(2) A number of devices may be used to produce the necessary trim characteristics. One popular method of meeting this requirement is through the use of control balance springs in conjunction with a small amount of built-in control system friction. Other methods include use of friction, magnetic brakes, bungees, and irreversible mechanical schemes.
(3) This regulation is not intended to require zero friction or zero breakout force in the control system, nor is it intended to require automatic control recentering. The regulation, in fact, specifically prohibits excessive high friction or high breakout forces which would produce undesirable discontinuities in the primary control force gradient.
b. Procedures.
(1) If comprehensive company flight test data are available, compliance with this requirement can quickly be found by spot checking extreme center of gravity loadings. Trim tests can ordinarily be done during the course of other flight test activities. To conduct the test, simply release the control at the required flight conditions and determine that the control does not move. The wordsiany appropriate speedi ordinarily include any speed from hover to VH. If the control system trim device might be subject to temperature or humidity effects, these should be investigated at a minimum of two altitude extremes and during several test phases.
(2) It a pilot controllable variable friction device is incorporated, compliance with this requirement must be shown at the minimum adjustable value. The maximum value of adjustable friction should not completely lock the flight controls.
(3) Continued compliance with this requirement should be assured through a production procedure. If minimum friction or centering springs are used, it is desirable for the manufacturer to include some adjustment capability for production differences.
The explanation and procedures discussed here are applicable for VFR approval under § 29.161. For additional FR trim requirements, refer to Paragraph 775 of this advisory circular.
82A. § 29.161 (Amendment 29-24) TRIM CONTROL.
a. Explanation. Amendment 29-24 to the regulation adds the additional requirement that the trim control be capable of trimming collective forces to zero.
b. Procedures. The trim requirement is intended to allow the pilot to release the controls for brief periods to perform other cockpit duties, and to provide the pilot with a reference cyclic position for the given flight condition. The collective should be balanced so that there is no tendency for the collective pitch to change when the collective is released. Any magnetic clutch, friction brake or similar device which modifies the collective characteristics should be capable of being overpowered by the pilot, when fully applied, without requiring excessive force.
Par 82 177
83. § 29.171 STABILITY: GENERAL.
a. Explanation. This section is intended to require a manageable pilot workload for the minimum crew under foreseeable operating conditions.
b. Procedures.
(1) Compliance with the requirements of this section can often be obtained for the VFR condition without any specific or designated flight testing. If the rotorcraft is marginal in regard to pilot strain and fatigue, the FAA/AUTHORITY pilot should be assured, through special tests if necessary, that the aircraft can be satisfactorily flown throughout the maximum endurance capabilities of the rotorcraft including night and turbulence conditions if those are critical. This test should be conducted with minimum required systems in the aircraft and with minimum flight crew.
(2) Reasonable failure conditions which add to pilot workload, strain, and fatigue should be evaluated (electrical, hydraulic and mechanical failures, etc.). The necessary times associated with flight with a failed system must be appropriate to the flight manual procedures for each failure. A failure condition requiring immediate landing would obviously require shorter evaluation time than a condition allowing continued flight to destination.
(3) FR approvals necessitate a careful evaluation of Paragraphs (1) and (2) above. In IFR operations, weather conditions frequently necessitate continued flight to destination or diversion to alternate airports with critical failures. Immediate landing may not be feasible. The evaluating pilot must assure pilot strain and fatigue are acceptable during typical flight profiles for each type of operation to be approved.
84. § 29.173 (Amendment 29-24) STATIC LONGITUDINAL STABILITY.
a. Explanation.
(1) This rule contains control system design requirements for both stability and control. Paragraph (a) contains the basic control philosophy necessary for all civil aircraft. Forward motion of the cyclic control must produce increasing speeds and aft motion must result in decreasing speeds. For rotorcraft this is accomplished with throttle and collective held constant. This requirement in no way assures aircraft stability. It is simply a control requirement which speaks to direction of control motion.
Rotorcraft with either highly stable or highly unstable static longitudinal stability characteristics can typically comply with the basic requirement for control sense of motion.
(2) The remainder of § 29.173, through reference to § 29.175, contains the basic control position requirements necessary to establish a minimum level of static 178 Par 83
longitudinal stability. Positive stability is found for conditions of climb, cruise, and autorotation in § 29.175 by requiring a stable stick position gradient through a specified speed range. A defined level of instability is permitted for the hovering condition.
b. Procedures.
(1) The control requirement of this section is so essential to basic flight mechanics that compliance may be tound during conventional flight testing for compliance with other portions of the regulations. No special or designated testing should be required.
(2) The procedures necessary to assure compliance with the stability requirements of this section are contained under § 29.175, Demonstration of static longitudinal stability. Refer to Paragraph 85 of this advisory circular for an explanation of detailed flight test procedures.
85. § 29.175 (Amendment 29-24) DEMONSTRATION OF STATIC LONGITUDINAL STABILITY.
a. Explanation.
(1) This rule incorporates the specific flight requirements for demonstration of static longitudinal stability. Specific loadings, configurations, power levels, and speed ranges are stated for conditions of climb, cruise, autorotation, and hover.
(2) Some rotorcraft in forward flight experience significant changes in engine power with changes in airspeed even though collective and throttle controls are held fixed and altitude remains relatively constant. For these cases, the guidance in § 29.173, which states that throttle and collective pitch must be held constant, is appropriate for administration of this rule, and the specified power in § 29.175(a), (b), and (c) should be considered as power established at initial trim conditions. This will result in slightly higher or lower torque readings at "off trim" conditions. Collective and throttle controls are held constant when obtaining data during climb, cruise, and autorotation tests.
(3) The effects of rotor RPM on autorotative static stability should be determined, and positive stability demonstrated for the most critical RPM. For Category A rotorcraft this requirement may be satisfied at a nominal RPM value. RPM values can be expected to change as airspeed is varied from the "trimmed" condition.
Manufacturer's recommended autorotation airspeed is ordinarily used for trim.
(4) Hovering is considered a flight maneuver for which the pilot repeatedly adjusts collective to maintain an approximately constant altitude above the ground. For hover stability tests, collective and throttle adjustments are made as necessary to Par 84 179
maintain an approximately constant height above the ground. Also, a limited amount of negative longitudinal control travel is allowed with changes in speed.
b. Procedures.
(1) Instrumentation.
Sensitive control position instrumentation is mandatory. Engine power parameters should be recorded at trim. For testing of minor modifications or when using a "before and after" method, a tape measure or a stick plotting board may be utilized. A stick plotting board consists of a level surface with a clean sheet of paper on it and attached to the cockpit or seat structure. The installation must not interfere when the flight controls are fully displaced. A recording pencil is attached to the cyclic control by an offsetting arm in such a manner that it can be pushed down on the board to record relative cyclic position at key times during test maneuvers. The Figure 85-1 plot is a typical presentation of longitudinal static stability.
(її) Other necessary parameters include pitch attitude, pressure altitude, ambient temperature, and indicated airspeed (pace vehicle or theodolite speed for hover tests). For hover tests, hover height (radar altitude if available), and surface winds should be documented. Two-way communications with a pace vehicle is highly desirable. Ground safety equipment is desirable.
(2) Ambient Conditions. Smooth air is necessary for stability testing. Allowable wind conditions for hover stability testing are the same as those for hover controllability tests and are described in that section (Paragraph 81). Extrapolation is covered in Paragraph 58 of this advisory circular.
(3) Loading. Aft center of gravity (CG) is ordinarily critical for longitudinal stability testing, although high speed flight and hover should be checked at full forward CG and maximum weight. At aft CG, light or heavy weight conditions can be critical.
The manufacturer's flight data should be reviewed to determine critical loading conditions.
(4) Conducting The Test.
(i) The rotorcraft should be established in the desired configuration and flight condition (climb, cruise, autorotation) with the required power and rotor speed at the trim airspeed. The collective stick should be fixed in that position, usually by applying sufficient friction to insure that it is not inadvertently moved. For autorotative tests, a rotor speed should be selected so that the variations in rotor speed as airspeed and altitude change do not exceed the allowable limits. This point is recorded as the trim point. Airspeed is then increased or decreased in about 10-knot increments, stabilizing on each speed and recording the data. At least two points on each side of the trim speed should be taken.
180 Par 85
(її) The cruise test should be conducted by varying airspeed around the desired altitude with throttle and collective fixed. This should be accomplished by first determining V (level flight speed at maximum continuous power) at the test altitude.
Then reduce power to establish a level trimmed condition at 0.9 VH (or 0.9 VNE if lower).
This point is then recorded as the trim point.
(ill) For climb and autorotation tests, conduct fixed collective tests through an altitude band (usually +2,000 feet), first increasing airspeed as data points are collected, then decreasing speed through the same altitude band. It will probably not be possible to obtain the required data on one pass through the altitude band. If repeated passes are required, a trim point should be taken at the beginning of each pass unless very sensitive collective pitch position information is available in the cockpit. Generally, it will be possible to acquire all the high speed points on one pass and the low speed points on the second.
(iv) If extremely precise results are required, an alternate method of testing can be used to acquire the data at a constant altitude. For cruise, data can be obtained by alternating airspeeds above and below the trim speed to arrive in the vicinity of the test altitude as the point is recorded. This method results in very precise data because collective and throttle are not moved as airspeed is changed at a constant altitude. A typical sequence of speeds that could produce these results would be: 150 (V), 135 (0.9VH) trim speed, 125, 145, 115, 155, 105, and 165.
(v) For rotorcraft with high rates of climb, a series of climbs, each at a different speed, may be required through a given altitude, utilizing sensitive instrumentation to assure collective position is the same for each data point. In autorotation, a similar case arises and a series of descents, each at a different speed, may be required through a given altitude band, using sensitive instrumentation to assure a repeatable collective position.
(vi) Hover tests should be conducted by maintaining an approximately constant altitude above the ground at the hover height established for performance purposes. The test altitude above the ground may be increased to provide reasonable ground clearance during rearward flight. Groundspeed is varied using a pace vehicle, theodolite, or other velocity measuring equipment. A pace vehicle is an aid in maintaining an accurate hover height. The pilot can accurately maintain height by controlling his sight picture of the pace vehicle (level with the roof, antenna, etc.).
Hover stability tests are ordinarily conducted in conjunction with hover controllability tests because instrumentation and facilities are essentially the same.
(vii) Normally climb, cruise, and autorotation tests should be conducted at low, medium, and high altitudes. See Paragraph 55 for guidance on interpolation and extrapolation. High speed stability has been critical during cold weather testing. In two Par 85 181
recent models, VE at cold temperatures has been limited by the stability requirements of § 29.176(b). Cold weather testing should be accomplished or a conservative approach for advancing blade tip Mach number should be used to limit cold weather VNE to tip Mach number values demonstrated during warm weather testing.
(viii) Hover stability should be verified at low altitude and, if required, at high altitude. Refer to Paragraph 55b(2) for guidance on expansion and extrapolation of altitude.
182 Par 85
• CRUISE
• CLIMB
• AUTOROTATION
• HOVER
SHADED POINTS REPRESENT TRIM 100 90 000000 000000 A A 20 10 20 40 60 80 100 120 140 160
CALIBRATED AIRSPEED (KNOTS) FIGURE 85-1. STATIC LONGITUDINAL STABILITY Par 85 183
86. § 29.177 (Amendment 29-24) STATIC DIRECTIONAL STABILITY.
a. Explanation. This rule requires that positive static directional stability be demonstrated at the trim airspeeds defined in § 29.175. The trim speed for climb is Vy and for cruise is 0.9VH or 0.9VNE (whichever is less). For autorotation that airspeed defined by the midpoint of the speed range specified in § 29.175(c) may be used.
b. Procedures.
(1) Tests for static directional stability require instrumentation for pedal position and sideslip angle. Lateral cyclic control position instrumentation should be provided for IFR certification tests. To obtain accurate sideslip angle and airspeed information, a "yaw boom" is usually installed for the purpose of mounting a sideslip vane and swiveling airspeed pitot head outside the main rotor downwash region of influence.
Special care should be taken to ensure that the yaw boom installation has been verified to be structurally adequate and free of dynamic instabilities for all combinations of airspeed and rotor speed likely to be experienced during the static directional evaluation. For some installations, the instrumentation yaw boom may influence the flying qualities of the rotorcraft itself. Thus, it is advisable to correlate yaw string displacement or slip indicator ball widths of skid with yaw boom sideslip angle, and then repeat a few critical points with the yaw boom removed.
(2) For some rotor system designs, the main and tail rotor flapping angle may be a critical instrumentation requirement for static directional testing. Both main and tail rotor flapping may increase dramatically at high airspeeds with increasing sideslip angle. Therefore, for rotor systems exhibiting this characteristic, flapping should be monitored carefully during the sideslip maneuver to avoid exceeding limitations. Static directional stability is normally defined in terms of pedal displacement required to maintain a straight flight path sideslip. A single-rotor rotorcraft flying in coordinated flight will exhibit a small inherent sideslip due to tail rotor thrust and fuselage/main rotor sideforces. This condition is normally taken as trim with the inherent sideslip angle noted. Airspeeds should be the trim values described above. A generally accepted technique follows:
(i) Stabilize at the trim point, and note indicated airspeed.
(il) Record trim conditions including inherent sideslip. Maintain fixed collective and throttle for the remainder of the maneuver.
(ill) Smoothly yaw the aircraft with directional control and coordinate with lateral control to establish the desired sideslip angle. A steady heading can best be ensured by maintaining a track over a straight landmark on the ground such as a section line or straight segment of powerline or highway.
184 Par 86
(iv) Note airspeed immediately upon completion of the yaw maneuver.
There may be a small change from the trim airspeed. Fly the new airspeed while maintaining a constant heading, and record indicated airspeed, control positions (directional at a minimum), sideslip angle, rotor speed, rate of descent, amount of ball deflection, and bank angle. The pilot should note the physical sideforce feel experienced. A minimum of two sideslip data points on each side of the trim point should be obtained to adequately define the slope of the pedal displacement versus sideslip angle relationship.
Smoothly return the aircraft to the inherent sideslip angle. Static directional stability plots can be expected to differ slightly on either side of the inherent sideslip angle. Positive static directional stability is indicated by increased left pedal displacement for a larger right sideslip and, conversely, increased right pedal for a larger left sideslip angle.
87. § 29.181 (Amendment 29-24) DYNAMIC STABILITY: CATEGORY A ROTORCRAFT.
a. Explanation. This section requires that Transport Category A rotorcraft, certificated under Amendment 24 of FAR 29, demonstrate positive damping for short-period oscillations (5 seconds or less) at forward speeds from Vy to VNE with the cyclic, collective and directional controls held in the desired test condition or released by the pilot. This requirement would prevent persistent or divergent short-period oscillations and thus alleviate the pilot workload to actively dampen oscillatory motions for all types of operations.
b. Procedures.
(1) Tests for short period dynamic stability are carried out in the same manner as for IFR (reference Paragraph 775 of AC 29-2A) except the oscillation need not be damped as heavily (i.e., to ½ amplitude in not more than one cycle). Similarly pulses and doublets may be used to generate an upset condition that would be expected to be encountered in moderate turbulence for that particular rotorcraft.
(2) Tests should be conducted at the critical gross weight, altitude, center of gravity, rotor RPM, and power conditions during routine climb, cruise, and descent condition for speeds from Vy to VNE. This test must be conducted with the minimum amount of stability augmentation approved for continued safe flight. Consideration should be given to optional equipment that are to be mounted externally.
(3) This requirement is not applicable to transport category rotorcraft certificated as Category B only. The requirements for this situation are unchanged.
88.-95. RESERVED.
Par 86 185 (thru 193)
SECTION 5. GROUND AND WATER HANDLING CHARACTERISTICS
96. § 29.231 GENERAL (GROUND AND WATER HANDLING CHARACTERISTICS).
a. Explanation. The rule states: "The rotorcraft must have satisfactory ground and water handling characteristics, including freedom from uncontrollable tendencies in any condition expected in operation." In addition, §§ 29.235, 29.239, and 29.241, contain specific requirements concerning ground and water handling characteristic evaluations.
b. Procedures.
(1) During the flight test program and the F&R program (§ 21.35(b)(2)), the rotorcraft will be subjected to evaluations at various weight and CG conditions. Any uncontrollable tendencies found during these test programs must be corrected.
(2) Controllable or damped vibrations or oscillations on the ground or in the water are acceptable, provided the design limits of the rotorcraft are not exceeded.
(3) Any significant vibration or oscillation characteristics found during tests should be described in the test report, and the rotorcraft flight manual should contain appropriate descriptions and procedures to describe and either avoid or handle significant characteristics.
(4) For rotorcraft equipped with wheel gear, the evaluation should include takeoff, landing, and taxi at the maximum airspeed and ground speed CG extremes. If a nose or tail wheel lock/swivel control is installed, each position should be evaluated for limiting takeoff, landing, and taxi speeds. Maximum substantiated speed values should be included in the RFM as limitations.
(5) For water operations, the wave height and frequency or "sea state" should be included as a limitation or, if no limit was reached during testing, the demonstrated values should be placed in the Performance Section of the Rotorcraft Flight Manual.
Information or limits on the allowable "sea state" for rotor startup and shutdown should also be included.
97. § 29.235 TAXING CONDITION.
a. Explanation. The rotorcraft is designed for certain landing load factors (§§ 29.471 and 29.473). The rotorcraft must not attain a load factor in excess of the design load factor when taxied over the roughest ground that may reasonably be expected in normal operation at the expected taxi speeds. This rule applies to wheel landing gear equipped rotorcraft.
194 Par 96
7/30/97/ AC 29-2B
b. Procedures. The structural substantiation data contains the allowable design limits for the rotorcraft. A calibrated accelerometer or load factor "g" meter should be installed, as near as practicable to the rotorcraft CG, to record the maximum vertical load factor attained. Instrumentation of the landing gear and/or related structure may also be an acceptable means of showing compliance.
(1) Calibrated instrumentation should be installed to record the maximum loads or maximum vertical load factor attained during the taxi tests.
(2) The taxi surface should be evaluated for compliance with the rule.
Corrugated surfaces, as well as broken or uneven surfaces, in accordance with the rule, should be used.
(3) Representative typical taxi speeds, up to the maximum selected by the applicant, should be attained over the selected taxi surfaces.
(4) A light and heavy rotorcraft weight condition should be evaluated.
(5) Limitations appropriate for the rotorcraft design should be included in the flight manual. If these tests indicate that it is unlikely that limit load factors will be attained while taxiing, flight manual limitations may not be necessary.
(6) Pertinent taxi information obtained from these test conditions may be included in normal procedures of the flight manual.
98. § 29.239 SPRAY CHARACTERISTICS.
a. Explanation. The intent of this requirement is to evaluate by demonstration that water spray does not obscure visibility (day or night) or damage the rotorcraft during normal waterborne operation (for those rotorcraft which have waterborne or amphibious capability).
b. Procedures.
(1) The following maneuvers should be evaluated in ambient conditions up to the proposed sea state or wave height for operation.
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Con- Rotor Altitig. Condition Weight CG RPM tude Remarks Taxi Max Optional Max SL Speeds up to maximum proposed for water operation.
2 Hover Max Opt Max Determine critical hover height, any.
Takeoff Max Opt Max SL Unstick at maximum proposed water operation speed.
4 Land Max Opt Max SL Touchdown at maximum proposed for water operation.
5 Shutdown Opt Opt SL Shut down the rotorcraft.
6 Start Max Opt Max SL Start engines and release rotor brake.
(2) The maximum sea state or wave height evaluated under this rule should be stated and included in the limitations section of the flight manual.
(3) The effect of saltwater contamination and deterioration of turbine engines and other component parts of the rotorcraft should be considered in accordance with § 29.609 and Paragraph 244 of this advisory circular. Information on saltwater effect and attendant corrective action should be provided in the flight manual, if appropriate, and in the maintenance manual.
99. § 29.241 GROUND RESONANCE.
a. Explanation
(1) The rule states: The rotorcraft may have no dangerous tendency to oscillate on the ground with the rotor turning. This rule is a flight requirement that pertains to demonstrating freedom from dangerous oscillations on the ground. CAR Part 7, predecessor to FAR Part 29, originally contained a "strength requirement," under § 7.203, requiring ground vibration tests. This test would identify critical vibration frequencies and modes of the rotorcraft. CAR Part 7, Amendment 7-4, effective October 1, 1959, removed this ground vibration requirement because the agency concluded that if any major component has a natural frequency which could be excited by some operating parameter, such a condition would be revealed in the course of other ground and flight tests. The Federal Aviation Administration (FAA) apparently 196 Par 98