national advisory committee for aeronautics no. 638. …

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NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS

NO. 638.

TANK TESTS OF E(ODEL 36 BLYSNQ-BOAT'HULL

By John M. Allison Langley Memorial Aeronautical Laboratory

r 1 TBa.phiru$ on March 1938

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RATIONAL ADVISORY COMMITTEE KOR AERONAUTICS

TECHNICAL NOTE NO. 638

TANK TESTS OF MODEL 36 FLYING-BOAT HULL

By John M. Allison

SUMMARY

N.A.C.A. model 36, a hull form with parallel middle body for half the length of the forebody and designed particularly for use with stub wfnga, was tested according to the general fixed-trim method over the range of practical loads, trims, and speeds. It was also tested free to trim with the center of gravity at two different positions. The results are given in the form of nondimensional coefficients.

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The resistance at the hump was exceptionally low but, at high planing speeds, afterbody interference made the performance only mediocre.

INTRODUCTION

Model 36 was designed and built for use in an inves- tigation of the water performance of stub wings. A rather small beam was used, because the stub wings were expected to take part of tho load at low speads. The model was made with a flat deck and the sides of the hull were made parallel forward of the step for half the length of the forobody, in order that tho stub wings might be fitted to tho sidos of the hull by merely squaring the root ends. A hull with parallel sides near amidships has a further ad- vantage from the standpoint of the designer of a transport flying boat because it enables him to maintain the maximum seat and aisle width over the greater portion of the cabin length.

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The model was included with several others sent to the propeller-research tunnel for aor-odynamic tests. The results of these tests (reference 1) showed that the air I resistance throughout a wide range of angles of attack was fairly good, as compared with two other models with flat decks. The hull was not tested with the stub wings attached.

2 N.A.C.A. Totihnical Note No. 638

After publication of the,wind-tunnel tests, requests fof.the results of the hydrodynamic tests were received by the Committee. The tosts were subsequently made of the hull without the stub wings.

DESCRIPTION OF MODDL

The principal linoa of the modol are shown in figuro 1, and tho offsets are given in table I. The mo;iel TX,~ shaped from a horizontally laminated shell of pine and was finished .in gray enamel, wet-sandad and polished to give a smooth.surface. Tolerances offsets bo3.oT the chine.

The particulars of

Length: Over-all'. . . . , To second step . . . Forebody . . . . . ,

Maximum beam . , . . .

D-ead rise at step . . .

Angle of afterbody keel

Angle of tail extension

Center of moments above

the model.are as follower

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100.00 in, -80.00 in.

50.00 in.

14.00 in.

2o"

6-l/4'

loo

14.06 in.

10.00 in.

0.56 in.

of ho.32 inch were- held on all

keel at step .

Centor of momants forward of stop . . . .

Dopth of main stop . . . . . . : . . .

Forebody: Percentage of over-all length Percentage of length to second:itip'

Beam: Percentage of over-all length . Percentage oT IengXh to second.itop' Pmertientage- of fo-rebody length . . . ,

.50.0 62.5

14.0 i7.5 28.0

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N.A.C.A. Technical;' Note.No. 638 3

Center of moments above keel at step: Percentage of over-all-length ......... 14.1 Percentage of length to second step ...... 1'7.6 Percentage of forobody longth ......... 28.2

Conter of moments forward of step: Percentage of over-all length . . . . . . . . . 10.0 Percentage of length to socund step i .', . . . 12..5 Percentage of forcbody Pongth . . . , . . . . . 20.0

Depth of step, porcontago of beam . ! , ! . . . . .4.0 -.

The hull has a long straight forebody keel and a chine flare on the forebody only. The angle of afterbody keel and the depth of step are in accordance with N.A.C.A. design practice on somewhat similar forms.

APPARATUS AND METHODS

The N.A.C.A. tank and its original.equipment are described in reference 2. The suspension of the model and the method of measuring the trimming moment have since been altered as described in reference 3. .

The test program included both' general fixed-trim and specific free-to-trim tests. In the general fixed- trim test, the modol is towed at constant speed and trim while the load is varied to cover the useful range. Suf- ficient trims are investigated to determine the minimum resistance at all speeds and the resistance at zero trim-

:ming moment at low speeds.

In the specific free-to-trim test, the gross load of tha modol corresponds to a reasonable gross-load coeffi- ciont for a prosont-day transport flying boat. A cali- brated hydrofoil simulates the lift of the wing at a constant angle of attac,k and is set to make the model leave the water at a speed corresponding to the take-off speed of the assumed flying boat. -.---

The model is ba_l‘ande&d“about the center of moments so that this' p'oint becomes' the center .a?' grtivity -arid-'The trim

' is influenced only by the water and air forces 'acting.. The trim of the full-size flying boat, however, is affected by aerodynamic moments not simulated in the-test set-up.

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4 N.A.C.A. Technical Note.N0..638

RESULTS .

The nondimensional coefficients used in the presenta- tion of the data are as follows:

L04a coefficient, *CA = A/wb=

Resistance coefficient, CR = R/wb3

. Speed coefficient, v /d-T .

cp =

.Draft .coofficiont, Cd = d/b

,,Rise cooffie5ont, C, = r/b

Trimming-momont coofficiont, CM = M/wb4

whcro A

W',

is the load on tho w;iLter, lb.

specific weight of the water, lb./cu. ft. (65,2 for these'tests).

:, beam.of hull, ft.

. . water resistance, lb.

speed, ft./set. .

trimming moment, lb.-ft.

acceleration of grav.ity, ft./sec.a

draft at main step, ft.

rise of the center of gravity of the model, ft.

The data for the fixed-trim test are presented in figures 2 to 7; resistance coefficient CR, trimming- moment coeffic'ient CM, and draft coefficient Cd are

plotted against speed coefficient Cv with load cooffi- Cient CA as a paramotor. The center of momenta was that shown in figure 1. ,:

The characteristics of the model at best trim wore obtai%ned by cross-plotting resist,ance coefficient and trimming-moment coefficient against trim at selected

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N.A.C.A. Tochriical Note NP; 638 5

values of speed coafficiont, with load coof.ficient as a paramotor. .Minimum resistance coefficient, best trim (trim for minimum resistance), and trimming-moment coefficient at best trim were determined for each speed coefficient. Three of these cro8s plots are shown in figuro .8. Resistance coefficient and trimming-moment coefficient at boat trim are plotted in figures 9 and 10, r3spectively; best trim is plotted against speed coefficient in figure 11.

Resistance coefficient CR is plotted against CA with Cv as a parameter,. in figures 12 and 13 for free ,' to trim and best trim, respectively, The free-to-trim ' curves of figure 12 are supplemented by a plot of trim against speed coefficient with load coefficient as a parameter for convenience in determining.the,trim at low sp'eods. Figures 12 and 13 are useful in making take-off- time tind distance calculations, -.-

The results of the specific free-to-trim test are presented in nondimensional form in figures14 and 15. Figure 16 shows the effect upon resistance and trim of moving th.e center of gravity nearer the step.

Trimming-moment coeffi&ient and draft coefficient at rest are plotted in figures 17 and 18, respectiv&ly; These curves are useful in calculating longitudin&i sta- bility and in determining water-lines of the hull for various static conditions. . .'.'

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DISCUSSION

Resistance characteristics.- The hump resistance of model 36, as determined from the curves of CR against cvs based upon best trim (fig. 9) is exceptionally low, The fact that the nondimensional coefficiedts u’z&dmWere based,upon the beam alone makes it difficult to compare fairly the hull forms of quite.different length-beam ra- tip, because the longer hull will have a greater bottom area for a given boam and will therefore be able to-carry larger loads at-the hump; The resistance at planing speeds is not so favorable as,that at the hump, but it __ compares well with successful' American hulls.

A comparison of figure 14 with figure 9 shows that;

6 N:A.C.A. Technical Note No..638

with the center of gravity 10 inches forward of the step, the hump resistance, free'to,trim, was about 21,percent higher than that'corrosponding to best trim and the trim was 2.70 greater than best trim, Trim at high speeds was much too low. The position of the center of gravity was therefore moved 3-l/8 inches aft.to a point 6-7/8 inches forward of the step in order to dotermino the affect on frdo-to;trim characteristics. Fimmo 16 shows how tho ro- sistance at high speeds was reduce.d by the change, The hump resistance was, however, almost 40 percent higher than that for best trim, and the trim was 4.30 abovo best trim. The thrust momont and elevator-control moment would have to be taken into consideration boforo a final rocom- mondation as to tho posttion of the contor of gravity could bo mado.

gorpoising.- Porpoising was encountered at about 20 foot por second in the tosts of tho model havfng tho center of gravity 10 inches forward of tho stop. In ordor to measure resistance in the porpo.ising rogion, it was nocossary to apply hoavy damping in pitch. With the conter of gravity 6-7/8.inches forward of tho step, thoro was but littlo tondoncy to porpofse. It should bo notod that the model sot-up is not dynamically sfmilar to full- scale conditions and any conclusions as to whether or not porpoising will occur at full-scale should be made with caution.

Trimming-mOmQnt characteristics.- Tho curvoi of trin- ming-moment coefficient at best trim against speed coefficient (fig. 10) show that, with the hull heavily loaded, large negative moments ar.e produced at low speeds and large positive moments are oncounterod just above hump spoed. At spoods above ,Cv = 4.0, tho moments havo do- crossed to such an oxtent that they may bo counteracted with the olevators.

Spray charactoristics.- Photographs of the modol running froo 'to trim at low spoods aro shown in figuro 19. In figures 19(a) and 19(b); at a spoed just below hump speed, the stern is riding heavily, fn tho wator and throm- ing up a roach. In figuros 19(c) and 19(d), at a spood just above hump spoed, tho pointed afterbody is just touch- ing the water, but thQ tail oxtension is clear. Figures 19(a) and 19(f) show the model running at a slightly higher speed; the tail extension and the pointed end of the afterbody are n-ow both clear of the water, and planing of tha

N.A.C,A. Technical Note No. 638 7

forebody has been established. The trfm shown in the photographs is from 2 o to 3O greater than best trim.

Photographs of the model running fixed in trim.aro shown in ffguros 20 and 21. Hump conditions wfth heavy load are shown in figures 20(a) and 20(b). The trim is slightly abovo best trim. Hump conditions with a smallor load arc shown in figures 20(c) and 20(d) with trim -slightly undor best trim. (See fig. 8.) At the hump, even with a very heavy load, the bow wave is not thrown high enough to be objectionable. In figures 20(e) and 20(f) , the model is running near best trim with planing well established. The pictures show water striktng the afterbody, indicatfng that there fs some afterbody interference. In figures 21(a) and 21(b), the model fs shown running at a highor planing speed. It will bo noted by comparing ffguros 21(b) and 20(f) that afterbody interference is worse at the higher speeds and lfghter loads.

Comparison with model 35A.- In figure 22, the load- resistance ratios of models 36 and 35A (reference 4) are compared. L/b

Both these models have high longth-beam ratios: of model 36 (takfng-- L as the distance from F. P. to

the sternpost at the end of the afterbody) is 5.70 and L/b of model 35A is 6.15. Inasmuch as their length-beam ratios are of the same order, the models may be compared fairly on the basis of the nondimensional coefficients based upon beam alon. The load-resistance ratios of ffg- uro 22, taken at three speeds, with a wide range in loads, show that model 36 is hotter at the hump for the heavier loads and model 36A is better fn the planing speed Tanges. The superiority of model 35A at high speeds is due to the better clearance of the afterbody, especially at light loads.

CONCLUDING REMARKS

The form of model 36 has many of the characteristics favorable for low hump resistance: rather small dead rise; moderate angle of afterbody keel; moderate depth of step; long, straight forebody undersurface; and high length-Beam ratio. Several of these features, involving the posftion of the afterbody with respect to the forebody, affect the resistance at high speed advorsoly when thoy improve it at hump Speed. Good all-round performance depends upon ad- justing the various factors until a satisfactory compromfse

8 N.A.C.A. Technical Note No, 638

is reached. Each flying-boat design requires a different compromise. If the total air-plus-water resistance of a contemplated design using tho hull form of model 36 gfves a critical condition of OXCQSS thrust at high spoods with a large amount of excess thrust at hump .SpQQd, thon the aftorbody clcnranco could be increased to improve high- speed porformnnco at the oxpcnso,df hump-SpQQd performance.

Modol 36, in common 'with most conventfonal hulls, has a tendency to trim higher than best trim at the hump, for practical positions of the center of gravity. Unpub- lished skoleton tests of the model with stub wings show that t,he stubs act'to roduco the trim and the sproad be- twoon free to trim and best trim. Furthor tosts with various stubs and stub posi'tions are contomplatod.

Langley Memorial Aeronautical Laboratory, , National Advisory Committee for Aeronautics,

Langley Field, Va., January 26, 1938.

REFERENCES

1. Hartman, Edwin P.: Tho Aorodynnmic Drag of Flying-Boat Hull ModQl6 as Moasurod in tho N.A.C.A. 20-Foot Wind TUnnQl - I. T.N. No. 525, N.A.C.A., 1935.

2. Truscott-, Starr: Tho N.A.C.A. Tank - A High-Speed Tow- ing Basin for Tosting Models ,of,.Soaplano.Floats, T.R. No. 470, N.A.C.A., 1933.

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3. Allison, John M.: Tank Tosts of a'Modo1 of the Hull Of thQ Navy PB-1 Flying Boat - N.A.C.A. Mod01 52. T+N. No. 576, N.A.C.A., 1936.

4. Dawson, John R.: Tank Tests of Throo Modo'Ls of Flying- Boat Hulls of tho Pointed-Step Typo with Dffforont Angles of Dead Riso - N.A.C,d. Model 35 Sorios. T,N. No. 551, N.A.C.A., 1936.

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Station

-- F.?. 114 112 1-

l-112 2-

z 5-

to 10,F lO&- 11 12 - I. 1 2 15- 16,F 16,A 17 18 - 19 20

Distmct from F.P.

0.W 1.25 2.50

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20.00 -‘25.00‘

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g: - $ci&

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_ g5.@Je go.co 95.00

100.00

TABLE1

offsets for M.A.c.A. Model 36 Plying-Boat ~u.ll (Inches)

Distance below base line

, I

Half-breadths

Keel !&gem of bott

flare - 4.50

10.65 e.oo ll.51

;*96; z2 -10:44 A50 10.77 __

12.50 10.82

11.94 11.39

__ - 10.85 LO.30

-.;*:z 8:66

5- r 4. 2

z;Dper I Tangency Main Chine of bottom chine

flare

0.15 I*33 l-55 2.08 2.62

-- 3*13 4.s

4.75 7.00

7.41 6.55 5-75

2:;: 3.78

3.25 2.76

r ::$I I

Cove Upper Rdius of chine bottom

-//

flare

0.69 1.30 2.34- 3.13

it :i;- 4.q -

5.00

r .

HalfbreaUth

T.PF i.L a-3

14.06"

.

/L---- 60.00’ I

+o.ol!s , Deck and ham line

11 12 j 14

PP- ohi

m

0.60’ 30.00” - m.ow-

1 F 100.00' q

Pl!ofils

figluel. -xodsl33. Prinolpsl llMs Ed dzlmaMions.

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X.A.O.A. reohnical mote 80.638 rig.2

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rigur0 a. -Hode136. Reslatsnos, triwaing-wmsnt, uxl draft ooeffioisntr, f - 3O

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1 a a 4 6 6 7 6 Qmed 00effifoiant, a,

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Tlgurr 3. - Yodel 20. Roalatsnos, trimni~-mamat, olrd draft ooeffloientp 7. 3'

P.A.O.A. Tsoimio~l Iote B0.638 TIC.4

0

31.a

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spee-d oorffloient, qr

Hodrlz4. Berirtuwe, trlllming-monmt, anl draft oorffioirntr, f - 7'

I.A.O.A. TsohuioU Pot8 YO. 688 riga .J5,6

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rigurr 5.- Yodel 'AS. Rmirtmor, trimaing-moimit, ti draft oorffioientr. T - 90

Tigure 6. - Model 38. Rwlatanor. trltiq-sownt, and draft 00sff10irntr.r - 11

I.A.O.A. TwhiWl lOtO x0. 689 ngmi7.u

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rigura 9. -xode1a6. Yrrlation 0: a, dth h at beet trim.

P.A.O.A. Toohnioal Yoto Y0.638

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- 1 _a _ 3 4 6 I --.. - speed 00013101011~, uv I I

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I I I I I I I I i I .a .4 .8 .a 1.0

LOM ooeffioient, Od

rigure ia.- Yodel 36. Ohart for detomiai 3

rrai*tums ad trim at rioro trimming aoment. 10 inohrn formard of tU #top.

rntrr of gravity

I.A.O.A. Teohnioel Iota IO. 038 Tlg.13

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TLgare 13. -Yodel 36. Ohut for dsterainlng rs#intaaoe rt beat trim.

I.A.O.A.Todmbal Ioto X0. 634 Slg.14

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8: . g.08

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go4 m

16 t I I I I I I I I I I I I

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ngtue 14. -xodel36. Renilts 0f n pe0ifi0 tent at sa.7 pounlm hitid load, osntsr of gravity 10 inches formad of the step.

I.A.O.A. TrotiOrl JOtO x0.688 rig. 18

1

8 P 1

spesd 00dfi01&, 0, -

rigue 16. -Modr13e8. Retits 0f apOpfiO tee at 6a.7 pcrrala iattid loti, oatsr of gravity B/ginohss forward Of ths **OP.

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11.6.O.A. Tbobnioal Botm 10.838 Figm.16,17

-a . .

- B

-m % .

0

!

I.A.O.A. TaoohPiobl HOtb PO.6a8

.s

I t I I I I I I I Trim, T ,da&$

/St I

.a

.1

0 .a -4 A .8 1.0 1.8 1.4 1.6 LOad OO~ffiOie~t, OA

Figure 18. - Yodel 30. Draft ooeffioientm rt red.

“I Yodel 38

7 \ ---Yodel 3%

6

a I

.ii

siga a., aa

sip23 aa. bdai 36. %%in.

lmon of load-reaistaaoe ratios of modelm 38 ~IU 3SA, at

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N.A.C.A. Technioal Note No.638 Fig. 19

(a) ov = 8.3, 7 = ?.?O,

(b) c, = 0.54.

T = 8X0, (d)

c* = 0.50 I

.

l

(0) % = 2.9, I- = 7.8g,

Figure 19. - Model 36. Spray photograph@, free-to-trim. Oenter of gravity 10 inohes forward of the step.

N.A.O.A. Teohnioal Note No.638 Fig. 20

(b) 0, = L.2 l

b) % = 4.22 , 7 = 5O,

Figure 20. - Model 36. Spray photographs, fixad trim

N.A.C.A. Teohnioal Note No.638 Fig. 21

.

l

(0) (d) ov = 5.74, 0 7=5, OA = 0.4.

Figure 21. - Model 36. Spray photographs, fixed trim, high epeeda .

national advisory committee for aeronautics no. 638. …

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