Note on hydrofoils for flying boats

I

Not on Hyd.rofoila for Flying Boats by

3.?. Gtt, Ph.D.

RIA.E. Ref _{ero/5..WJP./1 26}

1, Introduction

A recent M.A.E.E. report di.rects attention again to the possibility of improving the perfoni.nce. of flying boats by fitting hyd.rofoils. The purpose of this note is to1put on record. some Seaplane

Tk experience of hydrofóils 'and. to discuss its bearing on the genera].

prOblem

An R.A.E. report 2 fNovember, 1937 save a general discussion of

hyd.rofoilsand. indiated that tank tests.ere in progress.

Unfortwately,

these tests were never brought to any finality, largely because the

preliminary results wre very discouraging, and. none of thc3e closely, asociated. 'with the work i

_{no avdlablc,ut the writer is generally}

familiar vith what was done.

n extreme degree cf instability was encountered'in the tank tests

ni this has caused the 'writer to take a very pessimistic view of the

possibility bf developing hydrofoils.

Hence, the first part of this

note consists of an enumeration of difficulties which are, in fact, vxr serious,, b'4 .this is foalowed by a reconsid.eration of tke original -work of Guidoni -" which suggests that later workers have made a fundamental.

rrdstake in the arrangement 'chosen for their hydrofoils on the hull. C-uidorii clearly explainshow to arrange the hydrofàils, but he does not give his reasons very cle4riy, and this may explain why later workers do not seem to have taken his remarks seriously. The final conclusion

is that it would be rth while to make further' tank tests on hydrofoils.

2 Eeaplane Tank T'est on' Iyth'ofo±ls /

A certain nimiber of foree measurements were made on siñgl

hrdrofoils and systems of hydrofoils following the saue lines as the recent

extensive N..L.G.A. measurements

-' 5'

The speeds reached in he

Seaplane Tank prably niCver gave true cavitation but surface .cavitatiofi",

,due to air sucked down the- stiirs, was of frequent occurrence. , It is true

that this effeOt can :be largely reduced by a suitable choice of strut

section, for a fixed iinnersion- o' the hydrofoil, but in a flyingboat

tak-off the hydrofoil must ventuaJ.y rise to the surface and change ovcr from the hydrofoil condition to the planing condition, and. this change over is neither' more nor less then surface cavitation. The problem is

to make it t.ké place smoothly withOut any sudden loss oI lift. In our experience there was yOy' oftOn, if n&t alwys, some stage at which either typC of flow was possible and the thang over from hydrofoil flow tp

planing, flow was lia]le to take place sdenJ.y, the lift decreasing, often by As much as 5Cm, at the same time. Such an effect could have

catastrophic eslts ifl practice.

Similar sudden changes are found in the

'NoA.C..A test,s,'but1 their seriousness does not seem to be generally. ap)reciated.

-

. .

ROEAL AIRCRAFT ESTLISINT., FPLRNBOROUGH

/

Lab. v. Sheesbcuwid

Tecirnsc;ie

TeOhnical Note No. .Aero.1353 (Tank)

January, 1944..

a

UNCLA_{881Ff E} f

.T.N. No. Aero.1353 (Tank Jn extension of the surface cavitation effect 'was noticed.

Consider a hydrofoil running just sufficiently innersed. to prevent surface cavitation from ever occurring spontaneously in the tank. Un3ox these

conditions, if the water surface is touched just by the strut, complete

cavitation' verr often occurs ithimediately, witi corresponding lois of lift. This experiment has an obvious bearing on the fouling of the' hydrofoil

by seaweed or other floating matter and it is quite possible that a

really good design of strut may, by delaying surface cavitatibn, make the loss o,f lift due to fouling more, sudden and serious when it does occur.

ilydrotfoils for boats have been designed with sharp leading edges to cut

through floating atter

This would Olear the hydr'foil but would nt

prevent loss of lift due to 'surface cavitation.

Hence, it is clear that

the' stability of the hydrofoil system for a flying boat must be such that

a sudden loss of lift will not lead to serious consequences. 'It is perhaps

worth noting that, the amount of floating matter being fixed, danger

due to fouling would' be less for' very large hydrofoils, 'on large airz'ft,

than for small ones.

Before the force measur'erdents on hydrofoils had. progressed very

fr, an attempt was made to test system of hycirofoils on a complete flying

boat model. A"resistance model oscillated so vigorously at the higher

speeds that measurement were irrrposs±ble and. a dynamic model, after reasonable iehavior at low speeds, at .a moderate speed dived under.the water and.

broke up' so suddenly that no one Ieew what had happened. A similar result was obtained with a modified dynamic model.

These. results ilustrate

the dangers inherent 4in hythofOils, but they need not be taken,ore

seriously then that, for there is now little doubt that the system of hyd.rofqils tested was fundcxnentally unsuitable for 'a flying boato

It

consisOd of a main hydrofoil system on the forebody ahead' Of the C. G.,

with an awciliary systefri near the rear step, and was a straight copy from

an experiruental'motor boat.. It is convtnient to note here that the requirements for surface boat and. fiyin boat are' largely conflicting. The surface boat rises out of the ater but the hydrofoils remain

imnersed and should control the attitude of the boat. For a flying boat. the hydrofoil have to rise out of the water and should allow the pilot

cont±'oi'of attitude t high speeds. '

'Before leaving the "unfavourable evidence" it is worth noting

that therecent 'N.A.C.A. tests on hydrooils do not touch on the question

of stability and no.. account has been received of any tosts on a system of hydrofoils actl4aily applied to a flying boat model.

It is perhaps

siificant that a recent N.A.C.A. 'report (R No. 3F15) entitle..

1'Prelirninary tank tests on planing tail seaplane hu1ls', disnisses hydrofoils with the remark Hydrofoils in general, lead to serious

instability problems . . .

The Work Of C-uidoni . . .

The' bnly re'al evidence to. show that hydrofoils can be successfully applied to flying boats is Vo be fouid in the work of Cuid.oni, and it

is quite good. evidonce. (uidoni placed 'the main hydrofoil syntem a litt.e aft Of the C. G. and 'a smaller auxiliary hydrofoil system a long way

forward. According to his account, 'the auxiliary hydrofoil 'keeps the bows

u at low" speeds and soon comes out of the water as ,the fiyi5.g'boat

trs

, presumably on approaching the hump speed. Owing to the nose

do morneht it produccs,the main hydrofoil must not ii the hull out

of the water until the' speed. is such that the air forces take control 'of attitude. Simple 'arguments suggest t'iet"this position, for the main

hydrofoil, whih is 2irnila±' to the step pition on a. modern hull,

will give stability.

Suppose, that the flying bo.t is running. on the main

?.N. No. ero.i353 (Tank)

for ex?le, to surface cavitation occurs.

This will cause the boat to pitch to a greater engic, o increasiiig the incidence of both wing and hydrofoil, and

tending to restore the lift.

With a. suitably placed and

proportioned hydrofoil

this should givc stability, at loast against diving,

tiough a porpoise might result.

Cansider now the case of a flying boat running on a hydrofoil

piacedaheadof the c.c,

If a loss of lift ocáur

it wi].]. cause the

incidence to decrease and so reduce the lift still further.

The hydrofoil lift may even go negative with disastrous consequences.. Cuidoni

describes a hydxofoil system ahead of the C. G. a a practi.ca]. arrangement but says that, "control of the machine is not very easy", that he used

it in "only one test", and did not think it "advisable to adopt it".

With this arrangement "both landing and. take..off require the close

attention of the pilot".

This arrangement corresponds closely to the tank

models which dived, especially hen the rear hydrofoil has left the water, and as the cnditionz of tank tests tend to exaggerate the violence' of a

divergent diving instability, it is probable that the tank tests are

quite consistent with Cuidoni' a remarks.

A single hydrofoil placed ahead of the C.Gi is suggested in. the

MI A.E.E. repOit. T14s recomendation is' based on the R.AaE. 'ditching

tests of a Libcrator model with a hydrofoil placed in this position.

G.reat stress is plaoe on these experiments and it is deduced that a hyth'o.f oil wil]. increase stability. It cannot be agreed. that this is a

justifiable deduction from th cxperirçtent. A model which dived. violently stopped with an average deceleration of 6 - 8 g, was prevented from diving and stopped with a deceleration of about I g, by the use of a

lifting hydrofoil ahead of the C. C.. The hydrofoil was set at the extreme

incidence of 90 to fuselage datum to avoid any possibility of getting a,

negative incidence in the, water, and. 'it planed on the surface for thost of

the run so that no sudden loss of lift was possible. Diving during ditching is not instability in the usual sense and. decelorations of I g do not. occur in

an acceptable landing, so the' test has .little bearing on a nomial landing

and still less on a take-off.

General_ëmarks on.Hy&rofdils

Consider a flying boat equipped with a system of hydx'o foils conClsting of a main hydrofoil (or hydrofoil system) and n auxiliary hydrofoil either

placed ahead like a. "tail-first" aeroplane in the manner of Guidorii, or placed aft like a conventional aeroplane. Divide the take-off into

three stages as follows

::-(i ' Low speeds. Hull and. hyd.rofoils imnersed.

(2 Medium speeds. Hull' clear of water. }J.ydrofoils deeply irnnrscd..

(3 ' High speeds. Hydrofoils aroaching surface or' planing on surface.

These stages 'are convenient for discussion but do not necessarily represent the sequence of events.

Serious stability prcblems do not arise in stage (1).. Stage (2).

represents the top speed. of a hydrofoil .sürace boat and. would occur in. a

flying boat designed. to be lifted on to its hydrofoils at a low speed. Stability in thiacond.ition should. be attainable on the same lines as the stability of an ceroplane, whethel' the arrangement be "taii-fist" or

oonitithaJ

and, aft

the flrst tank model

& eraed,

tns

of that kind were 'involved, in the design ofthe second.. . Such considorat

gra&ally oase to' aply 'in stage (3) whçn surface effects occur and

fil conlDlctely when an auxiliary hydrofoil, placed aft, comes out of the

water. It has already been indicated that the !'tail_first" arrangement

jives' the best prospect of stability when' surface' cavitation _occurs.

-3-I. No

There is no danger of diving when an

axi]4ai7 hydofpil

placed.

forward comes out of the water .nd the afterbody of the hull will óheckt90 great an irIceaSc

in attitue.

In C.uidoni-' s seaplane s the au?dliaY hydrofoil. seems to have cthne

out of the vter in

stage (1) and a modlfieL st.ge (2) was delayed

until the air forces

could control attitude ±Icioe stability when

±'unningbn two hydro±'oilS may not have been necessaxy.

Guidoni claimed that hydrofoils gave more

stility aM were

better' in rough water than the usual seaplane float or flying boat hull. There is no reason for doubting these state!peflts relative to the flying

boat hulls of hi. day, but that dpe not say that hydrofoila would

compare favourab]-Y with the best modern flying boats. Again, rough

water is a vely

indefinite.t0I.

In what degre of rough water did

Gw.donJ- oprate Thus the claim that hydrofo1l iill iirprovc stability

seems to be' quit.e hypothetical .bu,t there seems to 'be good reasori

fo'

suposi.n,.. that they could be designed to give sufficient

stability.

Thus

it apears that

it would be worth while to.make further tank tests with

the object f developing an arrangement of hydrofoi5 which .vuld ve

stability on a flying

boat model and. it is recomnmend.ed. that this should

be done. It might reqyire a l'engthy investigati9fl

to sort out the

various factors roughly outlined abov and thc work should only be

under-taken on a fairly long teim basis.

ooclusiofl to be drawn from the recent M.A.E.E. report is really the same as was drawn from the

earlier R.A.E. report.

that

hydrof oils will have substantial advantags

if the stability

problem can be solvod Both reports treat the hydrof Oil from the .poirit of view of som

1zl)rovemen .t

th concntioa1al flyng

boat hull and the improvfleflt

clô.imcd 13 ccnTparat]VelY modest. !nether vie' been expressed from

time to .tirae; that to meko hydrofOils worth while they should lift

the' hull but of the water at a low sDeed,, so that a completelY strCFUfll ined

fuselage, without either steps or chines, could be used.

In a certain

sense this is

what. Guid.oni Eichieved., for. he regarded hydofOil5 as an

alterrmativG to stOps andchinCS and. his

floats were. diiCdbO.

provide buoyancy only. They were usually straight circular cylinders with crude

bows and stern attached. Sti-eainhined. shapes, with'hydrofoil attached,. t'end to suck down into the water to such an extent as to. neutrali8e the

lift of the

'hydrofoil.

It

.s inteiestifl3 to note that Guidoni' s straight

floats would. avoid

this trouble,

while the J.oi wing loadings of his

tiic

probably helped to lift the Doats out

of the rtcr at a

low enough speed. to keep spray dpwn, so that the absence of chine would not be serious.

In any furtherSwait n hydrofoils the extreme view of a .streall3].ifled

'flying "boat without either steps or chines should not be wholly disregarded'

It is a vexy long step from the scalanes of Guidoni' a

tine to the

flying

boat .of to-day but-his cylindrical hydrofoil floats. would make ax exceflent Shape for a pressuri3ed flying boat and. so would meet an

objectiOL to the convertiOflal ,.flyiig boat hull.

ReferCflCe

No. .Aut Title

1' Naylor and'

The t'akeoff and landing of a flying boat with a

Smith

hydrofoil. 1/LA..

repoxt No WRc..i.69, NovezrCr, 191.3. '

T.N0 No. Aero.1353(Tank)

Cooithes and Note on the ossihI1itf of fitting k'rdrofoils

Davies ti a flying boat. hull.' R.A.E. Report No. BA.114O

November, 1937 .(337,s358)..

3 Guidoni ' "Fifteen years of naval aviation. Journ.' RyAero.

Soc. 19284

Waid.and. Lnd Preliminary tests in. the N.A.C.A. tank to

invcstigate the Fundamental Characteristics of

HydrofOils. N.A.C.A. Confidential report _{(5223,s..)49)..}

Benson and Land. .tnInvestigation of Hy&rof.oils in the N.A.C.L Tank. I. '-.Effect of.Dihedral and Depth of Sifon3ersion.

N..A.C.A. Confidential Report _(6t.89, S.L.71f)..

6 Land. Characteristics of an N.A2C.A. _66, S.209 section

hydrofoil at several depths.

N.A.C.A.. Confidential Report. .

a.cPh.il arid Model tests of the alighting o landp].anes on the

Ross sea,

P.rt 3,

. Liberator. R.E.RLIport Icz. Aro.i77O.

Circulation C':,

.I.

. D.D.E.h.1. . J1D./R.D.T. A.D./R.DaT.1. . . R.D.T.'1(c) . .' ..'

A.D./R.D. S. (ct.on copy) -. .

M.A.E.. -. T.D.A&nin.(2 + i). R.T.P.2 (12) ' : Director ' ' . D.D.R.E. S.M... Lib rary . . T/A . . :Aero. (1) . ' . Aero. .(T) .(2) . j. : File. /