requirements for ground resonance prevention means. A ground vibration survey was not reinstituted by the adoption of § 27.663. Compliance with § 27.663 does require investigation and substantiation as stated.
(2) “Ground resonance” is a mechanical instability of the aircraft while in contact with the ground, often when partially airborne. Stated another way “ground resonance” is a self-excited mechanical instability that involves coupling between the in-plane motion of the rotor blade and the motion of the rotorcraft as a whole on its landing gear (reference “Aerodynamic of the Helicopter,” Gessow & Myers, page 308).
It is caused by the motion of the blade in the plane of rotation (called in-plane vibration) coupled with a rocking or vertical motion of the aircraft as a whole. The tires, landing gear, and rotor pylon restraint structure act as a spring with a vibration frequency which coincides or couples with the natural in-plane frequency of the blade about a real or effective drag hinge in the plane of rotation. When the frequencies of the two mot ions (rotor and airframe) approach each other and couple, a violent shaking of the rotorcraft may occur which, if undamped, could result in the destruction of the rotorcraft.
(3) Ground resonance can occur due to flexibility in the rotor pylon restraint system as well as with landing gear flexibilities. This mode of vibration or resonance can happen in flight (called air resonance) as well as on the ground and should be addressed in the certification program. The evaluation should include variations in stiffness and damping that could occur in service to the rotor pylon restraints.
(4) Ground resonance may be prevented by placing the first order in-plane vibration frequency above the rotor turning speed.
(5) For such configurations which are not susceptible to ground resonance (first order in-plane frequency above rotor turning speed), a simple rotor RPM run-up and run-down with appropriate cyclic control displacement (i.e., excitation of any inherent vibrations) is adequate demonstration that a ground resonance condition does not exist.
Unhinged “rigid” rotors, such as Bell Helicopter two-blade designs, are this type of rotor system.
(6) For configurations that are susceptible to ground resonance (i.e., first in-plane frequency is below the rotor turning speed), ground resonance is generally prevented by dampers on the blade acting in the plane of rotation, dampers on the landing gear (sometimes serving as oleo struts), or proper placement of the landing gear frequencies combined with rotor and/or landing gear dampers.
(7) Elastomeric components (in the rotor pylon support system, possibly in the landing gear, and possibly in the rotor head) are significantly affected by ambient temperature prior to warm-up. Their damping characteristics require thorough investigation for the range of rotorcraft operating environment as noted in § 27.663.
b. Procedures.
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(1) Under all conditions, any oscillations which may be introduced should be damped. However, no instability should occur at any operating condition such as during RPM changes from minimum to maximum and idle to maximum. For rotorcraft with wheel gear, uneven taxi surfaces in conjunction with particular taxi speeds, may excite ground resonance and should be evaluated by taxiing on typical surfaces. This evaluation may be conducted in conjunction with the tests of § 27.235. In operation, the resonance characteristics should be checked during takeoff and landing at zero speed and during run-on landings using various power values.
(2) For those aircraft equipped with Stability Augmentation Systems (SAS), all ground resonance investigations should be conducted with SAS on and SAS off. This includes the hovering and running takeoffs and landings, taxi tests, and specific ground resonance tests noted herein. Consideration should be given to conducting tests in various SAS configurations such as roll channel on and pitch channel off, where such configurations are possible and authorized.
(3) For each rotorcraft configuration tested, the aircraft should be positioned on the ground in flat pitch with the rotor stabilized at the minimum practical rotational speed or optionally at a speed shown analytically to have significant margin from indicated resonant conditions. Control system inputs should be used to disturb the system for evaluation of subsequent damping.
(4) For each incremental increase in rotor speed and for each rotor speed setting at increments of collective pitch settings, cyclic and collective inputs should be investigated prior to proceeding to the next rotor speed setting. These inputs should cover the appropriate range and combinations of amplitude and frequency. The collective pitch setting increments should range from flat pitch to light on the landing gear prior to fully airborne, depending upon the test sequence for minimum risk.
(5) Cyclic pitch inputs should be made either by the pilot through the cyclic stick or through a signal-generating device working in conjunction with the cyclic controls.
For each frequency of input, amplitude of the inputs should be increased incrementally and ultimately should be large enough to generate responses representative of normal ground and flight operation on the rotor and support system. The inputs should continue for a time sufficient to obtain representative responses, typically time sufficient to execute five complete circles of the cyclic stick (about neutral) at the selected frequency.
(6) The excitation frequency should be such as to excite the blade in-plane frequency. Rotor speed settings should be increased to 1.05 times the maximum power-on rotor speed. Collective pitch settings should be increased in increments of not more than 20 percent to maximum collective or alternately to the collective setting required to become partially airborne (when the cyclic is displaced as noted).
(7) Typically, articulated rotor aircraft have natural frequencies on the blade in lag of approximately 0.3 times the power-on main rotor RPM. Soft in- plane rotors have Page B - 65
natural frequencies approximately 0.7 times the main rotor RPM. Therefore, for example, for a rotorcraft with an in-plane frequency of 0.3/rev, operating at 300 RPM, and with 6 inches of total lateral cyclic stick displacement, the stick should be rotated for 5 revolutions in a 0.6-inch-diameter circle at ((1-.03) x 300 RPM) or 3.5 cycles per second to attempt excitation of possible resonant frequencies. At the conclusion of the excitation, the cyclic stick should be returned to the neutral position while continuing the recording of data listed in paragraph b(13).
(8) The excitation process should include cyclic excitation inputs from the directional and longitudinal controls if critical for the type of rotorcraft being evaluated.
(9) If onset of ground resonance is encountered, one possible corrective action is to increase the collective pitch and rotor speed and become airborne. However, lowering the collective pitch and applying the rotor brake (if installed) or rolling off the throttles has been effective for some designs and is considered a satisfactory procedure if resonance can be consistently stopped.
(10) With the rotor speed stabilized, landing should be made at a touchdown speed which minimizes risk.
(11) Special Considerations.
(i) The influence of variables, including environmental effects, corresponding aircraft component characteristic changes, operational parameters, and surface conditions should be investigated over the ranges proposed for certification.
Additionally, the potential of misservicing and possible failure modes should be evaluated. For ground resonance qualification, where practical, variations from the baseline test configuration may be accomplished by ground run (§ 27.663(b) requires investigation of probable ranges of damping), analyses, component tests, aircraft shake test, the specification of special operational procedures in the rotorcraft flight manual, or a combination thereof. Detailed and rational analyses showing acceptable correlation to the baseline tests, and for which the input parameters were verified by drawings, calculations, component static or dynamic tests, or by aircraft shake tests simulating the conditions/configurations in question, may be used to limit testing to only those variables and operational conditions showing marginal or unacceptable system damping. All operational limitations should be clearly stated in the rotorcraft flight manual. A report of the analytical results and/or test results should be submitted per § 27.663.
(ii) Potential instability while airborne, called “air resonance,” may occur due to the dynamic coupling of the rotor flexibility and the pylon restraint flexibility. The same considerations apply to air resonance as to ground resonance except that the pylon restraint variables replace the landing gear variables. Air resonance should be addressed in the certification program.
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(iii) When operating on the ground, there may be a tendency for the aircraft to exhibit a “ground bounce.” For many configurations, this is a benign, although undesirable phenomenon which may be aggravated by pilot induced oscillations (PIO), particularly if there is little or no friction on the collective.
(12) Rotorcraft with fully articulated rotor heads and landing gear oleos in either skid or wheel configuration have tendencies for ground bounce to occur when light on the oleos, either just prior to takeoff, just after landing contact, or during a power assurance check. This bounce may induce ground resonance, particularly if the intensity of the bounce is aggravated by PIO. The corrective action is either to lift off to a hover or to positively lower the collective and remain on the ground..
(13) Instrumentation and Data Acquisition.
(i) Atmospheric Conditions (to be manually noted):
(A) Altitude.
(B) OAT.
(C) Wind velocity.
(ii) Aircraft Configuration (to be manually noted):
(A) Gross weight.
(B) C.G.
(C) Tire pressure.
(D) Landing gear oleo pressure.
(iii) Instrumentation (for recording during test).
(A) Main rotor RPM.
(B) Time history of cyclic control fore-and-aft and lateral stick position.
(C) Time history of collective control stick position.
(D) Time history of rotor damper motion.* (E) Time history of pylon component motion.* (F) Time history of landing gear (oleo) motion.* (G) Time history of aircraft motions.* *As required to obtain modal damping Page B - 67
SUBPART B - FLIGHT
MISCELLANEOUS FLIGHT REQUIREMENTS AC 27.251. § 27.251 VIBRATION.
a. Explanation.
(1) Each part of the rotorcraft must be free from excessive vibration under each appropriate speed and power condition (rule statement).
(2) This flight requirement may be both a qualitative and quantitative flight evaluation. Section 27.571(a) contains the flight load survey requirement that results in accumulation of vibration quantitative data. Section 27.629 generally requires quantitative data to show freedom from flutter for each part of the rotorcraft including control or stabilizing surfaces and rotors.
(3) Review Case No. 70 (reference FAA Order 8110.6) contains a policy statement concerning compliance with this rule. This policy statement is condensed here for convenience:
“The rotorcraft must be capable of attaining a 30° bank angle (turn), at VNE, with maximum continuous power (maximum continuous torque) without encountering excessive roughness/vibration. The FAA/AUTHORITY requires the maneuver demonstration to provide the pilot with some maneuver capability at VNE and further to provide the pilot some margin away from roughness when operating in turbulence.”
(This maneuver may result in a descent or a climb.)
(4) Section 27.1505 pertains to VNE determination. Section 27.1509 pertains to rotor speed limits determination.
b. Procedures.
(1) During the company flight test program, the rotorcraft is flown to the appropriate rotor and airspeed limits at several weights to prove that the rotorcraft is free from excessive vibration under appropriate speed, power, and weight conditions.
The flight loads survey quantitative data (reference § 27.571) and the applicant’s qualitative and quantitative flight test data must also prove compliance with the requirement prior to issuing an authorization for official FAA/AUTHORITY flight tests.
(2) The flight load survey data obtained under § 27.571(a) will contain measured data concerning proof of freedom from flutter and excessive vibration.
Pertinent critical flight conditions will be reinvestigated during FAA/AUTHORITY flight tests. The specific condition or conditions necessary to demonstrate compliance with § 27.251 vary with the rotorcraft design and with the minimum and maximum rotor Page B - 68
speeds, VNE and VD speeds, and weight and CG position. An illustration of the speed and RPM demonstration is shown in figure AC 27.251-1. (Also see paragraph AC 27.251b(4).)
(3) The airspeed and rotor speed limits investigated and established under §§ 27.33, 27.1503, 27.1505, and 27.1509 are also investigated and made a matter of record in the flight loads survey data. During the official FAA/AUTHORITY/TIA flight tests, critical parts of the rotorcraft may have limited instrumentation to reinvestigate and confirm that the critical conditions investigated during the flight load survey are satisfactory and do not result in excessive vibration. Use of instrumentation is optional if the flight loads data are conclusive.
(4) FAA/AUTHORITY policy for certification (Review Case No. 70) requires a “rotor roughness” flight demonstration of a 30° bank angle left and right at maximum continuous power (MCP) (maximum continuous torque which may be in excess of the maximum continuous temperature limit) at VNE. To provide the pilot with some margin from roughness, the FAA/AUTHORITY requires maneuver demonstrations of 30° banked turns at VNE without encountering excessive roughness. The maneuver should be conducted with the rotor speed at the minimum RPM and maximum RPM limits.
During the flight load survey, this condition should be investigated and data recorded to ensure hazardous loads are not encountered for this “unusual” condition. As indicated, the flight condition will be reinvestigated during the FAA/AUTHORITY flight tests. See paragraph AC 27.251b(2) for illustration of this speed and RPM demonstration.
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0° Bank, Autorotation Level Forward Flight, 105% Max RPM Power-Off g 4 • 30° Bank, Power-On Power-On Max RPM Power-Off 02 Max RPM Power-On д б ROTOR RPM - Min RPM Power-On ш 8 45 Min RPM Power-Off 01 95% Min RPM Power-Off a 3 YNE 1.11 VNE
AIRSPEED
1. Autorotation at 1.11 VNE (AR) at minimum placard rotor speed.
2. Autorotation at 1.11 NNE (AR) at maximum placard rotor speed.
3. Autorotation at NNE (AR) at power-off minimum design limit rotor speed.
4. Autorotation at NNE (AR) at power-off maximum design limit rotor speed
5. Forward flight 1.11 VE at minimum power-on rotor speed.
6. Forward flight 1.11 VNE at maximum power-on rotor speed.
7. Right and left turn at VE at maximum power-on rotor speed with 30° bank angle.
8. Right and left turn at VE at minimum power-on rotor speed with 30° bank angle.
Note: VNE (AR) may be less than NE • FIGURE 27.251-1 DEMONSTRATION POINTS Page B - 70
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CHAPTER 2. PART 27
AIRWORTHINESS STANDARDS
NORMAL CATEGORY ROTORCRAFT
SUBPART C - STRENGTH REQUIREMENTS
STRENGTH REQUIREMENTS - GENERAL AC 27.301. § 27.301 LOADS.
a. Explanation.
(1) The rule is a general statement concerning limit and ultimate loads and the application of these loads to the rotorcraft.
(2) Ultimate loads are limit loads multiplied by the prescribed factors of safety.
(3) The specified loads are specified to be distributed appropriately or conservatively and significant changes in distribution of the loads, as a result of deflection, are specified to be taken into account.
b. Procedures. The design criteria report and/or design loads report should contain data that comply with the rule.
AC 27.303. § 27.303 FACTOR OF SAFETY.
a. Explanation.
(1) Unless otherwise provided by Part 27, a factor of safety of 1.5 is required and is applied as stated in the rule. This safety margin will ensure that the design strength of the rotorcraft is greater than the design loads contained in Part 27.
(2) Other rules, §§ 27.561(b)(3) and 27.787(c), specify use of defined ultimate inertial forces for protection of occupants.
b. Procedures.
(1) The design criteria report and/or design loads report should contain data that include the appropriate factor of safety.
(2) The factor of safety multiplies the limit external and inertial loads. The rule does allow the application of this factor to the resulting “limit internal” stresses if it is more conservative.
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AC 27.305. § 27.305 STRENGTH AND DEFORMATION.
a. Explanation.
(1) This general rule defines, in relative terms, allowable deformation for limit and ultimate loads.
(2) The structure is required to be able to support, in a static test, ultimate loads for 3 seconds without failure, or dynamic tests simulating actual load application may be used.
(3) Section 27.307 concerns proof of the structure and requires certain specified tests. This rule also allows substantiation by structural analysis. See paragraph AC 27.307.
b. Procedures. Any test results, static or dynamic, should satisfy the limitations or acceptance criteria contained in the rule.
(1) Any test proposals submitted for approval that are used to demonstrate compliance with sections of Part 27 should contain the criteria stated in the rule.
(2) Any test results reports shall contain data and information showing the test results comply with the standard.
AC 27.307. § 27.307 (Amendment 27-3) PROOF OF STRUCTURE.
a. Explanation.
(1) The rule requires compliance with the strength and deformation requirements for each critical loading condition. Certain tests must be conducted as specified. Additional tests for new or unusual design features may be required as noted in § 27.307(b)(6).
(2) Structural analysis rather than load tests may be used only if the structure conforms to those for which experience has shown the structural analysis method to be reliable.
b. Procedures.
(1) The design criteria and/or design loads report should contain typical or representative loading conditions from which the critical loading conditions will be selected for analytical substantiation in structural (static and fatigue) reports, dynamics (vibration and stability) reports, and in fatigue, static, dynamic, or operational test reports.
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(2) Whenever tests are used or required, a test proposal or plan should be approved prior to the tests. The test article should have received conformity inspections and should have been accepted by the FAA/AUTHORITY for the test. Test fixtures and instrumentation should also be acceptable to the FAA/AUTHORITY (using DERs as appropriate) prior to the start of the test. The quality control office of the applicant or other qualified personnel may be authorized to conduct inspections of the test fixtures and instrumentation rather than the FAA/AUTHORITY or DER performing this task. The test proposal may be used to define and to authorize the means to accomplish inspection of the test fixtures and instrumentation. Unnecessary drawings such as test fixture details or layering of approvals are not intended or envisaged by this policy.
Drawings, sketches, or photographs have been used by the FAA/AUTHORITY to control and to ensure correct location, direction, and magnitude of loads and other critical test parameters.
(3) Structural analysis has been accepted for rotorcraft in place of static tests.
Generally, the rotorcraft airframe should have natural frequencies remote to predominant rotor excitation sources, including higher harmonics, to avoid undesirable and possibly excessive vibration and potentially high operating stress levels due to this vibration. During the flight load measurement program conducted under § 27.571, critical loaded areas or critical joints may be instrumented with strain gages or other stress strain measuring devices. This actual flight data should be compared to the analytical data to verify accuracy.
(4) Paragraph (b) of the rule specifies certain tests. Test proposals should be approved prior to conducting official FAA/AUTHORITY tests. Other paragraphs in this advisory circular pertain to those tests.
AC 27.307A. § 27.307 (Amendment 27-26) PROOF OF STRUCTURE.
a. Explanation. Amendment 27-26 adds the requirement to account for the environment to which the structure will be exposed in operation. This change is intended to codify recent FAA/AUTHORITY and industry practices for the consideration of environmental effects in showing “proof of structure.”
b. Procedures. All of the policy materials pertaining to this section remains in effect with the following additions:
(1) For either tests or an analysis, environmental effects are now explicitly required. Consideration of loss of strength and stiffness of metals with elevated temperatures and loss of strength and stiffness of composite materials from exposure to heat, moisture, or other operational environments is now required and should be documented in analyses and test reports.
(2) MIL-HDBK-5F; AC 20-107B; or MIL-HDBK-17, Vol. I, Rev 1E; Vol. II, Rev. D; Vol. III, Rev E (or later versions) are acceptable sources of data and procedures Page C - 1
to show compliance with environmental effects of metallic and composite materials, respectively.
AC 27.309. § 27.309 DESIGN LIMITATIONS.
a. Explanation.
(1) The rule requires an orderly selection and presentation of the basic structural design limitations of the rotorcraft. The applicant is required to establish these structural limitations to facilitate design of the rotorcraft.
(2) Refer to the rule for the specific requirements.
b. Procedures.
(1) The design criteria and/or design load report should contain the design limits specified.
(2) These items are structural design limits. Other requirements may result in narrowing the ranges of type design limits or in reducing limits. It is not necessary to revise structural design criteria limits to agree with more conservative operational limits established during the certification program. The operational limits may be subsequently expanded by additional flight tests to agree with design limits.
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