The engines would have variable nozzles, which would be fully opened on take-off and landing, so that the exhaust would be slow and relatively quiet, and narrowed when the aircraft was cruising, providing the necessary speed.
【核心词汇】
variable a.易变的，多变的
exhaust n.排气，废气，排气管
relatively ad.相当地，相对地 take-off起飞
【结构分析】
本句主句为The engines would have variable nozzles。其后的定语从句比较复杂，以逗号后and为切入点，得到which would be fully narrowed...，关系代词which指代先行词nozzles在从句中作主语。其中so that引导的结果状语从句so that the exhaust would be slow and relatively quiet说明(nozzles)would be fully opened on take-off and landing情况发生时所产生的.结果，句末的现在分词结构providing the necessary speed作(nozzles)would be narrowed when the aircraft was cruising情况发生时的伴随结果，其中when引导的从句作时间状语。
【参考译文】
引擎将配有各种各样的尾喷管，在起飞和降落时这些尾喷管会全部打开，这样喷气速度会下降，噪音也会更小;而在飞机巡航过程中，尾喷管将变小以保证必要的飞行速度。
