naca tn 215 the calculation of wing float displacement in single-float seaplanes

7/27/2019 NACA TN 215 the Calculation of Wing Float Displacement in Single-float Seaplanes

1/6

TECErJ IC1>.L NOTES

rAT IONAL. DVISORY COMMITTEE FOR AERONAUTICS.

No. 215

THE CALCULATION OF WING FLOAT DISPLACEMENTIN SI TG E-FLOAT SEAPLANES.

By Edward P. ~ a r n e r ,~ t a s s a c h u s e t t s In s t i tu te of Technology.

March , 1925.


2/6

NATIOIAL ADVISOR! OO}.1HITTEE FOR AEROTAUTICS.TEC':1NICAL TOTE NO. 215.

THE CALOULATION OF WING FLOAT DISPLACEMENTIN SI1JGLE-FlOAT SEAPLAlJES .

By Edward P. '1arner.

The l a t e ra l s tabi l i ty of a l l s ingle- f loa t seaplanes andf ly ing boats except those of the Dornier type with l a t e ra l extensions on the hul l i t se l f must be assured by the use of auxi l iary f loats a t the wing t i p s these f loats being mounted highenough so tha t they make contact wi th t he water only when th eseaplane is heeled over to one side or the other and so that only one of the wing f loats can be in contact with the water a t agiv en time.

In calculat ing the proper size for such wing f loa ts , thes tabi l i ty of the main centra l f loat i s often entirely neglected,the posi t ion of center of buoyancy being assumed independent ofthe angle of incl inat ion . The weight carried by one of the aux-i l ia ry f loats in s t i l l ~ a t e r and s t i l l a ir can then be calcu-lated direct ly by takin moments around the center of buoyancywith the nachine in the inclined posi t ion, and the to ta l volumewhich the wing f loats should have can be calculated i f i t i s assumed tha t a fixed reserve of buoyancy i s desi rable there , justas i t i s for the main f loat . I t i s then necessary only to mul-t ip ly the we i gh t carried by the wing f loa t under ideal conditions

,J"


3/6

[ . A ~ C . A . Technical Note No . 215 2by a fixed constant to find the desirable to ta l displacement.The applicat ion of this me thod has been explained in deta i l withcalculat ions for a number of specific examples, in a recent pub-l icat ion.(Rcference 1).

The use of a fixed reserve of buoyancy has several possibledisadvantages in cer tain s ~ e c i a l cases. In the f i r s t place, i ti s l iable to lead to incorrect resu l t s for machines with a verylow center of gravi ty , such as f lying f lonts . The ins tab i l i tyof the main f loa t a lone is small in such cases, and even a 100percent reserve may represent a very small absolute addit ion tothe size of the wing f loat . I t wil l be observed tha t the re-se rve buoyancy of the wing f loats is exceptionally large on mostf lyin g boats which have proven sat is factory in actual service.On the NC , for example, the reserve was 490 percent. A methodwhich would give a c o n 8 t ~ n t independ ent of size and height ofc . g . would obviously be desirable . Second, and by similar rea-soning , the use of a constant reserve of buoyancy takes no ac-count of the stab i l i t y of the main f loa t . It is conceivablethat two dif feren t f loats might be designed for a given machine,and that the ne gat ive l a te ra l mctacentric height with one ofthem would be 3 fee t , while with the other i t would be only 3inches. Obviously, i f t he centra l f loa t i t se l f has a consider-abl e measure of s tabi l i ty the size of the wing f loa t s can be re-duced. Third, the method involving a constant reserve of buoy-ancy cannot be applied a t a l l in those cases where there i s a


4/6

T.A.C.A. T e c h ~ i c a l No te rO . 315 3small posit iv e metacentric height. A hull of the Dornier type,for example, might b e so desi gn ed "as to locate the metacenter afew inches above the c. g . There would then be no weight car-ried on the wing f loat when the seaplane was at res t under idealconditions, as the machine would f loat on an even keel, but,nevertheless, wing f loats would actually be required under serv-ice conditions to provide a suff icient ly large reserve of s ta-bi l i ty .

In seeking another me t hod i t seems lo gical to turn direct lyto the concept of metaeentric height , always used in investi-gating the s tab i l i ty of a single floating object. ~ e n i t issaid that a twin f loat seaplane has a l a te ra l metacentric h e i g ~ tof 12 feet the implicQtion i s that t he oenter of gravity couldbe raised 12 feet from i t s actual posit ion before the machinewould become la tera l ly unstable in s t i l l water a nd would fa i l toreturn to i t s original posit ion of equilibrium i f s l i ght ly dis-turbed therefrom. A f ic t i t ious raising of the center of gravitymay be used in an analo gous manner in determining the size of thewing f loat . Instead of s tat ing that such a f loat must have 100%reserve of buoyancy, i t may be specified simply that the f loatmust be la rge enough so that i t would hold the wing out of wateri f the center of gravity were raised x feet from i t s presentposi t ion, and that procedure overcomes a l l of the principal ob-jections jus t stated against the other method.

Obviously, the height by which i t should be possible to


5/6

l AC.A. Technical Note N o ~ 215 4raise the center of gravity should bear some defini te re la t ion tothe desirable metacent r ic height of a twin-float system. Diehlhas shown (Reference 1) that present pract ice endorses the useof twin-floats so set that thc transverse metacontric height i sequal to 13 + . 002 W, where W is the to ta l weight of the a i r -plane in pounds . Manifestly, however, i t would be unnecessaryto allow for ra is ing the c .g . by that fu l l amount whcn auxi l iaryf loa ts are employed, as the auxi l iary f loats are normally out ofthe water and the incl inin g moments when running through wavesare therefore much less than those aris ing vlhen there are twomain f loats of equal size continually carrying the seaplane.Comparison of the charac ter is t ics for exist ing seanlanes, simi-la r to the compar i son made by Diehl in deriving h is constants ,suggests that on the average the center of gravity should beassumed raised by abou t one-half of the desirable metacentricheight as givcn by Diehl 's formula, but tha t the exact rat ioshould be a funct ion of the angle of incl inat ion to submerge thewing f loat completely and tha t i t should vary l ineal ly from avalue of .8 a t 30 incl inat ion down to one of .2 a t 12 0 , remain-ing constant for angles of incl inat ion higher than that . Fig . 1shows the nature of the variat ion.


6/6

N. A. C. A. Techni c[l,l No to No . 2i5 5

- - -j.8 I- } - - - - - + - - ~ . _ _ r

- I - - - - ~ - -. 6 - ~ +-I

~ . - -

Fig. 1

The formula for the displacement of each wing f loa t thenbecomes:

= W[GM + k (13 + .002 W)Jtans

where ~ i s t h e to ta l buoyancy of a wing f loa t , Vi the wei ghtof the seaplane, GM i t s negative metacentric hei gh t, k thecoeff ic ient plot ted in Fig . 1, :l> the angle of t i l t to submergea wing f loa t , and s the distance from each wing f loat to theplane of symmetry of the seaplane .

Reference1. W. S. Diehl: Stat ic Stabi l i ty of Seaplane Floats and Hulls.N.A.C .A. Techni cal Note No. 183. 1924.

naca tn 215 the calculation of wing float displacement in single-float seaplanes

Documents