Wind Tunnel Studies on Hover and Forward Flight Performances of a Coaxial Rigid Rotor

Chang Wang,Guichuan Zhang,Minqi Huang,Xianmin Peng,Haowen Wang,Min Tang

doi:10.3390/aerospace8080205

Chang Wang, Guichuan Zhang + Show 4 more

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https://doi.org/10.3390/aerospace8080205

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Abstract

The aerodynamic performance of a reduced-scale coaxial rigid rotor system in hover and steady forward flights was experimentally investigated to gain insights into the effect of interference between upper and lower rotors and the influences of the advance ratio, shaft tilt angle and lift offset. The rotor system featured by 2 m-diameter, four-bladed upper and lower hingeless rotors and was installed in a coaxial rotor test rig. Experiments were conducted in the Φ3.2 m wind tunnel at China Aerodynamics Research and Development Center (CARDC). The rotor system was tested in hover states at collective pitches ranging from 0° to 13° and it was also tested in forward flights at advance ratios up to 0.6, with specific focus on the shaft tilt angle and lift offset sweeps. To ensure that the coaxial rotor was operating in a similar manner to that of the real flight, the torque difference was trimmed to zero in hover flight, whilst the constant lift coefficient was maintained in forward flight. An isolated single-rotor configuration test was also conducted with the same pitch angle setting in the coaxial rotor. The hover test results demonstrate that the figure of merit (FM) value of the lower rotor is lower than that of the upper rotor, and both are lower than that of the isolated single rotor. Moreover, the coaxial rotor configuration can contribute to better hover efficiency under the same blade loading coefficient (CT/σ). In forward flight, the effective lift-to-drag (L/De) ratio of the coaxial rigid rotor does not monotonously change as the advance ratio increases. Increases in the required power and drag in the case with a high advance ratio of 0.6 leads to the decreasing L/De ratio of the rotor. Meanwhile, the L/De ratio of the rotor is relatively high when the rotor shaft is tilted backward. The increasing lift offset tends to result in reduced required rotor power and an increase in the rotor drag. When the effect of the reduced rotor power is greater than that of the increased rotor drag, the L/De ratio increases as the lift offset increases. The L/De ratio can benefit significantly from lift offset at a high advance ratio, but it is much less influenced by lift offset at a low advance ratio. The forward performance efficiency of the upper rotor is poorer than that of the lower rotor, which is significantly different from the case in the hover flight.

Highlights

One of the future helicopter development directions is to achieve high-speed flight
Based on the Momentum Theory developed by Leishman [1] that has been utilized in optimizing the aerodynamic design of a coaxial rotor [15,16,17,18] or propeller [19] in hover and axial flight conditions, the inflow of the lower rotor contains its own induced velocity and the wake of the upper rotor, resulting in Scyoanltrfoorlsa[7co].aTxhiael trhortuorstucsoinefgfiacifernetesvoofrtthexe mupeptheor dan[2d0l]o. wer rotors under the coaxial rotor condTithioenFMarevaloluweerofththaen stihnagtleofistohleatseidngrloetoisrofliartsetdinrcorteoarseast athsethseamcoellpecotwiveer pciotnchsurmiseesd, rdeuacehtiongthietsimntaexrfiemreunmcevbaelutwe eoefn0.t5h9e(uFipgpuerre a7n).dTlhoiws emr arxoitmorusmthvaatlucaeuissesclothsee troottohraet fofifecaierlniecry rtootodrescrweiatshesaims tihlaer rsootloidr iitnyfl[o2w1] ibnuctreloasweesr
The rotor system was trimmed, followed by measurements of forces and moments of both rotors as well as investigations into the effects of shaft tilt, advance ratio and lift offset on performance and interference of rotors

Summary

Introduction

One of the future helicopter development directions is to achieve high-speed flight It has been challenging for a conventional helicopter to fly faster than 360 km/h because of the lift that needs to be balanced on the advancing and retreating sides of the rotor. During high-speed forward flight, the lift is mainly borne by the advancing side of the rotor disk, the coaxial rotors balance the rolling moment and counter torque, and the stall on the retreating blade would be much less important. The rotor speed is lowered to keep the advancing blade tip below the sound barrier [3,4] This would result in an enhanced high-speed forward flight capability and an improved cruise efficiency whilst retaining the hover efficiency and maneuverability of the helicopter. It is imperative for the theoretical research and engineering design of coaxial rigid rotors to implement a coaxial rigid rotor wind tunnel test to study rotor lift offset and mutual interference of coaxial rotors

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