Aircraft Engine Attachment

7/21/2019 Aircraft Engine Attachment

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Lord Corporation LL-6505

Aircraft Engine Attachment and Vibration Control

By Jesse DePriest, Lord Corporation


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Ai rcraft Engine Attachment and Vibration Control

Jesse DePriestSection Leader, FW Engine Installation Engineering

Lord Corporation

ABSTRACT

Controlling the vibration and internal cabin noise levels of fixed wing aircraft has long been a challengeand never-ending trade off of system performance variables. A presentation of the fundamental aspectsof vibration and how it relates to fixed-wing aircraft engine attachment is made. Available technologiesrelated to engine vibration treatments are presented with a preferred design approach.

INTRODUCTION AND STATEMENT OF THE PROBLEM

The intent of this paper is to provide the reader with a fundamental background to the enginevibration/noise problem in modern aircraft and present the available solutions that can be used to treatthe engine vibration problem. Additionally, a design approach that provides technology options to theaircraft OEM throughout the design and flight test phases of the program is outlined.

All mounting systems need to accomplish two basic functions: 1) constrain motion, and 2) providevibration isolation and noise reduction. Constraining Motion refers to limiting the relative motionbetween two structures created by thrust, g loads, weight, and torque. Providing isolation andreducing noise involves minimizing the transmission of vibration from one structure to another so as toreduce the transmitted noise into the cabin area.

To provide the first basic function, the mounting system must be stiff to minimize relative motions. Inorder to minimize transmitted vibration (or noise), the mounting system must be dynamically soft(Reference 1). This inherent problem sets up competing objectives that require compromise and flexibility

in the engine attachment design. This basic issue, along with the need for longer service lives andreduced costs, is the reason for new technology development.

ENGINE VIBRATION SOURCE

In an aircraft engine installation, an imbalance in the rotating machinery creates oscillating forces appliedto the structure, thus generating structural vibration, as depicted in Figure 1. The consequence of therotating imbalances of the engines manifests itself through the structural vibration of the fuselage, whichinduces noise in the cabin as shown.

structural

airborne

Figure 1. Generation of noise from rotating imbalance in wing mounted engines

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At the engine and airframe interface, there are several paths that vibration can take to enter into thecabin. The primary path is at the mounting structure point (point C1 in Figure 2 below) and is the focalarea or choke point, at which to treat the vibration. This paper focuses only on this path and source ofvibration input.

Figure 2. Primary and flanking paths of vibration for fuselage mounted engines

NOISE

The noise at frequencies related to engine vibrations is usually produced at levels much higher than noiseproduced by sources such as external airflow, air conditioning, or accessories. These sources generallycreate the broadband noise levels, whereas the engine rotating imbalance creates specific tones of theirfundamental frequencies and harmonics. Figure 3 below, shows a typical noise spectrum of themeasured data of the internal cabin noise spectrum. Clearly shown are the tonal penetrations of theengine vibrations. This higher noise, produced by the engine vibration through the structure, presents the

most likely need for isolators in an attachment system.

0 50 100 150 200 250 300 350 40030

40

50

60

70

80

90

100

Frequency (Hz)

N1 - Low Speed

Turbine

N2 - High Speed

Turbine

Back of Cabin

Front of Cabin

Figure 3. Typical internal cabin (measured data) noise spectrum of an aircraft with fuselage mounted engines

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The noise generated in an aircraft cabin will have an uneven distribution of energy over the audiblefrequency range (20 Hz to 20 kHz). By aggregating the energy over the audible frequency range a soundpressure level (SPL) is attained. If each frequency is given the same relative importance, the result is anoverall SPL in dB (Lin) (as shown in Figure 3). When the frequency values are given weighting based ontheir importance, the result is an overall SPL in dBX (where X represents the weighting curve used). A-Weighted (dBA) apply to low noise levels as for an audibility test and C-Weighted (dBC) are adapted tonoisy environments (Reference 2), such as most aircraft cabins.

TRANSMISSIBILITY AND EFFECTIVENESS

Transmissibility is a common term used when discussing vibrating systems, but is more correctly usedwith rigid masses and foundations that do not exist on aircraft.

Effectiveness is a comparison of an attachment systems performance with an isolator to that of a hardmount, and is a more appropriate term to describe the vibration isolation (or noise reduction) realized.

Figure 4 is a representative curve of effectiveness. The figure plots the ratio of mount stiffness (Km) andstructure stiffness (Ks) to the amount of noise or vibration reduction realized. This shows that as themount stiffness decreases relative to the structure, greater isolation performance is realized. As thefigure shows, passive systems generally provide up to 10 dB reductions and active systems are effectiveenough to produce 25 dB reductions.

Stiffness

10

60

50

40

30

20

10

0.01 .1 1

Active

Passive

.001

Normalized Mount Stif fness, r = Km/Ks

dB

Re

ductioninSPLandVibration

1020 log (1+1/r )

mk

Velocity

Source

Noise

Air fram eStructureStiffness

Mount

Engine

ks

Figure 4. Effectiveness curve

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Another representation of the attachment system performance is shown by the reduction in thetransmitted force through the structure. Figure 5 is a force versus frequency plot (aft location alone), of anengine hard mounted to the strut (without an isolator). Note the magnitude of the forces in both thevertical and lateral directions around the operating frequency range (60 to 70Hz).

10090807060504030201000

1000

2000

3000

4000

Vertical

Force (y)

Lateral

Forc e (z)

Frequency (Hz.)

Forc e

(N)

Figure 5. Measured data of force Vs frequency at the aft mount location of a typical wing mounted engine on a commercial aircraft -

hard mounted

Figure 6 is a force versus frequency plot (aft location alone) of the same configuration as Figure 5, exceptthat the engine is attached to the strut with an elastomeric isolator. Note the substantial reduction in peakforces coupled with separation from the operating frequency.

0 1009080706050403020100

100

200

300

400

500

Vertical

Force (y)

Lateral

Force (z)

Freque nc y (Hz.)

Forc e

(N)

Figure 6. Measured data of force Vs frequency at the aft mount location of a typical wing mounted engine on a commercial aircraft -

with isolator

Figures 5 and 6 clearly show that force and frequency are drastically reduced by the use of an isolator inthe attachment system, and is indicative of the effectiveness relationship between the stiff structure andthe soft isolator.

AVAILABLE TECHNOLOGIES

Noise and vibration treatments can be separated into two categories, passive and active. Passive

treatments include resilient materials (rubber or wire mesh), Fluidlastic mounts, Tuned VibrationAbsorbers (TVA), and many different cabin wall and interior treatments. Active systems, which requirecontroller electronics, consist of three main types; Active Isolation Control (AIC), Active Noise Control

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(ANC), Active Structural Control (ASC). Additionally, these various technologies can be combined inseveral ways to create a very effective hybrid system (Reference 2).

The vibration and noise treatment systems can also be separated into three categories based on howthey are applied in the application. The available technologies are presented in the three categoriesdefined below.

1. Vibration isolation systems are those that are placed directly in the primary load/vibrationtransmission path.

2. Structural Control systems are those that treat the structure (and are attached to it), attacking thevibration along its primary path, but is not directly in the primary structure/load/vibration transmissionpath itself.

3. Noise Control is a special active system, which does not treat vibration, but rather actively cancelsnoise in the cabin space, locally near the passengers.

VIBRATION ISOLATION

Hard Mounted Structure

A structure used to attach an engine to an airframe has a certain level of flexibility. A hard mountedsystem is not infinitely rigid and therefore can be somewhat effective in reducing vibration as it travelsthrough the structure. The attachment structure stiffness characteristics, whether hard mounted or usingan isolator, are key to the systems ability to isolate vibration.

Figure 7: Example of hard attachment structure

A direct, hard-mounted attachment structure offers the aircraft manufacturer a significant advantage inthat they carry the static loads in a very well defined and understood way and can last the life of theaircraft. However, in the design of a hard mounted structure, it is difficult to include the necessaryflexibility (and damping) required to provide adequate vibration isolation. A hard mount can be designedto be soft, but it is very difficult to change the spring rate if necessary for final tuning. Because anentirely metal, hard attachment structure has little or no damping, it is best represented as a simple spring

in a mechanical model.

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Resilient Materials

Stiffness and damping (as depicted as K and C respectively in Figure 8) are the basic properties of aresilient material, which provide its effectiveness in a mount. The stiffness of a resilient material likerubber is measured in terms of modulus.

Structure

Engine

K

C

Figure 8. Mechanical analogy of a passive, resilient material mount

While we can measure modulus in compression, tension, or shear, the static modulus G is most oftenused in isolator design. Modulus under dynamic conditions (like spring rate) is a complex quantity whichin shear is referred to as G* and can be broken down into two components; the elastic modulus (G') andthe loss modulus (G"). The measured modulus is temperature, strain, and frequency sensitive. Inherentdamping is measured by the ratio of G"/G', and this ratio is also called tan or loss factor. Long lastingvibration isolation systems demand an elastomer with high resilience or low tan .

The primary life limiting aspect (in a non-hostile environment) of any resilient material used in an engineattachment system is drift, creep, or set. These are the permanent deformations a resilient materialinherits while statically loaded over time. Drift or creep are generally caused by a shear load on themount and set is typically caused by a compressive load. A good resilient material will allow for isolatordrift or set, with the maximum clearance-to-snub, equal to two times the static (un-deflected) isolatordeflection (2g clearance from the un-deflected state).

Resilient material mounts offer a trade-off between static deflection and vibration isolation, in the sensethat improved vibration isolation results at the expense of greater static deflection.

The biggest advantages to elastomeric mounts are that they:

1. provide the spring and damping for good vibration isolation, and are much more efficient than metal

springs2. have a proven application history, and are the lowest cost solution compared to other technologies3. carry load in multiple directions (the rubber geometry is designed so that it can carry load in

compression and shear)4. provide easily variable spring rates5. provide impedance mismatch for high frequency noise attenuation6. have a low dynamic to static spring rate ratio compared to a metal spring (rigid structure)

In some relatively rare cases, Rubber-To-Metal (RTM) or elastomeric mounts cannot be used. Thesecases include extreme high and low temperature environments. The majority of elastomeric engine

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mountings utilize organic elastomers for maximum service life, minimum size and best overallperformance characteristics. These elastomers are generally intended for use in a temperature

environment from -65 F to 250 F (up to 310 F for very short periods of time). Some synthetic

elastomers are also used in temperature environments from -65 F to 375 F and situations where therubber is susceptible to attack by aircraft engine fluids. Elastomeric isolators are designed for a maximumlife equal to a multiple of the engine overhaul interval. This approach has provided up to 20,000 hours of

flight time in some applications.

Figure 9: Example of RTM mount

Figure 10: Example of RTM attachment system

Wire-mesh mounting systems, much like RTM, are a passive approach to vibration isolation using curled,formed, and compressed stainless steel wire. The advantage to using wire-mesh mounts is that theyprovide the spring and damper for good isolation, can handle very broad range of temperatures, and arenot affected by any type of engine fluid. The disadvantage to wire mesh mounts is that they produce arelatively high dynamic to static spring rate ratio, are susceptible to relatively high permanent set (drift)deflections, and are usually unidirectional. Wire mesh isolators typically provide several engine overhaul

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intervals of use, and have been successful in applications with service lives of over 8,000 flight hours.These parts can be designed for a maximum life of well over 15,000 flight hours.

Figure 11: Example of wire mesh mount and attachment system

FluidlasticIsolators

For the same static motion limits, fluid isolators can be designed to provide a dynamic stiffness valueapproximately 25% that of the stiffness of traditional elastomeric isolators at a specific frequency. This isachieved by using an inertial fluid mass, which acts like a tuned mass absorber.

The fluid mass is designed to respond to engine dynamic motion and to cause small internal mountpressure differences that help the mount to be moved more easily when loaded by the engine. Thereduced force in the structure reduces the engine-generated noise in the cabin.

A mechanical analogy for a Fluidlasticisolator is shown below in Figure 12.

K

Structure

Engine

CM

Figure 12. Mechanical analogy of a Fluidlasticisolator

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The major advantages to Fluidlasticmounts are the same as RTM mounts. In addition, they provide a

notch in the transmissibility curve and can achieve higher static spring rates. Generally, a Fluidlasticmounts static stiffness can be three times as high as the dynamic stiffness. The disadvantages to

Fluidlasticmounts are the same as RTM, plus they stiffen (dynamically) at frequencies higher than thetuned, notch frequency (see Figure 23). This stiffening effect at higher frequencies is generally notsignificant, as the frequencies at this level are not the primary or contributing disturbance.

Fluidlasticisolators have the same life limiting aspect as traditional rubber to metal mounts, and havebeen successful in applications with service lives of nearly 8,000 hours. These parts have been designedand are expected to last for 10,000 flight hours.

Figure 13: Example of business jet Fluidlasticmount

Figure 14: Example of regional jet Fluidlasticmount

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Active Isolation Control (AIC)

Active Isolation Control systems (depicted in the mechanical analogy in Figure 15) introduce electro-mechanical actuators (represented as F in the figure below) into the mounts.

K

Structure

Engine

C

F

Figure 15. Mechanical analogy of Active Isolation Control

The AIC system commands these actuators to minimize the vibration and noise signals fromaccelerometers or microphones. These actuators create forces that directly oppose imbalance forces,thereby reducing vibration. Therefore, engine vibration (and subsequent noise) is literally cancelled(Reference 1).

Figure 16: Example of AIC actuator

These systems are very effective in minimizing the trade-off between constraining motion and providingvibration isolation. Active isolation mounts can have virtually zero dynamic stiffness at the vibrationfrequencies, and yet the static stiffness can be quite high.

Active isolation control is a full feedback system that can be used either with an isolator or with a hardmount. In using actual cabin noise information, the actuator forces can compensate for flanking paths,such as bleed-air lines, linkages, fuel lines, and hydraulic lines. Additionally, because AIC

has constant

monitoring and adjustment, the system adapts to changes in engine speeds, power settings, and flight

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levels. When coupled with a passive RTM system, the failure mode is benign. If there is a completepower failure, the system will behave similar to a passive mount. The same result will occur if there is acomplete software failure.

The drawback to these systems is their cost and weight relative to a passive system. The majoradvantages to AIC are that it can be added to the aircraft after flight-testing (if the passive mount hasbeen designed so that it can be converted later) and it is highly adjustable. The passive part will besomewhat heavier (larger) than if it was designed as a purely passive device. When coupled with arubber mount, AIC has the same life limiting characteristics as a RTM mount. When incorporated withouta rubber element, the isolation system can be designed to last for nearly the life of the aircraft.

Active Fluid Isolators (AFI)

Active Fluid Isolation, as shown in Figure 17, is a combination of a passive fluid mount and an integratedelectro-mechanical actuator with controller. With the addition of an actuator, the pressure inside theelastomeric fluid chambers can be controlled, thereby controlling the dynamic stiffness of the mount(Reference 2).

K F

Structure

Engine

CM

Figure 17. Mechanical analogy of an active Fluidlasticmount

The advantages of the active fluid mount are same as the passive fluid mount. However, the Active

Fluidlastic mount increases the level of isolation (by deepening the notch or reducing the dynamicstiffness) and widens the frequency range that it treats (see Figure 23). The dynamic stiffness can becontrolled to nearly zero at the engine vibration frequencies through the use of the actuator and controller.The disadvantage is their cost and weight relative to a passive system. Again, the life limitingcharacteristic of an AFI system is the resilient material and can be designed to last for multiples of theengine overhaul interval.

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STRUCTURAL CONTROL

Tuned Vibration Absorbers (TVAs)

Tuned vibration absorbers (TVAs) are passive vibration absorbers that attach to the vibrating structure.This is a simple mass on a spring system typically using elastomer as the spring. Various geometries and

material properties can provide a wide range of capabilities treating vibration problems up to about 600Hz.

Figure 18: Example of elastomeric TVA

A TVA is tuned to a discrete frequency. This disturbance frequency will cause the TVA to enterresonance. The resonating TVA will generate a force back into the structure that cancels out theunwanted vibration or force. When correctly placed on the engine attachment structure, the TVAeffectively increases the impedance of the structure at that tuned frequency. Passive TVAs are capable of4 to 6 dBA reductions in noise in the aircraft cabin.

Active Structural Control (ASC)

Active structural control, as shown in Figure 19, uses electro-mechanical actuators, that are attached tothe structure, as close as possible to the vibration source, which cancels the vibration before it reachesthe fuselage.

K

Structure

Engine

C M

F

Figure 19. Mechanical analogy of active structural control

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Figure 20: Example of ASC actuators

This system processes both cabin noise information and engine signals in a central computer that drivesthe actuators in order to ensure optimal noise reduction throughout the flight cycle. The adaptive controlalgorithm, allows the system to react almost instantaneously to engine throttle changes.

Figure 21: System diagram of ASC or AIC

The technological principle to ASC is the same as ANC or AIC. The difference is that the actuators inASC are not directly in the vibration path, but are instead, attached to the primary structure/vibration pathinputting a force to the system that inherently cancels the unwanted vibration. All components of an

active system are designed to be replaced on condition, and can last for the majority of the aircraftlifetime.

ACTIVE NOISE CONTROL (ANC)

Active Noise Control systems utilize loudspeakers inside the cabin to create a secondary noise field,which cancels the primary field due to the engines or propellers. For an ANC system to create globalreductions, one of two criteria must be met.

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First, the acoustic response must be lightly damped and possess low modal density in the frequencyrange where the noise must be reduced. When this occurs, a few actuators can be used to reduce noiseat all points throughout he cabin. Secondly, speakers should be placed within a quarter wavelength ofdiscrete sources.

Unfortunately, neither of these criteria can generally be met in aircraft. Depending on the size of thecabin, the transition from sparse to dense modal response typically occurs at a frequency less than 50Hz.Since most aircraft sources such as turbofan engines or propellers produce noise at frequencies above50 Hz, global noise reductions are not possible using the first criteria. Further, since the sources aredistributed rather than discrete, the second criteria can rarely be used. (Reference 3).

If global noise control cannot be achieved, then local control can be utilized. Local control involvescreating zones of quiet around the control microphones. The size of the zone of quiet is related to thefrequency being controlled. In general, the radius of the sphere of quiet, will be roughly one-tenth thewavelength of the sound. At 200 Hz, the radius of the sphere of quiet is 6 inches. It is possible to enlargethe zone of quiet by a number of techniques including using multiple microphones. However, if thefrequency were 2,000 Hz, the zone of quiet would be too small to be practical.

CabinCockpit

Microphones

SpeakersControllerReference Signals

Figure 22: System diagram of ANC

Although ANC has its limitations, it can be very effective for controlling low-frequency noise typical ofturboprop aircraft. An ANC system for the Beech King Air provides up to 12 dB spatially averagedreduction in the propeller-induced noise, producing dramatic subjective improvements in passenger andcrew comfort (Reference 3).

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COMPARING TECHNOLOGIES

Figure 23 below, is a representative chart of the available technologies, and their relative performancecharacteristics, based on the relationship of the dynamic stiffness. The passive RTM stiffens slightly withincreased frequency, whereas there are significant softening effects with each of the other technologies.

Frequency

K

FluidlasticActive Isolation

ControlActive Fluidlastic

Passive RTM

Figure 23. Performance (dynamic stiffness vs Frequency) comparison of the available technologies.

APPLICATIONS PERSPECTIVE TO ISOLATION SYSTEMS

PROPELLER DRIVEN AIRCRAFT

Piston Engines

Propeller driven aircraft produce a high level of vibration due to propeller unbalance. Piston engineaircraft also have firing pulses creating unique vibration environments coincident with the number of firingpistons. Typical piston engine speeds are on the order of 40 Hz with one half order at 20 Hz, and systemnatural frequencies of about 10 to 14 Hz.

Typically, piston aircraft are lower cost aircraft, and therefore do not incorporate higher performing (and

higher cost) isolation/mount systems such as Fluidlasticor NVX. Therefore, the design goal for apiston aircraft using rubber-to-metal (RTM) mounts is to isolate the first order vibration. The one-half ordervibration is normally not isolated, because the necessary static deflections in a passive RTM mount wouldbe too high in order to attain the dynamic stiffness necessary to affect the half order disturbance.

Turbo Props

By far, the largest source of noise in turboprop aircraft is the extremely large acoustic pressures thatoriginate from the propeller blades and strike the fuselage. Engine attachment solutions will not be ableto control this noise that occurs at the blade pass frequency and its harmonics. However, TVAs andactive structural control (ASC) attached to the cabin wall and active noise control (ANC) can reduce thisnoise (References 1 and 3). A less dominant, but still significant source of vibration is mass andaerodynamic imbalance forces that occur at the once-per-revolution frequency (Reference 4). Enginemounting technology can effectively reduce this source of vibration.

The relative light weight and slow speed of the engine/propeller system results in relatively largedeflections that need to be incorporated into the isolator design. Typical turboprop applications havepropeller speeds from 10 to 30 Hz and system natural frequencies of about 6 30 Hz (Reference 4)which make mount designs difficult for traditional RTM technology.

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TURBO FANS

Most turbofan engines have two rotating spools (N1 and N2). Together, they create a composite vibrationat the frequencies related to the spools rotating frequencies, N1 and N2.

Business jet applications typically have fan speeds of 175 Hz and a high-pressure compressor speed of

433 Hz. At these speeds, any eccentricity or tolerance differential in the shafts or bearings, as well asmass imbalance, will result in unbalance forces in the engine and engine casing vibrations. Largeraircraft applications have slower fan speeds (i.e.: Boeing 737~ 85Hz, 757~ 60Hz, and 767~ 50Hz).Through controls in the manufacture of the engine this unbalance can be minimized, but not eliminated.Balancing of the engine low-speed-shaft (N1) after engine assembly is possible to correct and minimizevibration due to unbalance of the shaft, however, this is not possible for the high-speed shaft (N2)(Reference 1).

Fuselage Mounted Turbofan Engines

Mounting of turbofan engines to the aft fuselage of aircraft is typically done on smaller aircraft, includingbusiness and some regional/commercial aircraft. This position is particularly challenging from a cabinnoise standpoint due to the close proximity of the engine to the passenger cabin. A very efficient isolationsystem is needed to reduce the transmission of engine induced vibratory forces into the aircraft structure.

Since the forward structure of the engine attachment scheme is very close to the pressure bulkhead atthe rear of the aircraft, any vibratory forces due to engine unbalance are readily converted to cabin tonalnoise. In the case of the aft engine attachment point, the increased distance from the cabin generallyresults in a less efficient transfer of vibratory forces to cabin noise. Also to be considered, are the loaddirections that need to be reacted at each mounting plane. Since the forward mount plane typically reactsthe engine torque, axial, lateral, and vertical loads, the loads driven into the structure at the front

attachment point are often larger. Generally, higher performing Fluidlasticmounts are successful at theforward attachment point and RTM mounts are employed at the aft point.

Wing Mounted Turbofan Engines

On wing mounted engine installations, the aft mounting point becomes a more prominent noise source,since it is closest to the main structure elements that transmit the noise to the cabin. The aft mounttypically attaches to the structure that is very stiff and is very close to the wing beam. This stiff primarystructure of the wing then acts as a very efficient transmission path for vibration and noise into the cabin.The front mounting structure, however, is farther away from the main transmission path and is usuallyless efficient in transmission of vibration and noise to the aircraft. Couple this with the softer structurestiffness due to the cantilevered pylon and it becomes clear that the aft isolator design needs to be veryefficient.

APPROACH TO DESIGN OF AN ISOLATION SYSTEM

Weight, cost, and acceptable vibration/noise levels are all primary considerations during the developmentof a new aircraft. The design of an engine attachment system must include these, but currently, there isno way to predict noise levels inside of the aircraft accurately enough to determine what technology to

include. Because this critical design information is not available, it is very common that the aircraft OEMmay want to defer the decision to include isolators until after flight test (to confirm the actual need for anisolation system). In cases where the aircraft OEM knows that they want to incorporate anisolation/attachment system, the analytical information is usually not adequate, or the required noise levelof the cabin is vague. In any event, the decision to include isolation systems can be difficult. Because ofthis, the isolation system/engine attachment provider must be very flexible in the design process and thesolutions that are offered.

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The preferred approach to the design of an engine isolation/attachment system calls for a continuousinterchange of ideas and information between the structure/isolator provider, the aircraft OEM, the enginemanufacturer, and the nacelle provider. The engineering extension and integration of these differentcompanies is essential to designing an optimum isolation system.

Through this preferred approach, data correlation between forces at the mounting point and noise in thecabin can be established. In order for this correlation to work, data on the force generated by the engine,data on the airframe structure characteristics (stiffness), and data on the optimal nacelle layout must beshared.

Defining and Understanding the Requirement

Basic information that is required to begin an isolation system design are engine system weight andinertia data, performance characteristics (thrust, turbine/propeller speed, number of pistons, fan speed)for various flight conditions, environmental conditions, the space available, and allowable static motions.

A basic understanding of the engine attachment scheme is required with detailed attachment locationsdefined.

However, the critical aspect in the early design/definition phase of any new project is to clearly establishthe goals. In order to best serve or meet these goals (considering many potential unknowns) it is veryhelpful to build in flexibility to the design concept.

Proceeding from the OEM Requirements and Selecting the Appropriate Technology

Most new aircraft can incorporate a yoke/attachment structure that is adaptable to all availabletechnologies. In this way, the customer (aircraft OEM) can select the appropriate solution to the vibrationand noise problem after the problem is adequately defined or confirmed. For example, an aircraft can bedesigned, built, and flight tested. Then, upon conclusion of the flight tests, decide what vibration andnoise treatment is the best for the application, without lengthy or expensive redesign efforts.

The best approach incorporates a single attachment structure (for the particular aircraft) that can

accommodate several of the available technologies. For example, a very statically stiff Fluidlasticisolator might be offered as the baseline system. Then after flight test, this isolation system can bemodified as necessary. The possible modifications include:

removal of the fluid (remains an elastomeric only mount)removal of the fluid and adding AIC actuators into the mount (converting it to an active isolationcontrol system)reverting back to a hard mountreverting back to a hard mount and incorporate active structural control on the attachment structureincluding hybrid (combinations of technologies) solutions such as adding TVAs

All of these options could be incorporated without changing any yoke or attachment structure design. Theengine attachment structure is critical to the system effectiveness (Figure 4). This includes anyattachment links, forward yoke, and hardware. For best optimization of the isolation effectiveness of thesystem, the isolator and the attachment structure should be designed concurrently (Reference 5).

Generally, a preliminary design concept will be developed with the airframer, engine manufacturer, andthe nacelle provider. It is preferable to design a yoke or attachment structure that can incorporate allavailable technologies if a change in performance is required later.

An acceptable engine attachment system design includes the following:

1. reduction in cabin noise and vibration to established goals2. long life of all components in the system (typically equal to engine TBO or multiples thereof)

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3. acceptability to the engine manufacturer4. reasonable size (space limitations) and weight5. proper selection of materials for the given environment6. metal parts designed for life of the airframe or infinite life (including limit/ultimate loads and damage

tolerance)7. costs at an acceptable level

CONCLUSION

Vibration and noise levels can be controlled in a variety of ways, from passive elastomeric mounts toactive vibration control systems, each with varying levels of complexity, performance, and cost. Theprimary benefit of an engine attachment and isolation system (beside the obvious primary purposeattaching the engine to the airframe) is the reduction in vibration and noise in the aircraft cabin byreducing the dynamic forces in the structure.

As in any equipment design, compromise and communication are a key activities. The optimal solution isthe one that best meets the total system requirements for static/structure needs, dynamic needs, andinstallation/maintainability needs without too much compromise in any one area. The best approach tothe system design is one that provides flexibility in the choice of technology without compromising thecertification or development schedule of the aircraft program. A common yoke or attachment structuredesign that can incorporate any vibration isolation treatment is optimal.

ACKNOWLEDGMENTS

This paper is a team effort from many individuals at Lord Corporation. Those that specifically contributedto this paper are Tom Law, Paul Herbst, Charlie Schroeck, Lane Miller, Jim Potter, Becky Weih, JerryWhiteford, Guy Billoud, Scott Miller, and Mark Norris.

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11. Pechter, L.S., H. Kamei, Design of Focalized Suspension Systems Autonetics, Anaheim, CA

12. Browne K.A., Predetermination and Control of Vibration in Aircraft Originating From the Engine, WrightAeronautical Corporation # 353, Patterson, NJ

13. Miller, L.R., M. Ahmadian, C.M. Nobles, D. Swanson, 1992. Modelling and Performance of an ExperimentalActive Vibration Isolator, ASME DSC VOL 38, Active Control of Noise and Vibration, Cary, NC

14. Landmann, A.E., H.F. Tillema, G.R. MacGregor, 1992. Application of Analysis Techniques for Low FrequencyInterior Noise and Vibration of Commercial Aircraft, NASA Contractor Report 189555, Hampton, VA

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Aircraft Engine Attachment

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