Abstract Numerous issues are yet to be discovered about the small tandem rotor aerodynamics in the vicinity of ground. In this paper, a multipurpose test rig has been used to test outwash patterns of a sub-scale model of a generic tandem rotor helicopter in ground effect using two total-static rakes at different positions ahead and aft of the rotor(s). The results showed that in tandem rotor, the change in the rakes distance from the rotors has mild influence on the outwash flow velocity. The interaction between rotor wakes and formation of the fountain flow impressed the outwash of the rotors and changed their behavior in comparison with a single rotor. For the elevation higher than z/2R = 0.8, the outwash averaged velocity of the rotors due to flow recirculation was negative. As the rotor elevation decreased, outwash accelerated by moving away from the rotor within the measurement area. Comparing the results of present study with the experiments for the CH-47D helicopter indicated that the overlap between the rotors was caused the outwash velocity be more than non-overlapping rotors. Tuft flow visualizations showed the evidences of a fountain flow between the rotors and that, in the vicinity of tandem rotors, the resultant outwash velocity was inward due to formation of the fountain flow and the consequent suction.

中文翻译：

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地效下亚尺度串联转子周围的流出流测量

摘要 关于地面附近的小型串联转子空气动力学还有许多问题有待发现。在本文中，一个多用途试验台已被用于在地面效应下使用两个全静力耙在旋翼前后不同位置测试通用串联旋翼直升机子比例模型的外洗模式。结果表明，在串联转子中，耙距的变化对出水流速的影响较小。与单个转子相比，转子尾流和喷泉流形成之间的相互作用给转子的外洗留下了深刻的印象，并改变了它们的行为。对于高于 z/2R = 0.8 的高程，由于流动再循环，转子的流出平均速度为负。随着转子高度降低，通过在测量区域内远离转子来加速流出。将当前研究的结果与 CH-47D 直升机的实验进行比较表明，旋翼之间的重叠导致外洗速度大于非重叠旋翼。簇流可视化显示转子之间存在喷泉流的证据，并且在串联转子附近，由于喷泉流的形成和随后的吸力，产生的流出速度向内。