Parametric studies on the channel type tension fitting in aircraft

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Parametric studies on the channel type tension fitting in aircraft

SHWETHA JAIN

Dept. of Civil Engineering, The Oxford College of Engineering

Bangalore, Karnataka State, India shwethajainjinesh@gmail.com http://www.theoxford.edu/

AMARNATH K

Prof. and Head, Dept. of Civil Engineering, the Oxford College of Engineering Bangalore, Karnataka, India sri.valuers@gmail.com http://www.theoxford.edu/

SUBRAMANYA SASTRY SS

AGM (Tech), Engineering Vertical, Infotech Enterprises Limited

Hyderabad, Karnataka, India

Subramanya.Sastry@Infotech-

Enterprises.com

http://Infotech-Enterprises.com

Abstract:

Tension Fittings are commonly classified into four types. They are Bathtub fitting, Channel fitting, Angle fitting, PI fitting and Double angle fitting. Tension fittings are conservatively sized as their weight is usually small relative to their importance. In this paper two analysis methods of Channel fitting are compared. Simplified method of approach to the design of channel fitting is studied in detail with respect to various parameters affecting the design. Margin of safety is calculated using two methods and compared. The two methods are compared and contrasted for their results prediction. Aluminum alloy 7050-T7452 is selected for the study.

Keywords: Tension fittings, Channel fittings and Angle Fittings

  1. INTRODUCTION

    A tension fittings are one in which the bolt is loaded primarily in tension and they are used in the various applications like removable wing root joints of fighter wings, wing root joints at the four corners of the wing box, circumferential production joints on assemblies of fuselage sections, axial load transfer of the fuselage stringer by using tension bolt to go through major bulkhead web rather than by cutting a large hole in the web and back-up fittings for support pylons, flap track etc., which have to go through wing box [1].

    Since the state of stress in a tension fitting is very complex, conventional structural analysis is extremely difficult to obtain with reasonable results. The methods used in this paper have rational basis and give loads agreeing well with available test data [2]. A fitting factor of 1.15 [1 2] is applied in the analysis while calculating the margin of safety. The analysis will show a minimum margin of safety of greater than 0.2 [1 2]. This condition is demonstrated for both yield and ultimate conditions. Eccentricity of the load is

    inevitable because of the shape of the fitting and various ruling dimensions. But the resulting stresses are kept to a minimum to give the required margin of safety. A minimum edge distance of 2.5D and pitch of 6D is considered in the present study. In this paper, diameter of the rivets is kept to a minimum and only fitting wall thickness is considered as the variable parameter.

  2. ANALYTICAL MODELING

    The various parameters of the channel fitting under study are described below.

    Type of the fitting: Channel Fitting Diameter of bolt: 0.562 in (14.27 mm) Diameter of rivet: 0.15625 in (3.97 mm)

    Size of the fitting: 2 inch by 2 inch by 4.5 in (50.8 mm * 50.8 mm * 101.6 mm)

    Ultimate load: 30 Kips (133.476 KN)

    A list of parameters used in this study is provided below.

    Pult Ultimate load on the fitting MS Margin of Safety

    ta – Fitting wall thickness tb – Fitting web thickness tc – Fitting end thickness Ag Gross area of web An Net area of web

    Fitting Factor (1.15)

    Pad end Load application end connected to the load bearing and dissipating member of aircraft Wall end End of the outstanding leg of the channel fitting connected to the backup structure Web end Web end of the channel fitting connected to the backup structure

    A typical representation of the channel fitting under study is shown in Fig. 1.

    Fig.1: Pictorial view of channel fitting under study

  3. LOADS AND BOUNDARY

    CONDITIONS

    The channel fitting under study is assumed to be connected firmly to the backup structure using Titanium rivets (Ti-6Al-4V).

    The loading on the beam is assumed in the form of a bolt load (numerical value is given under Section 2). This bolt load is a typical characteristic load of a class of channel fittings in fighter planes.

  4. MATERIALS

    The materials used in channel fitting under study are as follows.

    1. Fitting: Aluminium alloy 7050-T7452 (Hand Forging, AMS 4108)

    2. Bolt: Alloy steel (Fsu = 95 Ksi)

    3. Washer: Steel

  5. ANALYSIS

    Design of Fitting wall

    1. As a first step, the channel fitting is made of a ductile material (Aluminium, 7050- T7452).

    2. Check the channel fitting for limitation on dimensions.

    3. Determine the tension on the fitting wall using the relation fta = (P/An), where P is the ultimate load and An is the net tension area.

    4. Calculate the moment of inertia, centre of gravity of the channel section.

    5. Determine the eccentricities of the load from the centre of gravity of the channel section.

    6. Calculate the moment of the load due to the effect of eccentricity on the wall end and web end of the channel fitting.

    7. Calculate the bending tensile stress (fnx, fny) in the wall using the moment and moment of inertia of the fitting.

    8. Calculate the total maximum tension stress as ftu = fta +fnx +fny.

    9. Calculate margin of safety for ultimate load, MS = (Ftu/*ftu)-1 > 0.20, where is the fitting factor (1.15)

    10. Check all fasteners between fitting and attached member for failure against shear, tension and shear tension interaction modes of failure.

    Design of Fitting end

    1. Bending: Compute the ultimate bending stress in the fitting end, fbu = (kc*P/tc2). This bending stress is function of the aspect ratio of the fitting (a/b).

    2. Calculate the margin safety and make sure that its value is greater than 0.2 after using a fitting factor of 1.15.

    3. Calculate the ultimate shear stress through the bolt hole, fsu = (P/0.6Dw,otc).

    4. Calculate the margin of safety using the formula, MS = (Fsu/fsu)-1 > 0.2.

    Exact Method

    The exact method [2] differs from the approximate method as given below.

    1. Use of tension efficiency factor to account for the uniform and staggered rivet pattern in determining the wall tension stress ratio.

    2. Calculation of wall tension and bending stress ratio and interaction between the tension and bending.

    3. Effect of stress concentration factor on allowable bending stress.

    The margin of safety is computed using the exact method and compared with the results of approximate method.

  6. RESULTS AND DISCUSSIONS

    The results are presented in the form of graphs and tables. Table 1 and 2 show the margin of safety obtained for the Aluminium alloy 7050- T7452 under study. Table 3 shows the comparison of results between the approximate and exact methods. Figures 2 shows the variation of Margin of safety near the web end of channel fitting with the change in thickness of fitting wall. Figures 3 shows the variation of Margin of safety near the wall end of channel fitting with the thickness of fitting wall. Figures 4 shows the variation of

    Margin of safety near the pad end of channel fitting with the thickness of fitting wall. Figures 5 shows the variation of Margin of safety for shear through bolt hole of channel fitting.

    Table 1: Table of stess in the channel fitting

    0.30

    Margin of Safety (%)

    0.20

    0.10

    0.00

    -0.10

    MoS near wall end of Channel Fitting

    0.15 0.16 0.17 0.22

    Fitting wall thickness, ta, in

    Bending stress near

    web of

    fitting (ksi)

    Bending stress near wall end of fitting

    (ksi)

    Total stress near web of fitting (ksi)

    Total

    stress near wall end of fitting

    (ksi)

    0.15

    17.94

    35.89

    58.02

    75.97

    0.16

    16.69

    32.11

    55.37

    70.79

    0.17

    15.55

    28.88

    52.92

    66.24

    0.22

    11.14

    17.91

    43.09

    49.87

    -0.20

    -0.30

    Tthickness of Fitiing wall (inch)

    Fig. 3: Margin of safety near the wall end of channel fitting

    0.40

    Margin of Safety (%)

    0.30

    0.20

    0.10

    0.00

    -0.10

    -0.20

    -0.30

    -0.40

    MoS near pad end of fitting

    0.15 0.16 0.17 0.22

    Thickness of Fitting wall (inch)

    Table 2: Table of margin of safety at various location of the channel fitting

    Fig. 4: Margin of safety near the pad end of channel fitting

    Comparisonof approximate andexact solutions

    Sl. No.

    Description

    Approx. Method

    Exact Method

    Percentage

    difference

    1

    Gross area (sq.in)

    1.15

    1.15

    0.00

    2

    Location of C.G. from

    wall end (inch)

    0.76

    0.79

    3.17

    3

    Tension Efficiency

    Factor

    N / A

    0.82

    N/ A

    4

    Bending-Tension

    interaction

    N/ A

    0.3577

    N/ A

    5

    Stress in wall (ksi)

    26.20

    26.20

    0.00

    6

    Stress in end pad (ksi)

    48.38

    63.86

    24.24

    0.60

    Margin of Safety (%)

    0.50

    0.40

    0.30

    0.20

    0.10

    0.00

    MoS for Shear through bolt hole

    0.15 0.16 0.17 0.22

    Thickness of Fitting wall (inch)

    Table 3: Table of comparison of various results between approximate and exact methods

    Fig. 5: Margin of safety for shear through bolt hole of channel fitting

    The detailed study of results is presented below.

    0.50

    0.45

    Margin of safety (%)

    0.40

    0.35

    0.30

    0.25

    0.20

    0.15

    0.10

    0.05

    0.00

    MoS near web of Channel Fitting

    0 0.1 0.2

    Thickness of Fitting wall (inch)

    1. The study is presented for a channel fitting of cross section 2 inch * 2 inch (50.8 mm).

      Comparisonofapproximateandexactsolutions

      Sl.No.

      Description

      Approx. Method

      ExactMethod

      Percentage

      difference

      1

      Grossarea(sq.in)

      1.15

      1.15

      0.00

      2

      LocationofC.G. from

      wallend(inch)

      0.76

      0.79

      3.17

      3

      TensionEfficiency

      Factor

      N/A

      0.82

      N/A

      4

      Bending-Tension

      interaction

      N/ A

      0.3577

      N/ A

      5

      Stressinwall(ksi)

      26.20

      26.20

      0.00

      6

      Stressinendpad(ksi)

      48.38

      63.86

      24.24

    2. From Table 1, it is observed that as thickness of the wall of the fitting increases by 46.67%, bending stress near the web end of the fitting decreases by 37.9%.

    3. From Table 1, it is observed that as thickness of the wall of the fitting increases by 46.67%, bending stress near the wall end of the fitting decreases by 50.1%.

    4. From Table 1, it is observed that as thickness of the wall of the fitting increases by 46.67%, total stress near the web end of the fitting decreases by 25.73%.

    5. From Table 1, it is observed that as thickness of the wall of the fitting increases by 46.67%, total stress near the wall end of the fitting decreases by 34.35%.

      Fig. 2: Margin of safety near the web of channel fitting

    6. From Table 2, it is observed that that as thickness of the wall of the fitting increases by 46.67%, margin of safety near web end of fitting increases by 43% while it increases by only 24% from a negative margin.

    7. From Table 2, it is observed that that as thickness of the wall of the fitting increases by 46.67%, there is no improvement in the margin of safety in the rivets because it is independent of the wall thickness or web thickness. It is single shear value of the rivet and is governed by the diameter of the rivet rather than on the thickness of web or wall in bearing.

    8. From Table 2, it is observed that that as thickness of the wall of the fitting increases by 46.67%, margin of safety for bending at the fitting end increases from -29% to 28%.

    9. From Table 2, it is observed that that as thickness of the wall of the fitting increases by 46.67%, margin of safety for shear through bolt hole increases by 3.45 times the original value.

    10. Table 3 shows a comparison of the results obtained from the approximate and exact methods. The difference in the various computed quantities is indicated in percentage for comparison purposes.

  7. CONCLUSIONS

      1. Approximate method doesnt take into consideration the reduction in efficiency of the channel fitting due to the presence of rivet holes.

      2. Approximate method doesnt take into consideration the effect of bending-tension interaction in the channel fitting.

      3. Approximate method under predicts the stress in the end pad by 24.24% as compared to the exact method.

ACKNOWLEDGMENT

We sincerely thank and acknowledge the support and encouragement provided by Mr. M. Lakshmana Rao, VP, Engineering Vertical, Infotech Enterprises Limited, Hyderabad in bringing out this work in the form of a technical paper.

REFERENCES

  1. Niu, Airframe stress analysis and sizing, Second edition 1999.

  2. Lockheed Stress Memo 88a.pdf

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