endurance test, although the presence of the vibration aspects does not normally satisfy the vibration evaluations required by §§ 29.571 and 29.907. In fact, vibration modes may be changed and amplified by the tie-down restraints and the increased thrust to be expected from in-ground effects on the rotor system. These effects, although unquantified, are intended as a normal part of endurance testing.
Preproduction rotor blades have been successfully used in endurance tests but only after specific investigations of blade properties such as stiffness, inertia and inertia distribution, thrust and blade bending, and torsional frequency response have been carefully compared to assure validity of the test. The endurance test includes testing of the rotor control mechanism. Conformity of the rotors may be very significant to this aspect of the test.
b. Procedures.
(1) Section 29.923(b)(1) prescribes the takeoff portion of the endurance test.
This test involves a series of 5-minute repetitive runs at the torque and at the engine/rotor rotational speed selected by the applicant for the takeoff limit for the rotorcraft. These values of torque (manifold pressure, for reciprocating engines) and RPM should correspond to the red radials on the corresponding powerplant instruments, except on installations where uncompensated engine governor "droop" results in a higher rotational speed for lower powers. The requirement in this section for declutching the engine may be difficult to achieve if engine deceleration and rotor system deceleration rates are similar. In some cases, the engine fuel control deceleration schedule may be adjusted to achieve clutch disengagement, otherwise, an engine shaft brake mechanism may be needed.
(2) The torque and speed requirements for the optional 2½-minute one-engine-inoperative (OEl) tests should be interpreted as described above for the takeoff runs. If the test is conducted during warm ambient conditions, excessive engine gas temperatures may be required to achieve the torque and speed conditions required by this part of the test. Minor adjustments in the run schedule may be allowed to take advantage of cooler nighttime ambient temperatures. Addition of water/alcohol systems to increase engine hot-day power may be appropriate in some instances. Liquid nitrogen spray into engine inlets has also been used to depress inlet temperatures sufficiently to obtain test conditions.
(3) In § 29.923(c), (d), (e), and (f), the torque requirements should be interpreted as above; i.e., the run should be made with maximum continuous torque or percentage thereof, as specified by the subparagraph, and the rotational speed should be maximum continuous for Paragraph (d) and the lowest permissible "power-on" speed for Paragraphs (e) and (f). Rotor control cycling must be accomplished during the "maximum continuous" portion of the endurance run. The controls of concern are the flight controls; i.e., cyclic and directional controls for rotorcraft with tail rotor and single main rotor. The collective control is normally used to set power and is not involved in control cycling. During control cycling the controls may be cycled from stop Par 423 653
to stop, or a limited travel may be accepted if the travel produces the maximum fore and aft, left and right, and yaw thrust components of the rotors as measured in flight. One method of determining the required control displacement is to measure main rotor mast bending in level forward flight at maximum continuous power for the forward control displacement limit, and in level rearward flight at maximum continuous power (or the power associated with the maximum rearward flight speed to be expected) for the aft control displacement limit. Using the same mast bending instrumentation, with the rotorcraft in the ground tie-down situation, and with collective control set for maximum continuous power, displace the cyclic fore and aft to obtain the same mast bending as measured in flight. Similar measurements and control displacements may be used for sideward thrust components. Yaw control displacement should consider maneuver requirements in conjunction with sideward flight. Critical gross weight and center of gravity should be used to establish test conditions. These same procedures may be used to establish limited control positions required to comply with § 29.923(i) except that typical flight conditions to be used would be stabilized level flight at maximum continuous power, climb at maximum continuous power, and hovering, including stabilized sideward and rearward flight. Note that for § 29.923(i)(1) vertical thrust is required. Depending on the mast angle and center of gravity, this condition may not necessarily involve zero mast bending loads. Vertical thrust may be used during the takeoff run, including the runs at 2½-minute power and the overspeed run of § 29.923(h). One-engine-inoperative runs (§ 29.923(k)) should be conducted with the cyclic set for maximum forward thrust. For these runs and any run that does not specify the position for the yaw control, that control should be set to react main rotor torque.
(4) Section § 29.923(g) provides for introducing special tests into the endurance tests to demonstrate that the transmission and drive system can tolerate certain engine malfunctions to be expected. This was originally directed at demonstrating safety in the event of spark plug or magneto failures of reciprocating engines. Turbine engines normally do not exhibit failure modes suitable for substantiation by endurance testing; however, severe or abusive operating conditions which must be expected to occur in service should be defined and included in this test.
Conditions or phenomena to be considered should include but not be limited to moderate engine surge, abusive clutch engagements, torque mismatching, anticipated control mishandling, and so forth. Alternatively, repeating the takeoff run of § 29.923(b) may be appropriate. It is not intended that the special testing for 2½-minute power be repeated if a rerun of the takeoff power run is required by § 29.923(g).
(5) Section 29.923(h) requires overspeed testing at the torque which will produce maximum continuous power and at the maximum rotational speed to be expected. Normally this would be the maximum transient, power-on rotor speed available with speed controls operating. Special control adjustments for test purposes may be needed to achieve the required test conditions.
654 Par 423
(6) Section 29.923(i) requires stabilized flight control displacement according to a prescribed schedule. The control displacement should be the same as derived to show compliance with § 29.923(c)(2).
(7) Section 29.923() requires 400 clutch and brake engagements. These tests are prescribed to establish a level of reliability of clutch and brake components installed as a part of the rotor drive system of rotorcraft. The clutch tests apply to all clutches installed to comply with § 29.917(b)(3), and each such clutch must be tested. A rotor brake is not required for certification, although a brake of some type may be installed temporarily to facilitate conducting the clutch testing required by this section. Clutch disengagement is also required by this section, thus, malfunction of the disengagement feature would be a basis for discontinuance. Some rotorcraft configurations (those with single-spool turbine engines or reciprocating engines) include an additional clutch to decouple the engine from the drive system to facilitate engine starting. These clutches should also be exercised at least 400 times during this test.
(8) Section 29.923(k) sets forth the optional tests to be conducted if a 30-minute OEI rating is requested. It may be noted that the time for conducting this test replaces time deducted from the run of § 29.923(f). Flight control positions should be set for level flight or climb, whichever produces the maximum forward thrust component, and the antitorque system control should be set to react the maximum rotor torque. The torque and rotational speed values should be the maximum for which approval is requested.
(9) Section 29.923(m) normally is satisfied by the requirements of §§ 29.571 and 29.907.
(10) Section 29.923(n) requires special tests for rotor drive systems designed to operate at two or more gear ratios. Depending on the limitations and instructions proposed for operating at other gear ratios, additional tests (beyond the normal 200-hour schedule) or substitutions into the basic test should be conducted to quality the rotor drive system for operations at other gear ratios. The length of testing, torque and speed requirements, overspeed tests, and control positions for these tests should parallel the requirements of the basic endurance test.
(11) Section 29.923(o) requires the rotor drive system and rotor control mechanism to be in a serviceable condition at the end of the test. Verification of this requirement requires a complete disassembly and examination of the entire rotor drive system and rotor control mechanism. The disassembly itself should be closely monitored for evidence of adequate breakaway torque on all bolted fasteners. Samples of lubrication from oil sumps and filters should be retained for spectrographic analysis, and seals should be examined for possible damage due to test requirements. Care should be taken to differentiate between seal damage and bearing damage due to disassembly procedures so that the direct results of the test may be properly considered. Close visual observation of each tooth on each gear is necessary to affirm Par 423 655
proper load/contact patterns and absence of excessive surface stress or scrubbing motions. Bearings should be examined to verify that ball or roller paths are within limits, bearing cages are undamaged, and bearing balls or rollers and their races are free from pitting. Any evidence of bearing races turning or spinning in respective housing or bores probably indicates design or fit deficiencies. The applicant should have available wear limits data which include items such as distance across pins and tooth profile limits for gears. Many of these items require special, close tolerance inspection equipment and trained inspectors to determine compliance. In some instances bearings, clutches, oil pumps, etc., should be returned to the original manufacturer for a finding of serviceability. Localized overheating, usually exhibited by discolorations is an indication of an unsatisfactory condition. Should any of the items discussed above or other defects appear such that the component is unserviceable, a redesign which includes recognizable improvements should be required before authorizing a retest. To simply "try again" in hopes of success should not be accepted.
(12) This section also prohibits intervening disassembly which might affect test results. Generally, this simply means no disassembly whatsoever. However, some very limited disassembly can usually be conducted provided care is used to assure that items such as critical fastener torques or gear backlash controls are not disturbed.
c. Additional Test Considerations.
(1) Pressure Lubricated Gearboxes. The endurance test hardware can be adjusted/modified to sustain high-limit oil temperature and low-limit oil pressure in order to provide a basis for approval of the values listed as limits. A minimum of 20 hours at maximum continuous torque and maximum continuous rotational speed should be involved in the test. Other parameters such as minimum oil temperature and maximum oil pressure may more appropriately be evaluated by bench test. The significant points here are effects of extremely high oil pressure (due to the high viscosity of cold oil) on any positive displacement oil pump, on filters for possible collapse, on oil coolers for possible rupture due to internal pressure, seals, bypass valves, and most important, adequate lubrication of gears, bearings, etc., under conditions of minimal oil flow.
Normally, an operation restriction against exceeding idle power/speed conditions until significant warm-up occurs is prescribed. Individual component qualification tests may provide data to meet some of these aspects.
(2) The existing endurance test schedule does not necessarily provide for any asymmetric power inputs from multiengine drive system arrangements. For this situation, the drive system should at least be subjectively evaluated for possible hazards or excessive loads to be expected from asymmetric torque inputs and additional testing prescribed under the authority of § 29.923(g). The extent and severity of these tests should be established in consideration of the design peculiarities, the recommended operating procedures, and any OEl tests included in this test schedule.
656 Par 423
(3) Accessory Drives. Normally, all accessory drives on a gearbox will be loaded during the endurance test. Electrical load banks or other suitable methods may be used to assure that the generator drives are loaded and thus properly qualified.
Hydraulic pumps may be loaded by resetting hydraulic system relief valves to maintain limit pressure (load) continuously. If this condition is excessively severe, a method of load cycling may be appropriate. Note that accessory loads reduce the power available to the main rotor. Also, tail rotor loads are, insofar as the transmission is concerned, another large accessory. Care should be taken to assure that in-flight unloading of these accessory drives, including the tail rotor does not subject the main gearbox to loads significantly beyond those qualified by endurance tests.
(4) Gearbox Oil Tanks. Normally, gearbox oil is contained in an integral cast sump which, for other reasons, has sufficient strength to obviate the need for pressure tests. However, a subjective evaluation should be made to assure that detail design features such as sight gauges, filler caps, etc., offer adequate strength.
423A. § 29.923 (Amendment 29-26) ROTOR DRIVE SYSTEM AND CONTROL MECHANISM TESTS.
a. Explanation. Amendment 29-26 includes additional endurance test criteria for a new continuous OEI rating, and clarifies the torque and RPM relation intended for the various power ratings involved in the tests prescribed by this section.
(1) Section 29.923(a)(1) was amended to require that the test cycle be extended beyond 10 hours if OEl rating tests are included in the test program. This change was needed to maintain the cycle aspect of the test if OEI ratings are included.
(2) Section 29.923(a)(3) was amended to include rotational speed as a part of the test because the term "torque" by itself does not adequately define the test requirements.
(3) Section 29.923(b)(2), (f), and (k) were amended to add the test requirements for the new continuous OEl rating and retain, as an alternate, the 30-minute OE rating tests for those applicants who may request this rating. This change provided a regulatory test basis for qualifying the rotor drive system for optional OEI ratings.
(4) Section 29.923(g) was amended to remove the inference that the 2½-minute OEI runs should be repeated if the takeoff run is reconducted. Under these circumstances, additional testing for the 2/½-minute rating is unnecessary.
b. Procedures.
(1) The construction of this amendment is such that a series of runs, each at least (but not limited to) 10 hours in length, should be repeated 20 times for a total of at Par 423 657
АС 29-2B 7/30/97 least 200 hours of test. The time required to adjust power or to stabilize operating conditions for those conditions that require stabilization is not included. Figure 423A-1 shows a graphic representation of the 10-hour test cycle. Extension of the total test time beyond 200 hours (or extension of test runs beyond 10 hours) will occur if qualification for the 2½-minute, 30-minute, or continuous OEI optional ratings is proposed by the applicant for rotorcraft equipped with two or more engines. Also, compliance with § 29.923(g) may result in extended endurance tests if dynamic or malfunction conditions exist which adversely affect the endurance tolerance of the rotor drive system. Paragraph 29.923(a) should be interpreted as requiring test runs or cycles to be repeated in essentially the same sequence, although more than 10 hours may be needed to complete a run or cycle. This section also requires the test to be conducted "on the rotorcraft." This means a rotorcraft that is in conformity to the type design for which approval is requested. However, many nonconforming features, such as doors, some cowling and instrumentation, fuel tanks (alternate external fuel supply may be utilized), interior features, fire detectors, extinguishers, inlet ducts, exhaust baffles, etc., may be acceptable provided each item is technically considered and found to have no impact on the test results. Any significant deviations from the conformed rotorcraft type design should be coordinated with the cognizant FAA/AUTHORITY engineering staff and, if found acceptable, properly documented. The restraint (tie-down) arrangement used during the test should be arranged to react rotor thrust loads in lateral as well as vertical directions. The restraint should permit normal deflections due to rotor thrust in the engine and drive system support arrangement.
(2) The test torque and speed requirements of § 29.923(a)(3)(i) refer to the torque/speed combination (or power) values for which approval is requested. The requested torque/speed combination should not exceed the limits approved for the respective engine(s) to be used. An applicant may qualify the rotor drive system for torque values higher than those approved for the engine.
(3) In §§ 29.923(c), (d), (e), and (f), the torque requirements should be interpreted as above; i.e., the run should be made with maximum continuous torque or percentage thereof, as specified by the subparagraph; and the rotational speed should be maximum continuous for Paragraph (d) and the lowest permissable "power-on" speed for Paragraphs (e) and (f). Rotor control cycling should be accomplished during the "maximum continuous" portion of the endurance run. The controls of concern are the flight controls (cyclic and directional controls for typical rotorcraft). The collective control is normally used to set power and is not involved in control cycling. During control cycling, the controls may be cycled from stop to stop; or a limited travel may be accepted if the travel produces the maximum fore and aft, left and right, and yaw thrust components of the rotors as measured in flight. The frequency for cycling the controls is defined in §§ 29.923(c)(1), (2), and (3), and specified in Note 3 of Figure 423A-1.
One method of determining the required control displacement is to measure main rotor mast bending in level forward flight at maximum continuous power for the forward control displacement limit, and in level rearward flight at maximum continuous power (or the power associated with the maximum rearward flight speed to be expected) for the 658 Par 423A
aft control displacement limit. Using the same mast bending instrumentation, with the rotorcraft in the ground tie-down situation, and with collective control set for maximum continuous power, displace the cyclic fore and aft to obtain the same mast bending as measured in flight. Similar measurements and control displacements may be used for sideward thrust components. Yaw control displacement should consider maneuver requirements in conjunction with sideward flight. Critical gross weight and center of gravity should be used to establish flight test conditions. These same procedures may be used to establish limited control positions required to comply with § 29.923(1), except that typical flight conditions to be used would be stabilized level flight at maximum continuous power, climb at maximum continuous power, hover, and stabilized sideward and rearward flight. Note that for § 29.923(i)(1), vertical thrust is required. Depending on the mast angle and center of gravity, this condition may not necessarily involve zero mast bending loads. Vertical thrust may be used during the takeoff run, including the runs at 2½-minute power and the overspeed run of § 29.923(h).
One-engine-inoperative runs (§ 29.923(k)) should be conducted with the cyclic set for maximum forward thrust. For these runs and any run that does not specity the position for the yaw control, that control should be set to react to main rotor torque.
(4) Section 29.923(k) sets forth the optional tests to be conducted if a 30-minute or a continuous OEl power rating is requested. Flight control positions should be set for level flight or climb (whichever produces the maximum forward thrust component) and the anti-torque system control should be set to react the maximum rotor torque. The torque and rotational speed values should be the maximum for which approval is requested.
Par 423А 659
AC 29-2B
ROTOR DRIVE SYSTEM ENDURANCE 10+ HOUR CYCLE - REPEAT 20 TIMES NOTE 1 (TOTAL OF 200+ HOURS)
2.5 MIN OEI T.O.
30 MIN OEI M.C.
90% M.C. NOTE 3 80% M.C.
60% M.C. Fit. Idle 29.923 (c) MAXIMUM CONTINUOUS RUN 29.923 (d) 90% M.C. 29.923 (e) 80% M.C. NOTE 4 29.923 (f) 60% M.C. 20,022 g REPEAT T.O. RUN MALFUNCTION RUN OR 29.923 (h) OVERSPEED AT M.C. TORQUE 29,07 ₴ (k) EACH ENGINE 30 MIN O.E.I. OR CON'T O.E.I. O.E.I. RUN NOTE 2 Gr. Idle TAKEOFF RUN 29.923(6) , 7/30/97 Par 423A TIME - HOURS FIGURE 423A-1
7/30/97 423A TAKEOFF RUN PORTION OF ROTOR
DRIVE SYSTEM ENDURANCE TEST IF 2 1/2 MINUTE OEI RATING IS REQUESTED 2 1/2 MIN. OE NOTE 1 - T.O.
FIt. Idle #1 ENGINE 2 1/2 MIN. O m #2 ENGINE 2 1/2 MIN. OE #1 ENGINE 2 1/2 MIN. OEI #2 ENGINE 2 1/2 MIN. OEI Gr. Idle 10 20 30 40 AC 29-2B
TIME - MINUTES * NOTE 2 FIGURE 423A-2
AC 29-2B
TAKEOFF RUN PORTION OF ROTOR
DRIVE SYSTEM ENDURANCE TEST TO NIW NOTE 1 IF 30 SECOND/2 MINUTE OEI RATING
IS REQUESTED T.O.
* #1 ENGINE 30 SEC & 2 MIN OEI #2 ENGINE 30 SEC & 2 MIN OEI * GR. IDLE 10 20 30 40 50 * NOTE 2 TIME - MINUTES 7/30/97 Par 423A FIGURE 423A-3
Figures 423A-1. 423A-2, and 423A-3 Notes
1. If the 2½-minute OEl rating is requested, the following should be conducted for each engine: Demonstrate 2½-minute OEl power twice or 5 minutes per cycle for a total of 100 minutes at 2½-minute OEI power during the 200-hour endurance test. See Figure 423A-2 for a graphic description of the takeoff run for a two-engine rotorcraft using the 2/½-minute OEl rating (Refer to § 29.923(b)(2)). If the 30-second/2-minute OEI rating is requested, the following should be conducted for each engine: Demonstrate 30-second OEI power followed by 2-minute OEI power. After 2 minutes reduce and stabilize power to 30-minute or continuous OEI level. Once the power is stabilized reapply 2 minute OEI power.
This should result in 4½ minutes per cycle for a total of 10 minutes at 30-second OEI power and 80 minutes at 2-minute OEI power during the 200-hour endurance test. See Figure 423A-3 for a graphic description of the takeoff run for a two-engine rotorcraft (Refer to § 29.923(b)(3)). If either the 2½-minute or 30-second/2-minute OEl ratings are demonstrated, the takeoff run portion of Figure 423A-1 will be longer than 1 hour as shown in Figure 423A-2 or 423A-3, respectively.
Apply the rotor brake during the first minute of the 5-minute idle period. Conduct 400 brake applications during the 200-hour endurance test (§ 29.923(i)).
During the maximum continuous run, cycle the rotor controls 15 times per hour:
(Refer to §§ 29.923(c)(1) - (3)). The cyclic control should be cycled through maximum vertical thrust, maximum forward, maximum left, maximum right, and maximum rearward thrusts. The pedal controls should be cycled through maximum right, neutral, and maximum left positions. Each maximum cyclic and pedal control position should be held for at least 10 seconds. During the remainder of the test, set the yaw control to react to the main rotor torque, and set the flight controls to achieve:
Condition Portion of Test max vertical trust 20 percent max forward thrust 50 percent max left thrust 10 percent max right thrust 10 percent max rearward thrust 10 percent
4. The 60 percent maximum continuous run is 2 hours (Refer to § 21 29.923(f)), Par 423A 663
unless either 30-minute OEI or continuous OEl power is requested. In that case, the 60 percent maximum continuous run is 1 hour.
5. A 1-hour malfunction run (if deemed necessary) or the takeoff run is repeated (without OE portions). Refer to § 29.923(g).
6. The OEI run defined in § 29.923(k) is not required unless an OEI power rating is requested. If a 30-minute OEl power rating is requested, each engine in sequence should be run at the 30-minute OEI condition for 30 minutes. If a continuous OEI power rating is requested, each engine in sequence should be run at the continuous OEI condition for 1 hour. The total OEI run time may exceed the 1 hour shown in Figure 423A-1.
423B. § 29.923 (Amendment 29-31) ROTOR DRIVE SYSTEM AND CONTROL MECHANISM TESTS.
a. Explanation. Amendment 29-31 added § 29.923 (p) that defines qualification tests for lubricants used in the rotor drive system and control mechanisms.
Section 29.923(p) contains a requirement for a portion of the system qualification tests to be accomplished with specific lubricating oil temperatures and pressures.
b. Procedures. The requirements of Paragraph 29.923(p) should be met for all rotor drive system and control mechanism qualification tests. Additionally, these requirements should be met if certification of alternate lubricants for the rotor drive system is requested. Thirty hours of qualification testing is required on the gearbox in which the alternate lubricant(s) is proposed to be used. During this testing, the lubricant temperature is to be measured with the temperature probes that will be used in service. The lubricant temperature should be maintained at the maximum operating temperature established for the gearbox in service. For pressure lubricated systems, the gearbox lubricant pressure should be maintained at the minimum operating pressure that has been established for the gearbox in service. The lubricant pressure is to be measured in the same manner and with the same probes that will be used in service. During the 30 hours of testing required by § 29.923(p), the lubricant temperature and pressure should be applied and measured simultaneously. For one-engine-inoperative ratings, the test time should be extended by one engine failure cycle to include operation at the one-engine-inoperative rating for which approval is requested. Equivalent testing or comparative analysis of the proposed lubricant and the approved lubricant specifications may be used to approve an alternate lubricant.
423C. § 29.923 (Amendment 29-34 and 29-40) ROTOR DRIVE SYSTEM AND CONTROL MECHANISM TESTS.
a. Explanation. This paragraph, 423C, reflects changes made by Amendment 29-34 and 29-40. Amendment 29-34 added § 29.923(b)(3) that defines 664 Par 423A