The coaxial helicopter vibration reduction

Pnpcr N° 91

E · 16

THE

COAXIAL

HELICOPTER

VIBRATION

REDUCTION.

DR. B.N.BuRTSEV,

KAMOY HELICOPTER SCIENTIFIC

&

TECHNOLOGY COMPANY,

RUSSIA.

ABSTRACT

Theoretical and flight test results are presented. I t is shown that substantial reduction of alternating

forces acting on a helicopter

structure

and

reduction

of

vibration level are achieved by selection of

optimum configui~ation.

1.

INTRODUCTION

coaxial rotor

The problem of the helicopter vibration level reduction

i s most i)'l'i.portant 'loday. A l o t of technical appr-oaches has

been adopted lor the saJne as:

-aet~oelastic:. blade design;

-pendulum type vibJ~ation absorbe1~s for the blad~ root sect.ion;

-vibroisolali1·1g n~in r-otor and reduction gear mounts;

-active vibratiot1 C HHC , I BC ) •

level

reduction systems

The results of investigaliort in the field of tl1e

vibration level reduction, HHC and IBC systenlS in

particular, were published in many p-apers and contain

analyt.ical models, wind tunnel and flight lest data [ 2-14l. Tl·1e presented paper is devoted to tl·1e formulation ar1d

solut-ion of lbt! coaxial helicopter· vibt~atiorl lev~l r~duction

problem.

2. THE

FORMULATim~

OF

THE

PROBLH~

At fig.1 there are plotl8d alle1··r1alirlg foJ~ces acti.r1g

upon Lilt.-' coaxial rotors hubsf transmitlt::c-d via Lht~ m..-1i r1

rotors and the r'eductor g~ar· shafts and causing tht?

helicoplt~T vibr'alion. For· the sake tJf simplicity let us

examirte only lhf2' vertical .allt~r-nating for·ces:. upon lhe;o uppt.::or·

frequency 3w and corresponding phases l'u and 'l'L. At fig.1 there are presented the location of the blades in respect to each other, the direction of the upper and lower rotor rotation as well as the lower rotor No.1 blade azimuth

<hu

=

wt and the upper rotor No.1 azimuth 1'1U

=

wt + <f>

Some 25 years ago our engineers fol'mulated a simple but rather goog idea by changing the difference of the alternating forces phases C~u - il'L) on the upper and lower

rotors to make these forces act in a. counter-phase thus reducing the total alternating force and the helicopter vibr-ation level.

This appoach was verified at the Ka-25 helicopter flight testing in 1968.

3. FLYING

TEST

BED

Flying lest bed based on a helicopter was tested in

different main r·otor~s system. configurations. Nan1ely, 'P

angle-determining the upper rotor blade lead over th~

corresponding lower r-otor blade was set at different values

C1·ef. fig.1 ).

At a zero lead angle tp=O the corresponding lower and

uppet' rotor blades "meet" at a zero azimuth at the back in

the helicupler longituditlal plane. Wl1en tl·1e angle is r1ot zero rp

>

0 'lhe blades of the upper and lower rotors come

one over the other at the l e f t fi~om the helicopter synunetr·y

plane at an azumut..h 1-;/2 in the direction of lhe upper

rot.or rotation. At the cl1ange of tt1e uppet~ rotor blade

positior1 in respect to that of tl1e lower rotor blade the

differ·ence of the alternatir1g focces acting upon the upper

and lower~ rotot~ i s also changed.

The flight test results of the Ka-25 h~.:;licopt_~r·

pt··esent.eJ .at fig. 2. I t lrtay be s•:..·en that the ar!"!plitudt.? of

the ver·tic:3l vibr·ational ae:celt:-r·ations e:ons.Lder·ably dep2nds

upon the lead angle value at the flight speed app1~oac.:hing

the maximal and decreases by 3 . . . 4 t im•.?S in COiHpar-ision to

ll1at

of

ll1e

basic corlfiguralio11.

4.

~.-IATHEMATlCAL

HODELLING

The results: obtained by the ver·tic.al

altet~nat.ing forces on the coaxial

modelling

t~otor· hubs some

ir1lt:>r·8s.l.

The ULYSS-6 genet~al nLalhem.alic:al model of lh12 coaxial

rt)lor system has beera used whict1 was descr·ibed by ll·1e autor

i n his pap~~r pr·1:?Sf"":o'nted at the 17th Eur·opian Rotor·cr·;)ft For·um

i n 1991 [ 1 l .

The model represents the following aeror=-laslic factur·s :

-nonlinear motion 1!!quat.ions. :J.nd bound:.l!~y

condilior1s for the 6-bladed coaxial rolo1· including t_he joint ~~las.tic bl.-::~.de r·oot tor-~:;i.on ai. a flexible COI1lt··ol lir1kage;

-coaxial r·oloi' vor-tex blade modl:.·l;

-aerodynamic dala of lht? a i r f o i l .i.t1 .a compr'essib.lt.?'

air' flow and a dynamic s t a l l based on lhe I'esul Ls

of testit>g in a stationar-y str-eam and testing of

oscillating airfoil in the wind tunnt?-ls; - r i g i d fuselage .

Tl·1e 1·esulls of vertical

altert1aling fot'C~s

using the ULYSS-6 ar·e shown at fig. 3, 4.

calculation

At the left side of fig.3

lt1ere

at'e presented

tl1e

calculated

vector

diagrams

ol

the

vertical alter-nating

forces Fu and FL acting at a blade frequency 3w upor1 tt1e

coaxial rotor hubs. A

set.

of vector

pairs is

prest?nled whet'e

each pair cor-responds to some definite value of the lead

angle C p ). The vectors turn angle F u , FL i s equal to the

blade azimuth when the 3w frequency vertical forces reaches

i t s maximum value. Tbt:- vector length i s equal t.o the

alternating load maximum value.

I t may be seen that the upper rotor

an azimuth function depends l i t t l e up011

blades i11

respect to

each

other

(angle p

The lower

rotor fot'ce

FL C 4L

)

consider-ably upon r:~

.

The

,;.

phase is

L force the )

.

location as of amplitude depends

cl·1anged too. The

fot~ce vectot' turns in the direction of the upper t'otor

rotation approximately at. an angle +'/2. The vector· "follow"

the

points whez'e

the upper

rotoz~

bl.a,J0

t i p vot~texes

1neet

with t-he lower r·ol.or

blades

The sum .. m.ary ver·tical for·c:e tJf tht::o two t~otoJ-s

tl·.ansmitted lo the fuselago .is shown at the r·ighl side of

fig. 3. The sulmning up takes account of the upper· and lower·

r-otor blades location i n r·espect to each oth~r- determined by

a :L) angle. The sumrnary c:alculatt;:od for-{:.:8 has lh-2 lt-ast.

amplitude

at

p =60

degrees.

At

fligl·1t testi1·1g

tl·1e

lowest

v~r·tical vibr·alioi1 level was obser·ved ~l p ~ 30 deg.

Cr-ef. fi9. 3).

San11? calculations were d011e also for

nonvot'tex coaxial r·olot- system modE.•l i . e . with

fl.ow constat·1L all over the disc.

Tl·10Se

results

al fiq 4. ·It. may be seen that 3w frequency

forces values on the blades l1ave gr·owrt ]jy about

a

simpli tied

;).n induct i V(? arE:'? presented alternating 1.5 limes:o of the

the force phases do not. depend upon the location

blades in respect to each other. However· the summary

the smallest also al p =60 deg.

forces

5. THE

KA-50

HELICOPTER FLIGHT TEST RESULTS

At·t a r-eal fligl·tt side ar1d lor1giludir1al aller·r1aling

forces of 3w fr0quer1cy also act upor1 t~1e J·~~~licol)ler· alo1·1g

with lhe ver-tical for~ces which complicatt::-s tl·H2! .<H1clly~:.i.s..

Howevet' we n~rtage to acl1ieve a low vil)rati.or1 level al Kamov ht~licopter-s..

Th•?

blade a0roelas.Lit:: design ZJ.nd Lh1::- bl.<1d0

the vibr-ation pt'oblem ..

Fig.5 preset1ls ll1e vibraliotl level of out·· laslesl

ht?licopter Ka-50

. I'l

ro..a.y

be

obser-ved

t.h.at.

the

v.ibr·aliun

level alung a l l three .axises-ve-r'lical, longitudinal and

side-is ratl1er low.

6_ CONCLUSIONS

t.he field of

The investigalions carried out in

aeroelasticily and vibt~.ations .at the Kamov

pemi

t

to a chi evt2' a 1 ow level of out~ helicopters

Helicopter S&TCo.

coaxial

7.

ACKNOWLEDGEMENTS

The aulor

expr-esses

his sincere gratitude to

engineers contributing to the invesligation

and

ot' the r-esults: Mt-.A.Z.Vot'onkov, Hr.

Hr .. G.Y.Jakemenko.

R

~ r £

R

~

N C

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Bur ~s2v,B. N., "A-::?::.:·c. .. e(a.stl~C~ ty o f

the KanvJv

discussion B.P.Sobol,

h'e~ ic~·_c_.teJ.··

,C::.otor", P-::·oceedic·Lg o f t.-:...e 17th £urc,c;6:-a..n.. ,:;._:::_,tcn-c;;,··r.::.._ft

Ber-L i.-,·.~, ,:,eJ:·:r:..arLy, Sept. c:..;·-27, t£·Qt, pp ... F35--:.:S2.

2. Gu,or< .. :.,

c ..

P. ~ "BLade

h'o-:?Ltcopr_e't' VtC .. l"'at ion...s",

r:.o.

207-2 i 7.

Desi n,e Pcra.'~--'2 L rs :.._~h ~eli. 4-0th A;:.n;._..:.c:..l.. A.'i-:'S F .. :. .. l-·.1:'., ,'-;'..:.:.·v

. .;ff~-c t < ,-)..-.., '

J. :::>.::-....:-.

3. I. ?apo.uc:..s-s:'~ Lto~..r, P. ?. F:t'i.edr:ta.J·~n. C .. Ve·i'L.\c;.:tesr.:;....-~, ••,-.:_.:__.:._lol..ed Rotor/r'u..sel..aee Vib.r-at ion P..ed'u.c t ton L/st:.-~8 Nul.. t 1~p[e FJ:·&q~..:.en.c.y

BLa.d.~? Pitch Cont:col.n, Proceedine of the l7th £1.1.t·o-;:...•t?-:::J~

R.otorcro-ft Foru.;rt, Ber·L in, Ger·rtta.ny, Sept. 24-.:.::7, t99t,

pp. 76. t-76 .. -J:-4,

4. Reic.heJ:·t, G. "h'el. i.coptt!:'i' 1/ibJ:'·o.t. i(YtL C.J:.,,.~u_-o;_ -,·1 \/E•t'tica, Vol..5, ,\\::;

t.

pp. 1-20. 19-Si.

5. Lc~~).J.-'Y•P.G .. ''Hel. i.cc·pter Vi brat icn.: A

P&rsp&c f.. i. u~::-", AH5 .Jo,..rY·rtc.l, VoL. 29, No .. -~, Oc Lob~.~·l'

Tr~,.:_ hJtO J. C:~·:· l r:: C l.

6. h'am, N., "h'el..i.copter Individual.-Bl..ade-Con.t.rcl.

AppLications" 39th Ah'S Foru..·n.. St. Louis, Nissc-1n~i.

and 1Cs Nc.y 19S3.

1. R.iche2 .. , P., £isbrech..er, h'. D. o.r-td Kl.oppel., V.. ..Desin8 c~;d

FLie;ht Tests of Indiuidual. BLade Control. Actuator's",

Proceedine of !..he 16th Euro.~--ean Rotorcraft Fo:: .. Wll., Gl.asec~:.).

U.K., Sept. i8-2t,t990,pp. t t i . 6. t-11 t. 6. 9.

8. Chopra, I. , of Open-Loor:--·,

S:.~s t en'.S", Ah'S

and _7. L. i"tcCLoud. HA Nun'<..e:::iccL Si.m.ul.aticn Stt.:dy CLosed-Loop and Adapt t_,_)e N·ul. t lcycl. Lc Con .. t~ .. ol.

_]ourna7.., Vol.. 28, i'lo. i , _lan..-uary 1983, p,c. 63-77.

9. Robinson, L., c.nd Friedmann. P. P. , ''A Study, of FuYldcvrten[o..l.

Issues in Hieher Har·nwni.c ControL Lls-in.e; Aercetastic

Sim.ul.at ion, "AN5

_1o-urnal.,

Vol.. 36, No.2, Apri L1991, pp. 32-43.

1(). Lehn"-a.nn,G., .. Th.e Effect of h'iehe:: .. h'a..rmonic Co;·":..Lr·ol. Ch'h'C) t_"';n. a Fc··ur-Bl.od.es Nihg·el..ees h'odel. Poto·l''", Vert Lee. VoL.:;,', i"io. 3, 19.&5, .c. ... c. 273-2.:34

and Tec.L,R.,

i-. .'u.~··t:•tortic CorttJ.~.:.:,L: hlin..d T·u.nneL De ... 'tOi'tst.::··c.ticn of Fu.LL~.' ~ffec t.. i)_'e Vi.b~··at.cry J-t'-u .. l:.., Fo2··ce Supp~~ess-ic:n," Ah'S ..Journc.~,

~-'c,z.. 34~ :\\:_,_ 1, _,'a_r~ur.:.t.T":>li·;.".:iB9, pp. 14-25.

t3. Niao,l·/. Ho.rn:.c.'n L c \ilush i r~.-::; t_ on,

Pe>t:..-e::.··s, ..J. J-l. , C L i !'1.~"" , _;. h'. ,

cn·u.J Frye, H. i'·t. ,

Contr·ot (/1HC)

D. C. .June 1986.

.. Fl. i eh t Derru::.>r(.s-t l'·a t ion. o j

on. S-76", 42nd AJ-lS Achache, h'. , ''h'ieh.eJ.~ Con t .::··c· ~ Hi ~"':..1'-:~S-:: F C.J r ·,.J.rt~.., 14. Pol.ychrcn.i.c.dis, N. O..tuJ.

Contl·ul: FLieht Tests of an

F::c;.s-=.-a.rch G(:.z&L Le", 42nd AJ·{S

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