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Racteristics for Tandem Rotors in Forward Flight The 2nd Asian/Australian Rotorcraft Forum and th The 4 International Basic Research Conference on Rotorcraft Technology Tianjin, China, September 08-11, 2013 Parametric Effect Investigation on Aerodynamic Interaction Cha- racteristics for Tandem Rotors in Forward Flight Huang Shuilin Senior Engineer Science and Technology on Rotorcraft Aeromechanics Laboratory CHRDI, Jingdezhen, China (Tel：0798-8465668, E-mail：[email protected]) Xu Guohua Lin Yongfeng Professor Senior Researcher Science and Technology on Rotorcraft Science and Technology on Rotorcraft Aeromechanics Laboratory Aeromechanics Laboratory NUAA, Nanjing, China CHRDI, Jingdezhen China Huang Jianping Liu Zhangwen Deputy Chief Designer Assistant Engineer Science and Technology on Rotorcraft Science and Technology on Rotorcraft Aeromechanics Laboratory Aeromechanics Laboratory CHRDI, Jingdezhen China CHRDI, Jingdezhen China ABSTRACT An iterative free-wake computational method has been developed for the prediction of aerodynamic inte- raction characteristics between the twin rotors of a tandem helicopter. In this method, the mutual interaction effects between twin rotors as well as the rotor and wake have been included, and a rotor wake model, blade aerodynamic model and rotor trim model have been coupled during the process of solution. A new dual-rotor trim method has been presented to be suitable for the aerodynamic interaction calculations be- tween tandem twin rotors. By the present method, the blade aerodynamic loads and rotor performance on a tandem twin-rotor in the conditions of interactions have been calculated, and the comparisons with the available experimental data have been also made to show the capability of the present method. Then, the effects of such parameters as the longitudinal separation and axial separation between twin rotors on the aerodynamic interaction characteristics have been systematically analyzed. As a result, the conclusions which can be meaningful for guiding on structure layout of the tandem rotors have been obtained. Further- more, the rotor performance between the tandem rotors and a single rotor has been compared, and the cal- culated results indicate that the strongest interaction state does not appear in the hovering state, but in low- speed forward flight state. ping each other, which easily make the aft INTRODUCTION rotor under the vortex wake of the front one. The front rotor wake may either impact the Tandem helicopter has two rotors overlap- aft rotor directly or closely pass through the 358 The 2nd Asian/Australian Rotorcraft Forum and The 4th International Basic Research Conference on Rotorcraft Technology Tianjin, China, September 08-11, 2013 aft one, severe aerodynamic interaction oc- till 1995, Bagai adopted free wake method curs between twin rotors as well as the rotor to investigate wake dynamics of tandem ro- and wake. Compared to single rotor, the mu- tors as shown in literature[4] at the first tual interaction between twin rotors and time. The literature just analyzed the geome- wake causes a larger variation of the flow try characteristic of rotor wake without field and aerodynamic characteristics, and showing aerodynamic force of blade ele- makes the situation more complicated than ment and rotor performance. In China, a single rotor, and makes the aerodynamic large amounts of researches[5]~[9] on aero- analysis method significantly different from dynamic interaction for tandem rotors have ingle rotor. The aerodynamic interaction be- been performed which have made promising tween twin rotors finally affects the twin progress, but most of them are for coaxial rotors performance directly. As the new twin rotors[6][7] and side-by-side (tilt) twin generation helicopter is toward the direction rotors[8][9]. And less literature has investi- of more compact structure and bigger disk gated on parameter effect of twin rotors loading, the aerodynamic interaction be- layout. tween rotors becomes stronger. It will be a As helicopter lifting and control surface, critical advancement for helicopter design to rotor performance plays a decisive role in deeply carry out research on the twin rotors flight quality, reliability and comfort; more- aerodynamic interaction of tandem helicop- over, reduction of aerodynamic interaction ter. and improvement of rotor performance re- The researches on aerodynamic characteris- main always a goal in tandem helicopter tics of twin rotors of tandem helicopter design. However, exactly calculation of ro- started in about 1980’s, those researches al- tor performance is a challenge task, and the most aimed at general performance, stability main reason is that rotor performance not and control of tandem helicopter mainly by only has a high correlation with blade three- experiments[1][2], which spread less in re- dimensional effect, but also with correct spect of aerodynamic interference and rotor capture of rotor wake. Meanwhile, when performance within aerodynamic interac- blade tip Mach number is relatively large, tion. Later on, some researches investigated compressibility effects will rapidly increase aerodynamic interference of overlapping rotor power consumption. Further more, part of two rotors by adopting simple mo- even in steady forward flight, the blade rela- mentum theory and blade element theory, tive wind and attack angle are still unsteady for example, literature [3] deduced compu- because of blade rotation, especially when ting formula of performance for tandem ro- there exit mutual interaction between twin tors in hover, and drew a summary on some rotors. All this factors make the perfor- previous experiments. The main disadvan- mance calculation of two rotors more com- tage of momentum theory is that the in- plicated. duced velocity distribution across rotor disk Aerodynamic analysis of tandem helicopter is uniform, which significantly differs from can adopt some different methods. Recent the reality situation. From eighty’s of last years, CFD method has made a considerable century on, some researchers initially used progress in modeling rotor flow-fields and vortex theory to study aerodynamic interfe- performance, and some researchers begin to rence characteristics of tandem rotors. But choose advanced CFD methods to investi- 359 The 2nd Asian/Australian Rotorcraft Forum and The 4th International Basic Research Conference on Rotorcraft Technology Tianjin, China, September 08-11, 2013 gate flow-fields interaction of twin ro- induced velocity of rotor 1 (R1) and rotor 2 tors[5], which represents the development in (R2) correspondingly. the future. But CFD method requires much To obtain the correct wake structure, the calculating resources, and numerical dissi- governing equation of vortex line must be pation remains still relatively obvious, espe- numerically discreted and solved. Although cially that research on the aerodynamic in- equation (1) is relatively simple in form, terference characteristics with considering the solution is quite difficult. This is be- vortex as the main feature factor remains in cause the local velocity includes free-flow the development stage for twin rotors. Vor- speed, mutual induced velocity and self- tex theory (wake analysis) which directly induced velocity of wake vortex in tandem solves governing equation of vortex line rotors. And in physics, helicopter rotor could be a more convenient method. Thus, wake is unstable. This results in great diffi- this paper tries to establish a coupling itera- tive computational method of free-wake culties in solving the governing equation of which is suitable to aerodynamic characte- rotor wake. Previous methods mostly adopt [10] ristics analysis of twin rotors, and parameter explicit time marching method . One ma- effect on performance of tandem rotors is jor problem of explicit time marching me- emphatically investigated. thod is the numerical instability. After- For better researching the aerodynamic inte- wards, some researchers have attempted to raction characteristics of tandem rotors and partly overcome this problem by introduc- comparing with single rotor, the paper has ing numerical damping. But also rotor defined the additional power factor of twin wake is strained when introducing numeri- rotors. Then the variation rules with differ- cal damping, the condition of no load- ent twin rotor structures and layouts are sys- bearing is destroyed and the results can not tematically studied. be reliably used. Compared to explicit time COMPUTATIONAL METHOD AND marching method, Semi-implicit scheme MODEL relaxation method has better numerical sta- bility, this paper adopts relaxation me- Free Wake Model for Aerodynamic Inte- thod[11]. raction of Tandem Rotors When equation (1) is solved, we need to In accordance with the definition of free give initial value in the spatial domain and wake, vortex lines freely move at the local boundary conditions in the time domain, speed in the flow field, in mathematics, it namely: can be written as a first order ordinary diffe- ψψ= ψ + πψ rential equations (ODE),namely rr(,)(WW 2,) (2) dr(,ψψ ) W ==++Vr((ψψ , )) V v (( rψψ , ) rr(ψ ,0)=tip [(cos β00 cos ψα coss ++ sin β sin αs )i dt WW0 11 cosβ0 sin ψjk+− (sin β 00 cos αss cos β cos ψα sin ) ] vrvrvr12 ((ψψ ,WWW )++21 (( ψψ , )22 (( ψψ , )) (3) (1) Where rtip is shed location of tip vortex, Where r is the position vector of wake β0 is coning angle of the rotor, and αs is point, ψ is the blade azimuth angle, ψW is tip-path-plane tilt angle. the age angle of wake vortex. V0 is the ve- locity of free flow, v11
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