transition through the critical speed range and to avoid excessive nonsyncronous vibrations or instability when transitioning through the critical speed.
(3) A review of typical design practices and drive system arrangements discloses several types of shaft support and loading:
(i) Main rotor/mast/transmission assemblies rigidly mounted to the airtrame.
(ii) Main rotor/mast/transmission assemblies compliantly mounted to the airtrame.
(III) Main rotor supported through a bearing arrangement by a rigid nonrotating structure with a coaxial torque shaft driving the rotor.
(iv) Cross-shafting, interconnect shafting, tail rotor drive shafting which are generally supported by gearboxes at each end and by hanger bearings and couplings at intervals along the tail rotor drive shaft.
Engine to transmission shafting which, for compliant pylons, incorporates a flexible or geared coupling, to accommodate the misalignment and chucking.
(vi) Tail rotor/mast/gearbox supported on the tailboom or near the upper extremity of a vertical fin.
(4) With regard to compliant pylon mountings, recent developments in vibration control have led to rotor isolation wherein the fuselage is isolated from the rotor and transmission, resulting in improved vibration and system reliability. Rotor isolation systems typically entail the installation of isolation devices at the transmission-airframe interface. The crux of rotor isolation is providing adequate, low-frequency isolation without excessive relative displacement or loss of mechanical stability. Rotor isolation affects shaft critical speeds in the following ways:
(i) First, the transmission mounting configuration, system stiffness, and tuning requirements may result in different fore-and-aft and lateral natural frequencies, imposing additional analytical requirements. For compliant mounting, the response while transitioning through the fundamental or rocking modes is generally controlled by dampers or elastomeric elements.
(i) Second, the relatively high displacements permitted by the isolation system, depending on configuration, may result in variations in shaft misalignment and length thus adding further complexity to the analytical prediction of critical speeds.
b. Procedures.
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(1) Subcritical Shafting Designs. Three basic methods of qualification may be considered, with the required margins relative to the degree of assurance provided.
The margins are shown for guidance only.
(1) Analytical.
(A) Simplistic model(s) as shown in figures AC 27.931-1 and AC 27.931-2; 35-percent margin shown above maximum operating speed.
(B) Detailed model, taking into account significant variations in shaft stiffness, mass distribution, cone adapters, support bearing stiffnesses, support structure; 20-percent margin shown above maximum operating speed.
(ii) Analytical supported by tests. Analysis supported by shake test (rotating or nonrotating) or by bench test, where appropriate adjustments are made for differences between the bench and the aircraft; 15-percent margin shown above maximum operating speed.
Whirl test on the aircraft.
(A) For all cases, it should be shown that, under maximum permissible unbalance and at the maximum operating speed, the shafting and support structure has acceptable clearance and does not have excessive vibration.
(B) For compliant pylon mountings, damping of the rigid body rocking modes, which are often transitioned during runup to normal speed (and which are not critical flexing modes), may be verified by analyses, laboratory tests, or ground runup with the rotor at maximum permissible unbalance. Damping on the order of 5 percent equivalent viscous damping is generally acceptable.
(C) For tail rotor masts, the analysis should include fixed system structural response including tailboom, fixed control surfaces, and vertical fin. The frequency analysis will then contain both fixed system and rotating system modes. An energy analysis can then be used to identify whether the modes are predominantly fixed system or rotating system modes. Systems with up to 35-percent energy in the rotating system have been operated in the field without significant problems. For this type of shafting installation, it is advisable to avoid fixed system modes at multiples of shaft speed, particularly where highly non-isotropic mountings exist.
(2) Supercritical Shafting Design. Another facet occasionally encountered with shafting is the concept of normally operating at speeds above the critical speed, commonly referred to as "supercritical operation." To function properly, suitable dampers must be installed to enable the shaft to pass safely through the lower critical speed up to the operating speed, and speed controls should be devised to avoid any tendency to operate continuously at any critical speed. Accurate balancing of the rotating components will also decrease the energy to be dissipated into the damping Page E - 23
device during transition thereby increasing its serviceability and reliability. Note that damper design and locations become more complex as selected operating speed increases through the third or fourth critical frequency. Multiple node points will exist where dampers will not be effective. Production specimen testing at high speed/high torque conditions should include checks for shaft straightness until experience verifies that shaft deflecting is not significant. For systems utilizing squeeze film dampers at the support bearings, variations in oil pressure, flow restrictions, and the effects of bearing preload should be evaluated. The effects of shaft and unbalance and the proximity of the damper to bottoming under maximum unbalance should be evaluated.
(3) If the shafting configuration of the rotorcraft includes universal joints or misalignment couplings, a velocity differential will exist across the joint which creates sinusoidal torques and bending moments at both shafts at multiples of the rotation speed. To avoid amplification of these torques and bending moments, the design should preclude coincidence of critical speeds and multiples of normal speeds.
(4) Order 8110.9, Handbook on Vibration Substantiation and Fatigue Evaluation of Helicopters and Other Power Transmission Systems, also addresses this subject. This document is distributed to section level and above in all Regional Aircraft Certification Offices.
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M + 0,23m = first critical speed, RADISEC = shaft spring rate, LEVIN = 3EVL = modulus of elasticity = moment on inertia M = mass of weight, LB-SECHIN m = mass of shaft, LB-SEC?/IN FIGURE AC 27.931-1 CANTILEVERED SHAFT, FIRST CRITICAL SPEED DISTO Wo = a ET Ц L# Wa = first entical speed-RAD/SEC = mass per unt length E = Young's modulus = length between supports = inertia of shaft = a numerical constant for first critical speed, a = (pi)2 = 9.87 The numerical constant (a) for higher critical whirl modes or other shaft support systems may be derived from standard texts on this subject FIGURE AC 27,931-2 SHAFT BETWEEN SUPPORT BEARINGS, FIRST CRITICAL SPEED Page E - 25
AC 27-1B, Chg 4 3/31/2014 AC 27.935. § 27.935 SHAFTING JOINTS.
a. Explanation. This rule requires the design of shafting joints to include provisions for lubrication when such lubrication is necessary for operation.
b. Procedures. Review the design of the rotor drive system for universal joints, slip joints (splines) and other shaft couplings. Lubrication access points (Zerk fittings) should be required unless the design incorporates alternate provisions for lubrication acceptable to the FAA/AUTHORITY and shown valid by test or experience.
AC 27.939. § 27.939 (Amendment 27-11) TURBINE ENGINE OPERATING CHARACTERISTICS.
a. Explanation. This section provides guidance for evaluation of engine operation, engine inlet airflow distortion, and engine-to-drive system torsional stability. A satisfactory rotorcraft design for all three items should be established by the manufacturer early in the development program since changes in design to satisfy these requirements are typically very expensive and will adversely impact other basic design features. Introduction of full authority digital engine control (FADEC) controls has increased the complexity in evaluating engine operating characteristics with the need to investigate FADEC degraded or failure modes. The certification test plan should address the engine control being used. In addition, where manufacturing tolerances could affect engine handling and rotor governing, tests should be performed considering the worst tolerances regarding the tests to be conducted. The results of these evaluations are used in part to verify that the requirements for the engine installation Instruction Manual mandated by § 33.5(a) are satisfied.
b. Procedures.
(1) Turbine engine operation.
(1) Explanation. Smooth, stable operation of turbine engines is essential to safety and control of rotorcraft. This can be adversely affected by rotorcraft maneuvers, turbulence, high altitude, temperature, airspeed, and installation features such as the engine air inlet duct, exhaust duct, and the location with respect to other airframe items which induce or influence air flow through the engine. Powerplant control displacement rate can also be a factor, although most modern engines incorporate internal protection for this aspect. The engine's tolerance to these factors is reflected as the "stall margin" which is established by the engine manufacturer through design and test. However, this stall margin is applicable only to an engine with a specified inlet and exhaust and at specified altitude, temperature, and effective airspeed. Typically, the specified engine inlet duct is a symmetrical belmouth and the exhaust is a short straight duct of specified diameter and length. The stall margin, even under the above test conditions, usually varies with engine power, acceleration or deceleration, compressor air bleed, and accessory power extraction.
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(iI) Procedure. The official flight test plan should include requirements to investigate the engine operating characteristics for stall, surge, flameout, acceleration and deceleration response, and transient response (within approved limits) throughout the operating range of the rotorcraft. The results must show that no adverse characteristics are present, to a hazardous degree, during normal and emergency operation within the range of operating limitations of the rotorcraft and engine. Test configurations should encompass the critical engine power settings and design or rigging tolerances expected to be seen in service. In addition, normal and degraded engine operating control modes should be evaluated where applicable. Test conditions should include maximum airspeed-sideslip combinations, power recoveries, hover with wind from all directions including tailwinds and other maneuvers appropriate to the certificated operating envelope of the rotorcraft. In addition, recirculation of exhaust gases during hover or rearward flight can be critical for engine operation and should be evaluated. Particular attention should be given to flight and operating conditions that can be judged critical from review of data on engine inlet pressure and temperature distribution patterns and engine stall margin data if available. High altitude has typically been critical for these tests unless other critical areas of the flight envelope are identified. In addition, during rearward flight at high altitude, results may indicate unacceptable thermal distortions in the inlet due to reingestion. Stall, surge, or flameout which may be hazardous (i.e., causes loss of engine function, causes loss of control, causes severe torsional shock through the rotor drive system, or otherwise damages the rotorcraft) is unacceptable. The flight test program should include:
(A) Normal operation (hover, forward stabilized flight, collective inputs, rearward flight).
(1) Checks in hover. Hover with wind from all azimuths to the allowable wind limits, including tailwinds, should be evaluated. This evaluation should include maneuvers with rapid power changes. Recirculation of exhaust gases can be critical.
Particular attention should be given to flight and operating conditions that can be judged critical from review of data on engine inlet pressure and temperature distribution patterns and engine stall margin data. Operating at high density altitudes, especially engine operation during low-speed rearward flight, is more likely to result in unacceptable engine inlet thermal distortion due to reingestion of exhaust gas.
Behavior of the compressor control bleed air valves, if any, must be carefully checked by collective oscillations around the shut off or open bleed air valves operating points.
To mitigate the possible risk of power loss, a safe test build up should be considered, such as a build up in actual wind conditions before progressing to rearward or sideward flight.
(2) Checks in forward stabilized flight.
(1) Behavior of the engine must be checked during level flight, climb, and descent at various power settings in order to verify the:
(A) Governing stability (e.g., absence of engine parameters oscillations).
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AC 27-1B, Chg 4 3/31/2014 (B) Variation of rotor speed with the requested power.
(C) Engine matching on multi-engine helicopters.
(D) Accuracy of the engine parameters available to the pilot.
(E) Opening and closing thresholds of the bleed valve, if any.
(E) Effect of sideslip.
(ii) Stabilized flight conditions should be long enough to allow the engine to reach its thermal stabilization (typically 2 or 3 minutes) with the aim of:
(A) Assessing that the engine cycle (TOT and turbine inlet temperature) as installed is consistent with the engine certificated type design.
(B) Evaluating installation effects.
(3) Checks of engine behavior during collective inputs. Before conducting the below collective increase inputs and collective decrease inputs tests, the tested engines should be precisely defined, especially the gas generator turbine nozzle and free turbine nozzle sections (key point for the stall characteristics). Additionally, the acceleration controllers' settings (key points for acceleration and deceleration) should be identified.
1) Collective increase inputs.
(A) The collective increase inputs will verify:
(a) The transient behavior of the engine, the acceleration, and the minimum NR speed achieved.
(b) The values that the engines could reach in transients depending of the collective inputs.
(c) The effects of the collective anticipation, if any.
d) The rotorcraft handling qualities (amplitude of variation of pitch, roll and yaw, cross coupling, etc.) during the transients.
B) The flight test techniques consist of increasing collective from the limit of desynchronization (N2/NR needles are still matched) up to the collective pitch corresponding to the MCP. Depending on the altitude (speed of collective input Page E - 26.2
should be reduced when altitude increases) and the rotorcraft category (speed of collective input should be reduced when rotorcraft max weight increases), the duration of the maneuver should be from 1 to 3 seconds at low altitude to approximately 3 to 5 seconds at higher altitude.
(C) Some collective inputs should be made from desynchronized conditions (NR > N2) but should be carefully done as those maneuvers generally lead to larger reductions of Np and high free wheel or engine constraints.
(D) On multi-engine helicopters, collective inputs at lower speeds should be done with one engine at idle position to check the behavior of the other engines in case of an engine failure.
(E) Droop of N2/Np out of the green arc is permitted during the test, provided it is acceptable to the engine or airframe manufacturer for the purpose of the test. The goal is to check for generally satisfactory acceleration (no surge, no extreme N2/NR droop, no significant overshoot). Any N2/Np droop must not result in a condition that requires exceptional piloting skill, alertness, or strength to maintain safe flight.
(E) During those collective inputs, NR below the minimum NR should be avoided by lowering the collective. The reasons for poor engine acceleration should be determined.
(ji) Collective decrease inputs.
(A) Collective decrease inputs will verity:
(a) Combustion stability (i.e., that there is no flame-out).
(b) The engine behavior in transients and in particular the maximum free turbine speed, minimum oil pressure, etc.
(c) The rotorcraft handling qualities (variation of pitch and roll and yaw, cross coupling, etc.) during the transients.
B) Rate of collective lowering should be built up from 5 seconds down to under 3 seconds. Regarding N2 overspeed, a quick stop type maneuver that leads to an initial acceleration of the Np should be envisaged.
(4) Compressor stall investigation.
1) The turbine engine installation should not suffer from compressor stalls anywhere in the flight envelope. Previous tests as described above include some of the required areas, but additional testing is necessary. In stabilized conditions, the collective pitch is moved in slow and small oscillations in:
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AC 27-1B, Chg 4 3/31/2014 (A) Level flight with or without sideslip at different power settings.
(B) Pull up or in turn.
(C) Autorotation.
(D) Descent at different rates.
(ii) The maneuvers described in paragraphs b.(1)(ii) (A)(4)(i)(A) through (D) above check the engine susceptibility to air flow distortions. The test technique involves oscillations of the collective at various rates around the identified thresholds of the compressor control bleed valves.
(B) Degraded modes: Governor failures.
(1) The specific degraded mode testing for a hydromechanically controlled engine is different than for a digitally controlled engine. However, the basic principles for both types of engine controls include:
I) Review of the safety analysis, especially the FMEA.
(I) Determine the failures to be checked in flight, based on prior analysis or testing.
(iII) For each failure to be checked in flight:
(A) Evaluate the behavior of the engine when the failure occurs.
(B) Determine the acceptability of the procedure of identification of the degraded engine.
(C) Assess the human factor aspects of the machine interface (cautions, warning, etc.).
D) During the use of the degraded engine, consider the workload impact related to engine response in adjusting NR or power for approach and landing.
(E) Evaluate the adequacy of the RFM procedures.
(2) Helicopters equipped with FADECs are becoming increasingly integrated into the helicopter systems for engine control sensor inputs. For example, N2/NR values available could depend on parameters such as airspeed, altitude, or temperature as provided by the basic helicopter systems and sent to the FADEC. A Page E - 26.4
concern is the effects of degradations, discrepancies or failures in the basic helicopter systems on engine governing laws and power available. Therefore, a review of the FMEA before beginning engine failure or degraded mode evaluations should be performed.
(3) It is recommended to have special test equipment installed on the test engine in order to simulate failures to the FADEC or digital engine control. Test risk mitigation should include the evaluation of possible malfunctions of this special test equipment.
(4) As a minimum, the following typical failures or degraded modes, including engine control reversion that is based on safety analysis and ground test results, should be evaluated in flight:
(i) Failure causing the engine to increase power.
(I) Failure causing the engine to decrease power.
(II) Failure fixing the engine power at a static value.
(iv) Failure introducing oscillations.
(v) Failure introducing lower acceleration or deceleration.
(vi) Failure causing the N2/NR to increase or decrease.
(2) Vibration.
(i) Explanation. Engine airflow patterns are deflected or distorted by the presence of airframe inlet hardware, cowling, fuselage panels, and, to a degree, in almost all flight regimes. Additional items such as airframe installed particle separators, deflectors for snow, ice, or sand protection, and obstructions forward of the engine inlet, such as a hoist kit, could affect the engine air flow patterns. The rotating elements of the engine, particularly the compressor blades, will be subjected to a cyclically varying air flow as these elements move into and out of areas of deflected airflow to the engine. A corresponding aerodynamic load will be imposed on these engine elements. Since this loading is also cyclic, the possibility of critical frequency coupling with an engine component shall be investigated.
(il) Procedure. Typically, this evaluation would involve installation in the engine inlet of a special multiple probe, total pressure sensing system, and flight testing which largely follows that prescribed for evaluation of engine operating characteristics as described above. Data from these tests can be reduced to create a pressure map at the compressor inlet face which, in conjunction with compressor speeds, may be used to determine the frequencies and relative amplitudes of the cyclic air loading imposed on the engine compressor blades. The engine manufacturer either supplies the sensing probe or specifies its design and performance. Also, the engine manufacturer may Page E - 27
AC 27-1B, Chg 4 3/31/2014 evaluate the test results or publish acceptance criteria. A wave analysis may be involved in identifying higher order excitations. Engine exhaust ducts which include bends, noise suppressors, or other obstructions may require an evaluation similar to that discussed above for the engine inlet. The engine manufacturer should be consulted for instructions or approval of this aspect. High performance engines may also require an engine inlet temperature survey. Details of instrumentation and acceptance criteria should be provided by the engine manufacturer. Engines equipped with only centrifugal compressors are less likely to encounter frequency coupling and may not require this investigation. The engine manufacturer's recommendations should be followed in these cases.
(3) Torsional Stability.
(i) Explanation. Governor-controlled engines installed in rotorcraft are subject to a fuel control resonant feedback condition which could be divergent if not properly designed or compensated. This condition occurs when the response frequency of the governor on the engine is coincident with or close to a low order natural torsional frequency of the rotorcraft rotor drive system. Typically, these frequencies appear in the 3 to 5 CPS range. The manufacturer usually resolves torsional instability problems by introducing damping into the engine governor or fuel control. Provisions for this change must be supplied by or approved by the engine manufacturer. The final configuration may be a compromise between a lightly damped control, which will allow a positive but slow convergence of drive system torsional oscillations, and a highly damped control which exhibits excessive rotor speed droop or overspeed following rotorcraft collective control displacement.
(ii) Procedure. A ground and flight test program should be devised to evaluate the torsional response of the engine and drive system combination presented by the applicant. Instrumentation to record drive system torsionals should be applied to all major branches of the drive system. Engine parameters such as torque and power turbine speed should be recorded simultaneously with drive system parameters. The test program should include ground tie-down operation and flight operation across a range of engine power and rotor speeds while injecting control inputs as close to the first order drive system natural frequency as possible. Mechanical methods of making these inputs are not usually necessary if the desired frequency is in the 3 to 5 CPS range and the instrumentation readout confirms that the drive system was actually excited torsionally at its natural frequency. Control inputs should include collective, antitorque, and throttle. Also, cyclic inputs may be important on tandem rotor rotorcraft.
The acceptance criteria may be dependent on several items. Among these are rotor and drive system fatigue loading, engine power response characteristics, limitations established by the engine manufacturer, etc. The acceptance criteria are usually stated as a percent damping (minimum). Typically, 1 percent of critical equivalent viscous damping (or greater) is acceptable. In effect, this means that the free vibration response to a control input damps to ½ amplitude in 11 cycles or less.
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SUBPART E - POWERPLANT
FUEL SYSTEM AC 27.951. § 27.951 (through Amendment 27-9) FUEL SYSTEM--GENERAL.
a. Explanation.
(1) The term "fuel system" means a system which includes all components required to deliver fuel to the engine(s). This includes, but is not limited to, all components provided to contain, convey, drain, filter, shutoff, pump, jettison, meter, and distribute fuel to the engines.
(2) Paragraph (a) of this section is a general statement of the performance requirements for fuel systems and constitutes authority to require the fuel system to be adequate notwithstanding compliance with detail requirements listed in §§ 27.953 through 27.999 of this subpart.
(3) Paragraph (b) of this section requires fuel systems to be designed so that air will not enter the system under any operating conditions by either arranging the system so that no fuel pump can draw fuel from more than one tank or by other acceptable means.
(4) Paragraph (c) of this section sets forth a fuel system performance requirement intended to ensure that ice to be expected in fuel when operating in cold weather will not prevent the fuel system from supplying adequate fuel to the engines.
Although fuel system filters and strainers are the items in the fuel system most susceptible to clogging from ice particles in the fuel, this paragraph requires that the entire fuel system be shown to be capable of delivering fuel, initially contaminated with water and cooled to critical icing conditions, to the engine(s).
b. Procedures.
(1) For paragraph (a), the applicant should show compliance with the fuel system requirements of this subpart, except that if unusual fuel system arrangements or requirements exist which are not adequately addressed by these subparts, this paragraph may be used as authority to require special tests, analysis, or system performance needed for proper engine functioning.
(2) For paragraph (b), review the fuel system design with special attention to fuel tank selector valves, crossfeed systems, and multiple tank outlet arrangements to ensure that no fuel system configuration will allow air to enter the system. For questionable situations, the applicant should conduct ground tests and flight tests as necessary to verify compliance with this section.
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