F-16 Landing Gear Simulation for Structural and Reliability Analyses

8/11/2019 F-16 Landing Gear Simulation for Structural and Reliability Analyses

1/14

1

F16 Landing Gear Simulation for Structural and Reliability Analyses

H.W. Ng*, G.K. Neo, X. W. Xie and W.X. Chu

Nanyang Technological University

School of Mechanical and Aerospace Engineering

50, Nanyang AvenueSingapore 639798

*Corresponding author email: [email protected]

Abstract

The minor deformations on an aircraft landing gear and the associated wheel well doors

structures due to wear and tear over time, and various loadings on them has been studied

to determine their effects on landing gear operation reliability. The aircraft landing gear

system uses micro-switches to feedback the exact position of the parts and they operate

with close tolerances. Minor deformations on the landing gear and its associated wheeldoors structures may cause small misalignment between the moving parts due to various

aerodynamic loadings. These deflections are large enough to affect the micro-switches inflight but not easily detectable on ground during troubleshooting.

The structural components of the landing gears consists of: shock struts, drag and sidestays, actuator mechanisms, various linkages and pivot pins, up or down locks and

position sensors for cockpit indications. The wheel well and doors also have a large

number of parts such as alignment pins, hinges, locking mechanisms at points ofattachment and position sensors. These parts experiences cyclic and sometimes extreme

loads during service. Over time, due to in service wear and distortions, the functionaltolerances of critical pins, bushings and actuators degrade to affect the operability of the

landing gear mechanisms before scheduled overhaul are due.

Finite Element Method (FEM) and Hydraulic Simulation software together have thecapability to analyse localized detailed stresses and simulate possible small combined

relative movements of the various mechanisms due to wear and in service loadings of

landing gear components. This presentation describes the application of modeling andanalyses for in-service conditions to predict localized high stress areas or wear hot

spots and distortion modes of the landing gear and its associated wheel well doors

components.

A highly detailed CAD model of the structures of the F-16 port side main landing gear,

wheel well and doors have been constructed. The applicable loads besides the variousaerodynamic loads include air load, inertia loading due to mechanism acceleration and

deceleration and gyroscopic loads and induced by landing gear extension and retraction.

The loading cases also including vertical reaction landing loads and brake loads are

considered.


2/14

2

1. Introduction

The landing gear system is one of the aircraft most critical systems allowing it to taxi,

take off and land. A failure of the landing gear may result in aircraft on ground (AOG),

loss of mission and/or aircraft and endangerment of flight crew. All of the

aforementioned will also result in loss of operational capability. The F16 landing gearconfiguration has a multitude of joints to enable it to be retracted into the fuselage wheel

well. The large number of joints necessitated more maintenance compared to other types

of landing gear. According to the past defect trends, common defects found include thosedue to tolerance such as rigging problems, wear and tear in joints and tire worn to limit

and excessive stress causing shearing of bolt. Of this, a portion of the defects are related

to the main landing gear system looseness in pins and bushings of joints. It is thereforeessential to investigate the causes of the defects in the joints to improve the availability of

the aircraft and solve some of its reliability issues. Although the F16 landing gear is the

focus of the article, it is to an extent for illustrative purpose only, other aircraft are

equally amenable to the analysis methodology described.

2. Analysis Methodology

The main objectives of the methodology are:

2.1.Analyse the global stresses on a large model and extract local loads at bearings

for the load cases:

(i) aerodynamic drag due to relative air speed during take off and landing.(ii) inertia force due to retraction or extending mass of gear through 90

owithin

a time of 6 seconds.(iii) gyroscopic force, caused by rotating wheel swiveled through 90

oto lie flat

in the wheel well. Although a prebrake is often applied to stop wheel

turning after takeoff, it takes a few seconds to be activated.

(iv) vertical reaction force, a landing load factor Nz of 3 was used.(v) side reaction force caused by taxiing around a curve.

(vi) spin up force due to inertia when wheel spin up upon landing.

The operational cycles of the landing gear consists of the following phases:

Phase 1: Landing approach

Phase 2: TouchdownPhase 3: Taxiing roll

Phase 4: Taxiing turn

Phase 5: Take-offEach phase is a combination of the load cases selected from above. One of the most

severe phase encountered is during landing touch down which involves cases (i), (v)

and (vii). A landing gear system is designed comprehensively to withstand these

loads (refs. 1 and 2) and have adequate structural capacity for all combinations ofloads.


3/14

3

2.2.Analyse the inertia forces due to actuator actions during retraction and extension

for the local contact stress analysis at the connections at the ends of the actuator.This is a more precise analysis including interaction with the hydraulic system.

2.3.Analyse the local stresses due to contact between the bushes and internal pins.

2.4.Determine the deformation of the main landing gear components under various

loadings.

2.5.Determine the possible wear characteristics on the critical areas under various

loadings.

3. Landing Gear Description

The landing gear system of the F-16 is fuselage-mounted, tricycle-type and retractable.

The fuselage attachment is simply through three hard points, two main points carry in-line hinging allowing the landing gear to swing outward i.e. sideways from the side of the

fuselage. The third point located forward carry the drag stay connection. The wheel pathswing forward and is swiveled to horizontally fit flat inside the fuselage. (An animation

is shown in the presentation). Figure 1 (ref. 3) illustrates the main components of the

main landing gear.

A hydraulic system provide hydraulic power for normal operation of the landing gear,

which include extension and retraction, landing gear door opening and closing, braking,and nose wheel steering. The main landing gear and door subsystem has standard

landing gear structures on both sides of the fuselage. The main gear doors cover thewheel wells during flight and are hydraulically opened and closed.

Figure 1: Main landing gear components (ref 3).


4/14

4

The tension strut assembly consists of the tension strut, the axle fitting, wheel positioning

collar and link. The drag brace assembly is a folding stay with internal lockingmechanism at the folding joint. It consists of an upper drag brace, a lower drag brace, and

a downlock mechanism. It connects to the tension strut wheel positioning collar at the

lower end. The upper drag brace connects to the aircraft structure in the forward part of

the wheel well. It houses the downlock actuator and the toggle assembly.

4. Finite Element Analyses

Finite element analysis is divided into two phases; namely a global phase and local phase.

The global phase analyses the entire landing gear subjected to the above mentionedloading cases (i) to (v). The purpose of the global phase is to extract forces at the joints

to be later applied to the more finely meshed local component level i.e. the local phase.

Also, the global phase allows for the visualization of the motion during the extraction and

retraction. In this presentation, the wheel well structure and the attached doors are not

considered. They will be carried out in future to determine the in-flight deflections whenthey are subjected to air and dynamic loads.

4.1.Motion Model of Landing Gear

The main structural components of the main landing gear and wheel well are drawn usingPro-Engineer Wildfire 2.0 Software (ref.4). The main structural components include

main landing gear wheel well, main landing gear wheel well door, tension strut, shock

strut, upper and lower drag brace, axle, toggle, centrelink, wheel, link, collar, andbushing. A detailed CAD drawing of the main landing gear, and wheel well and door

are as shown in figure 2 and 3. The file is imported in Visual Nastran which performedthe motion simulation and finite element analysis. The motion file is shown in the

presentation. Figures 4 to 5 shows various positions during the motion of the landing

gear and wheel well. For the motion model to work well, as is clearly shown in the

animation, all the linkages and tolerance have been accurately set.

Figure 2: CAD solid model of detached landing gear wheel well door.


5/14

5

Figure 3: Solid models for the wheel well and extended main landing gear .

Figure 4. Motion model showing the landing gear fully retracted into wheel well, note

the wheel swivel.


6/14

6

MLG in fully extended state Retraction of MLG

MLG is fully retracted state Closing of Wheel well door

MLG fully retracted and wheel well door fully closed

Figure 5. Stopped motion screen shots at different points in the retraction cycle.

4.2.Global Stress Analysis of Complete Landing Gear

The final mesh of the FE model is shown in figure 6. A relatively crude mesh is applied

to the solid model in order to reduce computational times. Furthermore, the purpose of

the global model is not to obtained accurate stresses but to derive the joint forces. It is

noted that despite this the stresses along the struts and stays are relatively accurate. Thejoint forces are later extracted to be input into refined local component models which will

be elaborated in the next section to obtain the local contact stresses. For this phase of the

analysis, the landing gear is separately analysed by removing from the wheel well and thethree attachment hard points were rigidly fixed. Also all the joints are made rigid by

connecting pin to bush surfaces. Also, the landing gear is frozen into the fully extended

down and locked position.


7/14

7

Figure 6: Mesh of the main landing gear

4.3.Local Contact Stress Model

The contact stresses in pins and bushings are dependent on the joint geometry and

local forces applied. The local models are finite element models created for all jointswhich consisted of the bush and pin rolling contact parts (ref 5). These were aimed at

obtaining the high contact stresses during load transfer between the landing gear

structures during various phases of operation. It is intended in this presentation todescribe one such joint at the connection between the gear actuator and the tension

strut. In creating this model, contact elements were used to model the contact

between the pin and bush. By imposing symmetry condition, the joint is reduced to ahalf two dimensional model shown in figure 7, using finely meshed 2D solidelements. The maximum force was applied to the lower collar of the bush assembly

while the pin was restrained by spring elements representing the stiffness of the pin

under 3 point loading. The contact elements connect the lower semicircular surfacesof the bush and pin. Preliminary results are presented in section 6 because this work

is currently in progress.


8/14

8

Figure 7: The half two dimensional model of the bush to collar joint between gearactuator and tension strut pin. Note: the structure undeformed shape is shown in

white.

5. Hydraulic flow modeling

Hydraulic flow modeling is employed to investigate the behaviour of the hydraulicsystem in terms of pressure and flow rate when subjected to loads due to landing gear

extension and retraction (ref. 6). The analysis determine the load applied at the joints

during extension and retraction of the landing gear by hydraulic actuator. The actuationforce is dependent on the inertia of the combined structure and wheel assembly and the

hydraulic flow response. To determine these forces, the dynamic performance of the

landing gear hydraulics system during extension and retraction operations was examinedunder various conditions using FLOWMASTER (ref. 7). The aircraft landing gear

extension and retraction may be simplified as a constant load on the hydraulic system.

However in order for the model to be applicable to actual landing gear extension and

retraction, the resultant load required is non-linear because of the changing position andacceleration of the gear. It is the available pressure and flow rate delivered by the

hydraulic pump that will determine the acceleration and speed of extension or retraction

of the landing gear. On the other hand, the inertia of the landing gear resists the

acceleration imposed by the hydraulic actuator, which presented it as a dynamic load.Thus, the inertia of the system interacts dynamically with the hydraulic system, resulting

in force balance giving rise to the final acceleration of the gear. Hence, simple kinematicanalysis cannot be used to determine the loads carried by the landing gear and wheel

doors during extension and retraction operations.

Figure 8 shows the hydraulic system model representing the complete system controllingthe actuation and locking of the gear and wheel well doors during extended and retracted


9/14

9

phase for the main gear. In the presentation, the important item is the landing gear

actuator (circled in figure 8). The completeness of the system is necessary if it is toassess the capability of the system to cope with all the demands of the system.

Figure 8: Flowmaster model representing the hydraulic components including

actuators for the landing gear, wheel well door, uplock and downlock.

The Flowmaster simulation use the Visual Basic logic script to simulate the sequence

of movement of the Door Uplock, Door Actuator, Door Downlock, Landing Gear

Uplock, Landing Gear Actuator and Landing Gear Downlock, throughout both theextension and retraction of the landing gear.


10/14

10

5.1.Analytical model for the LG actuator load

An analytical model shown in Figure 9 is required to derive the actuator force FT

exerted by the hydraulic actuator in order to rotate the landing gear from a fully

extended position to a retracted position. The analysis of the applied force to

overcome gravity load and the inertia load ( i.e. the resistance by the mass M toaccelerate under applied force) gives the following equations.

Figure 9: Analytical model to derive the landing gear actuator load under applied

displacement, velocity and acceleration.

=

96.0

1cos

2

1 ACL (1)

( )( )( )( )( )

=

sin96.0

2 LL && (2)

( )( ) ( )( )( )( )( )

+=

sin48.0

cos48.0 &&&&&& LLL (3)

( )

=ACL

sin8.0sin (4)

IGT FFF += (5)

Finally, ( ) ( )( )( )

+

=

sin.12sin

cos &&BCT

LmmgF (6)

A B

FYF

FX90

mgx

mgy

mg

90


11/14

11

From the equations obtained above, it can be seen that with input values ofL , L& andL&& ,

all the unknown values in the FT equation can now be defined when substituted into

the ,& , && and sin equations. To obtain the values ofL , L& and L&& gauges (see figure

10 for the concept of gauges) are connected to the hydraulic cylinder inside the

Flowmaster network to measure the displacement, velocity and acceleration. A visual

basic script controller embedded the above equations to give the required actuator load.The Flowmaster solver iterates at each time step until convergence i.e. force balance is

achieved in which the actuator output force equal to the script calculated value.

Figure 10: Concept of defining a load on an actuator by means of a controller (rectangle)

and gauges (circles). The controller contains Visual Basic Script that processes the data

from gauges to calculate the applied load on the actuator.

6. Results and Discussion

Two different finite element models have been created, the first being a global model

consisting of all the major structural components and the second the local model forcontact stress evaluation in specific joints. Although it is linear elastic problem, the

global model problem is prohibitively expensive in computational time despitesimplifications such as rigid joints. The non-linear gap element local model can be run

quickly and yielded quick results.

6.1.Results of the Global Stress Analysis

The contour plots of von-mises equivalent stress are shown in figures 11 and 12 for the

load case of vertical reaction during touchdown. The mesh refinement on the mainstructure yielded adequate stress results. However, due to the rigid assumption on the

rotating joints, the stresses near to the pins and bushes are unreliable, so more detailed

analysis was carried out using gap elements (see local analysis results in next section).


12/14

12

Figure 11: Overall top and isometric view of von-mises equivalent stress for landing gearduring touchdown.

Figure 12: Von-mises stress contours on the wheel axle (left) and lower tension strut

(right) during touchdown.

6.2.Results of the Local Stress Analysis

Figure 13 shows the von-mises equivalent stress at the maximum load applied to the

actuator. The resulting contact stresses covers the area immediately below the pin. Thepeak stress is considerably below the allowable stress for yielding indicating that the


13/14

13

structure can still support further increase in load. However high loads can cause

deformation and progressive enlargement of the clearance gap between the pin and bushin other joints, leading to looseness or play in the joint. A more suitable measure of

ability to sustain further load is through strain limits or through deformation based

measure.

Figure 13: Von-mises equivalent stress at bushpin region.

6.3.

Results of Hydraulic Analysis

The actuator loading calculated by the Flowmaster simulation with the use of controller

script is shown in figure 13. In-flight conditions such as aerodynamic drag on the landinggear during extension/retraction have not been included. This and other scenarios may be

evaluated in future.

0 5 10 15 20 25 30 35 40

Time

0

10000

20000

30000

40000

50000

ExternalP

istonLoad

Figure 13: Plot of landing gear actuator load against time during extension (at 4 seconds)

and retraction (at 30 seconds) phases.


14/14

14

7. Conclusions

From the various loads and cases analysed, it can be seen that the bushing, pins, tension

strut and axle are components which experience higher stresses. The contact stress

between the pins and bushings are areas that experienced the highest von mises stresses

which currently under investigation. High contact stress is most likely to cause wear andtear, result in minor deformation of the parts and thus further work are required to

develop a wear quantification model. FEM together with hydraulic simulation software

have been applied successfully to determine critical areas of the landing gear mechanismin troubleshooting and identification of wear and distortion modes, as well as

understanding of the effects of the various loadings.

8. Acknowledgements

We are grateful to the Air Logistics Division, Republic of Singapore Air Force for the

opportunity to work on this project. We would like to thank their staff membersespecially; Ed Pang (former Head) Cpt. Phillip Ong and Ms. Foo Wei Ling for their

assistance and arrangement for site visits.

9. References

1. Conway, H. G.,Landing Gear Design, Chapman & Hall, London, 1958.

2. Currey, N. S.,Aircraft Landing Gear Design: Principles and Practices, AIAAEducation Series, Washington, 1988.

3. F-16 Aircraft information (Courtesy of AeB, ALD RSAF).4. Neo G.K. F16 Main Landing Gear Modelling and Simulation.School of

Mechanical and Aerospace Engineering, Nanyang Technological University.

Singapore.5. Chu W.X. Finite Element Analysis of Contact Stress in Aircraft Landing Gear

Bush to Pin Bearings. School of Mechanical and Aerospace Engineering,

Nanyang Technological University. Singapore.

6. Xin X.W. Simulation of Behaviour of Aircraft Landing Gear Hydraulic System.School of Mechanical and Aerospace Engineering, Nanyang Technological

University. Singapore.

7. Flowmaster User Manual (www.flowmaster.com).

Documents

F-16 Landing Gear Simulation for Structural and Reliability Analyses