Experimental study of a low modulus flutter model for strut foil pod configurations

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TECHNICAL REPORT 59-2

EXPERIMENTAL STUDY OF A LOW MODULUS FLUTTER MODEL FOR STRUT-FOIL-POD CONFIGURATIONS

By

T. T. Huang

July 1967

DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED

The research reported herein was carried out under U. S. Naval Ships'Systems Command General Hydromechanics Research Program,

administered by

Naval Ship Research and Development Center Prepared Under

'Office of Naval. Research Department of the Navy

Contract No... Nonr-11293(OO)

Reproduction in whole or in part is permitted for any purpose of the United State.s Government

}WDRONAUTICS, Incòrporated J_ -TABLE OF dONTENTS Pa g e ABSTRACT i INTRODUCTION 2 THE MODELS . 6

MODEL EST PROCEDURE 7

RESULTS 9

CONCLUSION 13

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-ii-LIST 0F FIGURES

Figure 1 - Dimensions of i/k Scale Built-Up Model Strut

Figure 2

- The Details of the Pod

Figure 3

-

The Pod and Wing

Figure k - Photograph of the 200 Swept Low Modulus Strut

With Rigid Pod

o

Figure - Photograph of the 20 Swept Low Modulus Strut

With Rigid Pod and Wing

Figure 6

-

Photograph of the Tert-ical Low Modulus Strut With Rigid Pod and Wing

Figure 7 - A View of the Model and its Supporting System

Figure 8 - The Flutter Velocites of the Low Modulus Strut

at Various Sweeps and Submergences

Figure 9

-

The Flutter Frequencies of the Low Modulus Strut

atTarious Sweeps and Submergences

Figure 10 - The Flutter' Velocities of the. Low Modulus Strut

With Various Pod and Foil Configùration

Figure. 11 - The Flutter Frequencies of the Low Modulus Strut

With Various Pod and Foil Configuration

Figure 12 - The Flutter Velocities of the 20° Swept Low Modulus

Strut with Pod and Foil at Various Angles of Attack and Dihedral

o

Figure 13 - The Flutter Frequencies of' the 20 Swept Low Modulus

Strut with od and Foil at Various Angles of Attack and Dihedral

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-111-LIST 0F TABLES

Pag e

Table i - The Predicted and the Measured Flutter

Velocities and Frequencies of the Built-Up

and the Plastic Model Struts 17

Table 2 - The Flutter Test Results of the 200 Swept

Low Modulus Strut With Variöu Pod and Foil Configurations

Table 3

-

The Flutter Test Results of the 20° Swept Low

Modulus Strut with Pod and Foil at Various

Angles of Attack and Dihedral 19

Table - The Weights of he Components and the Measured

Torsional and Bending NaturalFrequenCieS

ONR 8930/1

';

,

United Stetei o Americe

Department oF the Navy

OFfiCE OF NAVAL RESEARCH, BRANCH OFFICE

Keysin House, 429 Oxford Street

London, W I,

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ABSTRACT

The results of an experimental investigation of three low modulus built-up struts with. several rigid pods and foil con-figurations are presented.

.The scaled model struts 'were constructed using a.copper alloy spine coatedwith an extremelylow modulus silicone rubber. The spine and coating of the flutter models were designed so that the elastic axis, the center of gravity, the ratio of torsional stiff-ness to bending stiffstiff-ness and the mass density match those of the aluminum Grumman No. 3 strut (k). The modulus of the model.was

reduced to scale the model flutter speed at about i/k that of the prototype. The rigid pod and foil.were hydrodynamically. similar to2the uET configuratibn (io).

The tested results of the low moduiusstrut are in good

agreement with. the scaled values from the prototype. In addition, a .series of fluttér tests were conducted to study the effects of strut sweep angle, strut submergence, pod weight and foil angle of attack and dihedral on.the flutter characteristics of the strut.

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-2-INTRODUCTION

In the past few years, the field of hydroelastiCity has attracted a great deal of interest. Investigations of hydro-foil and strut flutter have been at the core of this interest because of its fundamental importance in the success of

hydro-foil craft. The phenomenon of aircraft wing flutter is now rather well understood ad satisfactory techniques for its pre-diction do exist. Unfortunately, these same techniques are not usable for the prediction of hydrodynamiC flutter phenomena. The difficulty arises from the considerably larger density of the

fluid surrounding hydrofoils compared to that surrounding air-craft wings. For aircraft wings, the.mass of the surrounding fluid is almost inconsequential compared to the wing mass. This is not true for hydrofoils and, in fact, the added mass caused by the water surrounding a hydrofoil is usually larger than the mass of the hydrofoiL Classical flutter theory predicts that

for ratios of foil mass to added mass below a certain value, crit flutter is not possible.

However, several occurrences of flutter have been systematically observed in tests of struts with mass ratios considerably below i..L . . It is clear that

cnt.

the classical theory is inadequate. If a satisfactory technique is to be developed to predict this catastrophic phenomenon, it is crucial that experimental and theoretical investigations be conducted to observe and describe the physics of low mass ratio flutter.

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Considerable progress has .been made in the understanding of some facets of the hydrodynamic flutter phenomenon. Herr (i) has, shown that the assumption of zero f 1u1d damping eliminates the

crit asymptote at low mass ratios.

However, this assump-tion has only a minor effect on the flutrter bOundary at the high mass ratios usually studied in aircraft flutter. Thus, it ap-pears that an accurate kno.wlede of the damping is considerably more important for hyd.Doelastic problems than for aeroelatic problems. The overestimation of the overall damping ratio by the classical theory has been measured by experiments conducted by Henry (2). Abramson and Langner (3) arbitrarily shifted the phase angle of the Theodorsen function in their computation.

Their results indicate that a phase. shift of approximately e = 300 is required in order to predict the flutter speed.

On the other hand, Baird, et. al. (k,5) have shown that pre-diction of strut flutter is very dependent on the number of modes used in the analysis. This study was somewhat inconclusive since increasing the number of modes assumed first improved the agree-ment with experiagree-mental results and then weakened the comparison. Baird's computations indicated that increasing the number of

modes considered, that is within the limitation of his study, resulted in monotoniclly decreasing flutter speeds. Baird!s analysis encountered difficulty with Oonvergence of the flutter speed with increases in assumed modes. nhis study, the assumed modes were those measured for the strut in air. The in-water modes were calculated from the measured in-air modes and the

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significantly different from the in-air modes. Baird's approach to the in-water modes or use of hydrodynamic strip theory may not be satisfactory. In addition, Dugundji and Ghareeb '(7) found that the mode shapes at flutter change from a standing wave type flutter at high mass ratio p. to a traveling wave type at low p.. Thus, modal analysis may not be feasible for the prediction of flutter at low p..

One exceptionally important phenomenon is thé flutter of strut-pod-foil systems typical of modern hydrofoil craft. It is fair to say that neither the prediction of flutter nor the flut-ter modeling of such systems is well developed. Mitchell and Rauch

(8)

discovered several surprising effects during the test-ing of the strut-pod-foil systemsused on the FRESH I. For in-stance, increasing foil angle of attack was stabilizing in the case of the main foil system and destabilizing for the tail foil system. This result was unanticipated since the geometries of these systems are very similar. The analytical predictions for flutter of the tail strut proved conservative but those of the main strut failed to predict an important flutter mode. Due to the extreme complexity of the geometries and suspensions of the strut-pod-foil system involved in these tests, it does not seem possible to draw any conclusion from these results.

It thus appears essential that, the designer has.' availabl,e means of determining the condtions for flutter and divergence instabilities. In light of' the current inadequacy of prediction techniques, this can be accomplished only by tests on properly

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and funds of such model construction be small enough so that de-sign variation can readily be made when necessary.

Since October .1, 1963, HDRONAUTICS, Incorporated has been engaged in the development and testing of scaled low, modulus fluttér models. Because of the complexity of the problem, we

limited our effort on the modeling technique. We hope that a useful theory may be developed from a large collectioñ of the test data of the flutter models. At least a reliable modeling technique can be developed to solve the immediate practical

prob-lem while the existing theory is not yet dependable.

The material, developed in the first phase of this work has been reported by Ho (9). The developed material is plastic and tungsten carbide powder mixture which has the desired properties of low modulus and high density. The results of flutter tests

are in good agreement with theresults of the Grumman struts (k,5). However, this material has creep, fatigue, and aging limitations. It may not be suitable for modeling the strut-pod-foil system which is designed 'to carry a steady load. Instead of using the plastic material to model the prototype, a built-up strut which is constructed by a copper alloy coated with an extremely low modulus silicone compound was used as the scaled model. Flutter tests have been conducted on this strut, and the results agree with previous results of the plastic strut

₍₉₎

and of the Grumman strut (24.). Furthermore, a series of flutter tests were conducted to study the effects of a rigid pod and a rigid pod and foil on flutter characteristics ofthe strut.

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-6-THE MODELS

The model consists of a low modulus strut and rigid pod and fully wetted foils. The configuration was designed to be hydro-dynamically similar to the

"E"

foil configuration

(io)

currently under consideration for the PGH by Code

6363,

NAVSEC (formerly

Code k20, Bureau of Ships). The specific féatures of these com-ponents are:

1. The low modulus built-up strut was constructed by

using a copper alloy coated with. a.n extremely low modulus sili-cone rubber. The strut nose was coated byplastic to avoid lead-ing edge flutter, but the plastic wäs so slit as. to eliminate any. stiffness arising from the plastic nose. The built-up strut has almost the same external.configuration as the alurninumGrumman No. 3 strut, (k) except the sharp leading edge of the Grumman

strut was rounded to reduce the susceptibility of the leading edge to.cavitation and flow separation. .The spine and coating were designed so that the elastic axis, the center of gravity, the torsion a.nd bending moduli, the mass density, and the ratio of torsional stiffness to bending. stiffness match those of the previouslytested plastic strut (9). Dimensions of the built-up

strut are given in Figure 1. Three low modulus built-up struts have been constructed, the onewith 150 sweep. was used totest

the flutter characteristics of the strut alone. The others are 200 _{swept and vertical struts. .They are used to study the effects} of the podand foil on the .strut flutter characteristics.

HYDRONAUTICS, Inc orporat ed

-7-The pod is a rigidseries-58 body of revolution with the length of 8.7+ inches. The pod was connected to the tip, of the low modulus strut. The pöd was constructed with in-ternal holes so that a foil weight could be inserted. The re-sultant pod weight system then has the same mass and yaw and inertial properties as the pod-foil system. Details of the pod are shown in Figure 2.

The foil isa rigid, untapered section, hydrody-namically equivalent to the 'E" foil (identical lift force slope and aerodynamic center line). Variations of foil angle of

at-tack and dihedral could be made during the test. The ordinates and the properties of the pod and foil are given in Figure

3.

Photographs of strut-pod-foil combinations are shown in

Fig-ures )4 through

6.

MODEL TEST PROCEDURE

The flutter tests were conducted in the High Speed Water Channel at HYDROÑAUTICS, Incorporated. The model was attached rigidly to the supporting structure through a sway force block gauge. The gauge was used for determining flutter frequency. A view of the model and its supporting system in the channel is shown in Figure 7. The overall test procedure was simple but systematic. The model was placed in the channel test section, and the speed of the water in the channel was increased gradu-ally until flutter took place. The low modulus strut was ini-tially aligned with the direction of flow so that it had no side

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-8-force until it began to flutter. The side force sensed by the block gauge was recorded by a Honerwell Visicorder. From the Visicorder trace, one can determine the frequency of flutter when it occurs. The velocity at which flutter occurred was

ob-tained from the manometer on the channel. Motion pictures were taken from :the side and the bottom of the channel during the flutter. The flutter obtained from these struts were all con-tinuous and regular.

The

150

swept low modulus strut was tested at 10, 9, 8 and

6 inches of submergenòe out of the 12 inches. total length. The

o o

o.

sweep angle was varied from:.l0 to ₂₅ at 5 increment for each

depth of submergence. This test was conducted to study the effect of sweep and submergence depth on the. single strut. The results were used to compare the results of the plastic (9) strut and the

Grumman ('ì-) strut in orderto determine the merit of the built-up strut. The results are shown in Table 1 andFigures 8

and9.

o

The 20 swept model strut was. tested to study the effects of various pod and foil configuration on the flutter character-istics o'f the low modulus strut. The strut and pod with and with-out foil weight at the depth of submergence

varyingfrom

6 to

10 inches was first tested. The strut and pod and foilat +2e,

o o o .

o , -2 , and -k angles of attack. was then tested at a similar

range of depth. The results of these tests are given in Table 2 and Figures 10 and li. Finally, the strut submergence depth was

o o

kept at 10 inches, three dihedral angles of the foil, O , +2 and were used, . and for each dihedral the foil angle of attack. was

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VF(ditd) = VF(G

varied from +60. to O at lo increments. These results are shown in Table 3 and Figure.s 12 and 13.

Further,. the natural frequencies of both heading and tor-sion modes of the various configurations were measured in air. These results, as well as the weights of the components are given in Table 4.

RESULTS

The predicted flutter velocity V in Table i was F (predicted)

obtained from the Grumman experirnental results (k) scaled to the model tested by

[

(EI)0

_AG

LGrummanJ Adi

where EI is the bending stiffness, and A the cross section area.

(EI) of the built-up strut obtained from .the deflection

model

measurement is 572 lb-in2 . (EI) given in Reference k is

Grumman

k.k6 x ic lb-in2 . The value of (A . /A ) here is 152.

Grumman model The predicted flutter frequency was calculated by

prototype F(odel,) .(prototype)

f

V

. L

model F(prototype) (model)

where

f is the flutter frequency, and

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-10-The measured flutter velocities shown in Table 1 agree with- the predicted values within 10 percent (the measured values are on the conservative side). The measured. flutter frequencies are in good agreement with the scaled values from the.prototype. It is important to note that although the -strit flutters at 100 sweep, a large divergence occasionally occurs during flutter. Thus, the

strut appears to be close- to the intersection of flutter and di-vergence boundaries at 100 sweep. The flutter- was much more easy -, to obtain in the present test than that in the prototype since

the present test was conducted in a stationary channel rather than a towing tank which can only provide a very-short range of

-constant test speed.

Since the built-up strut does not have creep, fatigue, and aging-limitation, the structure is so strong that failure never occurred even after -severe flutter and divergence. The experi-mental data-were repeatable and were in reasonable agreement with the scaled values -from-the prototype. Thus, it was decided to use theHbuilt-up strut in the flutter-study-of the

strut-pod-foil configuration.

-As- shown i-n Figure

8,

the strut flutter velocity increases with the increase of sweep angle for each depth of submergence,

and for a given sweep the flutter velocity of the s-trut alone de-creases with increasing submergence (see Figure 10). These trends were-also reported in the calculation by-Herr (l).an-d by-Squires

(12). It can be concluded that the theoretical trends of the

ef-fects of sweep, and submergence -on the flutter -velocity of -the strut alone are in go-od agreement with the- present data.

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The effects of the rigid pod and the Í'igid foil on the strut flutter characteristics are shown in Table 2 and Figures 10 and 11. The data presented are for the strut at 200 sweep. For zero swept

strut, the strut tends to diverge (at rather low speed,

15-20fps)

rather than flutter for all configurations shown in Table 2.

o

In. Figure 10, for the 20 swept strut and the pod with .the foil weight, the flutter was observed at rather low speed, and the increase of the submergence only increases the flutter veloc-ity slightly. However, the strut and the pod without foil weight, the strut flutters at higher speeds. than that with foil weight, and .the flutter velocity increases rapidly with increasing sub-'mergence similar. to the results by Squire's (12) who computed the

flutter velocity of a tpica1.strut-pod-foil configuration. .The generalized mass. density ratio .of the strut and pod without foil weight maybe slightly under its critical mass density ratio, and an increase of the foil weight in the pod,, the generalized mass density ratio of the system may exceed its criticalvalue. The measured drastic change of flutter characteristics from the strut and pod without foil.weight to that with weight is probably due to this effect (see Figure 8 of Herr (1.)). As shown in Fig-ure _9, the flutter frequencies for the strut and the pod with and without foil.weight are all decreased with increasing sub-mergence.

For the 20° swept strut, pod and foil no flutter was ob-served at the foil zero angle of attack up to 32 fps, but the strut began to diverge at about 26 fps 'when the foil was at +2°

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-12-angle of attack. However, at -2° and angles. o.f attack the strut fluttered within the test range.. . The flutter velocity

e-creases but the flutter frequency ine-creases with the increase of submergence. It is important to note that when thefoilis at

00 _{angle of attack, the foil damping prevents the} strut from fluttering within the test range, .whilethe strut and pod with and without foil weight all flutter within the test range. For the foil at positive angles of attack, the foil static loading tends to.reduce the divergence speed in the present configuration. However, when the foil is a negative angles of attack, the foil

static loading tends to increase the strut divergence speed but decrease the strut flutter velocity. The downward force caused by. the foil.at a .negative anglé of attack results in an apparent

stiffening of the strut.. This increase in apparent stiffness causes a higher strut natural frequency. and produces the above phenomena.

From the analysis of the motion .pictflre, the flutter of the tested struts is the bending-torsion type with. a few fundamental modes. .The flutter of the strut alone is. dominated by:thefirst bending and torsion modes and the bending.amplitude is consider-ably larger than the torsion amplitude. The flutter of the strut and pod both with and without foilweight is dominated by,the second bending andthe first torsion modes,and the torsion amp-litude is larger than .the bending ampamp-litude. T.he torsion

amp-litude of the strut arid pÒd s much larger, and its bending amplitude is much smaller than that of the strut alone. . In the flutter of the strut-pod-foilsystern, the first and the second bending and the first torsion can be identified. . Sometimes the modeshape change fromthe second bending to the first bending

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-13-was observed during th.e test. The bending andtorsion ampli-tudes are equally;large in the flutter of the strut-pod-foil

system. In one or--two cases the traveling wave modes appear to be-present.

CONCLUSION

The developed low modulus built-up flutter mo-d1 offers an attractive technique -fòr the study of hydroelasticit-y- since -it does -hot have creep, fatigue and brittle limitations, and the experimental data were repeatable and were in reasonable agree-ment with the scale values from the-prototype.

The theoretical prediction of the flitter of the complete strut-pod-foil system i-s far -from satisfactory. Thus, this re-liable model, technique is of fundamental importance -for -solving the immediate practical problem while the existing theory -is -not yet dependable -and is also promising for the development of a useful theory. The -flutter' characteristics such as mode shape and frequencyin the present test can be carefully-observed. Suchobservation may-be useful for the modal type analysis.

- Within the range of the present test, the flutter velocity

- of the strut alone increases -with the increase of sweep anale

-and submergence. For the case of--the 200 swept st'ut and-pod, the strut and pod with -foil weight flutters- at lower-speeds -than that without foil weight. Inthe test of the-strut-pod-foil sys-tem., ho-flutter was observed at the foil-zero angle of attack up

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-1h-to 32 fps, but the strut began -1h-to diverge at about 26 fps when

o o o

the foil was at +2 angle of attack. However, at -2 and -4 angles of attack the strut fluttered at velocities smaller than 35 fps.

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-15-REFERENCES

Herr, Robert W., "A Study of Flutter at Low Mass Ratios With Possible Application Hydrofoils," NACA Technical Note D-831, May

96l.

Henry, Charles J., "Hydrofoil Flutter Phenomenon and Air-foil Flutter Theory," Davidson Laboratory Report R-856, September 1961.

Abramson, H. N., and Langner, C. G., "Correlation of

Various Subcavitating Hydrofoil Flutter Predictions Using Modified Oscillatory Lift and Moment Coefficients,"Southwest Research Institute Technical Report Contract No. Nobs-88599, June 196k.

k. Baird, E. F., Squires Jr., C. E. and Caporali, R. L., "An

Experimental and Theoretical Investigation of Hydrofoil Flutter," lAS Paper No.

62-55,

Presented in New York, January. 1962.

5 Baird, E. F., Squires Jr., C. E. and Caporali, R. L.,

"Investigation of Hydrofoil Flutter," Final Report, Grumman Aircraft Eng. Corp., Reort No. .DA.1O-k80-3, February 1962.

6 Peller, R., and Figueroa, L., "Experimental Investigation of Su.percavitating Hydrofoil Flutter Phenomena," General Dynamics/Convair Report GDC-63-132A August

_1963.

Dugundji J., and Ghareeb, N., "Pure Bending Flutter of a Swept Wing in a High Density, Low Speed Flow," MIT Fluid Dynamics Research Group Report No. 6k-i, March 196k.

Mitchell, L., and Rauch, F. J., Jr., "Dynamic Tests of the i/k Scale Models of the 80 Knot Transiting Strut-Foil Sys-tems for the Fresh I Hydrofoil Test Craft," Draft Report, Grumman Aircraft Eng. Corp., Report No. DA M51-239, 1 August

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-16-Ho, H. W., "The Development and Testing of Low Modulus Flutter Models of a Base-Vented Strut," HYDRONAUTICS,

In-corporated Technical Report

1459l,

May

_1965.

Curtis, E. S.., "The Static Performance of a

1/5

Scale Mödel of the BuShips E Foil Configuration," HYDRONAUTICS, Incorporated Technical Report

507-2,

August

1965.

Bisplinghofí', R. L., Ashley, H., and Haifman, R. L., Aeroelasticity, Addison-Wesley Publishing Company, Inc.,

1955.

12. Squires, C.E. Jr., "Hydrofoil Flutter - Small Sweep Angle Investigation," Final Report, Grumman Aircraft Engineering

TABLE 1

The Predicted and the Measured Flutter Velocities and

Frequencies of the Built-Up and the Plastic Model Struts

* The predicted flutter velocities and frequencies

are calculated from the Grumman's measurements

(k)

scaled

to the models tested.

Angle Sweep Depth of Built-Up Strut Plastic Strut (9) Submergence

Ines

Flutter Velocity, VF fps Flutter Frequency, f, cps Flutter Velocity, fps Flutter Frequency, f, cps 25° 10' Predicted* Flutter Measured Flutter Predicted* Measured Predicted* Flutter Measured Flutter Predicted* Measured 40 33.0 6.25 6.0 27.5 27.0 4.3 4.4 25° 9" -34.0 -6.2 -250 8" None up to 35.0 -250 6" None up to 35.0 200 10" 33 30.0 4.35 4.6 22.0 22.5 2.9

3.0

200 3k

30.5

4.35 4.8 23.1 24.0 2.9 2.8 20° 8" -31.5 -4.9 -200 6" -None up to 35.0 10" 29.2 27 3.34 3.3 22 22 3.3 3.0 l5 9" -28 -3.5 -8" -31 -3.7 -o 15 o -Noneup to 35.0 -10° lO" 26.8 25 2.32 2.0 18.0 18.7 1.5 1.6 10° 9" 28 27 2.2 2.4 19.0 20.0 1.4 1.5 100 8" -30 -2.8 -o 10 o -Noneup to35.0

-TABLE

2

The Flutter Test Results of the

200

Swept

Low Modulus

Strut With Various Pod and Foil Configurations

* No flutter for and 00 of attack. (00:

maximum speed tested,

32 fps,

+2e: 26

fps began to diverge) Depth of Submergence in Inches

Strut and Pod Without

Foil Weight

Strut and Pod With

Foil Weight Stt_P0d_F0j1*

-2°

Angle of Attack -k° Angle of Attack Flutter Velocity

Vfs

Flutter Frequency f,cps Flutter Velocity VFfps Flutter Frequency f,cps VF fps f, cps VFI fps f, cps l0 3k

5.2

16.0

7.9

30.5

5.k

28.0

5.0

9" 30

5.8

15.0

8.2

31.0

4.5

29.0

4.6

811

25.5

8.7 14.0

8.3

34.5

4.2

30.0

4.4

6'

20 10.5 14.0

9.0

None up to

35.0

-33.0

3.6

HYDRONALITICS, Incorporated

19

-TABLE ₃

The Flutter Test Results of the 200 Swept Low Modulus

Strut with Pod and Foil at Vaiious Angles of Attack and Dihedral

Foil Angle of Attack

Dihedral Angle of the, 8

o 00 e 2° e fps f, cps _VF fps f., cps _VF fps f, cps +6° None up to 32.0 - Slowly Diverges at 33.0 - Slowly Diverges at 32.0 --i-k° None up to 32.0 - Slowly Diverges at 33.0 - Slowly Diverges at 32.0 -30 None up to 32.0

- None up to 33.0 - Slowly Diverges

at 32.0

-None up to 32.0

- None up to 33.0 - Slowly Diverges

at 32.0

-+10 _{None up}

to 32.0

- None u to 33.0 - Slowly Diverges

at 32.0

-o

O None up

0

32.0

- None up to 33.0 - Slight Flutter

32.0 --1° None up to 32.0 - 31.0 6.0 32.2 6.k --2° 30.5 30.5 5.6 31.5 .6.0 30 29.0 5.2 29.0 5.2 30.0 5.6 .j+o 28.0 5.0 28.0 5.1 29.0 5.k 26.5 k.9 27.0 5.1 27.5 5.3

-6°

26.0 .8 26.0

.9

. 27.0 5.0

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-20-TABLE

The Weights of the Components and the Measured Torsional and Bending Natural Frequencies in Air

I. The Weights of the Components

Strut: 0.48 lb.

Pod: 0.55l- lb.

Foil:

0.176

lb.

II. Measured Torsional Natural Frequencies in Air

Strut and Pod without Foil Weight:

16 cps

Strut and Pod with Foil Weight: 12.8 cps Strut-Pod-Foil: 12.8 cps

III. Measured Bending Natural Frequencies in air

Strut and Pod without Foil Weight:

3.3 cps

Strut and Pod with Foil Weight: 2.9 cps Strut-Pod-Foil: 2.9 cps

HYDRONAUTICS, INCORPORATED EXTERNAL ORDINATES COPPER ALLOY Cu O/ - NI 18 O/ ZN 17 O/ X _{SILICON RUBBER} SLIT PLASTIC 0.016" LEADING EDGE J_l

T

0.20-1

1 .35" NOSE RADIUS (y = 0.009") SECTION A-A = 1.4X105 IN4 J = 5.7X 10 IN4

A.1. = 0.27 IN2 (TOTAL AREA) As = 0.1032 IN2 (SPAR AREA)

E.A. =0.69 CHORD FROM NOSE CG = 0.583 CHORD FROM NOSE FIGURE 1

-

DIMENSIONS OF THE 1/4 SCALE BUILT-UP MODEL STRUT

X >1 0

0.

0.1" 0.020" 0 2" 0 030" 0 3" 0 032" 0 4" 0 034" 0.5" 0.036" 0.6" 0.038" 0.7" 0.040" 0 8" 0 042" 0 9" 0.044" 1 0" 0 046" 1 1" 0 048" 1.2" 0.050" 1.3" 0.052" 1.4" 0.053" 1 5" 0 054" 3.0" 0.054"

/8 R _3.062

2.400

3.665

3/8 R(REF)', 3/4 BALL END MILL (TYP) I

3.437

CAVITY FOR INSERTING FOIL WEIGHT

5. 959!' 5.209 4.779 3.510 k .531 -3.343

FIGURE 2 - THE DETAILS OF THE POD

I_J;

/

'J

/E

.0938 REAM FOR DOWEL PIN (ITEM-10) SEE NOTE i

437 I I

j

II I t 4 .024 .054

CAVITY FOR INSERTING FOIL WEIGHT

11-A-.003,4T. 5.250 6.875 + .625 4.875 4-2.875 ± .001 .031

1/4 R(REF), _{1/2 BALL END MILL}

I I I I

I i I I i

I

I' NO. 36(.1065) DRILLANDIAP FOR 6-32 UNC2B X .250 DEEP (SEE NOTE 2)

1.375 i

I

4.603

2.862

4.2'

THE WEIGHT OF POD = 0.554

lb

THE WEIGHT OF FOILS

0.17611)

TOTAL

07301b

THE WEIGHT OF STRUT =

0481b

3.900"

1 .285"

-.007 (REF)

2.150"

FIGURE 3 - THE POD AND WING

HYDRONAUTICS, INCORPORATED

X

/

LOWER SURFACE

LEADING EDGE RADIUS .007

"IB' - "IB" NACA - (93175) 0.8 FOIL

FIGURE 3 - (CONCLUDED) NOSE RADIUS .0893 TAIL RADIUS .0180 STATION DISTANCE FROM NOSE SERIES 58 RADIUS 0 0 .000 1/2 .437 .283 1 .874 .390 2 1.75 .520 3 2.62 .584 4 3.50 .618 5 4.37 .626 6 5.25 .607 7 6.12 .558 8 7.00 .464 9 7.87 .299 91/2 8.3 .183 10 8.74 .000

UPPER SURFACE LOWER SURFACE

X. Y X y 0.0000 0.0000 0.0000 0.0000 0.029 0.019 0.025 0.013 0.044 0.028 0.049 0.017 0.091 0.040 0.095 0.022 0.137 0.050. 0.142, 0.025 0.184 0.058 0.189 0.028 0.277 0.071 0.282 0.031 0.371 0.081 0.374 0.034 0.557 0.096 0.560 0.039 0.744 0.104 0.746 0.041 0.931 0.107 0.931 0.042 1.116 0.104 1.118 0.041 1.302 0.094 1.304 0.037 1.488 0.076 1.4.91 0.028 1.674 0.047 1.677 0.016 1.768 0.027 1.769 0.008 1.862 0.0000 1.862 0.0000

HYDRONAUTICS, INCORPORATED

FIGURE 4 - PHOTOGRAPH

OF THE 200 SWEPT LOW MODULUS STRiJI WITH RIGID POD

FIGURE 6 - PHOTOGRAPH

OF THE VERTICAL LOW MODULUS

STRUT WITH RIGID POD AND WING

FIGURE 5 - PHOTOGRAPH

OF THE 20° SWEPT LOW MODULUS STRUT

WITH RIGID POD AND WING

A,

FIGURE 7 - A VIEW OF THE MODEL

HYDRONAUTICS, INCORPORATED

40

30

20

FIGURE 8 - THE FLUTTER VELOCITIES OF THE LOW MODULUS STRUT AT

VARIOUS SWEEPS AND SUBMERGENCES

h = 10

Lh= 9"

h= 8'

NO FLUTTER WAS OBSERVED AT h = 6" WITHIN THE TESTED SWEEP

ANGLES FOR CHANNEL VELOCITY

UP TO 35 FPS A

'

%12'1

-VF 0 50 100 15° 20° 25° 30° A SWEEP ANGLE

HYDRONAUTICS, INCORPORATED

h=10"

£ h =

9"

h = 8"

BENDING NATURAL FREQUENCY IN AIR

50 100 _{15° 20° 25° 30°}

A SWEEP ANGLE

FIGURE 9 - THE FLUTTER FREQUENCIES OF TH1E LOW MODULUS STRUT

40 30 10

- A

STRUT SWEEP ANGLE

VA= 10° £A= 15°

1

STRUT

A=20°

rAL0NE A = 250 40 30 10. A a = = = = = = STRUT SWEEP ANGLE FOIL ANGLE OF ATTACK

A A _A A

20° STRUT AND POD WITHOUT

FOIL WEIGHT

20° STRUT AND POD WITH

FOIL WEIGHT 20° STRUT-POD-FOIL a = _20 20° STRUT-POD-FOIL O 211 411 8" 10" 121 2'' 411 6'' 8" 10'' 12'' h, SUBMERGENCE LENGTH h, SUBMERGENCE LENGTH

FIGURE 10 - THE FLUTTER VELOCITIES OF THE LOW

HYDRONAUTICS, INCORPORATED V) O- u-3 10 9 8 7 6 5 4 3 2

FIGURE 11 - THE FLUTTER FREQUENCIES OF THE LOW MODULUS STRUT WITH

VARIOUS POD AND FOIL CONFIGURATION

-.

A

V

AO

STRUT SWEEP

A =

100

A =

15 = 20°

A = 25°

I

J

ANGLE STRUT ALONE 211 411 6, 8l _{loll 12"} h, SUBMERGENCE LENGTH

HYDRO NAUTICS, INCORPORATED 11 10 9 8 7 6 4 3 2

-

A = STRUT SWEEP ANGLE

a = FOIL ANGLE OF ATTACK

A = 200 STRUT AND POD WITHOUT

-

FOIL WEIGHT

S A =

200 STRUT AND POD WITH FOIL WEIGHT

-

A = 200 STRUT-POD-FOIL o

o-2

A A = 20° STRUT-POD-FOIL o a = -4 o 211 411 611 811 loll 121 h, SUBMERGENCE LENGTH FIGURE 11 - (CONCLUDED)

HYDRONAUTICS, INCORPORATED 40 -30 0 20

>

10 8, DIHEDRAL ANGLE OF THE FOIL £ +2° +4 -2° -3° 4 5 -6° -7°

a, ANGLE OF ATTACK OF THE WING

FIGURE 12 - THE FLUTTER VELOCITIES OF THE 20° SWEPT LOW MODULUS STRUT

HYDRONAUTICS, INCORPORATED 8 7 6 5 4 2

FIGURE 13 - THE FLUTTER FREQUENCIES OF THE 200 SWEPT LOW MODULUS STRUT

WITH POD AND FOIL AT VARIOUS ANGLES OF ATTACK AND DIHEDRAL

8 DIHEDRAL ANGLE OF THE WING 12" +4 I I I e VF

'///i

-BENDING

OF STRUT, POD NATURAL AND WING FREQUENCY (IN AIR) O _iO _{-2° -3°} _40 50 -8

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1

i

1

1 1

UNCLASSIFIED

curitv Classification

DD

I JAN 64

FORM 1473

D2355 UNCLA SS IFI ED

Security Classification

DOCUMENT CONTROL DATA - R&D

(Security clasalfic o of title, body -of abstraot and indoxin annotation must be entered vhen the o'erall repor is ls:f,ed,

- I. ORIGINATIN G ACTIVI"Y (Corporate author)

EYDRONATJTICS, Incorporated

Piridell School Road, Howard County, Laurel, Maryland. 20810

-FLUTTER MODEL

Za REPORT SECURITY C LA5.FlCA l'ION

Unclassified

2b. GROUP

3. REPORT TITLE

EXPERIMENTAL STUDY OF A LOW MODULQS CONFIGURATIONS

FOR STRUT-FOIL-POD

4. DESCRIPTIVE NOTES (Typa of repàet IId lnIusive dates)

Technical Report

-5. AUTHOR(S) (Last na°ñie. first name, Initial)

Huang, T. T.

6 REPORT DATE

July 1967

7e. TOTAL NO. ÒF PAGES

21

7b. NO. OF REFS 12 8e. CONTRACT OR GRANT NO.

Nonr-293(00)

b. pROJECYNO.

a.

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Technical Report -59-2

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-This document has been approved for public release and sale; its ditribution is unlimited. .

. i'

11. SUPFLEMENTARY NOTES 12. SPONSORiNG MILITARY ACTIVITY

Naval Ship Research and Development Center, Dept. of the Navy

13. ABSTRACT .

-The results of an experimental investigation of three low modulus built-up struts with several rigid pods and foil

con-figurations are presented.

The scaled model struts were constructed using a copper alloy spine coated with an extremely low modulus silicone rubber. The spine and coating of the flutter models were designed so that the elastic axis, the center of gravity, the ratio of torsional stiff-ness to bending stiffstiff-ness and the mass density match those of the aluminum Grumman No. 3 st-rut (u). The modulus of the model was re-duced to scale the model flutter speed at about l/ that of the prototype. The rigid pod and foil were hydrodynamically similar to the TE1 configuration (lo).

The tested- results of-the low modulus strut are in good agreement with the scaled values from the prototype.- In addition, a series of flutter tests were conducted to study the effects of

strut sweep-angle, strut submergence, pod weight and foil angle of attac1 and dihedral on the flutter characteristics of the strut.

14.

UNCLASSIFIED

Security Classificlation

KEY WORDS

Low Modulus Flutter Model Strut

Pod Foil

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UNCLASSIFIED

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