- 90° 90° YAW DEGREES / 10° MAX 0°
AIRSPEED 30 KNOT LESSER OF
SIDEWARD VH OR VNE FLIGHT FIGURE AC 29.351-1 SAMPLE YAW/FORWARD SPEED DIAGRAM Page C - 12
(viii) During flight test evaluations, yaw angles have been measured using a yaw angle probe (swiveling vane type) on a nose boom. Both a visual readout for the pilot and a record, such as an oscillograph trace, have been used. This test may be conducted in the flight test program or in the flight load survey program. This record should confirm the yaw angle used in design as conservative with respect to operational and actual flight characteristics. This test is not a requirement however.
AC 29.351A. § 29.351 (Amendment 29-30) YAWING CONDITIONS.
a. Explanation. Amendment 29- 30 adds maximum sideslip angles to the existing § 29.351 for structural design purposes. The standard should apply to power -on conditions; not power-off, since VH is a part of the standard. For airspeeds up to 0.6 VNE, sideslip angles larger than 90° (or sideward flight) need not be considered. For airspeeds at VNE or VH (whichever is less), sideslip angles larger than 15° need not be considered.
b. Procedures.
(1) All of the policy material pertaining to this section remains in effect with the addition of the maximum sideslip limits of 90° and 15° specified above. The rotorcraft does not need to be capable of attaining these conditions. A revised yaw/forward speed diagram is presented in figure AC 29.351A-1.
(2) FAR § 29.351(b)(1) incorrectly references § 29.395(a) for maximum pilot forces. The correct reference should be § 29.397(a).
Page C - 13
go•
YAW DEGREES (LON GI TUDI N AL AXIS)
AIRSPEED +-15° MAX.
LESSER OF
VH OR VNE AC 29-2C, Chg 1 4/25/06 FIGURE AC 29.351A-1
SAMPLE YAW/FORWARD SPEED DIAGRAM Page C - 14
AC 29.351B. § 29.351 (Amendment 29-40) YAWING CONDITIONS.
a. Definitions.
(1) Suddenly. For the purpose of this section, ‘suddenly’ is defined as an interval not to exceed 0.2 seconds for complete control input. A rational analysis may be used to substantiate an alternative value.
(2) Zero Yaw. Normal, 1 -g, level flight condition with either zero bank angle or zero sideslip.
b. Explanation. The rule requires a rotorcraft “structural” yaw or sideslip design envelope. This sideslip envelope must cover minimum forward speed, or hover, to VH or VNE, whichever is less. The rotorcraft must be structurally safe for the thrust capability of the directional control system.
(1) The rotorcraft structure must be designed to withstand the loads for the specified yawing conditions. The standard does not require a structural flight demonstration. It is a structural design standard.
(2) This standard applies only to power-on conditions. Autorotations need not be considered.
(3) This standard requires the maximum allowable rotor RPM consistent with the flight conditions, including special operational rotor settings.
(4) For the purposes of this section, the analysis may be performed at international standard atmosphere (ISA) sea level conditions.
(5) The rotorcraft structure must be designed to withstand the loads for the specified sideslip conditions. This includes, but is not limited to:
(i) Main cabin, tailboom, and vertical control surfaces.
(ii) Tai l rotor structures, including the fitting attachments to the frame.
(iii) Windows, doors, and other transparencies.
(iv) Landing gear and retracting mechanism.
(v) Fairings and cowlings.
(6) Maxim um displacement of the directional control, except as limited by pilot effort (§ 29.397(a)), is required for the conditions cited in the rule. Control system limiting devices may be used, however the probability of failure or malfunction of these system(s) should be considered (see Figure AC 29.351B -2). This evaluation may Page C - 15
AC 29-2C, Chg 2 4/25/06 include Flight Manual Limitations, if failure of the system is reliably indicated to the crew.
(7) Both right and left yaw conditions should be evaluated.
(8) For vertical stabilizers, the airloads may be assumed independent of the tail rotor thrust (superpositioning).
(9) Loads associated with sideslip angles exceeding the values of Figure AC 29.351B-1 do not need to be considered. The corresponding points of the maneuver may be deleted.
c. Procedure. The design loads should be evaluated within the limits of Figure AC 29.351B-1 or the maximum capability of the rotorcraft, whichever is less; at speeds from zero to VH or VNE, whichever is less, for the following phases of the maneuver:
(1) With the rotorcraft at an initial trim condition (1 g level flight and zero yaw), the cockpit directional control is suddenly displaced to the maximum deflection limited by the control stops or by the maximum pilot force specified in § 29.397(a). This is intended to generate a high tail rotor thrust.
(2) While maintaining maximum cockpit directional control deflection, within the limitation specified in c(1) of this AC paragraph, allow the rotorcraft to yaw to the maximum transient sideslip angle or to the value defined in Figure AC 29.351B-1, whichever is less. This is intended to generate high aerodynamic loads.
(3) Allow the rotorcraft to stabilize at the maximum steady-state sideslip angle.
In the event that the maximum steady state angle is greater than the value defined in Figure AC 29.351B-1, the rotorcraft should be trimmed to the value of the angle using less than maximum cockpit directional control deflection.
(4) With the rotorcraft yawed to the static equilibrium sideslip angle specified in c(3) of this AC paragraph, the cockpit control is suddenly returned to its initial trim position. This is intended to combine a high tail rotor thrust and high aerodyna mic restoring forces.
Page C - 16
SIDESLIP ANGLE ( deg. ) 90 15 0 0.6 Vne Vh or Vne whichever is less
AIRSPEED FIGURE AC 29.351B-1
YAW/FORWARD SPEED DIAGRAM Page C - 17
FS 1 .5 1 .0 -~ I 10-9 Qj - Probability of being in failure condition j AC 29-2C, Chg 2 4/25/06
• For static strength substantiation, each part of the structure should be able to withstand without failure, the loads generated by the maneuver described in the rule multiplied by a factor of safety depending on the probability of being in this failure state. The factor of safety is defined in the figure below:
Qj = (Tj)(Pj) where:
Tj = Average flight time spent with a failed control limiting system j (in hours) Pj = Probability of occurrence of failure of the control limiting system j (per hour) Note:If Pj is greater than 10-3 per flight hour then a 1.5 factor of safety should be applied to all limit load conditions specified in this standard.
FIGURE AC 29.351B-2 Safety Factors for Probability of Failure AC 29.361. § 29.361 (Amendment 29-26) ENGINE TORQUE.
a. Explanation.
(1) The rotorcraft should be designed for limit engine torque values, as prescribed by the rule, to account for maximum engine torque, including certain transients and torsional oscillations. The rule recognized that reciprocating (piston) engines generate higher torque oscillations than turbine engines.
(i) A factor of 1.25 applies to maximum continuous power for turbine engines. Section 29.923 refers to torque output and § 29.927(b) refers to other torque output conditions for use in an “endurance test.”
Page C - 18
(ii) Torque factors are also specified for reciprocating engines having two or more cylinders in § 29.361(a)(2) or § 29.361(b) of Amendment 29-26. The appropriate torque factor applies to takeoff power torque as well as maximum continuous power and other power conditions.
(2) Amendment 29-26 introduced additional turbine engine installation considerations for the following:
(i) Engine torque loads associated with emergency operation of governor-controlled turboshaft engines.
(ii) Torque reaction loads from sudden turbine engine stoppage which is applied to the engine and the engine suspension and restraint system.
(3) Paragraph AC 29.549 concerns § 29.549(c) and (e) that contains design standards for engine mounts and adjacent structure for flight and landing and also flight with 2 ½-minute OEI power rating. Amendment 29- 26 added OEI power to the standard.
(4) Section 29.547(e)(1)(ii) concerns the application of limit engine torque to design of the main rotor structure.
b. Procedures.
(1) The engine torque associated with the maximum continuous power condition should be multiplied by the appropriate torque factor to obtain the engine torque value used for structural substantiation purposes of the rotorcraft.
(2) The torque values associated with the minimum power-on RPM limit should be used. Maximum power -on speed limit will result in a lower torque value when calculating torque from design horsepower values. However, due to piston engine power output characteristics, an engine may produce a higher torque at higher engine speeds contrary to the previous statement. The torque factor should account for this characteristic.
(3) For turbine engines limit torque values are determined for the four cases cited. Two cases are related to “endurance” test standards.
(4) For sudden stoppage of turbine engines the engine manufacturer can reasonably provide engine rotating inertia and deceleration time expected in the event of sudden engine stoppage which generates these critical loads in the engine mounting and restraint system. These manufacturer’s data should be acceptable for use in complying with this part of the design standard.
Page C - 19
SUBPART C - STRENGTH REQUIREMENTS
CONTROL SURFACE AND SYSTEM LOADS AC 29.391. § 29.391 CONTROL SURFACE AND SYSTEM LOADS - GENERAL.
a. Explanation. This general rule concerns requirements for design loads of tail rotors, control or stabilizing surfaces, and their control system.
b. Procedures. The design criteria and/or the design loads report must contain the loads dictated by the referenced rules. See paragraphs AC 29.395, AC 29.397, AC 29.399, AC 29.401, AC 29.403, AC 29.411, and AC 29.413.
AC 29.391A. § 29.391 (Amendment 29-30) CONTROL SURFACE AND SYSTEM LOADS - GENERAL.
a. Explanation. Amendment 29- 30 adds an explicit reference to § 29.427, Unsymmetrical Loads (paragraph AC 29.427), to clarify that substantiation for unsymmetrical loads is a general control surface requirement. A reference to § 29.399, Dual Control System (paragraph AC 29.399), is also added for clarification. In addition, §§ 29.401, 29.403, 29.413 were removed by this amendment since these references and requirements were adequately addressed in other standards.
b. Procedures. The referenced AC paragraphs become AC 29.395, AC 29.395A, AC 29.397, AC 29.399, AC 29.411, and AC 29.427.
AC 29.395. § 29.395 CONTROL SYSTEM.
a. Explanation. Control system design loads and the application of these loads are contained in this rule.
(1) Paragraph (a) of the rule specifies the way or means of reacting the design loads specified in §§ 29.397 and 29.399 (for dual control systems). The design loads must be imposed on any locks and stops and irreversible mechanisms in the control system. Both rotor blade horns and control surface horns must react without failure, the specified loads while the controls are in critical positions.
(2) Paragraph (b) of the rule specifies application of limit pilot forces or of the maximum loads that can be obtained in normal operation, including any single power boost system failure, whichever is greater. However, minimum limit pilot force 0.60 of the loads specified in §§ 29.397 and 29.399, may be used, as specified, in parts of the primary control system that are not stiff enough to react to the loads specified in the first part of Paragraph (b) of the rule. Note the objective for a rugged control system.
Page C - 20
(3) Control system design feature and test requirements are found in §§ 29.671 through 29.695. Bearing factors and fitting factors are specified in §§ 29.623 and
29.625, respectively.
b. Procedures.
(1) The design criteria and/or a design loads report that includes the primary control system design loads should be submitted for FAA/AUTHORITY approval.
(2) The rotorcraft control system may be tested to ultimate design loads or may be analyzed for the ultimate design loads. See paragraph AC 29.307.
(i) It is advisable that the applicant prepare a proposal describing the procedures and techniques to be used in the static testing of the control system which reflects compliance with the condition specified. It is further advisable that the FAA/AUTHORITY concur that the tests proposed achieve that objective. Omission of these steps may result in the need for retesting. The test results should be documented.
(ii) If tests are not conducted, a structural analysis of the control system is required. Appropriate factors from §§ 29.685(e), 29.623, and 29.625 must be used as specified. A structural analysis report should be used to document compliance with § 29.685(d)(1) and (4), and § 29.685(f).
(3) If a part of the control system is not stiff or rigid enough to react the design loads specified in § 29.397, that part of the system may be substantiated for lower loads as prescribed.
(i) The limit design loads are those loads specified in § 29.397;
(ii) The limit design loads are the maximum that can be obtained in normal operation, including any single power boost system failure, except for objectives stated for a rugged system; and
(iii) In lieu of a rational analysis, the limit design loads may be 0.60 of the loads specified in § 29.397.
(iv) For example, if a control surface servo tab or a small elevator is a part of the rotorcraft design, the control system for this part must be stiff enough to react the control surface loads without failure and to provide enough surface deflection to control the rotorcraft. These limit loads may be 60 pounds fore and aft and 40 pounds laterally on the cyclic control stick in lieu of a rational analysis and may be the maximum loads that can be obtained in normal operation.
(v) If a hydraulic power actuation or boost system is part of the rotorcraft design, the design limit load for the affected parts of the control system will be the Page C - 21
maximum output force of the boost at normal operating pressure added to the limit design loads resulting from the loads specified in § 29.397. If a single failure in the power portion of the hydraulic system results in actuator forces that exceed the maximum output force at normal operating pressure, the highest output loads must be used as noted in subparagraph (3)(ii). This hydraulic system failure standard is specified in § 29.695(a)(1) as well.
(4) Controls proof and operation test is required by §§ 29.307(b), 29.681, and
29.683. This test is conducted using the design limit loads approved under § 29.395(b).
See paragraphs AC 29.681 and AC 29.683.
AC 29.395A. § 29.395 (Amendment 29-30) CONTROL SYSTEM.
a. Explanation. Amendment 29- 30 clarifies that the loads in § 29.395(b) apply to power “control” systems not just power “boost” systems; and the limit pilot forces prescribed in § 29.397 are required to be applied in conjunction with the forces from normally energized power devices. The amendment may increase required loads for systems if operational loads may be exceeded through jamming, ground gusts, control inertia, or friction. If so, the system is required to withstand 100 percent of limit pilot forces specified in § 29.397, rather than 60 percent of the limit pilot forces as specified previously.
b. Procedures. The procedures of paragraph AC 29.395 continue to apply except that the increased loads in new paragraph § 29.355(b)(4) of 100 percent of limit pilot forces are specified for systems where operational loads may be exceeded by jamming, ground gusts, control inertia, or friction.
AC 29.397. § 29.397 (Amendment 29-12) LIMIT PILOT FORCES AND TORQUES.
a. Explanation. Design forces are contained in the rule.
(1) Primary controls, pilot and copilot, must be designed for the limit pilot forces specified in paragraph (a) of the rule.
(2) For other operating controls, such as flap, tab, stabilizer, rotor brake, and landing gear, design limit forces are specified in paragraph (b) of the rule.
b. Procedures.
(1) Design loads specified in the rule must be used in required structural tests and in any structural strength analysis of the control systems submitted in compliance with other rules.
(2) Operation tests of the control systems noted in other rules require application of these forces also.
Page C - 22
AC 29.399. § 29.399 DUAL CONTROL SYSTEM.
a. Explanation. Design limit loads are specified for dual control systems. Pilot effort forces applied in opposition and in the same direction are required for dual control systems.
b. Procedures.
(1) Design loads specified in the rule must be used in required structural tests and in any structural strength analysis submitted for compliance with the other rules.
(2) Operation tests of the control systems, noted in other rules, require application of these forces also.
AC 29.401. § 29.401 (Amendment 29-4) AUXILIARY ROTOR ASSEMBLIES.
a. Explanation.
(1) For rotorcraft equipped with auxiliary rotors, normally called tail rotors, an endurance test is required by § 29.923 and structural strength substantiation is required.
Section 29.401(b) specifically refers to structural strength substantiation for centrifugal loads resulting from maximum design rotor RPM. Due to the pitch feathering requirements, auxiliary rotors typically have detachable blades.
(2) The rotor blade structure must have sufficient strength to withstand not only aerodynamic loads generated on the blade surface, but also inertial loads arising from centrifugal, coriolis, gyroscopic, and vibratory effects produced by this blade movement.
Sufficient stiffness and rigidity must be designed into the blades to prevent excessive deformation and to assure that the blades will maintain the desired aerodynamic characteristics. As a design objective, the structural strength requirements should be met with the minimum material. Excess blade weight imposes extra centrifugal loads that may increase the operating stress levels. Blade weight and strength should be optimized. Even though a structural strength analysis for the blade design loads is required, a flight load survey and fatigue analysis are also required by § 29.571.
(3) Section 29.1509 defines the design rotor speed as that providing a 5 percent margin beyond the rotor operating speed limits.
b. Procedures.
(1) The endurance tests prescribed by §§ 29.923 and 29.927 require achieving certain speeds, power, and control displacement for the auxiliary (tail) rotor as well as the main rotor. The parts must be serviceable at the conclusion of the tests.
Page C - 23