COLLI, ANDREA

Gibertini, G.

Zanotti, A.

English

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DYNAMIC STALL INDUCED BY BLADE VORTEXINTERACTION IN HELICOPTER DESCENDING FLIGHTGiuseppe Gibertini, Andrea Colli, Alex Zanottigiuseppe.gibertini, andrea.colli, alex.zanotti @polimi.itDepartment of Aerospace Science and Technology, Politecnico di Milano, ItalyAbstractIn recent years studies have shown how blade-vortex interaction (BVI) can cause the onset of dynamic stall onhelicopter rotors. In order to investigate this phenomenon wind tunnel tests were performed on a rotor model indescent flight conditions. The anaylsis of the rotor global performances indicated a defect of thrust for a particularvalue of collective pitch angle that can be explained as the occurrence of dynamic stall induced by a perpendicularvortex interaction. This conclusion is supported by evidence from both flapping angle measurements and by theresults of numerical simulations performed using a code based on blade-element coupled with a vortex-particlewake model.List of symbolsCt rotor thrust coefficientSH;u rotor angle of attack, uncorrectedSH;c rotor angle of attack, corrected blade flapping angle, mean removed rotor Lock number advance ratio blade azimuth angle rotor solidityc collective pitch angles longitudinal cyclic pitch angle1 INTRODUCTIONThe possibility of a collision between the helicopter ro-tor blade tip vortex and a following blade in descend-ing flight is well known and it is, in general, associ-ated with noise production. Indeed this is one of themain sources of helicopter noise and has been widelystudied under this point of view. As a matter of fact,helicopter pilots try to avoid blade vortex interaction(BVI) conditions in order to reduce the noise propaga-tion to the surrounding area. Nevertheless, this eventcan occur and a collateral effect that has not yet beenwidely investigated is the possible triggering of retreat-ing blade dynamic stall.As a matter of fact, a few years ago, a previous se-ries of experiments carried out at Aerodynamic Labo-ratory of Politecnico di Milano on an oscillating 2D air-foil model demonstrated that a perpendicular vortexcoming from upstream can induce dynamic stall [9;4].In these experiments, the vortex issuing from the tipof the upstream airfoil (spanning just half test section)perpendiculrly impacted on the downstream oscillat-ing airfoil inducing a local remarkable variation of theangle of attack and therefore facilitating the onset ofdynamic stall. This observation led to the idea thata similar effect could happen on the real helicoptermain rotor in BVI conditions. Indeed in the last yearssome other authors, Chaderjian in 2017 [1] and Richezin 2018 [8] found similar effects by means of CFD sim-ulations on a complete helicopter rotor in BVI condi-tions. In particular, Chaderjian showed how a perpen-dicular BVI can induce dynamic stall on the retreatingblade in the case of a four-bladed rotor at  = 0:153and Ct = 0:00657, with a rotor angle of attack of 0:75°.Thus a wind tunnel test activity on a rotor model wasplanned, in order to experimentally assess that thisphenomenon is possible and to investigate its effecton rotor performances.2 WIND TUNNEL TEST SETUPThe experimental tests were carried out in the GVPM(Large Wind Tunnel of Politecnico di Milano), usinga four-bladed, fully articulated rotor model (see Fig-ure 1).The 4m x 3:84m test chamber was configurated inthe open jet mode. The test rig was based on a whirltower already extensively used in GVPM for previousresearches [2;3]. This whirl-tower is powered by an hy-draulic motor (maximum power 16 kW at 3000RPM)and equipped with a strain gauge balance allowing tomeasure the thrust while the torque was measuredby means of strain-gauges direclty fixed on the rotor-motor shaft. The whirl tower allowed to set the shaftangle of attack keeping the rotor head in the centerof the testing area. The rotor was equipped with aThis work is licensed under the Creative Commons Attribution International License (CC BY). Copyright © 2019 by author(s).Figure 1: Test rig in place in the Large Wind Tunnelset of four rectangular, untwisted carbon fiber blades,0:6m long and with a constant chord of 0:06m. Theblade section, constant along the span, was repre-sented by a NACA 0012 airfoil. The rotor had a ra-dius R = 0:8m, a solidity of  = 0:0955 and a Locknumber  = 4:77.The rotor hub swashplate was actuated by three lin-ear servo-motors connected to a control unit allowingfor manual setting by means of three knobs.The run-time measurement of pitch, flap and lag an-gles was made by means of Hall effect magnetic sen-sors. All the signals coming from the rotating part wereconnected to the DAQ system through a slip-ring. Therotor RPM too was measured by means of Hall effetssensor.3 TEST PROCEDURE3.1 Preliminary studyBefore starting with the test activity, a preliminarystudy was conducted investigating the trajectory ofthe wake tip vortices in order to find suitable interest-ing test conditions. This analysis made use of a sim-ple epicycloidal model [6] in order to find the possibleblade-vortex intersections. In particular the case of = 0:13 was analysed, close to the value of the testcase of Chaderjian [1] and characterized by a ratherlarge number of possible intersections.Figure 2 shows the possible intersection pointscome out from this analysis and coloured on the baseof the BVI typology, with black indicating perpendicu-lar BVI, red indicating parallel BVI, and blue indicatingoblique BVI.The perpendicular interaction on the external re-treating blade (indicated in the figure) was consid-ered the most interesting for this study. Assuming aCt = 0:007 (as in [1]) and making use of the Mangler-Squire [7] model for induced velocity, an angle of attackaround  2° was found for that BVI occurrence.3.2 Test campaignTo produce the desired descent flight condition, therotor shaft was inclined backwards resulting in a neg-ative angle of attack. As the application of wind tunnelcorrections would lead to a lower (in absolute value)angle of attack, the tests were carried out at two differ-ent shaft angle of attack (SH;u = 2° and SH;u = 4°)that included the estimated angle for the interaction.The tests consisted in a series of sweeps of collec-tive pitch angle c, ranging from 4° to 16°. For eachtest point the longitudinal cyclic pitch angle was man-ually adjusted to keep the flapping response as smallas possible.As a non negligible level of vibrations was expected(due to the possible onset of dynamic stall) the rotorangular velocity was kept relatively low (1200 RPM)This work is licensed under the Creative Commons Attribution International License (CC BY). Copyright © 2019 by author(s).Figure 2: Possible BVI points in the disk plane eval-uated by epicycloidal model (black for perpendicular,red for parallel and blue for oblique BVI)leading to a tip velocity of 100ms 1 (Mtip = 0:2875).4 EXPERIMENTAL RESULTSFor both the considered angles of attack it was foundthat at a particular value of the collective pitch angle(not the same for the two cases) a defect of the rotorthrust coefficient was apparent: the thrust suddenlydecreases then increases again showing a local valleyin the Ct - c graph as can be seen in Figure 3 andFigure 4. This effect could be explained as the effectof the dynamic stall triggered by the interaction withthe vortex, although its occurrence was for a Ct higherthan expected while the corrected angle of attack isclose to what estimated.In tables 1 and 2 the Ct obtained at high range ofmeasured collective pitch angles are listed with thecorrected value of incidence estimated by means ofHeyson method [5].c s Ct SH;c11:1° 6:4° 0:0107 2:5°12:0° 7:9° 0:0111 2:6°12:9° 8:9° 0:0101 2:6°14:0° 9:6° 0:0110 2:5°15:1° 10:8° 0:0112 2:5°16:0° 12:5° 0:0113 2:5°Table 1: Test points for SH;u = 4°0 5 10 1500:20:40:60:811:210 2c [°]Ct[ ]experimental dataFigure 3: Thrust coefficient in function of collectivepitch angle for SH;u = 4°,  = 0:130 5 10 1500:20:40:60:811:210 2c [°]Ct[ ]experimental dataFigure 4: Thrust coefficient in function of collectivepitch angle for SH;u = 2°,  = 0:13Figure 5 presents, for the case of SH;u = 4°, theblade flapping behaviour in the condition where theCt ”‘valley” (c = 12:9°) was found compared withthe behaviour at previous and next collective pitch an-gles. It can be see that from c = 12° to c = 12:9°the flapping becomesmuch more irregular with appar-ent spikes occurring any two rotor revolutions. Thesespikes disappeared again for c = 14°. Although thisobservation is not a direct evidence of the BVI inducedstall, it indicates that something of very particular ishappening at that condition.Further investigations by means of PIV techniqueand including more test conditions are forseen for thenext year at GVPM.This work is licensed under the Creative Commons Attribution International License (CC BY). Copyright © 2019 by author(s).0 2 4 6 8 10 12 2 1012 [rad][°]c = 12° c = 12:9°0 2 4 6 8 10 12 2 1012 [rad][°]c = 12:9° c = 14°Figure 5: Blade flapping behaviour measured for SH;u = 4°,  = 0:13c s Ct SH;c11:0° 7:7° 0:0104 0:6°11:9° 7:9° 0:0110 0:5°12:9° 9:3° 0:0113 0:5°13:9° 10:7° 0:0112 0:5°15:1° 12:9° 0:0104 0:6°15:9° 14:2° 0:0109 0:5°17:1° 16:2° 0:0111 0:5°Table 2: Test points for SH;u = 2°5 NUMERICAL SIMULATIONIn order to get a first confirmation of the experimen-tal results a series of numerical simulations were car-ried out by means of an in-house code based onblade-element coupled with a vortex-particle wakemodel. The code did not include a dynamic modelmethod and, furthermore, such a three-dimensionalphenomenon is not expectable to be correctly repro-duced by a quasi-2D approach as blade-element; nev-ertheless, this code can give an indication of the tipvortex trajectory.The simulation were carried out for the correspond-ing free flight conditions (i.e. at the corrected valuesof the angle of attack), for SH;u = 4°.Figure 6 shows the tip vortex path, confirming theperpendicular BVI in correspondence of the collectivepitch angle for which the Ct defect was observed.6 CONCLUSIONSAn experimental wind tunnel activity carried out in pos-sible BVI conditions showed a Ct defect for a par-ticular value of collective pitch angle that can be ex-plained as the onset of dynamic stall induced by a per-pendicularly interacting vortex. This hypotesis seemsto be confirmed by several observations and simpli-fied analytical models; nevertheless, the phenomenonrequires further investigation for a better physical in-sight. Furthermore, it is important to understand if sim-ilar behaviour can occour at lower values of thrust co-efficient more characteristic of the flight envelope.This work is licensed under the Creative Commons Attribution International License (CC BY). Copyright © 2019 by author(s).Figure 6: Tip vortex trajectory indicating blade-vortex interaction on the retreating blade, for the case of SH;u =4°,  = 0:13 and c = 12:9°, top view (left) and lateral view (right); direction of flight is to the left, the interestedblade is shown at  = 284°References[1] N. M. Chaderjian. Navier-stokes simulation of uh-60a rotorwake interaction using adaptive mesh re-finement. In Proceedings of the 73rd Annual Fo-rum of the American Helicopter Society, 2017.[2] G. Droandi, A. Zanotti, and G. Gibertini. Aerody-namic interaction between rotor and tilting wing inhovering flight condition. Journal of the Americanhelicopter Society, 60(4):1–20, 2015.[3] G. Droandi, G. Gibertini, D. Grassi, G. Campa-nardi, and C. Liprino. Proprotor–wing aerody-namic interaction in the first stages of conversionfrom helicopter to aeroplane mode. AerospaceScience and Technology, 58:116–133, 2016.[4] G. Droandi, G. Gibertini, and A. Zanotti. Perpen-dicular blade–vortex-interaction over an oscillatingairfoil in light dynamic stall. Journal of Fluids andStructures, 65:472–494, 2016.[5] H. H. Heyson. Rapid estimation of wind-tunnel cor-rections with application to wind-tunnel and modeldesign. 1971.[6] G. J. Leishman. Principles of helicopter aero-dynamics with CD extra. Cambridge universitypress, 2006.[7] K. Mangler and H. Squire. The Induced VelocityField of a Rotor. Aeronautical Research Coun-cil. Reports and memoranda. Ministry of Supply,Aeronautical Research Council, 1950.[8] F. Richez. Analysis of dynamic stall mechanismsin helicopter rotor environment. Journal of theAmerican Helicopter Society, 63(2):1–11, 2018.[9] A. Zanotti, M. Ermacora, G. Campanardi, andG. Gibertini. Stereo particle image velocimetrymeasurements of perpendicular blade–vortex in-teraction over an oscillating airfoil. Experiments inFluids, 55(9):1811, 2014.Copyright StatementThe authors confirm that they, and/or their company or organization, holdcopyright on all of the original material included in this paper. The authors alsoconfirm that they have obtained permission, from the copyright holder of anythird party material included in this paper, to publish it as part of their paper.The authors confirm that they give permission, or have obtained permissionfrom the copyright holder of this paper, for the publication and distribution ofthis paper as part of the ERF proceedings or as individual offprints from theproceedings and for inclusion in a freely accessible web-based repository.This work is licensed under the Creative Commons Attribution International License (CC BY). Copyright © 2019 by author(s).

Dynamic Stall Inducted by Blade Vortex Interaction in Helicopter Descending Flight

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Dynamic Stall Inducted by Blade Vortex Interaction in Helicopter Descending Flight

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