摘要
共轴刚性旋翼桨尖间距是涉及直升机飞行安全的关键问题,建立了共轴刚性旋翼综合气弹分析模型,开展了升力偏置、前进比、旋翼交叉角、提前操纵角以及高阶气动载荷等因素对桨尖间距影响的数值计算和分析研究.研究结果表明,升力偏置是影响桨尖间距最重要的因素,随着升力偏置量增大,桨尖间距呈线性减小趋势,而且旋翼拉力越大,趋势直线的斜率就越大,本质上桨尖间距是由桨根动态挥舞弯矩决定的,桨尖间距不会随飞行速度的增大而减小,旋翼交叉角对桨尖间距有一定的调节作用,提前操纵角和高阶气动载荷则对桨尖间距影响很小.
共轴刚性旋翼桨毂阻力占全机阻力50%以上,而且随着飞行速度增大,共轴桨毂阻力还会激增,减小桨毂阻力是共轴刚性旋翼高速直升机必须解决的关键问题之一,而减小上下旋翼安装间距就是减小桨毂阻力最简单和最有效的手段.但是另一方面,共轴刚性旋翼在工作状态需要承受很大的升力偏置载荷,共轴反转的上下旋翼桨叶会发生显著的弹性形变,过小的安装间距可能会导致上下旋翼桨尖发生碰撞,影响直升机的飞行安全,因此必须对共轴刚性旋翼的桨尖间距进行准确的评估和设计,保证在整个飞行包线内始终保持安全间
美国西科斯基公司在共轴刚性旋翼技术方面处于领先地位,已先后研制了XH-59A、X-2和S-97等多型共轴刚性旋翼高速直升
本文首先建立了共轴刚性旋翼综合气弹动力学分析模型,以及含升力偏置目标的共轴双旋翼配平计算方法,开展升力偏置、前进比、旋翼交叉角等多种因素对桨尖间距影响的数值计算和分析研究,揭示了不同因素对桨尖间距影响的作用机理和影响强弱,为共轴刚性旋翼设计提供理论支撑.
根据Hamilton变分原理推导旋翼气弹动力学方程,其中δU, δT, δW分别是动力学系统的应变能、动能和外力虚功变分.
(1) |
桨叶结构动力学模型基于中等变形梁理论,分别用u,v,w,ϕ来描述桨叶的拉伸、摆振、挥舞和扭转运动,根据中等变形梁弹性变形Green应变表达式以及桨叶上任意点位置矢量表达式,可以推导出桨叶应变能项和动能项的变分表达式.桨叶空气动力学模型基于二阶升力线理论,翼型气动力计算采用Greenberg准定常气动力模型,入流采用线性入流或者自由尾迹模型,进而可以得到气动外力虚功,各变分表达式如
(2) |
采用如
(3) |

图1 弹性桨叶15自由度梁模型示意图
Fig.1 Schematic diagram of 15-DOF beam model of elastic blade

图2 共轴刚性旋翼气弹动力学模型
Fig.2 Aeroelastic dynamic model of coaxial rigid rotor
对于共轴双旋翼结构建模,根据上下旋翼同轴反转的关系,可得到上下旋翼的旋转角速度和桨毂载荷具有如
(4) |
(5) |
共轴刚性旋翼的操纵是有冗余的,在本文中根据既定的操纵策略,固定旋翼提前操纵角Γ,采用
(6) |
(7) |
利用XH-59A风洞试验数据对本文计算模型进行验

图 3 XH-59A桨叶剖面挥舞弯矩载荷计算对比
Fig.3 Comparisons of flapping moment loads on XH-59A blade profile
本文计算所采用的共轴刚性旋翼基本参数如

图4 桨尖间距示意图
Fig.4 Diagram of tip clearance
在典型前进比0.25状态下,对不同旋翼拉力和升力偏置状态进行了配平计算分析,从

图5 配平操纵量随升力偏置变化
Fig.5 Variation of trimmed control with lift offset

图6 旋翼需用功率随升力偏置变化
Fig.6 Variation of rotor power required with lift offset

图7 桨尖间距随升力偏置量的变化
Fig.7 Variation of tip clearance with lift offset

图8 桨尖间距随桨毂滚转力矩的变化
Fig.8 Variation of tip clearance with rotor hub roll moment
在额定旋翼拉力状态CT=0.0135下,计算了三种升力偏置状态,桨叶挥舞弹性形变和桨尖间距随前进比的变化情况,前进比μ是表征旋翼前飞状态的无量纲参数(μ=V/Vtip),即直升机前飞速度与旋翼桨尖速度的比值.如

图9 不同前进比下的桨尖挥舞位移曲线
Fig.9 Blade tip flap curve at different advance ratios

图10 桨尖间距随前进比的变化
Fig.10 Variation of tip clearance with advance ratio
旋翼交叉角是共轴双旋翼特有的设计参数.如

图11 共轴双旋翼交叉角示意图
Fig.11 Diagram of coaxial rotor crossover angle
如下
与常规旋翼按照挥舞响应滞后角度来确定提前操纵角不同,共轴刚性旋翼的提前操纵角通常设置为可变的.
(8) |

图12 不同提前操纵角配平情况下上旋翼周期变距变化历程
Fig.12 Cyclic pitch angle of upper rotor with different advance control angle
气动干扰是共轴刚性旋翼比较突出的问题,需要考虑它对桨尖间距的影响,如

图13 入流模型对桨尖挥舞位移的影响
Fig.13 Effect of inflow model on blade tip flapping displacement

图 14 基于自由尾迹的桨尖位移谐波幅值
Figure 14 Harmonic amplitude of tip displacement based on free wake model
另外,共轴反转的上下旋翼还会产生频率为2Nb/rev的气动干扰脉冲载荷,同样的道理,这种现象对桨尖间距也基本没有影响.
(1) 升力偏置是桨尖间距最重要和最显著的影响因素.随着升力偏置量增大,桨尖间距呈线性减小趋势,而且旋翼拉力越大,趋势直线的斜率就越大,本质上桨尖间距是由作用在桨根的挥舞弯矩载荷决定的;
(2)前进比对桨尖间距的影响程度较弱.随着前进比的增大,桨尖间距没有减小的明显趋势,这个性质对于飞行安全控制是很有益的;
(3)共轴刚性旋翼桨尖最小间距一般出现在270°方位角附近.此时上旋翼(俯视逆时针)在后行侧下挥最多,下旋翼(俯视顺时针)在前行侧上挥最多,旋翼交叉角决定了上下旋翼桨尖可能发生碰撞的方位角,因此它具有一定的调节作用,以避开发生最小间距的方位角.但需要说明的是,共轴旋翼交叉角对直升机振动响应有非常重要的影响,需要综合考虑;
(4) 提前操纵角对桨尖间距没有影响.它只是影响纵横向操纵量的分配,但是分别对于上下旋翼来说,在配平状态下它们的变距操纵历程是相同的,那么桨叶的动力学响应也就相同,因此对桨尖间距没有影响;
(5)旋翼气动干扰等因素对桨尖间距的影响很小.桨尖间距问题是位移动力学响应造成的结果,主要由低频的1/rev升力偏置载荷控制,气动干扰引起的高频气动载荷对位移响应的贡献很小.
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