE OR SHEET CASTING NO. ASSY. NO. NAME AND LOCATION MATERIAL FIG. NO.
Base Assembly, Pilot's Collective Column 318 Par 250
251. § 27.623 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 application, 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 Paragraph 251b(7) below.
(2) A bearing factor, appropriate for the application, shall be used unless a larger factor is used.
(3) For reference, specific bearing factors are contained in § 29.685(e) for transport rotorcraft 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) for transport rotorcraft.
(4) MIL-HDBK-5D, Paragraph 8.3, refers to design standards for plan or journal bearings or bushings. These standards are found in Air Force Systems Command Design Handbook AFSC DH-2-1, Airframe, Chapters 3 and 6.
b. Procedures.
(1) Control system joint bearings are discussed under Paragraph 284, § 27.685, of this AC, but the bearing factors are noted here for convenience. For transport rotorcraft control systems, § 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 should not be exceeded. Use of this for normal category rotorcraft is recommended.
(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 a grease-lubricated plain bearing or bushing to account for the anticipated higher loads caused by pounding or vibration. See Paragraph 251b(6) for recommendations on 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 OR ILLUSTRATION, a bearing factor of
1.85 may be applied to the design loads on the softest material reacting the bearing loads. A different but appropriate factor will be acceptable. The design limit load may be calculated for the example of a 0.312-inch-diameter bolt in a 2-inch-long bearing.
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The bearing projected area is 0.312 × 2 = 0.624-square-inches. The design limit load is 3,000 pounds. The design limit bearing stress is 3,000 pounds/0.624-square-inch 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. See Paragraph 251a(4) for bearing factors.
(4) Military standard part specifications, MS 21240, "Bearing, Sleeve Plain, TFE Lined," and MS 21241, "Flanged Bearing, Sleeve Plain, TFE Lined," contain allowable 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 § 27.625, 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 should 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 necessary.
(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 0.0014 to 0.0040 inch.
252. § 27.625 FITTING FACTORS.
a. Explanation. A 1.15 factor is specified to ensure that the calculated load and stress distribution within any fitting is conservative. Application of the factor is excluded or is an exception as stated in the rule.
320 Par 251
b. Procedures.
(1) The factor may be applied to the calculated load or stress for the fitting.
(2) The structural design substantiating data should include the fitting factor and where applicable should include, but not be limited to, the rotor system. The rotor system includes the 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.
(її) 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 satety.
253. § 27.629 FLUTTER.
a. Explanation. The rule requires that the rotorcraft "be free from flutter under each appropriate speed and power conditioni
b. Procedures. Freedom from flutter is to be shown for the entire rotorcraft with special attention to the blades, fins, and stabilizers.
(1) Flutter is defined as an aeroelastic instability resulting primarily from coupling of flap and pitch bending modes.
(2) Freedom from flutter may be shown by analysis or by appropriately instrumented flight flutter tests.
(3) The flight load survey proposal submitted for compliance with § 27.571 may also contain tests to fulfill compliance with § 27.629.
Par 252 321
АС 27-1A 7/30/97
(4) Flight loads survey data or flight flutter test data should be reviewed to ensure that excessive oscillatory deflections of rotors or surtaces will not be encountered.
(5) Sensitivity analyses should be conducted to ensure that normal wear in the pitch change mechanisms of the main rotor blades and tail rotor blades does not reduce the effective stiffnesses sufficiently to cause flutter.
322 Par 253
254.-264. RESERVED.
Intentionally Left Blank 323 (thru 333) Par 254
SECTION 16. ROTORS
265. § 27.653 (Amendment 27-2) 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 else 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.
266. § 27.659 (Amendment 27-2) 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 §$ 27.571 and 27.629 are met.
(2) The structural substantiation should show static strength to meet the maneuver and gust loads of §§ 27.337, 27.339, and 27.341. In addition, the main rotor loads of § 27.547(c) should be substantiated. The fatigue strength of the mass balance devices (including structural supports) should meet the requirements of § 27.571.
(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.
267. § 27.661 (Amendment 27-2) ROTOR BLADE CLEARANCE.
a. Explanation.
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