Computer methods for preliminary design of surface effect vehicles

~ornpu~rrs & Structures,Vol. 3, pp. 529J41.

COMPUTER

Persamon Press 1973. Printedin Great Britain

METHODS SURFACE

FOR PRELIMINARY EFFECT VEHICLESt

DESIGN

OF

R. MUSKATand J. A. ERICSON Aerospace Corporation,

San Bemadino

Operations, San Bernadino,

California

92402, U.S.A.

Abstract-This report describes a computerized engineering analysis which determines the structural weight for surface effect vehicles. This structural program, together with a performance analysis program, and supporting data provide the means for generating rapid parametric studies and concept&. designs of surface effect vehicles (SE%%). The programs are useful in: (1) defining optimum SEV design parameters, (2) comparing data, methods and results from different sources, and (3) definining technology areas where significant improvements are needed. The structural program determines design loads such as water impact, hogging and sagging, and uses these, together with material properties and a structural model to calculate structural weights.

1. INTRODUCTION

1.1 Purpose The analysis and the computer program an SEV (Surface Effects Vehicle) performance mearzs for conducting

rapid parametric

discussed in this report, used together with program and supporting data, provides the studies and preparing conceptual designs of SEV’s.

These programs were developed to facilitate: (1) the selection of preferred values for SEV design parameters, (2) the comparison of data, methods and results from different sources, and (3) the definition cf areas where technology advancement may be desirable to validate design procedures or improve SEV performance. These programs are not intended to replace detailed engineering evaluation and analysis but rather to improve the completeness, precision, uniformity and speed of parametric and pre-design studies. 1.2

Program functions

The structural program calculates structural and skirt weights from basic vehicle characteristics and performance requirements. It uses structural parameters such as the number of beams and frames, allowable stresses, other material properties, minimum and maximum gages, and a variety of loading conditions. The structural weight fraction is used in the performance program. This program determines drag, power required, a vehicle weights breakdown, and propeller and lift fan sizes from the vehicle speed and water wave height, engine and propeller characteristics, cushion pressure and vehicle length-to-beam ratio. tFksented at the National Symposium on Computerized Structural Analysis and Design at the School of Engineering and Applied Science, George Washington University, Washington, D.C.,27-29 March, 1972. 529

R. MUSKATand J. A. ERDON

530

1.3 Optimum con&urations Optimum configurations are obtained by running multiple cases. To retain maximum flexibility, optimization loops are not included in these programs. The structural program can complete up to 10 cases per min using the program’s limited output option on the IBM 360 Mod 50. 1.4 Limitations The structural program was written primarily to study vehicles with an over-water performance requirement. However, an experienced user may be able to adjust wave constants, minimum gages and superstructure design loads to obtain a structural fraction for non-ocean-going vehicles. The program was designed to analyze amphibious craft rather than the captured air bubble (CAB) type of vehicle which has hard side walls in place of the flexible skirts on the sides of the amphibious craft. A more specific list of limitations and approximations is given in Section 5. 1.5 Program verification The structural program was calibrated using detailed SR.N4 structural data. Weights were obtained from the code using SR.N4 structural thicknesses; the constant in the nonload dependent weight term was adjusted to give agreement between the program-calculated total structural fraction and the actual SR.N4 structural fraction. Structural fractions were computed for_the SR.NS and the AALC-ClSO using the code and good agreement was found with the actual structural fractions (see Table 1).

TABLE1. Comparison of actual and

Vehicle

Gross weight

computedstructural weight

cllshion P-m

fractions

Structural fraction actual computed

AALC-Cl50

333,GQo

80

27

25

SR.N4

330,000

50

35

34t

SR.NS

13,000

24

35

38

tBa8eliie con6guration

2. STRUCTURAL MODEL 2.1 General The surface effects vehicle is modeled as a rectangular box. The analytic model has a length equal to the cushion length (L), width equal to the cushion width (B), and overall height (De) determined by either a preset value or by the required cargo hold height plus a calculated buoyancy tank depth (see Fig. la).

Computer Methods for Prehbary

Design of Surface Effect Vehicles

531

FRQST VlRl CAP

FIG. la. Structural model.

The structural model has three main decks; the top deck, the eargu deck, and the wet deck (vehicle bottom); two side walls, a forward and aft wall, water-tight compartment walls, beams and frames, and top trusses (see Fig. la). Additional cargo decks, between the main cargo deck and the top deck, are provided for by a weight allowance calculated within the program. The top deck is subdivided into three sectiuns as shown in Fig. la. By properly seIe&ng any combination of these three top deck portions, a variety of vehicle cross-sectional shapes can be obtained. Figure lb shows some of these and indicates several possible SEV applications. CROSS-SECTION

SIIMPLE APPLtCATiONS CARGO OR FERRY CRAFT

MiSSlLE CARRIER

WDAR CARRIER

FXCL1b. Structural program cross-sectional options.

532

R. MIJSKATand J. A. ERICSON

The number of frames and beams are key parameters because they determine the panel size for the wet deck. Frames are truss members that traverse the vehicle between the main cargo deck and the wet deck. Beams are truss members which run the length of the vehicle between the same two decks. Large beam and frame spacings result in heavy wet deck panels, due to the generally large water impact loads. Small spacings result in large total beam and frame weights and small deck panel weights. Therefore, the number of beams and frames must be optimized to obtain an appropriate structural weight. The top trusses (Fig. la), which carry the main vehicle longitudinal shear loads, are simple trusses with cap, vertical and slant member thicknesses which do not vary with vehicle length. The vehicle external side, forward and aft wall, and the water-tight compartment walls are plates of constant thickness. Bracing is added to the external walls, if necessary, to keep the plate thickness below an input maximum value. Stiffeners run across the vehicle to support the top decks. All truss members are assumed to be angle sections and the braces are modeled as channels. Either honeycomb sandwich panel or solid plate construction can be selected for the decks. Weights are calculated assuming constant material thicknesses over the entire cargo deck. Likewise, a single thickness (or set of thicknesses for honeycomb panels) determined for top deck 1 are multiplied by deck area to obtain the volume of material for weight purposes. A similar procedure is followed for top decks 2 and 3. The wet deck, however, has greater thickness near the front and sides of the vehicle than at the center to accommodate the greater water impact loads experienced there.

2.2 Loads and sqfety factors The program calculates water impact, hogging and sagging loads, cargo floor loadings and a superstructure green water load (see 3.2.2) to obtain weights for the variety of structural components which make up the structural model. A number of minimum gages are specified which determine structural thicknesses when loads are small. Maximum gages for honeycomb panels are included to reflect current practice in honeycomb panel design and fabrication. The primary loads (water impact, hogging and sagging) are calculated using the techniques presented in Elsley and Devereux’s book entitled ‘Hovercraft Design and Construction’ (Ref. [l]). These techniques are based on the British Civil Air Cushion Vehicle Safety Requirements (BCACVSR). The water impact load formula are derived from seaplane design practice. The formulation presented in the British safety regulations was modified to obtain a reasonable correlation between calculated accelerations and those measured from model and full scale test results from the British SR.Nl hovercraft. Safety factors are selected based on the design procedures in Ref. [2] and discussions with SEV contractors. The following safety factors are used: 1*Oto 1.2 on yield 1a5 to 1.75 on ultimate. Acceleration factors due to crash loads are not included.

Computer Methods for PreliminaryDe&n of Surface Effect Vehicles 2.3

533

Non-load dependent structure

To account for structural weight which is not a function of the primary loads, a non-load dependent weight allowance is calculated. This weight includes items such as internal walls and trim, loading ramps, stair wells and railings, propeller pylons, skirt mounting structure, etc. The allowance is assumed to be a linear function of vehicle volume with a constant determined so as to provide agreement with a baseline vehicle. The SR.N4 hovercraft structural data was used to evaluate the constant in the non-load dependent weight scaling equation, because of this vehicle’s large size and the availability of structural data. 2.4

Skirt

A skirt weight is calculated using a scaling relation based on SR.N4 skirt data. This relationship provides realistic values for BHC (British Hovercraft Corporation) type skirts, but does not necessarily provide good agreement with the weights of other skirt concepts.

3. PRINCIPAL

ANALYSIS METHODS

3.1. Overall dimensions The overall dimensions of the model are determined from three input quantities; the gross weight of the vehicle, WG; the length to beam ratio, L/B; and the cushion pressure, Pc or the cushion pressure to length ratio, Pc/L. If P,/L is one of the input values, then Pc is calculated utilizing the following equation:

3.1.1. Length and width (L, B). The length and width of the craft are evaluated by either of two means: 1

L = P&P,/L) - I B=L(L/B)-’

or 2

B=

wG J PC . (LIB)

L = B(L/B) .

The first set of equations is used if PcIL is given, whereas, the second set is employed is P, is known. 3.1.2. Depth (D,). The depth of the structure is subdivided into two distinct parts; the preset cargo hold depth (DJ and the buoyancy raft depth (DI).

The raft depth is either an input value DInr,” or a calculated value. This calculated value makes the total raft volume equal to the sum of twice the vehicle displacement and

R. MUSKATand J. A. ERKSON

534

the fuel volume. The factor of 2 is employed to maintain positive vehicle buoyancy after severe vehicle damage or water-tight compartment leakage. The equation used is:

D, =

max[($+g). Dl,,,i,,].

Thus, total structural depth (Do) is determined by adding the cargo hold depth (&) to the aforementioned raft depth (Oi). DO=D,+

D,.

3.2 Loah 3.2.1. Primary loads. The primary loads carried by the craft are assumed to be those

related to wave impact on the vehicle wet deck or bottom, hogging and sagging. The wave impact loads are based on formula for seaplanes given in the British Civil Air Cushion Vehicle Safety Requirements (Ref. [2]) and modified to obtain correlation with SR.Nl data, according to Elsey and Devereux (Ref. [I]). The uniform load used for design of the wet deck panels is assumed to be 44 per cent of the peak pressure (P) due to wave impact as specified in the safety requirement formulas. The magnitude of the load (P) varies linearly with the distance of the panel from the bow according to the following equation : P=@324K, V, V&44 where Kz is a function of hull station, and V,=craft

relative vertical velocity in ft/sec

Vc=craft total resultant speed relative to the water in ft/sec. As the vehicle may experience high yaw angles at high speed, the wet deck side panels are designed to carry the same water impact pressures as the bow panels. The craft is assumed to be supported by a wave at its center of gravity for the hogging condition. Whereas, in the sagging condition, each end of the craft is assumed to be sup ported by a wave. Both the hogging and sagging conditions are used to generate longitudinal bending moments. Sagging only is used to generate a lateral bending moment. Wave impact provides a longitudinal bending moment which is compared with hogging and sagging bending moments to obtain the maximum value used for design. Water impact loads also provide the design loads for the wet deck panels. In the bending moment calculations, the gross weight of the vehicle is assumed to be distributed over the rectangular vehicle platform in a uniform manner. For the hogging and sagging conditions, the waves are assumed to be point supports, which provides a degree of conservatism to the resulting bending moments. 3.2.2 Superstructure loads The superstructure is designed to statically support five feet of green water (i.e. a uniform load of 320 lb/f?, 5 ft x 64 lb/ft3). This value was determined through discussions with members of the SEV industry.

Computer Methods for Preliminary Design of Surface Effect Vehicles

535

3.2 3 Cargo loa&. The magnitude of the cargo load which is assumed to be uniformly distributed over the cargo deck is a percentage of the gross weight of the vehicle. The value employed generally varies between 30 and 50 per cent. 3.3 Weights 3.3.1 Deck weights. The cargo deck and wet deck analytic model consists of multiple panels spanning the raft beams and frames. The top deck model consists of panels which span the side walls, top trusses and stiffeners. Each panel is assumed to be supported all around its edges and to be uniformly loaded. The panels can be either of honeycomb or plate construction. The design analysis technique employed for the honeycomb panels is given in Refs. [3 and 41. For flat plate panels, the thickness is obtained utilizing the following equation (Refs. [5 and 61) for a thin plate with all edges supported and uniformly loaded: t=

3;w2 bALL(l + l*61a3)

J

Plate buckling is not analyzed. Whether honeycomb or flat plate panels are specified, both a minimum and maximum allowable thickness is used. A minimum thickness is required for compatibility with manufactured gauges and for live load considerations, i.e. to prevent damage to the decks by concentrated loads. A maximum thickness is also required for compatibility with manufactured gages. The weight of the decks is obtained from the product of the constant thickness times the area and the material density. 3.3.2 Weight for walls ancl water-tight compartments. All walls, including the watertight compartment walls are designed as fixed edge flat plate panels uniformly loaded to 320 lb/ft2 (5 ft of green water). The thicknesses are determined using the flat plate equation presented under deck construction. 3.3.3 Weight for braces. If a flat plate having the maximum allowable thickness (an input value) cannot support the design load, vertical braces are used in the external walls to decrease the panel size. Braces are added until the required plate thickness is less than the maximum allowable. The braces use a cross-section composed of two channels with stiffened flanges back to back and a cross-sectional area equal to 40 times the thickness. The coefficient of 40 is an average value of the ratio of area to thickness for the larger heavy gage sections given in Ref. [6]. The thickness of the fixed.uniformly loaded double channel beam is calculated using the following equation, which limits the bending stress to the allowable yield stress. 3 t,=

,/30M/(7420a,,)

The moment (M)in this equation is obtained from the uniform load (5 ft of green water) for one panel width applied along the full length of the brace.

536

R. MUSKAT and J. A. ERICSON

3.3.4 Weightjor st@ners. Stiffeners are utilized on the top decks to decrease the deck panel length to a value equal to the frame spacing. That is, a stiffener is located on the top deck at each frame location (Fig. la). The cross-sectional shape is assumed to be the same as for the aforementioned braces. The stiffener thicknesses and weights are calculated as previously stated for the braces.

3.3.5 Weight for trusses, raft beams and frames caps. The cross-sectional area of the caps for the top trusses and raft beams is determined such that the allowable compression or tension stresses caused by bending, at any location within the vehicle cross-section, are not exceeded by stresses due to either wave impact or hogging and sagging loads. Maximum bending stresses for either loading condition occur at a cross-section mid-way between the bow and stern. The cross-sectional area of the caps is taken as constant along the length of the craft using the value determined at the maximum load cross-section. The cross-sectional area of the raft frame caps is determined utilizing wave impact loads only, and the cross-section is assumed constant with vehicle width. 3.3.5.1 Vertical and slant members. The vertical and slant members are designed to carry the axial loads resulting from the shear generated by the water impact and hogging and sagging bending conditions. The required cross-sectional area for each member is determined such that the axial load on the member divided by the area is less than the allowable stress (P/A < uALL). The weights are calculated as the area times the length times the material density. 3.3.5.2. Optimization of frame and beam spacing. The number of frames and beams are optimized by running multiples cases with the structures program. As the edges of the cargo and wet deck panels are supported by the frames and beams, the panel size for the cargo and wet decks is determined by the number of frames and beams. The frames and beams also serve as the side wall supports for the water-tight compartments. Too few frames and beams result in large panel sizes, and therefore, heavy decks. Too many frames and beams give light weight decking, but heavy frame and beam weights. The purpose of the optimization procedure is to achieve minimum total structural weight while maintaining water-tight compartments with a length to width ratio of approximately three or less. The latter condition is arbitrarily imposed to eliminate minimum weight solutions with large numbers of frames and small numbers of beams. The long narrow water-tight compartments, which would result from large length-to-width ratios are undesirable because collision with an object could damage a large number of water-tight compartments by making a straight line cut along the length of the vehicle. The optimization procedure consists of making a series of runs with a variable number of beams and a constant number of frames. The first set of runs is made with the minimum frame spacing; succeeding sets are made with increased frame spacing. A typical result is shown in Fig. 2. A minimum frame spacing is required to accommodate components between frames (e.g. lift fan ducts) as well as to produce a low length to width ratio watertight compartments. In general, the minimum frame spacing (maximum number of frames) yields the minimum vehicle structural weight. 3.3.6 Skirt weight. The skirt weight is evaluated as a function of the vehicle perimeter (2 (L+ B) ) and the skirt height (HJ. The constant, 5.5 in the equation below, was selected to match the known skirt weights of the SR.N vehicles. Ws,=2(L+B)Hsx5-5

Computer Methods for Preliminary Design of Surface Effect Vehicles

537

52

i

36 0

6 No. of

,

I

12

18

I 24

beams

FIG. 2. Optimization of frame and beam spacing.

3.3.7 Weight.for load independentstructuralallowance. A load independent structural weight is included in the total weight. The magnitude of this weight is a function of the total craft volume: W,,=LxBx D,/3 The factor of one-third was selected to obtain a match between the total SR.N4 structural weight and the weight calculated by the program using all available SR.N4 structural charactersitics as input. 4. COMPUTER PROGRAM 4.1 Flow chart The analysis employed in the structural program is subdivided into the fourteen distinct sections as shown on the flow chart (Fig. 3). Loading conditions are calculated within each section. 4.2 Options Several options are available to the user within each of the above mentioned sections. In Section 1, the variable PJL, the cushion pressure to cushion length ratio, may be inputted instead of Pc. If PC/L is inputted, then the cushion length (~5) and width (B) are calculated from PC/L. Otherwise, L and B are evaluated using the gross weight of the craft and the cushion pressure. A second option in Section 1 allows inputting the total height of the craft. The alternative is to input the cargo hold height (D,) in which case the total height equals the cargo hold height plus the buoyancy tank height. Several options exist in Section 2. The user has the option of selecting five different cargo deck configurations, as shown in Fig. lb. This option is initiated by inputting a zero density for the honeycomb core material and the foil or plate material for any one or any combination of the three distinct top deck sections shown in Fig. la. In a second option, the width of each top deck can be varied by changing the number of stringer spacings contained within each top deck bay. This option, together with the option in

R. MUSKATand J. A. ERICSON

538

CALCULATE LENGTH

hDTH. SECTION

4 I

1

CALCULATE

CRAFT

FORWARD

AND

EXTRA DECK WEIGHT

I

B I

SECTION

II I

I CALCULATE

CALCULATE

CALCULATE SKIRT WEIGHT SECTION

12

CALCULATE

%:GF” SECTION

4

If-cl STRUCTURAL

P

CA,LCULATE

CALCULATE SIDE WALL WEIGHT SECTION

FRAME WEIGHT

SECTION

5

WATER-TIGHT COMPARTMENT WALL WEIGHT

SECTION

OUTPUT RESULTS

SECTION

15

10

0 EXIT

Fro. 3. Flow Chart.

Section 8, is used to produce the vehicle cross-sections shown in Fig. lb. A third option available in this section as well as in Sections 3 and 4 is specification of either honeycomb sandwich or flat plate type deck construction. This option is initiated by entering a zero core density if the deck is not to be a honeycomb structure. No options are available in Sections 5 and 6. In Sections 7 and 9, the user inputs the number of beams (in Section 7) and frames (in Section 9). In Section 8, the operator can eliminate either or both of the top trusses by inputting zero density for the truss material. In Section 10, the number of water-tight compartments is an input variable. In Section 11, the number of extra decks (N,,)

is an input variable.

In Section 12, the user has the option of inputting a skirt weight factor. If this factor is inputted as l-0, the skirt weight is evaluated using an equation (see 3.3.6) which is a function of the craft dimensions, and the skirt height and matches SR.N4, N5 and N6 data. If the skirt technology changes or the skirt material density differs from that used on the SR.N craft, the user must supply a non-unity value for this factor. No options exist in the remaining two sections.

Computer Methods for PreliminaryDesignof Surface Effect Vehicles

539

5. LIMlTATIONS Use of the program should be guided by the following limitations: (1) The structural model has a rectangular platform rather than the rounded bow and stern often used on operational craft. (2) The structural model size and cushion size are assumed equal. (3) The loads utilized in the design of the wet deck and super-structure are determined by sea conditions and are not applicable to an over-land only design. (4) Main structural members are designed for bending; shear stresses are not explicitly treated. (5) Structural members are designed utilizing yield theory. design is considered.

No ultimate (plastic)

(6) Skirts are scaled from SR.N data. (7) All walls are limited to flat plate construction. (8) No buckling analysis is done within the program.

6. RESULTS The structural program has been used to obtain structural weight fractions for performance analysis of various vehicle concepts and to study structural weight trends as a function of several key parameters. The cushion pressure, PC, and the vehicle gross weight, W,, are the two major variables that effect the structural weight fraction. However, other parameters such as the craft velocity, V,, and sea state conditions can strongly influence the structural weight and the trends shown in the following figures. Figure 3-1 illustrates the hyperbolic relationship between the structural weight fraction and the cushion pressure for a vehicle having a gross weight of 1400 tons and a cruise speed of 80 knots. This trend agrees with the results of several SEV contractor studies. One contractor has developed a scaling relationship which exhibits this same functional dependence on cushion pressure as shown by Fig. 4a. The relationship between the vehicle gross weight and the structural weight fraction is illustrated in Fig. 4b. This figure was developed using four discrete solutions that were optimized for beam and frame spacing. The straight line representation was used to reflect an average trend over the 1000-2000 ton gross weight vehicle range. The small variation from the line, as shown in the figure, were due to structural component size limitations (max and min gages) and the beam and frame optimization procedure. It is highly desirable to obtain more accurate and complete SEV structural weight details on vehicles currently in the detailed design stage to improve confidence in the accuracy of the structural weight fraction calculation. Based on numerical comparisons (Table 2.) the program provides a more accurate basis for structural fraction estimation than known scaling relations which have previously been used for conceptual design and parametric study.

R. MUSKATand J. A. ERICSON

540 60-

50 .S 8-

40-

t P z .P $ E ; ‘u 2 5,

30-

Range,

zo-

R= 4000

nm

Grass weight,

W, = 1400

VehrAe

speed.

cruise

tans

Vc = 80 knots

IO

t 0-L

70

100

Cushion

I

I/I,

40

I50 pressure,

32 28

/

24

Cushion

20

210 Psf

I

I

16

I4

ft’xlo-’

area.

FIG. 4a. The effect of cushion pressure on structural weight fraction

100

200 Cushion

II/II

20

I

1412 IO

300

500

I

I

I

6

4 tt*xio

600

Psf

8

Cushion area,

FIG. 4b. Contractor

400 PC.

pressure,

-3

scaling equation for structural weight fraction vs cushion pressure

Computer Methods for Preliminary Design of Surface Effect Vehicles 50x

6

D 4o.c

S ‘7 8 c‘

30x

,I-

,-

I-

z .p 3 20x 6 !i ‘, z v,

,-

Range,

R = 4000

Vehicle

cruise

Cushion

pressure,

Vc ~80 knots

Pc = 140 psf

IOX I-

1500

1000 Gross I

I

I

12

14

16

Cushion 4c.

nm speed,

C,_

FIG.

541

2000

weight, I

I

16 20 Oreo.

tons I

I

22

24

1

I

26 26

ft*x 10-3

The effect of gross weight on structural weight fraction. TABLE2. Structural fraction comparisons

Vehicle

Actual

Structural fraction Program Contractors’ scaling law

AALC-Cl 50

27

25

32

SR.N4

35

34

46

SR.NS

35

38

28

Acknowle&ements-The data, design practices and analysis incorporated in this program were derived from a variety of sources including ARPA studies, industry publications, the general SEV technical literature and personal communications. The authors would like to thank members of the Surface Effects Vehicle (SEV) industry who shared their expertise with the authors in structures, vehicle drag, and other areas. Their help made the computer program described herein possible. Special appreciation is extended to the Advanced Research Projects Agency under whose auspices this work was conducted. The study is reported in Aerospace Corporation report number TOR-O172(S2858)-2entitled Computer Codesfor the Preliminary Design and Parametric Analysis of Surface Effect Vehicles dated 31 October 1971.

REFERENCES [l] G. H. EUUY and A. J. DEVEREUX, Hovercrafr Design and Construction, Cornell Maritime Press Inc. [2] Provirional British Civil Air Cushion Vehicle Safety Requirements (February 1962). [3] Mechanical Properties of Hexcel Honeycomb Materials, Hexcel Products Inc., TSD-120 20 February (1964). [4] ROHRCorporation, Acihesive Bonding Design Handbook (1962). [5] R. J. ROARK,Formukzsfir Stress and Strain, McGraw-Hill, New York (1954). [6] Manual of Steel Construction, AISC. (Received 16 February 1972)