The advances in aircraft engine noise reduction and the increasing demand for quieter aircraft has led noise research to focus more attention on airframe noise. Landing gears and high lift systems have been known to significantly contribute to the total aircraft noise at approach idle conditions, especially in cases where high by-pass ratio engines are used, when noise from the engines is low enough that it could be considered comparable or lower than the noise from the airframe. Landing gears in service on today’s aircraft were not designed with noise impact in mind and contain a myriad of possible noise generating features. Today’s landing gear designer is challenged to consider noise among many other factors when designing efficient aircraft landing gear systems. It is a challenge further complicated if noise reduction solutions are required for retro-fit applications. Much of the experimental noise research conducted on landing system noise has centered around the understanding of noise generating mechanisms and evaluation of noise reduction concepts in model-scale environments such as wind tunnel tests. These experiments have provided valuable insight into landing gear noise sources. However, little effort has been made to integrate noise reduction research with full-scale landing gear design and evaluate noise reduction potential in a full-scale flight environment. The work conducted under the Quiet Technology Demonstrator 2 (QTD2) program marks a first step in the successful integration of noise research with landing gear design with the focus being to design, implement and evaluate noise reduction solutions in a full-scale flight environment. This paper discusses the design and analysis of a ‘toboggan’ shaped main landing gear noise reduction fairing for full-scale flight evaluation on a B777-300ER aircraft in the QTD 2 program. The fairing was selected for flight evaluation after a series of model-scale wind tunnel acoustic experiments were conducted in conjunction with full-scale feasibility studies. The fairing design addressed issues such as gear kinematics and stowing, brake cooling, ground operations and noise reduction potential. The design was supported by static stress analysis and flutter analysis to ensure that the fairing was flight worthy.
I. Introduction hile engine noise dominates aircraft noise at takeoff, researchers have found that airframe noise contributes significantly to approach noise for many aircraft. In addition to deployed flaps and slats, the landing gear has
been identified as a major source of airframe noise at approach.1 Landing gear noise sources are perhaps less well understood than slat and flap noise sources. Due to the complex gear flow field, identifying noise from specific gear structures is a difficult task. Developments in acoustic measurement techniques, such as phased arrays, combined
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with advances in flow field measurements and computational fluid dynamics (CFD) methods have made it possible for airframe noise researchers to gain insight into the relative strength of various landing gear noise sources.
Under the AST/QAT program, tests have been conducted to investigate aircraft landing gear noise. Full-scale and model-scale landing gear wind tunnel acoustics tests conducted at the Boeing Low Speed Aeroacoustic Facility (LSAF) showed that noise from local gear details is broadband and has significant amplitude and that details such as hydraulic lines, open bores in structural pins, and electrical harnesses contribute significantly to high frequency gear noise.1,5 Wind tunnel acoustics testing conducted on a 6.3% semi-span 777 model showed evidence of what might be landing gear-trailing-edge flap interaction noise.1 NASA Langley tests of a 1/10th scale gear model on the Energy Efficient Transport (EET) wing showed significant noise being generated when the landing gear door was installed.
All the above tests showed that local gear details, such as hydraulic lines, electrical lines, open bores in structural pins, and other features contribute significantly to noise from the landing gear. However, noise from specific gear components and contribution to the overall landing gear noise, were not investigated in detail in the above tests.
Wind tunnel acoustics experiments conducted by NASA Ames on a 26% 777 main landing gear model identified gear components contributing to the overall 777 main gear noise.2 This test identified the axles, bogie beam and brakes, braces and lock links, of the main landing gear to be major contributors to gear noise at approach overhead, a concern for noise certification (Fig. 1). Other contributions from gear components such as cable harnesses, torque links, aft wheel hubs, struts and the landing gear door were identified as having greater impact on overall approach gear noise at sideline observer locations (Fig. 1). The same gear model tested later on a B777 semi-span wing model in the NASA Ames 40ft-by-80ft wind tunnel showed that the noise with the gear stripped of all the local gear details (low-fidelity model), such as hydraulic lines and electrical lines, was 2-3 dB quieter than the gear with full-fidelity.6 This test also indicated that the noise from this low fidelity configuration was still 4-5 dB higher than noise with the gear removed.6
Other experiments, conducted in Europe under the Reduction of Airframe and Installation Noise (RAIN) program, provided further insight into landing gear noise generators and potential for reduction.3,4,7 Wheel hub caps tested on a A340 nose gear provided as much as 0.5 dB reduction in overall sound pressure levels (OASPL) at 500 Hz.4 A340 full scale studies also indicated that a properly designed gear door/strut fairing could provide as much as 3.5 dB reduction in OASPL in the forward and overhead observer angles.4
In 2003, a NASA research program was initiated to design and evaluate novel modifications to commercial transport aircraft landing gear systems with the objective of reducing radiated noise to the community by:
1. Evaluating noise reduction potential of landing gear modifications. 2. Determining implementation and integration details associated with incorporating noise-reducing features on
real aircraft. The joint program consisted of teams from NASA, Boeing, Goodrich and Virginia Tech (VPI). The program
consisted of a series of model-scale landing gear wind tunnel experiments accompanied by full-scale implementation and integration studies to identify promising noise reduction concepts for full-scale flight evaluation. Wind tunnel testing was conducted to identify 777 main landing gear noise sources on a 26% scale-model at the VPI stability wind tunnel. It should be noted that the 26% 777 scale model used for the experiments closely represented gear details found on a B777-200 main gear. It was not an exact replica of the full-scale article as it is impossible to represent the myriad of hoses, tubes, electrical lines in model-scale. In addition, the key difference between the B777-200 main landing gear and the B777-300ER main landing gear, which is the full-scale flight article, is the semi-levered actuation system used on the latter, which eliminates the need for the forward cable harness of the B777-200 main gear shown in Fig. 1. Noise reduction potential of novel concepts targeting major landing gear noise sources such as the axles, bogie beam and brakes, cable harness and torque link, braces and lock links, door/strut interface and wheel hubs were assessed in acoustic wind tunnel tests conducted at VPI (Fig. 1). Results from these wind tunnel tests are discussed in the paper by Virginia Tech.10 Issues of full-scale implementation and integration were addressed concurrently for many of the concepts tested in model-scale and incorporated in some of the concepts tested. The work resulted in the selection of a ‘toboggan’ shaped fixed ‘lower truck’ fairing for flight test on a B777-300ER aircraft main landing gear (Fig. 2) under the QTD 2 flight test program. The ‘lower truck’ region of the main landing gear comprises of components such as the axles, bogie beam, and brakes, key contributors to approach overhead noise. This paper discusses the design and analysis of this landing gear noise reduction device. The flight test results are documented in the companion paper titled ‘Airframe Noise Results from the QTD II Flight Test Program’.11
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II. Flight Test Fairing Design This section discusses the design of the landing gear ‘toboggan’ fairing tested in the QTD 2 flight test on a
B777-300ER aircraft. In addition to the ‘toboggan’ fairing an aligned gear configuration was tested in flight as well. Phased array measurements from the 26% 777 main gear wind tunnel tests showed that the relative strength of noise sources for the aligned gear configuration was lower compared to the baseline configuration at frequencies above a 1000 Hz full-scale (Fig. 3). Figure 3 shows that the relative strength of the noise source at the aft axle region is reduced when the gear is aligned with the tunnel flow.
The B777-300ER main landing gear is at a ‘toes-up’ attitude when deployed (Fig. 4b). The toes-up attitude of the gear serves an important function in that it is the first indicator that the aircraft wheels are on ground and is thereby used to signal the activation of other control systems required to bring the aircraft safely to rest. However, in the normal gear retraction cycle the gear moves to a ‘toes-down’ position before retraction into the wheel well. To fix the gear in this ‘toes-down’ attitude, hydraulic lines were re-routed at the flight test. With the positive angle of attack of the aircraft it was estimated that at approach the gear would be aligned with the free stream flow (Fig. 4a). The aligned gear configuration did not require additional design changes to be made to the main landing gear and therefore is not discussed further in this report.
Acoustic test results from early B777-200 flight tests indicated that the main landing gear was a dominant noise source compared to the nose gear for a B777-200 aircraft.9 The nose gear was not targeted for noise reduction in the QTD 2 flight test. Nevertheless, the nose gear is an important noise source, as seen from the results of the QTD 2 flight test11, and should be addressed in future studies.
‘Toboggan’ fairings were designed for both main landing gears of the B777-300ER flight test aircraft. Figure 5 shows an isometric view of the fairing designed using CATIA V5 by Goodrich. The ‘toboggan’ fairing consists of an aluminum curved forward shield (bull-nose), a forward aluminum lower panel assembly and an aft aluminum lower panel assembly. The fairing was designed so that it replaced a gravel shield that attaches to the electrical junction box of the existing B777-300ER main landing gear. The assembly is secured to the truck beam via band clamps and is supported using support brackets and support arms (Fig. 5 and Fig. 6). 26% scale-model 777 gear experiments indicated that increasing toboggan bull-nose arc length had minimal effect on noise reduction potential at approach overhead condition (Fig. 7). The phased array integrated sound pressure level deltas in Fig. 7 are relative to the baseline untreated gear and show little impact due to changing ‘toboggan’ fwd shield arc length. However, the wind tunnel test results also indicated that removing the bull-nose degraded the acoustic benefit realized from the toboggan with the bull-nose.
Scale model 777 gear experiments were also conducted to study the effect of varying ‘toboggan’ width on noise reduction potential (Fig. 8). The phased array integrated delta sound pressure level plot shown in Fig. 9 indicate that increasing ‘toboggan’ width improved noise reduction potential and that a width between medium and maximum width would provide significant noise reduction. A 0.095 distance ‘keep-out-zone’ with normalized distance 0.095 (on each side) from the centerline of the tire was defined by Boeing based on analysis of tire deflections under flat tire and pivot turn conditions. Note that all landing gear fairing dimensions are normalized. This limited the actual normalized width of the metallic fairing to be no more than 0.18, to avoid any chance of tire damage due to contact with metal (Fig. 10). In order to safely extend the fairing and improve noise reduction potential flexible edge treatments were incorporated into the design (Fig. 5). Edge treatments increased the normalized total width of the fairing to 0.22.
Design of such a fairing for a production environment would require addressing brake-cooling concerns in addition to tire damage due to deflections. A fairing covering the brake system could hinder rapid cool down of the brake assembly when an aircraft is parked at an airport terminal gate. This could in-turn lead to increased aircraft turnaround time resulting in higher operational cost due to these delays. Brake fairings, designed to address brake-cooling concerns, were tested in isolation and in combination with a minimum width ‘toboggan’ fairing in model-scale wind tunnel acoustics tests at VPI. Test results were encouraging showing improved noise reduction when brake fairings were combined with a minimum width ‘toboggan’ fairing as opposed to being tested in isolation.
Edge treatments, that were required to safely extend the ‘toboggan’ fairing width, were sandwiched in-between two aluminum plates on either side of the truck beam as shown in the sketch of Fig. 11. The normalized edge treatment thickness and width were 0.002 and 0.05 respectively. A 0.002 bend at the outer-edge was incorporated to address flutter requirements. The soft-edge consisted of an elastomeric silicone material with varying plies of polyester/fiberglass cloth stiffening material. The edge material was designed so that the ply count could be varied in one-piece to improve flexibility near regions where tire rub might occur. If tires were to touch the edge sections it would deflect out of the way and spring back to its original position causing no damage to surrounding gear
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components or the ‘toboggan’ fairing. A patent application on increasing fairing width through the use of soft-edge treatments has been filed.
A seal was located in-between the forward lower panel assembly and the aft lower panel assembly (Fig. 5). The seal was made from the same elastomeric silicone material as the soft-edge. The seal was required to accommodate the displacement of the center axle and movement of the ‘toboggan’ lower assemblies during flight (Fig. 12). It eliminated the need for a separate ‘toboggan’ center section and eliminated the gap between the forward and aft ‘toboggan’ assemblies that would have caused aerodynamic and noise concerns.
The ‘toboggan’ fairing was attached to the truck beam (bogie beam) via the junction boxes using brackets and band clamps as shown in Fig. 5 and Fig. 6. Based on results from stress analysis aft and forward load carrying members were added to the design to ensure safe operation in flight.
The weight of the design was not optimized as a requirement of the flight test. A composite fairing was considered in the preliminary design phase but was not pursued further due to lack of composite repair capabilities in the remote flight test location. The fairing was designed to be installed utilizing only gear jacks and removal of center wheels. This eliminated the need for use of large airplane jacks at the test site. The fairing was also designed for ground operations with aft-axle steering locked out, though solutions more applicable to production settings were investigated. Aft-axle steering on the B777 main landing gear provides improved turning capabilities in taxi conditions requiring tight radius turns. The Glasgow test site runways and taxiways were wide enough to be able to turn the aircraft with the aft-axle steering locked-out. The ‘toboggan’ fairing was also designed for retraction into the wheel well. Gear kinematics and the limited clearances available in the wheel well impose a major challenge to the design of noise reduction devices, especially in retro-fit applications. The noise reduction fairing was designed so that it was within the limits of the wheel-base so that it fits into the limited space available in the wheel well when retracted.
III. Analysis A static stress analysis and a aeroelastic analysis were conducted to support the design of the ‘toboggan’ fairings
for the flight test. The analysis was essential to ensure that the fairing would perform structurally at critical loading conditions in flight and to assure no damage to the junction box. The need for a flutter analysis was further elevated by the fact that flexible edge treatments were incorporated in the design. The ensuing sections discuss these analyses in greater detail.
A. Model Description A finite element model was made of the ‘toboggan fairing’ assembly (Fig. 13). The finite element model was
split in two sections; the forward section (Fig. 14a) and the aft section (Fig. 14b). These two sections of the assembly are not directly connected to each other due to the existence of the center seal at this interface (Fig. 5). The ‘toboggan’ fairing is basically a stiffened flat panel that covers the lower portion of the landing gear truck beam. The components of the assembly are shown in Fig. 15a and Fig. 15b. The two assemblies are then attached to the landing gear truck beam via the junction box and several support brackets (Fig. 5). The forward and aft ‘toboggan’ supports are also summarized in Table 1 and Table 2. The material properties used in the finite element analysis are shown in Table 3.
Support Type Supporting Figure Number 3 Band Clamps Fig. 15b Aft Tail Bracket Fig. 15b 2 Support Arms Fig. 15b
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B. Stress Analysis A static stress analysis was conducted for the structural components of the ‘toboggan’ fairing. Fatigue was not
addressed as it wasn’t a concern due to the short duration of the flight test. The finite element analysis was performed on the fairing assembly for 6 different inertia loads combined with 3 different pre-loads at the truck beam strap connections for a total of 18 load cases (see Table 4). The analysis addressed only the static equivalent vibration load cases as these were found to be more critical in comparison to the effect of aerodynamic pressure cases during the flight test.
The stress analysis concentrated on 3 key areas; ‘toboggan’ lower panel assembly attachments to the junction boxes and connection to truck beam, the support structure for the forward bull-nose assembly, and support structures for the aft ‘toboggan’ lower panel.
As indicated in the previous section the ‘toboggan’ fairing was attached to the junction boxes (J-boxes) which hangs from the B777-300ER truck beam by a series of band clamps (Fig. 5 and Fig. 6). Additional arms from the J-box lugs on the side of the truck beam provided additional stability. The ‘toboggan’ fairing replaced a gravel shield that hung from the J-boxes in the existing landing gear configuration. The friction between the band clamps, the truck beam and the J-box saddle was a significant design factor in the design and analysis of this configuration.
The forward shield of the ‘toboggan’ fairing assembly was supported by a bracket which attached directly to the truck beam through two bolts in the front axle (Fig. 5). The forward shield is completely supported by this bracket transferring very little load between the fwd shield and the rest of the ‘toboggan’ fairing. This bracket only assisted in stabilizing the forward ‘toboggan’ lower panel assembly in resisting a drag moment about the truck beam longitudinal axis.
The aft half of the ‘toboggan’ fairing assembly was also supported by a bracket at the aft end which supported against drag rotation about the truck longitudinal axis (Fig. 5).
The results of the finite element analysis indicated that the ‘toboggan’ fairing was structurally adequate for a flight test. The most critical elements of the fairing assembly were the attachment hardware. The fairing itself was
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relatively robust. The finite element model mesh density was sufficiently fine to directly read the stresses from stress contour plots.
Figure 16 and Figure 17 illustrate the stress contour plots at the bracket and forward bull-nose supports respectively. The results are presented for the lowest margin condition (Coefficient of Friction = 0.3). Table 5 summarizes the margin of safety for the critical components. All analyses assumed linear elastic materials.
C. Flutter Analysis An aeroelastic analysis was performed for the final design of the ‘toboggan’ fairing for the B777-300ER main
landing gear. The objective of the analysis was to determine if the fairing had any flutter problems within the flight envelope. The main aluminum region of the toboggan fairing is very stiff and was not evaluated. However, the softer edges of the fairing were evaluated. The flutter requirements were:
• Minimum Altitude – Sea Level • Maximum Altitude – 10,00 Feet • Maximum Speed – 320 KEAS Additional requirements were: • Fore/Aft Direction Modes > 20 Hz • Inboard/Outboard Direction Modes > 43 Hz The complete geometry for the ‘toboggan’ fairing was simplified to include only the edge treatments of the
fairing. The structural finite element model was constrained at the bolt locations and along clamped edges (Fig. 18). The edge treatment of the fairing consists of a thin fiberglass laminate molded within a thicker silicone elastomer material. The structural model consists of 2D shell elements that reference laminate (NASTRAN PCOMP) properties. These laminate properties include the thickness and material of each “ply”. The material properties used in this analysis is shown in Table 6.
The flutter V-F (velocity vs. frequency) chart for all modes is shown in Fig. 19. This figure illustrates how each
mode shifts in frequency with increasing speed. The flutter V-G (velocity vs. damping) chart for all modes is shown in Fig. 20. V-F charts were created for the results from the aeroelastic simulation at each speed. For flutter analysis, the damping value is defined as the damping required to achieve neutral stability of the coupled structural/aerodynamic system. Flutter occurs when the damping of a mode crosses zero and becomes positive. In reality, some structural damping is present within the structure.
• Positive Damping = Results in Flutter • Negative Damping = Stable From Fig. 20 it can be seen that at the maximum velocity of 320 KEAS the damping is negative. Based on the
results no flutter was found within the flight envelope.
Table 5: Margins of safety for critical components
Component P/N Load Case Material M.S. Strap Bracket 287W6123 aft_pre800_40g_down_2 AISI 304 .136
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A modal analysis was also performed on the toboggan fairing. The lowest observed frequency in the fore/aft direction was 22.6 Hz. This mode is on the bull nose of the forward section and is shown in Fig. 20. No critical modes were found in the inboard/outboard direction.
IV. Pre-Flight Checks Prior to flight a fit check of the ‘toboggan’ fairing was conducted at the Goodrich landing gear facility in Everett,
Washington. A gear swing test was also conducted at the Boeing factory to ensure proper retraction and stowing. The ability to retract the gear with the fairing installed provided the capability to perform takeoff and cruise noise testing of engine noise reduction devices on the same day and eliminated the need for an additional configuration change.
V. Conclusion Reduction of airframe noise is essential to reducing total aircraft noise levels to meet today’s stringent federal
and local airport noise restrictions. The highly turbulent and complex flow field in the vicinity of the landing gear makes it a major source of airframe noise. Developing solutions for mitigating landing gear noise poses a major challenge to the landing gear designer, especially for retro-fit applications. Landing gear noise reduction solutions have to withstand harsh environmental and operational conditions both on-ground and in-flight.
This paper describes the design and analysis of a main landing gear noise reduction fairing for flight test on a Boeing 777-300ER aircraft. The ‘toboggan’ shaped noise reduction fairing was down selected for flight test after evaluating many main landing gear noise reduction solutions in a model-scale wind tunnel environment. Feasibility studies addressing issues of design, integration and economics aided in the design of this flight test fairing. The fairing was successfully tested in the QTD 2 joint noise technology demonstrator program in 2005. The work performed in this research program have identified key issues that need to be addressed in future low noise landing gear design as well as retro-fit applications. This research program marks a first step in the successful integration of noise research with landing gear design and full-scale implementation.
Acknowledgments The success of this joint landing gear noise research program depended upon the teamwork and
communication between the many organizations and personnel involved. The flight test would not have been possible if not for the excellent design work by Mr. Rui Cruz of the Goodrich Landing Gear division and Mr. Joe Zecca of the Goodrich Aerostructures Group and many others who assisted in the design. The authors are appreciative of the leadership provided to the design activities by Mr. Dennis Martin of the Goodrich Landing Gear Division. The authors also would like to thank Mr. Virinder Duggal and his team from Goodrich Interior Products and Mr. Mark Eckert and his team from Goodrich Wheels and Brakes for the support in the assessment of innovative landing gear noise reduction concepts.
This work was supported in part under NASA Contract NAS1-03008. The authors gratefully acknowledge the support provided by Mr. Mehdi Khorrami of the NASA Langley Research Center who was the NASA technical monitor and the Boeing and VPI teams.
References 1. Stoker, R. and Sen, R., “An Experimental Investigation of Airframe Noise Using a Model-Scale Boeing 777,” AIAA
2001-0987, 2001. 2. Jaeger, S., and Burnside, N., “Microphone Array Assessment of an Isolated, 26%-Scale, High-Fidelity Landing Gear,”
AIAA 2002-2410, 2002. 3. Dobrzynski, W., and Buchholz, H., “Full-Scale Noise Testing on Airbus Landing Gears in the German Dutch Wind
Tunnel,” AIAA 97-1597, 1997. 4. Dobrzynski, W., Chow, L., Guion, P., and Shiells, D., “Research into Landing Gear Airframe Noise Reduction,” AIAA
2002-2409, 2002. 5. Stoker, R., “Full-scale Landing Gear Noise Test Results,” Presented at the NASA AST 3rd Airframe Noise Workshop,
Seattle, WA, November 1998. 6. Horne, C., Burnside, N., Soderman, P., Jaeger, S., Reinero, B., Storms, B., James, K., Arledge, T., “Results from the
STAR 26% 777 Aeroacoustic Test in the Ames 40- by 80 Foot Wind Tunnel,” Presented at the NASA QAT Airframe Noise Workshop, Hampton, VA, October 2002.
7. Heller, H., and Dobrzynski, W., “Airframe Noise: Recent Results from Experimental Studies on Landing Gears and High-Lift Devices,” Aeroacoustics Workshop-Project SWING, October 1999.
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8. Jaeger, S., Burnside, N., Soderman, P., Horne, C., James, K., “Acoustic Study of an Isolated, 26%-Scale, High-Fidelity, Landing Gear Model,” Presented at the NASA QAT Airframe Noise Workshop, Hampton, VA, October 2002.
9. Stoker, R. W., Streett, C., Burnside, N., “Airframe Noise Source Locations of a 777 Aircraft in Flight and Comparisons with Past Model Scale Tests”, AIAA 2003-3232, 2003.
10. Ravetta, P., Burdisso, R., Ng, W., Khorrami, M. R, and Stoker, R. “Screening of Potential Noise Control Devices at Virginia Tech for QTD II Flight Test,” AIAA 2007-3455, 2007.
11. Elkoby, R., Brusniak, L., Stoker, R., Khorrami, M. R, Abeysinghe, A., and Moe, J., “Airframe Noise Test Results from the QTD II Flight Test Program,” AIAA 2007-3457, 2007.
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