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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(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
shows that rotor pylon mount stiffness could affect ground resonance, the evaluation should include variations in stiffness and damping of the rotor pylon restraints that may occur in service (reference “Ground Vibrations of Helicopters,” M.L.. Deutsch, JAS, Vol. 13, No. 5, May 1946).
b. Procedures.
(1) The reliability of the means for preventing ground resonance may be substantiated as stated in the standard. An analysis report or a test proposal and subsequent test report may be used to show compliance. The probable range of variations, in service, of the damping action are an important part of the assessment.
The test may be conducted in conjunction with the testing required by § 29.241. See paragraph AC 29.241.
(i) Analysis and tests may be used.
(ii) Reliable service history of identical or closely similar systems may be used. The materials and fluids used, clearance or fits, seals, and physical installation are important items to be evaluated and considered for “closely similar” systems.
(iii) Testing of the complete rotorcraft may be used to prove that malfunction of a single means of the damping system will not cause ground resonance.
One method of demonstrating acceptable compliance is by removing all seals, if practicable, from one damper. Another method is to remove all or most of the fluid, in conjunction with considering the allowable ranges of damping of the other parts of the rotorcraft damping system and operating the rotorcraft throughout the rotor speed range from start to maximum rotor speed. Investigation of elastomeric dampers may require innovative test procedures and preliminary discussions of these prior to preparation of a test proposal. The rotorcraft cyclic control should be displaced as noted in paragraph AC 29.241 to assure that the possible rotorcraft resonance frequencies are excited. If vibrations are damped in all tests, the damping system is satisfactory. Each critical rotor damper and landing gear damper, which includes shock struts and tires, should simulate a malfunction to comply with the standard. The testing discussed, however, could become very extensive if one were to attempt to test all combinations of all maintenance adjustments of all components which contribute to the prevention of ground resonance, while at the same time rendering each of the pertinent components ineffective in turn and then repeating all of the maintenance tolerance testing each time.
Fortunately, rational analytical methods are available which will permit the evaluation of such combinations so that only the combinations with the least amount of margin used are physically tested.
(2) The pylon damper variation can affect ground resonance. The variations in stiffness and/or damping of pylon mounts should be evaluated except the pylon mounts on contemporary conventional rotorcraft may have little influence on “classical” ground resonance stability. The dynamics of the rotorcraft on its landing gear is generally established by the airframe properties and the landing gear properties under the Page D - 35
influence of the rotor system, with the “pylon” having little effect. For air or flight resonance, the rotor generally couples with the rigid body modes of the fuselage. For a specific design, a relatively simple analysis may be used to show the effect of the pylon mount system stiffness on air and ground resonance stability, and if not important, variations in the system may be omitted from the test program.
(3) The probable ranges of damping shall be established and investigated as prescribed and noted in paragraph (b) of § 29.663. An approved test proposal and test results report should be used for complying with § 29.663(b). For example, if a conventional wheel landing gear is used on the rotorcraft, the probable ranges of tire pressure or the lowest probable tire pressure should be stated in the test proposal and effects of the tire pressure investigated during the test. In addition, the effects of strut pressures should be investigated also. See paragraph AC 29.241, § 29.241, concerning tests and instrumentation of the test associated with complying with § 29.241. The instrumentation noted in paragraph AC 29.241 also applies to § 29.663(b).
(4) If the wheel landing gear is equipped with wheel brakes, the evaluation should include brakes “on” and “off.” The nose or tail wheel should be locked and unlocked if it swivels to evaluate any possible adverse effects of this feature.
AC 29.663A. § 29.663 (Amendment 29-30) GROUND RESONANCE PREVENTION MEANS.
a. Explanation. Amendment 29- 30 clarifies that analysis as well as tests may be used to show freedom from ground resonance after malfunction or failure of a single means of ground resonance prevention. This amendment primarily clarifies that the probable range of damping should be established as well as investigated.
b. Procedures. The procedures of paragraph AC 29.663 continue to apply with the addition of the need to document the establishment of probable range of damping of ground resonance prevention means. Acceptable tire and oleo minimum and maximum pressures as well as other identified factors should be documented in maintenance instructions if necessary to assure the desired characteristics.
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SUBPART D - DESIGN AND CONSTRUCTION
CONTROL SYSTEMS AC 29.671. § 29.671 (Amendment 29-24) CONTROL SYSTEMS - GENERAL.
a. Explanation.
(1) The rule requires that controls operate easily and smoothly and provide positive response of the rotorcraft to control input.
(2) In addition, the rule requires that incorrect assembly be prevented by special design features or special markings.
(3) After Amendment 29-24, November 6, 1984, the rule requires that the flight control system be designed such that the full range of flight control authority can be verified by the pilot before flight. This check would normally have to be completed prior to turning the rotor since control extremes typically cannot be reached with the rotor operating on the ground.
b. Procedures.
(1) Easy, smooth operations of controls are substantiated by the operations tests of § 29.683 and the FAA/AUTHORITY flight testing under TIA procedures.
Positive response of the rotorcraft to control inputs is also evaluated during company flight testing and FAA/AUTHORITY TIA flight testing to the requirements of §§ 29.141 through 29.175.
(2) To meet the requirement that incorrect assembly be prevented, the preferred method is providing design features which make incorrect assembly impossible. Typical design features which can be used are different lug thicknesses, different member lengths, or significantly different configurations for each system component. In the event that incorrect assembly is physically possible (because of other considerations), the rule may be met by the use of permanent, obvious, and simple markings. Permanent (durable) decals or stencils may be used.
(3) Design features of the control systems are checked when reviewing the type design drawings. During the proof and operation tests of §§ 29.681 and 29.683, the controls should be thoroughly reviewed for possible incorrect assembly and for any required markings supplied for compliance with this standard.
AC 29.672. § 29.672 (Amendment 29-24) STABILITY AUGMENTATION, AUTOMATIC, AND POWER-OPERATED SYSTEMS.
a. Explanation.
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(1) This rule requires that the pilot be made aware of stability augmentation, automatic, or power-operated system failures which could lead to an unsafe condition.
It should be understood that this requirement applies to stability augmentation and supplementary controls and not the primary flight control system, which is dealt with under § 29.695 and associated advisory material. Examples of clearly distinguishable warnings include, but are not limited to, an obvious aircraft attitude change following the failure or an audio warning tone. A visual indication itself may not be adequate since detection of a visual warning would normally require special pilot attention. The use of devices such as stick pushers or shakers is not acceptable as a warning means since the automatic flight control systems (AFCS) may provide a hands-off capability or normal helicopter vibrations could mask a control shaker. However, this rule is not intended to eliminate the use of such devices for other purposes. Examples of automatic control systems other than a stability augmentation system would be a pitch axis actuator used for the purpose of demonstrating compliance with longitudinal static stability requirements or a fly-by-wire elevator. The design of such systems must not interfere with completion of the control checks described in § 29.671(c). Further, for control systems where a series actuator malfunction could degrade control authority, a means should be provided to the pilot to determine actuator alignment (see § 29.1329(b)).
(2) The corrective flight control input following a system failure should be in the logical direction. For example, a malfunction resulting in a nosedown pitch of the aircraft should require a corrective cyclic control input in the aft direction. The system deactivating means does not have to be located on the primary flight control grips;
however, it should be easily accessible to the pilot. Consideration should be given to the consequences of inadvertent de-selection of the automatic stabilization system, especially if the deactivation control is mounted on a primary control grip. Malfunctions and subsequent recoveries must be shown throughout the operating envelope of the aircraft. In a case where control authority is decreased following a malfunction, a practical flight envelope must be defined wherein compliance with controllability and maneuverability requirements can be demonstrated. This practical flight envelope must be presented in the flight manual. Compliance with trim and stability characteristics is not required following a malfunction; however, a pilot workload assessment should be made to show that these characteristics are not impaired below that needed for continued safe flight and landing.
b. Procedures. A discussion of malfunction test procedures is presented in paragraph AC 29 Appendix B b(6). Controllability and maneuverability test procedures are addressed in paragraph AC 29.143.
AC 29.673. § 29.673 (Amendment 29-24) PRIMARY FLIGHT CONTROLS.
a. Explanation. This section basically defines primary flight controls as “those used by the pilot for immediate control of pitch, roll, yaw, and vertical motion of the Page D - 38
rotorcraft.” This section clarifies the application of § 29.1555 which requires markings for controls other than “primary flight controls or control(s) whose function is obvious.”
b. Procedures. The primary flight controls (e.g., cyclic stick, collective, and tail rotor pitch control pedals) are excluded from the marking requirements of § 29.1555.
AC 29.674. § 29.674 (Amendment 29-30) INTERCONNECTED CONTROLS.
a. Explanation. A new § 29.674 is added by Amendment 29-30 which requires that the rotorcraft be capable of safe flight and landing after a malfunction, failure, or jam of any auxiliary interconnected control.
b. Procedures.
(1) Section 29.674 requires that the rotorcraft be shown to be capable of safe flight and landing after a malfunction, failure, or jam of an auxiliary control interconnected with a primary control. The section does not apply to interconnected primary controls; e.g., cyclic and collective controls.
(2) Examples of auxiliary controls covered by this section may include certain autopilot or stability augmentation or trim system components. Section 29.1309 methods may be used in determining failure effects of autopilot and stability augmentation system components.
(3) If an engine control could jam and result in a collective control jam, these controls should be designed to relieve that connection.
AC 29.675. § 29.675 (Amendment 29-17) STOPS.
a. Explanation.
(1) Stops are required to prevent unrestrained movements of pilot/autopilot inputs from causing interferences or overloads.
(2) The rule requires that the stop must not appreciably affect the control system range of travel due to wear, slackness, or take-up adjustments.
(3) Each stop is required to withstand loads corresponding to design conditions.
(4) In addition, each main rotor blade, if appropriate for the design, must have stops to limit its travel about its hinge points. For rotors with hingeless design, stops may be provided as appropriate to limit blade travel. Loads which may result from the blade hitting the stops (during starting or stopping the rotor, or during any large but allowable pilot control inputs such as autorotation cyclic traverse or when subjected to ground gusts, etc.) shall not overload the stops nor any rotor component.
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